An announcement on the JAVAD GNSS website states “On December 21, 2012, we have tracked E6 B/C signal from all launched Galileo satellites, using TRE-G3T-E E6-band capable receiver.
“The following graphs shows SNR and ‘code-minus-phase’ combination of svn #11 (sat #81 on graph), svn #12 (sat #82) , svn #19 (sat #89) and svn #20 (sat #90). C/A stands for E1, P2 for E5B, CL2 for E6, L5 for E5A.”
The announcement includes a link to a short article describing how these codes were found. The Galileo E6 codes have not been published by the European Space Agency.
Galileo IOV-3 Broadcasts E1, E5, E6 Signals; Russian SBAS Luch-5B in Orbital Slot; EGNOS and Galileo in Emergency Call, Road Tolling; Compass ICD Rumored
Galileo IOV-3 Broadcasts E1, E5, E6 Signals
By Oliver Montenbruck, German Space Operations Center and Richard B. Langley, University of New Brunswick
After reaching its final position, the Galileo IOV-3 satellite started transmitting its first ranging signals on December 1. Within three days, the various carriers (E1, E5, E6) and associated modulations were activated, and full in-orbit testing is now in progress. Anyone with commonly available GNSS receivers can presently access the open signals in the E1, E5a, and E5b frequency bands as well as the wide-band E5 AltBOC signal.
According to statements made at the recent 6th ESA Workshop on Satellite Navigation Technologies (Navitec 2012) in Noordwijk, The Netherlands, the IOV-3 satellite, which is also identified as Flight Model 3 (FM3) and E19 after its pseudorandom noise code, will continue to use binary offset carrier modulation — specifically BOC(1,1) — on the E1 Open Service signals for the time being. In contrast to this, the first pair of IOV satellites has already started to use composite binary offset carrier modulation, which offers better multipath suppression in the received signal.
Right after its activation, IOV-3 could be tracked immediately by the global network of stations participating in the Multi-GNSS Experiment (MGEX; http://www.igs.org/mgex) initiated by the International GNSS Service (IGS).
Figure 1. Pseudorange errors of IOV-3 tracking at Tanegashima, Japan, using the E1 BOC(1,1) signal (top) and the E5 AltBOC signal (center). The elevation angle over time is shown in the bottom panel.
The high quality of the IOV-3 signals is illustrated by measurements collected by the Tanegashima station during a 10-hour pass of the satellite over Japan (see Figure 1). The E5 AltBOC pseudorange measurements in particular exhibit an exceptionally low noise and multipath level of better than 10 centimeters at mid- and high-elevation angles.
An attractive feature of the Galileo system is the availability of multiple signal frequencies, which opens up numerous prospects for precise positioning and scientific investigations.
Carrier-Phase Measurements
While the E6 signals foreseen for a future Commercial Service are not presently supported by geodetic receivers due to the lack of information on the transmitted codes and possible licensing issues, users can already benefit from the E5a and E5b signals in addition to E1. By way of example, the ionosphere-free and geometry-free linear combination can be formed from carrier-phase measurements on these frequencies. Results of some first tests using this combination for IOV-3 are shown in Figure 2, based on measurements made at four MGEX stations: CUT0 (Perth, Australia), GMSD (Tanegashima, Japan), KZN2 (Kazan, Russia), and SIN1 (Singapore).
The results provide an indication of carrier-phase noise and multipath effects but are free of long-term variations that have earlier been found in GPS L1/L2/L5 signal combinations.
It is anticipated that similar measurement quality will be obtained with the E1 and E5 signals of IOV-4, which were activated on December 12 and 13.
This level of performance highlights the potential benefit of Galileo signals in advanced triple-frequency techniques such as undifferenced ambiguity resolution and ionospheric monitoring.
Figure 2 The difference between the ionosphere-free carrier-phase combinations formed from E1/E5a and E1/E5b signals received at four MGEX stations: CUT0 (Perth, Australia), GMSD (Tanegashima, Japan), KZN2 (Kazan, Russia), and SIN1 (Singapore).
Russian SBAS Luch-5B in Orbital Slot
The second Russian satellite-based augmentation system (SBAS) satellite, Luch-5B, has now been positioned at its designated orbital slot of 16 degrees west longitude. The satellite had been in a drift orbit since its launch on November 2 at 21:04:00 UTC along with the domestic communications satellite Yamal-300K.
NORAD/JSpOC tracking data showed Luch-5B arriving at its geostationary position by about December 13. Figure 3 shows the footprint of the satellite with the elevation-angle contours at 30-degree intervals.
Luch-5B, the second of a set of three geostationary satellites being launched to reactivate Roscosmos’s Luch Multifunctional Space Relay System, is expected to use PRN code 125.
The Luch system will relay communications and telemetry between low-Earth-orbiting spacecraft, such as the the Russian segment of International Space Station, and Russian ground facilities. The system’s satellites also carry transponders for the System for Differential Correction and Monitoring (SDCM), Russia’s SBAS. The transponders will broadcast GNSS corrections on the standard GPS L1 frequency.
Luch-5A, launched in December 2011, resides in an orbital slot at 95 degrees east longitude. It began transmitting corrections on July 12, 2012 using PRN code 140.
Figure 3. Geostationary position of Luch-5B, carrying a transponder for the Russian System for Differential Correction and Monitoring.
EGNOS and Galileo in Emergency Call, Road Tolling
The Intelligent Transport Systems (ITS) World Congress in Vienna this fall drew attention to the multi-constellation advantages provided by Galileo during a session on eCall, the European initiative for safer mobility. “Galileo will provide accuracy and reliability in all the transport markets, but in the case of emergency rapid assistance, the positioning need is even more critical,” said Fiammetta Diani, market development officer at the European GNSS Agency (GSA).
A multiconstellation approach for eCall and similar initiatives will deliver better performance without additional costs. Yaroslav Domaratsky from NIS-GLONASS, the Russian national navigation services provider, confirmed that ERA-GLONASS, the Russian version of eCall, will benefit from multiconstellation. “Solutions including also Galileo are welcome in the Russian initiative.”
Satellite ITS applications in road transport cover much more than in-car navigation. They include road-user charging with satellite-based toll collection systems; in-vehicle dynamic route guidance for drivers; intelligent speed adaptation to control the speed of vehicles externally; traveller information systems; and fleet-tracking systems for better management of freight movements and goods delivery.
Road Tolling
European road-toll operators outlined how they plan to emply the European Geostationary Navigation Overlay Service (EGNOS) and Galileo to provide new tolling solutions.
Luigi Giacalone, managing director of Autostrade Tech, which provides the technology for the French Ecomouv project, said EGNOS will contribute to reliably collect taxes on the heavy trucks using the road charging scheme. “This is a tax, not a toll. It aims to collect a new tax reliably and fairly according to distance travelled, while dissuading fraud,” he said. “Thanks to GNSS multi-constellation, only 10 locations out of the 15,000-kilometer network need support beacons.”
Ecomouv, which Includes anti-jamming and anti-spoofing mechanisms, covers 600,000 French lorries and 200,000 foreign ones, and will run from July 2013 for 11.5 years. Giacalone said its performance target was 99.75 percent accuracy of the entire collection chain, and its trials had already 99.8 percent accuracy.
Miroslav Bobošík from SkyToll, which operates Slovakia’s electronic tolling operations, explained how the system was able to cover not only 570 kilometers of motorways, but also 1,800 kilometers of first class roads in the country. “We needed a flexible system to cover different roads in different circumstances. And also to be fair to drivers, so they pay only for what they use,” said Bobošík. “We cover all services, not just toll collection, but enforcement, and technological maintenance and repair.”
GNSS tolling means flexibility as well as feasibility for SkyToll: since its launch in mid-2010, many changes have been made to the operation of the network, but thanks to the technology, they were easy to make. And they were cheap, he said. “While it is difficult to compare costs with other country, SkyToll has the lowest cost per kilometer to operate,” he said. “GNSS is the best possible solution for electronic tolling system in Slovakia, and GNSS is the most suitable for ITS.”
Changing the Game
Volker Vierroth from T-Systems, the German IT services subsidiary of Deutsche Telekom, explained GNSS’s game-changing role: the availability of a huge variety of additional data linked to actual positions; more computing power, notably mobile and cloud-based; fast and reliable networks available now with broad coverage, most recently with the shift from 3G to 4G; and smartphones, powerful and versatile, surging to the fore.
“GNSS [in the form of EGNOS] has proved to be a reliable technology for large-scale road charging on complex networks,” he said. “Galileo will bring further improvements, and may become the cornerstone of future road applications.”
Compass ICD Rumored
As this magazine goes to press, unconfirmed reports from Shanghai state that the Compass Interface Control Document (ICD) will be released on December 27.
Such rumors surfaced in late 2010 and again in late 2011. An October 2011 GPS World newsletter reported “The long-awaited signal ICD for China’s growing GNSS will appear this month, according to representatives of the system who spoke in a “Compass: Progress, Status, and Future Outlook” workshop in September [2011].
“The ICD has been rumored to be available previously to receiver manufacturers within China, creating some disgruntlement among companies outside the country. A workshop panelist affirmed that GPS/Compass chips and receivers are being actively developed by many Chinese manufacturers and research institutes.”
Based upon lessons learned from the LightSquared situation, the author identifies important considerations for GPS spectrum interference standards, recommended by the PNT EXCOM for future commercial proposals in bands adjacent to the RNSS band to avoid interference to GNSS.
On January 13, 2012, the U.S. National Positioning, Navigation, and Timing Executive Committee (PNT EXCOM) met in Washington, D.C., to discuss the latest round of testing of the radiofrequency compatibility between GPS and a terrestrial mobile broadband network proposed by LightSquared. The proposed network included base stations transmitting in the 1525 – 1559 MHz band and handsets transmitting in the 1626.5 – 1660.5 MHz band. These bands are adjacent to the 1559 – 1610 MHz radionavigation satellite service (RNSS) band used by GPS and other satellite navigation systems. Based upon the test results, the EXCOM unanimously concluded that “both LightSquared’s original and modified plans for its proposed mobile network would cause harmful interference to many GPS receivers,” and that further “there appear to be no practical solutions or mitigations” to allow the network to operate in the near-term without resulting in significant interference.
The LightSquared outcome was a lose-lose in the sense that billions were spent by the investors in LightSquared and, as noted by the EXCOM, “substantial federal resources have been expended and diverted from other programs in testing and analyzing LightSquared’s proposals.” To avoid a similar situation in the future, the EXCOM proposed the development of “GPS Spectrum interference standards that will help inform future proposals for non-space, commercial uses in the bands adjacent to the GPS signals and ensure that any such proposals are implemented without affecting existing and evolving uses of space-based PNT services.”
This article identifies and describes several important considerations in the development of GPS spectrum interference standards towards achieving the stated EXCOM goals. These include the identification of characteristics of adjacent band systems and an assessment of the susceptibility of all GPS receiver types towards interference in adjacent bands. Also of vital importance to protecting GPS receivers is an understanding of the user base, applications, and where the receivers for each application may be located while in use. This information, along with the selection of proper propagation models, allows one to establish transmission limits on new adjacent-band systems that will protect currently fielded GPS receivers. The article further comments on the implications of the evolution of GPS and foreign satellite navigation systems upon the development of efficacious spectrum interference standards.
Adjacent Band Characteristics
The type of adjacent-band system for which there is currently the greatest level of interest is a nationwide wireless fourth-generation (4G) terrestrial network to support the rapidly growing throughput demands of personal mobile devices. Such a nationwide network would likely consist of tens of thousands of base stations distributed throughout the United States and millions of mobile devices. The prevalent standard at the present time is Long Term Evolution (LTE), which is being deployed by all of the major U.S. carriers. LTE and Advanced LTE provide an efficient physical layer for mobile wireless services. Worldwide Interoperability for Microwave Access (WiMAX) is a competing wireless communication standard for 4G wireless that is a far-distant second in popularity.
For the purposes of the discussion within this article, an LTE network is assumed with characteristics similar to that proposed by LightSquared but perhaps with base stations and mobile devices that transmit upon different center frequencies and bandwidths. The primary characteristics include:
Tens of thousands of base stations nationwide, reusing frequencies in a cellular architecture, with the density of base stations peaking in urban areas.
Base-station antennas at heights from sub-meter to 150 meters above ground level (AGL), with a typical height of 20–30 meters AGL. Each base station site has 1–3 sector antennas mounted on a tower such that peak power is transmitted at a downtilt of 2–6 degrees below the local horizon, with a 60–70 degree horizontal 3-dB beamwidth and 8–9 degree vertical 3-dB beamwidth.
Peak effective isotropic radiated power (EIRP) in the vicinity of 20–40 dBW (100–10,000 W) per sector.
Mobile devices transmit at a peak EIRP of around 23 dBm (0.2 W), but substantially lower most of the time when lower power levels suffice to achieve a desired quality of service as determined using real-time power control techniques.
As LTE uses efficient transmission protocols, emissions can be accurately modeled as brickwall, that is, confined to a finite bandwidth around the carrier.
Throughout this article it will be presumed that LTE emissions in the bands authorized for RNSS systems such as GPS will be kept sufficiently low through regulatory means.
The opening photo shows a typical base-station tower, with three sectors per cellular service provider and with multiple service providers sharing space on the tower, including non-cellular fixed point microwave providers. As a cellular network is being built out, coverage is at first most important, and many base-station sites will use minimum downtilt and peak EIRPs within the ranges described above. As the network matures, capacity becomes more important. High-traffic cells are split through the introduction of more base stations, and this is commonly accompanied by increased downtilts and lower EIRPs.
The assumed characteristics for adjacent band systems plays a paramount role in determining compatibility with GPS, and obviously lower-power adjacent-band systems would be more compatible. If compatibility with GPS precludes 4G network implementation on certain underutilized frequencies adjacent to RNSS bands, then it may be prudent to refocus attention for these bands on alternative lower-power systems.
GPS Receiver Susceptibility
Over the past two years, millions of dollars have been expended to measure or analyze the susceptibility of GPS receivers to adjacent band interference as part of U.S. regulatory proceedings for LightSquared. Measurements were conducted through both radiated (see photo) and conducted tests at multiple facilities, as well as in a live-sky demonstration in Las Vegas. This section summarizes the findings for seven categories of GPS receivers. These categories, which were originally identified in the Federal Communications Commission (FCC)-mandated GPS-LightSquared Technical Working Group (TWG) formed in February 2011, are: aviation, cellular, general location/navigation, high-precision, timing, networks, and space-based receivers.
Aviation. Certified aviation GPS receivers are one of the few receiver types for which interference requirements exist. These requirements take the form of an interference mask (see Figure 1) that is included in both domestic and international standards. Certified aviation GPS receivers must meet all applicable performance requirements in the presence of interference levels up to those indicated in the mask as a function of center frequency. In Figure 1 and throughout this article, all interference levels are referred to the output of the GPS receiver passive-antenna element. Although the mask only spans 1500–1640 MHz, within applicable domestic and international standards the curves are defined to extend over the much wider range of frequencies from 1315 to 2000 MHz.
A handful of aviation GPS receivers were tested against LightSquared emissions in both conducted and radiated campaigns. The results indicated that these receivers are compliant with the mask with potentially some margin. However, the Federal Aviation Administration (FAA) noted the following significant limitations of the testing:
Not all receiver performance requirements were tested.
Only a limited number of certified receivers were tested, and even those tested were not tested with every combination of approved equipment (for example, receiver/antenna pairings).
Tests were not conducted in the environmental conditions that the equipment was certified to tolerate (for example, across the wide range of temperatures that an airborne active antenna experiences, and the extreme vibration profile that is experienced by avionics upon some aircraft).
Due to these limitations, the FAA focused attention upon the standards rather than the test results for LightSquared compatibility analyses, and these standards are also recommended for use in the development of national GPS interference standards. One finding from the measurements of aviation receivers that may be useful, however, is that the devices tested exhibited susceptibilities to out-of-band interference that were nearly constant as a function of interference bandwidth. This fact is useful since the out-of-band interference mask within aviation standards is only defined for continuous-wave (pure tone) interference, whereas LightSquared and other potential adjacent-band systems use signals with bandwidths of 5 MHz or greater.
Cellular. The TWG tested 41 cellular devices supplied by four U.S. carriers (AT&T, Sprint, US Cellular, and Verizon) against LightSquared emissions in the late spring/early summer of 2011. At least one of the 41 devices failed industry standards in the presence of a 5- or 10-MHz LTE signal centered at 1550 MHz at levels as low as –55 dBm, and at least one failed for a 10-MHz LTE signal centered at 1531 MHz at levels as low as –45 dBm. The worst performing cellular devices were either not production models or very old devices, and if the results for these devices are excluded, then the most susceptible device could tolerate a 10-MHz LTE signal centered at 1531 MHz at power levels of up to –30 dBm. Careful retesting took place in the fall of 2011, yielding a lower maximum susceptibility value of –27 dBm under the same conditions.
General Location/Navigation. The TWG effort tested 29 general location/navigation devices. In the presence of a pair of 10-MHz LTE signals centered at 1531 MHz and 1550 MHz, the most susceptible device experienced a 1-dB signal-to-noise ratio (SNR) degradation when each LTE signal was received at –58.9 dBm. In the presence of a single 10-MHz LTE signal centered at 1531 MHz, the most susceptible device experienced a 1-dB SNR degradation when the interfering signal was received at –33 dBm.
Much more extensive testing of the effects of a single LTE signal centered at 1531 MHz on general location/ navigation devices was conducted in the fall of 2011, evaluating 92 devices. The final report on this campaign noted that 69 of the 92 devices experienced a 1-dB SNR decrease or greater when “at an equivalent distance of greater than 100 meters from the LightSquared simulated tower.” Since the tower was modeled as transmitting an EIRP of 62 dBm, the 100-meter separation is equivalent to a received power level of around –14 dBm. The two most susceptible devices experienced 1-dB SNR degradations at received power levels less than –45 dBm.
High Precision, Timing, Networks. The early 2011 TWG campaign tested 44 high-precision and 13 timing receivers. 10 percent of the high-precision (timing) devices experienced a 1-dB or more SNR degradation in the presence of a 10-MHz LTE signal centered at 1550 MHz at a received power level of –81 dBm (–72 dBm). With the 10-MHz LTE signal centered at 1531 MHz, this level increased to –67 dBm (–39 dBm).
The reason that some high-precision GPS receivers are so sensitive to interference in the 1525–1559 MHz band is that they were built with wideband radiofrequency front-ends to intentionally process both GPS and mobile satellite service (MSS) signals. The latter signals provide differential GPS corrections supplied by commercial service providers that lease MSS satellite transponders, from companies including LightSquared.
Space. Two space-based receivers were tested for the TWG study. The first was a current-generation receiver, and the second a next-generation receiver under development. The two receivers experienced 1-dB C/A-code SNR degradation with total interference power levels of –59 dBm and –82 dBm in the presence of two 5-MHz LTE signals centered at 1528.5 MHz and 1552.7 MHz. For a single 10-MHz LTE signal centered at 1531 MHz, the levels corresponding to a 1-dB C/A-code SNR degradation increased to –13 dBm and –63 dBm. The next-generation receiver was more susceptible to adjacent-band interference because it was developed to “be reprogrammed in flight to different frequencies over the full range of GNSS and augmentation signals.”
Discussion. Although extensive amounts of data were produced, the LightSquared studies are insufficient by themselves for the development of GPS interference standards, since they only assessed the susceptibility of GPS receivers to interference at the specific carrier frequencies and with the specific bandwidths proposed by LightSquared. If GPS interference standards are to be developed for additional bands, then much more comprehensive measurements will be necessary.
Interestingly, NTIA in 1998 initiated a GPS receiver interference susceptibility study, funded by the Department of Defense (DoD) and conducted by DoD’s Joint Spectrum Center. One set of curves produced by the study is shown in Figure 2. This format would be a useful output of a further measurement campaign. The curves depict the interference levels needed to produce a 1-dB SNR degradation to one GPS device as the bandwidth and center frequency of the interference is varied. The NTIA curves only extended from GPS L1 (1575.42 MHz) ± 20 MHz. A much wider range would be needed to develop GPS interference standards as envisioned by the PNT EXCOM. It may be possible, to minimize testing, to exclude certain ranges of frequencies corresponding to bands that stakeholders agree are unlikely to be repurposed for new (for example, mobile broadband) systems.
Figure 2. Example of NTIA-initiated receiver susceptibility measurements from 1998.
Receiver-Transmitter Proximity
The LightSquared studies, with the exception of those focused on aviation and space applications, spent far less attention to receiver-transmitter proximity. Minimum separation distances and the associated geometry are obviously very important towards determining the maximum interference level that might be expected for a given LTE network (or other adjacent band system) laydown.
Within the TWG, the assumption generally made for other (non-aviation, non-space) GPS receiver categories was that they could see power levels that were measured in Las Vegas a couple of meters above the ground from a live LightSquared tower. Figure 3 shows one set of received power measurements from Las Vegas. In the figure, the dots are measured received power levels made by a test van. The top curve is a prediction of received power based upon the free-space path-loss model. The bottom curve is a prediction based upon the Walfisch-Ikegami line-of-sight (WILOS) propagation model. The NPEF studies presumed that the user could be within the boresight of a sector antenna even within small distances of the antenna (where the user would need to be at a significant height above ground).
Figure 3 Measurements of received power levels from one experimental LightSquared base station sector in Las Vegas live-sky testing.
The difference between the above received LTE signal power assumptions has been hotly debated, especially after LightSquared proposed limiting received power levels from the aggregate of all transmitting base stations as measured a couple of meters above the ground in areas accessible to a test vehicle. After summarizing the aviation scenarios developed by the FAA, this section highlights scenarios where so-called terrestrial GPS receivers can be at above-ground heights well over 2 meters. The importance of accurately understanding transmitter-receiver proximity is illustrated by Figure 4. This shows predicted received power levels for one LTE base station sector transmitting with an EIRP of 30 dBW and with an antenna height of 20 meters (65.6 feet). The figure was produced assuming the free-space path-loss model and a typical GPS patch-antenna gain pattern for the user. Note that maximum received power levels are very sensitive to the victim GPS receiver antenna height.
Figure 4. Received power in dBm at the output of a GPS patch antenna from one 30 dBW EIRP LTE base station sector at 20 meters.
Aviation. The first LightSquared-GPS study conducted for civil aviation was completed by the Radio Technical Commission for Aeronautic (RTCA) upon a request from the FAA. Due to the extremely short requested turnaround time (3 months), RTCA consciously decided not to devote any of the available time developing operational scenarios, but rather re-used scenarios that it had developed for earlier interference studies. It was later realized that the combination of five re-used scenarios and assumed LightSquared network characteristics did not result in an accurate identification of the most stressing real-world scenarios. For instance, within the RTCA report, base stations’ towers were all assumed to be 30 meters in height. At this height, towers could not be close to runway thresholds where aircraft are flying very low to the ground, because this situation would be precluded by obstacle clearance surfaces. Later studies used actual base-station locations, from which the aviation community became aware that cellular service providers do place base stations close to airports by utilizing lower base-station heights as necessary to keep the antenna structure just below obstacle clearance surfaces.
The FAA completed an assessment of LightSquared-GPS compatibility in January 2012 that identified scenarios where certified aviation receivers could experience much higher levels of interference than was assessed in the RTCA report. The areas where fixed-wing and rotary-wing aircraft rely on GPS are depicted in Figures 5 and 6 (above the connected line segments), respectively.
Figure 5. Area where GPS use must be sssured for fixed-wing aircraft.Figure 6. Area where GPS use must be assured for rotary-wing aircraft.
Aircraft rely upon GPS for navigation and Terrain Awareness and Warning Systems (TAWS). Helicopter low-level en-route navigation and TAWS for fixed- and rotary-wing aircraft are perhaps the most challenging scenarios for ensuring GPS compatibility with adjacent-band cellular networks. In these scenarios, the aircraft can be within the boresight of cellular sector antennas and in very close proximity, resulting in very high received-power levels. The FAA attempted to provide some leeway for LightSquared while maintaining safe functionality of TAWS through the concept of exclusion zones (see Figure 7). The idea of an exclusion zone is that, at least for cellular base-station transmitters on towers that are included within TAWS databases, that it would be permitted for the GPS function to not be available for very small zones around the LTE base-station tower. This concept is currently notional only; the FAA plans to more carefully evaluate the feasibility of this concept and appropriate exclusion-zone size with the assistance of other aviation industry stakeholders.
Figure 7. Example exclusion area around base station to protect TAWS.
High-precision and Networks: Reference Stations. To gain insight into typical reference-station heights for differential GPS networks, the AGL heights of sites comprising the Continuously Operating Reference Station (CORS) network organized by the National Geodetic Survey (NGS) were determined. The assessment procedure is detailed in the Appendix.
Figure 8 portrays a histogram of estimated AGL heights for the 1543 operational sites within the continental United States (CONUS) as of February 2012. The accuracy of the estimated AGL heights is on the order of 16 meters, 90 percent, limited primarily by the quality of the terrain data that was utilized. The mean and median site heights are 5.7 and 5.2 meters, respectively.
Figure 8. Distribution of heights for CORS sites.
RALR, atop the Archdale Building in Raleigh, North Carolina, was the tallest identified site at 64.1 meters. This site, however, was decommissioned in January 2012 (although it was identified as operational in a February 2012 NGS listing of sites). The second tallest site identified is WVHU in Huntington, West Virginia at 39.6 meters, which is still operational atop of a Marshall University building. 223 of the 1543 CORS sites within CONUS have AGL heights greater than 10 meters, and furthermore the taller sites tend to be in urban areas where cellular networks tend to have the greatest base-station density.
High Precision and Networks: End Users. Many high-precision end users employ GPS receivers at considerable heights above ground. For instance, high-precision receivers are relied upon within modern construction methods. The adjacent photos show GPS receivers used for the construction of a 58-story skyscraper called The Bow in Calgary, Canada. For this project, a rooftop control network was established on top of neighboring buildings using both GPS receivers and other surveying equipment (for example, 360-degree prisms for total stations), and GPS receivers were moved up with each successive stage of the building to keep structural components plumb and properly aligned. Similar techniques are being used for the Freedom Tower, the new World Trade Center, in New York City, and many other current construction projects.
Other terrestrial applications that rely on high-precision GPS receivers at high altitudes include structural monitoring and control of mechanical equipment such as gantry cranes. At times, even ground-based survey receivers can be substantially elevated. Although a conventional surveying pole or tripod typically places the GPS antenna 1.5 – 2 meters above the ground, much longer poles are available and occasionally used in areas where obstructions are present. 4-meter GPS poles are often utilized, and poles of up to 40 ft (12.2 meters) are available from survey supply companies.
General Location/Navigation. Although controlling received power from a cellular network at 2 meters AGL may be suitable to protect many general navigation/location users, it is not adequate by itself. For example, GPS receivers are used for tracking trucks and for positive train control (the latter mandated in the United States per the Rail Safety Improvement Act of 2008). GPS antennas for trucks and trains are often situated on top of these vehicles. Large trucks in the United States for use on public roads can be up to 13 ft, 6 in (~4.1 meters), and a typical U.S. locomotive height is 15 ft, 5 in (~4.7 meters). Especially in a mature network that is using high downtilts, received power at these AGL heights can be substantially higher than at 2 meters.
Within the TWG and NPEF studies, the general location/navigation GPS receiver category is defined to include non-certified aviation receivers. One notable application is the use of GPS to navigate unmanned aerial vehicles. UAVs are increasingly being used for law enforcement, border control, and many other applications where the UAV can be expected to occasionally pass within the boresight of cellular antennas at short ranges.
Cellular. The majority of Americans own cell phones, and a growing number are using cell phones as a replacement for landlines within their home. Already, 70 percent of 911 calls are made on mobile phones. Although pedestrians and car passengers are often within 2 meters of the ground, this is not always the case. Figure 9 shows three cellular sector antennas situated atop a building filled with residential condominiums. The rooftop is accessible and frequently used by the building inhabitants. According to an online real estate advertisement, “The Garden Roof was voted the Best Green Roof in Town and provides amazing 360 degree views of downtown Nashville as well as four separate sitting areas and fabulous landscaping.” One of the sector antennas is pointing towards the opposite corner of the building. If the downtilt is in the vicinity of 2–6 degrees, then it is quite likely that a person making a 911 call from the rooftop could see a received power level of –10 dBm to 0 dBm, high enough to disrupt GPS within most cellular devices if the antennas were transmitting in the 1525–1559 MHz band.
Figure 9. Cellular antennas atop Westview Condominium Building in downtown Nashville.
This situation is not unusual. Many cellular base stations are situated on rooftops in urban areas, and many illuminate living areas in adjacent buildings. In recent years, New York City even considered legislation to protect citizens from potential harmful effects of the more than 2,600 cell sites in the city, since many sites are in very close proximity to residential areas.
Propagation Models
Within the LightSquared proceedings, there was a tremendous amount of debate regarding propagation models. Communication-system service providers typically use propagation models that are conservative in their estimates of received power levels in the sense that they overestimate propagation losses. This conservatism is necessary so that the service can be provided to end users with high availability. From the standpoint of potential victims of interference, however, it is seen as far more desirable to underestimate propagation losses so that interference can be kept below an acceptable level a very high percentage of time. As shown in Figure 3, some received power measurements from the Las Vegas live-sky test indicate values even greater than would be predicted using free-space propagation model. Statistical models that allow for this possible were used in the FAA Status Report. The general topic of propagation models is worthy of future additional study if GPS interference standards are to be developed.
Future Considerations
GPS is being modernized. Additionally, satellite navigation users now enjoy the fact that the Russian GLONASS system has recently returned to full strength with the repopulation of its constellation. In the next decade, satellite navigation users also eagerly anticipate the completion of two other global GNSS constellations: Europe’s Galileo and China’s Compass. Notably, between the GPS modernization program and the deployment of these other systems, satellite navigation users are expected to soon be relying upon equipment that is multi-frequency and that needs to process many more signals with varied characteristics. New equipment offers an opportunity to insert new technologies such as improved filtering, but of course the need to process additional signals and carrier frequencies may make GNSS equipment more susceptible to interference as well. Clearly, these developments will need to be carefully assessed to support the establishment of GPS spectrum interference standards.
Summary
This article has identified a number of considerations for the development of GPS interference standards, which have been proposed by the PNT EXCOM. If the United States proceeds with the development of such standards, it is hoped that the information within this article will prove useful to those involved.
Bow highrise under construction in Calgary, showing GPS receivers in use (photos courtesy Rocky Annett, MMM Group Ltd.)(Photo courtesy of Rocky Annett, MMM Group Ltd.)(Photo courtesy of Rocky Annett, MMM Group Ltd.)
Appendix: AGL Heights of CORS Network Sites
The National Geodetic Survey Continuously Operating Reference Station (CORS) website provides lists of CORS site locations in a number of different reference frames. To determine the height above ground level () for each site within this study, two of these files (igs08_xyz_comp.txt and igs08_xyz_htdp.txt) were used. These two files provide the (x,y,z) coordinates of the antenna reference point (ARP) for each site in the International GNSS Service 2008 (IGS08) reference frame, which is consistent with the International Terrestrial Reference Frame (ITRF) of 2008. These coordinates are divided into two files by NGS, since the site listings also provide site velocities and velocities are either computed (for sites that have produced data for at least 2.5 years) or estimated (for newer sites). The comp file includes sites with computed velocities and the htdp file includes sites with estimated velocities (using a NGS program known as HTDP).
The data files can be used to readily produce height above the ellipsoid, , for each site. This height can be found using well-known equations to convert from (x, y, z) to (latitude, longitude, height). Obtaining estimates of requires information on the geoid height and terrain data, per the relationship:
(A-1)
For the results presented in this article, terrain data was obtained from http://earthexplorer.usgs.gov in the Shuttle Radar Topography Mission (SRTM) Digital Terrain Elevation Data (DTED) Level 2 format. For this terrain data, the horizontal datum is the World Geodetic System (WGS 84). The vertical datum is Mean Sea Level (MSL) as determined by the Earth Gravitational Model (EGM) 1996. Each data file covers a 1º by 1º degree cell in latitude/longitude, and individual points are spaced 1 arcsec in both latitude and longitude. The SRTM DTED Level 2 has a system design 16 meter absolute vertical height accuracy, 10 meters relative vertical height accuracy, and 20 meter absolute horizontal circular accuracy. All accuracies are at the 90 percent level. Considering the accuracies of the DTED data, the differences between WGS-84 and IGS08 as well as between the ARP and antenna phase center were considered negligible. Geoid heights were interpolated from 15-arcmin data available in the MATLAB Mapping Toolbox using the egm96geoid function.
Lower AGL heights are preferred for CORS sites to minimize motion between the antenna and the Earth’s crust. However, many sites are at significant heights above the ground by necessity, particularly in urban areas due to the competing desire for good sky visibility.
Christopher J. Hegarty is the director for communications, navigation, and surveillance engineering and spectrum with The MITRE Corporation. He received a D.Sc. degree in electrical engineering from George Washington University. He is currently the chair of the Program Management Committee of the RTCA, Inc., and co-chairs RTCA Special Committee 159 (GNSS). He is the co-editor/co-author of the textbook Understanding GPS: Principles and Applications, 2nd Edition.
MY UNIVERSITY, the University of New Brunswick, is one of the few institutes of higher learning still using Latin at its graduation exercises. The president and vice-chancellor of the university asks the members of the senate and board of governors present “Placetne vobis Senatores, placetne, Gubernatores, ut hi supplicatores admittantur?” (Is it your pleasure, Senators, is it your pleasure, Governors, that these supplicants be admitted?). In the Oxford tradition, a supplicant is a student who has qualified for their degree but who has not yet been admitted to it. Being a UNB senator, I was familiar with this usage of the word supplicant. But I was a little surprised when I first read a draft of the article in this month’s Innovation column with its use of the word supplicant to describe the status of a GPS receiver.
If we look up the definition of supplicant in a dictionary, we find that it is “a person who makes a humble or earnest plea to another, especially to a person in power or authority.” Clearly, that describes our graduating students. But what has it got to do with a GPS receiver? Well, it seems that the word supplicant has been taken up by engineers developing protocols for computer communication networks and with a similar meaning. In this case, a supplicant (a computer or rather some part of its operating system) at one end of a secure local area network seeks authentication to join the network by submitting credentials to the authenticator on the other end. If authentication is successful, the computer is allowed to join the network. The concept of supplicant and authenticator is used, for example, in the IEEE 802.1X standard for port-based network access control.
Which brings us to GPS. When a GPS receiver reports its position to a monitoring center using a radio signal of some kind, how do we know that the receiver or its associated communications unit is telling the truth? It’s not that difficult to generate false position reports and mislead the monitoring center into believing the receiver is located elsewhere — unless an authentication procedure is used. In this month’s column, we look at the development of a clever system that uses the concept of supplicant and authenticator to assess the truthfulness of position reports.
“Innovation” is a regular feature that discusses advances in GPS technology andits applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering, University of New Brunswick. He welcomes comments and topic ideas. Contact him at lang @ unb.ca.
This article deals with the problem of position authentication. The term “position authentication” as discussed in this article is taken to mean the process of checking whether position reports made by a remote user are truthful (Is the user where they say they are?) and accurate (In reality, how close is a remote user to the position they are reporting?). Position authentication will be indispensable to many envisioned civilian applications. For example, in the national airspace of the future, some traffic control services will be based on self-reported positions broadcast via ADS-B by each aircraft. Non-aviation applications where authentication will be required include tamper-free shipment tracking and smart-border systems to enhance cargo inspection procedures at commercial ports of entry. The discussions that follow are the outgrowth of an idea first presented by Sherman Lo and colleagues at Stanford University (see Further Reading).
For illustrative purposes, we will focus on the terrestrial application of cargo tracking. Most of the commercial fleet and asset tracking systems available in the market today depend on a GPS receiver installed on the cargo or asset. The GPS receiver provides real-time location (and, optionally, velocity) information. The location and the time when the asset was at a particular location form the tracking message, which is sent back to a monitoring center to verify if the asset is traveling in an expected manner. This method of tracking is depicted graphically in FIGURE 1.
FIGURE 1. A typical asset tracking system.
The approach shown in Figure 1 has at least two potential scenarios or fault modes, which can lead to erroneous tracking of the asset. The first scenario occurs when an incorrect position solution is calculated as a result of GPS RF signal abnormalities (such as GPS signal spoofing). The second scenario occurs when the correct position solution is calculated but the tracking message is tampered with during the transmission from the asset being tracked to the monitoring center. The first scenario is a falsification of the sensor and the second scenario is a falsification of the transmitted position report.
The purpose of this article is to examine the problem of detecting sensor or report falsification at the monitoring center. We discuss an authentication system utilizing the white-noise-like spreading codes of GPS to calculate an authentic position based on a snapshot of raw IF signal from the receiver.
Using White Noise as a Watermark
The features for GPS position authentication should be very hard to reproduce and unique to different locations and time. In this case, the authentication process is reduced to detecting these features and checking if these features satisfy some time and space constraints. The features are similar to the well-designed watermarks used to detect counterfeit currency.
A white-noise process that is superimposed on the GPS signal would be a perfect watermark signal in the sense that it is impossible reproduce and predict. FIGURE 2 is an abstraction that shows how the above idea of a superimposed white-noise process would work in the signal authentication problem. The system has one transmitter, Tx , and two receivers, Rs and Ra. Rs is the supplicant and Ra is the authenticator. The task of the authenticator is to determine whether the supplicant is using a signal from Tx or is being spoofed by a malicious transmitter, Tm. Ra is the trusted source, which gets a copy of the authentic signal, Vx(t) (that is, the signal transmitted by Tx). The snapshot signal, Vs(t), received at Rs is sent to the trusted agent to compare with the signal, Va(t), received at Ra. Every time a verification is performed, the snapshot signal from Rs is compared with a piece of the signal from Ra. If these two pieces of signal match, we can say the snapshot signal from Rs was truly transmitted from Tx. For the white-noise signal, match detection is accomplished via a cross-correlation operation (see Further Reading). The cross-correlation between one white-noise signal and any other signal is always zero. Only when the correlation is between the signal and its copy will the correlation have a non-zero value. So a non-zero correlation means a match. The time when the correlation peak occurs provides additional information about the distance between Ra and Rs.
Unfortunately, generation of a white-noise watermark template based on a mathematical model is impossible. But, as we will see, there is an easy-to-use alternative.
FIGURE 2. Architecture to detect a snapshot of a white-noise signal.
An Intrinsic GPS Watermark
The RF carrier broadcast by each GPS satellite is modulated by the coarse/acquisition (C/A) code, which is known and which can be processed by all users, and the encrypted P(Y) code, which can be decoded and used by Department of Defense (DoD) authorized users only. Both civilians and DoD-authorized users see the same signal. To commercial GPS receivers, the P(Y) code appears as uncorrelated noise. Thus, as discussed above, this noise can be used as a watermark, which uniquely encodes locations and times. In a typical civilian GPS receiver’s tracking loop, this watermark signal can be found inside the tracking loop quadrature signal.
The position authentication approach discussed here is based on using the P(Y) signal to determine whether a user is utilizing an authentic GPS signal. This method uses a segment of noisy P(Y) signal collected by a trusted user (the authenticator) as a watermark template. Another user’s (the supplicant’s) GPS signal can be compared with the template signal to judge if the user’s position and time reports are authentic. Correlating the supplicant’s signal with the authenticator’s copy of the signal recorded yields a correlation peak, which serves as a watermark. An absent correlation peak means the GPS signal provided by the supplicant is not genuine. A correlation peak that occurs earlier or later than predicted (based on the supplicant’s reported position) indicates a false position report.
System Architecture
FIGURE 3 is a high-level architecture of our proposed position authentication system. In practice, we need a short snapshot of the raw GPS IF signal from the supplicant. This piece of the signal is the digitalized, down-converted, IF signal before the tracking loops of a generic GPS receiver. Another piece of information needed from the supplicant is the position solution and GPS Time calculated using only the C/A signal. The raw IF signal and the position message are transmitted to the authentication center by any data link (using a cell-phone data network, Wi-Fi, or other means).
FIGURE 3. Architecture of position authentication system.
The authentication station keeps track of all the common satellites seen by both the authenticator and the supplicant. Every common satellite’s watermark signal is then obtained from the authenticator’s tracking loop. These watermark signals are stored in a signal database. Meanwhile, the pseudorange between the authenticator and every satellite is also calculated and is stored in the same database.
When the authentication station receives the data from the supplicant, it converts the raw IF signal into the quadrature (Q) channel signals. Then the supplicant’s Q channel signal is used to perform the cross-correlation with the watermark signal in the database. If the correlation peak is found at the expected time, the supplicant’s signal passes the signal-authentication test. By measuring the relative peak time of every common satellite, a position can be computed. The position authentication involves comparing the reported position of the supplicant to this calculated position. If the difference between two positions is within a pre-determined range, the reported position passes the position authentication.
While in principle it is straightforward to do authentication as described above, in practice there are some challenges that need to be addressed. For example, when there is only one common satellite, the only common signal in the Q channel signals is this common satellite’s P(Y) signal. So the cross-correlation only has one peak. If there are two or more common satellites, the common signals in the Q channel signals include not only the P(Y) signals but also C/A signals. Then the cross-correlation result will have multiple peaks. We call this problem the C/A leakage problem, which will be addressed below.
C/A Residual Filter
The C/A signal energy in the GPS signal is about double the P(Y) signal energy. So the C/A false peaks are higher than the true peak. The C/A false peaks repeat every 1 millisecond. If the C/A false peaks occur, they are greater than the true peak in both number and strength. Because of background noise, it is hard to identify the true peak from the correlation result corrupted by the C/A residuals.
To deal with this problem, a high-pass filter can be used. Alternatively, because the C/A code is known, a match filter can be designed to filter out any given GPS satellite’s C/A signal from the Q channel signal used for detection. However, this implies that one match filter is needed for every common satellite simultaneously in view of the authenticator and supplicant. This can be cumbersome and, thus, the filtering approach is pursued here.
In the frequency domain, the energy of the base-band C/A signal is mainly (56 percent) within a ±1.023 MHz band, while the energy of the base-band P(Y) signal is spread over a wider band of ±10.23 MHz. A high-pass filter can be applied to Q channel signals to filter out the signal energy in the ±1.023 MHz band. In this way, all satellites’ C/A signal energy can be attenuated by one filter rather than using separate match filters for different satellites.
FIGURE 4 is the frequency response of a high-pass filter designed to filter out the C/A signal energy. The spectrum of the C/A signal is also plotted in the figure. The high-pass filter only removes the main lobe of the C/A signals. Unfortunately, the high-pass filter also attenuates part of the P(Y) signal energy. This degrades the auto-correlation peak of the P(Y) signal. Even though the gain of the high-pass filter is the same for both the C/A and the P(Y) signals, this effect on their auto-correlation is different. That is because the percentage of the low-frequency energy of the C/A signal is much higher than that of the P(Y) signal. This, however, is not a significant drawback as it may appear initially. To see why this is so, note that the objective of the high-pass filter is to obtain the greatest false-peak rejection ratio defined to be the ratio between the peak value of P(Y) auto-correlation and that of the C/A auto-correlation. The false-peak rejection ratio of the non-filtered signals is 0.5. Therefore, all one has to do is adjust the cut-off frequency of the high-pass filter to achieve a desired false-peak rejection ratio.
FIGURE 4. Frequency response of the notch filter.
The simulation results in FIGURE 5 show that one simple high-pass filter rather than multiple match filters can be designed to achieve an acceptable false-peak rejection ratio. The auto-correlation peak value of the filtered C/A signal and that of the filtered P(Y) signal is plotted in the figure. While the P(Y) signal is attenuated by about 25 percent, the C/A code signal is attenuated by 91.5 percent (the non-filtered C/A auto-correlation peak is 2). The false-peak rejection ratio is boosted from 0.5 to 4.36 by using the appropriate high-pass filter.
FIGURE 5. Auto-correlation of the filtered C/A and P(Y) signals.
Position Calculation
Consider the situation depicted in FIGURE 6 where the authenticator and the supplicant have multiple common satellites in view. In this case, not only can we perform the signal authentication but also obtain an estimate of the pseudorange information from the authentication. Thus, the authenticated pseudorange information can be further used to calculate the supplicant’s position if we have at least three estimates of pseudoranges between the supplicant and GPS satellites. Since this position solution of the supplicant is based on the P(Y) watermark signal rather than the supplicant’s C/A signal, it is an independent and authentic solution of the supplicant’s position. By comparing this authentic position with the reported position of the supplicant, we can authenticate the veracity of the supplicant’s reported GPS position.
FIGURE 6. Positioning using a watermark signal.
The situation shown in Figure 6 is very similar to double-difference differential GPS. The major difference between what is shown in the figure and the traditional double difference is how the differential ranges are calculated. Figure 6 shows how the range information can be obtained during the signal authentication process. Let us assume that the authenticator and the supplicant have four common GPS satellites in view: SAT1, SAT2, SAT3, and SAT4. The signals transmitted from the satellites at time t are S1(t), S2(t), S3(t), and S4(t), respectively. Suppose a signal broadcast by SAT1 at time t0 arrives at the supplicant at t0 + ν1s where ν1s is the travel time of the signal. At the same time, signals from SAT2, SAT3, and SAT4 are received by the supplicant. Let us denote the travel time of these signals as ν2s, ν3s, and ν4s, respectively. These same signals will be also received at the authenticator. We will denote the travel times for the signals from satellite to authenticator as ν1a, ν2a, ν3a, and ν4a. The signal at a receiver’s antenna is the superposition of the signals from all the satellites. This is shown in FIGURE 7 where a snapshot of the signal received at the supplicant’s antenna at time t0 + ν1s includes GPS signals from SAT1, SAT2, SAT3, and SAT4. Note that even though the arrival times of these signals are the same, their transmit times (that is, the times they were broadcast from the satellites) are different because the ranges are different. The signals received at the supplicant will be S1(t0), S2(t0 + ν1s – ν2s), S3(t0 + ν1s – ν3s), and S4(t0 + ν1s – ν4s). This same snapshot of the signals at the supplicant is used to detect the matched watermark signals from SAT1, SAT2, SAT3, and SAT4 at the authenticator. Thus the correlation peaks between the supplicant’s and the authenticator’s signal should occur at t0 + ν1a, t0 + ν1s – ν2s + ν2a, t0 + ν1s – ν3s + ν3a, and t0 + ν1s – ν4s + ν4a.
Referring to Figure 6 again, suppose the authenticator’s position (xa, ya, za) is known but the supplicant’s position (xs, ys, zs) is unknown and needs to be determined. Because the actual ith common satellite (xi , yi, zi) is also known to the authenticator, each of the ρia, the pseudorange between the ith satellite and the authenticator, is known. If ρis is the pseudorange to the ith satellite measured at the supplicant, the pseudoranges and the time difference satisfies equation (1):
ρ2s – ρ1s= ρ2a – ρ1a – ct21 + cχ21 (1)
where χ21 is the differential range error primarily due to tropospheric and ionospheric delays. In addition, c is the speed of light, and t21 is the measured time difference as shown in Figure 7. Finally, ρis for i = 1, 2, 3, 4 is given by:
(2)
FIGURE 7. Relative time delays constrained by positions.
If more than four common satellites are in view between the supplicant and authenticator, equation (1) can be used to form a system of equations in three unknowns. The unknowns are the components of the supplicant’s position vector rs = [xs, ys, zs]T. This equation can be linearized and then solved using least-squares techniques. When linearized, the equations have the following form:
Aδrs= δm (3)
where δrs = [δxs,δys,δzs]T, which is the estimation error of the supplicant’s position. The matrix A is given by
where is the line of sight vector from the supplicant to the ith satellite. Finally, the vector δm is given by:
(4)
where δri is the ith satellite’s position error, δρia is the measurement error of pseudorange ρia or pseudorange noise. In addition, δtij is the time difference error. Finally, δχij is the error of χij defined earlier.
Equation (3) is in a standard form that can be solved by a weighted least-squares method. The solution is
δrs= ( ATR-1A)-1ATR-1δm (5)
where R is the covariance matrix of the measurement error vector δm. From equations (3) and (5), we can see that the supplicant’s position accuracy depends on both the geometry and the measurement errors.
Hardware and Software
In what follows, we describe an authenticator which is designed to capture the GPS raw signals and to test the performance of the authentication method described above. Since we are relying on the P(Y) signal for authentication, the GPS receivers used must have an RF front end with at least a 20-MHz bandwidth. Furthermore, they must be coupled with a GPS antenna with a similar bandwidth. The RF front end must also have low noise. This is because the authentication method uses a noisy piece of the P(Y) signal at the authenticator as a template to detect if that P(Y) piece exists in the supplicant’s raw IF signal. Thus, the detection is very sensitive to the noise in both the authenticator and the supplicant signals. Finally, the sampling of the down-converted and digitized RF signal must be done at a high rate because the positioning accuracy depends on the accuracy of the pseudorange reconstructed by the authenticator. The pseudorange is calculated from the time-difference measurement. The accuracy of this time difference depends on the sampling frequency to digitize the IF signal. The high sampling frequency means high data bandwidth after the sampling.
The authenticator designed for this work and shown in FIGURE 8 satisfies the above requirements. A block diagram of the authenticator is shown in Figure 8a and the constructed unit in Figure 8b. The IF signal processing unit in the authenticator is based on the USRP N210 software-defined radio. It offers the function of down converting, digitalization, and data transmission. The firmware and field-programmable-gate-array configuration in the USRP N210 are modified to integrate a software automatic gain control and to increase the data transmission efficiency. The sampling frequency is 100 MHz and the effective resolution of the analog-to-digital conversion is 6 bits. The authenticator is battery powered and can operate for up to four hours at full load.
FIGURE 8a. Block diagram of GPS position authenticator.
Performance Validation
Next, we present results demonstrating the performance of the authenticator described above. First, we present results that show we can successfully deal with the C/A leakage problem using the simple high-pass filter. We do this by performing a correlation between snapshots of signal collected from the authenticator and a second USRP N210 software-defined radio. FIGURE 9a is the correlation result without the high-pass filter. The periodic peaks in the result have a period of 1 millisecond and are a graphic representation of the C/A leakage problem. Because of noise, these peaks do not have the same amplitude. FIGURE 9b shows the correlation result using the same data snapshot as in Figure 9a. The difference is that Figure 9b uses the high-pass filter to attenuate the false peaks caused by the C/A signal residual. Only one peak appears in this result as expected and, thus, confirms the analysis given earlier.
FIGURE 9a. Example of cross-correlation detection results without high-pass filter.FIGURE 9b. Example of cross-correlation with high-pass filter.
We performed an experiment to validate the authentication performance. In this experiment, the authenticator and the supplicant were separated by about 1 mile (about 1.6 kilometers). The location of the authenticator was fixed. The supplicant was then sequentially placed at five points along a straight line. The distance between two adjacent points is about 15 meters. The supplicant was in an open area with no tall buildings or structures. Therefore, a sufficient number of satellites were in view and multipath, if any, was minimal. The locations of the five test points are shown in FIGURE 10.
FIGURE 10. Five-point field test. Image courtesy of Google.
The first step of this test was to place the supplicant at point A and collect a 40-millisecond snippet of data. This data was then processed by the authenticator to determine if:
The signal contained the watermark. We call this the “signal authentication test.” It determines whether a genuine GPS signal is being used to form the supplicant’s position report.
The supplicant is actually at the position coordinates that they say they are. We call this the “position authentication test.” It determines whether or not falsification of the position report is being attempted.
Next, the supplicant was moved to point B. However, in this instance, the supplicant reports that it is still located at point A. That is, it makes a false position report. This is repeated for the remaining positions (C through E) where at each point the supplicant reports that it is located at point A. That is, the supplicant continues to make false position reports.
In this experiment, we have five common satellites between the supplicant (at all of the test points A to E) and the authenticator. The results of the experiment are summarized in TABLE 1. If we can detect a strong peak for every common satellite, we say this point passes the signal authentication test (and note “Yes” in second column of Table 1). That means the supplicant’s raw IF signal has the watermark signal from every common satellite. Next, we perform the position authentication test. This test tries to determine whether the supplicant is at the position it claims to be. If we determine that the position of the supplicant is inconsistent with its reported position, we say that the supplicant has failed the position authentication test. In this case we put a “No” in the third column of Table 1. As we can see from Table 1, the performance of the authenticator is consistent with the test setup. That is, even though the wrong positions of points (B, C, D, E) are reported, the authenticator can detect the inconsistency between the reported position and the raw IF data. Furthermore, since the distance between two adjacent points is 15 meters, this implies that resolution of the position authentication is at or better than 15 meters. While we have not tested it, based on the timing resolution used in the system, we believe resolutions better than 12 meters are achievable.
Table 1. Five-point position authentication results.
Conclusion
In this article, we have described a GPS position authentication system. The authentication system has many potential applications where high credibility of a position report is required, such as cargo and asset tracking. The system detects a specific watermark signal in the broadcast GPS signal to judge if a receiver is using the authentic GPS signal. The differences between the watermark signal travel times are constrained by the positions of the GPS satellites and the receiver. A method to calculate an authentic position using this constraint is discussed and is the basis for the position authentication function of the system. A hardware platform that accomplishes this was developed using a software-defined radio. Experimental results demonstrate that this authentication methodology is sound and has a resolution of better than 15 meters. This method can also be used with other GNSS systems provided that watermark signals can be found. For example, in the Galileo system, the encrypted Public Regulated Service signal is a candidate for a watermark signal.
In closing, we note that before any system such as ours is fielded, its performance with respect to metrics such as false alarm rates (How often do we flag an authentic position report as false?) and missed detection probabilities (How often do we fail to detect false position reports?) must be quantified. Thus, more analysis and experimental validation is required.
Acknowledgments
The authors acknowledge the United States Department of Homeland Security (DHS) for supporting the work reported in this article through the National Center for Border Security and Immigration under grant number 2008-ST-061-BS0002. However, any opinions, findings, conclusions or recommendations in this article are those of the authors and do not necessarily reflect views of the DHS. This article is based on the paper “Performance Analysis of a Civilian GPS Position Authentication System” presented at PLANS 2012, the Institute of Electrical and Electronics Engineers / Institute of Navigation Position, Location and Navigation Symposium held in Myrtle Beach, South Carolina, April 23–26, 2012.
Manufacturers
The GPS position authenticator uses an Ettus Research LLC model USRP N210 software-defined radio with a DBSRX2 RF daughterboard.
Zhefeng Li is a Ph.D. candidate in the Department of Aerospace Engineering and Mechanics at the University of Minnesota, Twin Cities. His research interests include GPS signal processing, real-time implementation of signal processing algorithms, and the authentication methods for civilian GNSS systems.
Demoz Gebre-Egziabher is an associate professor in the Department of Aerospace Engineering and Mechanics at the University of Minnesota, Twin Cities. His research deals with the design of multi-sensor navigation and attitude determination systems for aerospace vehicles ranging from small unmanned aerial vehicles to Earth-orbiting satellites.
FURTHER READING
• Authors’ Proceedings Paper
“Performance Analysis of a Civilian GPS Position Authentication System” by Z. Li and D. Gebre-Egziabher in Proceedings of PLANS 2012, the Institute of Electrical and Electronics Engineers / Institute of Navigation Position, Location and Navigation Symposium, Myrtle Beach, South Carolina, April 23–26, 2012, pp. 1028–1041.
• Previous Work on GNSS Signal and Position Authentication
“Location Assurance” by L. Scott in GPS World, Vol. 18, No. 7, July 2007, pp. 14–18.
“Location Assistance Commentary” by T.A. Stansell in GPS World, Vol. 18, No. 7, July 2007, p. 19.
• Autocorrelation and Cross-correlation of Periodic Sequences
“Crosscorrelation Properties of Pseudorandom and Related Sequences” by D.V. Sarwate and M.B. Pursley in Proceedings of the IEEE, Vol. 68, No. 5, May 1980, pp. 593–619, doi: 10.1109/PROC.1980.11697. Corrigendum: “Correction to ‘Crosscorrelation Properties of Pseudorandom and Related Sequences’” by D.V. Sarwate and M.B. Pursley in Proceedings of the IEEE, Vol. 68, No. 12, December 1980, p. 1554, doi: 10.1109/PROC.1980.11910.
Digital Satellite Navigation and Geophysics: A Practical Guide with GNSS Signal Simulator and Receiver Laboratory by I.G. Petrovski and T. Tsujii with foreword by R.B. Langley, published by Cambridge University Press, Cambridge, U.K., 2012.
A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach by K. Borre, D.M. Akos, N. Bertelsen, P. Rinder, and S.H. Jensen, published by Birkhäuser, Boston, 2007.
Beating up the backstretch neck and neck, tied for third in the GNSS race, Galileo and Compass today offer some signals and some satellites to GNSS users — as long as those users are researchers. Galileo has more going for it in the way of signals, while Compass holds an edge in the number of satellites. Without an interface control document (ICD) to guide user/researchers and most importantly manufacturers in the employment of its signals, Compass satellites, however they may increase, are practically useless to anyone outside China. A Compass ICD has been rumored before and is now rumored again. Wait and see before placing your bets.
The fourth Galileo in-orbit validation (IOV) satellite, Flight Model 4 (FM4), began transmitting signals on December 12, joining its co-launched confrère FM3, which began airing navigation signals on December 1. The FM4 spacecraft uses PRN code E20. As of this writing, FM3 is broadcasting E1, E5, and E6 signals, and FM4 is broadcasting E1 and E5 signals; we don’t know if and when FM4 E6 signals start(ed) until ESA tells us.
GPS World authors Oliver Montenbruck (German Space Operations Center) and Richard Langley (University of New Brunswick) have written an early analysis of the signals from FM3; this account will appear in the January issue of the magazine. A few selected excerpts from that article, and one figure:
“Anyone with commonly available GNSS receivers can presently access the open signals in the E1, E5a, and E5b frequency bands as well as the wide-band E5 AltBOC signal.
Figure 1: Pseudorange errors of IOV-3 tracking at Tanegashima, Japan, using the E1 BOC(1,1) signal (top) and the E5 AltBOC signal (center). The elevation angle over time is shown in the bottom panel.
“According to an ESA statement, FM3will continue to use binary offset carrier modulation — specifically BOC(1,1) — on the E1 Open Service signals for the time being. In contrast to this, the first pair of IOV satellites has already started to use composite binary offset carrier modulation, which offers better multipath suppression in the received signal.
“The E5 AltBOC pseudorange measurements in particular exhibit an exceptionally low noise and multipath level of better than 10 centimeters at mid- and high-elevation angles.”
After discussing and displaying some carrier-phase measurements of the Galileo FM3 E1, E5, and E6 signals, Montenbruck and Langley conclude; “This level of performance highlights the potential benefit of Galileo signals in advanced triple-frequency techniques such as undifferenced ambiguity resolution and ionospheric monitoring.”
Theoretically, the total of four Galileo IOV satellites now in medium-Earth orbit yield the minimum number needed to perform a 3D navigation fix, although no statement of initial — or even sketchy — operating capability has been issued by the European Space Agency (ESA), nor is one expected.
Antonio Tajani, vice-president of the European Commission (EC) and head of the EC directorate-general responsible for industry and entrepreneurship, continues to publicly maintain a “political objective [of] the delivery of the first services before the end of 2014,” based on 18 orbiting satellites. In a December speech, he revised the basis for that position slightly to say the civil Open Service (OS) could be declared operational with as few as 12 satellites.
The system operators had announced three dual-satellite launches in 2013, two dual-satellite launches and one four-satellite launch in 2014, hypothetically producing an operable constellation of 18 satellites by the end of the promised 2014. However, unconfirmed reports from Europe suggest that problems with manufacture of the next set of 14 Galileo satellites mean that no launches at all will take place until Q4 of 2013. Whether this will push out the service delivery date beyond 2014 or not remains open to conjecture.
Compass
Another matter open to conjecture and much speculation is whether the world will soon — or ever — see an interface control document (ICD) for China’s Compass system. More than a year ago, I wrote that “The ICD has been rumored to be available previously to receiver manufacturers within China, creating some disgruntlement among companies outside the country . . . GPS/Compass chips and receivers are being actively developed by many Chinese manufacturers and research institutes.” Indeed, conference presentations, leading to a published article in this magazine’s October issue, “What Is Achievable with the Current Compass Constellation,“ confirm that this is so.
And yet, the rest of the world neither has nor holds a Compass ICD.
The end-of-year rumor mill has kicked into gear again, though. A GNSS industry representative stationed in Shanghai, China sent this message recently to a U.S. colleague: “Latest unofficial news said that the Compass Interface Control Document (ICD) will be released on 27th this month, and will be available on the internet on 28th.”
The Research and Innovative Technology Administration (RITA) seeks an experienced Electronics Engineer interested in joining the Office of Positioning, Navigation, and Timing (PNT). RITA coordinates the U.S. Department of Transportation’s (DOT) research programs and is charged with advancing rigorous analysis and the deployment of cross-cutting technologies to improve our Nation’s transportation system. RITA serves as the lead Administration representing DOT on PNT matters, including development of departmental positions on PNT and spectrum policy and protection, and is responsible for representing and supporting the civil Departments and Agencies in PNT systems analysis and coordination, including PNT requirements and architectural development.
In this challenging role, you will serve as the program manager for the inland (terrestrial) segment of the Nationwide Differential Global Positioning System (NDGPS) Program and serve as the lead for radio frequency spectrum management and analysis functions in support of DOT’s technical requirements and policy development.
As the NDGPS Program Manager, you will be responsible for the technical, cost and schedule performance of the NDGPS, as managed currently through the United States Coast Guard under a Memorandum of Agreement, and serve as the chair of the NDGPS Policy and Implementation Team.
You also will lead radio frequency spectrum management and analysis tasks in close coordination with other DOT Operating Administrations and the National Telecommunications and Information Administration (NTIA). You will provide expert advice specifically on issues of harmful radio frequency interference and operational degradation to DOT operations and planned operations, and serve as DOT’s representative to the Interdepartmental Radio Advisory Committee (IRAC).
If you or someone you know has the experience and a demonstrable record of proven results, I encourage you or them to apply to this Washington, D.C.-based position. We are looking for a diverse pool of qualified candidates
The announcement is posted to the Public and to Merit Promotion eligible applicants on www.usajobs.opm.gov. Please know that Merit Promotion announcements are the vehicle through which Federal employees generally apply for Federal positions.
The Technische Universitaet Muenchen has reported that transmissions of the L1/E1 signal from Galileo satellite IOV-4 (FM-4) started at about 17:15:10 GPS Time December 12. The navigation signals of both of the recently launched in-orbit validation satellites have now been activated.
A number of stations in the Cooperative Network for GNSS Observation as well as some stations participating in the International GNSS Service’s Multi-GNSS Experiment are tracking IOV-4. The satellite is using PRN code E20.
If the commissioning schedule is similar to that of IOV-3, the E5 and E6 signals of IOV-4 should be switched on over the next few days.
Affordability, Capability, and Back-to-Basics Acquisition
Headshot: Keoki Jackson
By Keoki Jackson
The history of GNSS shows each year has always been more successful than the year prior, and in 2013 we expect the trend to continue. In the United States, the role of GPS will continue to expand, and the applications for our technology will reach sectors we never imagined. As our international partner countries continue to launch GNSS satellites, and user equipment develops further, our community will increase its globalization, and international cooperation will reach new heights.
At the same time, our industry will see its fair share of challenges. We anticipate several significant trends to be further defined next year.
First, in the satellite world, affordability will be the name of the game. There is no disputing that the U.S. government is in austere budget times, and the Air Force will be asked to do more in acquiring GPS space, ground, and military user equipment, with fewer resources. Industry will partner with the Air Force in this new reality, and on the satellite manufacturing side, industry and government will need to demonstrate reduced costs, while sustaining the constellation and posturing for future demands.
It is no secret that military operations depend on GPS, and adversaries are working aggressively to erode the GPS combat advantage with low-cost jamming devices, spoofing concepts, or cyber attacks. On the user demand side, we expect the need for anti-jamming capability to become even more critical for military users. We also expect users to demand better accuracy and integrity, both in the military and civil communities. In 2013, the United States must secure its critical modernization efforts to meet these demands and bolster the space, ground, and user architecture against potential threats.
For us at Lockheed Martin, the message is clear. The threats and demands for enhanced capability are real, but the budget to meet those demands is shrinking. This presents a challenge, but we believe 2013 is the year we meet the challenge and position for the future.
GPS III, the Air Force’s next generation GPS satellite system, is a central part of the modernized solutions for the challenges laid out above. GPS III is the most affordable way to meet the increasing demand from users, while also prudently posturing the enterprise for the future. In 2013, we intend to prove that.
Space acquisition has weathered painful challenges in the past — that is not news — but the Air Force laid out the GPS III acquisition plan to reverse the trend and regain acquisition confidence. Leveraging hard-won lessons, the Air Force instilled a “back-to-basics” acquisition approach to provide better mission assurance, cost confidence, and schedule predictability. The approach emphasizes early investments in rigorous systems engineering, industry-leading parts standards, and the development of a fully functional GPS III satellite pathfinder to retire risks early and lower overall program costs. These investments early in the GPS III program were designed to prevent the types of engineering issues discovered on other programs late in the flight vehicle manufacturing process or even on orbit.
Back to Basics
The question in 2013 will be, “Is back-to-basics working?” — and we intend to show continued evidence of success next year. We will complete work on the GPS III Non-Flight Satellite Testbed (GNST), our full-sized GPS III satellite prototype. We will ship it to Cape Canaveral Air Force Station, Florida, for pathfinding activities at the launch site as we complete integration of the first space vehicle in our highly efficient GPS Processing Facility. The GNST is used to identify and solve development issues prior to integration and test of the first space vehicle. This will be a major milestone, putting the GNSS community on the cusp of fielding a new generation of PNT capabilities through very efficient and affordable production for all GPS III satellites.
Further proving out the back-to-basics acquisition approach, in 2013 we will be converting our options to build the next eight GPS III satellites to a fixed price contract structure, rather than cost-plus. This transition will limit the government’s risk and significantly contribute to Air Force affordability goals. The back-to-basics acquisition strategy and the progress we have already made on our GPS III prototype give us high confidence in our ability to perform efficient and affordable fixed-price satellite production going forward.
As the austere budget environment is amplified in 2013, we will focus our attention on our GPS III program performance while aggressively pursuing affordability and efficiency initiatives to ensure we are providing great value to the end user while being the best possible stewards of the American public’s investment.
User Demands
Affordability is one challenge; the other is meeting user demands. While the first GPS III satellites will bring on significant new capabilities, including improved accuracy, better anti-jam power, and a new civil signal to be interoperable with international GNSS systems, we do need to continue planning for technology upgrades in the future.
The Air Force laid out the GPS III program from the very beginning with evolution in mind — and the GPS III satellites have pre-architected capacity to add new capabilities and technologies affordably and with low risk. The acquisition plan calls for technology insertion beginning on the ninth satellite. 2013 will be a critical year in finalizing the production schedule for the capability insertion program.
We look at technology insertion in two ways: technology to reduce costs and technology to increase capabilities. To that end, we are developing dual launch, higher anti-jam signal power for the military, a new search and rescue payload, a digital navigation payload with the capability to incorporate new signals after launch, real time command and control cross links to improve system accuracy and a host of other innovations.
The timing for when these new capabilities will be on ramped onto new satellites will be determined by user demands and technical maturity. In 2013, we will be working very closely with the Air Force to implement a low risk ongoing modernization program to ensure GPS III meets the needs of users for decades to come while maintaining or reducing the per unit cost of a GPS III satellite.
In the uncertain and challenging environment of 2013 and beyond, GNSS technology will certainly continue to improve. User demand will increase significantly, while the resources to meet those demands will remain stable or decline. It is a tough challenge, but the GNSS industry has not disappointed yet, and we do not expect anything different in 2013 and beyond.
Dana (Keoki) Jackson is vice president of Navigation Systems in Space Systems Company’s Military Space line of business for Lockheed Martin Corporation. He is responsible for leading all aspects of the next-generation GPS III navigation satellite program for the United States Air Force, as well as operations and sustainment of the GPS IIR and IIRM satellites. Prior to joining Lockheed Martin, he was a NASA research fellow at the Massachusetts Institute of Technology, conducting Space Shuttle flight experiments in the field of human adaptation to the space environment. He has a doctoral degree in Aeronautics and Astronautics fromthe Massachusetts Institute of Technology.
A Proactive Approach for More Efficient Spectrum Use
In my vision of the future of GNSS, I see a pressing need to manage radio-frequency spectrum more efficiently. This will drive the creation of official standards for GNSS receivers, and better design of those receivers with better filters at lower cost, to protect against out-of-band and near-band interference. This in turn will enable user to undertake widespread monitoring and reporting of in-band interference, and create the freedom for many technologies to explore wider and more productive use of all bands of the radio-frequency spectrum.
Spectrum Management
As a consequence of unprecedented technological development on all fronts and in many fields, the radio-frequency spectrum is very congested. All countries, and the United States in particular, must find ways to use this spectrum more efficiently. Licenses for spectrum bands are very expensive, and special interest groups do all they can to secure ownership of any part of the spectrum and to prevent others from competing with them. There is an intense struggle going on, both behind the scenes and in the public arena; it has been called “the spectrum wars.” These involve big companies, very high stakes, politicians, and special interest groups. The Federal Communications Commission (FCC) seems caught, powerless, in the crossfire between these powerhouses.
GNSS Interference
GNSS interference exists everywhere and comes from many different sources, identified and unidentified, intentional or unintentional. The 1-dB effect on GNSS of the proposed LightSquared signal is negligible compared to what already exists. The reason that the LightSquared plan encountered so much opposition was not because of its effect on GNSS. It was because of its effect on the competing business models of large companies and special interest groups.
With the tools that we have created and embedded in our receivers, everyone can easily see that widespread interference already exists in most places, especially in cities, and that interferences can easily be monitored and automatically reported. It seems no organization has ownership of regularly monitoring interferences on these bands and taking corrective actions. This is partly because the tools to easily monitor and report interferences did not exist earlier.
GNSS Receivers
Current GNSS receivers on the market and in use around the world rely on inadequate designs. The technology does in fact exist to overcome out-of-band interference problems such as LightSquared and many others commonly encountered in today’s congested radio-frequency environment. There is no reason to prohibit others from using bands near GNSS; this just makes spectrum use inefficient. Continued shipping of inadequate, inefficient receivers by current manufacturers only increases and compounds the problems encountered by users.
There are standards for manufacturing countless industrial goods — for example, something as ordinary as car tires or — but there is no standard for building GNSS receivers that will be used in critical applications.
So far, the FCC has been silent on this topic, and has not established guidelines for GNSS receivers that are used in critical applications. The civilian users of GNSS, such as the U.S. National Geodetic Survey, the U.S. Geological Survey, the Federal Aviation Administration, and so on, have criteria for all sorts of little equipment, but there is no criteria for GNSS receivers that they claim are so important for their job.
Instead of taking the proactive and productive approach of putting filters into the receivers that they use, these organizations advocate keeping spectrum bands adjacent to GNSS off-limits to other users. Manufacturers do not see any reason to make better receivers while such a powerful lobby protects them.
Interference monitoring and reporting is strongly desirable for places such as GNSS reference stations, or for users to see the interferences before they start a jog that they are tracking on their GPS-enabled personal training device — just as pilots check the weather before they take off.
Special Interest Groups, Politics, and Blind Followers
The problem that LightSquared encountered was that its proposal impacted the business models of special interest groups. Although we — that is, JAVAD GNSS in presentations before the FCC in Washington DC — showed that other interferences exist in cities, the FCC did not care, and GNSS magazine editors did not care. They just blindly followed what the special interest groups had planned for them.
Brad Parkinson, in his article “PNT for the Nation: Three Key Attributes and Nine Druthers” in the October issue of GPS World, did not even hint at guidelines for building GNSS receivers. This is similar to formulating guideline on how to build and clean the roads while having no guidelines on how to build tires that are going to ride on the roads.
In Parkinson’s long list of recommendations, there was no mention at all that we need to build better GNSS receivers and be able to monitor interferences. There are guidelines and standards for how build every little item, but none for GNSS receivers that are claimed to be so essential for our security and prosperity.
Military GPS receivers do not have protection against even one particular type of interference such as that posed by LightSquared — and the suggested approach was to bomb such interferences, which most admit that of course cannot be done. This is a bad attitude. The cost of a filter in a receiver is almost nothing. A precision bomb costs millions if you factor in development costs, and deployment and delivery puts the full cost even higher.
The case is similar for GNSS receivers used in commercial airplanes. Instead of pushing for a better GNSS receiver design, the FAA simply hopes that interference does not happen.
Conclusion
These are my predictions — and my strongest possible recommendations — for the future of GNSS.
The FCC will create standards for GNSS receivers.
GNSS manufacturers will be forced to build better receivers.
GNSS users will benefit from better receivers at a lower cost.
Interference monitoring and reporting will become a desirable feature of GNSS receivers.
Bands near the GNSS spectrum will be freed for more efficient use by all types of productive technology.
I am proud to be a part of the efforts to make these happen, against all odds.
Javad Ashjaee received his Ph.D. in electrical engineering from the University of Iowa. He was chairman of the Computer Engineering Department, Tehran University of Technology, 1976-1981. He began his GPS engineering career at Trimble Navigation, 1981–1986. Founder and president of Ashtech Inc., 1986–1995, the company that produced the first integrated GPS-GLONASS receivers; founder and CEO of Javad Positioning Systems, 1996–2000, which he sold to Topcon Corporation. He founded JAVAD GNSS in 2007, and is currently president and CEO. In 2010, the company introduced the integrated geodetic receiver TRIUMPH-VS, with a GNSS Interference Analyzer, capable of tracking current and next-generation signals of GPS, GLONASS, QZSS, and Galileo signals. In 2011, the company introduced a LightSquared-compatible GNSS receiver.
An experimental GPS receiver, built by Surrey Satellite Technology Limited (SSTL), has successfully achieved a GPS position fix at 23,300 kilometers altitude – the first position fix above the GPS constellation on a civilian satellite. The SGR-GEO receiver is collecting data that could help SSTL to develop a receiver to navigate spacecraft in geostationary orbit (GEO) or even in deep space.
GPS is routinely used on Low Earth Orbit (LEO) satellites to provide the orbital position and offer a source of time to the satellite. Spacecraft in orbits higher than the 20,000 km of the GPS constellation, however, can only receive a few of the signals that “spill over” from the far side of the Earth, meaning that the signals are much weaker and a position fix cannot always be secured.
With the support of the European Space Agency (ESA) and the ARTES 4 program, SSTL included the SGR-GEO receiver on the GIOVE-A satellite to prove that a receiver could achieve a position fix from a higher orbit. The SGR-GEO is adapted from SSTL’s SGR range of receivers and incorporates a high-gain antenna and a precise oven-controlled clock. It will demonstrate special algorithms to allow reception of weak signals and an orbit estimator intended to allow a near continuous position fix throughout orbit.
“The results from the SGR-GEO receiver are really encouraging,” said Martin Unwin, principal GNSS engineer at SSTL. “We’re getting higher signal strengths than anticipated and also acquiring side lobes from the GPS transmit antennas, which improves the availability of the usable signals for navigation. With the success of the SGR-GEO receiver, GPS, in combination with Galileo and GLONASS, could soon be helping navigate spacecraft much further away from Earth.”
The experimental GPS receiver onboard GIOVE-A has been inactive for six years while the satellite has been used for its primary purpose of transmitting prototype Galileo signals. GIOVE-A’s retirement in June 2012 has allowed the commissioning of the experiment and is now providing valuable data to SSTL and ESA in support of the future use of spaceborne GNSS receivers at GEO altitudes. Engineers at SSTL will continue operations, testing out, tuning and improving the receiver software onboard GIOVE-A to achieve the best possible performance.
Firing on all cylinders — to use a slightly outmoded technological metaphor — GNSS moved forward on virtually every front in the past month. GPS made major advances both on the ground and in space, Galileo took a giant step, Compass continued on its roll, GLONASS has good news pending in only a day or two (knock on wood), and GAGAN is settling into space. But the best news of all is a very quiet, indeed somewhat hidden item: the UK patent applications against the interoperative GPS/Galileo signal design appear to have been dropped.
Let’s eat dessert first, since life is uncertain.
Patent Dispute Evaporates
Vague rumblings emerged throughout spring and summer this year that two British technologists, backed by the U.K. Ministry Defense, had filed patents on the future interoperable GPS and Galileo binary-offset carrier signal designs. If granted and enforced, the patents would have severely disrupted modernization plans for both systems and levied unexpected costs upon receiver manufacturers. And in fact a company called Ploughshare Innovations Ltd. Started dialing up said manufacturers and asking for payment of royalties, based on the patent filings.
After significant uproar and negotiations before and behind the scenes, it now appears that the initiative has been quietly scuttled. The file on application number 11/774,412, Modulation Signals for a Satellite Navigation System, on the U.S. Patent Office’s website, now reads “Expressly Abandoned — During Examination.” The status is dated September 16, 2012, some time ago, but that I’m aware of, no parties involved, whether as filers or negotiators, ever made any kind of announcement about it.
Checking the European Patent Office and its registry — which by the way is no trivial task of website navigation — I found a note under the docket for EP1830199, Modulations Signals for a Satellite Navigation System stating “Patent surrendered.” Dated September 24, 2012. A few days later, another note: “Lapsed in a contracting state announced via postgrant inform. From Nat. Office to EPO,” with further information to the effect of “lapse because of failure to submit a translation or the description or to pay the fee within the prescribed time limit.” And for good measure, a final docket not on October 3, “Lapsed due to resignation by the proprietor.”
However abstruse and arcane, we’ll take good news however we find it. Another bullet dodged.
GPS Ground Segment Benchmark
The GPS Directorate announced on October 26 that the U.S. Air Force and Raytheon have successfully met all requirements to enter into the engineering and manufacturing development phase of the Next-Generation Operational Control System (OCX). OCX will replace the current GPS operational control segment in managing the satellite constellation and providing command and control for all modernized signals.
OCX is being developed and fielded in blocks of GPS capability, to align with GPS III and military equipment deliveries.
OCX Block 0, also known as the Launch and Checkout System, scheduled to be available in the fourth quarter of Fiscal Year 2014, will allow OCX to support the launch of GPS III satellites.
OCX Block 1, scheduled to transition to operations in the first quarter of 2016, will deliver the operational capability to command and control the entire GPS constellation including GPS II and GPS III satellites. This block will also control the legacy civil and military signals, as well as two modernized civil and military signals, L2C and L5.
OCX Block 2 will specifically support advanced capabilities for civilian and military signals, the international civil signal, L1C, and the military signal, M-Code. OCX Block 2 is currently synchronized with modernized signal broadcast and timing.
GPS Block IIF-3 satellite.
GPS Block IIF Satellite Rises, Reaches Station, and Transmits
On October 11, The L5 transmitter aboard GPS Block IIF-3 satellite SVN65/PRN24 was switched on, transmitting the civilian safety-of-life GPS signal, designed to meet demanding requirements for safety-of-life transportation and other high-performance applications.
A day earlier, SVN65 began transmitting L1 and L2 signals as PRN24 on October 8. A number of stations of the International GNSS Service are tracking the satellite. As of press date for this magazine (October 25) the satellite is included in broadcast almanacs although it is set unhealthy and will continue to be so until satellite commissioning is completed. The satellite is drifting towards its designated orbital position of Slot 1 in Plane A.
The launch of the GPS Block IIF-3 satellite took place as scheduled October 4, aboard a United Launch Alliance Delta IV rocket from Cape Canaveral, Florida.
Galileo Turns Four. Validation Satellites, That Is.
The Galileo control room.
On October 12, a Soyuz launcher carrying two Galileo In-Orbit Validation (IOV) satellites deployed its twins into orbit within four hours after take-off, at close to 23,200 kilometers altitude. They join two earlier IOV spacecraft launched in October 2011. Once all four are operational in space, they will provide the minimum number of satellites required for navigational fixes — enabling system validation testing when all are visible in the sky.
A week after the dual liftoff from Kourou, French Guiana, the two satellites completed the critical Launch and Early Orbit Phase on October 19-20.
Satellites FM3 and FM4 satellites were handed over from the joint ESA/CNES Launch and Early Orbit Phase (LEOP) team in Toulouse, France, to the Galileo Control Centre, Oberpfaffenhofen, Germany, from where Spaceopal will manage operations of the Galileo constellation.
Three orbit maneuvers were conducted for each satellite to start them on drift orbits towards their operational positions, where they are expected to arrive on November 10 (FM3) and November 12 (FM4) after a series of drift-stop and fine-positioning movements.
The satellites were configured into a secure mode shortly after handover. While underway to their final positions, they will also undergo a series of tests to confirm the performance of their subsystems before switching on the payload.
The satellites were built by a consortium led by the Astrium division of EADS, which produced the platforms and has responsibility for the payloads, while Thales Alenia Space handled assembly and testing.
Compass up to Eleven
The two BeiDou-2/Compass satellites launched on September 18 reached their circular medium-Earth orbits on October 1 and started transmitting navigation signals. Several stations participating in the International GNSS Service’s Multi-GNSS Experiment as well as some in the Cooperative Network for GNSS Observation started tracking the satellites on September 26.
Although semi-official rumors had circulated that China was preparing for the Compass G6 (G2R) satellite launch on October 25, we have not found any announcement that the event has occurred.
The November issue of GPS World will appear in a few weeks’ time, with a cover story on “What Is Achievable with the Current Compass Constellation?” The technical article by Chinese researchers gives data from a 12-station tracking network distributed through China, the Pacific region, Europe, and Africa. It demonstrates the capacity of Compass with a constellation comprising four geostationary Earth-orbit (GEO) satellites and five inclined geosynchronous orbit (IGSO) satellites in operation. The regional system will be completed around the end of 2012 with a constellation of five GEOs, five IGSOs, and four medium-Earth orbit (MEO) satellites. By 2020 it will be extended into a global system.
GLONASS News in a Day or Two
As we go to e-press with this e-newsletter on October 30, we look forward to a Russian rocket rising on November 2 with a Luch data-relay satellite payload to service the the Russian satnav system. The second of a set of three geostationary satellites launched to reactivate Roscosmos’s Luch Multifunctional Space Relay System, it will also carry transponders for the System for Differential Correction and Monitoring (SDCM), Russia’s satellite-based augmentation system. The transponders will broadcast GNSS corrections on the standard GPS L1 frequency using C/A PRN codes assigned by the GPS Directorate. According to the most recent announcement, it will be positioned at 16 degrees West longitude, joining Luch-5A, already in an orbital slot at 95 degrees East longitude.
GAGAN Unfolding
The Indian Space Research Organization announced on October 3 that orbit-raising maneuvers placed the GSAT-10 satellite, launched September 30, in an orbit with 35,000-kilometer high orbit, with an orbit period of 23 hours 50 minutes, and a designated location of 83 degree East. GSAT-10 contains a payload to support the Indian GPS and GEO Augmented Navigation (GAGAN) satellite-based augmentation system. The satellite will likely use PRN code 128.
Another Dispute Headed for Resolution?
Finally, another pink dawn on the horizon. The European Union (EU) and China will reportedly meet in December in Paris to discuss overlapping radio frequencies both plan to use for their future encrypted government/military satellite navigation services.
The meeting will be conducted under what the Joint Statement on Space Technology Cooperation specifies as the ITU Framework. ITU is the International Telecommunication Union of Geneva, a United Nations affiliate that regulates satellite orbital slots and frequencies.
The statement was signed as an annex to a broader EU-China summit held September 20 in Brussels. The two sides continue collaboration on satellite navigation despite the signal conflict, which has been a subject of debate for at least two years.
The 27-nation EU and China have agreed to continue the China-Europe GNSS Technology Training and Cooperation Center.
Figure 1. Distribution of the GPS+COMPASS tracking network established by the GNSS Research Center at Wuhan University and used as test network in this study.
Data from a tracking network with 12 stations in China, the Pacific region, Europe, and Africa demonstrates the capacity of Compass with a constellation comprising four geostationary Earth-orbit (GEO) satellites and five inclined geosynchronous orbit (IGSO) satellites in operation. The regional system will be completed around the end of 2012 with a constellation of five GEOs, five IGSOs, and four medium-Earth orbit (MEO) satellites. By 2020 it will be extended into a global system.
By Maorong Ge, Hongping Zhang, Xiaolin Jia, Shuli Song, and Jens Wickert
China’s satellite navigation system Compass, also known as BeiDou, has been in deveopment for more than a decade. According to the China National Space Administration, the development is scheduled in three steps: experimental system, regional system, and global system.
The experimental system was established as the BeiDou-1 system, with a constellation comprising three satellites in geostationary orbit (GEO), providing operational positioning and short-message communication. The follow-up BeiDou-2 system is planned to be built first as a regional system with a constellation of five GEO satellites, five in inclined geosynchronous orbit (IGSO), and four in medium-Earth orbit (MEO), and then to be extended to a global system consisting of five GEO, three IGSO, and 27 MEO satellites. The regional system is expected to provide operational service for China and its surroundings by the end of 2012, and the global system to be completed by the end of 2020.
The Compass system will provide two levels of services. The open service is free to civilian users with positioning accuracy of 10 meters, timing accuracy of 20 nanoseconds (ns) and velocity accuracy of 0.2 meters/second (m/s). The authorized service ensures more precise and reliable uses even in complex situations and probably includes short-message communications.
The fulfillment of the regional-system phase is approaching, and the scheduled constellation is nearly completed. Besides the standard services and the precise relative positioning, a detailed investigation on the real-time precise positioning service of the Compass regional system is certainly of great interest.
With data collected in May 2012 at a regional tracking network deployed by Wuhan University, we investigate the performance of precise orbit and clock determination, which is the base of all the precise positioning service, using Compass data only. We furthermore demonstrate the capability of Compass precise positioning service by means of precise point positioning (PPP) in post-processing and simulated real-time mode.
After a short description of the data set, we introduce the EPOS-RT software package, which is used for all the data processing. Then we explain the processing strategies for the various investigations, and finally present the results and discuss them in detail.
Tracking Data
The GNSS research center at Wuhan University is deploying its own global GNSS network for scientific purposes, focusing on the study of Compass, as there are already plenty of data on the GPS and GLONASS systems. At this point there are more than 15 stations in China and its neighboring regions.
Two weeks of tracking data from days 122 to 135 in 2012 is made available for the study by the GNSS Research Center at Wuhan University, with the permission of the Compass authorities. The tracking stations are equipped with UR240 dual-frequency receivers and UA240 antennas, which can receive both GPS and Compass signals, and are developed by the UNICORE company in China. For this study, 12 stations are employed. Among them are seven stations located in China: Chengdu (chdu), Harbin (hrbn), HongKong (hktu), Lhasa (lasa), Shanghai (sha1), Wuhan (cent) and Xi’an (xian); and five more in Singapore (sigp), Australia (peth), the United Arab Emirates (dhab), Europa (leid) and Africa (joha). Figure 1 shows the distribution of the stations, while Table 1 shows the data availability of each station during the selected test period.
Table 1. Data availability of the stations in the test network.
There were 11 satellites in operation: four GEOs (C01, C03, C04, C05), five IGSOs (C06, C07, C08, C09, C10), and two MEOs (C11, C12). During the test time, two maneuvers were detected, on satellite C01 on day 123 and on C06 on day 130. The two MEOs are not included in the processing because they were still in their test phase.
Software Packages
The EPOS-RT software was designed for both post-mission and real-time processing of observations from multi-techniques, such as GNSS and satellite laser ranging (SLR) and possibly very-long-baseline interferometry (VLBI), for various applications in Earth and space sciences. It has been developed at the German Research Centre for Geosciences (GFZ), primarily for real-time applications, and has been running operationally for several years for global PPP service and its augmentation. Recently the post-processing functions have been developed to support precise orbit determinations of GNSS and LEOs for several ongoing projects.
We have adapted the software package for Compass data for this study. As the Compass signal is very similar to those of GPS and Galileo, the adaption is straight-forward thanks to the new structure of the software package. The only difference to GPS and Galileo is that recently there are mainly GEOs and IGSOs in the Compass system, instead of only MEOs. Therefore, most of the satellites can only be tracked by a regional network; thus, the observation geometry for precise orbit determination and for positioning are rather different from current GPS and GLONASS.
Figure 2 shows the structure of the software package. It includes the following basic modules: preprocessing, orbit integration, parameter estimation and data editing, and ambiguity-fixing. We have developed a least-square estimator for post-mission data processing and a square-root information filter estimator for real-time processing.
Figure 2. Structure of the EPOS-RT software.
GPS Data Processing
To assess Compass-derived products, we need their so-called true values. The simplest way is to estimate the values using the GPS data provided by the same receivers.
First of all, PPP is employed to process GPS data using International GNSS Service (IGS) final products. PPP is carried out for the stations over the test period on a daily basis, with receiver clocks, station coordinates, and zenith tropospheric delays (ZTD) as parameters. The repeatability of the daily solutions confirms a position accuracy of better than 1 centimeter (cm), which is good enough for Compass data processing. The station clock corrections and the ZTD are also obtained as by-products.
The daily solutions are combined to get the final station coordinates. These coordinates will be fixed as ground truth in Compass precise orbit and clock determination. Compass and GPS do not usually have the same antenna phase centers, and the antenna is not yet calibrated, thus the corresponding corrections are not yet available. However, this difference could be ignored in this study, as antennas of the same type are used for all the stations.
Orbit and Clock Determination
For Compass, a three-day solution is employed for precise orbit and clock estimation, to improve the solution strength because of the weak geometry of a regional tracking network. The orbits and clocks are estimated fully independent from the GPS observations and their derived results, except the station coordinates, which are used as known values.
The estimated products are validated by checking the orbit differences of the overlapped time span between two adjacent three-day solutions. As shown in Figure 3, orbit of the last day in a three-day solution is compared with that over the middle day of the next three-day solution. The root-mean-square (RMS) deviation of the orbit difference is used as index to qualify the estimated orbit.
Figure 3. Three-day solution and orbit overlap. The last day of a three-day solution is compared with the middle day of the next three-day solution.
In each three-day solution, the observation models and parameters used in the processing are listed in Table 2, which are similar to the operational IGS data processing at GFZ except that the antenna phase center offset (PCO) and phase center variation (PCV) are set to zero for both receivers and satellites because they are not yet available.
Satellite force models are also similar to those we use for GPS and GLONASS in our routine IGS data processing and are listed in Table 2. There is also no information about the attitude control of the Compass satellites. We assume that the nominal attitude is defined the same as GPS satellite of Block IIR.
Table 2. Observation and force models and parameters used in the processing.
Satellite Orbits. Figure 4 shows the statistics of the overlapped orbit comparison for each individual satellite. The averaged RMS in along- and cross-track and radial directions and 3D-RMS as well are plotted. GEOs are on the left side, and IGSOs on the right side; the averaged RMS of the two groups are indicated as (GEO) and (IGSO) respectively. The RMS values are also listed in Table 3.
As expected, GEO satellites have much larger RMS than IGSOs. On average, GEOs have an accuracy measured by 3D-RMS of 288 cm, whereas that of IGSOs is about 21 cm.
As usual, the along-track component of the estimated orbit has poorer quality than the others in precise orbit determination; this is evident from Figure 4 and Table 3. However, the large 3D-RMS of GEOs is dominated by the along-track component, which is several tens of times larger than those of the others, whereas IGSO shows only a very slight degradation in along-track against the cross-track and radial. The major reason is that IGSO has much stronger geometry due to its significant movement with respect to the regional ground-tracking network than GEO.
Figure 4. Averaged daily RMS of all 12 three-day solutions. GEOs are on the left side and IGSOs on the right. Their averages are indicated with (GEO) and (IGSO), respectively.Table 3. RMS of overlapped orbits (unit, centimeters).
If we check the time series of the orbit differences, we notice that the large RMS in along-track direction is actually due to a constant disagreement of the two overlapped orbits. Figure 5 plots the time series of orbit differences for C05 and C06 as examples of GEO and IGSO satellites, respectively. For both satellites, the difference in along-track is almost a constant and it approaches –5 meters for C05.
Note that GEO shows a similar overlapping agreement in cross-track and radial directions as IGSO.
Figure 5. Time series of orbit differences of satellite C05 and C06 on the day 124 2012. A large constant bias is in along-track, especially for GEO C05.
Satellite Clocks. Figure 6 compares the satellite clocks derived from two adjacent three-day solutions, as was done for the satellite orbits. Satellite C10 is selected as reference for eliminating the epoch-wise systematic bias. The averaged RMS is about 0.56 ns (17 cm) and the averaged standard deviation (STD) is 0.23 ns (7 cm). Satellite C01 has a significant larger bias than any of the others, which might be correlated with its orbits.
From the orbit and clock comparison, both orbit and clock can hardly fulfill the requirement of PPP of cm-level accuracy. However, the biases in orbit and clock are usually compensatable to each other in observation modeling. Moreover, the constant along-track biases produce an almost constant bias in observation modeling because of the slightly changed geometry for GEOs. This constant bias will not affect the phase observations due to the estimation of ambiguity parameters. Its effect on ranges can be reduced by down-weighting them properly. Therefore, instead of comparing orbit and clock separately, user range accuracy should be investigated as usual. In this study, the quality of the estimated orbits and clocks is assessed by the repeatability of the station coordinates derived by PPP using those products.
Figure 6. Statistics of the overlap differences of the estimated receiver and satellite clocks. Satellite C10 is selected as the reference clock.
Precise Point Positioning
With these estimates of satellite orbits and clocks, PPP in static and kinematic mode are carried out for a user station that is not involved in the orbit and clock estimation, to demonstrate the accuracy of the Compass PPP service.
In the PPP processing, ionosphere-free phase and range are used with proper weight. Satellite orbits and clocks are fixed to the abovementioned estimates. Receiver clock is estimated epoch-wise, remaining tropospheric delay after an a priori model correction is parameterized with a random-walk process. Carrier-phase ambiguities are estimated but not fixed to integer. Station coordinates are estimated according to the positioning mode: as determined parameters for static mode or as epoch-wise independent parameters for kinematic mode.
Data from days 123 to 135 at station CHDU in Chengdu, which is not involved in the orbit and clock determination, is selected as user station in the PPP processing. The estimated station coordinates and ZTD are compared to those estimated with GPS data, respectively.
Static PPP. In the static test, PPP is performed with session length of 2 hours, 6 hours, 12 hours, and 24 hours. Figure 7 and Table 4 show the statistics of the position differences of the static solutions with various session lengths over days 123 to 125.
The accuracy of the PPP-derived positions with 2 hours data is about 5 cm, 3 cm, and 10 cm in east, north, and vertical, compared to the GPS daily solution. Accuracy improves with session lengths. If data of 6 hours or longer are involved in the processing, position accuracy is about 1 cm in east and north and 4 cm in vertical. From Table 4, the accuracy is improved to a few millimeters in horizontal and 2 cm in vertical with observations of 12 to 24 hours. The larger RMS in vertical might be caused by the different PCO and PCV of the receiver antenna for GPS and Compass, which is not yet available.
Figure 7. Position differences of static PPP solutions with session length of 2 hours, 6 hours, 12 hours, and 24 hours compared to the estimates using daily GPS data for station CHDU.Table 4. RMS of PPP position with different session length.
Kinematic PPP. Kinematic PPP is applied to the CHDU station using the same orbit and clock products as for the static positioning for days 123 to 125 in 2012.
The result of day 125 is presented here as example. The positions are estimated by means of the sequential least-squares adjustment with a very loose constraint of 1 meter to positions at two adjacent epochs. The result estimated with backward smoothing is shown in Figure 8. The differences are related to the daily Compass static solution. The bias and STD of the differences in east, north, and vertical are listed in Table 5. The bias is about 16 mm, 13 mm, and 1 mm, and the STD is 10 mm, 14 mm and 55 mm, in east, north, and vertical, respectively.
Figure 8. Position differences of the kinematic PPP and the daily static solution, and number of satellites observed.Table 5. Statistics of the position differences of the kinematic PPP in post-processing mode and the daily solution. (m)
Compass-Derived ZTD. ZTD is a very important product that can be derived from GNSS observations besides the precise orbits and clocks and positions. It plays a crucial role in meteorological study and weather forecasting.
ZTD at the CHDU station is estimated as a stochastic process with a power density of 5 mm √hour by fixing satellite orbits, clocks, and station coordinates to their precisely estimated values, as is usually done for GPS data.
The same processing procedure is also applied to the GPS data collected at the station, but with IGS final orbits and clocks. The ZTD time series derived independently from Compass and GPS observations over days 123 to 125 in 2012 and their differences are shown on Figure 9.
Figure 9. Comparison of ZTD derived independently from GPS and COMPASS observations. The offset of the two time series is about -14 mm (GPS – COMPASS) and the STD is about 5 mm.
Obviously, the disagreement is mainly caused by Compass, because GPS-derived ZTD is confirmed of a much better quality by observations from other techniques. However, this disagreement could be reduced by applying corrected PCO and PCV corrections of the receiver antennas, and of course it will be significantly improved with more satellites in operation.
Simulated Real-Time PPP Service
Global real-time PPP service promises to be a very precise positioning service system. Hence we tried to investigate the capability of a Compass real-time PPP service by implementing a simulated real-time service system and testing with the available data set.
We used estimates of a three-day solution as a basis to predict the orbits of the next 12 hours. The predicted orbits are compared with the estimated ones from the three-day solution. The statistics of the predicted orbit differences for the first 12 hours on day 125 in 2012 are shown on Figure 10.
From Figure 10, GEOs and IGSOs have very similar STDs of about 30 cm on average. Thus, the significantly large RMS, up to 6 meters for C04 and C05, implies large constant difference in this direction. The large constant shift in the along-track direction is a major problem of the current Compass precise orbit determination. Fortunately, this constant bias does not affect the positioning quality very much, because in a regional system the effects of such bias on observations are very similar.
Figure 10. RMS (left) and STD (right) of the differences between predicted and estimated orbits.
With the predicted orbit hold fixed, satellite clocks are estimated epoch-by-epoch with fixed station coordinates. The estimated clocks are compared with the clocks of the three-day solution, and they agree within 0.5 ns in STD. As the separated comparison of orbits and clocks usually does not tell the truth of the accuracy of the real-time positioning service, simulated real-time positioning using the estimated orbits and clocks is performed to reveal the capability of Compass real-time positioning service.
Figure 11 presents the position differences of the simulated real-time PPP service and the ground truth from the static daily solution. Comparing the real-time PPP result in Figure 11 and the post-processing result in Figure 8, a convergence time of about a half-hour is needed for real-time PPP to get positions of 10-cm accuracy. Afterward, the accuracy stays within ±20 cm and gets better with time. The performance is very similar to that of GPS because at least six satellites were observed and on average seven satellites are involved in the positioning. No predicted orbit for C01 is available due to its maneuver on the day before. Comparing the constellation in the study and that planned for the regional system, there are still one GEO and four MEOs to be deployed in the operational regional system. Therefore, with the full constellation, accuracy of 1 decimeter or even of cm-level is achievable for the real-time precise positioning service using Compass only.
Figure 11. Position differences of the simulated real-time PPP and the static daily PPP. The number of observed satellites is also plotted.
Summary
The three-day precise orbit and clock estimation shows an orbit accuracy, measured by overlap 3D-RMS, of better than 288 cm for GEOs and 21 cm for IGSOs, and the accuracy of satellite clocks of 0.23 ns in STD and 0.56 in RMS. The largest orbit difference occurs in along-track direction which is almost a constant shift, while differences in the others are rather small.
The static PPP shows an accuracy of about 5 cm, 3 cm, and 10 cm in east, north, and vertical with two hours observations. With six hours or longer data, accuracy can reach to 1 cm in horizontal and better than 4 cm in vertical. The post-mission kinematic PPP can provide position accuracy of 2 cm, 2 cm, and 5 cm in east, north, and vertical. The high quality of PPP results suggests that the orbit biases, especially the large constant bias in along-track, can be compensated by the estimated satellite clocks and/or absorbed by ambiguity parameters due to the almost unchanged geometry for GEOs.
The simulated real-time PPP service also confirms that real-time positioning services of accuracy at 1 decimeter-level and even cm–level is achievable with the Compass constellation of only nine satellites. The accuracy will improve with completion of the regional system.
This is a preliminary achievement, accomplished in a short time. We look forward to results from other colleagues for comparison. Further studies will be conducted to validate new strategies for improving accuracy, reliability, and availability. We are also working on the integrated processing of data from Compass and other GNSSs. We expect that more Compass data, especially real-time data, can be made available for future investigation.
UA240 OEM card made by Unicore company and used in Compass reference stations.
Acknowledgments
We thank the GNSS research center at Wuhan University and the Compass authorities for making the data available for this study.
The material in this article was first presented at the ION-GNSS 2012 conference.
Maorong Ge received his Ph.D. in geodesy at Wuhan University, China. He is now a senior scientist and head of the GNSS real-time software group at the German Research Centre for Geosciences (GFZ Potsdam).
Hongping Zhang is an associate professor of the State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing at Wuhan University, and holds a Ph.D. in GNSS applications from Shanghai Astronomical Observatory. He designed the processing system of ionospheric modeling and prediction for the Compass system.
Xiaolin Jia is a senior engineer at Xian Research Institute of Surveying and Mapping. He received his Ph.D. from the Surveying and Mapping College of Zhengzhou Information Engineering University.
Shuli Song is an associate research fellow. She obtained her Ph.D. from the Shanghai Astronomical Observatory, Chinese Academy of sciences.
Jens Wickert obtained his doctor’s degree from Karl-Franzens-University Graz in geophysics/meteorology. He is acting head of the GPS/Galileo Earth Observation section at the German Research Center for Geosciences GFZ at Potsdam.
