My mailbox is currently overflowing with comments and questions concerning rampant rumors that in the March 2011 time frame a U.S. military reconnaissance aircraft was forced to land during an annual major east Asian military exercise, known as Key Resolve, due to GPS jamming. The jamming reportedly took place along the northern portion of the 684-mile long Korean peninsula, with the jamming supposedly originating with the North Koreans. The jamming scenario should come as no surprise, but it is the emergency or forced landing due to loss of a GPS signal among other supposed “facts” with which I take issue.
The Rest of the Story
As a former USAF (United States Air Force) aviator, who spent literally thousands of hours in the cockpits and mission compartments of various and highly sophisticated reconnaissance aircraft, allow me to set the record straight on several important issues. First the reports that the plane was forced down or made an emergency landing due to loss of GPS are certainly inaccurate, an exaggeration, and a devious way to generate headlines. The journalist who initially reported the incident was simply seeking media attention and was unfortunately successful. The reconnaissance aircraft was not forced down by jamming or enemy interference but rather the aircraft commander took the most prudent action, both from a military and political vantage point, and it may well have saved lives.
Sordid Aviation and Military History
Lest we forget, historically civilian airliners have been harassed, intercepted and even shot down in this area of the world. Consider North Korea’s extreme and high-profile actions of late concerning the U.S and South Korean military as well as the civilian populace of South Korea are solely for the purpose of provoking a military response. Both the U.S. and South Korean military have shown remarkable restraint. This latest jamming incident is merely another in a long series of provocations by North Korea. Remember the North Koreans reportedly sank a South Korean military vessel recently, with all lives lost, because it was supposedly in North Korean waters. Authorities do not know, or have not said, for certain if the South Korean vessel experienced GPS jamming, but GPS readouts and coordinates have now become the defacto standard for proving or disproving the legitimacy of reported border incursions, whether by land, sea, or air.
To reiterate, the U.S. reconnaissance pilot took the prudent action once the GPS signal was reportedly jammed even though I can assure you the pilot (and crew if there were any) had numerous other means of navigation at their disposal. None of our reconnaissance aircraft depend solely on GPS for PNT information.
Unlike so many of the critical, uninformed responses I have read concerning this incident, I applaud the reconnaissance pilot for making the right decision. And since this was a reconnaissance aircraft, it is very possible the military gained all the necessary data before deciding to terminate the mission. Suffice it to say our SIGINT (SIGnals INTelligence) tools are extremely sophisticated.
Are We Too Dependent on GPS?
This incident reminds me that the 19th USAF Chief of Staff, General Norton A. Schwartz, provoked quite a furor just 20 months ago when he spoke of a troubling operational dependency on GPS that must be tempered by other technologies and capabilites lest we become too dependent on one technology that could be denied our warfighters at critical times. It was reported at the time, by yours truly in GPS World and others, that General Schwartz’s call for alternative or augmenting technologies was “driven by serious threats to GPS… Officials familiar with the issue would not discuss current threats; however, they confirmed the GPS has been jammed or interfered with recently.”
Course of Action
The correct course of action is not to limit GPS — just the opposite. Refine GPS; increase the overall signal strength and accuracy for all users by integrating GPS with other embedded PNT (Position, Navigation and Timing) and communications systems through the use of intelligent software-defined receivers capable of utilizing all PNT signals available.
The dynamic Perfect Handheld or embedded GPS Transceiver (PHGPST) that I originally wrote about in March 2007 has evolved. The PHGPST must now be capable of receiving PNT signals from GPS, GLONASS, Galileo, Compass, among others. It must be capable of receiving all the wide area and local area augmentation systems available globally, such as DGPS (Differential GPS), WAAS (Wide Area Augmentation System), and EGNOS (European Geostationary Navigation Overlay Service), just to name a few. Such a system would also utilize a chip-scale atomic clock (CSAC) and ingenious commercial systems such as Skyhook Wireless, which uses Wi-Fi and GPS carrier signals for immediate (under four seconds) PNT results, even indoors.
Of course, to provide any future PNT capabilities GPS and all other satellite-borne PNT systems must exist within the protected satellite navigation spectrum currently threatened by LightSquared and an apparently clueless FCC (Federal Communications Commission).
eLORAN
The current LightSquared debacle and the North Korean jamming incident certainly underscore the reasons for General Schwartz’s concerns. The fact that the U.S. military has recently decommissioned one of the primary and historically viable backups and augmentations for GPS, that was essentially too powerful to be easily jammed — and I am speaking of course of eLORAN — is another matter for another column. In my opinion, and it is an opinion shared by many in the know, decommissioning eLORAN was a major operational blunder induced by minor budget concerns that both the current administration and the Coast Guard need to remedy. I would very much appreciate your comments, pro and con, on the eLORAN debate. This is far from a dead issue. Drop me a line at [email protected]. I digress.
Historical Viewpoint: Lessons Learned
The entire incident with the North Korean’s supposedly jamming GPS and General Schwartz’s comments regarding our dependency on GPS brings to light navigation concerns, actions, and lessons we should have learned from another well-known general officer who served as the fifth chief of staff of the USAF and as the commander of Strategic Air Command (SAC). I am speaking of the famous General Curtis “Bombs Away” LeMay who had a well-known aberration for navigation devices that were not passive in nature or integral to the aircraft being navigated. And even though he was primarily a command pilot, General LeMay understood navigation; in 1940 he served as the navigator on the prototype Boeing XB-15 heavy bomber that when it first flew, in 1938, was the most massive and most voluminous aircraft ever built in the United States. Late
r in his career as USAF CSAF (Chief of Staff) General LeMay strongly advocated the introduction of satellite technology for navigation and pushed for the development of the latest electronic warfare techniques. However, for General “Iron Pants” (the XB-15 could fly unrefueled for over 20 hours) LeMay new technology was never allowed to overshadow or jeopardize the primary mission.
General LeMay was a big believer in the basics, especially celestial navigation, and I can testify from personal experience that just a few years past, long after the advent of GPS and LORAN (LOng RAnge Navigation), SAC navigators and crews routinely flew vast distances across oceans and continents with nothing but a sextant and a very busy and nervous navigator. General LeMay was also concerned about SIGINT and required SAC aircraft to routinely practice radio and signals silence, no signal emissions. Entire missions were frequently flown from takeoff to landing without a single radio call or signal being transmitted. There were totally radio silent air refuelings by SAC tankers and bombers. Consider that celestial, inertial, eLORAN, and GPS fall into the silent and SIGINT free category. The inveterate cigar chomping and garrulous General LeMay would undoubtedly have approved and championed these new technologies. But he would never have allowed the loss of one capability to compromise the overall mission, and thankfully that same attitude is still prevalent in our Air Force today. Hence the timely comments by General Schwartz.
Today SAC’s assets (SAC was disestablished as a USAF Major Command — MAJCOM — in June 1992 after the end of the Cold War) are divided among Air Combat Command (ACC), Air Mobility Command (AMC), and Air Force Global Strike Command (AFGSC). To my knowledge none of these MAJCOMs today require crews to carry sextants onboard their aircraft, and indeed many of the newer aircraft do not have sextant ports. Apparently manual aviation celestial navigation skills are no longer taught at the joint military navigation courses except to Navy and Coast Guard shipboard navigators/personnel. Perhaps a back-to-basics approach is needed in training as well as in operations.
LightSquared Debacle
While we should not be surprised that GPS jamming takes place, we should be surprised and indignant that the current FCC commissioner has initially authorized legal GPS jamming by LightSquared. I originally penned three articles about the FCC and the ridiculous chain of events that led to the LightSquared debacle, and then circumstances precluded me writing any further articles on the topic. What I can say now is the LightSquared terrestrial transmitters and receivers, if approved by the FCC, amount to FCC-sanctioned jamming that will cause mayhem among GPS users worldwide. This is no longer an issue confined to the CONUS (Continental United States). There are billions of dollars in economic and containment costs at stake as well as lost income and revenue, not to mention the potential loss of life, detailed in a recent FAA report. Approval of the LightSquared terrestrial plan would be a global catastrophe and I am incredulous that the administration and the FCC are still unsure of what action to take.
Way Ahead
It is really rather simple: LightSquared originally signed on to provide broadband communication capabilities via satellite to everyone in the U.S. They propose broadcasting in the spectrum allocated to satellite transmissions, and as long as they fulfill that mission at the nominal satellite power levels from orbit there is not an issue. In this originally approved LightSquared scenario, all users would have the capability to receive broadband signals everywhere they can now receive a GPS signal. As we all know, with ever more sensitive receivers you can now routinely receive GPS signals almost everywhere, even indoors. The proposed broadband satellite coverage area provides a huge customer base for LightSquared but apparently it is not enough. It becomes a matter of market dominance versus market share. The FCC needs to wake up and take immediate actions to curtail plans for all high-powered terrestrial transmissions in the protected satellite spectrum or face the disastrous consequences. The North Korean jamming headlines are bad enough; none of us want to read a headline that says “FCC GPS Actions Cause Huge Loss of Life as Airliners Collide.” This is far from over; write your Congressman.
Google is at it again. This time Motorola Mobility is on the buying block. What does this mean to the location-based services market? Another potential location platform market closed off? Some industry experts believe this is the case. In addition, Iridium and TeleNav are making LBS news with recent product launches and acquisitions.
The recent $12.5-billion Google acquisition of Motorola Mobility has some industry experts saying that the location market piece of pie is getting smaller every time the search giant makes a deal.
“I think with Google controlling both the hardware and software stack of the Android ecosystem it will be hard for any technology company to work with Motorola. They want to own the whole shooting match for themselves,” said Ted Morgan, Skyhook Wireless CEO.
Boston-based Skyhook is suing Google for allegedly using tactics to block Motorola Mobility and Samsung from contracts that use the company Wi-Fi-based tracking system in Android smartphones.
Many industry experts have said that the main makers of Google Android smartphones should feel challenged as well as the company has seemingly gone into business against them.
Google has made many moves into the location business in the last two years. It is trying to grab a large share of the European traffic market by offering real-time services in 13 European companies. Google shook up the navigation market with free navigation service for Android phones in 2009. Last month, LBS Insider detailed Google’s purchase of The Dealmap, which offers a location-based daily deal service.
Google’s acquisition of Motorola is another step in a development strategy that appears to be aimed at increasing the company’s ability to compete across multiple markets that are served by mobile computing, said Mike Dobson, Telemapics president, author of Exploring Local. “[This is] supplemented by the company’s ability to supply its customers proprietary content that can provide a unique and informed world view whether those customers are at home or on the road exploring new geographies,” he said.
Dobson says that Google clearly wants to compete on a level playing field with Apple and appears to feel that the only way they can do so is to acquire one of the premier manufacturers of mobile phones. “While Google had hoped to control the mobile market by developing Android, doing so has not allowed them the gather the strategic control of phone design, pricing, positioning, placement, or distribution,” he said. “Conversely, Apple has been able to bring mobile phones to the marketplace whose features, functionality, and looks have generated a design revolution that has enchanted consumers in a manner dissimilar to anything we have ever seen in the mobile marketplace.”
Although Motorola’s brand has been tarnished in recent years, it is clearly the case that they are an extraordinarily talented developer of popular mobile devices that continue to stretch to boundaries of the capabilities of the cell phone world, said Dobson, who believes that this is evidenced by the fervor of anticipation surround the current release of the dual-core, 4G LTE compatible Motorola Droid Bionic.
Motorola’s design team, however, does not appear to understand the consumer mobile phone market with the same ability to interleave design and hardware functionality that is the hallmark of all Apple products, including the iPhone. “Nor do I believe that Google has the capabilities, as of this time, at least, to remedy this situation,” he said.
Dobson said that Google’s proposed acquisition of Motorola, coupled with those like its acquisition of Zagat’s and proposed acquisition of ITA Software, an airline ticketing company, seems to indicate that Google is interested not only in providing the platform and OS, but also the common content that might be of interest to users of their mobile devices. “When Google’s control of key content is wrapped within the control of the delivery platform and nested within the Internet’s most successful advertising delivery platform, AdSense/AdWord, it would appear that Google will have advantages in the mobile world far superior to any company that currently exists,” he said.
Now that the U.S. government has blocked AT&T’s acquisition of T-Mobile, all eyes are on Google’s newest purchase. Dobson has said that while it is impossible to estimate the size and data usage total that can be attributed to location services, there is little reason to assume that it does not mirror the growing trend in data growth.
At the time the AT&T/T-Mobile deal was announced, Dobson told LBS Insider that if AT&T can advantage itself by easing its spectrum crunch through the acquisition, it could result in the company being more interested in navigation and LBS than in the past.
Iridium Making LBS Foray
As GPS World reported, McLean, Va.-based Iridium Communications announced that its Iridium Force strategy will include LBS and M2M to grow its personal mobile satellite capabilities beyond satellite phones. The new capability enables communication with Wi-Fi-enabled devices such as smartphones, tablets, and laptops. The Iridium Extreme, which is the company’s smallest, will be connected to online portals with GPS and LBS capabilities.
The company also says that Iridum Tracking Portals allow customers to access location monitoring that show real-time status and location, scheduling regular check-ins, geo-fencing, and other features.
In a July interview with LBS Insider, Patrick Shay, Iridium vice president and general manager for data services, said that the machine-to-machine market constitutes the company’s fastest growing segment. The company said it reached 500,000 total billable subscribers for its satellite voice and data services worldwide. The breakdown of subscribers includes 90 percent commercial customers and 10 percent U.S. government customers.
TeleNav Buys LBS Firm Goby
In a smaller acquisition, of which financial details were not disclosed, TeleNav purchased Boston-based Goby, a local and travel search startup that focuses on mobile applications — and will look at advertising revenue models.
TeleNav has been tight-lipped about the acquisition, only saying that they are impressed with the small company and its personnel and technology. Published reports indicate that the company, and 10 employees, are staying in Boston.
The first Galileo navigation satellite has arrived in Europe’s Spaceport in French Guiana, ready to begin preparations for launch on October 20, reports the European Space Agency (ESA). Packed within a protective, air-conditioned container, the satellite known as Flight Model 2 (FM2) landed at Cayenne Rochambeau Airport aboard an Antonov aircraft at 06:45 local time on Wednesday after departing from Thales Alenia Space Italy’s Rome facility where it was built.
A Thales and ESA team stood ready to receive FM2, having flown into French Guiana the previous week, along with all the testing and support equipment. The team loaded the satellite container on a lorry for transport to the Guiana Space Center, where it arrived at 10:00 local time and was moved into the preparation facility. It stayed there overnight for the temperature to settle before it was taken out of its container the following morning.
The FM2 satellite is due to be launched aboard a Soyuz ST-B vehicle on October 20, together with a second Galileo satellite called the Proto-Flight Model (PFM), now being readied for its own flight to French Guiana.
This will be the first launch of Russia’s Soyuz rocket from French Guiana, and the first Soyuz launch from a spaceport outside of Baikonur in Kazakhstan or Plesetsk in Russia. The launch will take place from a new facility 13 km northwest of the Ariane 5 launch site. French Guiana is much closer to the equator, so each launch will benefit from Earth’s spin, increasing the maximum payload into geostationary transfer orbit from 1.7 tonnes to 3 tonnes.
The first four Galileo satellites, built by a consortium led by EADS Astrium Germany, will form the operational nucleus of the full Galileo satnav constellation.
We have the United States GPS Industry Council, the Japan GPS Council, and the Korean GNSS Technology Council.
Anything missing?
The challenges facing the performance, navigation, and timing (PNT) community, which relies on GNSS amongst other things, are getting more numerous and complex, and Europe is the only major territory without a unified industry nexus where such challenges can be engaged. However, this is about to change.
From my background and current activity as CEO of DW International, an independent navigation consultancy with a strong interest in GNSS specifically, I have begun forming the European PNT Industry Council (EPIC) with other industry leaders to act as a focal point for the PNT community’s concerns and to help coordinate the effort for standardization and harmonization. Additionally, with issues such as the LightSquared debacle looming, it is key that European stakeholders have a voice on the global stage.
A recent survey that the nascent EPIC conducted jointly with Marketing Analytics highlighted the need for an organization such as EPIC. We asked key PNT figures around the globe about the issues concerning them and how these concerns should be addressed by EPIC. For such a diverse group of respondents (including representatives from state transport agencies, academic institutions, OEMs, independent consultancies, land survey companies, maritime, and aviation) there was clear agreement on the need for a European focal point for PNT to better facilitate interoperability and harmonization of standards among the current PNT activities being undertaken around the world. Sixty-six percent of respondents wanted an international forum for information exchange (that is, ideas, best practices, and lessons learned) where such issues as interoperability and harmonization could be addressed.
Sixty-three percent rated system-level PNT policy issues as a very important subject area for EPIC, while 56 percent rated standards for PNT in areas such as aviation, rail, and E112 as being very important. There is no shortage of issues to tackle, and EPIC will prove to be a key player in forming the coalitions required.
As one respondent put it, when asked about his priorities regarding PNT policy:
Galileo launch schedule;
Compass CPII and CPIII signal details and operational plans;
Information about GLONASS L3 and GLONASS CDMA plans, particularly ICD and frequency of planned L1 CDMA signal;
SBAS plans, such as EGNOS and GAGAN;
European regulatory plans that relate to navigation and positioning; E112, road user charging, tracking and logistics;
Standards for navigation and positioning applications, plus applications that rely on a position.
Whatever the appeal of a forum for the exchange of technical knowledge amongst professionals, it was also clear that respondents wanted EPIC to take action as well. One wrote:
“EPIC needs to be outcome/results oriented and not turn into a talkfest. Therefore issues such as LightSquared need to be addressed head on so that bureaucrats start listening to the science behind decisions and policies rather than commercially driven for short-term political expediency.”
Indeed, EPIC joined the chorus of organizations writing directly to the FCC calling for a rethink of the LightSquared issue.
I personally believe that with the industry councils active in the United States and Asia, EPIC is the third leg of the stool. PNT is such a dynamic world, with so many moving parts, that even large international organizations risk being left behind unless their interests are represented and the information they need is available in a consistent and practical fashion.
But more than that, PNT is a utility that needs to be protected, maintained, enhanced, and utilized. EPIC will ensure that those who want to, can.
The need is there. The stakeholders are there. It’s happening.
The most efficient use of spectrum the world has ever seen benefits more than a billion people today. Two billion tomorrow, when modernized and interoperable GNSS gets real. This massive installed base constitutes a source of innovative advantage and invaluable good will for the United States.The latter arises from the high degree of trust and confidence in the United States and its stewardship of GPS, one of the most successful — and perhaps only — simultaneous foreign aid and domestic economic stimulus programs ever created.
Smooth dealers operating inside a hedge, playing with other people’s money, want to make billions by raiding this national resource to provide video on cell phones to young audiences.
The Federal Communications Commission has acted in ways inconsistent with reasonable public expectations of a federal rule-making agency. Early on, it gave the appearance of buying into the LightSquared agenda, issuing a ruling with undue speed.
It has waived, explained, and proclaimed in ways that show an abject ignorance of radio frequency (there are conflicting reports as to whether agency technical staff was ever consulted by leadership prior to acceding to the LightSquared request) — and too clever by half. The chairman, rumored to be in line for the China ambassadorship, was careful not to sign the waiver himself, but have the deed done by a subordinate.
In this act, they ignored the inherent conflict in two competing national policy objectives: the National Space Policy and the Broadband Memorandum. Rather than taking time to reconcile crucial guiding principles, the waiver plots its own course.
The ruckus has gotten the president out on a limb, and now the agency must find a solution allowing him to crawl back before the election. Either that, or he and his advisors, including the FCC chair, will knuckle down and carry on regardless, saving political face in the short run while weakening national infrastructure and defensive capabilities.
Never underestimate politicians’ desire to save face. In many ways, it’s all they’ve got.
The best thing the GPS community can do during this quiet reloading period is to keep the letters and calls flowing to Congress: the safest and most fact-based action for the FCC is to conclude that the terms of the LightSquared conditional waiver have not been met and withdraw the license to deploy a terrestrial network in the 1525–1559 MHz band. This is the only approach fully consistent with both the National Space Policy and the Broadband Memorandum, as well as the FCC’s own regulations.
At this point, any actions taken by the FCC are subject to unpredictable political considerations.
By Andrei M. Shkel, Defense Advanced Research Projects Agency (DARPA)
The aggregated DARPA Microtechnology for Positioning, Navigation, and Timing (micro-PNT) program is pursuing a new wave of innovation focused on bringing to life revolutionary ideas and fabrication technologies on micro/nano/pico/femto/atto scales, packaging, ultra-low-power electronics, innovative algorithms, never-before-explored architectures, and exploitation of new integration paradigms.
After about two decades of harmonic investment in developments, potential users of so-called small technology for positioning, navigation, and timing (PNT) applications increasingly ask, “Are we there yet?” Clearly, some significant advances have been made, and we see a footprint of the technology in an ever-growing consumer electronics market full of interactive products enabled by inertial and timing microtechnologies. These products include accelerometers for gaming applications, gyros for auto safety, resonators for clocks, and more.
The question remains, however: Is the technology really on the level of what we consider to be precision navigation and timing, that is, is it capable of achieving an accuracy level of at least 10 meters in position and 1 nanosecond in time throughout the entire duration of missions that may range from minutes to hours to days? In reality, small technology remains several orders of magnitude short with respect to long-term stability, dynamic range, and accuracy compared to conventional technology, which is already known to perform adequately for many military applications.
Why does making inertial instruments and clocks small necessarily lead to degradation in performance?
We don’t yet have a complete answer to this question, and we are still working hard to disprove the contention that high-performance inertial micro-instrument is a contradiction in terms. We can make things small, but we cannot yet make them sufficiently precise and uniform; the accuracy of lithography-based manufacturing is on the order of 10–2–10–3 (the ratio of the average defect to the smallest feature size), while the accuracy of conventional manufacturing utilizing precision machining is two to three orders of magnitude higher, on the order of 10–5. We know we can deposit materials layer-by-layer with high precision, but we cannot make micro-devices truly 3D, as is readily achievable using conventional machining. We consistently have an excellent case for low-cost and bulk fabrication, but we cannot seriously challenge so-called boutique processes when it comes to achieving precision, structural complexity, and long-term stability.
We need new knowledge regarding the dimensional stability of materials. We also need a better understanding of material scaling, surface effects, energy-loss mechanisms, and the consequences of fabrication imperfections on the performance of micro-instruments.
PNT applications demand both unusual new fabrication technologies and new materials with special properties. To achieve the required phenomenal accuracy for precision navigation and timing, we need a new wave of innovation in design and refinement of many existing transducers. Future breakthroughs in microtechnology for PNT will likely rely on yet-to-be-exploited physics, new materials, highly specialized fabrication technologies and batch assembly techniques, selective wafer-level trimming and polishing, a combination of passive and active calibration techniques strategically implemented right on-chip, and introduction of innovative test technologies.
Need for Advanced Capabilities
PNT technology usage has doubled every five years since 1960, mostly due to GPS and the miniaturization of electromechanical components. Future PNT usage is expected to double every two years as a result of telecommunication, automobile navigation, robotics, and other commercial markets inserting micro-electromechanical systems (MEMS) technologies. The modern PNT paradigm is based on the assumption that space-based GPS is accessible most of the time to provide position, velocity, and timing information, enabling every user to operate on the same reference system and timing standard.
Today’s military systems increasingly rely on GPS, creating a potential vulnerability for U.S. and allied war-fighters should GPS be degraded or denied. When GPS is inaccessible, critical information with respect to position, orientation, and timing can only be gathered through self-contained onboard instruments: a local clock and two triads of inertial sensors (three accelerometers for position and three gyroscopes for orientation). The ideal solution would be a self-sufficient instrument not relying on any external information. Precision microscale clocks and inertial sensors are required to address the paradigm of self-contained PNT.
Clocks. Position and time have a relationship important to a broad spectrum of military applications, including communication systems that feature efficient spectrum utilization, resistance to jamming, high-speed signal acquisition, and an increase in the period of autonomous operation. Other important applications include surveillance, navigation, missile guidance, secure communications, identification friend-or-foe, and electronic warfare.
The emerging applications require new compact time-distribution systems technologies capable of achieving signal phase (time) common synchronization of better than 10–9 seconds relative to the Coordinated Universal Time (UTC) standard; intersystem synchronization of less than 10–8 seconds relative to battle group; and less than 10–9 seconds for interoperability, surveillance, and high-speed communications. Solid-state and atomic oscillators are the key components enabling time and frequency distribution for communication, navigation, and command and control systems.
To support emerging applications, we are interested in clocks with
signal phase (time) communication synchronization less (better) than 28 nanoseconds (ns) within 5 minutes (real time), UTC;
intersystem synchronization less (better) than 28 ns relative to other system nodes within 5 minutes (real time); and
local navigation/communication systems capable of time transfer less (better) than 28 ns, UTC.
The operational frequency mismatch (δf=f), where f is a nominal frequency and δf is a frequency deviation from the nominal, is a measure of oscillator quality and subsequently the quality of the frequency distribution system. Different applications can tolerate different levels of frequency mismatches. For example, for low-accuracy aircraft/land mobile platforms, the requirement for frequency mismatch is 10–12, while for intermediate land reference sites the requirement is an order of magnitude smaller, 10–13. For large time-division multiple-access (TDMA) systems, the tolerable frequency mismatch is on the order of 10–11.
Small size, weight, and power (SWaP) are critical metrics for portable time and frequency distribution systems. The target performance characteristic for low-power clocks and oscillators is long-term stability (aging), which need to be less than 10–11/month, with less than 1 W power consumption. It is desirable that the oscillators have small SWaP and preserve the level of long-term stability while surviving an inertial environment with accelerations on the level 10,000 g, where g is the gravity constant.
For comparison, the one-way satellite transmission from a GPS satellite in common view at two sites allows one to do accurate time transfer to within 10 ns, with a potential to achieve accurate time transfer of the order of 1 ns. Achieving an accuracy of time transfer on the level of 1 ns is loosely defined as precision timing.
Inertial Navigation Systems. The navigation-grade performance provided by inertial sensors is defined as an INS that accumulates an uncertainty in location not greater than one nautical mile (nmi), or 1.852 km, after one hour of navigation. The error in position is historically defined by the circular error probable (CEP) of 50 percent. The ability to achieve a CEP of 1 nmi in one hour (or 1 nmi/hour) does not translate to a unique performance requirement for a gyroscope and/or an accelerometer. Rather, it presents a trade-off in the overall inertial measurement unit (IMU) error budget. The trades can be generated within a family of gyroscope errors, such as gyro angle random walk (ARW) versus bias drift, or similarly within a family of accelerometer errors. For example, an IMU with gyroscope bias drift of 0.01º/hour combined with an accelerometer bias drift of 25 μg would guarantee a CEP of less than 1 nmi/hour, if no other errors are present. To generate the trade-off space for component performance, one efficient approach is to first generate the parameter space at the linear error covariance level, taking into account the bias drift of components, and subsequently perform more extensive modeling in a bounded trade-off space by a nonlinear Monte Carlo simulation.
The ability to navigate and keep precise timing has been an important factor in defining the military and economic power of nations for at least a millennium. For almost a century, the development of high-performance inertial instruments has been an extensive area of research. It is anticipated that the following level of performance will soon be achieved, significantly reducing navigation errors and enhancing military capabilities, within the next 5 to 10 years:
< 0.1 nmi/hour CEP for aircraft, vehicle, or spacecraft for attitude, guidance, and control;
< 1.0 nmi in 30 hours for ships;
< 0.4 nmi/hour CEP for missiles.
It is critical that future-generation INS systems be capable of operating through shock levels greater than 1,000 g.
Similar to clocks, the reduction of SWaP and cost (SWaP+C), while not compromising in performance, are the critical metrics for future development of IMUs. The current performance of state-of-the-art MEMS-based IMUs is on the level of tactical grade, with CEP approaching 100 nmi/hour. There is a great potential for achieving performance improvements that will subsequently enable platforms for personal navigation, precision navigation of small unmanned aerial vehicles (UAVs), unmanned underwater vehicles (UUVs), and GPS-free navigators for missiles. It is expected that the performance levels of chip-scale inertial instruments and clocks, shown in Table 1, could be achieved within the next 5 to 10 years, thus significantly enhancing military capabilities. The conservative estimations are projected by the Department of Defense’s Science and Technology List for Positioning Navigation and Timing. The aggressive estimates presume successful completion of the micro-PNT program described here.
The military has access to a currently specified accuracy of 21 meters (95 percent probability) from the GPS Precise Positioning Service (PPS). Accuracy can be improved after calibration for some of the GPS errors, for example, by utilizing optimal estimation techniques correlating GPS and INS signals. A CEP of less than 10 meters has been routinely achieved, with a potential to achieve accurate positioning on the order of 1 meter CEP.
Navigation, guidance, and automatic control are not the only military applications that could benefit from improvements in inertial sensors. Azimuth or north-pointing determination systems include celestial devices, magnetic compasses, and inertial sensors. Utilization of gyroscopes to precisely determine orientation has a number of benefits attributed to their immunity to magnetic fields, speed of acquisition, and potentially small SWaP+C. For this purpose, a variety of inertial equipment is being explored, including IMUs, attitude-heading reference systems (AHRS), and gyro-compasses. Providing an azimuth or north-pointing accuracy of less (better) than 0.5 arc minute multiplied by secant latitude has the potential to significantly enhance military capabilities for many targeting applications, especially for anticipated mobile platforms.
Current Research
This section provides an overview of the ongoing efforts funded by DARPA (Defense Advanced Research Projects Agency) under the micro-PNT program.
Clocks. The potential payoff of the precision-clock technology developed by the program will enable ultra-miniaturized and ultra-low power absolute time and frequency references for applications such as nano/pico satellite systems, UUVs, UAVs, wristwatch-size high-security UHF communicators, and jam-resistant GPS receivers.
There are currently two efforts within the micro-PNT program involving the development of clocks: Chip-Scale Atomic Clock (CSAC) and Integrated Micro Primary Atomic Clock Technology (IMPACT).
The goal of the CSAC effort is to create ultra-miniaturized, low-power, atomic time and frequency reference units that will achieve, relative to present approaches: more than 200× reduction in size (from 230 cm3 to <1 cm3); more than 300× reduction in power consumption (from 10 W to less than 30 mW); and matching performance (1 × 10–11 accuracy and 1 ns/day stability). This work, funded by DARPA since 2002, has been supporting 11 teams. The program is currently in its final phase and supports two performers, Symmetricom and Teledyne Scientific. Symmetricom has already demonstrated pilot units that are 1 cm3 in volume, consume on the order of 100 mW of power, and perform on the level of better than 30 × 10–11 short-term 1 sec instability (Allan Deviation) and 5 × 10–11/day (1.4 × 10–10/month) long-term frequency drift.
The IMPACT program seeks to improve the stability and accuracy of microscale atomic clocks by as much as two orders of magnitude. Atomic-clock performance is affected by buffer gases (nitrogen or argon), which are necessarily present in either rubidium- or cesium-based atomic clocks. Buffer gas atoms interact with alkali atoms and effectively shift the resonant frequency of atoms. Emerging atomic-clock technologies based on laser-cooled atoms and trapped ions could overcome the limitations of CSAC.
The goal of IMPACT is to create miniaturized, low-power, integrated micro primary atomic clock technology that will achieve significant reduction in size relative to conventional clocks, but slightly larger than CSAC (volume less than 5 cm3 in final package, excluding battery); significant reduction in power relative to conventional clocks, but slightly greater than CSAC (50 mW); and two orders of magnitude increase in performance relative to CSAC (frequency accuracy 1 × 10–13, Allan deviation at one-hour integration time, and stability characterized by 5 ns/day time loss). The work, funded by DARPA since 2008, currently involves four teams: Honeywell, Symmetricom, Sandia National Laboratories, and OE Waves.
The overall approach is based on sampling of atomic transitions at extremely low temperatures, requiring vacuum on the level of 10–9 Torr and the ability to trap atoms in a small volume. The technology has been previously demonstrated on a large scale, but transferring the technology to small scale is far from trivial, requiring major innovations. The effort has already demonstrated magneto-optical trapping in a 16 cm3 atomic cell, and chip-scale clocks implemented using cold atoms performing on the level, quality factor × signal/noise ratio ∼ 2.6 × 1010, time loss after 1 ms equal to 10–4 ns; after 1 second, 6 × 10–3 ns; after 1 hour, less than 10 ns; and after 24 hours, on the order of 100 ns. Frequency retrace was demonstrated at the end of the phase on the level of 10–11.
Inertial Sensors and Systems. There are currently three efforts within the micro-PNT program involving the development of inertial sensors and systems: Navigation-Grade Integrated Micro Gyroscopes (NGIMG), Micro Inertial Navigation Technology (MINT), and Information Tethered Micro Automated Rotary Stages (IT-MARS).
The NGIMG effort seeks to develop tiny, low-power, rotation-rate sensors capable of achieving performance commensurate with requirements for GPS-denied navigation of small platforms, including individual soldiers, unmanned (micro) air vehicles, unmanned underwater vehicles, and even tiny (for example, insect-sized) robots. By harnessing the advantages of microscale miniaturization, the NGIMG effort is expected to yield tiny (if not chip-scale) gyroscopes with navigation-grade performance characteristics: overall size less than 1 cm3 (no power source), power consumption less than 5 mW, ARW less than 0.001°/√hour, bias drift less than 0.01°/hour, scale factor stability on the order of 50 parts per million (ppm), full-scale range greater than 500°/sec, and bandwidth on the order of 300 Hz.
The NGIMG effort has been funded by DARPA since 2005, and work is currently being conducted by three teams: Northrop Grumman, Boeing, and Archangel Systems. The work has demonstrated several experimental prototypes (some, but not all, independently verified by the government) performing on the level of ARW 0.01°/√hour, and bias drift 0.05°/hour.
The MINT effort seeks to develop microscale low-power navigation sensors that allow long-term (hours to days) precision navigation in GPS-denied environments. The goal is to create high-precision, navigation-aiding sensors that directly measure intermediate inertial variables, such as velocity and distance, to mitigate the error growth encountered by integrating signals from accelerometers and gyroscopes alone. In addition to aiding sensors such as velocity sensors, the combination of microscale inertial sensors will be integrated to a form-factor of one or two integrated circuits. Such an integrated sensor suite will be incorporated into the sole of a shoe for accurate and precise velocity sensing using zero-velocity events during walking.
The final goal of MINT is to achieve an overall package and form-factor for a velocity sensor (excluding IMU) of less than 1 cm3, power consumption for the velocity sensor of less than 5 mW, 1-meter position accuracy after 36 hours of walking, and 10 µmeter/second velocity sensing bias per step. The effort has been funded by DARPA since 2008 and involves work by four teams: Carnegie Mellon University, Analog Devices, Northrop Grumman, and Case Western Reserve University/University of Utah. To date, the work has demonstrated positioning error on the order of 4 meters after 30 minutes of walking.
The goal of the IT-MARS program is to implement and demonstrate a MEMS-fabricated rotary stage providing a rotational degree of freedom to planar MEMS structures and sensors, thus enabling free rotation of micro-structures and micro-sensors relative to the package, with coupled power and signal transfer from the rotating platform to the package. The IT-MARS effort may enable highly accurate calibration of inertial sensors and serve as a micro-platform for carouseling of inertial sensors that further enable on-chip calibration and gyro compassing. The ultimate program goal is to achieve an overall volume of no more than 1 cm3, power consumption for actuation on the order of 10 mW, angle position absolute accuracy to within 1 milli-degree, maximum wobble of 10 micro-radians, a rotation rate of 360°/second, and reliability (run time of rotor) greater than 104 hours.
This effort, which has been funded by DARPA since 2009, supports three teams: UCLA, UC-Berkeley, and the Boyce Thompson Institute. The work has already demonstrated free rotated platforms, and future efforts will focus on manufacturability and precision control of the stage-rotation and reduction of wobbling.
New Initiatives
In January 2010, DARPA launched a coordinated effort focused on the development of microtechnology specifically addressing the challenges associated with miniaturization of high-precision clocks and inertial instruments. The new program, Microtechnology for Positioning, Navigation, and Timing (micro-PNT), aggregated the existing efforts (CSAC, IMPACT, NGIMG, MINT, and IT-MARS) and initiated four complementary new developments:
Microscale Rate Integrating Gyroscopes (MRIG),
Chip-Scale Timing and Inertial Measurement Unit (TIMU),
Primary and Secondary Calibration on Active Layer (PASCAL),
Platform for Acquisition, Logging, and Analysis of Devices for Inertial Navigation & Timing (PALADIN&T).
The overall goal of the new aggregated micro-PNT program is to focus all of these complementary efforts toward achieving one specific overarching goal: self-contained chip-scale inertial navigation (see opening illustration). The reduction of SWaP+C of IMUs and timing units (TUs) is the technological objective. The developments consider a number of operational scenarios, ranging from dismounted-soldier navigation to navigation, guidance, and control (NGC) of UAVs/UUVs and guided missiles. The new micro-PNT initiatives will increase the dynamic range of inertial sensors, addressed by the new MRIG effort; reduce the long-term drift in clocks and inertial sensors, addressed by the PASCAL work; develop ultra-small chips providing position, orientation, and time information, addressed by the TIMU effort; and provide a universal and flexible platform for the testing and evaluation of components developed within the comprehensive micro-PNT program, addressed by the PALADIN&T effort.
The primary goal of MRIG is to create a vibratory gyroscope that can be instrumented to measure the angle of rotation directly, thereby extending the dynamic range and eliminating the need for integrating the angular rate information; MRIG will thus eliminate the accumulation of errors due to numerical/electronic integration.
The final goals are to:
extend the dynamic range to 15,000°/second;
achieve drift repeatability on the level of 0.1°/hour (angle dependent) and 0.01°/hour (bias-dependent) under variable –55°C to 85°C thermal conditions;
achieve ARW of 0.001°/√hour, an operation range of 1,000 g with acceleration sensitivity of 10–5 degrees/hour/g, vi
bration sensitivity angle random walk of 0.01°/√hour per g/√Hz, and drift rate of 0.01°/hour per g2/√Hz.
These performance characteristics are thought to be achievable through development of precision 3-D fabrication technologies utilizing high-Q materials; development of wafer-level balancing and trimming techniques that reduce the effects of aniso-inertia (mass misbalance), aniso- compliance (stiffness misbalance), and aniso-damping (damping misbalance); and development of active control and an active calibration architecture.
These performers have been selected for the initial phase of the MRIG effort: Draper Labs, Honeywell, Northrop Grumman, Systron Donner, UC-Irvine, UC-Davis, UCLA, Cornell, University of Michigan, and Yale University.
The TIMU effort will address challenges associated with the development of a miniature (10 mm3), low-power (200 mW), high-performance (CEP on the order of 1 nmi/hour), and self-sufficient navigation system on-a-chip. The smallest state-of-the-art IMUs perform on the level of tactical-grade instruments (CEP on the order of 100 nmi/hour) and are about the size of an apple (more than 104 mm3). This effort intends to develop a technological foundation for a navigation-grade TIMU (CEP less than 1 nmi/hour and time accuracy of 1 nanosecond/minute) with a significant reduction in SWaP, potentially miniaturizing the TIMU to the size of an apple seed (10 mm3).
PASCAL will develop self-calibration technologies intended to eliminate long-term bias drift of inertial sensor and clocks. The grand challenge of this effort is to raise long-term bias stability to the level of 1 ppm.
This level of stability represents a two-orders-of-magnitude improvement compared to state-of-the-art inertial microsensors, currently at 200 ppm. The work will investigate an approach for fabricating sensors on an active layer that may serve as a calibration layer for micro-PNT systems.
The PALADIN&T effort will develop a universal platform for test and evaluation of early prototypes developed in the micro-PNT program. The effort will also simplify the uniform evaluation of pilot prototypes within the program and provide an early field demonstration, advancing the technology readiness level.
Conclusions
Current state-of-the-art microscale clocks and inertial instruments can provide the required level of precision only for missions having a duration of no more than about one minute. The micro-PNT program at DARPA is developing small SWaP+C inertial sensors for a variety of operational scenarios for missions ranging from minutes to hours. Current projects (CSAC, IMPACT, NGIMG, MINT, IT-MARS) mainly focus on navigation, characterized as missions of prolonged durations in relatively benign environments (a few hours of operation on a platform moving at relatively low speed, less than 100 km/hour).
The new initiatives (MRIG, TIMU, PASCAL, and PALADIN&T) target the challenges of missile guidance for precision engagement scenarios, short duration missions in highly dynamic environments (10 seconds to 3 minutes of operation at speeds of 1,000 km/hour and higher). Ongoing efforts and new initiatives explore new physical phenomena, high-quality factor materials, specialized fabrication technologies, and innovative approaches to system integration.
Disclaimer. The views, opinions, and findings in this article are those of the author and should not be interpreted as representing official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense. The document GPS0911 [DISTAR case 17952] is approved for public release, distribution unlimited.
Andrei M. Shkel received a Ph.D. in mechanical engineering from the University of Wisconsin-Madison and is a program manager in the Microsystems Technology Office at the Defense Advanced Research Project Agency (DARPA), and on-leave professor of mechanical and aerospace engineering at University of California, Irvine, where he is also the director of the UCI Microsystems Laboratory. He holds 15 U.S. and international patents (12 pending) on micromachined angle-measuring gyroscopes, wide-bandwidth rate gyroscopes, light manipulators and tunable optical filters, and hybrid micromachining processes.
Experimenting with GPS on Board High-Altitude Balloons
By Peter J. Buist, Sandra Verhagen, Tatsuaki Hashimoto, Shujiro Sawai, Shin-Ichiro Sakai, Nobutaka Bando, and Shigehito Shimizu
In this month’s column, we look at how a team of Dutch and Japanese researchers is using GPS to determine the attitude of a payload launched from a high-altitude balloon.
INNOVATION INSIGHTS by Richard Langley
IT IS NOT WIDELY RECOGNIZED that relative or differential positioning using GNSS carrier-phase measurements is an interferometric technique. In interferometry, the difference in the phase of an electromagnetic wave at two locations is precisely measured as a function of time. The phase differences depend, amongst other factors, on the length and orientation of the baseline connecting the two locations. The classic demonstration of interferometry, showing that light could be interpreted as a wave phenomenon, was the 1803 double-slit experiment of the English polymath, Thomas Young. Many of us recreated the experiment in high school or university physics classes. A collimated beam of light is shone through two small holes or narrow slits in a barrier placed between the light source and a screen. Alternating light and dark bands are seen on the screen. The bands are called interference fringes and result from the waves emanating from the two slits constructively and destructively interfering with each other. The colors seen on the surface of an audio CD, the colors of soap film, and those of peacock feathers and the wings of the Morpho butterfly are all examples of interference.
Interference fringes also reveal information about the source of the waves. In 1920, the American Nobel-prize-winning physicist, Albert Michelson, used an interferometer attached to a large telescope to measure the diameter of the star Betelgeuse. Radio astronomers extended the concept to radio wavelengths, using two antennas connected to a receiver by cables or a microwave link. Such radio interferometers were used to study the structure of various radio sources including the sun. Using atomic frequency standards and magnetic tape recording, astronomers were able to sever the real-time links between the antennas, giving birth to very long baseline interferometry (VLBI) in 1967. The astronomers used VLBI to study extremely compact radio sources such as the enigmatic quasars. But geodesists realized that high resolution VLBI could also be used to determine — very precisely — the components of the baseline connecting the antennas, even if they were on separate continents.
That early work in geodetic VLBI led to the concept developed by Charles Counselman III and others at the Massachusetts Institute of Technology in the late 1970s of recording the carrier phase of GPS signals with two separate receivers and then differencing the phases to create an observable from which the components of the baseline connecting the receivers’ antennas could be determined. This has become the standard high-precision GPS surveying technique. Later, others took the concept and applied it to short baselines on a moving platform allowing the attitude of the platform to be determined.
In this month’s column, we look at how a team of Dutch and Japanese researchers is using GPS to determine the attitude of a payload launched from a high-altitude balloon.
The Japan Aerospace Exploration Agency (JAXA) is developing a system to provide a high-quality, long duration microgravity environment using a capsule that can be released from a high-altitude balloon. Since 1981, an average of 100 million dollars is spent every year on microgravity research by space agencies in the United States, Europe, and Japan. There are many ways to achieve microgravity conditions such as (in order of experiment duration) drop towers, parabolic flights, balloon drops, sounding rockets, the Space Shuttle (unfortunately, no longer), recoverable satellites, and the International Space Station. The order of those options is also approximately the order of increasing experiment cost, with the exception of the balloon drop. Besides being cost-efficient, a balloon-based system has the advantage that no large acceleration is required before the experiment can be performed, which could be important for any delicate equipment that is carried aloft.
In this article, we will describe JAXA’s Balloon-based Operation Vehicle (BOV) and the experiments carried out in cooperation with Delft University of Technology (DUT) using GPS on the gondola of the balloon in 2008 (single baseline estimation) and 2009 (full attitude determination and relative positioning). The attitude calculated using observations from the onboard GPS receiver during the 2009 experiment is compared with that from sun and geomagnetic sensors as well as that provided by the GPS receiver itself.
Nowadays, GNSS is used for absolute and relative positioning of aircraft and spacecraft as well as determination of their attitude. What these applications have in common is that, in general, the orientation of the platform is changing relatively slowly and, to a large extent, predictably. Here, we will discuss a balloon-based application where the orientation of the platform, at times, varies very dynamically and unpredictably.
Balloon Experiments
Scientific balloons have been launched in Japan by the Institute of Space and Astronautical Science (ISAS), now a division of JAXA, since 1965, and it holds the world record for the highest altitude reached by a balloon — 53 kilometers. Recently, balloon launches have taken place from the Multipurpose Aviation Park (MAP) in Taiki on the Japanese island of Hokkaido. The balloons are launched using a so-called sliding launcher. The sliding launcher and the hanger at MAP are shown in FIGURE 1.
Balloon-Based Operation Vehicle. As previously mentioned, JAXA’s BOV has been designed for microgravity research. The scenario of a microgravity experiment is illustrated in FIGURE 2. The vehicle is launched with a balloon, which carries it to an altitude of more than 40 kilometers, where it is released.
Figure 2. Microgravity experiment procedure.
After separation, the BOV is in free fall until the parachute is released so that the vehicle can make a controlled landing in the sea. The BOV is recovered by helicopter and can be reused. The capsule has a double-shell drag-free structure and it is controlled so as not to collide with the inner shell. The flight capsule, shown hanging at the sliding launcher in Figure 1, consists of a capsule body (the outer shell), an experiment module (the inner shell), and a propulsion system. The inner capsule shown in FIGURE 3 is kept in free-falling condition after release of the BOV from the balloon, and no disturbance force acts on this shell and the microgravity experiment it contains.
The outer shell has a rocket shape to reduce aerodynamic disturbances. The distance between the outer and inner shells is measured using four laser range sensors. Besides the attitude of the BOV, the propulsion system controls the outer shell so that it does not collide with the inner s
hell. The propulsion system uses 16 dry-air gas-jet thrusters of 60 newtons, each controlling it not only in the vertical direction but also in the horizontal direction to compensate disturbances from, for example, wind.
Flight experiments with the BOV were carried out in 2006 (BOV1) and in 2007 (BOV2), when a fine microgravity environment was established successfully for more than 7 and 30 seconds, respectively.
Attitude Determination. Balloon experiments are performed for a large number of applications, some of which require attitude control. Observations from balloon-based telescopes are an example of an application in which stratospheric balloons have to carry payloads of hundreds of kilograms to an altitude of more than 30 kilometers to be reasonably free of atmospheric disturbances. In this application, the typical requirement for the control of the azimuth angle of the platform is to within 0.1 degrees.
JAXA is developing the Attitude Determination Package (ADP, see TABLE 1) for a future version of the BOV, which contains Sun Aspect Sensors (SAS), the Geomagnetic Aspect Sensor (GAS), an inclinometer, and a gyroscope. Each SAS determines the attitude with a resolution of one degree around one axis and the ADP has four of these sensors pointing in different directions. Inherently, this type of sensor can only provide attitude information if the sun is within the field of view of the sensor. The GAS also determines one-axis attitude. The resolution of magnetic flux density measured by the GAS and applied to obtain an attitude estimate is 50 parts per million. This results in an attitude determination accuracy of the GAS of 1.5 degrees with dynamic bias compensation. The inclinometer determines two-axis attitude with a resolution of 0.2 degrees.
Table 1. Sensor specifications.
Background GPS Experiment. DUT is involved in a precise GPS-based relative positioning and attitude determination experiment onboard the BOV and the gondola of the balloon. Not only is the BOV a challenging environment, but so is the gondola itself, because of the rather rapidly varying attitude (due to wind and — especially at takeoff and separation — rotation) and the high altitude. For a GPS experiment, the altitude of around 40 kilometers is interesting as not many experiments have been performed at this height, which is higher than the altitude reachable by most aircraft but below the low earth orbits for spacecraft. An altitude of about 40 kilometers is a harsh environment for electrical devices because the pressure is about 1/1000 of an atmosphere and the temperature ranges from –60 to 0 degrees Celsius. Furthermore, the antennas are placed under the balloon, which affects the received GPS signals. Later on, we will describe in detail two experiments performed in 2008 and 2009, respectively.
The GPS receivers on the first flight in 2008 were a navigation-type receiver, not especially adapted for such an experiment. The data was collected on a single baseline with two dual-frequency receivers. The receivers were controlled by, and the data stored on, an ARM Linux board using an RS-232 serial connection.
For the second flight in 2009, we used a multi-antenna receiver, for which the Coordinating Committee for Multilateral Export Controls altitude restriction was explicitly removed. This receiver has three RF inputs that can be connected to three antennas, so the observations from the three antennas are time-synchronized by a common clock. The receiver has the option to store observations internally, which simplified the control of the GPS experiment. We used three antennas: one L1/L2 antenna as the main antenna and two L1 antennas as auxiliary antennas.
Theory of Attitude Determination
In this section, we will provide background information on the models applied in our GPS experiment. More details can be found in the publications listed in Further Reading.
Standard LAMBDA. Most GNSS receivers make use of two types of observations: pseudorange (code) and carrier phase. The pseudorange observations typically have a precision of decimeters, whereas carrier-phase observations have precisions up to the millimeter level.
Carrier-phase observations are affected, however, by an unknown number of integer-cycle ambiguities, which have to be resolved before we can exploit the higher precision of these observations. The observation equations for the double-difference (between satellites and between antennas/receivers) can be written for a single baseline as a system of linearized observation equations:
(1)
where E(y) is the expected value and D(y) is the dispersion of y. The vector of observed-minus-computed double-difference carrier-phase and code observations is given by y; z is the vector of unknown ambiguities expressed in cycles rather than distance units to maintain their integer character; b is the baseline vector, which is unknown for relative navigation applications but for which the length in attitude determination is generally known; A is a design matrix that links the data vector to the vector z; and B is the geometry matrix containing normalized line-of-sight vectors. The variance-covariance matrix of y is represented by the positive definite matrix Qyy, which is assumed to be known.
The least-squares solution of the linear system of observation equations as introduced in Equation (1) is obtained using from:
. (2)
The integer solution of this system can be obtained by applying the standard Least-Squares Ambiguity Decorrelation Adjustment (LAMBDA) method.
Constrained LAMBDA. In applications for which some of the baseline lengths are known and constant, for example GNSS-based attitude determination, we can exploit the so-called baseline-constrained model. Then, the baseline-constrained integer ambiguity resolution can make use of the standard GNSS model by adding the length constraint of the baseline, ||b|| = , where is known. The least-squares criterion for this problem reads:
.(3)
The solution can be obtained with the baseline-constrained (or C-)LAMBDA method, which is described in referred literature listed in Further Reading. Later on, we will refer to the attitude calculated by this approach simply as C-LAMBDA.
For platforms with more than one baseline, the C-LAMBDA method can be applied to each baseline individually, and the full attitude can be determined using those individual baseline solutions. For completeness, we also mention a recently developed solution of this problem, called the multivariate-constrained (MC-) LAMBDA, which integrally accounts for both the integer and attitude matrix. Both approaches are applied in the analyses of the BOV data.
Onboard Attitude Determination. In this article, we also use the onboard estimate of the attitude as provided by the multi-antenna receiver. The method applied in the receiver is based on a Kalman filter and the ambiguities are resolved by the standard LAMBDA method. The baseline length, if the information is provided to the receiver a priori, is used to validate the results. For baseline lengths of about 1 meter, the receiver’s pitch and roll accuracy is about 0.60 degrees, and heading about 0.30 degrees according to the receiver manual. We will refer to the attitude as provided by the receiver as KF.
Flight Experiments
In this section, we will discuss our analyses of the GPS data from two of the BOV experiments.
Gondola Experimental Flight 2008. In September 2008, we performed a test of the ADP for a future version of the BOV and a GPS system containing two navigation-grade GPS receivers. The goal of the experiment was to confirm nominal performance in the real environment of the ADP sensors and GPS receivers on the gondola; therefore, the BOV was not launched. The data from the single baseline was used to determine the pointing direction of the gondola, an application referred to as the GNSS compass. The receivers and the controller were stored in an airtight container (see FIGURE 4) and the antennas were sealed in waterproof bags. The location of the two GPS antennas on the gondola is indicated in Figure 4. The baseline length was 1.95 meters. Both receivers used their own individual clocks, so observations were not synchronized. The trajectory (altitude) of this flight is shown in the right-hand side of Figure 4, with the longitude and latitude shown in FIGURE 5. This is a typical flight profile for our application. The flight takes about three hours and reaches an altitude of more than 40 kilometers.
Figure 4B. Single baseline experiment performed in September 2008, the flight trajectory (altitude).
First, the balloon makes use of the wind direction in the lower layers of the atmosphere, which brings it eastwards. During this part of the flight, the balloon is kept at a maximum altitude of about 12 kilometers. After about 30 minutes, the altitude is increased to make use of a different wind direction that carries the balloon back in the westerly direction toward the launch base in order to ease the recovery of the capsule and/or the gondola.
At the end of the flight, there is a parachute-guided fall over 40 kilometers to sea level, for both the gondola and the BOV (if it is launched), which takes about 30 minutes. In this experiment, we could confirm the nominal operation of some of the sensors and reception of the GPS signals on the gondola under the large balloon.
Gondola Experimental Flight 2009. In May 2009, the third flight of the BOV was performed. The three GPS receiver antennas and the other attitude sensors were placed on an alignment frame for stiffness, which was then attached to the gondola. Furthermore, we used a ground station to demonstrate the combination of GPS-based attitude determination and relative positioning between the platform and the ground station. As the motion of the system is rather unpredictable, we used a kinematic approach for both attitude determination and relative positioning.
Preflight static test: Before the flight, we did a ground test using the actual antenna frame of the gondola (see FIGURE 6). The roll, pitch, and heading angles for this static test are shown on the right-hand side of this figure. Due to the geometry of the baselines, the heading angle is more accurate. For this static test, we can calculate the standard deviation of the three angles to confirm the accuracy achievable for the flight test. These results are summarized in TABLE 2. For the baselines with a length of about 1.4 meters, we achieved an accuracy of about 0.25 degrees for the roll and pitch angles and 0.1 degrees for heading, which is as expected from the lengths and geometry of the baselines. Using single-epoch data, we could resolve the ambiguities correctly for more than 99 percent of the epochs (see TABLE 3). Also, the standard deviation of the receiver’s Kalman-filter-based attitude estimate (KF) is included in the table. The accuracy is, after convergence of the filter, similar to our C-LAMBDA result, although the applied method is very different. The Kalman filter takes about 10 seconds to converge for this static experiment, whereas the C-LAMBDA method provides this accuracy from the very first epoch. For completeness, the instantaneous success rate of the standard LAMBDA and MC-LAMBDA methods are also included in Table 3.
Figure 6. Static experiment: C-LAMBDA-based attitude estimates.Table 2. Standard deviation of attitude angles for static test.Table 3. Single-epoch, overall success rate for baseline 1-2 (static experiment).
Gondola nominal flight: Next, we applied the same GPS configuration on the gondola. An important difference with respect to the static field experiment is that the antennas were now placed under the balloon and inside waterproof bags (see the picture on the left-hand side of FIGURE 7). The right-hand side of Figure 7 shows the flight trajectory (altitude) of the experiment. At 21:05 UTC (07:05 Japan Standard Time), the balloon was released from the sliding launcher (Figure 1). In 2.5 hours, the balloon reached an altitude of more than 41 kilometers from which the BOV was dropped. At 23:55, the BOV was released from the Gondola, and at 23:59 the gondola was separated from the balloon. After the release of the BOV, the balloon and gondola ascended more than 2 kilometers because of the reduced mass of the system. For this flight, the attitude determination package and the GPS system were installed on the gondola to confirm the nominal performance of all the sensors.
Figure 7A. Full attitude experiment performed in May 2009, sensor configuration.Figure 7B. Full attitude experiment performed in May 2009, flight trajectory (altitude).
Using the new GPS receiver with three antennas, we are able to calculate the full attitude of the gondola. The roll and pitch estimates, from both C-LAMBDA and KF, are shown in FIGURE 8. The heading angle from the GPS-based C-LAMBDA and KF, and that from the GAS and SAS sensors are shown in FIGURE 9. As explained in a previous section, the four SAS sensors will only output an attitude estimate if the sun is in the field of view of a sensor. Therefore we can distinguish four bands in the heading estimate of the SAS, corresponding to the individual sensors (indicated in Figure 7 as SAS1 to SAS4).
Figure 8A. GPS results for roll angles during nominal flight.Figure 8B. GPS results for pitch angles during nominal flight.Figure 9A. GPS results for heading angle during nominal flight.Figure 9B. GAS and SAS results for heading angle during nominal flight.
The number of locked GPS satellites at the main antenna is shown on the right-hand side of Figure 7. Before takeoff, we saw that the number of locked channels varies rapidly due to obstructions, but after takeoff the number is rather constant until the BOV is separated from the gondola. Before takeoff, the GPS observations are affected by the obstruction of the sliding launcher and therefore ambiguity resolution is only possible on the second baseline (see Figure 8). Also, the GPS receiver itself does not provide an attitude estimation during this phase of the experiment. During takeoff, we see large variations in orientation of the gondola (up to 20 degrees (±10 degrees) for both roll and pitch), which can be estimated well by both C-LAMBDA and KF. Again, the Kalman filter takes a few epochs to converge (in this case, 15 seconds from takeoff), whereas the C-LAMBDA method provides an accurate solution from the very first epoch. After takeoff, the attitude of the gondola stabilizes and the C-LAMBDA and KF attitude estimates are very similar.
We investigated the difference between the attitude estimation from the different sensors during nominal flight. The mean and standard deviations of the differences are shown in TABLE 4. If we compare the C-LAMBDA and KF attitudes, we observe biases for all angles. This is something we have to investigate further, but the most likely cause for this bias is the time delay of the Kalman filter in response to changes in attitude, as we observed in the static experiment in the form of convergence time.
Table 4. Attitude differences (offset/standard deviation) for flight test of 2009.
The standard deviation for the difference in the estimates of roll, pitch, and heading is as expected. For the comparison with the other sensors, we use the C-LAMBDA attitude as the reference. Between C-LAMBDA and GAS/SAS, we observe a bias, most likely due to minor misalignment issues between the sensors. The standard deviations in Table 4 are in line with expectation based on the sensor specifications. During this part of the flight, we achieved a single-epoch, single-frequency empirical overall success rate for ambiguity resolution on the two baselines of 95.09 percent. As a reference, we also include in TABLE 5 the success rate for standard LAMBDA using observations from a single epoch. If we make use of the MC-LAMBDA method, the success rate is increased to 99.88 percent as shown in the table. The success rate is higher as the integrated model for all the baselines is stronger.
Gondola flight after BOV separation: After the separation of the BOV from the gondola, the gondola starts to ascend and sway. FIGURE 10 contains roll and pitch estimates for this part of the flight until the gondola separation. In the figure, we see large variations in the orientation of the gondola (up to 40 (±20) degrees for roll and 20 (± 10) degrees for pitch). It is interesting that after BOV separation, during the large maneuvers of the gondola caused by the separation, both KF and C-LAMBDA estimates are available but to a certain extent are different. Table 4 also contains standard deviations and biases between C-LAMBDA and KF for this part of the flight.
Figure 10A. GPS results for roll angles during nominal flight.Figure 10B. GPS results for pitch angles during nominal flight.
We conclude that the differences (standard deviation but also bias) between C-LAMBDA and KF — both for roll and pitch — are increased compared to the nominal part of the flight. This confirms our expectation that the Kalman-filter-based result lags behind the true attitude in dynamic situations, whereas the C-LAMBDA result based on single-epoch data should be able to provide the same accurate estimate as during the other phases of the flight.
Future Work
For the final phase of the experiment program, we would like to collect multi-baseline data from a number of vehicles. The preferred option for the experiment is three antennas (two independent baselines) on the BOV, and two antennas (one baseline) on the gondola. Furthermore, similar to our 2009 experiment, a number of antennas at a reference station could be used. The goal of the final phase of the program is to collect data for offline relative positioning and attitude determination, though real-time emulation, between a number of vehicles that form a network.
Acknowledgments
Peter Buist thanks Professor Peter Teunissen for support with the theory behind ambiguity resolution and, including Gabriele Giorgi, for the pleasant cooperation during our research. The MicroNed-MISAT framework is kindly thanked for their support. The research of Sandra Verhagen is supported by the Dutch Technology Foundation STW, the Applied Science Division of The Netherlands Organisation for Scientific Research (NWO), and the Technology Program of the Ministry of Economic Affairs. This article is based on the paper “GPS Experiment on the Balloon-based Operation Vehicle” presented at the Institute of Electrical and Electronics Engineers / Institute of Navigation Position Location and Navigation Symposium 2010, held in Indians Wells, California, May 6–8, 2010, where it received a best-paper-in-track award.
Manufacturers
The Attitude Determination Package’s Sun Aspect Sensor is based on photodiodes manufactured by Hamamatsu Photonics K.K.; the Geomagnetic Aspect Sensor is based on magnetometers manufactured by Bartington Intruments Ltd.; the inclinometer is based on a module manufactured by Measurement Specialties; and the gyro is manufactured by Silicon Sensing Systems Japan, Ltd. For the 2009 experiment, we used a Septentrio N.V. PolaRx2@ multi-antenna receiver with S67-1575-96 and S67-1575-46 antennas from Sensor Systems Inc. Details on the receivers and antennas used for the 2008 experiment are not publicly available. A Trimble Navigation Ltd. R7 receiver and two NovAtel Inc. OEMV receivers were used at the reference ground station. The ARM-Linux logging computer is an Armadillo PC/104 manufactured by Atmark Techno, Inc.
Peter J. Buist is a researcher at Delft University of Technology in Delft, The Netherlands. Before rejoining DUT in 2006, he developed GPS receivers for the SERVIS-1, USERS, ALOS, and other satellites and the H2A rocket, and subsystems for QZSS in the Japanese space industry.
Sandra Verhagen is an assistant professor at Delft University of Technology in Delft, The Netherlands. Together with Peter Buist, she is working on the Australian Space Research Program GARADA project on synthetic aperture radar formation flying.
Tatsuaki Hashimoto received his Ph.D. in electrical engineering from the University of Tokyo in 1990. He is a professor of the Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA).
Shujiro Sawai received his Ph.D. in engineering from the University
of Tokyo in 1994. He is an associate professor at ISAS/JAXA.
Shin-Ichiro Sakai received his Ph.D. degree from the University of Tokyo in 2000. He joined ISAS/JAXA in 2001 and became associate professor in 2005.
Nobutaka Bando received a Ph.D. in electrical engineering from the University of Tokyo in 2005. He is an assistant professor at ISAS/JAXA.
Shigehito Shimizu received a master’s degree in engineering from Tohoku University in Sendai, Japan, in 2007. He is an engineer in the Navigation, Guidance and Control Group at JAXA.
FURTHER READING
• Authors’ Proceedings Paper
“GPS Experiment on the Balloon-based Operation Vehicle” by P.J. Buist, S. Verhagen, T. Hashimoto, S. Sawai, S-I. Sakai, N. Bando, and S. Shimizu in Proceedings of PLANS 2010, IEEE/ION Position Location and Navigation Symposium, Indian Wells, California, May 4–6, 2010, pp. 1287–1294, doi: 10.1109/PLANS.2010.5507346.
• Balloon Applications
“Development of Vehicle for Balloon-Based Microgravity Experiment and Its Flight Results” by S. Sawai, T. Hashimoto, S. Sakai, N. Bando, H. Kobayashi, K. Fujita, T. Yoshimitsu, T. Ishikawa, Y. Inatomi, H. Fuke, Y. Kamata, S. Hoshino, K. Tajima, S. Kadooka, S. Uehara, T. Kojima, S. Ueno, K. Miyaji, N. Tsuboi, K. Hiraki, K. Suzuki, and K. M. T. Nakata in Journal of the Japan Society for Aeronautical and Space Sciences, Vol. 56, No. 654, 2008, pp. 339–346, doi: 10.2322/jjsass.56.339.
“Development of the Highest Altitude Balloon” by T. Yamagami, Y. Saito, Y. Matsuzaka, M. Namiki, M. Toriumi, R. Yokota, H. Hirosawa, and K. Matsushima in Advances in Space Research, Vol. 33, No. 10, 2004, pp. 1653–1659, doi: 10.1016/j.asr.2003.09.047.
• Attitude Determination
“Testing of a New Single-Frequency GNSS Carrier-Phase Attitude Determination Method: Land, Ship and Aircraft Experiments” by P.J.G. Teunissen, G. Giorgi, and P.J. Buist in GPS Solutions, Vol. 15, No. 1, 2011, pp. 15–28, doi: 10.1007/s10291-010-0164-x, 2010.
“Attitude Determination Methods Used in the PolarRx2@ Multi-antenna GPS Receiver” by L.V. Kuylen, F. Boon, and A. Simsky in Proceedings of ION GNSS 2005, the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation, Long Beach, California, September 13–16, 2005, pp. 125–135.
“Development of the Integrated Navigation Unit; Combining a GPS Receiver with Star Sensor Measurements” by P.J. Buist, S. Kumagai, T. Ito, K. Hama, and K. Mitani in Space Activities and Cooperation Contributing to All Pacific Basin Countries, the Proceedings of the 10th International Conference of Pacific Basin Societies (ISCOPS), Tokyo, Japan, December 10–12, 2003, Advances in the Astronautical Sciences, Vol. 117, 2004, pp. 357–378.
• Ambiguity Estimation
“Instantaneous Ambiguity Resolution in GNSS-based Attitude Determination Applications: the MC-LAMBDA Method” by G. Giorgi, P.J.G. Teunissen, S. Verhagen, and P.J. Buist in Journal of Guidance, Control and Dynamics, accepted for publication, April 2011.
“Integer Least Squares Theory for the GNSS Compass” by P.J.G. Teunissen in Journal of Geodesy, Vol. 84, No. 7, 2010, pp. 433–447, doi: 10.1007/s00190-010-0380-8.
“The Baseline Constrained LAMBDA Method for Single Epoch, Single Frequency Attitude Determination Applications” by P.J. Buist in Proceedings of ION GPS 2007, the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation, Fort Worth, Texas, September 25–28, 2007, pp. 2962–2973.
“The LAMBDA Method for the GNSS Compass” by P.J.G. Teunissen in Artificial Satellites, Vol. 41, No. 3, 2006, pp. 89–103, doi: 10.2478/v10018-007-0009-1.
“The Least-Squares Ambiguity Decorrelation Adjustment: a Method for Fast GPS Integer Ambiguity Estimation” by P.J.G. Teunissen in Journal of Geodesy, Vol. 70, No. 1–2, 1995, pp. 65–82, doi: 10.1007/BF00863419.
• Relative Positioning
“A Vectorial Bootstrapping Approach for Integrated GNSS-based Relative Positioning and Attitude Determination of Spacecraft” by P.J. Buist, P.J.G. Teunissen, G. Giorgi, and S. Verhagen in Acta Astronautica, Vol. 68, No. 7-8, 2011, pp. 1113–1125, doi: 10.1016/j.actaastro.2010.09.027.
Many maritime users today believe that GPS will always be available. This is simply not the case.
By Alan Grant, Paul Williams, George Shaw, Michelle De Voy, and Nick Ward, The General Lighthouse Authorities of the United Kingdom and Ireland
GNSS availability can be affected in many ways, through events or conditions that affect constellation health, the signal-in-space, or the reception of that signal. The primary means of positioning, navigation, and timing (PNT) employed in maritime applications, whether stand-alone or augmented, has well known vulnerabilities.
This article considers three specific threats and reports on how they may affect maritime safety: GNSS interference and jamming; constellation availability; and space weather events.
Interference and Jamming
There has been a marked increase in both the use and the availability of GPS jamming equipment in recent years. The implications are that jamming units may find their way onto ferries and around ports or harbors where they will interfere with the many systems utilizing GPS, thus affecting maritime safety.
GPS jamming units are widely available on the Internet, with current models already capable of jamming L1, L2, and L5 signals. While we report here on the jamming of GPS, all GNSS constellations would be affected in a similar manner.
To understand the effects of jamming and GPS service denial on maritime safety, the General Lighthouse Authorities of the United Kingdom and Ireland (GLAs) conducted two jamming trials, in collaboration with the UK Government’s Ministry of Defence (MOD), who provided and operated the GPS jamming units. For the safety of all GPS users, and in line with MOD regulations for the peacetime use of GPS jamming units, notice was given to all national bodies. In addition, the GLAs issued notices to mariners explaining that aids to navigation (AtoNs) using GPS in the vicinity of the trials location would be unreliable during the jamming periods.
Flamborough Head. The first jamming trial was conducted off the East coast of the United Kingdom near Flamborough Head. The aim of this trial was to understand the effect GPS jamming may have on ship-borne and shore-based equipment, GLA AtoNs, and also on the crew.
The Northern Lighthouse Board vessel Pole Star steamed between two known waypoints, through an area affected by the jamming signal. Data was recorded from two typical marine-grade GPS receivers installed on the vessel, along with an eLoran receiver that provided the true position throughout the trial.
The results identified three distinct states (Table 1) corresponding to the manner in which GPS-fed equipment responded to jamming conditions. When the jamming signal was sufficiently strong to prevent reception of GPS signals, a large number of alarms sounded on the bridge almost simultaneously, providing a potentially disconcerting and confusing environment for the mariner. However, the effect that represented the highest risk was the provision of erroneous data from some GPS receivers.
Table 1. Effects observed for the three states identified from Flamborough Head trials.
Figure 1 compares an erroneous position reported by a typical marine-grade GPS receiver with the vessel’s true location. In this figure, the light blue line shows the path taken between the two waypoints.
The colors of the plotted position points indicate vessel speed. The three states described in Table 1 can be seen.
State 1 is observed at either end of the passage where the solid blue line occurs; this is where the jamming signal strength is much lower than the GPS signal strength, and the GPS-fed systems are operating normally.
As the vessel approached the main lobe of the jamming signal, indicated by the red lines, it reached an area where the jamming signal was comparable with the received GPS signals, leading to State 2. During this state, erroneous data can be observed with the receiver reporting the vessel on land traveling at high speed.
As the vessel entered the main lobe of the jamming signal, State 3 was observed: the GPS signals were swamped by the jamming signal, and the receivers failed to provide an output. Then, as the vessel continued the passage out of the jamming area, one can observe the change in states as the ratios of jamming to GPS satellite signals decrease, and GPS is reacquired.
In the worst case, the GPS receiver reported a position some 22 kilometers away from the true location. The GPS receiver nevertheless declared the position valid. This position was made worse by the fact it was reported inland at a speed of more than 100 knots, while the trial vessel steamed steadily at 10 knots. Depending on how the resulting GPS positioning data is used, it could feasibly result in vessels changing course, through the use of an autopilot, and it could also affect the vessel’s reported position to the outside world. This would then not only affect the vessel’s situational awareness but also the situational awareness of vessels in the vicinity.
The errors observed in Figure 1 were also seen on the vessel equipment fed by the onboard GPS receivers. Erroneous positions were observed on the vessel’s electronic chart display and information system (ECDIS), on the automatic identification system (AIS) positions (where loss of position prevents the unit from calculating a range or bearing to nearby vessels, greatly affecting the crew’s situational awareness), and on the vessel’s radar (Figure 2).
The results observed during these trials gave an important example of what can happen to onboard equipment as well as the impact it can have on the mariner during periods of GPS jamming and service denial. It is clear that GPS denial caused by jamming can not only prevent PNT information from being calculated, it can also result in erroneous data being presented to the mariner.
Newcastle. A second series of demonstrations was conducted off Newcastle-upon-Tyne, on the North East coast of England, to communicate the importance of resilient PNT to a selected audience. The audience included a number of key decision-makers from European and UK governments, maritime industry, mariners, and other aids-to-navigation service providers. The demonstrations took place onboard the Trinity House vessel Galatea.
For this trial, the GPS jamming unit was installed onboard the Galatea and configured to jam GPS within a small
area around the vessel. As before, two typical marine-grade GPS receivers were installed along with an eLoran receiver; for this trial, a modified electronic chart display was also installed and altered to enable two position inputs to be displayed at the same time, to compare the reported GPS and eLoran positions in real-time.
Throughout the demonstrations differential Loran (dLoran) corrections were provided using a transportable reference station installed on the shore at South Shields, to mitigate the impact of temporal variations on the eLoran position. Differential-Loran corrections were generated by the reference station and sent to the GLAs’ eLoran transmitter in Cumbria for inclusion in the eLoran Loran Data Channel (LDC) broadcast. The eLoran receiver on the vessel received the broadcast and was able to extract and apply the corrections in order to obtain an eLoran position within 9 meters (95 percent).
One demonstration scenario showed the sudden effect of a strong jamming signal, designed to simulate a jamming unit being brought onto a ferry or other vessel. This took the vessel’s equipment directly to State 3: complete loss of GPS information with a large number of alarms sounding on the bridge. The loss of GPS data prevented the Galatea’s AIS and VHF units, among other systems, from operating correctly.
Before the second scenario was conducted, the jamming unit was stopped, and all of the GPS receivers integrated into the bridge equipment were allowed to reacquire satellites and fully recover. The second scenario was designed to reflect a vessel steaming towards a jamming source. The field strength of the jamming signal was slowly increased until State 2 was observed, with erroneous and often hazardously misleading information reported.
As with the Flamborough trials, erroneous GPS positions reporting unfeasibly high speeds were observed as shown in the OPENING Figure. However, significantly more subtle errors were seen: errors where the vessel’s reported position differed only very slightly from the true location and wandered around slowly. These subtle changes produce believable positions but hazardously misleading information (HMI). While the overall result of GPS jamming on Galatea was consistent with that observed on Pole Star, there were a few marked exceptions.
The effect of GPS jamming can be seen (Figure 3) on the erroneous positions reported by the trial vessel NLB Pole Star (center right) and also on the vessel DutchProgress (top left).
The ECDIS onboard the Pole Star reported erroneous positions and ultimately failed with the complete denial of GPS. However the ECDIS on the Galatea continued to track the vessel’s position due to an additional position feed from the vessel’s gyro, making it more resilient to jamming, but only in the short term until the gyro requires re-calibration. This is carried out with its built-in GPS receiver! In addition, the AIS transceiver on the Pole Star reported the vessel’s position erroneously due to jamming, and this was observed at shore-based traffic monitoring stations.
During the demonstrations on the Galatea, the AIS transceiver did not provide any erroneous position information, as can be seen in Figure 4. These differences show that the impact of GPS jamming will be different for each vessel and depends on the model, installation, and configuration of the onboard systems.
Effect of Jamming on Safe Navigation
To navigate safely, the mariner needs reliable, clear and trusted information about where the ship is and what is going on around it, so that any threat can be located and identified. While consideration is often given to threats such as areas of shallow water, obstacles, or other vessels; consideration is not generally given to the loss of positional information, timing, or situational awareness.
Loss of GPS-derived PNT information at sea results in the loss of the vessel’s ECDIS, AIS, GPS, and DGPS receivers, preventing the mariner from being able to position the ship and others around it through what are nowadays regarded as the normal means. In addition, the systems one would normally expect to be independent from GPS, and as such available for use in GPS-denied conditions, are also affected; namely the vessel’s radar and gyro-compass.
The radar takes a GPS input to provide a “North-up” setting and the gyro-compass uses GPS to stabilize drift error. Under GPS-denial conditions these units also enter an alarm state and should not therefore be used in that condition.
Clearly GPS jamming can significantly affect the safety of mariners. From these trials it can be seen that the extent of the impact varies from vessel to vessel depending on the equipment installed and the configuration selected.
Satellite Constellation. From the users’ perspective, GNSS availability is the percentage of time they can receive usable data from sufficient satellites in order to calculate their position. The reduction in the number of available satellites in the constellation will have a direct impact on the system’s availability.
A report from the U.S. Government Accountability Office (GAO) in 2009 predicted “significant challenges in sustaining and upgrading widely used [GPS] capabilities” due to delays in launching modernized GPS satellites. The GAO reported the probability of maintaining a constellation of at least 24 usable GPS satellites could reduce to 80 percent or less by 2011, and not return to 95 percent probability consistently until 2015. This could lead to reduced satellite numbers causing coverage “windows” where less than four satellites could be observed and as such reduced GPS availability.
A later report by the GAO indicates that the probability of maintaining a constellation of at least 24 operational GPS satellites is now expected to be 95 percent for the foreseeable future. This figure is based on the current launch schedule, and although the U.S. Air Force Space Command (AFSPC) has provided reassurances, the satellite launch program has in recent years experienced delays, and therefore the risk of reduced satellite availability still remains.
Following the 2009 report, the GLAs commissioned a study to investigate the impact a reduced GPS constellation would have on users in their waters. This study was conducted by the GNSS Research and Applications Centre of Excellence (GRACE) and was split into two parts. The first part was to analyze the impact theoretically and found that with a 21-satellite constellation, GPS coverage “windows” (for example, fewer than four satellites) could last for several minutes and cover a large proportion of the UK and Ireland (Figure 5). This can cause reduced GPS availability and therefore increased likelihood of position errors affecting maritime safety.
The second part of the study investigated the effects further through a dynamic simulation, investigating the effects should a vessel be position
ed off the coast of Belfast during one of the coverage windows. For this a marine-grade GPS receiver and a simulator were used to observe the effects. The study found that the number of available satellites fell below four for several minutes and the reported position data from the receiver appeared to freeze for up to 10 minutes.
If a mariner was traveling at a speed of 35 knots when the position input froze, his reported position would be in error by 10 kilometers from an outage lasting 10 minutes. These outages are significant, and mariners need to be informed of such risks to GPS (and GNSS in the future) before they occur, so they are prepared for any disruptions.
Space Weather. Space-weather events are a particular concern to GNSS availability due to their random nature. It is known that GNSS signals are delayed proportionally to the number of free ions as they propagate through the Earth’s atmosphere enroute to the receiver. The amount of ions in the ionosphere, the total electron count (TEC), is dependant on time of day, latitude, and solar activity, among other factors. During high solar activity, the number of ions in the atmosphere is much higher than at any other time. The greater the signal delay, the larger the errors are in the satellite’s pseudo range and hence the position error can be significant.
Variation in electron density along the GNSS signal path causes signal refraction that produces phase scintillation, introducing group delay that may cause large errors in the pseudorange measurement. Diffraction of the signal wave front induces amplitude scintillation — variations in signal amplitude — with strong fades possible, leading to a GNSS receiver losing signal tracking, and at worst the GNSS navigation solution may be lost.
Solar activity is cyclical, peaking at a maximum approximately every 11 years, during which periods GNSS performance can be severely degraded, especially at equatorial, auroral and polar latitudes. The next solar maximum is predicted to occur during 2013.
During quiescent periods of solar activity, ionospheric effects on GNSS can be managed such that the residual errors caused by the ionosphere do not generally pose a problem to maritime navigation performance.
The GLAs’ DGPS corrections significantly reduce common mode errors, including the effects of the ionosphere. However, at the peak of the solar cycle with high levels of sunspot activity, solar storms and flares, the application of ionospheric models and differential corrections may be less effective, and this could increase position errors and introduce an integrity risk to maritime navigation.
Maritime navigation systems and services that rely on GNSS are at greatest risk of disruption from the ionosphere during the period from 2011 to 2015. Even during a quiet solar maximum, the occurrence of individual sun spots could produce significant effects for discrete events. The effects vary with latitude, season, and time of day (the hours soon after sunset being most affected).
Space weather events have the potential to affect GNSS availability, either by affecting the performance of the satellites themselves or by preventing signal reception.
Mitigation. In general, a number of steps can be taken to help reduce the impact of these threats:
Increase awareness of GNSS vulnerabilities.
Detect incidents and warn the mariner when they occur.
Prevent incidents from occurring, where possible, through legislation and enforcement.
Reduce as much as possible the effects of incidents when they occur, through the hardening of GNSS technology.
Have alternative means of PNT, independent of GNSS.
Understanding that these threats exist and knowing what disruption they may cause is the first step to mitigating their effects, but this does not stop them happening. Being able to identify that an event is occurring and that the data being received from the receiver may not be true is an important part of mitigating the effects.
For jamming issues specifically, the use of GPS jamming units is illegal in the UK and Ireland; however, preventing them from being used is very difficult to achieve. Jamming units are small and easily hidden; however, port-side security and vessel security procedures should prevent jamming units from being used in these locations.
It is a different case, however, to prevent a jamming unit from being used at a coastal location or headland due to the remote nature of these areas.
Mitigating the effect of jamming can be achieved in a number of ways: by limiting the effect within the receiver by using anti-jamming techniques, or by hardening GNSS receivers. Ultimately the best mitigating activity is to not rely on GNSS PNT once the integrity of the data has been compromised.
For space weather events or cases of reduced satellite numbers, there is very little action the mariner can take to remedy the problem or stop it happening. The mitigating action here is one of awareness — information forewarning the mariner that such a condition is imminent, for example.
Monitoring and detection networks can assist in providing such notifications and real-time information on GNSS problems. The need for such a network across the UK and Ireland is the subject of a different GLA publication, but the GLAs support the discussion on a body to monitor GNSS performance and to take the lead in the dissemination of key information.
For periods where GNSS availability has been affected by mutual interference, jamming, space weather events or constellation issues, the best mitigating action is to use PNT information from a second source, one with dissimilar failure modes.
Mariners need to be prepared for GNSS failures and have access to PNT information through dissimilar systems. In addition, procedures covering what to do in the case of GNSS unavailability should also be provided and rehearsed. It is with this view that the GLAs firmly promote the use of all available means of navigation.
Conclusions
All three threats to GNSS availability reviewed here could affect maritime safety. The two trials observed presentation to the mariner of erroneous data, some of which could be considered hazardously misleading, along with the degradation of crews’ situational awareness. The main effects observed were:
The presentation of random errors leading to hazardously misleading information that could, depending on installation, cause a vessel to move off course.
The presentation of erroneous and potentially misleading data to other vessels and shore-based infrastructure.
The sheer number of alarms on the bridge of the vessel could be disconcerting and distracting for the mariner.
The loss of GPS-fed systems, which can create an unfamiliar bridge situation and remove safety-critical systems from operation.
A large number of bridge systems are integrated with GPS and enter an alarm state during periods of GPS outage.
The loss of GPS or a lack of integrity in the reported information leads to an unfamiliar situation on the bridge.
The crews of the Pole Star and the Galatea were expecting to lose GPS, were well-trained, and had primed other systems so they could navigate safely. In real life, there would be no advance notice, and the impact on the crew would be more severe.
The impact of low satellite numbers, as predicted in the 2008 GAO report, could produce poor constellation availability and a loss of PNT information for a considerable period of time. This could result in the same outcome as observed in the GPS jamming trials when entering State 3, where many systems on the bridge failed and entered an alarm condition.
Space weather events are difficult to predict both in terms of when they may occur and their severity. Events could affe
ct satellite positions, their operation, and the reception of their signals by the user, and are clearly a threat.
The GLAs strongly support the need for a resilient PNT solution, one that could continue to provide reliable information during such threats for the safety and benefit of all mariners.
Acknowledgment
This article is based on a paper given at the Institute of Navigation’s 2011 International Technical Meeting.
Alan Grant is a principal engineer for the Research and Radionavigation Directorate of the GLAs of the UK and Ireland, technical lead and project manager for all GNSS projects there. He has a Ph.D. from the University of Wales.
Paul Williams is a principal development engineer with the Directorate and currently technical lead of the GLAs’ eLoran Work Programme. He has a Ph.D. in electronic engineering from the University of Wales.
George Shaw is an engineer at the Directorate and holds a master’s degree in mathematics from the University of Cambridge.
Michelle De Voy is a development engineer for the Directorate, with an MSc in oceanography from the University of Southampton and an MSc in satellite positioning from the University of Nottingham.
Nick Ward is research director of the General Lighthouse Authorities of the UK and Ireland, with responsibility for strategy and planning of research and development.
The U.S. Air Force 50th Space Wing’s 2nd Space Operations Squadron took command and control of the second GPS Block IIF satellite on August 19. SVN-63 (PRN 01) was set healthy on August 23.
The total of 12 next-generation GPS IIF satellites built by Boeing will provide improved accuracy through advanced atomic clocks, a longer design life than legacy GPS satellites, and a new signal, L5, that will benefit civil aviation and safety-of-life applications.
The Space and Missile Systems Center’s GPS Directorate at Los Angeles Air Force Base remained in control of the satellite during a 30-day on-orbit check-out period before hand off.
The constellation is more robust and capable than at any other time in its history, the GPS Wing said. Members of 2 SOPS operate the largest Department of Defense satellite constellation via the Master Control Station and a worldwide network of monitoring stations and ground antennas.
Recalls IIA to Duty. For only the second time in a quarter century, Air Force officials plan to transition a decommissioned GPS satellite back to active status. 2 SOPS staff noticed in late May that the clock on the GPS IIA SVN-30 was starting to malfunction. 2 SOPS engineers and counterparts at Boeing and Aerospace Corp. developed a plan to bring SVN-35 back in to service to replace the ailing bird. The 18-year-old satellite was decommissioned from active service in 2009 to make room for the eventual deployment of the latest GPS Block IIR vehicle; however, its navigational signal continued to function properly.
“We keep on-orbit spares for exactly this purpose,” said Lt. Col. Jennifer Grant, 2 SOPS commander. “The robustness of our current constellation and the recent completion of the Expandable 24 architecture provide us with the flexibility to perform replacements like this with minimal impact to global users.”
OCX Hits Bump: Does Not Pass Preliminary Design Review
The next-generation GPS Ground Control system (OCX) under the direction of prime contractor Raytheon did not pass the recently concluded initial Preliminary Design Review (PDR).
Not passing this critical PDR inspection so early in the OCX process and in the current fiscal environment (Congress has already trimmed the modernization budget and shifted elements to the right) constitutes a blow to the GPS modernization effort. It adds to the worry concerning the OCX-GPSIIIA gap having to do with the ability to launch the Lockheed-produced GPS IIIA space vehicles (SVs) and payloads that are scheduled to be ready for launch a full 14–16 months before the OCX ground system was originally scheduled to be able to control the launch.
That timeline undoubtedly stretches to the right with this development.
The PDR is a formal inspection by the government acquisition agency — the Air Force’s GPS Directorate in this case — of the high-level architectural design of the OCX automated systems and the associated C2 software. The PDR, critical for any military project but especially so for the new GPS Ground C2 system, is conducted to achieve confidence that the design satisfies the functional and nonfunctional requirements and conforms with the overall enterprise architecture. Overall project status, proposed technical solutions, evolving software products, and all associated documentation are reviewed at a high level during the PDR to determine completeness and consistency with contractual standards. The PDR also serves to raise and resolve any technical and/or project-related issues, and to identify and mitigate project, technical, security, and/or business risks affecting continued detailed design and subsequent development, testing, implementation, and operations and maintenance activities.
Typically during a PDR the government has several choices concerning the outcome. It can:
Approve
Approve conditionally
Withhold approval
Disapprove or fail the program.
In this case, the government chose to withhold approval and not approve conditionally or formally fail until all PDR action items are reviewed.
LightSquared Interference
For the first time in several months, there is little in the way of concrete news to report on this topic — as of press date August 24. The Federal Communications Commission weighs its options and scrutinizes the further data that it has requested: the number and lifespan of GPS receivers that will be interfered with, and the number of terrestrial base stations LightSquared plans to deploy. Here are highlights from the “LightSquared Watch” webinar on August 18:
GPS is arguably the most efficient use of spectrum the world has ever seen; almost a billion people benefit from the GPS signal that is available today. This use represents a massive installed base and source of innovative advantage for the United States. Most importantly, it represents a high degree of trust and confidence in the United States and its stewardship of GPS. — Scott Pace
Misinformation is rampant, and the pressure for action before analysis characterized the early stages of this process. History was reinterpreted, and the facts twisted to fit desired reality. We have heard lawyers’ assertions versus engineers’ judgements — with only the latter supported by verifiable data. — Jules McNeff
Launches Round the World
China launched a fourth inclined geosynchronous orbit (IGSO) satellite in the Beidou/Compass navigation system on July 26. Its orbit is currently centered on an East longitude of about 93 degrees, some distance away from the other three IGSO satellites. Plans call for completion of a 14-satellite constellation by 2012.
A single GLONASS-M satellite was set to be launched on August 26. Five further GLONASS launches are planned this year: a triple and a single GLONASS-M launch in October, and the second GLONASS-K1 satellite in December.
The first two Galileo In Orbit Validation satellites are set to be launched from French Guiana on October 20, with two more following them into orbit by mid-2012.
For the past several months, controversy has raged over the revelation that Apple and Google tracked mobile subscriber location movements and stored that information in an unencrypted file on the handset, where it was potentially vulnerable to hacking and other inappropriate usage. The resulting Location-gate scandal highlights the sometimes tenuous control of mobile subscriber information versus the business objectives of dominant platform and applications providers. These business objectives may include immediate revenue opportunities from the subscriber being tracked or broader self-interest initiatives, such as collecting marketing data that may be valuable to third parties like advertisers, or building subscriber-reported Wi-Fi access point databases.
Furthermore, while much has been written about the privacy impacts of the collection and use of consumer location information, few articles have clearly outlined the technologies behind Apple and Google’s tracking activities. It is important to fully explore and understand these technology methods, and how they differ from other location technologies in use, in order to properly evaluate the threat posed by Location-gate and to develop responses that maintain privacy while enabling the benefits of location-based services.
Location, Tracking, and Storage
iPhone and iPad subscribers had previously been aware that Apple tracked their location via GPS, because the company notified subscribers when an app required the use of GPS to identify location, and asked them to opt-in. However, soon after Location-gate erupted, Apple’s vice president of software technology, Bud Tribble, testified to Congress in May 2011 that Apple also had been tracking device locations over time using triangulation between nearby Wi-Fi access points and wireless base stations. Triangulation is the moderately accurate method in which the mobile device measures the nearby cell site or access point identifications and possibly signal strengths, typically pinpointing device location to within a few hundred meters.
Following this revelation, Apple’s initial response was that “users are confused” and that it was simply “maintaining a database of Wi-Fi access points and cell towers around your current location…to help your iPhone rapidly and accurately calculate its location when requested.” Soon after Apple location tracking activity was revealed, it became known that Google was doing essentially the same thing, although to a slightly lesser degree (Android phones stored only the 50 most recent coordinate fixes and up to 200 Wi-Fi access-spot locations), and using a similar triangulation method without the subscriber’s explicit knowledge. Google Android devices also have GPS capability.
Why, if both OS providers embedded or leveraged GPS in their phones, would they resort to a less accurate location method, triangulation?
Neither company has provided an answer. We know that the triangulation method uses less battery power than GPS, conserving battery life for other uses while filling in performance holes for GPS in urban and indoor environments. Also, unlike with GPS, mobile subscribers are either not able to disable triangulation or must disable it separately. More relevant is the fact that triangulation allowed the OS providers to identify location automatically and track it over time in the background without the subscriber’s knowledge, for purposes such as building and maintaining a subscriber-reported database of Wi-Fi access points.
From a privacy perspective, there is a dramatic difference between tracking someone’s location over time (the bread crumb trail that Apple and Google used), versus locating one’s position for a specific purpose and handling the location information only within the confines of a secure wireless network. Useful applications that are universally accepted, such as E911 for safety-of-life situations, employ the latter method.
Other players in the mobile ecosystem, such as wireless network operators, have collected subscriber location information as well, but not by storing it in the device as historical files in the same way that Apple and Google did. Some information exists on the network side in association with billing records for calls (call detail records or CDRs), but this is not bread-crumb tracking of cell-IDs. E911 calls have records stored for use by public safety agencies, but most users never make an E911 call. Other messages containing coarse location may exist on a transitory basis (for example, location area updates), but these are not typically aggregated or stored for later processing.
Depictions of location information stored on handset and in operator network.
Alternative Geo-Location Methods
There exist location methods that provide far greater privacy and security than the location tracking and handset storage that Apple and Google have utilized. Standard methods exist for performing location using the wireless service provider’s network elements. These are called control-plane methods, which follow standards developed by 3rd Generation Partnership Project (3GPP) and 3GPP2. Other standard methods exist using IP transport from the client phone to a location server. These are called user-plane methods, such as the Secure User Plane Location (SUPL) standard from the Open Mobile Alliance (OMA). Both control- and user-plane location standards incorporate mechanisms for data security and user privacy. These standard control- and user-plane methods differ from the proprietary methods used by many client applications and OSs, which are inherently user-plane in nature but with non-standard implementations.
Methods using a client application with handset-based location on the mobile device, also called user-plane methods, bypass the carrier’s wireless network elements and instead rely on an IP connection to transmit information from the client application to a server on the Internet. These user-plane location methods, such as client applications for handset-based A-GPS, as discussed, are already widely in use for location-based services. Handset applications are inherently vulnerable to hacking and privacy intrusions, as the recent spate of mobile viruses on Android has highlighted.
A-GPS is highly accurate at identifying location in direct line-of-sight conditions with the satellites (open sky conditions), as found in suburban and rural areas, but performs less well in challenging dense urban and indoor environments. GPS in the phone can be easily disabled by the end user, and the receiver chip in the handset can cause significant battery consumption when used in demanding applications, such as navigation and monitoring geo-fences. A-GPS, as used by wireless network operators for navigation and other location-based services, does not usually store unencrypted files of historical location information in the handset, as Apple and Google did.
Alternative, network-based, or control-plane, methods make use of the wireless services provider’s network elements to keep location information wholly behind the security of the operator’s firewall, employing highly standard protocols for security and privacy. Control plane location methods are used for today’s safety-of-life applications, like E911, where security and privacy are prime considerations.
One example of a network-based location technology that can work in control-plane is RF pattern-matching (RFPM), which is the only high accuracy, software-based, scalable location solution that requires no additional hardware changes/additions to the mobile device or at the base stations. It compares mobile measurements (signal strengths, signal-to-interference ratios, time delays, and so on) against a geo-referenced database of the mobile operator’s radio environment. RFPM boasts a 100 percent security record for subscriber mobile location information it produces, for critical applications such as E911 emergency call and law enforcement location applications.
Location information for growing consumer uses deserves the same privacy and security protections that other standards-compliant control-plane solutions provide for today’s mission-critical and safety-of-life location applications. RFPM works extremely well in non line-of-sight conditions such as dense urban and indoor environments, where GPS-based solutions face challenges. RFPM also offers low battery consumption and geo-fencing capabilities, which makes it ideal for providing location for the growing opportunity in location-based advertising and other location-based services (widely believed to be the true driver behind Apple and Google’s location tracking activities).
As Location-gate clearly illustrates, there is no shortage of methods to identify and track one’s location via mobile device. Now that the issue has been raised, it is imperative that the entire mobile ecosystem — network operators, OS providers, regulators, and subscribers — clearly understand what methods are used, when one’s location is being identified and tracked, and what is being done with that data. Breadcrumb trails are useful if you’re trying to find your way out of the forest, but not if Big Brother is tracking you.
Marty Feuerstein is chief technology officer of Polaris Wireless, where he leads research into new products, algorithms, system performance, and regulatory activities. He has a Ph.D. in electrical engineering from Virginia Tech.
“GPS can no longer evolve fast enough. Satellite-based systems cannot maintain the speed of development now required for the hyper-fast evolutionary pace of modern applications and devices. For positioning for the future, it has become exceedingly clear that GPS now needs a terrestrial component.” — from a Locata Corporation prospectus
A large number of companies and engineers have thrown billions of dollars at trying to improve GPS in urban and indoor applications,” states Locata Corporation co-founder Nunzio Gambale. “From a technological perspective, Locata has created something completely new: the capability to autonomously create a GPS-style system on the ground.”
Members of the GNSS community can see for themselves at Locata’s coming-out party at ION-GNSS 2011, including release of a Locata signal interface control document (ICD). GPS World took an advance look at the technology in a June trial of the demo that all ION attendees can see. This article presents these reports, after an outline of the technology.
The key to Locata’s positioning system is the signal generated by the Locata transceiver, or LocataLite, to synchronize its time to other LocataLites in a network. Locata creates a network that, according to the company, “is in almost perfect synchronization” without using atomic clocks. Each transmitter dynamically synchronizes with other Locata transmitters using a patented method called Time-Loc. Gambale says that a Locata network currently locks to about 2 nanoseconds.
Each LocataLite base station has an uninterrupted range of approximately 10 kilometers, with indoor signal penetration similar to that of a mobile phone tower.
The company emphasizes that its transceivers are not pseudolites, but devices that create TimeLoc synchronization, and thereby enable an autonomous synchronized network that, locally, looks like GPS. The local constellation is under local control, and can therefore be designed for deployment at any power, any frequency, or any density required by an application.
The networks can scale easily. The term “local” can mean a room or warehouse (100s of m2), a campus or open-cut mine (10s of km2), an airport terminal area with approach and landing routes (100s of km2), or a wide area, range, or city (1,000s of km2)
Gambale sees markets for Locata’s technology in defense, mining, emergency services, construction, and security. Locata is designed to integrate with existing GPS technology, as simply another constellation. This means an approprieate GPS-Locata receiver can use the satellite signal when outside the range of a Locata network. To a combined GPS-Locata chip, the LocataLite will appear as another satellite.
The company sold its first Locata network in July 2005. Locata has signed partnership agreements of various kinds with Leica Geosystems and Newmont Corporation (mining), the U.S. Air Force, the Advanced Navigation Technology Center of the Air Force Institute of Technology, and several other firms under non-disclosure terms. There was an initial test deployment at Holloman Air Force Base in May 2008, as a truth reference system spanning a test area of about 52 by 15 kilometers.
For high-multipath environments such as indoors and warehousing, the company’s latest development is a new antenna called a TimeTenna, which it will demonstrate at ION-GNSS.
Future research and development will focus on the miniaturization of the Locata receiver. Work has begun on a combined GPS-GLONASS-Locata chip that can be integrated initially into professional and industrial devices, and eventually into consumer devices such as mobile phones.
Locata plans to work with integrators only, not with end users, making the technology available to qualified partners developing receivers and applications. The ICD is the first step in Locata’s technology rollout.
LocataLites awaiting boards. Each LocataLite transmits four PRN signals.
A Long Time Coming
Eric Gakstatter, Survey editor
You may have heard the Locata name pop up over the past several years. It would be in the news, then back underground into stealth mode. About five years ago, I heard some interesting rumors about its technology but I decided not to take them seriously until I saw some real products.
Two years ago, I sat down with Nunzio Gambale, Locata CEO, at the ION-GNSS conference. At last year’s ION, I talked with him again. At that point, I understood the potential impact of Locata’s technology — if it worked as advertised. I again told myself that before I spent more time on it, I wanted to see a product introduced to the market based on Locata technology. In January of this year, it happened.
Leica Geosystems introduced its terrestrial GPS Augmentation Network for the mining industry, based on Locata technology. To me, that was a pivotal point. Leica is a reputable company and wouldn’t introduce a product without a thorough vetting.
I contacted Nunzio and we had further discussions. I wanted to see the technology in action — hard to do since Locata is based in Australia, I’m in Portland, Oregon, and an early installation occurred in South Africa. Fortunately, the company’s need to do a real run-through of its demo on site, prior to ION, meant that I got what I wanted to see, right on my doorstep: a Locata preview at the Oregon Convention Center in June.
The Technology
Essentially, Locata has developed a system that is very much GPS-like in that one has a network of reference stations (LocataLites) that interface to an unlimited number of rovers. One major difference is that there is no space segment. It doesn’t need or use satellites. Essentially, each reference station behaves like a satellite on the ground, with the rover moving around inside the polygon formed by the reference stations. The rover position is accurate to the centimeter level.
The value of the Locata receivers is that they don’t need a clear view of the sky to operate like a GPS receiver does. Yes, that means centimeter-level positioning indoors, where RTK GPS doesn’t work due to satellite visibility constraints, as well as outdoors.
Sound cool? It is. I saw it working indoors at the Oregon Convention Center. Locata staff set up a large room with Locata reference stations around the perimeter. They had two different rovers: one mounted on a small push cart and the other on a golf cart. We were able to move the rovers around the room freely and view the updated coordinates at 1 Hz intervals (although it’s capable of much faster update rates).
The Challenges
The new TimeTenna (see facing page) is large. Today that form factor is required to handle the high-multipath indoor environment. Locata is working on a scaled-down version, although it’s not unreasonable to envision the current model being mounted on a forklift or other vehicle if it was mechanically hardened. The antenna for Locata’s outdoor systems (for mining and other less hostile environments) is much lower profile and similar to a standard GPS antenna.
The Locata system requires that you manage a network of Locata reference stations. Similar to an RTK network, the Locata system is based on a network of reference stations around the project area. The baseline distances can be quite long (tens of miles), but nevertheless, one must install and manage the network much as one would a GPS RTK network, albeit with much less IT department involvement than a GPS RTK network.
Lastly, Nunzio Gambale wholeheartedly agrees that Locata’s technology is still developing. He likens it
to where GPS was in 1990. I tend to agree. The antenna technology needs to reduce in size and the system architecture needs to be vetted for reliability in production environments. But keep in mind that Leica and the U.S. Air Force’s 746 Test Squadron have already bought into Locata’s technology in a big way.
Although I don’t pretend to have the technical understanding that some of the others in the room possessed during the June demo, I did hear one of the sharper engineers exclaim “genius” at one point, referring to the design.
It’s certainly worth a close look as Locata’s technology continues to develop and be deployed. I think the day isn’t far away when we will see a system from Locata that will allow a user to transition seamlessly from centimeter-level positioning outdoors using RTK GPS to centimeter-level positioning indoors without breaking a step.
Now I’m a Believer
Tony Murfin, Professional OEM editor
I was invited to Portland in late June to preview an operational system which promises to help GPS in tough signal situations and work well indoors. While Europe, China, India, Japan, and of course Russia are all working to get more operational satellites in space, Locata in Australia has quietly been perfecting its terrestrial navigation system. I say perfecting because skeptics and naysayers have criticized Locata and what was seen as a pseudolite system with a rather lengthy development cycle. But nothing speaks as loudly as an operational system adopted and fielded by Leica Geosystems or a contract with the U.S. Air Force to get people’s attention back in the right place, even though Locata would claim it is only just getting started.
As I walked into the Portland Convention Center I was certainly apprehensive as to how any GPS-like system could function well within the massive concrete and steel building. When I found the smiling Locata group tucked away in one of the side ballrooms, it didn’t take long before I became a believer. Those wall dividers that allow the Convention Center to reconfigure rooms are apparently referred to as Acousti-Seal 931 Steel Operable Wall panels — yep, perfect multipath reflectors. So to see totally repeatable few centimeter positions in this cavern was not what I was anticipating.
The ballroom’s carpeted floor had been carefully laser-surveyed with a matrix of 5-meter squares, with a high-precision dot marking each grid intersection. LocataLite stations were set up at each corner and one in the middle at the far end, each with three antennas. A master station at the left corner of the entry wall originated the TimeLoc signal, and on each station one antenna pointed to an adjacent station, over which TimeLoc synchronization was cascaded around the network. This is a key feature of the ground network, allowing it to become fully synchronized and also to be extended or reconfigured at will.
Of course, when you run your own ground network it helps to be able to run at power levels significantly higher than GPS, so it’s easy for each station to communicate with another, provided they roughly have line-of-sight of each other — kind of like having to actually see a GPS satellite to get it into your GPS position solution. If you have some buildings or bushes or trees to contend with, having higher power available makes things easier, especially if you want an RTK carrier solution.
The secret to working indoors appears to be the TimeTenna phased-array antenna that Locata demonstrated in the steel-clad ballroom. With this top-hat-like antenna mounted on a wheeled cart along with a receiver and laptop, and positioned over one of those surveyed locations on the carpet, we could easily see that positions within less than 5 centimeters were consistent and solid. As a truth system, the company also had a motorized laser scanner pumping out centimeter-level positions on a parallel measurement system, and it was clear that there was excellent centimeter-level correlation.
But don’t take my word for it. Come to Portland for the ION-GNSS conference, September 20–23, and see the Locata demo for yourself — you’ll be impressed too!
Then there is the sole-source U.S. Air Force contract that has Locata updating an existing network to provide independent reference positions over 2,500 square miles of the White Sands Missile Range in New Mexico. The Air Force apparently needs to know how its navigation systems work when it turns on localized GPS jamming. The Locata system is designed to give the Air Force better than the specified <18-centimeter position accuracy in GPS-denied environments.
In August, Locata cleared the final USAF critical design review milestone for the wide-area White Sands Missile Range deployment. This is clearly a good sign that Air Force wants to continue with the next-generation Locata system. With GPS denied on this range, test vehicles will likely be constrained to inertial-only navigation, but with a LocataLite receiver onboard pumping out high-accuracy position measurements, the Air Force will no doubt have plenty of location data to track dynamic performance under GPS jamming conditions.
Another application that Locata has been investigating involves airborne trials in Australia, where initial results indicate position accuracy of less than 3 meters at up to 50 kilometers. The trials have involved a ground network with six base stations spread over a roughly square area of 1,500 square kilometers.
A University of New South Wales test aircraft equipped with precision GPS, inertial reference system, and laser scanner for truth reference use flew to within 3 to 49 kilometers of the reference stations at around 7,000 feet, producing the reported <3-meter code solution. Trials data is still being analyzed to produce a higher accuracy carrier solution, and Locata expects to issue these results at ION.
Airborne Reference Equipment
Leica has apparently been working with Locata for some time. The proof-of-concept installation at a 300-foot deep diamond mine in South Africa and a production set-up at a gold mine in Western Australia are going strong.
The gold-mine installation has now been extended to two pit sites using 15 LocataLite transmitters in total. LocataLite receivers are mounted on vehicles, atop drills and shovels, and all run off the multi-pit Locata network. The mobile units not only carry LocataLite receivers, but also precision Leica GNSS receivers running off side-by-side antennas. As time progresses, the ultimate solution will use integrated multi-constellation/LocataLite receivers: the Locata signals integrated into a combined satellite+terrestrial receiver position solution, using a single integrated antenna.
It’s easy to envisage such an integrated receiver and antenna where the Locata ground-network signals are used as just another local constellation. The investment to get to such a receiver would of course have to be justified by a whole proliferation of Locata networks. This would seem to be on the way, given the significant progress that Locata has now unveiled.
Will It Fly — Literally?
William Shears, aviation engineer
If you are an aviation satellite navigation enthusiast, you probably noticed this hasn’t been an auspicious year for aviation GNSS or for GNSS applied to any other user segment that needs highly reliable GNSS service. Between personal privacy jammers, instances of accidental interference, and the big chill sent through the community by the LightSquared debacle, many are asking if GNSS is now or ever will be reliable enough to be a sole means of position and time for safety-of-life applications.
A few years ago, the very idea that ordinary people would want to own GPS jamming devices and that they would be easily obtainable on the Internet would have been considered absurd. Similarly, the idea that the U.S. government would not vigorously protect GPS from interference was just not credible. But here we are in mid-2011 and the vulnerability of GNSS to interference has come home to roost, in several very big ways. This new awareness of the weaknesses of GNSS has led the U.S. Federal Aviation Administration (FAA) and civil aviation authorities of other countries to start rethinking their long-term strategies with respect to satellite navigation.
Even well before LightSquared crept into the consciousness of the GPS community and then burst forth as the apocalyptic specter that threatens to virtually end the utility of GPS in North America, the FAA had begun a study to consider the need for an alternate positioning, navigation, and timing (APNT) system to support critical aviation needs. The idea being that as the U.S. air traffic management system transitions to become increasingly dependent on management of traffic via four-dimensional trajectories, reversion to a non-trajectory based mode (for example, controllers vectoring aircraft as they do today) would become unfeasible. Hence, airplanes will need a very reliable source of 4D positioning and outages for any extended period of time due to interference, or anything else will be unacceptable. The FAA set about studying what level of performance would be required for a system intended to back up GNSS in the future. Other countries began to follow suit, and whereas the concept of an APNT was obscure a year and a half ago, it has become a significant point of discussion at the International Civil Aviation Administration (ICAO) as well as within various countries, including the United States, Australia, and several in Europe.
At first blush, the Locata system would seem to be a ready-made solution poised to fulfill aviation’s need for a GNSS backup system. In fact, acting as an independent backup (and/or an augmentation to) GNSS is one of the main motivations in Locata’s development. The technology seems to have promise in meeting the aviation community’s needs for an APNT. Locata is relatively mature technology that has demonstrated accuracies well in excess of what is required of an APNT meant to back up GNSS for enroute, terminal, and non-precision approach operations. Perhaps even precision approach and landing could be supported. Also, the system is very flexible, which suggests that service coverage could be tailored as needed around important airports. The system has significant redundancies built in, including multiple frequencies, multiple antennas for path diversity, and the ability for the network to reconfigure which LocataLite uses which other LocataLite for time synchronization.
Given this flexibility and redundancy, it should be possible to configure a system that provides highly reliable service where it is needed. Another major advantage of the Locata technology for aviation is the higher signal power level that comes from using terrestrial signals rather than signals from space. In theory, a Locata system would be more robust to interference than space-based GNSS signals.
Some people are indeed thinking about Locata for aviation use. Locata has conducted flight trials in Australia using a prototype demonstration network of six LocataLites covering an area of more than 1,500 square kilometers around Cooma airport in Australia. Locata has reported code positioning solutions of better than 3 meters at ranges up to 50 kilometers, and will present higher accuracy carrier-phase solutions at ION. The U.S. Air Force is also preparing to use Locata in an aviation environment as an independent truth reference.
At the ICAO Navigation Systems Panel (NSP) meeting in May 2011, the Australian panel member presented a paper outlining the general need for an APNT. The paper included a description of Locata as an example of what an APNT solution might look like. However, it is interesting that the paper fell short of proposing that the panel pursue Locata as the solution or to suggest that any standardization of a solution for APNT begin immediately. In spite of all the potential advantages discussed above, the Locata system faces a major obstacle before it can practically be used in aviation applications: standardization.
The first aspect of standardization that is likely to be a huge impediment for Locata (or any other APNT proposal, for that matter) is spectrum. The Locata systems implemented to date have been designed to operate in the 2.4 GHz unlicensed industrial applications band. For Locata to support safety-of-life applications, national aviation authorities will require that an APNT system use spectrum that is properly allocated for use in a safety-critical aeronautical navigation system, that is, spectrum allocated for Aeronautical Route Navigation Services (ARNS). Spectrum allocated as ARNS is afforded special protection from interference. Coordination of services in or near ARNS spectrum is often difficult, time-consuming, and expensive. For example, coordination between civil aviation use of the 108–118 MHz band (used for instrument landing systems, or ILS, and VHF omnidirectional range, or VOR) and FM broadcasting in the 88.1–107.9 MHz band produces real costs and restrictions to be borne by the FM broadcasters. Consequently, any proposal to convert non-ARNS allocated spectrum to ARNS is likely to be met with significant opposition.
Spectrum is a finite resource, and virtually all spectrum is already in use by someone. So, the reality is that a future APNT will likely have to be implemented in some existing ARNS spectrum, since a new global allocation of spectrum for ARNS is an unlikely proposition.
The current allocations for ARNS include:
108–118 MHz (ILS/VOR),
960–1215 MHz (DME/Mode-S/ADS-B/SSR/JTIDS/MIDS),
1556–1626 MHz, and
5.1–5.25 GHz.
All indications are that a Locata system could be could be operated at these frequencies. However, services that already exist in those bands will continue for the foreseeable future. So, to be viable, a Locata system would have to coexist in one of these bands with other existing systems, that is, not interfere with the operation of those other systems. Such coexistence has yet to be demonstrated either by analysis or test.
After suitable spectrum has been identified, the next major hurdle for Locata is standardization of the signal-in-space to the degree that supports interoperability of equipment produced by different manufacturers in different countries. The Locata ICD released at ION-GNSS 2011 is a good step in the right direction. But for an aviation application, a great deal more would need to be specified, including details about the waveform (spectral mask, out-of-band emissions, and so on), the protocols for producing the signals, and the standard protocols for the application of data to derive a position solution. A clear allocation of responsibility between the ground processing and airborne processing will need to be defined so that system integrity can be analyzed and assured.
At the international level, such standardization activities can take a decade or more. The length of time required depends on the maturity of the system that is proposed for standardization. The existence of a similar standard, with perhaps a significant user base and operational experience also helps (for example, an IEEE standard or RTCM standards). So, again, the ICD is a good start.
Beyond the technical aspects of standardization, there are political and institutional aspects that can often be more formidable barriers. Issues with spectrum have already been mentioned. Beyond that, there are issues with intellectual property. Creating aviation standards based on proprietary technology is unpopular although not unprecedented. Proposals for standardization are more likely to be successful the fewer strings, such as licensing agreements or fees, that are attached. This is a challenge since companies that have worked hard to develop cool new technology are often reticent to give away their intellectual property in the name of standardization.
Given all the barriers, how does new technology ever get implemented in civil aviation? Typically, applications begin in one of two ways:
in support of war.
in support non-safety related industrial applications.
The military has historically pioneered many technologies (radar, DME/TACAN, GPS) that would probably not have been developed otherwise. Even after the initial military experience, there is typically a period of time when the new technology is used in a non-safety-critical capacity to support some commercial objective. In the case of Locata, some potential applications would be flight-test position-reference systems, high-precision photogrammetry, high-precision positioning for crop dusting, and any other applications that require a highly robust, high-accuracy position solution in a well-defined region where interoperability and certification are not issues. Those are relatively small niche applications, which may provide some valuable operational experience.
However, serious movement towards adopting Locata as a standard for APNT is unlikely to happen without the support of at least a couple of large countries. Even a large user base with equipage does not guarantee that countries will adopt the technology or that air navigation service providers will authorize the use of the technology for safety-critical applications. For example, many carriers are equipping with broadband Internet equipment to provide service to the passenger cabin. Yet, there is no serious discussion of using that datalink capability for safety-related communications. Similarly, a very large number of aircraft are equipped with Aircraft Condition and Reporting System (ACARS) datalink, yet use of that system is largely limited to non-essential Airline Operational Communication (AOC) applications.
So will Locata fly? I believe that is entirely up to Locata and other companies that work with Locata to address the initial military and niche airborne positing markets. Operational experience gained by such early adopters will be critical in laying the groundwork for the support that will be needed from large states like the United States, Australia, China, and those in Europe, if Locata is to be a player in the longer-term international standardization of APNT.
In the near term, Locata is already serving the aviation community by demonstrating the art of the possible relative to what a ground-based navigation system based on modern technology could be.
New technology and wireless carrier openness now make real-time access to telephone location information available to the enterprise with no application required on the mobile device.
Yes, that’s right: no application required! Cloud-based location, offered via direct connections to wireless operators, changes the playing field for enterprises to introduce instant operational efficiencies. Marrying location insight, privacy controls, and multi-modal communications through network application programming interfaces (APIs) provides enterprises with flexibility, cost savings, and time-to-market advantages. Whether delivering geo-targeted promotions, dispatching services, verifying worker activities, or performing other location-relevant actions, businesses now have cross-carrier access to location information for more than 85 percent of U.S. wireless subscribers — instantly!
This enables businesses to go app-less with no costly, time-consuming deployment and maintenance of handset applications. Additionally, no specialized hardware is required. Location through carrier networks also assures secure and tamper-proof delivery of the location information since no potentially hackable client software is involved in the generation or delivery of that information. It comes straight from the carrier network over secure connections.
Cloud-based deployment, such as that available through TechnoCom’s Location Platform, opens up location intelligence to all device types, including both smartphones and feature phones. This removes a huge barrier that exists with the existing smartphone-only applications and enables businesses to immediately tailor their workflows and business processes to utilize the knowledge of real-time location from a secure and dependable source. Development cycles and costs are a fraction of those required for smartphone applications, such as Droid or iPhone apps, and the adoption hurdle of user download initiation is eliminated.
Simplification of access and deployment paves the way for adoption and finally opens the floodgate for location-based services to be implemented on a large scale across all wireless networks. This is analogous to the inflection point that cross-carrier text messaging access and interoperability had on cellular text messaging adoption rates in the ’90s.
By leveraging technology similar to that in the carrier networks and proven for use in 911 emergencies, businesses instantaneously benefit when new data is exposed by wireless operators, such as device capabilities, presence, rate plan status, roaming status, and so on. Enterprises may immediately harness this insight with upgrades to their server applications and no new technology deployments required in the field. That offers businesses a huge return on investment as they integrate once and consume enhancements dynamically. Location from the cloud opens up a new, instant intelligence frontier that was not possible for businesses to leverage just last year.
This unprecedented access to location information comes with a responsibility to comply with industry-accepted privacy controls. To make this easy on enterprises that are not expert in such policies, TechnoCom Location Platform provides carrier-approved privacy management functionality, and we work hand-in-hand with our customers to ensure their implementations are in line with best practices established by CTIA.
Tapping into location from other mediums such as VoIP, Wi-Max, NFC, Wi-Fi will increase the ubiquity of cloud-based location access even further. As devices get smarter and more powerful, better communications, device intelligence, and positional awareness will catapult businesses to yet another level of efficiencies in interacting with their mobile users, workers, and assets.
Mario Proietti is co-founder and chief executive officer of TechnoCom Corporation, and a member of the Editorial Advisory Board of GPS World magazine. He has a master’s degree in electrical engineering from the University of Southern California. TechnoCom delivers cross-carrier location services to enterprises through its location platform’s web services APIs. The company also integrates location technologies into wireless networks, products, and software, and works with wireless carriers to enable E911 and location-based services.
My mailbox is currently overflowing with comments and questions concerning rampant rumors that in the March 2011 time frame a U.S. military reconnaissance aircraft was forced to land during an annual major east Asian military exercise, known as Key Resolve, due to GPS jamming. The jamming reportedly took place along the northern portion of the 684-mile long Korean peninsula, with the jamming supposedly originating with the North Koreans. The jamming scenario should come as no surprise, but it is the emergency or forced landing due to loss of a GPS signal among other supposed “facts” with which I take issue.
