Likely none of us needs a reminder as the upcoming leap second has been all over the news outlets for the past few days. But just to provide the details again, read this article.
Presumably, all GPS receiver manufacturers have checked to make sure their receivers will handle the leap second properly. However, at least one late-model high-end receiver from a leading manufacturer is currently reporting incorrect advance leap second information in its data files.
The European Satellite Services Provider (ESSP), the EGNOS system operator and EGNOS safety-of-life service provider, announced in a service notice dated 22 May that there might be an interruption in service for a 72-hour period should the leap second not be managed correctly.
AGI, a company that develops commercial modeling and analysis software for the space, defense and intelligence communities, has warned: “The consequence of failing to accommodate this event is that orbit in-plane motion and corresponding Earth orientation will both become inaccurate by at least one second until the leap second is properly implemented. This will also affect estimating orbits using time sequences of observations spanning this leap second event. GEO satellites might be inaccurate to about 3 km and LEO satellites to about 8 km. How great the discrepancy will be depends on how long one waits to implement the leap second. The probable inaccuracies may be within the collision keep-out zones of many satellites, causing either false alarms or totally missed threat detections.”
And it has also been reported that some computer operating systemsmight hang due to improper handling of the leap second.
An article on the upcoming leap second for the popular press may be found here. And, in case you missed it, a recent Physics Today article on the leap second and its future can be found here.
UPDATE: The Interfax news agency has announced that the rescheduled launch date for SES-5 from the Baikonur Cosmodrome, originally scheduled for June 18, is August 6, 2012.
The launch is being delayed due to a problem with a first stage subsystem on the Proton launch vehicle. The rocket has been rolled back to the assembly building for further tests.
SES-5 is also known as Sirius 5 stemming from the development of the Sirius satellite constellation by Nordic Satellite AB, now owned by Luxembourg's SES.
The satellite carries a transponder for the European Geostationary Navigation Overlay Service (EGNOS). The transponder is intended to eventually replace or one of those on the currently used EGNOS satellites (Inmarsat 3-F2 at 15.5 degrees west using PRN 120, Artemis at 21.5 degrees east using PRN124, and Inmarsat-4-F2 at 25 degrees east using PRN 126 and designated for industry tests).
Unlike the present L1-only EGNOS satellites, SES-5 will have transponders on both the L1 and E5 frequencies similar to the setup on the Wide Area Augmentation System satellites, which broadcast on L1 and L5.
SES-5 is to be stationed at 5 degrees east longtiude.
A second SES satellite with EGNOS transponders is under construction. The SES Astra 5B satellite is scheduled for launch in the second quarter of 2013 and will be positioned at SES Astra's 31.5 degrees east orbital position.
Role Switch. On March 22 and 23, Inmarsat-4-F2 at 25 degrees east using PRN126 and Artemis at 21.5 degrees east using PRN124 switched roles. PRN126 became an EGNOS operational signal-in-space satellite while PRN124 became the test satellite, transmitting message type 0. PRN120 and PRN126 returned to service around 17:00 UTC on Tuesday, June 26.
According to an EGNOS service announcement dated April 3, the switch was due to the aging state of the Artemis satellite.
The European GNSS Agency (GSA) Wednesday published a contract notice in the Official Journal of the European Union inviting operators to bid for the provision of EGNOS services over the 2014-2021 period. This contract will consist in operating, maintaining and upgrading the EGNOS system infrastructure, and ensuring the continuous and safe provision of the three services offered by EGNOS.
The new EGNOS service provision contract is planned to be awarded in 2013 and is aimed at guaranteeing the provision of EGNOS services for eight years starting on January 1, 2014, without service interruption. The future EGNOS operator shall become certified for provision of the EGNOS services according to the Single European Sky (SES) regulation. The requests to participate shall be transmitted to the GSA by July 16 and the deadline for submission of initial tenders is expected to be in November 2012.
The GSA is carrying out this procurement on behalf of the European Commission and is expected to become responsible for the management of EGNOS from 2014.
All the documentation related to this call for tender can be found on the GSA procurement link here.
The European Geostationary Navigation Overlay Service (EGNOS) improves the accuracy of GPS by using 34 ranging and integrity monitoring stations (RIMS) that receive signals from the U.S. GPS satellites. Four mission control centers handle data processing and differential corrections counting and six navigation land earth stations manage accuracy and reliability data for sending to the three geostationary satellite transponders for relay to end-user devices.
EGNOS offers 3 services:
Open Service: free and open for anyone with an EGNOS-enabled GPS device.
Safety-of-life Service: provides an integrity message warning the user of any malfunction of the GPS signal in 6 seconds. This is essential when satellite navigation is used for applications where lives are at stake. EGNOS was certified for civil aviation in 2011.
The EGNOS Data Access Service (EDAS): provides EGNOS information in real time over the internet.
EGNOS is the first pan-European satellite navigation system. Similar services are provided in North America by the Wide Area Augmentation System (WAAS) and in Japan by the Multifunctional Satellite Augmentation System (MSAS).
UPDATE: The Interfax news agency has announced that the rescheduled launch date for SES-5 from the Baikonur Cosmodrome, originally scheduled for June 18, is August 6, 2012.
The launch is being delayed due to a problem with a first stage subsystem on the Proton launch vehicle. The rocket has been rolled back to the assembly building for further tests.
SES-5 is also known as Sirius 5 stemming from the development of the Sirius satellite constellation by Nordic Satellite AB, now owned by Luxembourg’s SES.
The satellite carries a transponder for the European Geostationary Navigation Overlay Service (EGNOS). The transponder is intended to eventually replace or one of those on the currently used EGNOS satellites (Inmarsat 3-F2 at 15.5 degrees west using PRN 120, Artemis at 21.5 degrees east using PRN124, and Inmarsat-4-F2 at 25 degrees east using PRN 126 and designated for industry tests).
Unlike the present L1-only EGNOS satellites, SES-5 will have transponders on both the L1 and E5 frequencies similar to the setup on the Wide Area Augmentation System satellites, which broadcast on L1 and L5.
SES-5 is to be stationed at 5 degrees east longtiude.
A second SES satellite with EGNOS transponders is under construction. The SES Astra 5B satellite is scheduled for launch in the second quarter of 2013 and will be positioned at SES Astra’s 31.5 degrees east orbital position.
Role Switch. On March 22 and 23, Inmarsat-4-F2 at 25 degrees east using PRN126 and Artemis at 21.5 degrees east using PRN124 switched roles. PRN126 became an EGNOS operational signal-in-space satellite while PRN124 became the test satellite, transmitting message type 0. PRN120 and PRN126 returned to service around 17:00 UTC on Tuesday, June 26.
According to an EGNOS service announcement dated April 3, the switch was due to the aging state of the Artemis satellite.
Free downloadable software Toolkits at www.egnos-portal.eu can help cell-phone and handheld receiver developers enhance location and timing applications with GPS corrrection data from the European Geostationary Navigation Overlay Service (EGNOS) satellite-based augmentation system.
The Toolkits include software packages, demo applications, and supporting materials, enabling application developers, researchers, university students, and others to create, use, and maintain EGNOS-capable positioning applications.
For handheld receiver manufacturers and mobile-phone developers, the Toolkit contains free source code for easy integration of EGNOS capabilities into a smartphone, and all the necessary files for the demonstration application, for use as a basis for a new application, as well as core libraries, to integrate enhanced EGNOS positioning capability into an existing application.
For the simply curious, an EGNOS Toolkit provides a means of exploring and understanding the entire chain from the raw GNSS satellite signal to enhanced EGNOS positioning data.
The development kit provides an easy way incorporate all EGNOS corrections and integrity capabilities, allowing developers to perform real EGNOS integration directly into a smartphone. It works with different operating systems, including Android, Apple, and RIM.
Static and kinematic tests show that EGNOS performs well in both cases: “The EGNOS SDK provides an average increase of 30 percent in position accuracy over GPS alone,“ according to developer DKE Aerospace.
EGNOS Software Development Kit provides a software receiver to enhance GPS positions, displaying position accuracy increases on average of 30 percent.
DOT Blank Stare on LightSquared
The U.S. Department of Transportation (DoT) responded to a Freedom of Information Act (FOIA) request by GPS World for its recommendations to the National Telecommunications and Information Administration (NTIA) regarding LightSquared interference with GPS. The DoT wrote, “We are withholding two pages [of thirteen relevant pages] in part and eleven pages in their entirety,” and enclosed two completely blacked-out pages.
Kathy Ray, DoT FOIA officer, added, “We have determined that the release of the redacted and withheld portions would foreseeably cause harm to the government’s deliberative process.”
The blacked-out DOT letter is dated August 25, 2011. How it differs from the agency’s July 21 “LightSquared Impact Assessment,” publicly available courtesy of the U.S. House of Representatives Committee on Science, Space, and Technology, cannot, of course, be known.
The Department of Homeland Security wrote in response to GPS World’s FOIA request, “We conducted a comprehensive search of files with the Science and Technology Directorate’s Homeland Security Enterprise and First Responders Group, and Cyber Security Division for records that would be responsive to your request. Unfortunately, we were unable to locate or identify any responsive records.”
The National Institute of Standards and Technology of the Department of Commerce replied, “NIST has no documents that are responsive to your request.”
The Department of the Interior provided the same documents that were previously made public by the House committee.
The National Aeronautics and Space Administration made a similar determination, but did not send a document, referring instead directly to the committee’s public website.
PNT Board Hears Proposal for LightSquared Solution
The November 9 meeting of the National Space-Based Position Navigation and Timing (PNT) Advisory Board in Alexandria, Virginia got several earfulls regarding the LightSquared/GPS controversy. One of seven speakers on a two-hour panel, Javad Ashjaee, president and CEO of JAVAD GNSS, demonstrated his company’s newly developed filter technology that he said could protect GPS receivers from LightSquared broadband network interference.
As Ashjaee stated, the proposed solution does not protect against interference from the so-called high-10 signals, one of two bands (the other is known as the low-10) for which LightSquared has received a conditional waiver. Unless and until a solution for the terrestrial high-10 signals is found, LightSquared transmissions in that band will still interfere with the GPS signal. The technical solution proposed by JAVAD GNSS addressed only the low-10 band.
Proposed filter to “harden” high-precision GPS receivers against Lightsquared Lower 10 (click to enlarge.)
The JAVAD GNSS proposed fix consists, in simplified form, of a ceramic filter followed by a series of surface acoustic wave (SAW) filters.
A PDF of Ashjaee’s 76-slide Powerpoint demonstration, without his verbal explanations and commentary, along with other presentations from the board meeting, are available at www.pnt.gov/advisory/2011/11/. A December 8 GPS World webinar reprised the same presentation, and the download at env-gpsworld-integration.kinsta.cloud/webinar includes audio of Ashjaee’s remarks.
Ashjaee said that his company’s testing of its own filter methodology found no GPS signal loss due to a low-10 (10L) signal power of –10 dBm. An “Ultimate Test: Special Zero Baseline” put receivers on a Moscow skyscraper with multipath from both above and below. One antenna fed two receivers (zero baseline). One receiver used standard filtering and the other the new filters. He said that over 15 hours of testing the average carrier-phase error between the two receivers was 0.2 millimeters, and the average code difference was about 5 centimeters.
JAVAD GNSS has started production of what Ashjaee calls “LightSquared-compatible” Triumph GNSS receivers. He brought 40 units to the PNT Board meeting. The company will begin manufacturing “LightSquared-integrated” receivers in May 2012, for RTK positioning using the proposed LightSquared broadband network for high-speed communication, if and when it is deployed.
Fellow presenter Jim Kirkland, vice president and general counsel for Trimble Navigation, pointed out that such filters represented a potential solution only for one class of high-precision receivers. Whether it would work for other classes of high-precision receivers had yet to be verified. Kirkland said that even if further independent testing shows that the filter solution is viable at the lower 10 MHz of the spectrum, retrofits would be costly and time consuming.
Questions regarding cost and responsibility of retrofit, should the solution prove practical, were not discussed at length at the meeting, nor was any solution proposed.
LightSquared executive vice president Martin Harriman did not directly answer a question as to whether his company intends to develop the upper 10 MHz for which it has been given a conditional waiver.
Scott Burgett, software engineering manager for Garmin International, said, “It is almost impossible to design new products compatible with LightSquared’s proposed system without knowing its technology’s end state.” He estimated 10–15 years to properly retrofit Garmin devices, which are widely distributed in general aviation, personal navigation, car navigation, and other sectors, so that they could coexist with LightSquared.
The panel was moderated by Tom Stansell of Stansell Consulting, who concluded, “I think we learned, thanks to Javad, about a very clever solution to a particular problem for a particular range of products — the products he is most familiar with. It may or may not fit in some of the other applications.
“What we have not addressed is the elephant in the living room,” Stansell continued. “That is the cost, and time delay, and changeover process if LightSquared is allowed to go forward. Will it be the lower 10, upper 10? That has to be resolved. There are very large questions remaining to be discussed, and [they] may or may not be fully solved in a short period of time.”
Constellation Updates
Where Is Compass ICD?
The long-awaited signal interface control document (ICD) for China’s Beidou/Compass GNSS has not yet appeared, despite an announcement at the ION-GNSS conference by Chinese delegates that ICD document v1.0 will be published in 2011, “probably” in the month of October. When it does appear, it should be available for download on the Compass website, www.beidou.gov.cn (as yet without an English version), also at www.compass.gov.cn.
The delay in publishing a document may reflect a system very much in formulation, with ongoing discussions among the principal parties to its design, with different views on system architecture and possibly even final signal structure. This was one possible conclusion that could be inferred — a dynamic system in formation and growing rapidly — from varying reports given by different Chinese representatives, governent and academic, at the ION Compass session.
There was some disagreement among panelists at that time as to, for example, the final targeted number of satellites in the system: either 30, or 35.
The ICD has been rumored to be available previously to receiver manufacturers within China, creating some disgruntlement among companies outside the country. One of the ION panelists affirmed that GPS/Compass chips and receivers are being actively developed by many Chinese manufacturers and research institutes.
The next BeiDou/Compass launch, which will be for the system’s fifth inclined geosynchronous orbit satellite, is expected during the first few days of December, according to web discussions. As of press time for this magazine, there had been no official announcement on the Chinese official government BeiDou website, www.compass.gov.cn.
The site has posted Chinese and English versions of a document titled “Report on the Development of BeiDou (COMPASS) Navigation Satellite System (V1.0)” by the China Satellite Navigation Office. The pages are viewable as separate images.
Galileo Under Control
Europe’s first two in-orbit validation satellites reached their final operating slotss 23,222 kilometers above Earth, have been activated, and are now undergoing tests of their navigation payloads, reports the European Space Agency (ESA).
Marking the formal end of their Launch and Early Operations Phase, control of the satellites passed on November 3 from the French space agency (CNES) center in Toulouse to the Galileo Control Centre in Oberpfaffenhofen, Germany.
Oberfaffenhofen, operated by the German Aerospace Center (DLR), will be in charge of the satellites’ command and control for the whole of their 12-year operating lives. The navigation signals are being checked out by ESA’s ground station in Redu, Belgium, where a 20-meter antenna measures the shape of the signals to a high degree of accuracy. Once the navigation payload is fully checked out and activated, a second Galileo Control Centre in Fucino, Italy, will oversee all navigation services. All activities are performed under contract to SpaceOpal, a joint subsidiary of DLR and the Italian company Telespazio.
GLONASS as Expected
The Satellite System Mission Control Center of the Russian Ministry of Defence, with the ISS-Reshetnev Information Computation Center, established communication with the three GLONASS satellites launched November 4. The satellites are earth- and sun-oriented, and their subsystems are functioning properly.
According to NORAD tracking, the three satellites were inserted into Plane 1. This was expected as there are only seven active satellites in this plane, whereas the other two planes have a full complement of eight satellites. Orbit slot 3 in Plane 1 is currently vacant. According to Nikolay Testoyedov, ISS-Reshetnev general designer and director general, the new satellites will ensure the operation of a complete 24-satellite GLONASS constellation, and allow creating the necessary orbital reserve.
GPS GEO-MEO Floated
In a presentation titled “Analysis of Alternatives for Future GPS Architecture; Considerations for Constellation Sustainment,” made to the U.S. PNT Advisory Board on November 9, Kirk Lewis, senior advisor from the Institute for Defense Analyses (IDA), put forth the concept of “boosting” GPS III payloads onto commercial geostationary Earth-orbit (GEO) satellites.
After concluding that the current program of launches and orbit costs extending into the Block III-C generation is not sustainable, Lewis presented several alternatives, but quickly eliminated two that involved low-Earth-orbit satellites and non-space options, due to technical, scheduling, and performance issues. Remaining in play are “potential and realistic” GEO and mid-Earth orbit (MEO, the configuration of the present GPS constellation) options, used individually or in combination.
IDA analysis found that two GEO satellites, separated by 15 degrees or more longitude, supplied almost the same signal performance as adding six MEO satellites. The presentation is available at www.pnt.gov/advisory/2011/11/.
First Overload Interference/Desensitization to GPS Receivers, Systems, and Networks Report to FCC
The joint working group co-led by the U.S. GPS Industry Council and Lightsquared, investigating potential problems of LightSquared/GPS interference, delivered its first monthly report on March 15 as directed by the FCC. The report (PDF) lays out a schedule for receiver selection and testing and names 34 members, two working group co-chairs, and four information facilitators of a technical working group (TWG) supervising and analyzing the assessment of GNSS receivers operating under conditions of a dense national network of high-powered cell-phone transmitters. “TWG members represent a diverse group of interested parties including equipment and chipset manufacturers, aerospace/aviation companies, wireless providers, engineering firms, public safety, and various federal agencies. Additionally, several individuals have volunteered to be advisors to the TWG,” said the report.
The TWG held its first meeting on March 3 in Arlington, Virginia, and via a conference bridge for members around the globe who were unable to attend in person. In that and subsequent teleconferences, the TWG focused on the first seven items from the Work Plan:
Establish pertinent analytical and test methodologies and assumptions underlying the test regime: definition of harmful interference, relevant information regarding terrestrial broadband network, interference analysis assumptions, and evaluation of potential test methodologies.
Select categories of receivers and receivers to be tested.
Develop operational scenarios.
Establish methodology for analyzing test results.
Derive test conditions based on the established operational scenarios.
Write test plan and procedures.
Identify and engage appropriate test facilities.
LightSquared provided technical details to the TWG regarding the equipment planned for its terrestrial broadband deployment, including the channelization plan, output power, out-of-band emission (OOBE) characteristics, and emissions mask.
The GPS community is concerned that desensitization/overload due to strong signals outside of the GPS band may cause GPS receivers to operate in a non-linear mode with reduced gain (that is, gain compression) for the desired GPS signal. Other receiver impairments may also arise as a result of the nearby strong signals.
The TWG has agreed to move forward with a combination of laboratory-based and field-based testing programs. Field testing will be performed at outdoor test locations using transmitters, filters, and antennas similar to those that LightSquared plans to deploy in its commercial operations.
Other items of interest in the report:
Definition of Harmful interference at the GPS/GNSS/Augmentations/L-Band Receiver. “The TWG members have discussed a number of receiver parameters related to the definition of harmful interference. In the FCC Rules, harmful interference is defined as ‘interference which endangers the functioning of a radionavigation service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocommunication service operating in accordance with [the ITU ] Radio Regulations.’
“Harmful interference affects different types of receivers in different ways. The key factors that pertain to the functioning of GPS receivers and/or whether service is degraded, obstructed, or interrupted are accuracy (position, velocity, time), availability (ability to perform a given function), coverage (within what space can a function be performed), integrity (what is the probability that the results are correct), and continuity (what is the probability that a given function can be completed). Metrics for harmful interference are developed from an understanding of the consequential relationship between negative impacts and receiver parameters, which include effective C/N0, PVT accuracy, time to first fix, loss of lock, cycle slips, etc. The signal conditions to be taken into account are defined in the GPS Standard Positioning Service (SPS) Performance Standard, 4th Edition, Interface Specifications (ISs), GPS policy, and both the present and planned future signal environments will be considered.Environmental and field conditions in which GPS receivers operate will also be considered.
“It should be possible to assess interference impact, up to that which includes harmful interference, using metrics in terms of receiver parameters that include measurable changes in effective C/N0 as well as position accuracy, time to first fix, loss of lock, cycle slips, etc. Related to this discussion is whether there is any margin that could be budgeted for terrestrial broadband operation, and if so, what that amount could be. When considering systems guaranteed for safety-of-life operations, there may be very little or no margin.
“There is general agreement within the TWG that the device testing protocols should include changes in effective C/N0 and degradation of other key performance measures so as not to exclude data that might be relevant for the post-testing analytical phase using operational scenarios.
Overload interference/desensitization at the GPS/GNSS/Augmentations/L-band Receiver. “Desensitization/overload due to strong signals outside of the GPS band may cause the GPS receiver to operate in a non-linear mode with reduced gain (i.e., gain compression) for the desired GPS signal; there may also be other receiver impairments caused by strong signals outside the GPS band. The TWG will consider these mechanisms further after testing is underway and sufficient samples are available to adequately assess such mechanisms.”
Evaluation of Potential Test Methodologies. “The TWG has agreed to move forward with a combination of laboratory-based and field-based testing programs. Laboratory tests are repeatable, allow for the creation of a fully controlled environment and the ability to test multiple scenarios and many devices in an efficient, repetitive manner. Field tests expose devices to a real-world environment where measurements can be performed at various distances and morphologies from terrestrial broadband network sites in order to gauge the effects of distance and physical environments on terrestrial broadband signal strength and potential interference. One advantage of field testing is that it captures a complete, live test environment comprehensively and helps develop keener testing or analysis insights that modeling cannot offer. The major disadvantage or concern is that field testing uses the present environment, not the environment that might exist at some future or past time. Interference testing analysis has to consider worse-case assumptions, and not only the current test reality.
“Laboratory testing will be performed either using conducted testing, where devices are connected directly to transmission sources via 50 ohm connectors, or through radiated testing in anechoic or other radiated emissions chambers. While conducted testing is the preferred laboratory methodology, anechoic chambers will be used where conducted testing is not practical, is not recommended by the manufacturer, or where connectorized devices cannot be made available within the established test timeline.
“Field testing will be performed at outdoor test locations that will utilize transmitters, filters, and antennas similar to those that will be deployed by Lig
htSquared in its commercial operations.”
The TWG identified seven categories of receivers that it considers representative of non-military GPS user equipment operating in the United States: aviation, cellular, general location/navigation, high precison, timing, space-based receivers, and networks.
Seven sub-teams are focusing on these receiver categories. The sub-teams are responsible for determining device selection and prioritization criteria, defining operational scenarios, listing testing conditions and test plan procedures, and recommending appropriate test facilities.
Save Our GPS Coalition Forms
Representatives from a variety of industries and companies have formed the Coalition to Save Our GPS to resolve what it terms a serious threat to the national positioning, navigation, and timing service: the FCC conditional waiver to Lightsquared allowing expansion of terrestrial use of the satellite spectrum immediately neighboring that of GPS, potentially causing severe interference to millions of GPS receivers.
“GPS is essential to Americans every day — it’s in our cars, the airplanes in which we fly and the ambulances, police cars, and fire trucks that help keep us safe. It’s also used in many industrial applications and even synchronizes our wireless, computer, and utility networks,” the group stated. “LightSquared’s plans to build up to 40,000 ground stations transmitting radio signals one billion times more powerful than GPS signals as received on earth could mean 40,000 ‘dead spots’ — each miles in diameter — disrupting the vitally important services GPS provides.”
The Coalition (www.SaveOurGPS.org) includes representatives from aviation, agriculture, transportation, construction, engineering, surveying, and GPS-based equipment manufacturers and service providers.
Initial members of the coalition are the Aeronautical Repair Stations Association, Air Transport Association, Aircraft Owners and Pilots Association, American Association of State Highway and Transportation Officials, American Rental Association, Associated Equipment Distributors, Association of Equipment Manufacturers, Case New Holland, Caterpillar Inc., Edison Electric Institute, Esri, Garmin, General Aviation Manufacturers Association, Deere & Company, National Association of Manufacturers, OmniSTAR, and Trimble. More members are expected to join in the near future.
The following is from a statement issued by the coalition:
“[In] The unusual waiver granted in January to LightSquared by the FCC . . . the usual FCC process of conducting extensive testing followed by approvals was not followed. Instead, the process was approve first, then test. Additional safeguards are needed, so the coalition recommends:
“The FCC must make clear, and the NTIA must ensure, that LightSquared’s license modification is contingent on the outcome of the mandated study. The study must be comprehensive, objective, and based on correct assumptions about existing GPS uses rather than theoretical possibilities.
“The FCC should make clear that LightSquared and their investors should not proceed to make any investment in operating facilities prior to a final FCC decision (or at least make it explicit that they do so at their own risk). While this is the FCC’s established policy, it failed to make this explicit in its order.
“Further, the FCC’s, and NTIA’s, finding that ‘harmful interference concerns have been resolved’ must mean ‘resolved to the satisfaction of preexisting GPS providers and users.’ Resolution of interference has to be the obligation of LightSquared, not the extensive GPS user community of millions of citizens. LightSquared must bear the costs of preventing interference of any kind resulting from operations on LightSquared’s frequencies.
“This is a matter of critical national interest. There must be a reasonable opportunity for public comment of at least 45 days on the report produced by the working group.”
WAAS Official Again
The Federal Aviation Administration (FAA) announced on March 18 that WAAS PRN 135 has resumed normal operations. “The WAAS team recently received the final report from Lockheed Martin on the failure of Galaxy 15,” reported FAA GNSS program manager Leo Eldredge. “After a review of that report, the team determined that the satellite was ready to be returned to operations.”
The FAA said that PRN 135 is currently located at ~120°W and enroute to its final destination of 133.1°W, but is now broadcasting operational corrections that can be used by both aviation and ground users, including those in Northwest Alaska.
In April 2010, satellite operator Intelsat reported it had lost contact with PRN 135 (named Galaxy 15) and it was drifting uncontrolled. At that time, the FAA reported that it would drift out of WAAS service within a few weeks. Instead, PRN 135 remained within a usable condition/location, although drifting east, until December 2010, when it ceased operating. On December 23, Intelsat reported that the power from the Galaxy 15 battery completely drained during its loss of Earth lock and the baseband equipment command unit reset, as it was designed to do. Shortly thereafter Galaxy 15 began accepting commands, and Intelsat engineers began receiving telemetry in the operations center.
Intelsat determined that static electricity charge caused the initial failure, and has uploaded new software to prevent the event from occurring again. There are now three operational WAAS GEO satellites:
◾ PRN 133 located at 98°W.
◾ PRN 135 located at 133.1°W (currently at ~120°W); will arrive at 133.1°W on or about April 4, 2011.
◾ PRN 138 located at 107.3°W.
EGNOS SOL Operational
The European Geostationary Navigation Overlay Service (EGNOS) was declared operational for safety-of-life (SOL) services on March 2. The service consists of GPS corrected signals intended for transport applications, particularly aviation, where lives could be endangered if the performance of the navigation system is degraded.
The SOL coverage area, expected performances, and conditions of use are described in the EGNOS Safety-Of-Life Service Definition Document (SDD, see env-gpsworld-integration.kinsta.cloud/egnosSOL). The two operational EGNOS satellites — Inmarsat-3-F2/AOR-E at 15.5 degrees west longitude using PRN code 120, and Artemis at 21.5 degrees east longitude using PRN code 124 — now transmit Message Type 2, indicating that the signals are available for safety-critical purposes.
Air-navigation service providers can now publish SBAS precision approach procedures, localizer performance with vertical guidance (LPV), based on EGNOS. On March 22, EGNOS operator European Satellite Services Provider published the first EGNOS LPV approaches for use at Pau Airport, near the Pyrénées in southern France.
EGNOS improves accuracy and provides integrity to the GPS signal over most of Europe and parts of North Africa. The system uses a monitoring network of 40 ground stations to provide the corrections with 99.9 percent availability over the core service region. Accuracy is measured by GPS user equivalent range error typically about 4.2 meters after EGNOS corrections for GPS signals from satellites at a 5-degree elevation, and 2.4 meters for satellite signals arriving from a 90-degree elevation. If reliability falls below a minimum level, EGNOS users are alerted within six seconds.
Russian SBAS Satellite Passes Transponder Tests
The Luch-5A geostationary communication satellite under construction has successfully completed a cycle of transponder tests. The satellite includes a transponder for the System for Differential Correction and Monitoring (SDCM), the Russian satellite-based augmentation system. SDCM will provide integrity monitoring of
GPS and GLONASS satellites and differential corrections and analyses of GLONASS performance: real-time differential corrections with horizontal accuracy of 1–1.5 meters, vertical of 2–3 meters.
An Assisted-GNSS Solution Uses the EGNOS Data Access Service
By Kevin Sheridan, Tomas Dyjas, Cyril Botteron, Jérôme Leclère, Fabrizio Dominici, and Gianluca Marucco
For use in billing drivers in road-user charging schemes, onboard units employing GNSS must meet stringent reliability and availability requirements, and at the same time, be based on low-cost equipment systems. The SIGNATURE unit includes an assistance service which provides ephemeris data and corrections from EDAS, optimized for user location.
As roads become more congested, governments and regional authorities seek better ways to manage their existing networks. Road-user charging (RUC) is increasingly promoted to tackle the congestion challenge: charging drivers a fee, perhaps on a monthly billing basis, derived from the distance their vehicles have traveled, time of travel, and whether congested roads were used.
Recording trip information with a GNSS receiver in an onboard unit (OBU) provides a convenient and flexible means to support automated fee collection. For GNSS positioning to be used as the basis for billing drivers, however, it must meet stringent reliability and availability requirements, and at the same time be based on low-cost equipment.
We have developed a prototype to provide both the positioning availability and integrity required for this application. The Simple GNSS Assisted and Trusted Receiver (SIGNATURE) includes an assistance service that provides ephemeris data and corrections from the European Geostationary Navigation Overlay Service (EGNOS) Data Access Service (EDAS), optimized for the user location. Assistance messages are sent to OBUs that can either host an experimental receiver or a commercial-off-the-shelf (COTS) receiver. Data from the receiver is processed with application-specific navigation algorithms on the OBU that aim to improve the integrity of the position solution relative to standard solutions.
Field trials have assessed the contribution that assistance can make to positioning performance, and illustrate options for enhancing standard assistance solutions. Enhancements to assistance encompass modifications to the message content and alternative means of communications, showing the benefits and feasibility of a broadcast service. The impact of including EGNOS corrections through a broadcast assistance service in urban areas is also under investigation.
GNSS Road-User Charging
RUC has the potential to reduce congestion, lower vehicle emissions, and generate revenue streams for infrastructure improvement. It can ensure that revenues are based on actual road usage, creating a financial incentive for changing driving behavior. This might include lower overall use of private cars and, in particular, reducing peak-time travel levels in urban areas by effectively spreading out the morning and evening rush hour. RUC can also encourage commuters to use alternative forms of public transport.
To automate the process of collecting charges, electronic fee-collection (EFC) systems have been developed based largely on dedicated short-range communications (DSRC). In a DSRC solution, a simple tag on the vehicle receives a signal when it passes a roadside beacon and a charge is computed accordingly. Cameras with automatic number-plate recognition (ANPR) technology are also widely used, mainly as an enforcement tool.
Both technologies rely on fixed roadside infrastructure. As charging schemes to date have focused on specific areas (individual cities) or road infrastructure (major motorways, tunnels, and bridges) this type of technology provides an adequate solution.
To meet future policy goals, however, this is not feasible. More extensive charging schemes covering greater areas, more road types, more classes of vehicle, and which will vary charges depending on location and time of day require a far more flexible solution. Flexible schemes require the positioning element to be onboard the vehicle. GNSS-based devices, possibly augmented with other sensors, have been identified as the best option to achieve this.
Using GNSS, the OBU tracks the location of the vehicle, and this is matched against the road network to calculate a charge. A GNSS solution can support many different charging strategies including time distance and place (TDP) based charging for road sections, geographic areas, and cordon schemes. While GNSS offers great potential, several operational and performance limitations have prevented more widespread adoption. Operationally, OBUs are relatively expensive, fraud prevention is potentially complex, and charging schemes must also accommodate infrequent users. GNSS performance is limited in terms of the ability to provide sufficiently accurate positions with high availability and integrity in all operating conditions.
To be fully flexible and to target congested areas, OBUs must work in all environments including urban areas. Urban-canyon problems, with satellite signals blocked and reflected, are well documented. In some cases, not enough signals are available to determine a position, and when there are enough satellites, the ranges will be prone to errors and the geometry is likely to be poor. Signals are more likely to be available in the longitudinal direction of the street, but with few or no satellites on either side of the vehicle. Signal blockage is a particular problem when the GNSS receiver is started up, and it attempts to decode satellite ephemeris data. This requires around 30 seconds of uninterrupted tracking with a relatively strong signal for each satellite, and in an obstructed urban environment it may take many minutes to determine the first receiver position.
Charging schemes typically aim to charge for at least 99 percent of road usage. If a typical journey length is 30 minutes, this means that only 18 seconds with no usable position solution can be tolerated and hence time to first fix (TTFF) must be below 18 seconds, and ideally much lower.
When positions can be determined, they must be sufficiently accurate to identify the correct road segment that the vehicle was on, and they must be reliable. Reliability, or integrity, becomes critical if road users are to be charged on the basis of GNSS-derived positions. Users must have confidence that they are being charged correctly for schemes to be effective and to gain public acceptance.
Whilst GNSS-based solutions are entering the market, for example in Germany and Slovakia for heavy goods vehicles, barriers to wider adoption remain. Many countries are considering GNSS-based road pricing, and they all face similar challenges in ensuring the accuracy, integrity, and availability of a GNSS-based solution for nationwide deployment.
SIGNATURE Solution
The principal objective of the SIGNATURE project is to prototype a GNSS-based solution for flexible road-user charging that can provide the required high integrity and high availability in a cost-effective and scalable manner.
This robust, high-availability, high-integrity solution is delivered firstly through providing reliable assistance (A-GNSS) data from EDAS to optimize receiver acquisition and tracking capabilities and reduce TTFF, and secondly through implementation of embedded GNSS reliability algorithms into an OBU, providing assurance of positioning information (Figure 1, at top).
These features are intended to make a positive contribution in terms of the key RUC performance criteria, as defined by the GNSS Metering Association for Road User Charging:
Accuracy: right cost per trip
Integrity: probability and amount of overcharging
Availability: amount of charged usage.
Assistance Server. An assistance service supplying suitably equipped OBUs is one means to maintain rapid TTFF and meet the requirement for high positioning availability. The most significant contribution assistance can make to TTFF is to provide the
ephemeris data, which takes around 30 seconds to download from a satellite signal. Assistance data can also reduce the frequency search space when a receiver is acquiring signals, as the expected Doppler frequency can be computed from the approximate receiver and satellite positions.
The assistance server in SIGNATURE is based on EDAS, currently available as a beta version. EDAS allows a user to plug into EGNOS to receive the data collected by all the current EGNOS Ranging and Integrity Monitoring Stations (RIMS). This makes it possible to access EGNOS data when there is no clear sight to the EGNOS geostationary satellites, which can often be the case in urban areas, particularly at higher latitudes. As well as supplying EGNOS messages, EDAS also provides GPS observation and navigation (broadcast ephemeris) data, the key component as far as an assistance service is concerned. By recording the ephemeris data received at the extremities of the monitoring network, it is possible to ensure that the current ephemeris for any GPS satellite in view to users over Europe is available and can be supplied in an assistance message. Other data streams provided by EDAS can also be used to enhance the assistance solution.
The main enhancement tested in SIGNATURE was the use of EGNOS corrections within the assistance message. EGNOS today consists of a space segment of three geostationary satellites broadcasting correction and integrity information in the L1 band. Three sets of corrections are broadcast to users:
Fast corrections: used to compensate short-term disturbances in GPS signals, generally attributable to satellite clocks;
Long-term corrections: used to compensate for the longer-term drift in satellite clocks and the errors in the broadcast satellite orbits
Ionospheric corrections: broadcast as a grid of vertical delays (GIVD) from which a user receiver can determine a slant correction to be applied on each range measurement to compensate for the delay experienced by the signal as it passes through the ionosphere.
Fast and long-term corrections are added to the ephemeris data in the assistance message. Rather than relaying the GIVD data to the OBU and letting the receiver reconstruct the ionospheric grid and calculate slant corrections, this is done within the assistance server. A slant correction is provided for each satellite that will be in view at the user location. This approach is valid provided the OBU updates the corrections regularly enough to take account of the changing satellite elevations and ionospheric conditions. It provides a significant saving in terms of processing and memory consumption at the OBU, while still delivering the accuracy benefit of the EGNOS ionospheric data. To correct for the tropospheric delay, a zenith value (ZTD) determined from the RTCA model is also included in the assistance message. Mapping this zenith value to a slant correction to be applied to satellite ranges is a straightforward process easily accommodated on the OBU.
Figure 2 shows how data from EGNOS RIMS is collected at the assistance server at NSL in Nottingham, UK, and then used to generate messages. In this case, the assistance data was provided for trials conducted in Brussels. The figures at the bottom of the plot are the EGNOS correction values provided for all 10 GPS satellite in the positioning solution.
Figure 2. Schematic of assistance solution.
Further enhancements are also possible using the GPS observation data provided through EDAS. Firstly, for areas close to RIMS, a local differential solution can be applied using standard DGPS techniques to provide pseudorange corrections rather than wide-area EGNOS corrections. This has the potential to give greater accuracy for certain areas and is under investigation. By combining EDAS data sources, a GNSS performance monitoring and prediction service has also been created (Figure 3). This provides an assessment of GPS and GPS+EGNOS positioning performance (accuracy, availability of corrections, integrity) at known reference stations as well as monitoring the availability of EDAS data from its central server. Monitoring of this kind can be used as a further tool to identify system-level problems that might significantly degrade user positioning solution, perhaps to a level at which charges could not confidently be applied. It can also aid the enforcement process, as a diagnostic tool to identify if missing or misleading data from an OBU could be due to a system-wide fault or to a more localized source.
Figure 3. GNSS performance monitoring using EDAS.
This approach relies on the approximate user position being known at the assistance server. To maintain the validity of the corrections, it would also require a receiver to update its assistance data at a much high rate than would usually be the case. For a large-scale operation this would be unfeasibly expensive using cellular communications (GSM/GPRS), however it would be possible using a broadcast assistance approach. Using a radio data service (RDS) broadcast for example, ephemeris data and EGNOS corrections could be provided on a continuous basis. RDS is an auxiliary signal to the FM radio broadcast system and is used routinely for supplying travel information to in-car navigation systems. As data is broadcast from known locations and over a definable coverage area, messages can be generated that are applicable for all users receiving data from a given transmitter. A drawback of RDS is that it has a relatively low bandwidth, and it takes approximately 3.5 seconds to broadcast an ephemeris message for a single satellite. A further argument against RDS as a long-term solution is that analog radio signals are progressively being phased out in favour of digital alternatives. With the far greater bandwidth of digital audio bßroadcasting (DAB), ephemeris data for 12 satellites could be broadcast in less than 1 second.
We are evaluating alternative message content and transmission options to determine if real benefits can be gained by providing additional content, other than the ephemerides, in the assistance message.
Onboard Unit. The SIGNATURE OBU (Figure 4) is based on a single-board computer (SBC) offering a high degree of flexibility. Developed by ISMB, it can host alternative receivers and positioning algorithms and manipulate different assistance data with a high degree of configurability. It is a powerful platform for developing and assessing OBU devices and their component parts, particularly as it allows lots of useful diagnostic data to be logged.
Figure 4. SIGNATURE Prototype Onboard Unit (OBU).
The OBU hosts a bespoke receiver which exploits the continuous availability of assistance data available through a high-speed data packet access connection and does not attempt to decode navigation data directly from satellite signals. This allows its design to focus on rapid signal acquisition with high sensitivity and to achieve a rapid TTFF even in areas where conventional receivers struggle. The SIGNATURE prototype has been designed using the well known SAT-SURF & SAT-SURFER platform.
The receiver, developed by the EPFL, implements massive parallelization by making use of the fast Fourier transform, leading to a processing power equivalent to approximately 650,000 equivalent correlators. The minimum sensitivity in acquisition is –145 dBm, obtained using coherent and non-coherent integrations. Thanks to the massive parallelization and the assistance, TTFF at –145 dBm is still below 3 seconds.
Positioning Algorithms. The OBU hosts positioning algorithms designed by NSL to provide high accuracy, availability, an
d integrity through exclusion of outlying measurements and provision of quality metrics (horizontal protection levels or HPLs). Numerous positioning algorithms and outlier detection strategies are being investigated. These include consistency checks applied to raw measurements and computed positions and receiver autonomous integrity monitoring (RAIM). EGNOS corrections are applied to improve accuracy and integrity indicators (user differential range error indices) are used as coarse fault-detection barrier. Consistency checks on measurements include differencing pseudoranges between epochs and checking that this rate is below a defined threshold. A RAIM algorithm is then applied to detect and exclude outliers before measurements enter the main navigation filter. Positions and velocities determined by the filter are then checked again as a further fault barrier. Checks at this stage identify if speeds are within expected ranges for the application and whether height changes are reasonable, for example.
The RAIM algorithm is based on the maximum normed residual method. For the detection procedure, the test statistic is calculated based on weighted sum of the squares of the residuals. This test statistic undergoes a globaltest (chi-square distribution), and is tested against thresholds that are computed based on the probability of false alarm (Pfa) and degrees of freedom (number of measurements minus number of unknowns). The exclusion procedure is based on an outlier detection technique also known as data-snooping, which is based on normed residuals and applied within the range domain. This technique uses measures of internal and external reliability, where the internal reliability gives estimates of how reliable the outlier detection procedure is, while the external reliability gives estimations of the influence of an outlier.
In the final step of the exclusion procedure, the maximum normed residual is tested against a critical value based on the normal inverse cumulative distribution, which in turn depends on the Pfa, and a decision is made on whether or not to exclude measurements. Having performed fault detection and fault exclusion until no further outliers are found, an HPL is calculated. This is the maximum horizontal position error that is guaranteed by the algorithm not to be exceeded, in accordance with the required probabilities of missed detection and false alarm. HPL is a function of visible satellites, expected error characteristics, and user geometry. Measurements which have been screened using the RAIM fault detection and exclusion are then processed in a Kalman filter.
Within the project, many alternative algorithms and configurations are being tested. As well as using RAIM in a snapshot mode to screen measurements entering the Kalman filter, fault detection can also take place within the innovation sequence of the filter itself. Weighting strategies that consider signal-to-noise ratios (SNR) as well as satellite elevations are also being used. This combined weighting is useful in reducing the impact of measurements affected by multipath in urban areas where simple elevation dependent models are often not applicable. The ultimate aim is to produce a robust GNSS positioning solution optimized for RUC in urban areas that balances the requirements of providing high availability with high integrity.
Test Methodology
The SIGNATURE end-to-end solution was tested in a series of field trials in the UK and Italy between April and July 2010. Trials took place in a range of operating conditions from rural areas with open skies to dense urban environments. In all trials, assistance data was provided from the service center in Nottingham, with messages tailored for the designated test area. The OBU recorded real-time position solutions as well as logging all raw measurements. Journey records can be sent back to the service center over a GPRS connection or can be downloaded back at the office. This allowed alternative solutions to be applied to the original datasets in post processing.
The position solutions were assessed through comparisons with high-accuracy GNSS reference solutions provided by additional onboard equipment and through processing with a map matcher (NSL’s Matchbox). Each journey record from a trial was compared against the known reference journey record to determine charging availability, accuracy, and integrity.
Using this approach, it is possible to assess whether improvements in the OBU position output are significant in terms of matching the vehicle location correctly to more road segments and with higher confidence. From direct comparisons between OBU positions and a high-accuracy reference solution alone, it is not possible to determine the significance of any changes in the OBU output in terms of final charging performance. Extensive trials of GNSS OBUs in London, for example, did not observe a relationship between location error (from OBUs) and performance at road segment level (map-matching) as map-matching can compensate for many errors. A strong relationship was observed between data availability and performance, though. Ultimately it is important to consider how successfully vehicle position can be related to charging objects, be they road segments, cordons or virtual toll-gates.
The objectives of the field tests were to:
Demonstrate that all elements of the end-to-end solution work as expected.
Assess the impact of assistance on TTFF.
Evaluate benefits of EGNOS data.
Investigate alternative positioning algorithms to optimize availability and integrity.
Demonstrate the feasibility of broadcast assistance using RDS.
Results
Field trials around Nottingham and Torino tested all elements of the solution. The tests confirmed the successful generation, transmission and use of assistance data, including EGNOS corrections. Position solutions determined onboard were transferred back to the service centre and processed with a map matcher. Figure 5 shows an example from a test in Nottingham city center, correctly identifying all the road segments travelled on.
Figure 5.Journey record view from Nottingham test. (Click to enlarge.)
Assess Impact of Assistance on TTFF. To examine the benefits of assistance, a series of trials were conducted to compare the TTFF of a consumer-grade receiver typically used in road applications against the performance of the SIGNATURE receiver that is assisted in all cases. They assessed TTFF for the COTS receiver in the following modes:
Hot Start: receiver has up-to-date almanac and ephemeris information so only needs to obtain timing/ranging information from each satellite to return its position fix;
Warm Start: receiver has the almanac information stored in its memory, but it does not have any ephemeris information. It also has approximate time and position knowledge. It can use this information to search for satellites but will then need to demodulate the ephemeris data from acquired signals;
Assisted: ephemeris provided over OMA-SUPL standard channel.
Table 1 shows the results from testing the receivers in open sky and urban conditions, specifically chosen to assess an extreme acquisition environment. In these tests when no valid ephemeris is available on a receiver at start-up, it takes an average of 28 seconds to determine a first position fix in open sky conditions. This increase to an average of more than 2 minutes in the worst-case urban environment as the receiver struggles to decode the navigation message on weak, noisy, and intermittent signals. With assistance, the SIGNATURE receiver maintains a rapid TTFF, outperforming the COTS receiver. The slower TTFF in the assisted COTS case may be partly due to the OMA-SUPL standard procedure
which is based on a more complex than the simple data transfer used in the SIGNATURE procedure. The COTS receiver is also decoding navigation subframes to determine signal transmission time. This can take up to 6 seconds depending on the point in the transmission cycle that acquisition begins.
Tests have also been carried out using a signal generator to control the strength of the received signal to assess acquisition and tracking sensitivity. At –145 dBm, the SIGNATURE receiver takes an average of 1.1 seconds to acquire 4 satellites and determine a first fix, and 5.1 seconds to acquire 12 satellites.
Positioning Algorithms. A variety of configurations have been investigated in the positioning algorithms, including applying outlier-detection routines at different stages of the solution and comparing snapshot and filtered approaches. Figure 6 shows a simple example of how the RAIM algorithm has been effective in detecting and excluding outlying measurements contaminated by multipath. By removing these meaurements and re-computing the OBU location, better position estimates are obtained.
Figure 7 shows the accuracy and integrity of the SIGNATURE solution assessed using a high-grade GNSS/INS reference in Nottingham city center. In this case, the horizontal accuracy is 4.4 meters (95 percent), and the computed protection level is shown to bound the actual position error with the required confidence.
Figure 7. Position error and protection level, Nottingham city center.
In rural, semi-urban, and urban (Nottingham) conditions, a positioning solution has been demonstrated that supports all charging accuracy, integrity, and availability requirements.
Further tests were also conducted in the center of London, in a worst-case obstruction environment. In this area the current solution falls just short of the requirements defined for this project. In such cases, better performance could be obtained using a hybrid solution making use of additional sensor inputs, but this will increase equipment costs and potentially installation costs, too. A more practical approach may be to simplify charging schemes in the densest urban environments, using zones and cordons rather than using more detailed approaches that require a continuous high-performance positioning solution to be maintained in all conditions.
Benefits of EGNOS Data. The SIGNATURE solution has the ability to provide EGNOS data to positioning algorithms on the OBU and to vary the rate at which this information is updated and used. Field tests have assessed the potential benefits of this source of data in various environments, starting from the case in which EGNOS messages are continuously available for the positioning solution and then investigating how any beneficial effects lessen as the data is provided less frequently. The greatest benefit from EGNOS was derived by applying corrections prior to performing the RAIM FDE algorithm. This led to more consistent measurements and produced lower HPL values. Figure 8 shows a comparison for a Nottingham test in which a GPS-only solution is compared against an EGNOS solution in which a full set of corrections is provided.
Figure 8. HPL GPS vs GPS + EGNOS.
This reduction in HPL values through the application of EGNOS corrections is clearer when the distribution of HPL values falling into discrete bins is assessed (Figures 9 and 10). Similar levels of relative improvement have been found through using this approach in all test datasets. The significance of these improvements can only be judged against the detailed specifications of a particular charging scheme.
Figure 9. HPL distribution GPS.Figure 10. HPL distribution GPS + EGNOS.
Conclusions
Using an assistance service based on EDAS, it is possible to achieve a TTFF of a few seconds, which supports the high availability requirements of RUC. Field trials showed that providing EGNOS information over the assistance data link had an integrity benefit. Applying corrections prior to a RAIM algorithm leads to more consistent measurements and reduces HPLs. Robust positioning solutions have been developed and implemented on the OBU, and a test methodology has been put in place to assess the impact on charging availability, accuracy, and integrity. Results indicate that GNSS-based road charging offers the performance and flexibility to meet current and future requirements, provided availability and integrity issues are properly taken into account.
Acknowledgments
The SIGNATURE project has received funding from the European Community’s Framework Programme (FP7/2007-2013) under Grant Agreement No. 228237 and is supervised the European GNSS Supervisory Authority (GSA). Full details of the project can be found at www.gnsssignature.org. Any views expressed here are entirely those of the authors and do not necessarily represent the EC.
Manufacturers
The SIGNATURE receiver is based on the Terasic Altera DE3 System with a high-density Stratix III FPGA (EP3S260), and on the Rakon GRM8652 high-performance front end.
Kevin Sheridan is technical manager at Nottingham Scientific Limited (NSL),where he works on development of robust GNSS positioning solutions for urban applications. He has a Ph.D. from University College London.
Tomas Dyjas is a navigation engineer at NSL where he develops and tests positioning algorithms for an experimental OBU for road-user-charging (RUC) and evaluating novel integrity approaches for aviation.
Cyril Botteron manages research and project activities of the GNSS and UWB research subgroups at the Ecole Polytechnique Fédérale of Lausanne (EPFL) in Switzerland. He received a Ph.D. from the University of Calgary.
Jérôme Leclère is a Ph.D. student at EPFL. His research focus is on acquisition and high-sensitivity GNSS receivers.
Fabrizio Dominici is the head of technologies for Galileo/EGNOS applications and embedded systems at Istituto Superiore Mario Boella (ISMB). He received a master’s degree in communications engineering from Politecnico di Torino.
Gianluca Marucco received a master’s degree in electronics engineering from Politecnico di Torino. His research interests include multipath mitigation techniques for future Galileo receivers and real-time performance monitoring services for EGNOS.
Using the Augmentation System with GPS-Equipped Mobile Phones
By François Boullete, Boris Kennes, Michaël Mastier, and Lee Banfield
GPS corrections from the European Geostationary Navigation Overlay Service can improve the positioning accuracy and user experience of GPS-enabled mobile phones, even if EGNOS satellites are not visible and even when the GNSS chipset in the phone does not support satellite-based augmentation systems.
Today, more than 20 percent of mobile phones in use in Europe include a GNSS chipset, and the penetration is expected to exceed 50 percent in the next 5 years. Despite its success in other sectors such as agriculture since the launch of its Open Service in October 2009,
EGNOS has received limited adoption in location-based services (LBS) and consumer applications, due to two main obstacles. First, the signals from the three EGNOS geostationary satellites that are easily received in open-sky environments are difficult to receive in cities, due to masking by buildings. Second, most GNSS chipsets embedded in today’s mobile phones are GPS-only without SBAS support, or use SBAS for ranging only, a function not supported by EGNOS at this stage.
The European GNSS Agency (GSA) and the European Commission (EC) supported the work described here to provide mobile phone operating system and application developers with a library of functions to allow them to benefit from EGNOS in all their applications. It works by receiving correction data via mobile communication networks when EGNOS satellites are not visible to the user device and even when using a standard GPS chipset, overcoming these two main obstacles for adoption.
Targeted mobile operating systems now include Nokia Maemo, Google Android, and Microsoft WinMobile. Further work will extend to this list to other compatible platforms.
This article demonstrates the feasibility and shows the performance of a software-based EGNOS solution and seeks to create awareness among mobile operating system and application developers on EGNOS.
User Benefits and Constraints
Although the sources of GPS positioning errors in urban areas are mainly due to multipath and GPS satellites availability, SBAS corrections on GPS satellites clocks and orbits and ionospheric correction model can still add value in case of moderate multipath environment characteristics. Although GPS stand-alone accuracy is nowadays generally sufficient, it is expected to degrade in the next couple of years as solar activity increases. Availability of free EGNOS corrections delivered via the mobile communication network will help maintain accuracy during these high solar activity periods.
The limited visibility of EGNOS satellites in urban areas requires the use of the mobile communication network to retrieve the EGNOS corrections. This can be perceived at the first sight as a drawback to the proposed solution as it involves communication costs. However, the required bandwidth is negligible compared to today’s mobile applications such as music and video streaming; further, mobile operators increasingly offer smartphones with unlimited data-access packages.
Implementation Overview
Implementation of EGNOS in current-generation mobile phones requires the introduction of a new library of functions at the software level that will allow application developers to get the best possible accuracy in their application regardless of the underlying algorithms used for position calculation. Such a library of functions can eventually be integrated directly in the application programming interface (API) of the phone operation system. At this point, application developers will simply request a position using the API, and the API will return the EGNOS improved position.
The main computations performed by this EGNOS library (see Figure 1) can be summarized as:
Reception: the GPS user position, satellites used, and their elevations and azimuths in NMEA format are requested to the phone’s GPS chipset, and the EGNOS correction message and Klobuchar ionospheric model parameters are received from a distant server (for example, EGNOS Data Access Service EDAS) using the communication link available at the mobile phone;
Preparation: collected input data are decoded and prepared for next step;
Calculation: the new position corrected by EGNOS is calculated by re-creating the line-of-sight or design matrix (using user position and satellite geometry), applying the EGNOS fast, long-term (including clock), and ionospheric corrections (included in the EGNOS message) and subtracting the Klobuchar ionospheric correction that was (assumed to be) applied at chipset level;
Output: the EGNOS corrected position is encoded in NMEA format and returned to the application.
Figure 1. Overview of EGNOS library implementation.
Data Access via the Internet
The EGNOS correction message and Klobuchar ionospheric model parameters are requested by the mobile phone to a distant server. Although the parameters and ephemeris data are stored on the phone’s GPS chipset once it has decoded the messages from GPS satellites, this data is not made available to other phone applications, hence the need to recover it from a remote source. Today, two alternative servers are available: the EGNOS Data Access Server (EDAS) developed by the EC and Signal-in-Space through the Internet (SISNeT) developed by the European Space Agency (ESA).
SISNeT’s advantage is the simplicity of the message (hundreds of bits per second) and the availability of specific functions that allow requesting all the necessary data for our application. However, SISNeT messages are produced from EGNOS signals in space, not from the ground segment: an EGNOS receiver installed at ESA’s ESTEC center receives the signals, demodulates them, extracts the correction message, and re-broadcasts it via the Internet. The reliability and availability of this approach depend upon the good reception of EGNOS signals at this site. Interference or EGNOS broadcast failure could disrupt service.
Unlike SISNeT, EDAS takes the EGNOS correction message directly from the EGNOS system, which guarantees higher service reliability and availability. Nevertheless, the EDAS message is complex and contains much more than the data required for the present application (hundreds of kilobits per second). Therefore a direct connection to EDAS would be inadequate. As a result an EDAS proxy needs to be interfaced between the EDAS server and the mobile platform in order to filter the data flow and extract only the required data. This proxy provides the same kind of messages and functions as SISNeT, whose specifications are ideal for such an application, however it is using data directly from the EGNOS system and not from EGNOS signals in space, improving reliability. In addition, planned EDAS improvements include the provision of such a simplified service directly from the server, removing the need for a proxy.
Independently of the data server used, the mobile platform must retrieve the EGNOS correction messages, and the Klobuchar ionospheric model parameters. The correction message is composed of a number of different message types (MT) as defined in the SBAS standard established by the International Civil Aviation Organization. For our application, the most important messages are:
MT1, the PRN mask that shows to which satellites (PRN) the data contained in the other, subsequent messages are related;
MT2-5, containing data to correct rapid variations in the ephemeris and clock errors of the GPS satellites. The important bits for us in these messages are the fast corrections for each satellite used to calculate the user position;
MT25, with data to correct long-term vari
ations in the ephemeris errors and clock errors of the GPS satellites;
MT18, the ionospheric grid points (IGP) mask that associates ionospheric corrections in MT26 with the IGPs to which they relate;
MT26, providing data to compute the ionospheric corrections for the IGPs present in the IGP mask. In particular it contains the grid ionospheric vertical delay.
The eight Klobuchar ionospheric model parameters must also be obtained from the distant server (using, for example, the GPS_IONO request with SISNeT).
Corrections from GNSS Chipset
The correction algorithm on the phone takes the original position provided by GNSS chipset and identifies the GPS satellite measurements which were used in this computation. It then determines a pseudorange correction for each of the GPS satellites used, and using knowledge of the user-satellite geometry, translates these to a combined position-domain correction.
Most mobile phones’ operating systems allow access to the NMEA sentences from the GNSS chipset using native API functions, for example, onNmeaReceived() with Google’s Android. In order to apply the EGNOS correction algorithms developed in this paper, the minimum required NMEA sentences are GGA, GSA, and GSV.
To construct pseudorange corrections, the Design matrix containing of line-of-sight vectors to the satellites is reconstructed using the elevation and azimuth data. All EGNOS corrections for the satellite orbit and clock errors and the ionospheric delay are applied in this range domain. The algorithm assumes that the Klobuchar model will have been applied to correct for the ionospheric delay in the original GNSS chipset positioning solution. Therefore it provides an adjustment to this original correction to exploit the greater accuracy of the EGNOS ionospheric data. Finally these range corrections are propagated into the position domain using the Design matrix. This provides a 3-dimensional position shift to apply to the original chipset position.
Implementation with Google’s Android
To obtain NMEA strings from an Android phone requires the ‘onNmeaReceived’ function, a function of the LocationManager class. The LocationManager uses the function ‘requestLocationUpdates’ to get a continuous update of the position input, which in this case is GPS. To implement the LocationManager, a LocationListener must be implemented either by the current activity or as a variable. The ‘onNmeaRecieved’ function will be called every second from the instant the Android’s GPS is switched on. The function provides the NMEA strings with a timestamp using the phone internal clock. This timestamp is not derived from GPS and should be used only for logging.
The HTC Legend produces the $GPGSV, $GPGGA and $GPGSA messages that are needed for the application. The Legend also produces $GPRMC and $GPVTG strings. The $GPGSV provides the elevations and azimuths needed for the algorithm, the $GPGGA provides the time, original position and number of satellites in the fix and the $GPGSA provide the PRN numbers of the satellites used in the fix.
For the present testing, necessary data are received via a TCP/IP connection to the SISNeT server (the EDAS proxy server described previously can be used in exactly the same way). For a snapshot solution a continuous connection is not needed and all the information is collected via ‘GETMSG’ and ‘GPS_IONO’ calls. ‘GETMSG’ calls get the last of a specific message type going back up to 30 messages. The types 0,1,2,3,4,5,18,24 and 26 were needed to provide the information for the position domain correction matrices. Only the last message types 0,1,2,3,3,4,5 were needed with type 18 needing 4 and many more of type 24 and 26.
The ‘GPS_IONO’ message gets the current Klobuchar values. By asking for all of the specific message types, almost instantly all the information is gained without having to wait for the 3 minutes Ionospheric grid cycle (message types 18 and 26) and the variable speed, dependant on number of satellites, complete slow correction set. Once the data has been downloaded from the server the connection is closed.
A streamed input could be used with the above approach by continuing to receive data after the initial connection and not closing the connection until the application using the service requested. This would require a continuous stable connection to a high speed mobile network and a limited use of the internet from other applications. As mobile technology improves this will not be a problem but is difficult to achieve with GPRS and 3G networks at present.
Figure 2 shows the current application running on the HTC Legend phone with corrected positions displayed alongside the original GPS positions.
Figure 2. Application running on HTC Phone.
Test Results
Before testing the implementation of the concept on a mobile platform, some initial tests were performed on an offline basis in order to assess the impact of the position correction and verify the approach. This was achieved through the use of 30s data recorded at continuously operating IGS reference stations, freely available over the internet. The data was processed using an in-house PVT engine designed to be representative of LBS implementations, in order to produce stand-alone and conventional EGNOS solutions. The algorithm described in this paper was then applied to the stand-alone solutions, after downloading EGNOS data from ESA’s EGNOS Message Server (EMS) which allows access to past broadcast messages, to produce a third set of solutions. The accuracy of each solution set was then computed based on the precise coordinates of the reference station made available by the IGS. Whilst this approach replicates the mobile phone correction algorithm it should be noted that there is less uncertainty involved in this offline approach as we can ensure that the assumptions made regarding the original PVT solution are valid. We must assume that the phone chipset PVT is a snapshot solution (no filtering) using the Klobuchar ionospheric model and an elevation-dependent weighting scheme.
The plots from Figures 3, 4, and 5 show the errors in position estimates obtained from a 24-hour dataset recorded at the HUEG IGS station in Huegelheim, Germany on May 5, 2010. Table 1 shows the statistics associated with the figures.
Figure 3. Stand-alone GPS horizontal positioning performance over 24 hours at HUEG IGS station.Figure 4. Conventional EGNOS horizontal positioning performance over 24 hours at HUEG IGS station.Figure 5. Position domain EGNOS horizontal positioning performance over 24 hours at HUEG IGS station.TABLE 1. Horizontal positioning performance statistics from 24hr HUEG IGS station analysis.
The results demonstrate that the conventional EGNOS solution improves the horizontal positioning performance of GPS, with an improvement in the 95th percentile of around 2 meters in this example. Importantly, it can be seen that the position domain EGNOS algorithm achieves a similar level of performance to conventional EGNOS. This can be seen more clearly by comparing the instantaneous horizontal error over this period from the three alternative solutions, as shown in Figure 6. It is clear that the position-domain EGNOS correction shown in yellow reduces the horizontal error of the GPS solution (red) in a similar way to conventional EGNOS (blue).
Figure 6. Time series of horizontal positioning errors for stand-alone GPS, conventional EGNOS, and position domain EGNOS solutions at HUEG IGS station.
Similar behavior was found in other datasets tested. With the ability of the algorithm to replicate conventional EGNOS performance verified, we assessed the performance when integrated on an HTC Legend phone. The key differences here were the real-time connection to the EGNOS data server and the uncertainty in the assumptions made regarding the chipset positioning algorithm.
Testing began by assessing the performance of the application over a static point. Two precisely surveyed points were used for this purpose at four separate time periods. The test method simply involved holding the phone over the point (vertical accuracy was not assessed) and requesting a corrected solution from the application, along with the original GPS chipset solution. The chipset applies stand-still detection to avoid generating multiple GPS positions for a single user location which would be unnecessary in typical phone applications. To generate a sample of position estimates therefore the phone was repeatedly moved away from the reference point then returned to it over the test period. This makes the collection of very large datasets over extended periods impractical. The samples from the four test periods were combined in order to generate results with greater statistical significance. 261 samples were collected to produce the results shown in Figures 7 and 8, and the statistics in Table 2.
Figure 7. Stand-Alone GPS Horizontal Positioning Performance from online static point testing.
Figure 8. Position Domain EGNOS Horizontal Positioning Performance from online static point testing.TABLE 2. Horizontal Positioning Performance from online static point tests.
The results indicate a small improvement in horizontal accuracy as a result of the position domain EGNOS correction. The statistical significance of these results is perhaps questionable given the limitations of the test method and relative small sample size. The reduced level of improvement compared to the offline tests is thought to be due to imperfect assumptions made about the chipset positioning algorithm. The correction algorithm must make many assumptions about the way in which the original GPS position has been computed by the phone chipset. These include assumptions on the measurement weightings used, an assumption that a filtered solution is not applied, assumptions that no additional sensors or systems (accelerometers, digital compass or cellular positioning) influence the computed position, and also assumptions that all information reported in the NMEA strings is accurate. Further work seeks to determine if the algorithm can be improved to better replicate the processes applied in the initial GPS solution in order to make a more significant improvement.
The phone GPS positioning achieves similar levels of accuracy to processing single-frequency data collected at an IGS station. This level of accuracy would be more than adequate for most LBS applications in which the main requirement is to be able to reliably relate a user location to a map or imagery feature. With increasing solar activity over the next few years, leading to larger ionospheric delays on satellite signals, the performance of standard GPS solutions will degrade, making the benefits of the more accurate and timely EGNOS corrections more significant.
Conclusions and Way Forward
By a relatively simple translation method, EGNOS data may be mapped into the position domain, allowing a user position solution to be corrected for signal-in-space (satellite orbit and clock) and ionospheric errors detected and predicted by EGNOS. User position solution provided by the phone chipset may be corrected in near-real time based on data downloaded from a distant server.
The method replicates conventional EGNOS performance (corrections applied at the pseudorange level) when all assumptions regarding the stand-alone GPS user position are valid. Ongoing work seeks to determine if the correction algorithm can be enhanced to provide a greater level of improvement to GPS positions on the phone platform. Ideally, it should be able to provide improvements similar to those produced when EGNOS data is applied in a conventional manner in the position solution. Developers would need to judge the significance of any potential improvement for their intended application.
The EC has launched a project to port this EGNOS library to other mobile platforms and complement it with additional functions that are needed by the application developers and that can bring user benefits. The software library can be obtained free upon request to [email protected].
Acknowledgments
Special thanks to Nottingham Scientific Ltd. for its work on this topic and cooperation in preparing this paper. This article is based on a paper presented at ION-GNSS 2010.
François Boullete was market development officer at the European GNSS Agency at the time of this work. He holds a diploma in project management from HEC and a diploma in engineering from Ecole Centrale.
Boris Kennes is R&D and market monitoring officer at the European GNSS Agency. He has a background in engineering and strategy consulting.
Michaël Mastier is policy officer at the European Commission in the Galileo/EGNOS applications unit. He has an engineering education and diploma in public works from ENTPE in Lyon, and a computer science post-graduate diploma from Saint-Etienne University, France.
Lee Banfield is a software engineer at Nottingham Scientific Limited (NSL) in the UK. He has developed applications which use EDAS data to provide EGNOS corrections, GNSS assistance messages and GNSS performance metrics for a range of road and LBS applications.
We now definitively declare “curtain up!” on the second act of the human and technological drama, Interoperable Global Navigation Satellite Systems, by many authors, directors, and actors, upon the global stage. It happened on August 2 with removal of the message 0 (“Do Not Use in aviation”), by the European Satellite Services Provider, from the European Geostationary Navigation Overlay Service (EGNOS) signal. It enables EGNOS use for en-route and lateral guidance approaches.
The first act of our drama, of course, saw the U.S. GPS achieve operational capability, not to mention enthusiastic worldwide reception and application. We might have declared the second act open upon GLONASS operability, intermittent though that has been — but GLONASS is not (yet) interoperable with GPS. We might have marked its beginning in 2003, when the European Council of Transport Ministers approved the Galileo program, or in 2006, when Galileo launched its first satellite and broadcast its first signal in space. But no useable navigation message has yet emanated.
The EGNOS signal is essentially a corrected GPS signal. Still, its certification for use in aviation embodies an international, interoperable navigation signal from space.
(Legalistic note: “After an operational period of three months [following August 2], the EC will declare the Safety of life (SoL) service available to the aviation community, enabling the publication of precision approach procedures with vertical and lateral navigation guidance (APV) based on EGNOS. At that time, European air navigation service providers will be in position to implement satellite-based precision approaches . . . .)
Chairman Mao said “The march of 10,000 li begins with a single step.” We have taken more than a few steps, though still at the beginning of our journey. Curtain’s up, vistas are wide. Let’s keep moving.
After reviewing current performance of WAAS, EGNOS, and MSAS, the authors present expected future performance, including the benefits of GPS L5. They evaluate the impact of the Indian GAGAN and Russian SDCM systems on global coverage and examine southward expansions for the original three SBASs. Finally, a look at the impact of a second constellation of navigation satellites and the performance for a user taking advantage of two core constellations.
By Todd Walter, Juan Blanch, and Per Enge, Stanford University
The Wide Area Augmentation System (WAAS) monitors GPS and provides both differential corrections to improve accuracy and associated confidence bounds to assure integrity. The first satellite-based augmentation system (SBAS), it was commissioned for service in 2003. Japan’s MTSAT-based Satellite Augmentation System (MSAS) was commissioned in 2007, and the European Geostationary Navigation Overlay Service (EGNOS) was declared operational in 2009, with safety-of-life service commissioning expected in mid-2010. Two other SBASs are in the developmental stage: India’s GPS Aided Geo Augmented Navigation (GAGAN) and Russia’s System for Differential Corrections and Monitoring (SDCM) have fielded equipment and plan to become operational in the next few years.
Coming improvements will expand SBAS coverage areas and strengthen their performance. In the near term, these include more monitoring stations and algorithmic enhancements, with incorporation of a second civil signal in a protected aeronautical band and new GNSS constellations in the long term.
An SBAS utilizes a network of precisely surveyed reference receivers, located throughout its coverage region. The information gathered from these reference stations monitors the GNSS satellites and their propagation environment in real time. Availability of SBAS service is a function of two quantities: the arrangement of the pseudorange measurements used to determine the user’s position, referred to as geometry; and the quality of each individual measurement, referred to as the confidence bound. Although very small confidence bounds can make up for poor geometries, and strong geometries can overcome large confidence bounds, both values are generally required to be good to obtain high availability.
Geometry is determined purely by the locations of the ranging satellites relative to the user. Currently the basic geometry is provided by the GPS constellation. Historically it has exceeded commitments, and there are currently 29 healthy satellites in orbit when only 21 are nominally guaranteed. However, as satellites are taken off-line in critical orbital slots, the quality of the geometry can degrade significantly. There could be short duration losses of service daily at some locations. Since the goal is to provide service more than 99.9 percent of the time, these outages can have a dramatic impact. WAAS currently mitigates this concern by adding geostationary satellites with a ranging function virtually identical to the GPS satellites. These satellites are always in view and improve the overall geometry, although they do not eliminate the problem completely.
The confidence bounds relate to the expected error sources on the range measurements. Currently three error sources are corrected via broadcast to the user: satellite clock error, satellite ephemeris error, and delay error due to propagation through the ionosphere. These error sources are described by two confidence bound terms: the user differential range error (UDRE) for the satellite errors, and the grid ionospheric vertical error (GIVE) for the ionospheric errors.
For single-frequency SBAS, this last error source is the most significant. Users may sample the ionosphere anywhere in the service volume, but the SBAS only has measurements from its reference station locations. Thus, there is always the possibility of undetected ionospheric disturbances. This leads to larger confidence bounding terms and lower availability.
The combination of geometry and confidence bounds yields the protection levels (PL). PLs are the real-time confidence bound on the user’s position error. To match aviation requirements these are broken into a vertical protection level (VPL) and a horizontal protection level (HPL). Each SBAS guarantees that the user’s actual position error will be smaller than these values 99.99999 percent of the time. The PLs are calculated in real-time using stored and broadcast information. They must be compared to the maximum allowed value for a desired operation. The upper bounds are called alert limits (AL) and they are fixed numbers whose values depend on the operation.
In this article we are interested in the localizer performance with vertical guidance (LPV)-200 approach with a VAL of 35 meters and HAL of 40 meters. Currently, LPV aviation approaches can only be accomplished with a WAAS GPS receiver. Performance of an LPV approach allows minimums as low as 200 feet above ground level before a missed approach must be executed. As of January 2010, there were 1,930 published WAAS LPVs, with plans to add 300 per year in the United States.
Because GPS and SBAS generally perform better at horizontal positioning than vertical, the requirement that the VPL be below the VAL is nearly always the limiting constraint for these operations.
Methodology
To determine the global availability and the effect of potential improvements, we used our Matlab Algorithm Availability Simulation Tool (MAAST). This tool uses almanac data to calculate the position of the satellites for each specified epoch. The almanac chosen for this study corresponds to the GPS almanac broadcast on April 8, 2009, when there were 30 healthy satellites. However, PRNs 25 and 32 were removed to simulate a condition with 28 healthy satellites. MAAST also implements the WAAS integrity algorithms to calculate the corresponding UDRE and GIVE values. Finally, it uses these values to implement the airborne algorithms specified in the minimum operational performance standards (MOPS) for SBAS. The MOPS specifies user algorithms for determining the protection levels. For these simulations, the VPL and HPL are calculated about every 5 minutes and every two and a half degrees across the globe.
MAAST does a good job of predicting WAAS behavior. It is less accurate when predicting other systems’ performance. EGNOS has developed its own monitoring receivers and integrity algorithms and has different criteria for assigning a satellite a particular UDRE value and assigning each ionospheric grid point’s (IGP’s) GIVE value. Nevertheless, both systems are designed to meet ICAO requirements for integrity, and their performance should be somewhat similar. In observing EGNOS coverage plots and comparing them to MAAST predictions, we do see differences. However, the size of the coverage region and approximate boundaries are reasonably close and provide an idea of performance if not an exact map.
The MSAS algorithms are based upon the same algorithms used in earlier versions of WAAS. Therefore, MAAST should be slightly more accurate in modeling its performance. GAGAN uses the same prime contractor as WAAS and therefore similar algorithms may be expected. Less is known about the intended SDCM algorithms and therefore the modeling of this system faces the largest uncertainty. Again, the MAAST predictions should be viewed as indicative rather than precise. Individual availability maps will not be completely correct, but relative performance improvements should be properly indicated.
Current Systems Status
Currently WAAS is in its full LPV-200 performance (FLP) phase. It consists of 20 WAAS reference stations (WRS) in the conterminous United States (CONUS), in addition to seven in Alaska, one in Hawaii, one in Puerto Rico, four in Canada, and five in Mexico for a total of 38. The station locations are shown as blue circles in Figure 1. There are three WAAS master stations (WMS) and two geostationary satellites (GEOs). The GEOs are the Intelsat Galaxy XV satellite
at 1338 W and the Telesat ANIK F1R satellite at 1078 W.
FIGURE 1. Existing SBAS reference networks, consisting of 38 reference stations for WAAS, 34 for EGNOS, and 8 for MSAS.FIGURE 2. Simulation results from MAAST for availability of LPV-200 provided by current systems.
As can be seen in Figure 2, availability of LPV-200 service is very high for most of North America. In general, this performance meets the goals for the system. However, in some regions performance is lower than the 99 percent minimum target. The West Coast, Alaska, and Southern Mexico all suffer from reduced availability.
MSAS is in its initial operating phase. It consists of six ground monitoring stations (GMSs) on the Japanese Islands, one in Australia, and one in Hawaii (magenta triangles in Figure 1). There are two master control stations (MCSs) and two Multifunction Transport Satellite (MTSAT) geostationary satellites at 1408 E and 1458 E.
Because of the limited network size, the GEO UDREs for MSAS are set to 50 meters and therefore do not benefit vertical guidance. Further, the limited ionospheric observations offer little availability of LPV-200 service as can be seen in Figure 2. As a result, vertically guided operations have not yet been authorized based upon MSAS. The Japanese Civil Aviation Bureau (JCAB) has studied performance improvements that could allow it to provide LPV-200 operations. Until then, MSAS provides only lateral navigation.
EGNOS is also in its initial operations phase. It consists of 28 ranging and integrity monitoring stations (RIMS) in Europe, one in Turkey, three in Africa, one in North America, and one in South America (green squares in Figure 1). There are four master control centers (MCCs) and two GEOs, the INMARSAT Atlantic Ocean Region-East (AOR-E) satellite at 15.58 W and the ARTEMIS satellite at 21.58 E.
For a variety of reasons, EGNOS has chosen to implement its GEO satellites without a ranging capability. Thus, for our simulations we have set them as data-links only and do not model a ranging capability. EGNOS also currently implements Message Type 27 (MT-27) rather than Message Type 28 (MT-28) as do WAAS and MSAS. MT-27 restricts the use of low UDRE values to a box centered on the European region. Its borders can be discerned in Figure 2. Currently it has little impact on LPV-200 service, but if EGNOS is to expand its coverage, it may become a limiting factor. Availability of LPV-200 service is very high for most of Europe. However, there is a desire to expand coverage to more reliably cover Iceland, Scandinavia, Eastern Europe, and the Mediterranean and South Atlantic regions.
Near-Term Improvements
EGNOS is fielding additional reference stations in the Canary Islands, Northern Africa, and the Middle East. In the longer term, MT-28 is being considered as a replacement for MT-27. In our modeling we added seven new RIMS, shown in Figure 3, and implemented MT-28. We also improved the ionospheric mask by including additional IGPs. We did not update GEO locations nor did we model ranging capability that could further enhance performance. By comparing
FIGURE 3. Improved SBAS networks. The newly added reference stations are marked by yellow filled squares for EGNOS and yellow filled triangles for MSAS.
Figure 4 to Figure 2 improvements can be seen, in particular expanded LPV-200 operation to the south.
FIGURE 4. Improved single frequency SBAS coverage for the original three SBAS.
The future of MSAS improvements is less certain, with no firm commitments for major service enhancements. We have chosen to model fairly aggressive enhancements based upon studies made by the Electronic Navigation Research Institute in Japan. We have added 10 new reference stations in Japan and made the ionospheric threat model less conservative, in line with current WAAS algorithms. Together, these improvements offer good vertical guidance coverage over Japan.
These improvements extend coverage in the vicinity of the reference station networks, but are unable to push availability much beyond. This is primarily due to the limitations of the ionospheric corrections. Because strong gradients can exist outside of the viewing area of the network, tight confidences cannot be provided to those regions.
SBASs model the ionosphere as a thin 2-dimensional shell 350 kilometers above Earth. This works well for quiet mid-latitude and polar ionosphere. However, equatorial ionosphere often has significant vertical structure that is not well replicated by the SBAS message. The resulting confidence bounds are then too large to reliably provide LPV-200 capability. No certified algorithm capable of bounding the equatorial ionosphere is known to the authors. Instead, it is recommended that SBASs in equatorial areas wait for the forthcoming L5 signal to provide vertical guidance in their regions.
GPS L5
The next GPS satellite to be launched will contain a new civil signal, L5, centered at 1176.45 MHz and in a protected aviation band. As such, it will be approved for use on aircraft. When the L5 signal is used in combination with L1, the ionospheric delay for each line-of-sight can be directly estimated. This will dramatically lower the uncertainty of the pseudorange measurement. Thus, if the SBAS is upgraded to provide corrections appropriate for an L1/L5 user and the user similarly upgrades his or her avionics, SBAS service can be dramatically improved.
Another important advantage of the second civil frequency is its relative immunity to ionospheric storms. Because the users are now directly eliminating the amount of delay they actually experience, they are no longer affected by shortcomings in the MOPS ionospheric model. The weaker effect of scintillation may have some impact; however, we do not expect to lose vertical guidance altogether. Furthermore, the availability of two civil frequencies offers protection against unintentional interference. If either L1 or L5 is jammed, the user still has access to guidance on the available frequency.
At the moment there is no MOPS for an L1/L5 user, so any ground or user algorithms will have to be speculative. We propose basing future L1/L5 algorithms on the existing L1-only algorithms. Instead of using L1-only pseudorange measurements, the user forms the ionosphere-free combination. For the confidence term representing the total pseudorange error on a line-of-sight, the ionospheric correction terms and airborne multipath terms are replaced with a single value representing airborne noise and multipath for the ionosphere-free combination.
For a single frequency user, each line-of-sight has four confidence terms that are summed together to obtain the total confidence. These terms correspond to: the satellite clock and ephemeris corrections (σflt), the ionospheric correction (σUIRE), the airborne code noise and multipath (σair), and the troposphere (σtrop). The total one-sigma confidence bound for a particular line-of-sight is the root sum square (RSS) of these four terms:
(1)
When a user has access to two civil frequencies, they can remove the ionospheric effects by forming the iono-free combination of the two pseudoranges:
(2)
where f1 and f5 are the L1 and L5 frequencies (1575.42 MHz and 1176.45 MHz) respectively. If σ1 and σ5 are comparable then the iono-free combination has roughly three times as much noise as either single frequency term, but is substantially smaller than σUIRE . Furthermore, satellites do not need a grid correction to be used, thus satellites farther from the network and IGP mask can be incorporated into the position solution. The dual-frequency confidence bound for a single satellite is then given by
(3)
where σair is used in place of σ1 and σ5 in (2).
For the VPL we propose adding nominal bias terms to handle observed signal biases and non-Gaussian behavior of the underlying error terms. By including these terms it is possible to reduce the net impact of these biases on the user. Further, we propose tailoring the VPL equation to the most significant remaining threat to the user: single satellite fault modes. The L1-only VPL equation is appropriate for threats that affect many signals simultaneously as may happen with the ionosphere or troposphere. However, with the user directly eliminating ionospheric effects, the most significant threats come from satellite fault modes. As these faults are rare, they are unlikely to affect more than one ranging measurement at a time. Therefore, a VPL can be constructed to explicitly account for such a threat. We recommend that the dual frequency VPL take the following form:
(4)
where KHMI and σ5 is the Gaussian tail factor corresponding to the probability of Hazardously Misleading Information, s3,i is the projection of the pseudorange error onto the vertical position estimate, sff is the fault free overbounding sigma, biasnomis the nominal bias bound, Kfault is the Gaussian tail factor accounting for the probability of fault, and biasfault is a bound on the magnitude of all satellite faults. The H0 condition corresponds to the most likely condition of no faults present. The H1 condition corresponds to the unlikely event of a fault on the dominant satellite. The final VPL is the maximum across both conditions.
Because the faulted bias term covers the satellite faults the fault-free sigma term, σff, can be much smaller than the current total value (1), or the dual frequency version (3). Further, since the probability of fault is small, Kfault can be much smaller than KHMI . The net result is that the proposed VPL is smaller than the existing VPL for the same conditions. To model L1/L5 availability we chose the following parameters:
KHMI = 5.33
Kfault = 2.33
σ 2ff = (σflt / 3 ) 2 + σ 2iono_free + σ 2trop
biasnom = 0.5 m
biasfault = 5.333 x σflt
Other values follow the single frequency MOPS specifications as normally implemented by MAAST.
Given these parameters, the H1 hypothesis nearly always dominates the VPL calculation. We have used a nominal weighting scheme to optimize for accuracy. It is possible to deweight the dominant satellite to improve availability. We will be looking at practical methods for determining more optimal weighting for the VPL given in (4). However, there is a concern that such optimizations could harm accuracy. The potential benefits vs. risks will be studied.
The improvement in performance for a dual-frequency user can be seen in Figure 5. The coverage is significantly expanded. Now each region is robustly covered with large margins surrounding their intended service regions. However, coverage is still limited to the areas around these first three SBASs.
FIGURE 5. Potential dual frequency coverage of the first three SBASs including network improvements.
GAGAN and SDCM
Two additional SBASs are currently under development that will extend coverage to more regions. India is developing GAGAN. Currently it has eight Indian reference stations (INRES) all in India (blue diamonds in Figure 6). There is one Indian master control center (INMCC), and plans to use the GSAT-4 as its initial GEO. The GSAT-4 is planned for launch in 2010 and will be located near 82° E. The geomagnetic equator passes through India and it therefore faces the full impact of equatorial ionosphere. The advent of L5 will allow GAGAN to obtain high LPV-200 availability that is unlikely to be achievable for single-frequency users.
FIGURE 6. The networks of five SBAS systems are shown. In addition to the reference stations from Figure 3, the 8 Indian stations are shown as blue diamonds and the 19 Russian stations are shown as red stars.
Russia is developing SDCM. It now has nine operational measuring points (MPs) and has plans for at least 10 more locations, all in Russia (red stars in Figure 6). There are also plans to use three GEOs: Luch-5a planned for launch in 2010 and to be located near 16° W, Luch-5b planned for launch in 2011 and to be located near 95° E, and Luch-4 planned for launch in 2013 and to be located near 167° E.
Figure 7 shows the combined dual-frequency coverage of all five systems, WAAS, EGNOS, MSAS, GAGAN, and SDCM.
FIGURE 7. The combined dual frequency availability of the five SBASs is shown.
The vast majority of land masses in the northern hemisphere are now well covered by at least one of the SBASs. Figures 6 and 7 clearly highlight that the majority of development has occurred in the northern hemisphere. In fact, only two reference stations have been placed below the Equator.
Southern Hemisphere
If SBAS is to provide a global solution, its coverage must extend into the southern hemisphere. There have been many discussions with representatives of countries in the southern hemisphere. Further, the United States has had testbed receivers in South America for nearly 15 years. Europe has fielded receivers in Africa. Australia investigated its own variant of SBAS called the Ground-based Regional Augmentation System (GRAS). However, we are not aware of concrete plans for development in this hemisphere.
We anticipate that discussions will eventually evolve into firm plans and that either independent SBASs will be developed in these regions or existing SBASs will expand their networks southward. We have chosen to assume that WAAS, EGNOS, and MSAS will expand their networks to extend LPV-200 coverage to the southern portion of their GEO footprints. This is but one of many possible scenarios. The pr
oposed expansion shown in Figure 8 is not based on any plans, but is based on the notion that civil aviation authorities will want to obtain global coverage. The assumed new southern reference stations are shown as yellow-filled circles for WAAS in South America, yellow-filled squares for EGNOS in southern Africa, and yellow-filled triangles for MSAS in and around Australia. Advantages of dual frequency allow us to have much less dense networks for the expansions, in addition to allowing LPV-200 capability to be obtained in equatorial areas.
FIGURE 8. The networks of the five SBAS systems including hypothetical expansions into the southern hemisphere
Figure 9 shows the combined dual-frequency coverage for these SBASs with the expanded network. Now nearly all land masses have good LPV-200 coverage. Note that we have not attempted to optimize these networks to assure coverage to all land masses, not have we tried to find the minimum number of stations that offer this capability.
FIGURE 9. The combined dual frequency availability of the SBASs with the southern hemisphere stations is shown.
Added Core Constellations
Galileo is envisioned as compatible with GPS in that each satellite provides ranging using signals covering the L1 and L5 frequencies with similar modulations. Although the final specifications are not yet set, it is envisioned that Galileo satellites will provide a service that is fully interoperable with the GPS civil signals. Thus, we can approximately model Galileo satellites as being equivalent to GPS satellites in different orbits. In parallel, China is developing the COMPASS system whose signals are also planned to be compatible with GPS.
The Russian GLONASS system has been operational for many years. However, its current signal structure makes it less suited for incorporation into avionics. There are modernization plans to broadcast L1 signals that are more in alignment with the other constellations. Thus it, too, may one day be incorporated into SBAS. We believe that SBASs will someday broadcast satellite clock and ephemeris corrections for GPS and one or more other core constellations. These corrections will remove any difference in the reference times or coordinate frames between the two systems, allowing the corrected signals to be considered fully interchangeable.
Adding 24 or more extra ranging sources will have tremendous benefit for all civil GNSS users. The user’s geometry would be very robust to the loss of one or two satellites. Adding one or more core constellations has the potential to significantly improve SBAS coverage. We chose to model the addition of one constellation, by combining the almanac we used for GPS with one that had been proposed for Galileo. For these scenarios, MAAST is modeling 55 medium earth orbiting navigation satellites in addition to the GEOS used by each SBAS. Because the orbital repeat period is approximately 10 sidereal days for Galileo, the simulated time step and total run time were each increased by a factor of ten.
Figure 10 shows the improved coverage when the reference stations shown in Figure 6 are used. The additional satellites fill in many potential coverage gaps and now, compared to Figure 7, the SBASs all have even more reliable coverage well beyond their reference networks. Indeed, the Northern Hemisphere is now essentially fully covered. Figure 11 shows the results when the expanded networks of Figure 8 are incorporated. Compared to Figure 9, the southern hemisphere is much more reliably covered. The remaining gaps could easily be filled in with just a few more reference stations if full global coverage were desired.
FIGURE 10. The combined dual-frequency, LPV-200 coverage of the five SBAS systems with both GPS and Galileo.FIGURE 11. Combined dual-frequency LPV-200 coverage, SBASs with GPS and Galileo and the southern hemisphere stations.
Conclusions
For single-frequency SBAS the coverage is limited to areas very close to the monitoring station network. However, each region can obtain very good LPV-200 coverage within their desired service area. The addition of GPS L5 makes vertical guidance largely immune to ionospheric disturbances, and permits SBAS coverage to extend into equatorial areas. Independence from the ionospheric grid also allows service to extend farther away from the core network regions. When new Indian and Russian systems are commissioned, a very large fraction of the northern hemisphere will have LPV-200 coverage.
With dual frequency, LPV-200 coverage can be established with comparatively sparse networks in South America, Africa, and around Australia. Additional dual-frequency core constellations such as Galileo, Compass, or GLONASS could greatly expand coverage to well outside the original reference network regions. As GNSS capability is improved and expanded, we anticipate that SBAS coverage may one day provide nearly global LPV-200 or better service capability.
Acknowledgments
The authors acknowledge support of the FAA Satellite Product Office. However, the opinions and potential future scenarios reflect those of the authors and are not necessarily representative of the FAA.
Todd Walter is a senior research engineer at Stanford University. He has been active in the development of the Wide Area Augmentation System and related systems around the globe. His focus is on the provision of certified integrity for aviation applications.
Juan Blanch is a research associate at Stanford University, where he works on integrity algorithms for GNSS. He holds a Ph.D. in aeronautics and astronautics from Stanford.
Per Enge is professor of aeronautics and astronautics at Stanford, where he directs the Stanford GPS Research Laboratory. He has a Ph.D. from the University of Illinois.
Four Galileo in-orbit validation (IOV) satellites scheduled to launch next year have already missed their first pad date.The European version of Russia’s Soyuz rocket is now scheduled to carry the four IOV satellites into orbit in two launches in November 2010 and early 2011, as announced by European Space Agency (ESA) Director-General Jean-Jacques Dordain on October 9.
Both launches had been set for earlier in 2010, but ESA has encountered difficulties with the satellites, built by a consortium led by Astrium Satellites and Thales Alenia Space. Introduction of Russia’s Soyuz rocket at Europe’s Guiana Space Center in French Guiana, on the north coast of South America, has also been repeatedly delayed.
The European Union and ESA plan to select a builder for the remaining 28 satellites late this year. Final bids from 11 companies bidding for on six Galileo work packages are expected by November 11.
Experimental Satellite Moved. In July and August, Surrey Satellite Technology Ltd (SSTL) repositioned GIOVE-A, the first Galileo test satellite, to an orbit 113 kilometers above the orbit that the operational Galileo navigation satellites will occupy.
Since its December 2005 launch, GIOVE-A has achieved all of its mission objectives and remains in excellent condition well beyond its design life of two years, SSTL stated.
The test satellite secured the Galileo frequency filings with the International Telecommunication Union (ITU), collected data to characterise the medium-Earth Orbit (MEO) environment, and flight-proved technologies such as highly accurate atomic clocks.
GIOVE-A remains fully operational, and has sufficient propellant remaining for further maneuvers. A further repositioning exercise may be performed to raise the orbit higher still before GIOVE-A is finally decommissioned.
SSTL and its new owner, OHB of Germany, jointly form one of the two consortia now bidding for the development and construction of 28 satellites for the operational Galileo service.
EGNOS. The European Commission (EC) declared on October 1 the official start of operations by the European Geostationary Navigation Overlay Servic (EGNOS), with its Open Service available free of charge to businesses and consumers. EGNOS is Europe’s first contribution to satellite navigation and a precursor of Galileo, the global satellite navigation system in development.
EGNOS is a satellite-based augmentation system that improves the accuracy of satellite navigation signals over Europe. The system is composed of transponders aboard three geostationary satellites hovering high above the Eastern Atlantic and the European continent, linked to a ground network of about 40 positioning stations and four control centers, all interconnected. The EGNOS ground stations receive signals sent out by GPS satellites. Information on the accuracy and reliability of these signals is relayed to users via the geostationary satellite transponders. This allows them to determine their position to within two meters in real-time, according to EC spokespersons.
The EGNOS coverage area includes most European states and has the built-in capability to be extended to other regions, such as North Africa and European Union neighboring countries.
The commission seeks to support new applications in sectors such as agriculture (high-precision spraying of fertilizers) and transport (for example, automatic road-tolling or pay-per-use insurance schemes). EGNOS can also support much more precise personal navigation services, both for general and specific uses, such as systems to guide blind people and to improve signal reception in urban areas.
EGNOS will be certified for use in aviation and other safety-critical areas in compliance with the Single European Sky regulation. Through EGNOS a safety-of-life service is expected to be in place by mid 2010. This service will provide a valuable warning message informing the user within six seconds in case of a malfunction of the system. A commercial service is under test and will also be made available in 2010.
EGNOS operations are managed by the European Satellite Services Provider, ESSP SaS, a company based in Toulouse, France, founded by seven air navigation services providers. A contract between the EC and ESSP SaS covers management of the EGNOS operations and maintenance until the end of 2013.
The EGNOS Open Service is accessible, without service guarantee or resulting liability, to any user equipped with a GPS/SBAS compatible receiver within the EGNOS coverage area. Most receivers sold today in Europe meet that requirement. No authorization or receiver-specific certification is required.
GLONASS Signal Generates Slip
A planned late-September launch of a three new GLONASS-M satellites from the Baikonur space center was postponed due to a problem with signals emanating from a previously launched GLONASS-M satellites, now on orbit. Initially, a new launch date of October 29 was set by Roscosmos, the Russian space agency, but no word had yet come at press time regarding investigation of a problem with the signal generator aboard the orbiting satellite, detected in late August. The spacecraft was taken out of service on August 31.
GPS Wiggles: SVN49, CNAV
The GPS Wing held an extraordinary session at ION GNSS in Savannah, Georgia, September 23, frankly explaining the SVN 49 satellite’s problem and probable solutions.
SVN49, the IIR-M) + L5 civil-signal satellite, will be set healthy in the coming months and it will be useable, the GPS Wing said. Its L1 an L2 signals contain a pseudorange error that remains within specifications for compliant GPS user equipment.
On the ground, a receiver sees from this satellite both a direct signal and a weaker reflected signal, which looks like a multipath component. According to models, if the direct and reflected L1 signals are in phase at zenith, a standard code-correlating receiver will measure a C/A-code pseudorange that is 1.62 meters too long. The error becomes smaller as the elevation angle drops, reaching zero at an elevation angle of about 42 degrees, and then rising slightly as the elevation angle drops to zero.
During audience input following the Savannah panel presentations, Javad Ashjaee of JAVAD GNSS proposed simply turning the satellite on as is and using it as an opportunity, given the “defined multipath” that it effectively transmits, to study multipath and other phenomena. JAVAD GNSS Triumph receivers have demonstrated the ability to remove almost all anomalies and satellite multipath from the SVN49 signal.
An as-yet-unconfirmed report has it that U.S. Air Force representatives and others, in an informal meeting after the session, came to a provisional agreement as to the best course. However, this has not yet worked its way through channels nor been announced.
New Message. The first test of the CNAV navigation message format to be used in the future on Block IIR-M and IIF satellites was announced at the September CGSIC meeting in Savannah, and will begin soon. A Type 0 message will be broadcast on the L2C signal by SVN49. By the end of the year, this message is to be switched on, on all IIR-M satellites. However, this initial message type will not contain useful information for end users.
Message Type 0 consists of a 12-second, 300-bit long message including the preamble, satellite pseudorandom noise (PRN) number, message type ID (=0), GPS time of week, a sequence of alternating 1s and 0s, and a cyclic redundancy check (CRC) parity block. The GPS time of week will change every 12 seconds, as will the CRC bits.
Penny Axelrad Honored
Penina Axelrad, professor of aerospace engineering sciences at the University of Colorado, received the Institute of Navigation’s 2009 Kepler Award for her “contributions in the field of satellite navigation and dedication to the education of future generations of navigation engineers.”
Axelrad has done advanced research in topics including receiver autonomous GPS integrity monitoring (RAIM), GPS bistatic radar, satellite formation flying using GPS, GPS-based orbit and satellite attitude determination, and multipath characterization, modeling, and mitigation.
She received a Ph.D. in aeronautics and astronautics from Stanford University and S.B. and S.M. degrees from the Massachusetts Institute of Technology. She has taught for 17 years at the University of Colorado.
The European Geostationary Navigation Overlay Service (EGNOS) recently passed flight trials in Limoges, France with flying colors, according to the European Space Agency (ESA).
EGNOS, a venture between the ESA, the European Commission and Eurocontrol, is the first step in Europe’s satellite navigation plans, paving the way for Galileo. EGNOS supplements GPS data, offering more accurate vertical positioning data to pilots, similar to systems already in operation in the United States. The system can provide a precision of better than two meters, according to the ESA.
In the most recent EGNOS flight trials, a French civil aviation authority test plane was specially equipped to make tests using EGNOS at an airfield in Limoges, France. It made a number of approaches and landings using the new procedures, in each case aligning itself with the runway’s axis and then following a descent path to touchdown.
Inside the plane, which is normally used for calibration of airport systems in France, the method of analyzing the quality of the EGNOS signals was done by comparing the landing phases guided by satellite with landings using traditional means, such as the plane’s Instrument Landing System (ILS).
The results of Limoges trials demonstrate again that EGNOS signals allow approaches and landings that meet the safety standards that govern international air traffic, the ESA says.
One of the main advantages of EGNOS is that it is available everywhere without the need for ground infrastructure and it provides vertical guidance procedures for every runway, the ESA says. Furthermore, the cockpit data display is the same as that of ILS, so there are no familiarization problems for the pilots and no additional training costs.
Currently in pre-operational service, EGNOS will be certified in 2008 for safety-of-life applications such as air traffic control. It will be comptible and interoperable with similar systems elswhere in the world, according to the ESA.
Four Galileo in-orbit validation (IOV) satellites scheduled to launch next year have already missed their first pad date.The European version of Russia’s Soyuz rocket is now scheduled to carry the four IOV satellites into orbit in two launches in November 2010 and early 2011, as announced by European Space Agency (ESA) Director-General Jean-Jacques Dordain on October 9.