One of the satellites in the Chinese domestic satellite navigation system, Beidou, is no longer in geostationary orbit and appears to have been abandoned.
According to information from the U.S. Space Command, the orbit of Beidou 1D was raised by around 130 kilometers on February 18, 2009. This may have been an attempt to place the satellite in a graveyard or disposal orbit. Such a maneuver is carried out by spacecraft operators when a satellite reaches the end of its life due to a malfunction or some other reason. However, the recommended boost height for geostationary satellites is about 300 kilometers, where a satellite is above the zone used to reposition active geostationary satellites and also provides a buffer for natural orbit variations due to solar radiation pressure and other causes. Beidou 1D may not have had sufficient propellant to reach desired orbit height.
In its current orbit, Beidou 1D is drifting westward at a rate of about 4.5 degrees per day and has already completed one circuit of the Earth. On July 17, it was positioned just west of the Greenwich meridian.
China launched Beidou 1D in February 2007; according to the Xinhua news agency at the time, the satellite was to serve as a backup to the three satellites already in orbit, perhaps replacing the first Beidou satellite, Beidou 1A, when necessary. Subsequent reports did indicate that Beidou 1A appeared to have malfunctioned.
It is not known what kind of malfunction Beidou 1D suffered or whether its signals have been switched off. Accurate detailed information about the current status of the Beidou domestic system is difficult to obtain.
China has plans to improve its domestic navigation system and to develop a global system known as Beidou 2 or Compass. Its first medium Earth orbit satellite, Beidou M1, was launched on April 13, 2007, followed by a geostationary satellite, Beidou G2, on April 14, 2009.
— Richard Langley
Galileo, Too, Has Accounting Problems
The European Union’s Galileo program has been ill-prepared and badly managed, according to a report by the European Court of Auditors released on June 29. These defects have set back development by five years, it believes.
The report also criticizes the Union’s 27 individual member states for counterproductive promotion of their respective national aerospace industries. The auditors conclude that the original public-private partnership plan was “inadequately prepared and conceived” and “unrealistic.”
The European Commission (EC) “must considerably strengthen its management,” advice the EC has evidently taken to heart. For the last year, contract negotiations by the European Space Agency (ESA) have taken place under the watchful eye of an EC program manager.
Contracts. On June 15, ESA signed contracts for the procurement of so-called long-lead items required for the construction of the constellation with Astrium GmbH and OHB Systems, the latter a German company and the former a German-French partnership with British involvement. Both Astrium (€7 million) and OHB (€10 million) contracts relate to parts for equipment of the satellite platforms and navigation payloads. Award of the satellite contracts themselves is planned to take place by the end of 2009.
ESA and Arianespace contracted for launch of the first four operational Galileo satellites on two Soyuz launch vehicles from Europe’s Spaceport in French Guiana. The four IOV satellites will be placed in orbit by end of 2010.
Conversations at the Paris Air Show seemed to indicate that ESA and the EC may divide the satellite construction contract into two stages to permit a later modification of the design, and that they may also divide the first satellite contract between the two bidders, Astrium and OHB, as an insurance policy to reduce the possibility of further development delays, and as a boon to design flexibility.
The Astrium CEO sharply criticized this option, saying it would increase overall program cost. The OHB CEO seemed more sanguine, lauding ESA’s move as likely to maintain a competitive environment.
Yowza!!, an application designed for the latest GPS-enabled iPhone 3G and 3GS models and iPod Touch, brings relevant coupon offers to customers based on their location.
“Any time you insert a concept such as location into a marketing program, you end up with a far more compelling value proposition,” states Mike Wehrs, president of the Mobile Marketing Association.
Sales and discount offers via Yowza!! can be updated in real-time and targeted by region or store location. “The phone will deliver a list of stores within one mile that have offers on Yowza!!,” said August Trometer, co-founder of the recent startup. Users show the barcode and digital mobile coupon on their handset at checkout to redeem the discount on their purchase.
“We work directly with merchants; they provide us with their latitude and longitude, we get the GPS coordinates, do a database search with a proprietary algorithm,” said Trometer. “The phone constantly goes back and forth between our app, touching data from our database. When the person touches their location, it touches a new set of data in the database. The phone will work with them to keep delivering the closest store. There’s a lot of work on the database end of things.”
One drawback of the app is that it has to be turned on to work — it does not sit in the background, waiting to be activated by incoming offers. “Users have to give the application access to their GPS coordinates,” explained Trometer. “But the power of the device and all the applications it brings make it silly to turn off the location capability.”
Retailers that have signed with Yowza!! include Sears, McDonald’s, The Container Store, and more. Unlike traditional forms of couponing such as newspaper ads, Yowza!! offers can be updated in real time and targeted by region or store location.
Trometer expects to announce Yowza!! capability through other GPS-equipped phones: Blackberry Storm, Google’s Android-based phone, and the Palm Pre. “All three makers allow developer access to the GPS and this is very important, it’s crucial, obviously. They also have a high-res screen, which is a requirement for our scannable barcode that the user shows to the merchant.”
Referring to GPS handsets that lack a high-res screen, he claims “The other phone manufacturers really have an uphill battle right now.”
Whose GPS? The source of the GPS chip within Apple’s iPhone remains a mystery. “Even people who have done teardowns of the devices, the chips are completely blank,” says Trometer.
“There are so many possibilities, we’re just scratching the surface right now with what can be done,” Trometer said. “The mind reels with the things that can be done with that.”
>> SURVEY & CONSTRUCTION
Hemisphere, Juniper Jointly Offer DGPS Receiver for Demanding Environments
Juniper Systems and Hemisphere GPS offer the XF101 DGPS receiver for the Archer Field PC, designed to deliver sub-meter DGPS to location-based applications in demanding environments.
According to the companies, the Hemisphere GPS XF101 DGPS receiver provides: Crescent GPS technology for sub-meter accuracy; COAST technology to maintain accuracy during temporary loss of differential signal; optional external antenna for centimeter-level accuracy; low power consumption; modular connection for rapid field use; real-time or post-processed DGPS data collection; and multipath minimization.
The XF101 with the Archer is priced at less than $2,500. It fully supports mobile GIS applications such as ESRI ArcPad and OnPoz GNSS Driver.
>> AVIONICS
NovAtel Receiver for Next-Gen WAAS
NovAtel announced receipt of a contract from the U.S. Federal Aviation Administration (FAA) to develop the next generation Wide Area Augmentation System (WAAS) reference receiver, the GIII. Total contract value can go up to $9.7 million.
NovAtel has worked with the FAA WAAS program since 1995, providing and supporting two previous generations of reference receivers for the WAAS ground network. The technology refresh will add support for new L1C, L2C, and L5 signal capabilities, on a qualified RTCA DO-178B software and DO-254 hardware platform. The WAAS GIII receiver program is scheduled to be completed over the next three years, and will include growth provision for further signal capability such as Galileo. As many as 14 receivers will be produced in the GIII development and qualification program.
>> FLEET TRACKING
AT&T, Trimble Fleet Management
AT&T has broadened its fleet and mobile asset management portfolio with the latest version of Trimble’s GeoManager solution, which helps reduce fuel and maintenance costs by enabling operators to manage their vehicle assets more efficiently.
Trimble GeoManager enables transportation and field-service fleet operators to track their mobile workers and assets through software and GPS modems running on AT&T’s wireless network. GeoManager integrates GPS, wireless data communications, and a browser interface to help manage mobile workers, the mobile worker’s work, and the mobile worker’s assets.
AT&T and Trimble have jointly offered fleet-tracking solutions for several years. The GeoManager update features improved map and status, new landmark uploads, WLAN usage, schedule report enhancements, driver logs, and organizational hierarchy modifications.
>> TIMING
Timing Vulnerability Concern Grows
Industrial and enterprise users in telecommunications and utilities privately express concern over revelations from the April Government Accounting Office (GAO) report, “Global Positioning System: Significant Challenges in Sustaining and Upgrading Widely Used Capabilities.” The GPS signal is used for synchronizing almost all global computer networks belonging to the military, utilities, banks, telecomms, television companies, and many more.
Backup? What Backup? These same companies point to a continued lack of commitment on the part of the U.S. government to stable and reliable backup for GPS. As long ago as 2007, in comments before the Department of Transportation, wireless carrier Sprint Nextel stated: “Sprint Nextel Corporation respectfully requests that the U.S. government continue to operate and invest in the LORAN-C and eLORAN systems. Should the DOT and DHS decide to decommission the LORAN-C system, Sprint Nextel recommends that the agencies delay doing so until the eLORAN system is fully operational. Sprint Nextel and other communications providers use the frequency signals of the Global Positioning System, LORAN, and atomic clocks for multiple levels of redundancy and diversity in their networks. Therefore, Sprint Nextel urges the DOT and DHS to carefully weigh decisions which might impact LORAN’s availability to the nation’s voice and data communications networks.
“The loss of a primary reference source (PRS) can negatively impact a telecommunications network, and those impacts can vary from minor short-term noise impairments to long-term network-wide outages. Both traditional wireline services and newer wireless services require a precise frequency reference for basic service delivery . . . . The continental U.S. portion of the Sprint Nextel network requires a PRS at thousands of switch sites, interconnection sites
, and cell tower sites to ensure reliable service delivery.”
Deadlock on Capitol Hill. Competing resolutions to either discontinue or adequately fund LORAN and eLORAN continue fencing in Congressional subcommittees in both chambers. Nothing has changed since Sprint commented two years ago — aside from a potential rise in the susceptibility of GPS to jamming, unintentional interference, and decreased availability.
GAO REPORT, FIGURE 5. Probability of maintaining constellation of at least 18, 21, and 24 GPS satellites based on reliability data as of March 2009 and a two-year GPS III launch delay.
>> TIMING
Telecom Clock from EndRun
EndRun Technologies announced a Telecom Clock Option for its Meridian Precision GPS Timebase, which provides accurate and stable GPS-synchronized outputs for military communications, aerospace, broadcast, engineering and calibration laboratories, telecommunications, and more.
The option was designed as a plug-and-play module that can supply any combination of E1, T1, J1 and/or composite clock outputs. An alarm output is also available and single-satellite mode (SSM) is supported. The Telecom Clock Option can be installed in EndRun’s GPS or CDMA-based Meridian and Tycho product lines.
I had a great visit at the ESRI User Conference earlier this month. If you recall last year, I wrote:
“As much as surveyors, engineers, and constructors may not appreciate geographic information systems (GIS) technology, at some point everyone should attend at least the ESRI Survey/Engineering Summit and the first couple of days of the ESRI User Conference held every summer in San Diego, California. This is not a GIS sales pitch. It’s a networking sales pitch. When other conferences are struggling to maintain attendance levels, the ESRI conferences seemingly never fail to grow in attendance. This year, it attracted some 15,000 people from 120 countries. That means gobs of GIS people, and also gobs of surveyors and engineers.”
The statement rang true this year too. Even in today’s economy where conferences are severely impacted or even cancelled due to travel budget cuts, the ESRI User conference still attracted ~11,000 people this month.
On another note, I think conference organizers are getting the message. People just can’t justify attending so many conferences. Next Spring, the ACSM (American Congress for Surveying & Mapping) is combining with the GITA (Geospatial Information & Technology Association) conference in Phoenix, AZ. Instead of 1,000-1,500 for each conference, it’s a larger event at 2,000-3,000. Even more interesting is talk in the rumor mill about a joint conference including ACSM and the ESRI Survey Summit in 2011. Include the GITA conference with those and that makes a lot of sense to me.
As usual, there were many things happening at this year’s ESRI UC conference and I attended many briefings. I’ll try to stay focused on the highly GPS/GNSS-related subjects:
Javad GNSS. One of the bigger news items on the GPS front was the joint Javad/ESRI effort in developing an ArcPad extension for Javad’s line of receivers. The demonstration was very cool. We loaded up a local map (San Diego) from their server located in Moscow (Russia) then took a Javad RTK receiver outside with the data collector (running ArcPad w/Javad’s extension). I collected data on a few points. The data was sent off to Moscow from the data collector (via GPRS while we were outside) to update the map. After we walked back into the convention center, the demonstrator clicked the workstation “refresh” button and viola, the map was updated with the points I collected at the cm-level.
According to the JAVAD engineer, “we make it look easy.” I agree. There’s a lot of heavy-lifting going on in the background to make this happen. With the heavy-lifting done, it still needs a bit of tweaking. There weren’t any quality control indicators (RMS values) on the data collector for the operator to reference and also ArcPad doesn’t recognize GLONASS satellites so while the GNSS receiver was utilizing GPS and GLONASS, ArcPad only reported GPS satellites. The operator really does need to know what’s going on before tapping on the STORE button. But, 95% of the work is done and the heavy lifting is complete so I don’t doubt they will finish off the last 5% in short order.
Topcon Positioning Systems. I’ve had a few questions from readers on Topcon’s new GRS-1 receiver. Is it single frequency? Is it dual frequency? Is it for GIS? Is it for survey?
The answers are Yes, Yes, Yes and Yes.
The entry-level GRS-1 is a single-frequency hand-held GIS data collector. That’s about US$5,000.
Add US$4,000 and you get a 5cm high accuracy GIS receiver.
Add another US$2,500 and you have a full-blown, cm-level RTK rover.
There are other options beyond this (eg. GLONASS), but I think you get the picture as I did. It’s a full L1/L2 GPS and GLONASS receiver. You pay to have features activated (plus some added hardware/software).
I haven’t tried one yet so I couldn’t tell you how it performs, but it’s worth a look.
Juniper Systems. Although they don’t design GPS receivers, their Archer hand-held is starting to show up in a lot of places. Hemisphere GPS has designed the XF-101 receiver as a plug-in for the Archer as well as having a similar model for the Trimble/TDS Recon and Nomad hand-helds. Javad was also offering the Archer with their systems. IkeGPS also introduced a new hand-held mapping system named the Ike1000 that is based on the Archer.
Geneq. Their flagship product, the SXBlue GPS, seems to be gaining momentum in the GIS marketplace. They have introduced a new model that utilizes the OmniSTAR correction service called the SXBlue II-L GPS. Their use of WAAS (via Hemisphere GPS Coast technology) and performance under tree canopy has created some buzz.
Trimble Navigation. It’s hard to leave Trimble out of the conversation, but nothing really new in the GPS product area. However, they continue their run of acquiring companies with the latest being Farm Works Software in the precision agriculture industry. In 2009, they’ve acquired four niche-market companies.
Magellan Professional. Introduced an upgrade to support ArcPad 8.0 for post-processing on their Mobile Mapper 6 hand-held for sub-meter accuracy. FYI: Magellan consumer GPS products is no longer part of Magellan Professional. Rumor has it that Magellan Professional will revert back to the Ashtech brand name of the1990’s.
I gave a presentation at the ESRI UC on Tuesday morning as part of the Survey (SUR) track. I focused on three core issues listed above. You can view my presentation here.
I’ll stick to the highlights…
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p>SVN-49 troubles. It’s broke and will never be as good as the other Block IIR-M satellites. Don’t expect it to be declared healthy in the immediate future. Even if a patch is developed and it’s declared healthy, it’s likely that pseudorange-based safety-of-life applications like SBAS (WAAS, EGNOS, MSAS) and NDGPS will not incorporate it into their solutions. Although more study is necessary, it appears that carrier-phase applications (cm-level real-time and post-processing) will be able to utilize SVN-49.
Solar Cycle 24. NOAA reports that the number of sunspots during the next solar cycle (2009-2020) will be the fewest since the 1920’s. That doesn’t mean the next solar cycle will be any easier on GPS than the last one. On the contrary, it could be worse for GPS. No one knows at this point. High performance GPS L1 receivers are the most exposed. Those utilizing NDGPS, WAAS and OmniSTAR’s VBS service need to be watchful. You can sign up to receive alerts from NOAA giving a three-day forecast of activity. NOAA predicts the peak of the next solar cycle will be in May 2013. Note that typically the geomagnetic activity that most affects GPS occurs after the peak. Links and more details are in the presentation.
GAO Report. I wrote an article on this subject back in June as it relates to medium and high precision users. You can read it here. High precision users will be affected more than other users because high precision GPS receivers perform better with a lot of observables. A loss of 2-3 GPS satellites can be significant and require users to begin using GPS mission planning software again to optimize the use of field time. Survey receivers using GPS and GLONASS will be less affected. The presentation references a report from the University of New Brunswick that takes a look at how GLONASS can compensate for a loss of GPS satellites.
Earlier this month I attended the Defense Installation GeoSpatial Information and Services (IGI&S) conference in Dallas. Although not a large conference, it is tightly focused and aimed directly at the GIS community that supports military installations. The conferences were initially started by the Air Force, but attendance by other branches has grown since they all have common issues and many have joint chains of command. The first keynote speaker was Lora Muchmore, director of Business Enterprise Integration, Office of the Deputy Undersecretary of Defense for Installations and Environment. She addressed the historic difficulty that DOD had keeping track of buildings, equipment, and personnel. The problem was exacerbated because each branch had their own inventory systems that were not interoperable with systems of other branches. Mrs. Muchmore concluded by emphasizing the increasingly important role that IGI&S is playing to improve the Department’s real property inventory, literally transforming the way resource and management decisions are made at the highest levels.
David LaBranche, the Defense Installation Spatial Data Infrastucture (DISDI) program manager, addressed how DISDI is working with the National GeoSpatial Intelligence Agency (NGA) to put IGI&S data and systems in place that are complimentary but of course different from intelligence community needs. They are working toward interoperability between the joint services and encouraging greater use of unclassified, shared, off-the-shelf products to build “installation situational awareness” without the expense of custom products and services.
The complexity of joint bases, base realignments and closures (BRAC), environmental issues, and interrelated activities with other federal agencies such as Department of Homeland Security (DHS), DEA, CDC and others highlighted the need for interoperability and shared spatial data. However, the opposite has occurred as systems grew in size and complexity. So the overarching theme of the conference was getting back to basics with standards and consistent base-to-base data and systems.
This concept of snapping back to where we started reminded me of a dysfunctional ship that I was assigned to early in my Navy career. The ship was a mess with poor morale, poor performance, and a very unhappy crew. A new CO arrived who made no immediate changes but instead carefully studied the ship for two months. He then called us officers together in the wardroom and gave us a very simple order. He told everyone to get a copy of the ship’s SORM, read it and put each division and department back in line with it. The SORM stands for Standard Organization and Regulations of the U.S. Navy. The long standing but continuously updated Navy manual is a guide that clearly identifies how a Navy ship should be organized and operated. It was created through decades of lessons learned and practical experience. Our new CO explained how any organization will morph over time away from it roots. Sometimes the changes grow out of operational necessity and some changes are driven by the personalities and talents of crew members. Regardless of the source, these changes can ultimately twist the organization into dysfunction.
As a junior officer, I was somewhat skeptical that this would have any major effect but I was wrong. Within three months the ship was functioning like a fine watch and there was actually harmony in the wardroom and among the crew. Of course this wasn’t the only change; the CO was a very gifted and uplifting leader with a superb memory. In that same three month period, through his “management by walking around” he learned the name, home town and family situation of every member of the 300 man crew. The CO became so respected and admired that eventually any crewmember would walk off a cliff for him. Getting back to the SORM roots certainly helped.
Many of the conference sessions addressed the same themes of interoperability and cleaning up the basics. The Marine Corps GeoFidelis program was highlighted by David LaBranche as a model, built with a focus on interoperability while also addressing the institutional issues of data handling and organization. Frances Railey, the GeoFidelis GIO, explained the details of the Marine Corps system and how it was designed to meet customer needs including a very detailed Data Access & Release Guide. Peter Len, the Gi&S manager for the Naval Facilities Engineering Command (NAVFAC) Pacific, explained the unique challenges he faces with facilities scattered across the Pacific including Guam and the Marianas. He discussed how GIS was used for encroachment planning and other base management issues.
The conference also included 22 vendors who demonstrated new products and services that also addressed the conference themes of interoperability. AutoDesk, which is one of the leaders in BIM modeling, had personnel demonstrating new tools that bridge the gap between CAD and GIS into the BIM environment. Bentley, another leader in BIM modeling, demonstrated BIM solutions that addressed life cycle facility management including energy and lighting. Their OGC compliant systems work with many different BIM model formats. NGA staffers demonstrated Palanterra, a Google-like online system used by emergency management people such as DHS for special event planning and response.
Woolpert demonstrated a new capability that can quickly create 3D building interior models from video clips taken from different view angles. I’ve seen this done with laser point clouds but this was the first time I saw video used to build 3D models. Kaya explained their GIS services including web engineering applications for facility and installation management as well as tools to build real property master plans. Pictometry demonstrated a new image service that will be launched this summer. Through the web service federal users will be able to view or download all Pictometry imagery, both ortho and oblique views, through secure portals. The service will be very affordable because imagery is accessed only when and where needed. This should be a boon for emergency responders.
ERDAS demonstrated web services that reduce a series of complex image/spatial analysis functions to simple and intuitive user tools and products. This screen capture below is an example intended for use by a tank commander. In one operation this service combines slope data, image analysis vegetation data, and feature data to build “drive/no drive” zones. This is very similar to work many of you have done using ESRI’s Model Builder with Spatial Analyst, but the ERDAS system is faster and takes full advantage of their 30 years experience in image analysis. “Wiping” the image shows the drive/no drive areas over a topo map.
ERDAS Apollo – WPS in Action.
The one nagging concern I had that seemed to need more attention was emergency preparedness. When I addressed the topic with attendees and presenters they talked about how their IT people had remote backup systems for installation data and how backups occurred regularly. I have this little gut twitch that occurs when I hear that IT people are backing up my data. Please take this first hand experience from someone who almost lost 10 years of GIS data because of an administrative error.
In a previous assignment our agency had a catastrophic server farm failure that resulted in the total loss of our entire SDE database. Backup? There was no backup. Some IT personnel view GIS data, especially imagery, as a huge data set that doesn’t need to be backed up often, if at all. They don’t see GIS data in the same light as financial data and may treat it differently so, through a misunderstanding, the IT people were not backing up the SDE database.
Luckily, a year before the loss, we placed all our GIS data and imagery on portable hard drives as part of our portable emergency GIS we set up in support of regional disaster preparedness and mutual aid. One set of those hard drives was in our emergency GIS locker in our office; the other set was 25 miles away in a secure location (see my January 2008 Geointelligence Insider column, What Can You Do for Your Country). At the risk of sounding like a broken record, or for those of you born in the post-vinyl period, “a continuously looping file,” I say what many others have said: “back up your data.”
Stepping down from my soap box, I can say that this conference was an exceptional conference that was rich in information and not overwhelming like some of our mega-conferences. For the vendors many good leads were generated and for the participants a lot of useful information was presented.
England’s CSR plc and U.S.-based SiRF Technology Holdings, Inc., have completed their merger, ending years of speculation over what may become of SiRF, a pioneering maker of GPS receivers that had become financially troubled during the current economic downturn.
“In bringing together the combined capabilities and broad range of CSR and SiRF technologies and platforms, we have created a new force in the industry and a world class organization with the commercial, technical and operational scale to build on CSR and SiRF’s existing customer relationships and deliver the next generation of connectivity and location enabled products,” said Joep van Beurden, CSR CEO. “Our strategic goal is to address the existing and emerging needs of our combined customer base for connectivity and location technologies. The potential applications and benefits to the end user of connectivity plus location are only just starting to open up, and these exciting new opportunities will be driven by our unique combination of leading location technologies and connectivity solutions.”
SiRF co-founder Kanwar Chadha echoed those sentiments. “CSR and SiRF have a shared vision of using innovation to bring the benefits of wireless connectivity and location to mainstream consumers and enterprises and to enable new and exciting user experiences,” said Chadha, now a CSR board member and chief marketing officer. “We believe that through this merger, our customers and consumers will derive benefits from a much stronger player whose focus is on delivering best in class connectivity and location platforms.”
For CSR’s customers, the merger with SiRF means CSR’s Connectivity Centre products are augmented by GPS technologies, including assisted GPS (A-GPS), dead reckoning, and location centric platforms, the companies said. Meanwhile, SiRF’s customers will see enhancements to SiRF’s location platforms with CSR’s Connectivity Centre capabilities.
The enlarged CSR group will have its global headquarters in Cambridge, United Kingdom, with SiRF’s headquarters remaining in San Jose, California, which will also serve as CSR’s U.S. headquarters. The combined CSR group is now among the top 10 fabless semiconductor companies, with a combined customer list including six of the top seven handset manufacturers, the top five personal navigation device makers, the top two automotive telematics suppliers, and other auto and consumer electronics providers, CSR said.
CSR plc of Cambridge, UK, and SiRF Technology Holdings Inc., of San Jose, California, on June 26 completed the merger between SiRF and a wholly owned subsidiary of CSR. The merger resulted in “creating a provider of connectivity and location platforms and a company with the scale, technology, and strategy to enable its customers to address the exciting and emerging opportunities in mobile markets,” according to a company statement.
The company said that customers of the enlarged CSR group will be able to deliver new user experiences of connectivity and location technologies in a diverse range of devices such as mobile phones, personal navigation devices, in-car navigation and telematics systems, laptop and netbook PCs, mobile internet devices, digital cameras, gaming machines, cellular accessories, and consumer electronic devices.
“In bringing together the combined capabilities and broad range of CSR and SiRF technologies and platforms, we have created a new force in the industry and a world-class organization with the commercial, technical and operational scale to build on CSR and SiRF’s existing customer relationships and deliver the next generation of connectivity and location enabled products,” said Joep van Beurden, CEO of CSR. “Our strategic goal is to address the existing and emerging needs of our combined customer base for connectivity and location technologies. The potential applications and benefits to the end user of connectivity plus location are only just starting to open up, and these exciting new opportunities will be driven by our unique combination of leading location technologies and connectivity solutions.”
“CSR and SiRF have a shared vision of using innovation to bring the benefits of wireless connectivity and location to mainstream consumers and enterprises and to enable new and exciting user experiences”, said Kanwar Chadha, co-founder of SiRF and newly appointed board member and chief marketing officer of CSR. “We believe that through this merger, our customers and consumers will derive benefits from a much stronger player whose focus is on delivering best in class connectivity and location platforms.”
“Technology innovation represents the foundation for both CSR’s and SiRF’s success in the market place,” said James Collier, co-founder, board member and Chief Technology Officer of CSR. “We look forward to combining the complementary expertise of our teams to take innovation to the next level in our multifunction radio and system platforms to address emerging customer and market needs.”
For CSR’s customers, the merger with SiRF means CSR’s Connectivity Centre products are augmented by GPS technologies that are well respected and enjoy widespread adoption, the company said, while SiRF brings to CSR a strong IP portfolio in GPS and assisted GPS (A-GPS), dead reckoning, and location centric platforms. The enlarged CSR group will have its global headquarters in Cambridge, UK, with SiRF’s headquarters in San Jose becoming CSR’s U.S. headquarters.
By Richard Langleuy, with an additional note by Oliver Montenbruck
The GPS Wing and its contractors have traced the cause of pseudorange errors on L1 and L2 broadcast by the newest GPS satellite, SVN-49, to the manner in which the L5 signal demonstration payload was added to the satellite. Signal leakage between the two input ports of the antenna coupler network for the satellite’s array of 12 helical antenna elements, reflected from the L5 filter and then transmitted, creates a second signal with a delay of approximately 30 nanoseconds, and the appearance of a multipath component.
While testing an adjustment to the signal-in-space to minimize the effect of the problem on receiver navigation solutions on Earth, the GPS Wing is interested in hearing from manufacturers and the user community concerning the different impacts of SVN-49 signals on the wide range products and applications in operation, before reaching a final decision on what to do with the satellite prior to setting it healthy.
The seventh modernized GPS Block IIR satellite was launched on March 24, 2009. Called SVN-49, its sequence number in the long line of GPS satellites, or PRN01, after its pseudorandom noise code identifier, this satellite is special. In addition to the equipment required to transmit the legacy GPS C/A-code and P(Y)-code signals and the new civil L2C-code and military M-code signals on the standard L1 (1575.42 MHz) and L2 (1227.6 MHz) frequencies, SVN-49 carries an L5 demonstration payload. L5 is the new civil signal to be transmitted on 1176.45 MHz by Block IIF and succeeding generations of GPS satellites.
The demo payload was included to claim the frequency, which was allocated by the International Telecommunication Union before the August 26, 2009, deadline. The deadline had been imposed seven years earlier when the GPS Joint Program Office (the forerunner of the GPS Wing) applied for the frequency. The Block IIF program schedule had slipped a bit and as a safeguard (and one which eventually saved the day), the demo payload was developed and assigned to SVN-49.
Shortly after the L1/L2 system on SVN-49 was activated on March 28, it became clear that the satellite had a small problem. The pseudorange data obtained by U.S. Air Force Space Command’s 2nd Space Operations Squadron (2 SOPS) monitor stations had larger than normal errors. Typically, the errors have a random characteristic, with a mean of zero and a peak-to-peak variation of two meters or so. But the SVN-49 ionosphere-corrected errors reached a level of about four meters and when they were plotted against the elevation angle of the satellite as viewed at each monitor station, a clear trend emerged (see Figure 1).
FIGURE 1. Ionospheric-refraction-corrected SVN4-9 pseudorange residuals from data collected at 2 SOPS monitor stations (courtesy GPS Wing).
Although larger than normal, the errors still fell within the accuracy tolerances specified for GPS signals. Nevertheless, the anomalous behavior of SVN-49’s signals was a cause of concern, and the GPS Wing and its contractors mounted efforts to find the cause.
Payload Source. They traced the source of the problem to the manner in which the L5 demo payload was added to the satellite. To understand the problem, we need to examine how the L1 and L2 signals are transmitted by a GPS satellite.
A primary and defining characteristic of GPS signals is that the received signal power should be approximately the same at any location on the Earth’s surface within view of the satellite. In other words, we should receive about the same signal power when a GPS satellite is overhead (and closer to us) as when it is low on the horizon (and further away). Any major variation in signal level seen by a receiver is typically due to the gain pattern of the receiver’s antenna.
To achieve a uniform power density at the Earth’s surface, a GPS satellite uses an array of 12 helical antenna elements, with an inner ring of four elements and an outer ring of eight, fed by an antenna coupler network (see Figure 2). The L1 and L2 signals are fed into the coupler through one of its two input ports: port J1. The inner ring of elements transmits most of the L1 and L2 power from J1 with a broad pattern, while the outer ring transmits a sharper pattern but with a weaker signal and a different phase. The net effect of this arrangement is to reduce the radiated power from the inner ring as seen at high elevation angles and boost it for lower elevation angles thereby achieving an almost uniform power density.
FIGURE 2. L-band antenna element locations (courtesy GPS Wing).
The antenna coupler’s other input port, J2, is used on SVN-49 to feed the L5 signal to the antenna array after first passing through a filter and a 162-inch (411-centimeter) cable. Most of the power from J2 goes to the outer ring, with less going to the inner ring — the inverse of the power distribution from J1. This is why initial reports of L5 signal acquisition noted its high directivity with much weaker signals at low elevation angles compared with the L1 and L2 signals. But this behavior was expected.
Not expected was the effect of the L5 filter and its associated cable run on the L1 and L2 signals. It turns out that some of the L1 and L2 signal from J1 exits the J2 port, is reflected from the L5 filter, and then is transmitted from the J2 port with a delay of approximately 30 nanoseconds. With hindsight, the J1 to J2 signal leakage and reflection from the L5 filter should have been prevented.
On the ground, a receiver sees both the direct signal and the weaker reflected signal, which looks like a multipath component. The GPS Wing and its contractors have attempted to model the effect of the reflected signal on GPS receiver measurements. According to their models, if the direct and reflected L1 signals are in phase at the zenith, then a standard code-correlating receiver will measure a C/A-code pseudorange that is 1.62 meters too long. The error becomes smaller as the elevation angle drops, due to the drop in power level of the reflected signal, reaching zero at an elevation angle of about 42 degrees, corresponding to a null in the antenna pattern and then rising slightly as the elevation angle drops to zero (see Figure 3).
FIGURE 3. Model of the differences between the SVN-49 L1 delayed (multipath) and direct signals (courtesy GPS Wing).
P(Y), L2, and L2C. The same error behavior is expected for L1 P(Y)-code pseudoranges. Maximum L2 P(Y)-code pseudorange errors are modeled to be zero if the direct and reflected L2 signals are in quadrature, or to have maximum values of about plus 0.95 meters if the direct and reflected signals have the same phase, and minus 1.1 meters if they have the opposite phase. Ground tests should confirm which of the three possibilities describes the actual signals. The L2C signal is expected to behave in a similar manner to the L2 P(Y) signal.
If ionosphere-free pseudoranges are computed from the L1 and L2 pseudoranges, the maximum errors are predicted to be 4.14, 2.66, and 5.84 meters for the quadrature, in-phase, and opposite-phase L2 direct and reflected signal possibilities.
The models also predict an effect on carrier-phase measurements, but these are very much smaller: a maximum error of 6.8 millimeters on L1 and 4.8 millimeters on L2.
It is not possible to fully fix the problem. The GPS Wing and its contractors are looking at ways to minimize the effect of the problem on receiver navigation solutions. One
experiment under assessment is to adjust the broadcast navigation message ephemeris of the satellite by placing the antenna phase center about 152 meters above the actual position of the satellite, while compensating with a satellite clock offset. Such navigation message adjustments reduce the peak-to-peak variation of the error by about a half; they do not eliminate it.
Status Quo? Another possibility is to broadcast the signal as is, without attempts to compensate for the error. It would then be up to the user to determine how best to use the signals. Initial indications show that certain receivers with advanced multipath mitigation correlators can essentially filter out much of the multipath component (see Narrow Correlators Screen Error section below). Receivers with standard correlators could use the SNV-49 signals but assign a higher uncertainty to the measurements when they are combined with those from other satellites.
The GPS Wing is interested in hearing from manufacturers and the user community concerning the impact of SVN-49 signals on products and applications before coming to a final decision on what to do with the satellite before setting it healthy, and a briefing and interview process has begun to obtain that information. The decision is expect by mid-September.
— Richard B. Langley, University of New Brunswick
Narrow Correlators Screen Error
The typical variation of SVN-49 multipath errors over time is illustrated in Figure 4 for semi-codeless P(Y)-code measurements on the L1 and L2 frequency from a commercial test receiver near Munich, Germany. SVN-49 was visible for roughly 6 hours at this site and reached a peak elevation angle of 80 degrees. The errors are most pronounced on L1 where they vary between –0.5 meters near the horizon and +1 meter near the center of the pass. L2, in contrast, is notably less affected. Here, multipath errors caused by signal reflections in the satellite are well below 0.5 meters in amplitude and cannot be clearly distinguished from local multipath at the receiver site.
FIGURE 4. Typical SNV-49 multipath errors for semi-codeless P(Y)-code tracking on L1 (top) and L2 (bottom) from a conventional correlator (using JAVAD GNSS Triumph receivers.)
While the example shown in Figure 4 is representative for many receivers currently tracking the new GPS satellite, a few receivers are able to filter out the satellite multipath component due to the use of special multipath-mitigation techniques. While implementation details are mostly proprietary, it is commonly known that strobe or double-delta correlators can effectively counteract short-range multipath when using an extremely narrow correlator spacing. The effectiveness of such techniques is shown in Figure 5 for C/A-code and L2C-code tracking by the same test receiver. Obviously, multipath errors are well below the thermal noise in this case and the measurement errors can hardly be distinguished from those of other GPS satellites.
FIGURE 5. SVN49 multipath errors for C/A-code (top) and L2C-code (bottom) tracking using special multipath-mitigation techniques with 20-nanosecond correlator spacing (using JAVAD GNSS Triumph receivers.)
From a practical point of view, users will probably have to decide on their own whether to employ receivers with advanced multipath-mitigation capabilities, whether to apply elevation-angle-dependent measurement corrections (primarily for L1 code measurements), or whether to simply accept the moderate degradation of the SVN-49 measurements. In view of the wide variety of receivers in use and considering their varied applications, a unique solution to the SVN-49 problem is probably not feasible, and care should be taken before applying a priori “corrections” that might cause more harm than good.
(Editor’s Note: The data used to track the anomalies of SVN-49 were gathered using JAVAD GNSS Triumph receivers.)
In the April edition, an article titled “DAGR Extended” covered news from the Space & Missiles Center regarding the GPS Wing awarding a follow-on contract to Rockwell Collins to provide Defense Advanced GPS Receivers (DAGR).
At the end of the article appeared an “Unofficial Word,” which made derogatory and inaccurate remarks about the use of the DAGR.
We are disappointed that the staff of GPS World did not contact us for a response to the accusations made in the article. Had you contacted us, our response would have been the following:
The DAGR provides the only means for dismounted soldiers or special operators to obtain location information of sufficient accuracy, reliability, and integrity for targeting purposes. Our warfighters use the DAGR to call in close air support missions, which the DAGR delivers GPS-guided munitions with pinpoint accuracy through its Advanced Laser Range Finder and Fire Support functions. The DAGR also provides unique Gun Laying Azimuth Determination applications.
Use of a commercial GPS in these circumstances would entail significant risk that would be totally unacceptable. No other handheld GPS is authorized, nor should it be authorized, for use in military targeting operations.
In the combat theater, our soldiers and special operators are working in extremely difficult conditions — environmental conditions where the DAGR functions consistently and provides warfighters with the information they vitally need.
On April 30, we celebrated the delivery of the 300,000th DAGR, which is proven testimony to the utility and reliability of the product.
In the future, we’d appreciate an opportunity to respond firsthand.
— Robert Haag
Senior Director, Soldier Solutions
Rockwell Collins
Don Jewell, Military & Government Editor, replies:
I could not agree with you more. At the same time, I totally disagree with your comment that our remarks were “derogatory and inaccurate … about the use of the DAGR.”
The conclusions drawn in that “Unofficial Word” (not, by the way, written by me) came directly from several industry and government warfighter panels (many of them attended by Rockwell Collins), face-to-face interviews, letters, and a plethora of personal e-mails from warfighters over the last 24 months. The results were unanimous: the DAGR, according to our warfighters who have opted not to use it, is too big, too heavy, has limited battery life, a black-and-white screen, is basically obsolete, and has a very difficult, definitely not user-friendly interface. Our interviews and correspondence show that the DAGR, as a standalone device, has been replaced by various GPS handheld or wrist-mounted units, Garmin and Trimble primarily.
How can I then agree with your comments? Because your letter very carefully only defends the use of the DAGR as an embedded device. Indeed it has been our experience that the only warfighters that consistently give the DAGR high marks are the soldiers using the DAGR as an embedded device: those responsible for directing fire — bombs or artillery on target. In a recent interview session with more than 40 soldiers, only the soldier responsible for directing fire said that he used the DAGR in any capacity. He stated, “For directing fire I use my DAGR because it has the necessary interface for laser designators and communications to direct fire. Other than that, I depend on my Garmin, as does everyone else I know, for a personal GPS unit. The Captain uses the DAGR as an input to the Blue Force Tracking (BFT) system that stays in the Humvee or Stryker vehicle.”
As I, and many others, in many articles, have said all along, the DAGR as an embedded piece of equipment, with dual frequencies, encryption, and approved government interfaces, serves a necessary and critical function: supplying BFT information and directing fire. As you correctly point out, it is the only approved government PNT source for directing fire. That is a good thing; all information and interfaces needed for the direct-fire mission have been worked out and do not need to be duplicated. Warfighters directing fire use the DAGR because there is no alternative, but for every other purpose for which they rely upon handheld GPS equipment, the DAGR is found seriously wanting. Suffice it to say the design is more than 14 years old, and the unit was dated when first released.
I have spoken to several Rockwell Collins representatives about my concerns and those of the warfighters over the years, and usually they do not dispute the DAGRs’ shortcomings. However, recently I was shown a picture, by a senior Rockwell Collins representative, of a new Rockwell Collins government GPS unit that impressed me as much as a simple picture of a GPS unit can. I asked for more information and a unit to review and I am still waiting. My problem, and I say this in all sincerity, is not with Rockwell Collins, as I know you built the DAGR to outdated government specifications that were generated in the early 1990s; by Moore’s law that is more than seven generations old by today’s standards. My primary concern is the safety and welfare of our warfighters. I know you can do much better, but the antiquated and non-responsive government acquisition system has prevented you from making changes and updating the poorly designed user interface. Rockwell Collins makes tremendous radios and avionics, which I used successfully throughout my 30-year military career, except for PLGR and DAGR units, which I consistently found to be inferior.
I consider myself a sophisticated GPS user and have tested more than 80 individual GPS units from manufacturers around the globe, yet I find your equipment and interface totally confusing. So please help me. Send me the new proposed government equipment with the color screen, the new interface, and hopefully new capabilities, and I will gladly review it in the magazine.
Several of my articles have helped gain waivers from the U.S. government for official use of thousands of commercial and civilian GPS handheld units in theater, mostly military-hardened Trimble units. If you have a great new handheld unit, then please send me an example to review and I will do that. Maybe we can get official waivers to use it in theater. I sincerely hope that is the case.
The soldiers, sailors, and airmen of the U.S. military have voted by purchasing their own units or by obtaining waivers. Even the newest recruits, whose low salary qualifies them for state assistance and food stamps, spend their money on commercial GPS units. As a very distinguished friend and world-renowned GPS expert said recently in a public forum, “You may not know it, but there has been an unofficial competition among military users for GPS handheld units, and Garmin won.” You have delivered 300,000 DAGRs, but how many of those units are actually in use today as stand-alone devices?
GAO Report
In my opinion the “GAO Questions GPS Health” article in the June issue focuses too much on the IIF as the potential problem. The May 14–15 National Space-Based PNT Advisory Board meeting heard a presentation from the DoD on GPS issues and challenges. During the briefing, Brigadier General Hyten acknowleged (as asserted in the GAO report) that there are three somewhat equally scary risks: delay of IIF, delay of OCX contract award, and delay of GPS IIIA. In the GAO report, the real doomsday scenario (in the 2015–2017 time frame) was from a two-year slide on the GPS IIIA program. You should also be aware that the graphs in the GAO report don’t account for two mitigation tools the DoD has in reserve: retired satellite still in space that could be revived (there are three at the moment), and power management as a means to extend satellite life.
I’m less worried about the first graph in the report that shows a dip in the 2010 time frame than I am about the catastrophic dip in the second chart around the 2015 time frame. I think we have a good chance of having fired our silver bullets by that time and will be much more constrained with respect to available mitigations. It is good you are writing about this as it raises awareness of the issue which could aid in the development of a more robust risk mitigation plan before this becomes a crisis.
I have been somewhat troubled by the anti-IIF program bias in the overall dialog on the subject. I don’t have full visibility or historical knowledge of what all went wrong there; what I do know indicates there was plenty of culpability to go around between the contractor and the government. I am concerned that too much focus on publicly spanking IIF will detract from fixing the root causes of the dilemma we are in: the requirements development processes and acquisition programs applied to GPS are broken. That is exacerbated by a lack of stable policy with respect to the long-term strategy for GPS development and sustainment. There are definitely lessons to learn from the IIF experience. But the difficulties associated with that program should be seen for what they are: symptoms rather than the root cause.
In the litigious society that we have become, it is not surprising to see GPS as a regular fixture in many civil and criminal proceedings in our nation’s courts. A new and growing outlet for the legal profession, it has also engaged many of the older GPS pioneers who, instead of just retiring, have found a relatively lucrative way to spend their free time. They now form the cadre of GPS expert witnesses, without whom many of the cases involving positioning could not be settled equitably.
These brave individuals must of necessity remain nameless, because all have signed non-disclosure orders regarding the details of any case they may be or have been working on. Even the public record of adjudicated cases affords but a small peek into the activities of these unheralded witnesses. Most civil cases are settled before trial, often with confidential terms, and many criminal cases plead out, so there is little to find in a search of public records for cases involving significant aspects of GPS.
Civil matters usually fall into one of the following categories:
misuse or misappropriation of intellectual property (IP), for example, patent infringement;
liability for accidents; or
product liability for latent defects.
Criminal matters involve some sort of tracking of suspects or felons, or use of GPS for evidence of an alleged perpetrator’s location at the time of the crime. The use of GPS in these instances comes smack up against the public’s right to privacy. In some states, many of these cases are thrown out for lack of warrants allowing use of GPS tracking, while in other states warrants are not required. In 2007, the 7th Circuit U.S. Court of Appeals held that no warrant was required, as did a court in Wisconsin. But the New York State Court of Appeals found the opposite on a 4–3 vote. It is likely that the U.S. Supreme Court will have to determine if such warrantless tracking of suspects violates the Fourth Amendment to the Constitution.
Patents. Most IP cases involve patent disputes wherein the patent in question in some way uses GPS or is itself a GPS component. An application relating to mapping in a car or the way differential GPS is performed provide examples of the former, while a method for improved receiver signal-processing would be of the latter type. These lawsuits are very contentious because experts from each side will disagree on what to others might seem to be obvious. These experts must opine on the meaning of the claims in the patent, the validity of the patent, and the likelihood that the device in question actually infringes on the patent. The cases are expensive to litigate and take a long time to come to an end. Many are settled just before going to trial.
During the pre-trial process, the expert witness must conduct research, provide reports, and testify in depositions. Early on, the expert will testify before a federal judge at proceeding called a Markman hearing, wherein each side presents his interpretation of the words in the patent claims that are in dispute. It is up to the judge to decide what the words mean. Lawyers refer to this as claim construction and how the claims are “construed.” If the case does go to trial, the experts testify in open court, usually before a jury.
Navy versus Air Force. A civil case well known to me involved whether or not GPS receivers would perform during and after the week-number rollover (WNRO) that occurred in the summer of 1999. This case came about as an adjunct to the hysteria involving Y2K. But it was a real concern to the tracking company and its customers, who had deployed thousands of GPS receivers, some in high-risk areas. They had valuable cargo and people at risk if their GPS failed.
The tracking company asked the receiver manufacturer if the units would operate through and after WNRO. The receiver company really didn’t know and delayed answering long enough that the exasperated tracking company commissioned a U.S. Navy test facility to experiment with a GPS simulator and the receivers in question to see what would happen. In the meantime, the receiver company told the tracking company that the Air Force expected everything to go ahead normally, that is the uploads performed at the Master Control Station in Colorado would continue on the same routine during WNRO as it had in the past, namely at least daily uploads. The Air Force would not guarantee that it would happen that way because its specification allowed for uploads plus or minus three days from the end of the week. As such, the receiver company told the tracking company it couldn’t guarantee the upload would be timely, but not to worry.
The tests by the Navy showed that if the uploads was early or late, there would be adverse consequences. One version of the receiver would stop operating for several days after the upload, and another version would stop operating and never recover. As a result of these tests, the tracking company purchased replacements and then sued the receiver company for the costs, claiming a latent defect in their products. The jury ruled for the tracking company and ordered the receiver company to pay for the replacement receivers.
Crash Course. Another case involved a fatal accident caused by the crash of an automobile company’s test van into an open-structured, desert racing car. The test van had GPS onboard as it was performing experiments. The data showed the speed and location of the van up to the time of the collision, and that was enough to cause a settlement.
GPS has figured in countless cases of property incursions where GPS survey data has been used to prove exactly where one property begins and another ends.
Probably the most celebrated and precedent-setting cases occurred in 2001, when a driver sued a rental-car company because it levied a $450 surcharge when a concealed GPS unit indicated he was speeding while driving the rental car. The judge threw out the case because the rental company failed to disclose that it had hidden GPS unit in the car, and that it had no right to collect a fine for speeding as only a government entity could do so.
Several ongoing cases involve patent disputes about GPS applications and receiver designs, but all are subject to non-disclosure restrictions.
Suspect Tracking. In the criminal arena, a large number of cases involve GPS use to track suspects. That sort of data was used to help convict Laci Peterson’s husband of murder in a recent and celebrated California trial. Today, courts all over America are pondering whether the covert use of GPS tracking is an invasion of privacy and should require a warrant before police can use it.
Authorities use GPS quite openly to keep track of felons, child molesters, parolees, indicted suspects out on bail, people sentenced to home restraint, and so on. Supposedly, in these cases the person has already broken the law so their rights are abrogated. Or, they may have signed an agreement giving consent to such tracking in exchange for their conditional release.
In one instance, a paroled sex offender in Florida was rearrested when the tracking company informed the sheriff that he was not where he was supposed to be. After an examination of the data and with help from Google maps, it was determined that if the tracking company’s data was correct, the parolee had to be traveling at 90 miles per hour across a field where there was no road. He was released forthwith.
Law enforcement routinely uses GPS to locate stolen cars equipped with services such as OnStar.
In Malibu, California, two fishermen were stopped by fish and game deputies and charged with illegal taking of lobsters. The officers had photos and onboard GPS fixes to present in court. Unfortunately for the district attorney,
the wily defense claimed that since magnetic north had moved more than 100 meters since the maps that Fish and Game relied on were made, the maps were not accurate, and therefore the GPS data was inaccurate. The jury did not seem interested in science, the law, or the facts, and it acquitted the lead defendant. His partner chose to plead to a lesser charge and was fined, while the boat owner went free.
Market Outlook. It is highly likely that litigation regarding IP will grow as more companies profit from GPS technology, in many instances not knowing that someone holds a patent on which they could possibly be infringing. Criminal proceedings will increase as well, now that GPS tracking is relatively inexpensive for law enforcement to deploy. Meanwhile legislatures and high courts ponder how to deal with potential violations of privacy and the need for warrants.
LEN JACOBSON is a consultant to the GPS industry and has participated as an expert witness in many cases involving GPS. He is the author of the book GNSS Markets and Applications, published in 2007.
Using Microwaves and Laser Ranging for Precise Orbit Determination
By Erik Schönemann, Tim A. Springer, Michiel Otten, and Matthias Becker
Though Galileo’s GIOVE-A is a test satellite not necessarily ready for scientific use, orbit analyses with a reduced accuracy can help to identify weaknesses and suggest improvements. This month, the authors share work being carried out to precisely determine the orbit of GIOVE-A using SLR and microwave observations. This preliminary investigation will benefit the procedures to be implemented for the future Galileo constellation.
INNOVATION INSIGHTS by Richard Langley
WE USE THEM FOR LISTENING TO MUSIC, for routine surgeries, for making a point in a presentation, and even for hanging pictures straight. Of course, I’m talking about lasers. Invented in 1960, the laser (an acronym for light amplification by the stimulated emission of radiation) has become ubiquitous in modern society. Every CD and DVD player has one. Many printers use them. But lasers are also used in a wide range of industrial and scientific applications including determining the orbits of satellites through satellite laser ranging (SLR).
In the SLR technique, pulses of laser light from a ground reference station are directed at satellites equipped with an array of corner-cube retroreflectors, which direct the pulses back towards a collocated receiving telescope. By accurately measuring the two-way travel times of the pulses and knowing the location of the station and other operating parameters, the positions of the satellites can be determined. A network of SLR reference stations around the globe is used to monitor the orbits of satellites over time and their variations have been used by scientists to improve our knowledge of the Earth’s gravity field; to study the long term dynamics of the solid Earth, oceans, and atmosphere; and even to verify predictions of the General Theory of Relativity.
The first SLR measurements were obtained from the Beacon Explorer-B satellite, which was launched in October 1964. Since then, dozens of satellites equipped with corner-cube retroreflectors have been launched including a number of radio-navigation satellites. Every GLONASS satellite is equipped with retroreflectors and two GPS satellites have been equipped—SVN35/PRN05 and SVN36/PRN06. The COMPASS-M1 satellite in medium Earth orbit carries retroreflectors, as do both GIOVE-A and –B, the Galileo test satellites.
Precise orbit determination of radio-navigation satellites using SLR has the advantage of being unaffected by any onboard satellite electronics and associated signal biases. Radiometric observations of a satellite’s microwave signals, on the other hand, are influenced by the satellite’s clock, for example, and its effect must be estimated to obtain precise (and accurate) satellite orbits for navigation and positioning. Therefore, a comparison of SLR- and microwave-derived orbits can be very useful for studying the performance of the data measurement and orbit-determination processes of both techniques.
In this month’s column, we take a look at some work being carried out to precisely determine the orbit of the GIOVE-A test satellite using SLR and microwave observations. This preliminary investigation will benefit the procedures to be implemented for the future Galileo constellation.
“Innovation” is a regular column that features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, who welcomes your comments and topic i deas. To contact him, see the “Contributing Editors” section on page 6.
The navigation office of the European Space Operations Centre (ESOC) is engaged in various activities using observations of the Galileo test satellite, GIOVE-A (Galileo In-Orbit Validation Element-A), recorded at the Galileo Experimental Sensor Stations (GESS). The work includes the assessment of the quality and performance of GIOVE satellite observables and the testing and improvement of orbit-determination software. These activities support the long-term goal of advancing the scientific applications of the future Galileo constellation.
Since the launch of GIOVE-A on December 28, 2005, various tests have been carried out to analyze the quality of the new code (pseudorange) and carrier-phase observables derived from tracking the satellite’s microwave signals. All of these tests demonstrate the advantages of the new signal structure compared to that of legacy GPS signals. In general, the reduction of the noise by factor of 4-5 as well as a reduction of the code multipath by approximately a factor of 1.2 (GPS C1C versus GIOVE-A C1B/C1C) could be seen.
As the comparison of observations is done indirectly (GPS and GIOVE-A have different orbits) and the databases used for most analyses published up to now is sparse, a deeper analysis of the signal quality parameters seems appropriate, especially as data quality has a direct impact on the precision of orbit determination. Our analyses, presented in the first half of this article, are based on a broad base of data from most of the stations in the GESS network. Because of the difficulty in accessing the phase multipath directly, we first evaluated the signal strength and the code multipath, which gave the first hint of the multipath behavior. In order to compare GPS and GIOVE-A data directly, only data received from the same elevation angles and azimuths were used. Subsequently, we present an analysis of the phase residuals derived by precise point positioning.
The second part of this article focuses on the precise orbit determination or POD of the GIOVE-A spacecraft. The Navigation Package for Earth Observation Satellites (NAPEOS) software used at the ESOC Navigation Support Office allows microwave (radiometric) and satellite laser ranging (SLR) observations to be used either separately or together. The two methods are different due to different tracking networks and the different sensitivity of the observables to atmospheric effects and in their noise levels. We will present the orbit results focusing on internal orbit consistency checks and SLR validation of the microwave-based orbits.
Data Analysis
We first describe the procedures used for analyzing the microwave data followed by those used for the SLR data.
Microwave Analysis. For the GIOVE-A signal analysis and precise orbit determination we used the RINEX data from all of the GESS stations available from the GIOVE archiving facility (see TABLE 1). All stations are equipped with GPS/Galileo antennas, built by Space Engineering S.p.A. and Galileo Experimental Test Receivers (GETRs), built by Septentrio. The data, containing tracking data of all GPS satellites and the GIOVE-A satellite, is given in the RINEX 3.00 data format with a sampling interval of 1 second. To save on storage space for the long-term analyses, such as orbit determination, the RINEX data is decimated to 30-second samples and Hatanaka-compressed, using a test version of the Hatanaka software for the RINEX 3.00 format.
The signal analyses shown here were carried out using GNU Octave, an open-source program for performing numerical computations similar to Matlab, and different scripts developed by the Institut für Physikalische Geodäsie at the Technische Universität Darmstadt. These analyses cover a selection of the designated Galileo signals recorded by the GESS within the time span from December 16 to 27, 2006. Within this time period, the current GPS signals, as well as the GIOVE-A signals E1 and E5, shown in TABLE 2, were recorded. The table also shows the signal components as well as the RINEX observation-type identifiers, which we use in this article.
The stations used for the analyses show a quite similar level of performance in general. There are stations with different behaviors for single signals, as for example GIEN with a stronger code multipath behavior on C1B and C1A, but no station with a considerably different performance level could be identified. The averaging over the data from all sites reduces the station-dependent effects such as multipath and the atmosphere to a large extent, and gives a good indication of the mean signal performance.
The analyzed phase residuals were taken from the processing carried out for the second part of this article. Hence, they include observation data over an extended period of 149 days and were limited to the GIOVE-A C1C/L1C and C7Q/L7Q signals.
This extended data period is from December 12, 2006 (day of year 346), until May 26, 2007 (day of year 146). During this interval, there is a period where no GIOVE-A data was available due to maintenance of the spacecraft. This gap occurred from February 12 to 28, 2007. So in total we have analyzed 149 days of microwave data. Because there are some differences between the results before and after this gap in February, many of the statistics are given for the first and second part separately. The first part covers December 12, 2006, until February 11, 2007; the second part covers March 1, 2007, until May 26, 2007.
We performed the precise orbit determination using the NAPEOS software, a general-purpose software package for orbit determination, prediction, and control, supporting all phases of an Earth-observation mission in terms of mission preparation and operations.
For the GIOVE-A analysis, the three main NAPEOS programs we used are GnssObs, Bahn, and Multiarc. GnssObs reads, cleans, and decimates the RINEX data and converts the data into the NAPEOS internal tracking-data format. The NAPEOS tracking-data format contains the ionosphere-free linear combination, for both code and phase, of the RINEX observations. For GPS, the ionosphere-free linear combination is based on the combination of C1P and C2P code and L1P and L2P phase measurements. GIOVE-A offers several different observables allowing for many different ionosphere-free observations. For most of the work presented in this article, we have used the ionosphere-free linear combination of the C1C and C7Q and L1C and L7Q observations for code and phase respectively.
The next module, Bahn, performs the parameter estimation. In this step, we use the ionosphere-free code and phase observations at a sampling interval of 5 minutes, and we have applied an elevation angle cut-off of 5 degrees. The data is processed in batches of 24 hours, thus resulting in 1-day-arc solutions. The estimated parameters in these daily solutions are the GIOVE-A state vector (position and velocity), five dynamical orbit parameters from the extended Center for Orbit Determination in Europe (CODE) orbit model, a GIOVE-A clock offset for each epoch, all receiver clock offsets for each epoch, one GPS-GIOVE-A “intersystem bias” parameter per day for each station except for a selected reference station, and the carrier-phase ambiguities (integers not resolved). The station coordinates are estimated but tightly constrained (1 millimeter) to their a priori value. We obtained the a priori station coordinates by combining the full set of daily solutions.
Despite the fact that the 13 GESS stations provide very good global coverage, it is expected that 24-hour solutions will not give the most precise GIOVE-A orbit estimates. To generate longer arc solutions, we have used the Multiarc program. This is a tool that has recently been added to the NAPEOS software package. It allows for a rigorous combination of normal equations, also referred to as normal equation stacking, which are generated by Bahn. During the normal equation combination, the satellite orbit parameters may also be rigorously combined, thus effectively leading to multi-day orbital arcs. For the work presented in this article, we have used Multiarc to generate solutions with arc lengths of 1, 2, 3, 4, and 5 days. We also used Multiarc to compute accurate a priori station coordinates by stacking all available 1-day normal equations.
Satellite Laser Ranging
Besides the 13 GESS stations, GIOVE-A is also tracked by more than 17 different SLR stations around the world. For most periods of the mission, the tracking has been consistent enough to allow for GIOVE-A POD using only the SLR data. As the SLR data is completely independent of the microwave data, the resulting orbit solutions will be to a large extent independent as well and thus can be used to give an indication of the achieved precision of the different microwave solutions.
The orbit determination strategy used for the SLR solutions is very similar to the one used for the microwave orbits with the main difference being the increased arc-length of 7 days. The same satellite parameters are estimated as with the microwave solutions: the GIOVE-A state vector and five dynamical orbit parameters from the extended CODE orbit model. No further parameters need to be estimated and all corrections applied to the SLR data are according to the International Earth Rotation and Reference Systems Service 2003 standards and, for station coordinates, we used those from the rescaled International Terrestrial Reference Frame 2005 solution. As the noise level of the SLR data is very low, the measurements can also be directly used to give an indication of the precision of the radial position components of the different microwave solutions by computing the SLR residuals without using them in the estimation process itself.
Combined Microwave and SLR Analysis. In this step, the SLR data was added to the microwave data in the 24-hour solutions. For the data weighting, we used 100 millimeters for SLR and 1000 millimeters and 10 millimeters for GIOVE-A and GPS code and phase observables respectively. The only change in the analysis strategy in this case was that we now processed the SLR data in 24-hour solutions and not in 7-day batches. All the processing options remained as described in the two previous sections. The resulting 1-day solutions, or rather the associated normal equations, were used in Multiarc to generate combined solutions of different arc lengths.
Microwave Data Quality
We now take a detailed look at the quality of the microwave data in terms of signal-to-noise ratio (SNR), code-tracking noise and multipath, carrier-phase-tracking noise, and carrier-phase residuals.
Signal-to-Noise Ratio. The SNR (or equivalently carrier-to-noise-density ratio, C/N0) is strongly dependent on the satellite transmitter, the signal path through the atmosphere, and the receiver configuration (ground station, antenna, receiver, cable, etc.). Hence the SNR cannot be seen as an absolute value. The SNR is specific to the position, the equipment, and the time. Furthermore, the determination of the SNR values depends on the receiver and the firmware used. As a result, SNR values from different receivers cannot be readily compared. Nevertheless, using only one type of receiver, assuming similar effects on all the different signals at the same epoch, and taking averages over a long time span, we expect the relationships among the signals to be constant. Based on this assumption, we can use the SNR values given in the GESS RINEX files without adjustment.
To compare the GPS with the GIOVE-A SNR values, we ordered the corresponding SNR values of all stations on all days by satellite position into a grid with widths of 5 degrees in azimuth and 5 degrees in elevation angle. For the evaluation, we took the grid cells occupied by both GPS and GIOVE-A values and computed the median over all the cells of equal elevation angle. The median per elevation-angle bin for each signal is shown in FIGURE 1.
FIGURE 1. Signal-to-noise ratio, GPS versus GIOVE-A
As can be seen from the figure, the signal strength of the GIOVE-A C8Q observable ranks best, followed by the GPS C1C, GIOVE-A C7Q, C5I/C5Q, C1A, and C1B/C1C. The weakest signal is found for the GPS C1P/C2P observable, with a maximum signal strength of 40 (receiver-dependent unit, approximately dB-Hz) at the zenith. Comparing the GPS open signals versus GIOVE-A, GPS C1C is considerably stronger than the GIOVE C1B/C1C. According to the GPS and Galileo interface control documents, GIOVE-A C1B/C1A should show up with a stronger signal strength than GPS C1C. The power levels guaranteed on the Earth’s surface are -160 dBW for GPS and -158 dBW for the future Galileo satellite signals except for the BOC(10,5) and BOC(n,m) modeled signals, for which a power level of even -155dBW is guaranteed. But looking at the SNR values shown in Figure 1, we see that the GIOVE-A C1B/C1C is worse by approximately 4 dB than the GPS C1C. But keeping in mind that GIOVE-A is an experimental satellite, an increase of the signal power for the future operational Galileo satellites should improve the signal performance above that shown in this article.
Code-Tracking Noise. For signals containing data and pilot components, as in the case of those from GIOVE-A, the code-tracking noise can easily be computed as the difference between the data and the pilot signal. The advantage of this computation scheme is that both signals are influenced by identical error sources (atmospheric errors, multipath errors, receiver errors, etc.). Based on the assumption of equal uncertainties in the two components, we divided the resulting noise values by the square root of two to specify the noise level of each part according to the laws of error propagation. TABLE 3 shows the code-tracking noise for the two analyzed GIOVE-A codes sorted by elevation angle. The median code-tracking noise is 0.62 meters for C1B/C1C and 0.35 meters for C5I/C5Q, for observations below an elevation angle of 5 degrees. For the C1B and C1C code measurements, the noise median stays below 0.2 meters for an elevation angle above 25 degrees, whereas the median for the C5I and C5Q code measurements for elevation angles above 35 degrees even comes down below 0.1 meters. The results discussed above are consistent with the code-tracking noise values published previously.
Code Multipath. We computed the relative code multipath effects as code minus phase differences assuming the amplitude of phase multipath to be insignificant compared to the amplitude of the code multipath. Ionospheric effects were taken into account by using the phase measurements on two frequencies in the usual way:
In this equation, CMPx is the estimate of the multipath error on the code, Px and Lx are the code and phase measurements of the same frequency, while Ly is the phase measurement used to correct the frequency-dependent ionospheric effect. The constant, , describes the relationship of the ionospheric behavior for the two frequencies.
In order to compare the code multipath level of GPS versus GIOVE-A, we sorted the multipath values using a grid covering the sky with widths of 5 degrees for both elevation angle and azimuth as before. FIGURE 2 shows the median standard deviation of the code multipath values, derived in each grid cell per day and station, versus the elevation angle. No significant difference between GPS C1C and GIOVE-A C1B and C1C, the open code signals on G1/E1, could be found. The code multipath behavior of the GPS precise codes are comparable with the GIOVE-A C5I, C5Q, and C7Q, whereas the C8Q shows the least code multipath effects closely followed by the GIOVE-A C1A, the public regulated service signal.
FIGURE 2. Code multipath, GPS versus GIOVE-A
Carrier-Phase-Tracking Noise Analyses. In the same manner as that carried out with the code, we computed the GIOVE-A carrier-phase-tracking noise as the difference of the two components (pilot minus data). To accommodate the effect of error propagation, the resulting errors were divided by the square root of two. The resulting phase-tracking noise values were sorted by elevation angle and can be found in TABLE 4.
In conformity with the theory that the phase-tracking noise is independent of the modulation scheme, both signals (L1B/L1C and L5I/L5Q) show the same results in units of cycles. Looking at the results in units of distance, GIOVE-A L1B/L1C shows up with a mean phase noise of 0.7 millimeters and L5I/L5Q with 0.9 millimeters. These values confirm those of previous studies.
Carrier-Phase Residuals. Phase residuals contain the phase tracking noise, multipath, as well as all unmodeled remaining errors such as antenna calibration inaccuracy and tropospheric effects. The magnitude of the residuals can be seen as an indicator for the observation and model accuracy as well as for measurement quality.
The following analyses are based on the ionosphere-free linear combination (GPS L1C/L2P, GIOVE-A L1C/L7Q), computed with NAPEOS. The analyses include data of the 13 GESS over a period of 149 days.
To compare the GPS and GIOVE-A residuals, we sorted them into a grid with a width of one degree in both satellite azimuth and elevation angle. Only data in overlapping grid locations were compared to make sure the data was affected in a similar way by multipath or other disturbances.
To properly interpret the results, we should mention that for GIOVE-A, 0.06 percent of the ambiguities (2501) were not fixed correctly whereas for GPS all ambiguities were fixed correctly. Looking at the GIOVE-A observations that were correctly fixed, we find a significantly larger number of rejected observations. The number of rejected observations is less by one third for GPS (6 percent) as for the GIOVE-A (9 percent) data.
Due to the small number of GIOVE-A observations for elevation angles above 86 degrees, the outlier-cleaned mean as well as the standard deviation at this elevation-angle range are not meaningful. For all elevation angles, GIOVE-A residuals show a lower standard deviation than GPS, indicating a superior performance of GIOVE-A signals.
Phase and Code Validation in Processing. Looking at the quality of the code and phase measurements on the different signals, it is conspicuous that GIOVE-A C1A/L1A and C8Q/L8Q rank best, whereas for the current processing of GIOVE-A data, usually the C1C and C7Q signals are used. This leads to the question of which is the best signal combination for GIOVE-A. Hence, we processed 10 days of GIOVE-A data, using different signal combinations. Presently the processing of the C8Q/L8Q signals is not yet implemented in NAPEOS. However, we were able to process the GIOVE-A C1A/L1A – C7Q/L7Q combination. The root-mean-square (RMS) of the code results were reduced by a factor of approximately 1.4 using L1A/C1A compared to L1C/C1C, whereas the RMS of the phase observations showed only a minor improvement. Furthermore, there is a higher number of rejected observations with L1A/C1A. Further analyses have to be carried out to evaluate the potential benefits of the different signal combinations.
Orbit Quality
In this section, we assess the quality of our precise orbit determination solutions. We have three sets of different orbit solutions. Set 1 is made up of the 7-day solutions based solely on SLR observations. Set 2 consists of the solutions based on the microwave observations using 1- to 5-day arcs. Set 3 consists of the solutions based on a joint analysis of the microwave and SLR observations also using 1- to 5-day arcs.
First, we assess the orbit quality by looking at the internal consistency of the solutions. For the two sets using microwave observations, the internal orbit consistency is done using an orbit fit. This will not tell us much about the absolute quality of the solutions but it will indicate the optimal arc length and whether adding the SLR observations to the microwave data improves the orbit estimates.
Secondly, we validate the orbits by determining the SLR residuals. Of course, the solutions that used SLR observations should perform better than the microwave-only solutions. However, the validation of the microwave orbits against the SLR observations will give us a good impression of the absolute accuracy of our orbits.
As a third test, we compare the best orbit (best arc length) of each of the three sets (set 1 only has one arc length) against each other. This should give us another indication of the quality of the orbits.
Internal Orbit Consistency. To determine the internal orbit consistency of the different solutions we make an orbit fit. For this orbit fit test, we used the middle 24 hours of two consecutive solutions and fit one 48-hour arc through these two parts. The satellite orbit was modeled by estimating the satellite state vector and all nine parameters of the extended CODE orbit model. The RMS of this fit gives us an indication of the internal consistency of the orbit estimates. For longer arcs, the RMS of fit should go down because the solutions are not fully independent of each other. So a lower RMS for the longer arc solutions is expected. On the other hand, this means that if the RMS does not go down with increasing arc length that we have reached the limit of our modeling capabilities. Furthermore, comparing the internal orbit consistencies of equal length solutions will tell us which solution has a better internal consistency. The results of this internal orbit consistency check are given in TABLE 5. The table gives the mean of the 2-day RMS over all processed days. The mean is given separately for the first and second part of the observation interval (see above) and also for the total observation interval.
Table 5 shows several interesting results. First of all, it shows that the results of part 2 of the observation interval are significantly better than the results from part 1. The reason for this is unclear since the statistics from the 1-day solutions, such as the residual RMS and number of observations, did not change significantly after the observation gap. The improvement, however, is very significant. The second observation is that the results including the SLR data are significantly better compared to those using only the microwave data. This is true for all arc lengths! As expected, we see a significant improvement of the internal consistency when going from 1-day arcs to 3-day arcs. The 4-day arcs show only a slight improvement compared to the 3-day arcs. The 5-day arcs do not show a significant improvement. This indicates that with the current observations and modeling techniques, the optimal arc length for precise orbit determination seems to be around 3 to 4 days.
SLR Validation. In this section, we look at the SLR residuals obtained from the different orbit solutions. We generated a clean SLR dataset by using the SLR-only orbit to remove any outliers in the SLR observations. The total number of valid SLR normal points for the entire period is 3520 observations from 17 different SLR stations. (A normal point is an average of a number of individual laser returns.) The number of observations for the first part of the observation period is 796 points from 12 stations and for the second part, there were 2724 normal points from 17 stations. For two of the three solutions, the SLR data has been used in the orbit determination process so the residuals will give a too-optimistic indication of the orbit quality.
As can be seen from TABLE 6, the 3-day solution based on the microwave-only data has the lowest SLR residuals and indicates a radial precision of around 100 millimeters. A similar behavior can be seen in the microwave plus SLR solution with the exception of the 1-day solution (and to a smaller extent also the 2-day solution) where the orbit solution is mainly driven by the SLR data, but the quality as can be seen from the internal consistency of the orbit is poor. Interestingly, there is a large improvement in SLR residuals for the microwave plus SLR solution, although the number of SLR data points is only 2 percent of the total tracking data in the combined solution. The values for the SLR-only solution are included in the table to give an indication of the lowest possible SLR residuals one could expect by combining the microwave and SLR data.
Orbit Comparison. To get an indication of the overall orbit quality, the best solutions were compared against each other for the second period of observation. TABLE 7 gives the RMS differences between the SLR only (SLR), 3-day microwave only (micro), and the 3-day microwave and SLR solution (micro+SLR) for the radial, along-track, and cross-track position components as well as the norm (3D).
As expected, the largest difference is between the SLR-only and microwave-only solutions giving a total (norm) orbit difference of 652 millimeters. As a major part of the SLR tracking from GIOVE-A comes from European stations, the quality of the SLR solutions is directly correlated with the ability of the European stations to track GIOVE-A. Bad weather over Europe can lead to data gaps for more than 24 hours, affecting the orbit quality. It is interesting to see the large impact the SLR data has on the combined solution. As mentioned before, the SLR data is only around 2 percent of the total tracking data but has a significant impact on the orbit solution as can be seen from the difference between the microwave-only and microwave-plus-SLR solution.
Based on the analysis presented above, we conclude that the 3-day solution using both microwave and SLR observations has provided the best orbit estimates.
Conclusion
The analyses of the observation data quality (signal quality) confirmed the good results from prior analyses for code multipath behavior and code noise. GPS C1C and the GIOVE-A C1B/C1C show a comparable multipath behavior, whereas the GPS precise codes C1P/C2P are comparable to the GIOVE-A C5I, C5Q, and C7Q. The least code multipath behavior could be found for GIOVE-A C8Q observable, closely followed by the GIOVE-A C1A. Based on this, the combination C1A/L1A – C8Q/L8Q should show the best noise behavior within the data processing scheme.
The results given in this article demonstrate that the 13-station GESS network allows us to determine the orbit of the GIOVE-A satellite quite accurately (~200 millimeters) using only microwave observations. The SLR validation of the microwave orbits gives an RMS of 100 millimeters (one-way range RMS). This result gives an absolute value for the orbital error. Of course, the SLR observations mainly tell us something about the radial orbit errors; the along- and cross-track errors could be much higher. To obtain accurate GIOVE-A orbit estimates, we need to keep the orbits and clocks of the GPS satellites, tracked simultaneously with the GIOVE-A satellite, fixed using the International GNSS Service (IGS) final orbit and clock products. Furthermore, an arc length of 3 days should be used. The microwave-based orbit estimates may be improved by adding the available SLR observations into the orbit-estimation process. Although there are relatively few SLR observations, they do have a significant positive effect on the orbit estimates, improving the internal consistency from 52 to 41 millimeters. Also, the validation of the orbits using the SLR observations shows a significant improvement. However, this is not an independent validation because the same SLR observations were used in the orbit determination.
The results presented in this article, even though based on observations from the GIOVE-A test satellite, can be considered as a first attempt towards establishing an optimal data processing approach for the future Galileo satellite constellation.
Acknowledgments
This article is based on the paper “GIOVE-A Precise Orbit Determination from Microwave and Satellite Laser Ranging Data – First Perspectives for the Galileo Constellation and Its Scientific Use” presented at the 1st Colloquium on the Scientific and Fundamental Aspects of the Galileo Program, held in Toulouse, France, October 1-7, 2007.
ERIK SCHÖNEMANN studied geodesy at the Technische Universität Darmstadt (TUD), Germany, writing his diploma thesis at the University of New South Wales, Sydney, Australia. Since receiving his diploma from TUD in April 2005, he has been working for the Institute of Physical Geodesy at TUD on GNSS station calibration and validation and analyses of GIOVE-A and GIOVE-B data.
TIM SPRINGER received his Ph.D. in physics from the Astronomical Institute of the University of Berne (AIUB) in 1999. He has been a key person in the development of the Center for Orbit Determination in Europe, one of the IGS analysis centers, located at AIUB. Since 2004, he has been working for the Navigation Support Office (NSO) at the European Space Operations Centre (ESOC) of the European Space Agency (ESA) in Darmstadt. In this group, he has led the development of the new ESOC GNSS software, which is used for most GNSS activities at NSO including GIOVE-A and -B analyses.
MICHIEL OTTEN obtained a degree in aerospace engineering from Delft University of Technology in 2001. He has been working for ESOC’s NSO since 2002. His main role within NSO is the precise orbit determination of low Earth-orbiting satellites equipped for SLR, DORIS, and GPS tracking. He is also responsible for ESA’s International DORIS Service Analysis Centre activities.
MATTHIAS BECKER is a full professor of geodesy and director of the Institute of Physical Geodesy, TUD. He received his diploma and Ph.D. in geodesy from TUD in 1979 and 1984, respectively. He is responsible for research and teaching in the fields of physical geodesy and satellite geodesy.
FURTHER READING
• GIOVE-A
“Meet GIOVE-A: Galileo’s First Test Satellite” by E. Rooney, M. Unwin, A. Bradford, P. Davies, G. Gatti, V. Alpe, G. Mandorlo, and M. Malik in GPS World, Vol. 18, No. 5, May 2007, pp. 36–42.
“Galileo Signal Experimentation” by M. Hollreiser, M. Crisci, J.-M. Sleewaegen, J. Giraud, A. Simsky, D. Mertens, T. Burger, and M. Falcone in GPS World, Vol. 18, No. 5, May 2007, pp. 44-50.
• GIOVE Tracking Network
“GIOVE Mission Sensor Station Receiver Performance Characterization: Preliminary Results” by M. Crisci, M. Hollreiser, M. Falcone, M. Spelat, J. Giraud, and S. La Barbera in Proceedings of Navitec 2006, the 3rd ESA Workshop on Satellite Navigation User Equipment Technologies, Noordwijk, The Netherlands, December 11-13, 2006.
• GIOVE Tracking Performance
“Performance Assessment of Galileo Ranging Signals Transmitted by GSTB-V2 Satellites” by A. Simsky, J.-M. Sleewaegen, M. Hollreiser, and M. Crisci in Proceedings of ION GNSS 2006, the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation, Fort Worth, Texas, September 26-29, 2006, pp. 1547–1559.
“Code and Carrier Phase Tracking Performance of a Future Galileo RTK Receiver” by T. Pany, M. Irsigler, B. Eissfeller, and J. Winkel in Proceedings of ENC-GNSS 2002, the European Navigation Conference, Copenhagen, Denmark, May 27-30, 2002.
• Multipath Mitigation in Modernized GNSS
“Comparison of Multipath Mitigation Techniques with Consideration of Future Signal Structures” by M. Irsigler and B. Eissfeller in Proceedings of ION GPS/GNSS 2003, the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, September 9-12, 2003, pp. 2584–2592.
• GIOVE Orbit Determination
“Estimation and Prediction of the GIOVE Clocks” by I. Hidalgo, R. Píriz, A. Mozo, G. Tobias, P. Tavella, I. Sesia, G. Cerretto, P. Waller, F. González, and J. Hahn in Proceedings of the 40th Annual Precise Time and Time Interval (PTTI) Meeting, Reston, Virginia, December 1-4, 2008.
• Satellite Laser Ranging
“GIOVE’s Track: Satellite Laser-Ranging Campaigns” by M. Falcone, D. Navarro-Reyes, J. Hahn, M. Otten, R. Piriz, and M. Pearlman in GPS World, Vol. 17, No. 11, November 2006, pp. 34–37.
“The International Laser Ranging Service: Current Status and Future Developments” by W. Gurtner, R. Noomen, and M.R. Pearlman in Advances in Space Research, Vol. 36, No. 3, 2005, pp. 327–332 (doi:10.1016/j.asr.2004.12.012).
“Laser Ranging to GPS Satellites with Centimeter Accuracy” by J.J. Degnan and E.C. Pavlis in GPS World, Vol. 5, No. 9, September 1994, pp. 62–7.
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In my last issue, I proclaimed the start of GPS/GIS month, with a focus on the subject in three of my newsletters. This is the second in that series. The first column can be read here. Also, I’m hosting a webinar June 30 to discuss using GPS receivers and technology for GIS data collection. In my last newsletter I discussed the use of consumer GPS receivers for GIS data collection. Remember the analogy I used…a Volkswagen Beetle wasn’t designed to run in a Formula One race? This column is going to focus on the Formula One cars, not the Volkswagen Beetles. In other words, it will focus on the GPS receivers on the market that are designed for GIS data collection. I will refer to them as GPS/GIS receivers.
What differentiates a GPS/GIS receiver from any other GPS receiver?
The number-one differentiator is that GPS/GIS receivers are designed do a better job of optimizing tracking and accuracy in areas where GIS data collection is performed. The operative term is “are designed.” Specifically, engineers who designed GPS/GIS receivers do so with different design criteria than engineers who design consumer GPS receivers and even survey GPS receivers. For example, a GPS/GIS receiver must be designed to operate where GIS data is collected and with reasonable accuracy. On the other hand, consumer GPS receivers are designed to track in tough conditions, but at the expense of accuracy. Furthermore, survey GPS receivers hold accuracy as the number-one priority so they sacrifice the ability to track in many environments.
The following matrix illustrates my point (1 = Highest priority design consideration, 5 = Lowest priority design consideration):
There are thousands of designers of consumer GPS receivers (Garmin, TomTom, Magellan, etc.) and probably only 10 designers of GPS receivers for surveying (Trimble, Leica/NovAtel, Topcon, Magellan Professional, Septentrio, JAVAD GNSS, NavCom, etc.). There are even fewer designers of GPS/GIS receivers — less than 10 (Trimble, Magellan Professional, Topcon, Geneq, Sokkia, Hemisphere, JAVAD GNSS, ViaSat).
The market for GPS/GIS receivers is a complicated one. That’s the primary reason why there are only a few manufacturers. Here are some of the reasons why it is complex:
Users require a GPS receiver that will work effectively in many different and challenging environments such as under trees, in mountainous areas and near buildings. There is not one product on the market that will meet every user’s requirements.
Users have various needs for the type of GIS data collected. For example, some only need two or three attributes for a utility pole and others may need to collect dynamic line segments such as speed zones and road lane types.
There is not an effective way for manufacturers to distribute such products. The traditional survey instrument dealers (not all) are not typically trained or experienced in GPS/GIS technology. Since there is not an effective distribution channel, the alternative is to create a grass-roots distribution channel, which is very time-consuming.
There are many factors to consider when attempting to determine what sort of GPS/GIS data collection system best fits a user’s requirements. Here are some in order of priority:
Budget. One could argue that data collection requirements should be #1. Maybe, but that depends on what stage of planning you’re in. If you are in the budget planning phase and are able to influence it, then I agree that user requirements should be the first priority. However, the vast majority of people I encounter are given an established budget to work within. In that case, budget should be #1 because it’s a waste of time to consider solutions outside of the budget constraint.
Accuracy. When I ask a potential GPS/GIS user what their accuracy requirement is, the typical answer is “as accurate as I can get”. Of course, you can imagine the ensuing conversation…Me: Well, Ok, you can achieve results around a centimeter. Them: That’s great. A centimeter is perfect. Me: Ok, here are the cost and training requirements. Them: Wow, why is it so expensive??????? Me: There is a direct relationship between accuracy and cost. The more accurate you want, the more expensive it’s going to be. Them: Well, Ok, we reeeeally only need to be within about three feet. Me: Do you need elevation values within three feet? Them (now leery of the response to their answers): Will those cost more? Me: Yes, probably quite a bit more. Them: No, we don’t need elevations.
Data collection requirements. Essentially, consumer GPS receivers and survey GPS systems “think” in terms of points. More specifically, consumer GPS receivers operate in terms of waypoints and survey GPS systems operate in terms of point averaging.
Some of the more sophisticated survey GPS systems offer Field-to-Finish (F2F) capability whereas points are automatically connected to form a line back in the office such as with curbs and property lines.GIS data collection systems are different. GIS “sees” the world in one of three ways; points, lines (or polylines) and areas (or polygons). All have some level of database information attached. For example, a fire hydrant is a point on a map but there is also information in the GIS about that fire hydrant such as condition, last inspection date, etc. A parcel is a polygon on a map but there is also information in the GIS about that parcel such as ownership, tax id, etc.
Additionally, there are several methods to record all three.For example, a wetland biologist may be mapping the perimeter of a wetland area but wants to “take points” on certain habitat nests he/she sees while walking the perimeter. Some of the more powerful GIS data collection software is built so the biologist can temporarily suspend mapping the perimeter and be allowed to map the next site and resume mapping the perimeter when point recording is finished.
Using the proper data collection software that matches the user requirements can save a significant amount of time and energy.
Data collection conditions. This is the biggest “gotcha” for GPS/GIS receivers. A certain GPS receiver designed for GIS data collection may perform flawlessly in the open-sky and works perfectly well for uses such as agriculture or other open-sky environments. However, most uses consist of some or all work done in “less-than-ideal” GPS conditions. Tree canopy is the biggest culprit. In that scenario, receiver performance can differ significantly. Some won’t track at all in those environments and some will track very well, but accept excessively noisy satellite measurements (which significantly degrades accuracy). The best ones are designed with a keen balance of satellite tracking and accuracy – with settings the user can change depending on the environment.
Why are GPS/GIS receivers so much more expensive than consumer GPS receivers?
Part of the reason that consumer GPS receivers are adapted to GPS/GIS data collection is the significant difference in cost. A consumer GPS receive
r can be purchased for well under US$200. The entry level price for a GPS receiver with comparable accuracy, but with GIS data collection features is four times that. Furthermore, the entry level price for a GPS/GIS receiver capable of sub-meter accuracy is about $2,000.
There are several specific and justifiable reasons for the price difference, but suffice to say that significantly more design engineering, technical support and sales effort is involved with GPS/GIS receivers. Furthermore, the volume of GPS/GIS receivers is miniscule compared to consumer receivers. If there were tens of millions of GPS/GIS receivers manufactured and sold every year, the price would be under US$200 each. But the GIS market just isn’t that large. Therefore, GPS/GIS manufacturers have to charge more per unit to account for engineering, technical support and sales overhead.
Lastly, as mentioned above, there are not very many manufacturers of GPS/GIS receivers. Lack of competition usually results in higher prices to the end user.
What sources of GPS corrections are available?
Autonomous (no differential correction applied) GPS is pretty accurate these days…on the order of a few meters. For this reason, consumer GPS receiver manufacturers tend to leave out information on GPS corrections in their specifications. Their rationale is that consumers don’t really care as long as they can navigate effectively.
However, the GPS/GIS receiver market is much more concerned with accuracy. Therefore, some sort of GPS correction source is highly recommended and necessary to achieve the desired accuracy.
There are essentially two types of GPS corrections: real-time and post-processing.
Throughout the 1980s and 1990s, post-processing was the dominant method of correcting GPS data. Even then, 2-5 meter accuracy was the norm for GPS/GIS receivers after post-processing was applied. Sub-meter GPS technology (using GPS/GIS receivers) only became possible towards the end of the 1990’s. Users were accustomed to going through the post-processing exercise (downloading base station data, QAing post-processed data, etc.). At that time, the only option for using real-time corrections were commercial services such as OmniSTAR.
In the mid-1990s, the U.S. Coast Guard (USCG) established the DGPS system that broadcast real-time GPS corrections free of charge along the US coastlines and major waterways. The user only needed to purchase equipment (beacon receiver) to receive the signal. The success of that program lead to the U.S. Department of Transportation (DOT) to expand the program to cover inland regions that were out of the USCG domain. That was the GPS/GIS user’s first taste of free DGPS corrections…and they liked it because it eliminated the time-consuming (and sometimes painful) process of post-processing.
The break-out milestone for real-time corrections came in 2003 when the Federal Aviation Administration (FAA) declared the Wide Area Augmentation System (WAAS) operational. WAAS took real-time GPS corrections to another level of simplicity. Not only is WAAS free of charge to users, but unlike the USCG DGPS and commercial DGPS services, it’s broadcast on the same frequency as GPS. This means that no extra antenna or receiver is required to utilize the signal. Furthermore, it’s broadcast nation-wide in the US where ever the WAAS satellites are visible to the user. Due to the success of WAAS, several other regions in the world have deployed similar systems; EGNOS in Western Europe, MSAS in Japan/Korea and GAGAN in India.
Finally, in the early part of this decade, local networks of reference stations began springing up. These are called RTK Networks. While built primarily for users of survey GPS receivers who require cm-level accuracy, there is a growing population of GPS/GIS users who are connecting their GPS/GIS receivers to these networks to obtain GPS corrections. However, the costs can be expensive. Some network operators charge a fee to access their network and the user must also have a data subscription with a wireless provider (GSM or CDMA) which has a monthly fee associated with it — similar to a mobile phone.
The Future is Clear
The trend is clearly towards using real-time GPS corrections no matter which source is used. The time consumed by post-processing and the expense of maintaining software and training requirements adds too much overhead in most applications for organizations to consider it.Although not the dominate correction technology any longer, post-processing in the GPS/GIS segment still has a niche – the so-called “sub-foot” niche. While the majority of GIS applications are satisfied with “sub-meter” (or even 1-3 meter) accuracy, there are certain applications where “sub-foot” accuracy is required. With these receivers, the users must post-process against several reference stations or tie into an RTK Network.
Integrated “All-in-one” GPS/GIS receiver or separate stand-alone receiver?
In the GPS/GIS receiver market, there are clearly two types of systems. The “All-in-one” receivers have the GPS receiver, antenna and data collector built into a hand-held format. These are products such as the Trimble GeoXT/XH, Magellan Mobile Mapper CX/6 and Topcon GMS-2.
The “stand-alone” receivers are a “black box” which houses only the GPS receiver, GPS antenna and optionally a battery. Other devices such as PDAs, tablet computers and notebook computers receive GPS data from these stand-alone receivers typically via Bluetooth interface or cable connection. These are products such as the Trimble ProXT/XH, Geneq SX Blue, Sokkia GIR1600, Hemisphere A100 and Javad GISMore.
There are advantages and disadvantages to both.
“All-in-one” receivers house everything one needs in a single hand-held unit. The advantage is that the data collector, GPS receiver, antenna, battery system, etc. are all designed by one company to work together. On the other hand, designing all of these components into a single hand-held can make for a somewhat heavier unit. Also, PDA technology is evolving rapidly. “All-in-one” receivers aren’t updated nearly as fast as PDA technology so an “All-in-one” unit may have an out-dated operating system and/or processor if the design is a few years old.
“Stand-alone” receivers are separate receivers that send GPS data to a PDA, tablet computer or notebook computer via wireless Bluetooth or cable connectio
n. The advantage of these systems is flexibility. On one project, they can be interfaced to a PDA. On the next project, they can be interfaced to a notebook computer running different mapping software. They aren’t affected by the advancement of PDA, operating system or computer processor technology.
The Final Analysis — GPS/GIS Receivers for GIS Data Collection
There a myriad of GPS receiver technologies being used for GIS data collection. It’s a complex industry. Some receivers being used are purpose-built and others have been adapted from other industries like consumer GPS.
There is no magic formula to determine which GPS receiver will work best because it really depends on the user’s requirements and in GIS, the user requirement vary greatly. “Try before you buy” is the best advice to follow when going through the equipment/software selection process.
If you have time, I’m conducting a GPS/GIS receiver webinar on June 30 (next Tuesday) at 10:00 a.m. Pacific time. I will continue the discussion of GPS/GIS receiver selection. Register for the webinar here.
