From Dec. 4-5, 2024, the National Space-Based Positioning, Navigation and Timing (PNT) Advisory Board met to discuss GPS-related topics. The PNT Advisory Board provides independent advice to the U.S. government on GPS-related policy, planning, program management and funding profiles in relation to the current state of national and international satellite navigation services. A March 28, 2024, GPS World article by Dana Goward highlighted that the PNT Advisory Board has been providing the government with independent expert advice about GPS and PNT for 20 years. He highlighted that the Board is chaired by retired Admiral Thad Allen and has six subcommittees.
This newsletter will highlight a topic that the emerging capabilities, applications and sectors subcommittee discussed at the final PNT Advisory Board meeting of 2024. The presentation title is “GPS High Accuracy and Robustness Service (HARS).” A white paper on the topic and the Dec. 4, 2024, presentation by Shachak Pe’eri, Ph.D., NOAA/NOS/National Geodetic Survey (NGS), can be found on the PNT Advisory Board website.
According to the document, the board prepared the white paper to support recommendation number PNT27-04-ECAS, which is to develop and implement a GPS HARS delivered to users via the Internet. The HARS concept was approved at the PNTAB-27 meeting (Nov. 16-17, 2022) and formally submitted to the National Space-Based PNT EXCOM co-chairs via Memorandum on Jan. 27, 2023.
Recommendation PNT27-04. (Photo: Presentation by John W. Betz, PhD Member, National Space‐Based PNT Advisory Board on May 29, 2024)
The November and December Advisory Board meetings are recorded, and individuals can listen to the entire meeting. The Board’s website provides links to the meeting agenda and presentations. Pe’eri’s presentation on HARS started at 10:30 am on Dec. 4 (2:04 on the recording).
During the meeting, the PNT Advisory Board officially stated that it supports the HARS Concept described by NOAA. Of course, the Board also stated that it has no money, but the Board’s stamp of approval of the concept is very important. Now, it is up to NOAA’s NGS to work with other federal agencies, such as NASA’s Jet Propulsion Laboratory (JPL), to work out the details and resources. By leveraging NASA’s real-time Global Differential GPS (GDGPS) System infrastructure and NOAA’s service delivery platforms, a high-accuracy, resilient service that ensures delivery of precise, reliable and secure GNSS corrections for a wide range of scientific and commercial applications can be built for the nation.
So, what exactly is the GPS High Accuracy and Robustness Service (HARS)? The following is a statement from a Jan. 27, 2023, PNT Memo:
“Implementing a GPS High Accuracy and Robustness Service: To augment GPS and overcome some inherent limitations of space-based PNT, the USG should provide a service comparable to the European Union’s Galileo HAS that provides signal corrections than enable better than one-meter level accuracy, as well as cryptographically-protected satellite navigation message data bits for integrity processing. The U.S. should develop and implement GPS HARS, based on the capabilities developed by the JPL for GDGPS, to be made available to users over the Internet.”
The white paper describes the problem and the solution as the following:
“The problem: GPS is falling behind other Global Navigation Satellite Systems (GNSSs) such as Europe’s Galileo and China’s Beidou. GPS has adopted an approach of allowing augmentation by third-party systems (such as Assisted-GNSS in mobile phones, WAAS for aviation accuracy and integrity, and commercial RTK for precision users), rather than providing specialized advanced services itself. Also, the data message modulated on the GPS signals is fragile. Environmental effects or malicious actions can prevent a receiver from reading the information or manipulate what is read, limiting the robustness of the GPS signals. Currently, GPS is the primary system in almost all GNSS chips, even chips made in Europe or Asia. That is: chips are designed to acquire GPS signals first, then signals from other systems. But Galileo and BeiDou are deploying high accuracy services that provide sub-meter position accuracy, enhancing satnav use in many civil applications. The absence of any plan for GPS to offer a similarly high accuracy service could cause GNSS chips to begin using Galileo or BeiDou, rather than GPS, as the primary system. A switch away from GPS as the primary PNT system is a problem for the US Government because it will lose its strategic advantage. Existing commercial chips are used in many strategically important US assets, such as airlines, ships, and organizations that support the US military. Once these chips change their architecture to Galileo-first or BeiDou-first, these strategic use cases depend on these services. It is one step in the direction of not having a GNSS system at all and borrowing the system of another power, exactly the situation that Europe and China were in before they built their own systems. GPS would no longer be the “pre-eminent space-based PNT service” called for in Space Policy Directive 7.”
“The Solution:A high accuracy and robustness service (HARS) provides information to user receivers, reducing errors and enhancing the ability to operate in challenging conditions. The PNT Advisory Board has identified a solution that the U.S. government can provide a HARS without adding cost and complexity to GPS itself; instead, the needed information from government or government-sponsored organizations can be obtained and provided over the Internet to properly equipped receivers. The result would be a world-class HARS at a small fraction of the cost or time, compared to implementing it on new GPS satellites. The HARS would provide cryptographically-protected robust (resistant to jamming and spoofing) GPS for critical infrastructure and would enable new applications (such as lane-dependent route guidance in automobile navigation and emergency vehicle guidance, GPS-only precision positioning of drones) that extend the societal benefits of GPS. HARS would be secure and less sensitive to radio noise and disruptions, including spoofing.”
The following are a few slides from Pe’eri’s presentation highlighting the need for HARS. He mentioned that there are six regional high accuracy systems and one global service that is already operational or in development.
Six regional HAS and one global HAS are operational or in development at this time. (Photo: NOAA/NGS)
NOAA’s presentation by Pe’eri was in response to a request by the Advisory Board. The Board was interested in learning more about the funding and operating a public service that can provide robust real-time GPS corrections. Summarized in three bullets:
High-Accuracy: Real-time corrections to GPS orbit parameters and clocks to enable more accurate positioning solutions.
Robustness: Nav Data (ephemeris) can be cryptographically signed and delivered on the same channel.
Service: Delivered over the Internet and is free to all users.
The HARS could be accomplished by employing the expertise, knowledge, and capabilities of NASA’s JPL and NOAA’s NGS.
NOAA has the authority to provide real-time operational services and regularly collaborates with other federal and state agencies and local communities. NGS manages and distributes the NOAA CORS Network (Foundation and Cooperative CORS). NASA JPL collects GNSS data and generates products with high accuracy.
NGS expertise and knowledge. (Photo: NGS/NOAA)
NASA’s GDGPS is a complete, highly accurate and extremely robust real-time GNSS monitoring and augmentation system. The CCDIS website states, “Employing a large ground network of real-time reference receivers, innovative network architecture, and real-time data processing software, the GDGPS System provides sub-decimeter (<10 cm) positioning accuracy and sub-nanosecond time transfer accuracy anywhere in the world, on the ground, in the air, and in space, independent of local infrastructure.”
JPL expertise and knowledge. (Photo: NASA)
By leveraging NASA’s real-time GDGPS System infrastructure and NOAA’s service delivery platforms, NGS and JPL can build a high-accuracy, resilient service that ensures delivery of precise, reliable and secure GNSS corrections for a wide range of scientific and commercial applications.
Pe’eri’s presentation highlighted HARS benefits to the daily operations of users of geospatial data.
HARS benefits for users. (Photo: NOAA/NGS)
The HARS concept is extremely important to the U.S. GPS user community, where the number of users is increasing every day. A 2019 Department of Commerce (NIST) study, “Economic Benefits of the Global Positioning System (GPS),” highlighted the economic damages a GPS outage would have on the agricultural industry.
The 2019 NIST study, “Economic Benefits of the Global Positioning System (GPS),” determined that $1.4 trillion in U.S. economic benefits from GPS. The study stated that a 30-day widespread outage could erode less than $1 billion in economic value per day. The study also highlighted the impact a GPS outage would have on Agriculture, stating that during planting season, economic damages in the agriculture sector could increase 30-day losses to $15 billion due to lower yields.
Table ES-1 and figure ES-1 from the 2019 report highlight the economic benefits of GPS for private sector use.
Table ES-1: Summary of economic benefits of GPS for private-sector use, 1984 to 2017. (Photo: NIST) Figure ES-1: Time series of GPS’s economic benefits for the private sector. (Photo: NIST)
I would encourage others to look at the PNT website, especially the Advisory Board website, to obtain information about space-based PNT. Other recommendations and letters from the Advisory Board to the Executive Committee (EXCOM) can be found on the PNT and Advisory Board websites. The webpage provides the Advisory Board’s recommendations on ways to improve GPS and national GPS management. The recommendations are published in the interest of public transparency.
The President’s National Space-based Positioning, Navigation and Timing (PNT) Advisory Board has warned United States leaders that the nation is highly vulnerable to disruption of GPS services. Also, national PNT issues have not received sufficient priority and attention for the last 20 years, and no one is accountable for system performance.
The overwhelming majority of Allen’s message, though, dealt with GPS and U.S. PNT being vulnerable, the importance of PNT to the nation’s safety and security and the failure of the government to do the things it said it should and would do. It says:
“America’s continued over-reliance on GPS for PNT makes critical infrastructure and applications vulnerable to a variety of well-documented accidental, natural and malicious threats.
…our conclusion is that PNT, in general, and GPS, in particular, have not been accorded their rightful prominence in the national policy agenda.
Simply put, the Board believes that the 20-year-old framework for GPS governance and the current policy statements establish neither the priority that the system deserves nor sufficiently clear accountability for its performance.”
The reason for this was assessed to be that the leadership and governance structure established by 2004’s NSPD-39 and confirmed in 2021’s SPD-7 was not working.
Allen gave a recent policy document on critical infrastructure as an example. All critical infrastructure sectors use PNT, and most depend on it.
“These findings were reinforced just earlier this year by the release of the National Security Memorandum on Critical Infrastructure Security and Resilience (NSM-22, April 30, 2024). We were surprised to discover that GPS is nowhere mentioned in that important document.”
While not mentioned in the memo, PNT was also not mentioned in national cybersecurity documents issued last year. This is despite timing being essential to the operation of IT systems, and time and location data being key elements in many applications.
Allen’s memo also emphasized that the real issue is not about GPS, but overall national PNT capability. He went so far as to say, “Space Policy Directive 7 (SPD-7), U.S. Space-based PNT Policy, must be revisited and the term ‘space-based’ should be removed from its title” (emphasis added).
Reinforcing this message, the following table comparing U.S. and Chinese national PNT systems was included in the memo.
“The Board believes it is time to take a fresh look at our approach to PNT governance and establish a clear strategy for re-establishing an unquestioned position of leadership for the United States. To be successful, such a strategy requires a governance structure characterized by clearer authority and accountability. Ideally, the administration should propose legislation to Congress that would support this goal with a clear mandate (authorization) and resources (appropriations) adequate to the task.”
Direct the U.S. Space Force to establish a way for “good enough” monitoring of GPS L5 signals using existing capabilities and set L5 signals healthy subject to a “use at your own risk” caveat, just as is done with L2C today. We recommend establishing a deadline of September 2024.
Direct federal Chief Information Officers to acquire and install multifrequency dual-system (GPS-Galileo) receivers to complement GPS use. The Department of Transportation and the Department of Homeland Security should actively encourage critical infrastructure users to adopt multifrequency dual-system (GPS-Galileo) receivers.
Assign a lead agency and single individual with clear responsibility and authority for the end-to-end prompt detection, characterization, and removal of significant sources of interference to GNSS in the United States.
Allen’s memo comes on the heels of a similar warning to United States leaders about the nation’s vulnerability to GPS disruptions. On July 18, the
Quickly prototype a GNSS interference detection and reporting system.
Implement an internet-based High Accuracy and Robustness Service (HARS)for GPS.
Relax export controls that currently restrict use of adaptive anti-jam antennas.
These are just three of the efforts the U.S. government is pursuing as a result of recommendations from the President’s National Space-based Positioning, Navigation and Timing (PNT) Advisory Board.
For 20 years the PNT Advisory Board has been providing the government independent expert advice about GPS and PNT.
Established by presidential directive in 2004 and administered under the Federal Advisory Committee Act by NASA, its charter has been regularly renewed. The charter provides that the board shall:
Be composed of experts from outside the United States government.
Seek input from state and local governments, industry and academia on developments in the application of space-based PNT technologies.
Evaluate national and international needs for changes in space-based PNT capabilities and assess possible trade-offs among options.
Provide independent advice and recommendations to the National PNT Executive Committee (co-chaired by the Deputy Secretaries of Defense and Transportation) on policy, system requirements, and program needs.
While “space-based” is in its name and charter, the board has long recognized that terrestrial assets also can play an important role in serving PNT users by augmenting, reinforcing, and complementing GPS. The use of complementary systems, for example, could help demotivate intentional jammers and spoofers and help safeguard users during any interference event. Thus, the board often considers a wide range of capabilities and systems.
The board also discusses policy, education, international relations and other issues important to the PNT community. As one board member commented, “Technology doesn’t exist in a vacuum. It is developed by, and intended to serve, people. If you don’t recognize that, you are missing most of the picture.”
The current board’s membership includes an impressive array of experts in PNT policy and technology. Its 29 members include a former governor, a retired admiral, three retired generals, GPS’ original chief architect, a former undersecretary, a former assistant secretary, three former presidents of the Institute of Navigation (ION), three international members and experts from across academia and industry.
Chaired by former Coast Guard Commandant Admiral Thad Allen, the board’s primary efforts are driven by its six subcommittees, reflecting a holistic approach to effective PNT:
Strategy, Policy & Governance
Protect, Toughen & Augment
Emerging Capabilities, Applications & Sectors
Education & Science Innovation
International Engagement
Communications & External Relations
While the subcommittees meet in fact-finding sessions to gather data, the PNT Advisory Board’s deliberations are public. Semi-annual meetings inWashington, D.C. and other locations may be attended by anyone, either in person or virtually. Announcements on the board’s webpage and in the Federal Register provide details before each meeting. By law, the minutes of each meeting are available to the public, and video recordings of meetings are normally posted as well.
Input from the public about PNT issues of concern is also welcome to inform the board’s current and future deliberations. Information on how to send input will be posted with the meeting announcement here.
According to board member Jeff Shane, former undersecretary at the U.S. Department of Transportation (DOT), the PNT Advisory Board is evidence of government at its best. “The very fact that the board was established underscores our government’s willingness to hear and consider the widest variety of views and input. It should be a source of optimism, and even pride, for the entire PNT community.”
National Space-based PNT Advisory Board
The next meeting will be from 9:00 a.m. to 6:00 p.m. MDT, April 24, 2024, and from 9:00 a.m. to noon April 25 at The Antlers Hotel in Colorado Springs, Colorado.Click here for information on a reception on April 23, featuring Gen. David Thompson.
ByBradford W. Parkinson Aeronautics and Astronautics Professor Emeritus (recalled) Stanford University
Brad Parkinson
We, of the PNT universe, have been hearing a rather continual message of doom from the media regarding the fragility of the GPS (or GNSS) signals. In a way, they are right. The received GPS signal is 1/10th of 1 millionth of 1 billionth of a Watt. It can be susceptible to jamming and spoofing.
In response, the U.S. government has sponsored major studies and some competitive tests of techniques to augment or possibly replace GPS. I applaud such queries, but also would strongly advocate more balance in efforts to increase robustness of positioning, navigation and timing (PNT).
Specifically, I argue for increased emphasis on well-known techniques that can greatly toughen GNSS receivers to both jamming and spoofing. Some of these techniques are deliberately denied to civil users by government policy.
Background
The PNT Advisory Board (PNTAB) is a panel of national experts who report to the PNT ExCom. The ExCom is comprised of the deputy heads of the nine U.S. government departments with the largest stakes in PNT. The PNTAB has a starkly simple and well-stated goal:
To meet its overarching goal, the PNTAB has developed a three-legged strategic framework, known as “PTA”: “We must protect, toughen and augment GPS to ensure that it continues to provide economic and societal benefits to the nation.”
Most current U.S. government efforts have been focused on the third of the PNTAB strategic legs: augmenting the GPS system. These system augmentations include: modernized Loran (eLoran), fiber-optic distribution of time, and ranging to low-Earth-orbit (LEO) satellites (particularly the swarms of communications satellites). In general, these system augmentations offer no hope of being equivalent to GPS in terms of availability and accuracy.
However, augmentations have the advantage of either being less vulnerable to interference, or highly proliferated in case of satellite outages. As supplements, or in an emergency, they can perform a very valuable role, but with nowhere near the equivalent performance of normally operating GNSS, which can routinely provide worldwide, 24/7 precisions better than decimeters in the dynamic real-time kinematic (RTK) mode. In the United States, GPS also offers continuous, real-time integrity assessments courtesy of the FAA1. Europe has a similar, compatible integrity system called EGNOS, and there are other regional system augmentations.
In summary, the current PNTAB assessment regarding these substitutes is:
“No current or foreseeable alternative to GNSS (primarily GPS) can deliver the equivalent accuracy (static down to millimeters) and worldwide, 24/7 availability.”
Toughening User Equipment
Toughening, the second leg of our assurance strategy, includes all aspects of GPS enterprise vulnerability — satellites, ground control and user equipment. For this article, I am focused on toughening the user equipment. I would argue that we have largely under-emphasized, or been prohibited by national policy from using, well-known and widely available user equipment toughening technology.
The main vulnerabilities of GPS receivers are jamming and spoofing of the received signals. Familiar anti-jam (A/J) methods can substantially overcome the inherent weakness of GPS signals to defeat deliberate jamming and spoofing. As I outline here, such measures can reduce a jammer’s effective radius by a factor of more than 100 and reduce the effective jammer area by a factor of 10,000 compared to the unprotected receiver.2
Thus, these methods are also deterrents, because they can render ineffective such hostile (or possibly inadvertent) acts. Further, the technology that provides this significant toughening is available now or will be within a few years, rather than the many years required by some alternative, system-level augmentations.
Toughening techniques (A/J improvements) are traditionally calibrated as the improvement in the amount of jamming that can be tolerated, measured by the jamming-to-signal power ratio (J/S) expressed as decibels (dB). However, for this discussion, I will also use a different, more intuitive, measure. This metric is the Denial Radius Reduction Ratio (DRRR):
DRRR = (radius of jammer denial after J/S measure applied)/(jammer radius without improvements)
For example, a 15-dB improvement in J/S would lead to a DRRR of 0.178.3 In other words, the 15-dB improvement has reduced the denial radius to about 18% of the line-of-sight radius that would be denied to an untoughened receiver. Note that the simultaneous use of techniques is generally multiplicative. For example, simultaneously applying technique #1 with a DRRR1 of 0.5 and technique #2 with a DRRR2 of 0.3, would result in a DRRR1&2 of 0.3 *0.5, or an overall DRRR of 0.15. This is the advantage of using this metric to describe the A/J improvements. 4
Baseline Case
For our basis for comparison, we will consider the L1 C/A signal in full accuracy (State 5) tracking mode and a 1-kW noise jammer.5,6 For this situation, the line-of-sight jammer could deny GPS to a radius of about 560 kilometers. A discussion of the lower accuracy State 3 tracking is included below.
It is useful to consider toughening techniques in four major categories.
Toughening Category 1: Signal Processing. With L1 C/A, GPS receivers can improve jamming resistance, albeit with loss of ranging (tracking) accuracy, by using code tracking mode – State 3. This reduces a line-of-sight jammer’s denial radius (DRRR) to about 0.29 (a 10.7 dB improvement).
Toughening Category 2: Inertial Components and Very Stable User Clocks. This includes miniature micro-electromechanical (MEMS) components up to high-grade inertial measurement units (IMUs) and quartz to chip-scale atomic clocks (CSACs). These techniques enable narrower tracking filters and longer averaging, as well as allowing navigation through regions when GPS is denied. The range of DRRRs is 0.40 down to 0.10. We will use a nominal value of 0.18 (a 15-dB improvement).
Toughening Category 3: CRPAs. Controlled reception pattern antennas (CRPAS) are digital, multi-element, phase-steered antennas. They represent well-understood and available technology; they have been used in large surface-search radar systems for many decades.7 They can be used in null-steering or beam-steering modes.8 The number of antenna elements could range up to dozens. Potentially, they could produce DRRRs down to .01 — that is, a 99% reduction of jammer radius to 1% of the unprotected GPS receiver value.
Unfortunately, the U.S. government does not allow more than three-element CRPAs to be manufactured or sold for civil use. 9 This is due to some very old International Traffic in Arms Regulations (ITAR). For our nominal example, we will assume the restriction has been relaxed and use a CRPA of about 20 elements, which should produce a DRRR of 0.06 (a 25-dB improvement).
Toughening Category 4: Signal Alternatives. This category includes alternative modulations at 1575 MHz (L1C, Galileo or other GNSS) and alternative frequencies (L5, L2, Galileo). Note that the modern signals generally offer significantly improved signal-processing toughening as well as increased power.
Using L1C in State 3 compared to L1 C/A in State 5 would yield a DRRR of 0.10. (a 20.3-dB improvement). The L1C international signal should be operational on GPS by mid-decade. The L5 signal, at 1176 MHz, is clearly the most capable of the civil GPS signals in terms of jam resistance. L5 also should be declared operational by mid-decade. As the use of LEO communication satellites matures, their use may also fit this category.
Summary of Receiver Toughening Options. Quantification of the selected, nominal receiver augmentations are summarized in FIGURE 1 for both full accuracy (State 5, centimeter-level accuracy in RTK) and for less accurate code tracking (State 3, meter-level accuracy). These results are shown with a logarithmic scale to accommodate the wide range of denial radii.
Figure 1. Effect of receiver augmentations on accuracy for both State 5 and State 3. (Image: Brad Parkinson)
The example shows that a 1-kW hostile jammer’s denial radius10 can be reduced by a factor of about 100, using the conservative example augmentations of inertial and CRPAs. Because area is proportional to radius squared, the effective denial area of an augmented receiver would be 1/10,000th of the unaugmented receiver, using the example values.
Reverting to code-only (State 3) tracking, it enables operating through higher levels of jamming, albeit with less ranging precision. All these receiver augmentations and tracking techniques would also offer a significant defense against any attempt to spoof (deceive) the position measurement. Again, none of these techniques are new; we demonstrated the capabilities at the original GPS Joint Program Office in 1978, more than 40 years ago. Today, many competent manufacturers are offering toughened GPS receivers with combinations and variations of these techniques.
GPS jamming tests at White Sands have caused aircraft interference, which could be largely avoided with toughened receivers. Here, M-code is tested on Joint Light Tactical Vehicle platforms in 2020. (Photo: Joe Bullinger/U.S. Navy)
Meeting Increasing Threats
Threats of both jamming and spoofing seem to have accelerated. Devices to perform these illegal acts are freely advertised on the internet. In fact, we read of incidents both in the United States and abroad.11 Near White Sands Missile Range in New Mexico, there have been GPS air traffic control outages due to authorized military operational jamming exercises. Such interruptions could be largely avoided if more robust (toughened) GPS receivers, with the enhanced jam resistance techniques outlined here, were in use.
News reports also highlight the spoofing issue. Hardening against this threat is also a task for toughening. A serious spoofing sequence usually starts with a strong jamming signal to cause the user’s receiver to break lock, followed by a strong false GNSS signal that causes false lock by the receiver. Using the false signal leads to a false position, of course. The first line of defense is to avoid the break-lock threat. Failing this, numerous self-check and authentication schemes can be used to avoid false positions.
A conclusion is that avoiding the break-lock jamming is a first line of defense against a spoofing attack. Of course, the toughening techniques to avoid this are the main subject of this paper. One well-known expert has stated that, for a well-designed receiver, a spoofing attack might deny the measurement of position, but should never cause false PNT. I will leave further discussion of spoofing to other authors.
Returning to disruptions of service in general, some have suggested many interference occurrences have gone unreported, because the typical user would not know where to make such a report. To remind the reader, the official reporting center is online at www.navcen.uscg.gov/?pageName=gpsUserInput.
In addition, the U.S. Federal Communications Commission (FCC) has repurposed a portion of the spectrum adjacent to the main GNSS L1 frequency (1575 MHz). The agency is converting the license holder’s original authorization to transmit a weak space-transmitted signal into a much stronger terrestrial system, potentially with thousands of transmitters. Extensive testing of civil GPS receivers by the U.S. Department of Transportation demonstrated that the planned repurposing will interfere with many existing receivers. Some observers call this disruption “legal jamming.”
Such a new spectrum use could have grave impacts on those existing receivers, notably aviation (especially helicopters and UAVs) and first providers. On the other hand, installing toughened replacement receivers would make the users virtually immune to this threat.
So, this begs the question: If the receiver toughening techniques are so effective, why are they not more prevalent?
Barriers to Adoption
Let’s examine the potential resistance to more extensive use of receiver augmentations.
Knowledge. This involves underestimating the threat to PNT and not understanding that toughening techniques are available. As mentioned above, threats to the fragile GNSS signals are growing.
There seems to be little interest in the U.S. government to monitor and suppress interference in the United States. Internationally, the reported incidents continue to increase.12 It is also reported that certain European aircraft manufacturers have installed advanced, deeply integrated inertial systems with civil GNSS receivers to defeat or “flywheel” through radio-frequency threats (particularly in the Middle East).
As this threat trend continues, GPS manufacturers and users must realize that many of these solutions will take time to authorize, implement and install. It appears that the media are not aware that not only are the toughening techniques outlined here feasible, but many manufacturers have product offerings that address these threats. Having off-the-shelf solutions will give the PNT user the opportunity to retrofit and defeat such threats.
Cost. The cost for a receiver to revert from State 5 to State 3 is zero, and all receivers that use Code 5 (for example, RTK) would naturally have this built in. Regarding use of other frequencies (such as L5) and modulations (L1C) rather than the original L1 C/A, there is some small cost associated, including the additional antenna for L5. Note that all modern cellphone chips, such as Qualcomm’s, have this capability — including integrated carrier-phase measurements — in a chip that is estimated to cost about $5. A potential barrier is that the L5 and L1C signals are not yet declared operational, but these newer GPS signals should be operational within about five years.
The costs of many inertial components (accelerometers and gyros) have plummeted in the last few decades with the proliferation of MEMS devices, particularly into cellphones and automobiles. Their power consumption has also decreased while their performance has steadily improved. Full IMUs are much more expensive, but are already installed on many commercial aircraft. Robust toughening with inertial sensors can be achieved, but requires deep integration and careful engineering.
Depending on their complexity, CRPA antennas can be a costly receiver augmentation. Very high-speed (330 MHz is available), 16-bit, A-to-D converters are at the heart of most of these phased-array devices. Some are priced at about $150 each. Applications with a high premium for PNT availability in the face of interference — such as commercial aircraft and cargo ships — should find them affordable. Aircraft manufacturers have resisted retrofitting existing aircraft with larger diameter CRPA antennas because of costs. For some of these applications, integration costs can be more than the costs of the receiver itself, particularly if not included in the original manufacture.
As the yearly sales of fully toughened receivers increase, the economies of scale should significantly reduce unit costs. Each application will make its own determination of affordability, based on risk.
Government restrictions. Civil use of CRPAs with four or more elements is restricted by ITAR. These are well-meaning restrictions on technologies that could be used against the United States by hostile military forces. Unfortunately, the phased-array antenna techniques are not only well understood and tested, but relatively inexpensive components are widely available on the open world market. In particular, the restriction on the number of CRPA elements for civil use should be completely removed. All potential enemies are well aware of the beam-steering method and have ready access to the parts to build them. Thus, the restriction is only harming civil users without affording any apparent improvement in general military posture.
Certified aviation receivers need approval for deep integration of inertial systems and multi-element CRPAs. (Photo: JasonDoiy/iStock/Getty Images Plus/Getty Images)
Gaining permission: FAA flight certifications. To be used in commercial aircraft operations, navigation equipment must be certified by the U.S. Federal Aviation Administration (FAA). Current, certified GPS aviation receivers have rudimentary toughening techniques, but gaining approval for deep integration of inertial systems and multi-element CRPAs must be completed. It is gratifying to hear that work is underway to do this.
Any civil solution for the United States must expand integrity monitoring beyond GPS to include all GNSS, and must be operationally included in the FAA’s integrity monitoring with WAAS.
Recommendation
In describing resistance to interference, I have introduced the idea of DRRR – Denial Radius Reduction Ratio. Also, I have used a 1-kW white-noise jammer as a standard threat for calculating the denial radius of various GPS receiver configurations. My recommendation is that equipment manufacturers specify their receiver offerings by stating their equipment’s denial radius against a “standardized” 1-kW EIRP white-noise jammer.
Summary
Media reports of interference to GPS may be accurate, but they generally do not recognize that available toughening techniques can largely defeat those interference threats. While exploring systems-level replacements or augmentations (such as LEO ranging or Loran) is worthwhile, GPS (or GNSS) still offers the greatest capability in combined terms of accuracy, integrity and coverage.
The goal of all PNT providers — GPS operators, certifiers and manufacturers — should be assured PNT, with the expected accuracy and availability. The described toughening techniques to do that have been known for decades, but have not been generally adopted by many critical civil users. Many manufacturers do offer civil products under existing government constraints.
The purpose of this article was to describe and advocate the solutions available to increase the robustness and toughening of civil GPS receivers. For example, readily available toughening augmentations for civil receivers can reduce the denial radius of interference by 99% or more. This implies that any denied area would be squeezed down to 1/10,000th of that experienced by an unaugmented receiver.
The payoff is high, and should be affordable to many high-end, safety-of-life users. Therefore, a renewed focus on toughening of GPS receivers is overdue. We discussed barriers to rapid adoption but, more than the specifics, it is crucial to fully and urgently embrace the goal of toughening receivers, particularly removing the ITAR restriction on antennas.
Opinions expressed in this article are those of the writer and should not be construed as the official position of the PNTAB or any U.S. government organization.
Notes
1. While not a part of the U.S. Department of Defense’s GPS operation, the FAA’s integrity signal (WAAS) is a GPS-type signal directly available and being used by almost all modern GPS receivers, including cellphones.
2. This is the ration of Denial Radius between, for example, unaugmented L1 C/A in State 5 and augmented L1C in State 3. Please see later footnote and graph.
3. Calculated as 10 (–15/20)
4. Of course, the more traditional dB measure of jammer resistance can (in most cases) be simply summed to estimate the total effectiveness. The use of DRRR gives a more intuitive calibration, particularly for non-technical persons who may not be at all familiar with dBs.
5. State 5 is the tracking mode that provides full accuracy; it requires tracking both the PRN code and the reconstructed carrier. It is required for RTK positioning, which is usually used for automatic control of machines or vehicles. It is most vulnerable to interference. Less vulnerable is State 3 tracking, which only provides code tracking, with precisions of perhaps a few meters.
6. Deliberate jamming using “matched spectrum” GPS-like modulations have also been employed in the Middle East. The toughening techniques described are also generally applicable, with appropriate rescaling. The matched spectrum is fundamentally used to improve the jammer spectrum efficiency.
7. See Michael Jones, “Anti-jam systems: Which one works for you?”, posted on gpsworld.com on June 14, 2017, for a survey of manufacturer offerings at that time. Named companies generally continue to offer improved, jam-resistant products.
8. Phased-array antennas, by their nature, distort phase and would probably have to be calibrated for precise operations such as RTK. Fortunately, we understand that this problem has been reportedly addressed and largely solved by the U.S. Navy’s JPALs program.
10. The denial radius results shown can be easily scaled for weaker or more powerful jammers. The scaling goes as simply the square root of the power ratio of a different size jammer to the 1-kW example. A 10-watt jammer is 1/100th the power of the example. The denial radius would then be one tenth of the example, which is the square root of 1/100.
11. “Ships have reported an increasing number of cases of significant GPS interference and jamming in recent months. The geographic areas with more than one reported incident include the eastern and central Mediterranean Sea, the Persian Gulf, and multiple Chinese ports.” (Source: www.gard.no/web/updates/content/30454065/gps-interference-and-jamming-on-the-increase).
“North Korea is using radio waves to jam GPS navigation systems near the border regions, South Korean officials said. The broadcasts have reportedly affected 110 planes and ships and can cause mobile phones to malfunction.” (Source: www.bbc.com/news/world-asia-35940542).
12. “Reports of GPS outages submitted by pilots from the cockpits of commercial flights show that disruptions to the navigation system, which was created and is maintained by the U.S. government, are now standard occurrence on the flight routes between North America and Europe and the Middle East, according to data from the European Organization for the Safety of Air Navigation, known as Eurocontrol.” Fortune Magazine, Nov. 1, 2020.
Given that space is increasingly a congested and contested arena, should the U.S. government establish a new office to manage both space-based and terrestrial-based PNT systems?
John Fischer
“The U.S. government already has the National Executive Committee for Space-Based PNT (see gps.gov), which coordinates policy among all the branches of government. There is also a PNT Advisory Board, which includes some international members to inform the committee, and a National Coordination Office to execute the policies. This is in addition to the new U.S. military’s Space Command under which GPS operates. So, space-based issues are being addressed, but maybe more could be done to coordinate terrestrial-based PNT systems with space-based ones.” — John Fischer
Ellen Hall
“As new technology advances, government, along with its policies and laws, struggles to keep pace. This was certainly true of the internet and cybercrime. This is also true of terrestrial-based PNT, where new technology emerges in areas not currently covered. Policy gaps and overlapping technologies need to be addressed and coordinated. It certainly seems that this would be one of those areas that could benefit from oversight. Space-based PNT is currently addressed by the National Executive Committee for Space-Based PNT, among others.” — Ellen Hall
Bernard Gruber
“The threat to U.S. space-borne assets and the signals they generate is very real. Both commercial and military users of GPS continue to seek independent alternatives to current PNT systems that are diverse and robust. The National Coordination Office supports the National Executive Committee (NEC) for Space-Based PNT. I believe the role of the NEC should be broadened to include terrestrial and alternative PNT as a first step to charter/secure a new or expanded program office.” — Bernard Gruber
Editorial Advisory Board
Tony Agresta, Nearmap
Miguel Amor, Hexagon Positioning Intelligence
Thibault Bonnevie, SBG Systems
Alison Brown, NAVSYS Corporation
Ismael Colomina, GeoNumerics
Clem Driscoll, C.J. Driscoll & Associates
John Fischer, Orolia
Bernard Gruber, Northrop Grumman
Ellen Hall, Spirent Federal Systems
Jules McNeff, Overlook Systems Technologies
Terry Moore, University of Nottingham
Mitch Narins, Consultant
Bradford W. Parkinson, Stanford Center for Position, Navigation and Time
In January’s issue, he listed 10 questions from a PNT expert perhaps unfamiliar with eLoran.
These are important questions that must be asked of any technology, especially one under consideration to augment and back up our essential, but very weak and vulnerable, GNSS signals.
Yet the expert’s concerns pale in comparison to the essential questions about GNSS and PNT facing the United States and the West.
While I look forward to answers to the “10 questions” as a part of our ongoing professional dialogue, there are two important points of context we all need to keep in mind.
A Broad Consensus
First, Mr. Luccio’s assertion about eLoran being a part of the solution is more than reasonable. It also has a lot of impressive support from a wide variety of authoritative sources.
In 2008 and 2015, after much study each time, the U.S. government decided on and committed to building eLoran systems. Also, the U.S. government-sponsored National Space-based Positioning, Navigation and Timing (PNT) Advisory Board recommended eLoran in 2010 and 2018 as a part of securing the nation’s critical PNT capability.
In 2021, the U.S. Department of Transportation told Congress that wide-area terrestrial broadcast was a necessary part of a national PNT architecture. They later commented that infrastructure required per coverage area would be a key selection criterion for that broadcast technology. In other words, a system like eLoran.
Overseas, support for Mr. Luccio’s statement on eLoran is even stronger.
The United Kingdom has long endorsed eLoran and operates an eLoran transmitter as a timing reference.
Russia operates Chayka, a version of Loran.
Available information points to Iran’s terrestrial PNT system being a form of Loran or eLoran.
China and South Korea have long had Loran-C systems, and both are in the process of upgrading to the eLoran standard.
Each of these countries has publicly announced that it operates Loran/eLoran as a matter of national security in case space-based systems are jammed or destroyed, and to generally avoid overdependence on space-based PNT signals.
So, Mr. Luccio’s assertion was not at all revolutionary. Given all the studies, recommendations and existing uses, it would be surprising if he did not consider eLoran a part of the solution.
The Important Questions
Second, modern keying, encryption, authentication and other tech advances will help make all PNT technologies much safer and more resilient than they would have been decades ago, Loran and eLoran included.
Yet all will still have their strengths and weaknesses.
The most important questions we must ask are about how to establish the right level of national PNT security. These include:
What is the right combination of technologies and systems with different delivery and failure modes that complement and reinforce GNSS and each other?
How can the systems be efficiently and effectively implemented?
How can the services they provide be easily accessed and widely adopted to ensure all parts of society are protected?
Countries such as China have answered these questions and are well down the path to implementation and wide adoption. Their robust national PNT architectures support easier rollout of 5G, rural broadband and other systems. They also serve as solid tech infrastructure upon which to build myriads of technologies and applications yet to be conceived.
Those nations not so advanced must accelerate their efforts. Otherwise, they must resign themselves to perpetually coping with GNSS vulnerabilities, including the possibility of attacks, and an eventual second or third place in the world because of their shortsightedness.
Tells Space Advisory group spectrum issues a concern for all
Speaking to a gathering of space industry leaders, Dr. Brad Parkinson, the original chief architect for establishment of the Global Positioning System, outlined the threats to GPS signals posed by a proposal from Ligado Networks that is before the Federal Communications Commission (FCC).
The proposal, which was initially approved, has been pending since 2011 over concerns about its impact on reception of GPS signals.
When asked why the proposal has been pending for so long, Parkinson speculated a combination of the enormous sums of money potentially involved and difficulties for the FCC in arriving at an equitable solution were to blame.
“I hope they find some spectrum for Ligado,” he said. “Just not the spectrum they are asking for.” Referring to the powerful proposed transmissions in frequencies adjacent to those used by weak GPS signals, he said “You don’t want to put a rock band next to a retirement home.”
“You don’t want to put a rock band next to a retirement home.”
Parkinson was speaking to the National Space Council’s User Advisory Group (UAG). The UAG was chartered under the Federal Advisory Committee Act in December 2017. Membership includes the CEOs of Lockheed, Boeing and other companies with interests in space, the Governor of Alabama, five former astronauts, and other leaders in space technology and policy.
This was the first time the UAG’s agenda focused on GPS and positioning, navigation, and timing issues. Parkinson was invited to discuss the activities of the U.S. National Space-based Positioning, Navigation, and Timing Advisory Board. The UAG was interested in learning from the PNT Advisory Board’s experience, and about some of the issues it has encountered over the 15 years since its establishment in 2004.
Parkinson used the PNT board’s experience with adjacent band compatibility and the Ligado proposal as an example of a particularly thorny issue. He said the Ligado issue was an ongoing concern that the UAG should share.
Brad Parkinson shared his concerns over lack of Ligado decision with the National Space Council’s User Advisory Group (UAG). (Photo: Rebecca Zia)
Also that spectrum issues writ large should be of great concern to all users of space as the competition for spectrum from terrestrial users will continue increase. He called the Ligado proposal “a grave threat to GPS,” especially for aviation and high-precision users.
During the course of the presentation Parkinson asked for the UAG’s support of the PNT Board’s recommendations on the Ligado proposal. He also asked the group to endorse the National PNT Executive Committee’s adjacent band compatibility methodology to any future such proposals.
Parkinson’s presentation concluded with a discussion of how GPS is increasingly being used to support activities in space. It has long been used for booster guidance during the initial phase of space flight.
Relatively recent work using side lobe signals from GPS satellites has caused engineers to recognize a Space Service Volume for GPS. This allows use of the signals for mid-course launch corrections, satellite station-keeping, rendezvous, and potentially trans-lunar navigation.
The U.S. Air Force will load a new signal feature, designed to make spoofing detectable, aboard a satellite that will broadcast it from space as a security overlay for the GPS L1C signal, but not until 2022 at the earliest.
The Chips Message Robust Authentication (Chimera) is now in testing under the auspices of the Air Force Research Laboratory (AFRL), getting ready to fly on the Navigation Technology Satellite 3 (NTS-3), which will trial a number of new PNT techniques and technologies.
Chimera inserts encrypted digital signatures and watermarks within the L1C signal. A GPS receiver with the requisite additional capability for this purpose can then detect whether the signal is real or fake and also authenticate the location of a GPS receiver that is remotely located.
This key feature could provide a defense against hacking by blocking access from anyone unable to prove they are at an anticipated or licensed site. Hacking, of course, is a growing threat to all sorts of infrastructure: financial, security, utility grid and more.
Presentation slide from PNT Advisory Board briefing by Logan Scott.
Consultant Logan Scott first proposed the Chimera technology in 2003, when he affirmed that “Some of the spoofing detection measures in wide use offer a false sense of security. Authenticatable signal architectures are needed.” In June, he made a presentation to the PNT Advisory Board: “The Role of Civil Signal Authentication in Trustable Systems.” The two slides accompanying this article appeared in that presentation.
“Chimera represents a fundamental paradigm shift in PVT security paradigms,” Scott related in a subsequent conversation. “Trust takes time and memory on a personal level and, in this case, in GNSS signals, too.
“You don’t trust somebody as soon as you meet them. Over a period of time, you get to know them. If you can’t remember anything, you can’t develop trust either.”
“In the GNSS world, there are a lot of applications where you don’t need output in real time,” Scott said. “For example, to align an inertial. The inertial provides the real-time aspect. You don’t want to send anything to the IMU that is factually incorrect. When building to aid inertial, I can afford to have a delay from real time as long as I tell it where it was 10 seconds ago. The power of that is, if I don’t have to give real-time output, I can ponder and think about things.
“If a spoofer attacks, there’s an evolution that happens there. If I, as the receiver, can see the developing scenario, and how it starts to look at little screwy, I can stop and not send anything to the IMU that might corrupt it.”
How It Works. The core concept of Chimera involves the satellites sending encrypted watermarks, encoded into the signal by the satellite. After a slight delay, the satellite sends the key used to generate those encrypted watermarks. Once a key is sent, the system changes the key.
Since the receiver has already recorded the signal with its watermarks before the key is sent, spoofers cannot know the correct key ahead of time, in time to insert correct watermarks of their own. This means that any spoofed signals can be easily spotted: either the subsequent key won’t match up with the spoofed watermarks, or there will be no watermarks at all.
“Another reason it’s hard for someone to generate these watermarks on their own is because the signal is buried below the noise,” added Scott. “The watermarks are hidden.”
A number of different time delays between signal and key are possible within this concept and within the general set-up of GPS. Scott and the AFRL have, for various practical reasons, provisionally settled on a 6-second delay on the fast watermark channel and a 3-minute delay for the slow watermark channel.
The signal enhancement could be incorporated into the Wide Area Augmentation System (WAAS). This has yet to be fully determined, but this route would lead to a faster implementation of Chimera. Scott thinks that going the WAAS route could bring Chimera capability into action within two years.
The AFRL, however, is looking at a much longer timeline. The NTS-3 satellite, where it first intends to test Chimera, will not launch until 2022 — three years hence. And that’s only a test, not an enactment or a system-wide implementation.
Slide: Logan Scott
Verification. One key benefit for commercial entities, particularly those in financial infrastructure and other systems that increasingly fall victim to hacking, is that Chimera gives them the ability to verify customers’ or partners’ locations before granting any kind of access. The customer’s or other erstwhile user’s GPS receiver would record the full signal, including the watermarks, and transmit that data to the company, entity or data center needing location verification, before the keys are published. Each combination of watermarks and signals is unique to the place where it was recorded, thus it is possible to tell whether the user is actually where they say they are, or in an authorized or pre-identified location before granting access or accepting further input (such as commands).
Scott claims that Chimera affords a 99.9% probability of detecting spoofers. “I have a 99.9% chance of detecting that the watermark is not there, because they don’t know how to generate it. This is based on how you’re processing the signal. It’s designed to be very flexible in how the receiver uses the signal.”
Just One Problem. Receiver manufacturers will have to develop new Chimera-capable receivers, and customers will have to buy them. An additional cost for the added processing, above and beyond that required for normal GPS operation, is unavoidable.
And a Hiccup. Chimera, while an acronym, is as a name perhaps not a totally felicitous choice. In Greek mythology, the chimera is a fire-breathing female monster with a lion’s head, a goat’s body, and a serpent’s tail. These historic ancestors have evolved into the word’s more current use: a thing that is hoped or wished for but that is in fact illusory or impossible to achieve.
AFRL Wants Your Opinion. The Air Force Research Laboratory seeks feedback from the PNT community on the Chimera enhancement for the L1C signal. The specification is here. And, you can download a comment form
On Aug. 10, the National Space-Based Positioning, Navigation, and Timing (PNT) Advisory Board, the government’s GPS expert board, sent a letter to the National Executive Committee for Space-Based PNT (a multi-agency body that steers GPS policy) that concluded, “We strongly recommend your opposition to the Ligado proposal.”
The letter sprang from a unanimous vote five days earlier to oppose allowing Ligado Networks to use spectrum neighboring the GPS band for terrestrial communications.
Ligado possesses licenses to broadcast on two satellite bands located adjacent to the GPS frequencies. The company has been seeking permission from the Federal Communications Commission (FCC) to repurpose these licenses from satellite-based use to ground-based use from powerful tower transmitters.
Ligado said in May it would lower the power in its proposal for the 1526–1536 MHz band to 9.98 dBW to avoid interference with certified aviation receivers. However, the PNT Advisory Board reiterated its opposition, saying that even if the transmissions’ power was lowered to just under 10 watts, it “will create totally unacceptable interference for a great number of GPS users in the United States.”
From the Letter: “This risk is far too great, and far too many questions remain, for Ligado’s proposal to be approved. While there are many broadband alternatives (Ligado would be a very small percentage of this national asset), there is only one GPS. Any impairment to current and future uses is clearly contrary to the national interest. Therefore, implementation of their recently proposed ~10-watt operating scheme will create totally unacceptable interference for a great number of GPS users in the United States. In fact, despite power limits in their current amended application, it is probable they could still be allowed to increase this power over time. This would be even more destructive to GPS users.
“We believe avoiding degradation over at least 90 percent of the region near Ligado transmitters is the absolute minimum protection for GPS receivers in each class. This would be a hypothetical 90 percent Protection Evaluation. This is not an endorsement of this level since, of course, all users would prefer 100 percent protection. The Department of Transportation (DOT) Adjacent Band Compatibility (ABC) study is the only validated test to verify degradation at various received power levels.
“Those results inform that to insure degradation not exceed 10 percent of the Region (90 percent Protection) for High Performance receivers, either:
Ligado maximum power can be no more than .0036 watts at the 400-meter spacing they had earlier planned. Tolerable power would be 3/10ths of 1 percent of their proposed ~10 watts. Or
the closest spacing of Ligado transmitters is 20,000 meters (over 12 miles) for their proposed ~10 watt power level (see Figure 1).”
Figure 1. The PNTAB strongly believes that 90 percent is the minimum Area Protection Criterion (maximum 10 percent degradation). (Chart: PNT Advisory Board)
The U.S. Defense Advanced Research Projects Agency (DARPA) Microsystems Technology Office is soliciting research proposals for the development of a new class of atom-based systems using integrated photonics and trapped atoms to enable high-performance, robust, portable clocks and gyroscopes.
The military researchers are asking industry to develop relatively simple portable photonic integrated circuits (PICs) for high-performance position, navigation and timing (PNT) devices as an alternative to GPS for when satnav signals are not available.
A PIC or integrated optical circuit, similar to an electronic integrated circuit, integrates multiple photonic (having to do with light) functions, providing capabilities for information signals imposed on optical wavelengths, typically in the visible spectrum or near-infrared, 850–1650 nanometers.
A-PhI Program
The Atomic-Photonic Integration (A-PhI) program seeks to develop trapped-atom based, high-performance PNT devices, reducing the complexity of these atomic systems by using PICs. According to the DARPA document, the PICs will replace the optical assembly behind devices such as sensitive and accurate angle sensors and clocks, while still enabling the necessary trapping, cooling, manipulation and interrogation of atoms.
A-PhI aims to demonstrate that compact PICs can replace the optical bench of conventional free-space optics for high-performance trapped-atom gyroscopes and trapped-atom clocks without degrading the performance of the underlying physics package.
Physics
Atomic systems using trapped atoms have the potential to be made portable while maintaining their accuracy due to the atomic trap’s small size and the inherent isolation a trap offers an atomic system from the environment, especially from acceleration.
Currently, these systems are bulky, heavy, and not notably portable, because of the complexity of the optical systems used to create the trap.
In the past, efforts to miniaturize the hundreds to thousands of optical components in such benchtop systems have relied on removing optical elements, miniaturizing the remaining elements, and tightly integrating them in a small package.
The products deliver degraded performance with the need to maintain very tight optical alignment, causing both poor environmental robustness and poor tolerance to design errors. Effective miniaturized atomic systems cannot be achieved at a reasonable cost with this approach.
Recent developments in PIC research suggest that on-chip optical frequency combs based on microresonators, optical frequency synthesis, novel on-/off-chip coupling, wavelength demultiplexers, and on-chip phased arrays for dynamic manipulation of light fields can replace optical systems with readily manufacturable, low-cost chips without the alignment sensitivity of conventional free-space optics.
Gyroscopes
A-PhI also seeks to develop proof-of-concept trapped atom gyroscopes, a matter-wave analog of the interferometric fiberoptic gyroscope. Such a miniaturization effort could generate an order of magnitude improvement in angular sensitivity and dynamic range over current free-space products.
A-PhI hopes to develop portable, high-performance, navigation and timing systems: the miniaturization of the optics of atomic systems without a decrease in performance. Subsequent work, the RFP asserts, will be required to incorporate the necessary compact and robust lasers and electronics to achieve a fully functioning, high-performance, portable PNT system.
By Bradford Parkinson Vice-chair, U.S. PNT Advisory Board
In the coming months, the U.S. Federal Communications Commission (FCC) may allow high-powered, ground-based, communication transmitters to broadcast at a frequency near GPS L1. U.S. Department of Transportation (DOT) tests have shown that such transmitters effectively become jammers for many existing GPS receivers.
I believe that this possibility is the greatest current threat to the position, navigation and timing (PNT) community.
L1 is the primary band for GPS as well as for similar GNSS. For example, the international signal called L1C is to be centered at L1, albeit with wider spreading than the current L1 civil signal, C/A.
Why is this of critical importance? An economics study that only considered a small subset of benefits concluded that the U.S. alone realized $65 billion per year in direct economic value. A more complete recent study for the UK, extrapolated to the U.S., estimated the total impact of the loss of GPS to be over $3 billion per day for a five-day outage — a far greater rate. Virtually all GPS applications rely on the signals at L1. Thus, any threat to GPS is not simply an inconvenience, it would have great potential to do economic harm.
The PNT Advisory Board (PNTAB)has been trying to protect PNT, particularly GPS, and at the same time accommodate Ligado, a company that has requested repurposing of nearby spectrum. At our November meeting, we reviewed the Ligado proposal and framed a response that will be made public in due time. Meanwhile, these observations and conclusions are my own.
History
In 2011, LightSquared proposed that existing restrictions on its existing frequency authorization in the Mobile Satellite Service (MSS) band (a faint signal, satellite-to-ground) be waived so that the band is effectively repurposed to allow for high-power terrestrial transmissions.
The company has two space-to-ground authorizations in the 1525–1559 MHz band (1526–1536 MHz and 1545–1555 MHz) very close to the GPS primary frequency (L1 at 1575MHz). Initially it requested repurposing to ground transmission of 42 dBW (15.8 kW).
Faced with tests and analysis that showed this would be very destructive to GPS, it proposed to abandon the closer band and reduce power in the further band to 32 dBW, or 1580 Watts.
Ligado filings suggest a spacing of approximately ¼ mile between transmitters. A GPS receiver would find even these weaker signals 5 billion times the power of GPS at the maximum range of ¼ mile.
Most PNT users would be much closer.
International criterion
To ensure ranging accuracy, the international standard for interference to GPS is a 1-dB increase in noise levels. In conventional terms, this max allowable 1 dB is a 25.8% increase in background noise. The power of the weak GPS signal is only about 1% of the background radio noise. Sophisticated signal processing algorithms allow the signal to be reconstructed.
The result: the international 1-dB standard is equivalent to a 25% reduction in GPS radiated power.
Two additional points
The 1 dB is not simply to protect signal lock, it is to protect ranging accuracy. Most GPS receivers will stay locked for higher levels of interference but lose high precision. This is particularly a problem for high-precision receivers, which need relative timing to sub-nanosecond accuracies.
These measurements are equivalent to the time it takes light to travel ¼ inch. Protecting such accuracies is of paramount importance to PNT users and applications.
Allowing such maximum degradation from a single source is not the whole picture. There are many other potential sources of interference and attenuations of the GPS signal. For example, foliage may reduce the GPS signal.
A receiver must cope with all of these difficulties. Allowing a single cause, such as the Ligado repurposing, the 25.8% equivalent reduction might be considered quite generous, but it is the accepted International Standard.
Ligado has specifically rejected this criterion, largely because testing has shown that the Ligado repurposing would then be unacceptable for many PNT user classes.
To support its rejection of the International Standard, Ligado has repeatedly alleged that five of the major manufacturers are in complete agreement regarding its repurposing. This is a substantial distortion. The record was set straight by Brian Ramsay of MITRE at the November PNTAB meeting: “Four of the five parties that reached agreements with Ligado (except for Topcon Positioning) support the 1-dB Interference Protection Criterion (IPC) in comments filed in response to this Public Notice.”
Further support was highlighted by Captain Robyn Anderson: “In June 2017, the Air Force produced a white paper on the 1-dB IPC that explained the relationship between harmful interference (levels that affect GPS receiver performance) and the 1-dB IPC (keeps interference below a level that would cause harmful interference).”
Lightsquared’s motivation in 2011 was clear: a $10 billion windfall profit (estimated increased value of the spectrum on open-market auction). The FCC did not confirm Lightsquared’s modified request, and in 2012 the company went into bankruptcy.
Reorganizing as Ligado and emerging in December 2015, it continued to pursue repurposing of its spectrum, sponsoring tests by Roberson and Associates, and tests at National Institute of Standards and Technology (NIST)/National Advanced Spectrum and Communications Test Network (NASCTN) to establish test procedures.
Both groups of tests were carefully reviewed by our PNTAB who found serious flaws. In general, Ligado rejected the 1-dB criterion and did not accept the need to protect all classes of users, particularly high-precision receivers. In addition, it did not consider the new GPS L1 signals (L1C and L1M), nor did it check the impacts on the international GNSS. The PNTAB assembled a 14-point summary of deficiencies and requested updates and corrections for the flaws.
NASCTN’S response did not really address the points, or claimed that there were no funds to correct the problems. The PNTAB then developed a Six-Point Criteria for acceptable interference testing,summarized as:
Accept and strictly apply the 1-dB criterion.
Verify interference for all classes of receivers.
Test and verify for all operating modes.
Focus analysis on worst cases.
Include the new GNSS signals.
Include GNSS expertise and openly publish results.
Image: PNTAB
We believe it is a very reasonable set that aims to protect PNT users and our economic benefits. In its sponsored tests, and in representations to the FCC, Ligado has consistently overlooked a basic facet of radio ranging: it is ranging accuracy, not simply locking onto a signal, that is the fundamental objective for PNT.
Both Ligado test sets clearly failed on all six points.
DOT ABC tests
While the Ligado-sponsored tests were neither independent nor adequate, the Department of Transportation, led by Karen VanDyke, sponsored a very complete set of independent tests; these are the most credible estimates of harmful interference. The ABC results have been made public. The PNTAB’s six points were published after DOT testing had begun, but DOT expanded and modified their effort to satisfy the criteria. The DOT conclusions, based on modeling real-world antennas and propagation patterns, are shown in Table 1.
TABLE 1. DOT ABC test results. Maximum tolerable effective radiated power (EIRP) for classes of the most susceptible GPS receivers for modified Ligado proposal (P2) of 1.58 kilowatts. In red are the factors that Ligado P2 exceeds the maximum tolerable radiated power. (Chart: GPS World)
At 100 meters, all classes of receivers tested had results that would exceed the 1-dB threshold, even for the reduced power level (P2, 1580 Watts) that has been the most recent filing. The shaded square is particularly troublesome. It shows that, for the most susceptible high-precision receivers, the Ligado proposed power exceeds the 1-dB threshold by over 200,000. This result is particularly damning for the proposed repurposing, because it is this class that produced the highest payoff in the recent Department of Commerce Study — over $30 billion per year.
PNT operations at risk
These are examples of unintended and potentially hazardous consequences of repurposing.
UAVs. Unmanned aerial vehicles (drones) will fly very close to the dense array of transmitters that Ligado would deploy. They usually require GPS for flight control. Even more important, if we are to monitor them and keep them from collisions, GPS offers the only viable techniques with 3D accuracy and almost 100% availability.
Precision survey. This is routinely used in urban areas for building construction and is a major source of productivity gains. These survey receivers are all high precision and routinely make measurements to better than ¼ inch.
Helicopters. These are found in urban area at all altitudes. They are used for law enforcement, rescue and passenger transportation. GPS is mainly used for general navigation.
Public safety vehicles. Fire, police and ambulances use GPS for both navigation and dispatch tracking. In a city, they would drive in and out of susceptible high-interference zones.
The PNTAB believes the DOT results are representative, accurate and credible. The National Coordinating Office for PNT also sponsored an evaluation of all testing to date. A summary report is now in coordination, as a combined Department of Defense (DOD) and DOT effort.
The DoD, which uses GPS in the national airspace for routine flight, testing, training, guiding rocket launches, and for humanitarian rescue missions, has opposed repurposing. The Air Force reported, “Results from the DOD ABC Assessment support the conclusions drawn from Department of Transportation’s ABC Assessment.”
November PNTAB meeting
At our November meeting, the board invited Ligado to make a presentation on its repurposing proposal. The invitation said: “Specifically describe your implementation plan, with a corresponding test plan addressing the issues we have openly raised. We request you specifically focus on those regarding the potential for interfering with any GPS/GNSS services that operate in the protected space-to-Earth L-band (1559–1610 MHz). Included should be all modes of operation and the use of all current and future GNSS signals.”
Valerie Green, executive vice president and chief legal officer of Ligado Networks, represented Ligado. In the run-up to the meeting, the Six-Point Criteria had been sent to Ligado. Green did not address the six points at all.
She did offer to reduce initial power to “the safe power level in the 1526–1536 MHz channel ranges from 9 to 13 dBW EIRP nationwide,not just near airports.”
FIGURE 1. Potential impacts on high-performance receivers. Red: loss of lock of all satellites. Yellow: loss of lock of low-elevation satellites. Green: 1-dB degradation. (Chart: PNTAB)
The 13 dBW corresponds to initial power levels of 19.95 W. However, Ligado has made clear in its FCC filings that it ultimately still wants a full 32 dBW base-station transmit power level, consistent with typical 4G/LTE networks.
The initial reduced power sounds like a major move in the right direction, but further questioning revealed two major issues:
Tower Spacing. Green was very evasive on the spacing of transmitter towers. Clearly, at the reduced power level, greater density would be needed to carry the original data bandwidth. At about 1/100th the power, density would have to increase by a factor of 100, and the spacing would have to decrease to 1/10th for the same data output rate.
Green referred us to an earlier filing which specified 0.25 mile, but did not clearly state that this was the plan; she claimed the details were proprietary. If this fundamental parameter, spacing, is not specified, it is hard to see the basis for the FCC evaluation of any new proposal. If the transmitter spacing is reduced to less than 1/10th of a mile, the sources of potential harm would be multiplied in a very worrisome way.
Future power constraint.A public presentation does not ensure that Ligado will actually file and agree to abide by those power constraints indefinitely. Board members pressed Green on the permanence of the power constraint.
She suggested it would be tied to the RTCA Minimum Operational Performance Standard. Revising the MOPS takes many years, if not decades, both to formulate and to implement. Retrofitting the commercial aircraft fleet is very expensive and time-consuming.
Further, her statement focuses only on commercial aircraft, ignoring the high-precision classes as well as future signals.
A modified summary chart (Table 2) for the lower power, based on the DOT ABC test results, shows that even at the lower power, the threshold for high-precision receivers is exceeded by a factor of over 3,000 at 100 meters. In fact, only cell phones, which are relatively inaccurate, could operate at 100 meters without exceeding the threshold.
TABLE 2. Results of DOT ABC test with Ligado transmitters constrained to 19.95 Watts (13 dBW). This illustrates that the International Interference Limit is exceeded many times over at 100 meters for certain high-precision receivers, highlighted in red. (Table: GPS World)
With these expectations and uncertainties, the PNTAB did not find the new revision acceptable to the PNT community.
Three fundamental issues
Ligado has steadfastly not accepted the realities of non-interference.
1 dB. Acceptance of the 1-dB (25.8% noise increase) International Interference standard is fundamental to protecting GPS applications throughout the country.
All current and future uses. Users of great concern are emergency services, helicopter and general aviation, UAVs, and precision survey and machine control. For example, many of the underground utilities in the U.S. have been mapped with precision, GPS-based, geographic information receivers. This application requires sub-meter accuracy and operates in both rural and urban environments.
Ligado has tended to simply focus on certified aviation, claiming that protecting that class of user is enough. The PNT community rejects that view. All current and future PNT users must be protected.
Worst–case interference. The recent round of testing was largely in a laboratory. Extrapolating to the real world must examine the situations with greatest interference. For example:
Number of simultaneous interfering transmitters. A single transmitter situation is not typical; three or more are apt to be in range. The additive power must be considered.
Propagation models. Propagation models for communications differ from those for evaluating potential interference to a navigation signal. For assured communication, a typical model assumes transmitted signal fall-off a little faster than 1/(distance squared). Ligado would naturally prefer to use this model, which is far from worst-case for interference. The early round of tests in Las Vegas verified the communications model would vastly underestimate interference levels, by factors of 10 or more. A more realistic model must be used.
Degradation Radius. This is the size of the circle within which the International Standard is violated for receivers in a specified class. If the spacing of transmitters is 400 meters, and the degradation radius is 200 meters, virtually all receivers are in the degradation zone. Ligado suggested an appropriate degradation radius is 250 feet for aviation (approximately 100 meters). Thus, they claim the PNT community should tolerate violation of the standard when closer than 100 meters to their transmitters. At 400 meters spacing, 25% of the area would be in violation.
But the ABC test results reveal a much graver situation. They show that, for the current Ligado proposal (1580 watts), the degradation radius is over 14 kilometers for high-precision receivers. See Figure 2.
The 1-dB criterion is the correct, accepted and somewhat generous allocation of interference that can be accepted by the PNT community. We would hope that the FCC would continue to insist on this standard.
PNT users must, yet again, defend the spectrum vigorously. Most of us are scientific and technical people. We are not used to discussions that deliberately avoid the technical issue or deny scientific evidence. We reject arguments that violate the fundamental laws of physics.
The currently filed proposal, 1580 Watts at spacing of ¼ mile, is unacceptable. It will do grave harm to many important PNT applications
We must be very leery of the new proposal by Ligado of 9–13 dBW. It still would violate the 1-dB criterion at 100 meters for many PNT users.
Moreover, the company history has been to bait and switch; it has an authorization for MSS Ancillary Terrestrial Component (MSS ATC) stations to fill the gaps in satellite coverage with ground transmitters. These must operate in conjunction with the space-to-ground link that made them effectively self-limiting. However, in 2011, it almost succeeded in switching this to a ground-only system, which would have achieved a huge financial windfall.
Open-air verification
If the FCC continues to consider this proposal, there is one step that it should take before granting it. It should require Ligado to deploy an array of transmitters in its advocated configuration, and run real-world, open-sky testing to assess the harm that may result, particularly to high-precision accuracy.
Such testing was done when the issue was first raised in 2011 and conclusively demonstrated unacceptable interference. Nothing has really changed from the baseline that was tested and found unacceptable then.
The company should carry the full financial burden of such a verification, under PNT supervision. The government, having already spent millions of dollars to defend the spectrum, should not bear the cost of such retesting.
Without this confirmation, it is hard to conceive of putting GPS and PNT at significant risk to satisfy investors who want to flip a company, after gaining “rezoning” permission for their spectrum.
From 20,000 feet altitude
If we examine the situation without the technical details, we have this: Fundamentally Ligado wants to provide service using its allocated frequency band for an unlimited number of Internet-of-Things installations.
It is not proposing a small, fixed number of transmitting towers located in isolated regions, but rather an accelerating deployment of private networks, many of which will be close to commercial and essential infrastructure where GPS use is critical.
It seems unrealistic that Ligado can or will reliably guarantee that these widespread installations will be continually adjusted and monitored to avoid GPS interference.
I believe the concept of allowing the installation of transmitting towers that, by design, will interfere with normal GPS use at some distance away opens the door to tacit approval of short-range (or not-so-short-range) GPS jammers.
While I can commend the entrepreneurial spirit, the Ligado proposal seems very reckless indeed. The incremental value of an additional broadband transmitting system when there are at least five already in existence seems trivial compared to the potential damage done to the modern utility named GPS.
I sincerely hope the FCC can find a spectrum swap or deny outright the current Ligado application.
Ligado Networks will appear and present at the National Space-Based Positioning, Navigation and Timing Advisory Board’s (PNTAB) meeting on Nov. 15 in Southern California.
Ligado and its predecessors have sought to install high-powered ground transmitters that have been shown to harm and overwhelm GPS signals and receivers in their general vicinity. The controversy has simmered for at least eight years without resolution.
That final resolution will ultimately be taken by the Federal Communications Commission (FCC), although congressional participation is also conceivable, since national infrastructure security is involved.
Meeting Locale. The PNTAB meeting will take place Wednesday, November 15, 2017, 9:00 a.m. to 5:00 p.m.; and Thursday, Nov. 16, 9 a.m. to 1 p.m., at the Crowne Plaza Redondo Beach & Marina Hotel, 300 North Harbor Drive, Redondo Beach, California, approximately a half hour’s drive south of Los Angeles International Airport. The meeting will be open to the public up to the seating capacity of the room. Visitors will be requested to sign a visitor’s register.
From June 28, 2017, PNTAB presentation by Brad Parkinson.
The central issue in this long-running fight is the as-yet unknown — though uniformly predicted by the various rounds of testing over the last eight years — effects of Ligado signals on a huge installed industrial and governmental base of GPS receivers, some of which are essential to the nation’s critical infrastructure.
Ligado Networks, the current-day incarnation of once-bankrupt LightSquared, seeks FCC permission to apply the satellite-based frequency licenses it owns to be broadcast from a ground-based network. This would put a powerful nearby signal immediately adjacent to the much weaker, more distantly emanating GPS signals, and by the way, those from other GNSS as well. Tests in 2011 and further testing in 2016 demonstrated these powerful signals interfering with GPS receivers.
Brad Parkinson
The Ligado appearance comes in response to an open letter, posted on Oct. 10 by PNTAB First Vice-Chair Brad Parkinson, inviting Ligado CEO Doug Smith to speak to the Advisory Board. That invitation itself emerged after a season of what have been termed “attack” statements issued in various forums by Ligado, which were in turn stimulated by two early-summer letters:
1. A June 27 letter from the American Geophysical Union, Aerospace Industries Association, American Meteorological Society, Aircraft Owners and Pilot s Association, Airlines for America, General Aviation Manufacturers Association, International Air Transport Association, Iridium Communications, Thales USA and other organizations (totaling 22) to the FCC opposing Ligado’s request.
“The undersigned organizations . . . write to reiterate that the threat of harmful interference from Ligado’s proposed ancillary terrestrial component (“ATC”) service remain real and persistent. Contra ry to the assertions in Ligado’s FCC advocacy and recent media blitz, its proposed terrestrial operations continue to pose a significant interference risk to numerous parties . . . . The risks to these critical services are very real and, consistent with the public interest, cannot be brushed aside.
That letter further notes that “Ligado seeks the ability to sell its spectrum to the highest bidder, underscoring the uncertainty of any prospective value of the services it has on previous occasions suggested it may provide. There is a clear effort by Ligado to downplay the significance of the technical concerns it continues to receive from numerous directions.”
2. A July 5 letter from the PNT Advisory Board to Deputy Secretary of Defense Robert O. Work and Deputy Secretary of Transportation Jeffrey A. Rosen, the co-chairs of the National Executive Committee for Space‐based Positioning, strongly opposing the Ligado proposal.
From June 28, 2017, PNTAB presentation by Brad Parkinson.
“The revised [Ligado] proposal to the FCC is fundamentally unchanged from a previous proposal reviewed in 2011. Extensive government testing in 2011 and in 2016, clearly shows that both proposals cause definitive harmful interference to many classes of GPS receivers.”
“All GPS stakeholders should be wary of any incremental approaches to deploying mobile broadband services in the mobile satellite systems (MSS) band. For example, initial services could operate at reduced power levels on a temporary basis to protect only a subset of GPS users, before moving to full — power levels that will cause widespread interference to many other classes of GPS users. Regulatory decisions must be based on the ultimate end-state of any systems proposed for operation in the bands adjacent to GPS, and must protect all classes of GPS users. Unfortunately, the latest industry proposal does not acknowledge the legitimacy of, and the need to protect, dozens of precise applications of great national importance.”
From June 28, 2017, PNTAB presentation by Brad Parkinson.
Round Two. The struggle has been a prolonged one, with many twists and turns, however coalescing into two main periods of activity:
2011-12, when the first round of tests showed then-LightSquared’s proposed network would overload the vast majority of GPS receivers. The Federal Communications Commission (FCC) tabled the proposal, and the company, holding spectrum licenses whose value could range far into the billions of dollars, filed for bankruptcy.
2016–18. LightSquared emerged from Chapter 11 in 2015 as Ligado Networks, positing a modified network plan, but one whose organizing concept remains unchanged, causing deep and continued alarm over GPS interference. 2017 tests, conducted by a firm and a government organization hired by Ligado, essentially reconfirmed the 2011 results. The tests found that the proposed ground towers would significantly interfere with GPS receivers as far away as 4 to 5 kilometers, “killing them dead” in the words of one expert who reviewed the test data.
Parkinson’s October 10 letter invites Ligado CEO Doug Scott “to provide the committee with clear up-to-date design information. . . . How might the system as now envisioned be deployed? How many ground terminals are needed, for example, and where would they be?”
Previous LightSquared and Ligado presentations have been long on promise but short on details. In fact, sound technical underpinning has not been communicated.
From June 28, 2017, PNTAB presentation by Brad Parkinson.
Parkinson writes “we would therefore encourage you to specifically describe your implementation plan , with a corresponding test plan address ing the issues we have openly raised . We request you specifically focus on those regarding the potential for interfering with any GPS /GNSS services that operate in the protected Space – to – Earth L band (1559 – 1610 MHz) . Included should be all modes of operation and the use of all current and future GNSS sign als. Without these specific technical details and corresponding evaluations, we can only conjecture as to what you are really proposing .”
Later, he affirms “our focus is to provide advice based on deep engineering and related expertise . As you know, interference to GPS/GNSS can adversely affect numerous safety – of – life systems , other vital national assets, and applications comprising over $60 billion of annual U.S. productivity benefits .”
Parkinson and the PNTAB have had better luck securing a Ligado appearance than did GPS World magazine. In August of this year, Ligado’s senior vice president and chief engineer for radio access technologies thrice declined an invitation to give a brief Expert Opinion for the September issue on the question: How can the safety, security, and full utility of GNSS applications be ensured while evolving best, most efficient use of limited, very valuable electromagnetic spectrum?
Just a Refresher. The PNTAB meeting will be held Wednesday, November 15, 2017, 9:00 a.m. to 5:00 p.m.; and Thursday, November 16, 2017, 9:00 a.m. to 1:00 p.m., at the Crowne Plaza Redondo Beach & Marina Hotel, 300 North Harbor Drive, Redondo Beach, CA, approximately a half hour’s drive south of Los Angeles International Airport. The meeting will be open to the public up to the seating capacity of the room. Visitors will be requested to sign a visitor’s register.
Ligado is by no means the only item on the Committee’s docket, but is very likely to be the pièce de résistance. The full agenda for the meeting includes:
Update on U.S. Space-Based Positioning, Navigation and Timing (PNT) Policy and Global Positioning System (GPS) modernization.
Prioritize current and planned GPS capabilities and services while assessing future PNT architecture alternatives with a focus on affordability.
Examine methods in which to Protect, Toughen, and Augment (PTA) access to GPS/Global Navigation Satellite Systems (GNSS) services in key domains for multiple user sectors.
Assess economic impacts of GPS/GNSS on the United States and in select international regions, with a consideration towards effects of potential PNT service disruptions if radio spectrum interference is introduced.
Review the potential benefits, perceived vulnerabilities, and any proposed regulatory constraints to accessing foreign Radio Navigation Satellite Service (RNSS) signals in the United States and subsequent impacts on multi-GNSS receiver markets.
Explore opportunities for enhancing the interoperability of GPS with other emerging international GNSS.
Examine emerging trends and requirements for PNT services in U.S. and international fora through PNT Board technical assessments, including back-up services for terrestrial, maritime, aviation, and space users.
Jamming Signals Criminal Activity in Intermodal Ports
By Logan Scott
More than 25 million containers pass through U.S. intermodal ports every year, with port operations valued at more than $1 billion per day. Measured in 20-foot equivalent units (TEU), the World Bank reports that worldwide, more than 600 million TEU passed through intermodal ports in 2012: 155 million through Chinese ports, 95 million through the EU ports and 43 million through U.S. ports.
The Port of Long Beach alone handled 6,820,806 TEU in 2014. GPS is a central component of automated port operations, but because GPS is widely used in asset tracking and monitoring, it has also become a target for denial-of-service attacks. If we look to the history of computer security, the initial attacks were mostly nuisances, but as criminals figured out how to monetize attacks, the attacks became more damaging, more sophisticated and more profitable.
In January, the U.S. Coast Guard held a public meeting on Maritime Cybersecurity Standards at Department of Transportation headquarters in Washington, D.C. Brett Rouzer, chief of Maritime Critical Infrastructure and Key Resources Protection, Coast Guard Cyber Command, described how a major East Coast intermodal shipping facility was degraded by a GPS disruption for more than seven hours. Two ship-to-shore cranes ceased operation due to loss of position, and two others were degraded. Ports are highly automated; ship-to-shore cranes are just one of the container-handling systems critically reliant on GPS. Fully automated ports providing services for unmanned container ships, trucks and trains lie within the realm of feasibility in the near future.
Rouzer did not specify the motivates for the disruption, how the attack was mounted, or if the shipping facility was even the intended target of the attack (I suspect it was not). Jamming is not a highly selective process, and it can affect numerous unintended targets.
In June 2014, I reported to the PNT Advisory Board on how every third or fourth truck on Highway 30B near Portland (Oregon) International Airport was radiating at or near the GPS L1 frequency. This highway leads to several nearby Port of Portland intermodal terminals west of the airport. The Federal Bureau of Investigation recently reported that “In 46 reported incidents, the thieves placed one or more GPS jammers in cargo containers with stolen automobiles” (italics mine). High-end automobiles command premium prices in foreign markets and are stolen and shipped out of the country within hours, usually via intermodal container. Active jammers can affect not only the automobile’s GPS tracker, but also trackers on other containers, ship’s navigation systems, straddle carriers and ship-to-shore cranes. Again, jamming is not selective.
Of particular note as cited above, criminals are beginning to use multiple jammers. Car theft rings are not unique in this. According to the Pharmaceutical Cargo Security Coalition in July 2014, “a tractor and trailer hauling $2 million worth of pharmaceutical products was stolen from a truck stop in Cartersville, Georgia, with the thieves deploying two separate GSM jammers.” The criminal’s motivation is that tracking devices can be hard to find and disable; just because you found one doesn’t mean that there isn’t another. The use of multiple jammers in criminal enterprise is indicative of a threat escalation where bad actors are seeking higher effect. This could lead to higher jamming powers and so on; and also more collateral damage.
Response
What is a correct and measured response to threats against navigation and timing? The key is to be on the lookout for emerging threats and to have a flexible response. Early detection usually yields a more effective and lower cost response; witness Ebola and ISIS. Following a public health model would seem to offer better prospects for protecting access to PNT. To this end, I would argue that situational awareness is the first important step.
One of the most striking comments that Sarah Mahmood (DHS) made at last June’s PNT Advisory Board meeting was about how backup systems are often not activated or used because the GPS receiver fails to recognize that there is a problem. As we move towards resilient PNT architectures, one of the most critical needs is to be able to distinguish good signals from bad signals and act accordingly.
Most GNSS receivers already have fairly advanced jamming detection capabilities by virtue of having an automatic gain control. Sudden changes in precorrelation input power levels are not normal and can indicate jamming or RF spoofing. Many GNSS receivers, particularly those that go into embedded mobile applications, also have sophisticated spectrum- and temporal-analysis capabilities, used mainly for diagnostic purposes in looking for interference sources from other components of the device. This same capability can be used in detecting and fingerprinting jammers. We already have the smoke alarms; we must amplify their use and visibility to the wider community of GNSS users and beyond.
Detection
One notable aspect of the port incident was the duration: more than seven hours. Rapidly finding and disabling the jammer was clearly a problem in this case. The old adage is that to find a stationary source (jammer) you need to be moving, and to find a moving source, you need to be stationary. Trucks and trains entering or leaving a port all pass through gates that can act as a simple chokepoint for detecting and finding active jammers. Properly hardened ship-to-shore cranes and straddle carriers can also act as a chokepoint. Straddle carriers used in moving containers around the yard and between modes could be very good at finding stationary jammers.
There are numerous relatively low-cost approaches for finding jammers in support of enforcement actions. One additional point: law enforcement officials need to be better educated as to why they should be interested in jammers; jammers point towards a crime much like smoke points to a fire.
Given the economic criticality of port operations and the concentration of assets (and asset trackers), we may see increased incidence of GPS disruptions. The situation is not critical yet, but it does bear watching. If jamming events increase or it takes too long to find and disable jammers, improved operational resilience will be needed.
Inertial measurement units are already used in many critical applications, but they don’t offer long-duration capability. They drift. Using adaptive arrays in critical equipments is another possibility, but they are not a panacea. Adaptive arrays are physically large, and standard null-steering approaches are not compatible with RTK processing. Precise positioning systems based on GNSS require specialized antenna-receiver designs to achieve a high level of jam resistance.
While I strongly believe eLoran is an urgently needed augmentation for resilient wide area navigation, it is not capable of the centimeter-level precision required for machine control, for example ship-to-shore cranes and straddle carriers.
High-precision local-area positioning systems based on optical systems, RFID and/or Locata-style systems may be the best approach for creating a defense in depth.
And then there is the cybersecurity question, which I will leave for another day.
Note: A video of the Coast Guard meeting is on YouTube. Rouzer’s talk starts at 36:30, with the port jamming incident mentioned at 48:51.
Logan Scott has 35 years of military and civil GPS systems engineering experience. He is a consultant specializing in radio frequency signal processing and waveform design. At Texas Instruments, he pioneered approaches for building high-performance, jamming-resistant digital receivers. He is a co-founder of Lonestar Aerospace, an advanced decision analytics company in Texas. Logan is a Fellow of the Institute of Navigation and holds 37 U.S. patents.
