GPS World

Tag: GPS Block IIF satellite

  • The System: GPS L5, the Real Stuff

    The System: GPS L5, the Real Stuff

    By Oliver Montenbruck, Andre Hauschild (DLR/GSOC), Stefan Erker, Michael Meurer (DLR/IKN), Richard B. Langley (UNB), and Peter Steigenberger (TUM)

    The L5 signal of the new Block IIF satellite shows a very favorable signal strength (Fig. 1), which is somewhere in between the L1 and L2C signal strength for the employed antenna and slightly higher than that of the GIOVE-A/B satellites. While the L5 test signal of the second-last Block IIR-M satellite (PRN1/SVN49) is transmitted through a narrow beam antenna and shows a steep variation with elevation angle, the new satellite exhibits an almost constant flux irrespective of the boresight angle.

    Following the successful launch of the first Block-IIF GPS satellite (PRN25/SVN62) on May 28, 2010 (UTC), and the activation of the legacy signals on June 6, users around the world have eagerly awaited the first transmission of PRN25 signals in the L5 band.

    In June, at last, the L5 payload was activated for more than five hours transmitting nominal signals with the PRN25 ranging code. This enabled standard tracking receivers to collect the first real L5 measurements from the new satellite.

    Scientists of the German Aerospace Center (DLR), the University of New Brunswick (UNB), and the Technische Universität München (TUM) spotted the first L5 data at 15:17:11 UTC from a station in Fredericton, Canada, followed a second later by stations in Japan, Singapore, the Canary Islands, and Germany. The stations are part of the CONGO network, which is the first global network of tri-band (L1/E1, L2, L5/E5a) GNSS receivers monitoring the GPS, GLONASS, GIOVE, and SBAS satellites. For background on the CONGO network, see the September 2009 GPS World article.

    Fig.1 Carrier-to-noise-density ratio of GPS (left) and GIOVE-A/B signals measured at the Wettzell station on June 17, 2010. Red curves refer to signals in the L5/E5a band and include data from the PRN1 test satellite and the new PRN25 satellite.
    Fig.1 Carrier-to-noise-density ratio of GPS (left) and GIOVE-A/B signals measured at the Wettzell station on June 17, 2010. Red curves refer to signals in the L5/E5a band and include data from the PRN1 test satellite and the new PRN25 satellite.

    The L5 signal of the new Block IIF satellite shows a very favourable signal strength (Fig. 1), which is somewhere in between the L1 and L2C signal strength for the employed antenna and slightly higher than that of the GIOVE-A/B satellites. While the L5 test signal of the second-last Block IIR-M satellite (PRN1/SVN49) is transmitted through a narrow beam antenna and shows a steep variation with elevation angle, the new satellite exhibits an almost constant flux irrespective of the boresight angle.

    Fig. 2 Multipath plots of L1 C/A code, semi-codeless L2 P(Y) code, and L5 code tracking for the Singapore station of the CONGO network (10-second smoothing).
    Fig. 2 Multipath plots of L1 C/A code, semi-codeless L2 P(Y) code, and L5 code tracking for the Singapore station of the CONGO network (10-second smoothing).

    While the new Block IIF satellite has not yet been set healthy and made available for public use, the early measurements collected on June 17 already demonstrate good tracking quality. This is illustrated in Fig. 2, showing the so-called multipath combination for pseudorange measurements from L1 and L2 legacy signals (the upper two panels) as well as the new L5 signal for Singapore, which had continuous visibility of PRN25 during the period of interest. Except for low elevation angles that are affected by strong multipath from structures in the vicinity of the antenna, root-mean-square tracking errors well below 30 centimeters were obtained for all signals.

    Fig. 3 L5 spectrum of PRN25 collected on June 17, 2010 with a 30-meter high-gain antenna at Weilheim, Germany.
    Fig. 3 L5 spectrum of PRN25 collected on June 17, 2010 with a 30-meter high-gain antenna at Weilheim, Germany.

    In addition, the GNSS signal monitoring facility at DLR’s ground station in Weilheim has been used to record high-rate radio-frequency samples and spectra of the new signal, a snapshot of which is shown in Fig. 3. The raw sampling also confirmed that the L5 signal of PRN25 comprises both in-phase and quadrature modulation (in contrast to the PRN1 test signal, which contains a Q-component, only).

    To the regret of U.S. scientists, the first publically traced L5 signals were only transmitted when the satellite was over Europe and Asia (see Fig. 4). Nevertheless, the test transmission provided an excellent sneak preview of what we can expect when the regular transmission starts. The satellite is presently expected to be set healthy and to start regular service by the end of August at the latest.

    Fig. 4. The ground track of PRN25 during the transmission of L5 signals on June 17, 2010. Also indicated is the footprint of the satellite showing the 0°, 30°, and 60° elevation angle contours at the beginning of the transmission. The ground track is almost centered over Diego Garcia, one of the GPS monitoring stations.
    Fig. 4. The ground track of PRN25 during the transmission of L5 signals on June 17, 2010. Also indicated is the footprint of the satellite showing the 0°, 30°, and 60° elevation angle contours at the beginning of the transmission. The ground track is almost centered over Diego Garcia, one of the GPS monitoring stations.

    Equipment. The CONGO network stations use JAVAD GNSS Triumph Delta-G2T/G3TH receivers. A Leica AR25R3 chokering antenna is used at Wettzell, while the Singapore station is equipped with a Leica AX1203+ GNSS antenna. The L5 spectrum was recorded with an Agilent PSA E4443A vector signal analyzer.

    Beidou G3

    China launched another Beidou/ Compass satellite, named G3, on June 2. By June 9, its apogee kick motor had placed the satellite in geostationary orbit at 84°38’ east, according to NORAD tracking reports.This is close to the position initially occupied by G2 (83°30’) before it started drifting. By June 9, G2 had drifted to 64°29’. By June 11, G3 had started transmitting signals on three frequencies.

    China now has two properly functioning geostationary satellites in its second-generation system, out of a total of five it expects to place by 2012 for a regional operating system; also needed for this concept are four mid-Earth orbit satellites (one currently aloft), and five inclined geosynchronous orbit satellites (zero in orbit now). A planned global system would require 5, 27, and 3 satellites in GEO, MEO, and IGO orbits, respectively, by 2020.

    Current regional-system signals on three frequencies use quadrature phase shift keying. Global-system signals will be binary offset carrier waveforms.

    Opinions on SVN-49

    The public comment period on proposed mitigation options for GPS satellite IIR-20M (SVN-49) ended May 28, and comments are viewable at www.regulations.gov under RITA Docket 2010–0002. Among others, the U.S. GPS Industry Council, NovAtel, Garmin, Septentrio, Raytheon, Boeing Commercial Airplanes division, General Motors OnStar, the European Commission, the MITRE Corporation, STMicroelectronics, the German Space Operations Center, and Cessna Aircraft have all filed comments expressing a preference for one option or another.

    Unfortunately for the U.S. Air Force and the GPS Wing, no clear consensus emerges. Indeed, differences of opinion naturally follow the respective orientation of each company or organization toward their customers’ or members’ specialized needs.

    Devote It to Science. Perhaps in recognition of this imbroglio, at the Air Force Space Command- Industry Exchange on June 15, Lt. Colonel Todd Parks briefed the PNT Functional Capability Team, explaining that the Air Force now was soliciting from industry “innovative applications” for the SVN-49 signal in space.This echoes a suggestion by Javad Ashjaee at last year’s unprecedented ION/ USAF session on SVN-49, where he proposed that the signal be used for studying multipath.

    A website article at env-gpsworld-integration.kinsta.cloud/49opinions recaps commentary and preferred options from several companies and organizations.

    The potential mitigations are each designed to reduce the impact of the unique nature — that is, errors — of the SVN-49 signal to a portion of the user segment. They are (so far):
    1. Set healthy with current 152- meter antenna phase center (APC) and associated clock offsets.
    2. Set healthy with factory APC offset.
    3. Users switch to multipath-resistant receivers.
    4. Modify receiver software to use look-up table corrections.
    5. Increase user range accuracy (URA) index to a minimum value of 3.
    6. Remove data modulation from L2 P(Y)-code, and
    7. Change L2C PRN code to a “unique sequence.” (6 and 7 are considered a pair, to be jointly implemented for desired effect.)
    8. Change SVN-49 from PRN-01 to PRN-32.
    9. Use spare health code so future users could use SVN-49 despite unhealthy setting. For background on the SVN-49 situation, see Richard Langley’s Expert Advice column from August 2009. Briefly, the pseudorange data broadcast by the satellite contains larger than normal errors that vary according to the elevation of the satellite above the horizon.

    The comments filed by the U.S. GPS Industry Council (USGIC), available as a PDF file at both URLs listed in this story, are the most detailed and extensive across all the options. However, the stated preference of the USGIC for Option 9 does not necessarily reflect agreement across all sectors of industry. As the USGIC points out, “Options 1 through 8 propose to designate SVN 49 as healthy using techniques that enable mitigation for some user applications, but that are unable to also mitigate adverse impacts to otherusers.”

     

     

     

     

  • Block IIF: Follow-on, or Failure?

    A few short weeks ago, the U.S. GPS program had its posterior firmly planted in the catbird seat. Government spokespeople in international fora looked on benignly as European, Chinese, and Russian GNSS programs struggled to resolve their issues and meet their heady challenges. All was well with the world. A new GPS satellite launched, a segment of radio-frequency spectrum secured for a promising new signal, a next-gen satellite shipped to the Cape, and the next-next-gen program nearing successful preliminary design review (since completed).

    In the blink of an eye, the world is turning.

    A progression of seemingly unrelated events began to affect GPS outlook.

    • While successfully broadcasting the new L5 signal, IIR-M (20) also began generating “out of family” measurements on L1 and L2 at low elevations.
    • The long-withheld Independent Assessment Team (IAT) report on eLoran appeared, unanimously recommending that “the U.S. government complete the eLoran upgrade and commit to eLoran as the national backup to GPS for 20 years.” While in itself this is good news — that is, if you believe in backing up critical systems — it does not augur well that a two-year Freedom of Information Act fight had to be waged to pry the report loose from know-nothings in the Department of Homeland Security, and that the vaunted Obama administration, heralded as a breath of change, had earlier come down on the same-old same-old government side of taking Loran out.
    • Then, the motherlode. The U.S. Government Accountability Office (GAO) issued a report on the future of GPS, characterizing the constellation as susceptible to falling below full operational capability between 2010 and 2018.
    • It turns out that while a IIF payload did travel to Cape Canaveral on May 7, this was solely for the purpose of preliminary launch-system compatibility testing. The satellite itself is not ready to operate in space, and in fact the IIF currently at the Cape is just a placeholder. Or, to use press-release verbiage, to “serve as a risk-reduction pathfinder for SV1 processing later this year.” The real satellite, the IIF that may, repeat may, go into orbit at the end of this year or early next year, continues in critical payload testing at the contractor facility.
    • Here’s a bright spot, at long last. Brad Parkinson, the first GPS Program Office Director, chief architect and advocate for GPS, has a plan for mitigating possible GPS brownouts — the gaps in service that may occur if the constellation should fall below quorum. Parkinson states that “It is possible that the constellation will be at a level of less than 24 satellites. I would like to focus on the options that would help reduce this risk.”

    Parkinson cites two principal causes for the current at-risk status of GPS service. “The first is that the generation of replacement satellites called IIF has been greatly delayed.  A substantial part of the reason for this is that the contract for IIF satellites was placed during a period when DOD imposed a grand experiment in contracting.  In addition there were some changes to the satellite to modernize its design, but the bottom line is the satellite has been on contract since 1996 and will not be launched until 2010. The design is quite old, and the capability of the satellites does not meet the latest requirements.”

    The second cause is protracted delays by the decision-making and budgeting processes in getting Block IIIA going. These issues have now been resolved, and Parkinson points out that both reasons “are now a matter of history. The current issue, that should concern us all, is: what options should we pursue to substantially reduce the risks of brownout.”

    Parkinson makes three recommendations in his personal presentation to the PNT Advisory Board meeting; the same presentation was also submitted as written testimony to the Congressional hearing following on the GAO report. Download the full Powerpoint file, with written details.

    “In my view, there are three major options for mitigating brownouts. Fortunately, these  options could be done together. These are:

    1. To reactivate the previously retired GPS satellites that are still operating in normal GPS orbits.
    2. To speed up the GPS IIIA development space (expedite the milestone approvals).
    3. To develop a simplified GPS IIIA based design, Spartan satellite (IIIS) that would not include the extra payloads, and, once designed, could be built quickly and launched into space with two satellites on a booster. This would be done in parallel with the current program.”

    Dr. Parkinson adds that “There is a fourth option, which may have been offered by some. This is to restart or expand the GPS IIF production line. The apparent advantage of this is that the GPS IIF is close to its first launch. Some might think major advantage would have been the fact that it is already designed. Weighing against this advantage is the fact that the design and the parts are obsolete. Virtually all the boxes and components would have to undergo a major redesign. Furthermore, the design is still untried, and was developed during an era of flawed procurements.”

    Counterpoint. Boeing says its engineers are working “very closely with the Air force and its team” and that the company has taken “aggressive steps to resolve the technical issues on IIF with a strong emphasis on mission assurance.” It maintains that it is on track to deliver the first IIF satellite, ready for launch, later this year.

    “Boeing’s GPS IIF satellites,” the press release continues, “will deliver more capability and improved mission performance to military and civilian users. . . . Design changes were required to ensure performance over the satellite design life and these have caused schedule delays, but these changes are in the final phase of implementation and a fully integrated satellite (SV1) has already successfully completed the thermal-vacuum test program — the most stressing system level test. SV2 was shipped to the Cape (Canaveral) on May 6 to perform system-level compatibility tests and serve as a risk reduction pathfinder for SV1 processing later this year.”

    The Department of Defense also made a presentation to the May 14–15 National Space-Based PNT Advisory Board meeting, and in it highlighted three risks: delay of IIF, delay of the ground control segment (OCX) contract award, and delay of GPS IIIA.

    Some in the GNSS community feel that the GAO-generated furor focuses too much on Block IIF and not enough on these other unknowns. They foresee a strong likelihood that the IIF satellites will get aloft on time, suitably “following on,” as they have been named. The real scary part will come later, in the 2015-2017 timeframe when GPS IIIA doesn’t get into orbit in sufficiently quick
    numbers.

    Further, the GAO report did not account for two mitigation tools that the DoD has in reserve: three retired satellites still in space that could be brought back into operation, and power-shedding as a means to extend satellite life.

    Back to the Mitigation Talk.Coming up are some of the strongest words Parkinson employed in the PNT Advisory Board presentation: further congenital defects.

    “While the Air Force has undertaken a very rigorous test program,” read the presentation notes, “it is still conceivable that we will find further congenital defects. The IIF satellite lacks the powerful military signal that will be extremely helpful against potential hostile jammers. In addition, it does not broadcast the new international signal L1C. Because of the extensive redesign it seems probable that the satellite would have to be re-competed. Finally, this would be a major near-term budget hit in a period when the IIF satellite is still over running its budget.”

    Not Even Half the Picture. GPS program planners have one of the most complex tasks going. They must consider many other issues in addition to keeping an integer number of satellites flying. Dual handling of the space and ground segments while both undergo modernization so that they remain in phase with each other, further synchronization with military user equipment on its own track of development, operating under a leadership and decision-making structure that lacks unity at the top, structuring future interoperability with other GNSS neither aloft nor complete in their signal-structure design — and then the various PR issues involved with servicing a worldwide, multinational, multi-industry, multi-requirement customer base.

    Personally, I feel much more comfortable here in my armchair.

    And despite all the grim news this month, I remain confident that GPS will continue to lead the field of GNSS, providing exemplary service round the clock, round the world.

  • Parkinson Prescribes Remedy for GAO Report Alarm

    Brad Parkinson, the first GPS Program Office Director, chief architect and advocate for GPS, submitted written testimony to Congress on mitigation options for possible GPS brownouts. His presentation comes in reference to the recent GAO report highlighting the risk that the GPS constellation may fall below the minimum level of 24 satellites required for full operational capability.  In his opening, Parkinson states that “GAO correctly points out the possibility that the GPS constellation will be reduced to less than the current number of 30 to 32 satellites.  In fact, it is possible that the constellation will be at a level of less than 24 satellites. I would like to focus on the options that would help reduce this risk.”

    Parkinson chides those who may not have been paying attention over the last two years, at least. “It should be noted that the risk of brownouts has been repeatedly pointed out by the independent review teams,” he states, referencing the the Defense Science Board, the GPS Independent Review Team, and the Pos-Nav Timing Advisory Board, who have all stated all that “30 satellites is the correct number.” He points out that the European Galileo program and the Chinese Compass system have also arrived at that number.

    “Although brownouts would only be ‘officially’ declared at levels below 24, anything below the current level of 30 satellites is a cause for concern. The potential economic impact if the number were below 24 may be quite serious.”

    To rectify the situation, Parkinson first gives a history lesson. The first GPS satellite went from contract award to launch in 44 months. “The keys to success were a streamlined approval chain (all the way up the OSD chain), severe restrictions on any contract changes, and an integrated product team.” He believes that GPS IIIA can achieve the same — given the same playing conditions.

    Spartan. He does throw in one twist not currently in the plans: “To develop a simplified GPS IIIA based design, Spartan satellite (IIIS) that would not include the extra payloads, and, once designed, could be built quickly and launched into space with two satellites on a booster.  This would be done in parallel with the current program.”

    Parkinson appears to advocate complete abandonment of the IIF line. “The reason is simply that the satellite design is old and relies on parts that are no longer available.  In addition, the satellite, while providing the older signals, does not meet current requirements.”

    He closes with a final admonition. “Above all, the senior decision making chain has to become a part of the solution. This means that they do everything in their power to help the program office achieve the needed schedule.”

    Click here for the full Powerpoint file of Brad Parkinson’s presentation, including detailed notes.

    Footage of live testimony given at the Congressional hearing is also online.

     

  • GPS at Risk: Doomsday 2010

    The United States Government Accountability Office (GAO) issued on May 7 an alarming report on the future of GPS, characterizing ongoing modernization efforts as shaky. The agency appears to single out the IIF program as the weak link between current stability and ensured future capability, calling into doubt “whether the Air Force will be able to acquire new satellites in time to maintain current GPS service without interruption.” It asserts the very real possibility that “in 2010, as old satellites begin to fail, the overall GPS constellation will fall below the number of satellites required to provide the level of GPS service that the U.S. government commits to.”

    Prepared at the request of the U.S. House of Representatives’ Subcommittee on National Security and Foreign Affairs, Committee on Oversight and Government Reform, and titled “Global Positioning System: Significant Challenges in Sustaining and Upgrading Widely Used Capabilities,” the report concludes that “it is uncertain whether the Air Force will be able to acquire new satellites in time to maintain current GPS service without interruption. If not, some military operations and some civilian users could be adversely affected.”

    “In addition,” the report summary continues, “military users will experience a delay in utilizing new GPS capabilities, including improved resistance to jamming of GPS signals, because of poor synchronization of the acquisition and development of the satellites with the ground control and user equipment. Finally, there are challenges in ensuring civilian requirements for GPS can be met and that GPS is compatible with other new, potentially competing global space-based positioning, navigation, and timing systems.”

    Among the report’s principal recommendations is a proposal often made in past years by a range of experts, but never implemented: the Secretary of Defense should appoint “a single authority to oversee the development of GPS, including space, ground control, and user equipment assets, to ensure these assets are synchronized and well executed, and potential disruptions are minimized.”

    While the Department of Defense (DoD) concurred with this recommendation, and while quite possibly it might effectuate the streamlined decision-making and corollary processes to remedy the highlighted deficiencies, it would run counter to the integral “dual-use” principle of GPS as dedicated to both civil and military users. Such a move could thus conceivably and adversely affect the interests of civil users.

    The full report can be downloaded from the GAO website.

    Testimony from invited GPS providers and users before a related National Security Subcommittee hearing (“GPS: Can We Avoid a Gap in Service?”), some of which is briefly encapsulated within this news story, can be downloaded.

    Why GAO Did This Study. A highlights document attached to the GAO report asserts that GPS “has become essential to U.S. national security.” The GAO conducted its own analysis of Air Force satellite data, in addition to interviewing key officials and analyzing program documentation. Specifically, the agency assessed progress in:

    • acquiring GPS satellites
    • acquiring the ground control and user equipment necessary to leverage GPS satellite capabilities
    • coordinating efforts among federal agencies and other organizations to ensure GPS missions can be accomplished.

    Gloomy Outcomes. Based on the most recent satellite reliability and launch schedule data from March of this year, the estimated long-term probability of maintaining a constellation of at least 24 operational satellites falls below 95 percent during fiscal year 2010 and remains below 95 percent until the end of fiscal year 2014, at times falling to about 80 percent. Program officials provided no evidence to suggest that the current mean life expectancy for satellites is overly conservative, the GAO stated.

    The results of fewer than 24 operational satellites could include:

    • Intercontinental commercial air carriers may have to delay, cancel, or reroute flights.
    • Enhanced-911 response to emergency calls could lose accuracy, particularly operating in urban and mountainous environments — exactly where emergencies tend to be most dire and hardest to locate.
    • Accuracy of precision-guided munitions could decrease, forcing the military to use larger munitions or use more munitions on the same target to achieve the same level of mission success, and increasing the risks of collateral damage. The urgent desire to decrease or eliminate collateral damage to civilians in or near conflict zones has often been cited by the founders of GPS as one of their key motivations in envisioning the program.
    • Both standard positioning service and precise positioning service could suffer, impacting large numbers of civil users, both professional (for example, surveyors) and casual (users of location-based services via cell phones) in moderately mountainous areas, in large cities, and under forest foliage.

    Block IIF at the Crux. Cristina T. Chaplain of the GAO presented the report to Congress, stating, “In recent years, the Air Force has struggled to successfully build GPS satellites within cost and schedule goals; it encountered significant technical problems that still threaten its delivery schedule; and it struggled with a different contractor. As a result, the current IIF satellite program has overrun its original cost estimate by about $870 million and the launch of its first satellite has been delayed to November 2009 — almost three years late.”

    The GAO reports cites specific problems with the IIF satellites contracted to Boeing. During the first phase of thermal vacuum testing in 2008, one of the test payload’s transmitters failed; consequently, the program suspended testing in August 2008 to identify the causes and take corrective action. Other hang-ups include maintaining the proper propellant fuel-line temperature, delaying final integration testing, and re-design of the satellite’s reaction wheels, used for pointing accuracy, because of on-orbit failures on similar reaction wheels on other satellite programs. Overall, about $10 million additional have accrued to program, according to the GAO.

    “Further, while the Air Force is structuring the new GPS IIIA program to prevent mistakes made on the IIF program, the Air Force is aiming to deploy the next generation of GPS satellites three years faster than the IIF satellites. GAO’s analysis found that this schedule is optimistic, given the program’s late start, past trends in space acquisitions, and challenges facing the new contractor.

    “Of particular concern is leadership for GPS acquisition, as GAO and other studies have found the lack of a single point of authority for space programs and frequent turnover in program managers have hampered requirements setting, funding stability, and resource allocation.

    “If the Air Force does not meet its schedule goals for development of GPS IIIA satellites, there will be an increased likelihood that in 2010, as old satellites begin to fail, the overall GPS constellation will fall below the number of satellites required to provide the level of GPS service that the U.S. government commits to. Such a gap in capability could have wide-ranging impacts on all GPS users, though there are measures the Air Force and others can take to plan for and minimize these impacts.

    “In addition to risks facing the acquisition of new GPS satellites, the Air Force has not been fully successful in synchronizing the acquisition and development of the next generation of GPS satellites with the ground control and user equipment, thereby delaying the ability of military users to fully utilize new GPS satellite capabilities.

    “Diffuse leadership has been a contributing factor, given that there is no single authority responsible for synchronizing all procurements and fielding related to GPS, and funding has been diverted from ground programs to pay for problems in the space segment. DoD and others involved in ensuring GPS can serve communities beyond the military have taken prudent steps to manage requirements and coordinate among the many organizations involved with GPS. However, GAO identified challenges in the areas of ensuring civilian requirements can be met and ensuring GPS compatibility with other new, potentially competing global space-based positioning, navigation, and timing systems.”

    Staving Off Disaster. In the course of its interviews with key officials, the GAO learned of and reports on some alternatives that have been examined. The Air Force Scientific Advisory Board considered the use of smaller GPS satellites in 2007. These could be developed more quickly and at lower cost. The board concluded that while small satellites could at some point serve to augment GPS capabilities, they would require a different and much more extensive ground control segment, program development would take too long, and necessary changes to user equipment would render the whole scheme cumbersome.

    The effects of satellite power loss over time, due to harsh space conditions, could be mitigated by shutting down satellite subsystems when not needed, reducing power consumption, also by shutting off a secondary (unnamed) GPS payload. DoD has long been reluctant to take either measure absolutely, particularly the second one, but according to testimony (see below) has been implementing both practices on an intermittent basis.

    Day in Congress. Other GPS community representatives testified to the House Oversight and Government Reform’s subcommittee on National Security and Foreign Affairs, alongside GAO spokesperson Chaplain.

    According to Lt. Gen. Larry D. James, Commander, 14th Air Force, Air Force Space Command, and Commander, Joint Functional Component Command for Space, U.S. Strategic Command, the Space Command maintains the required minimum of at least 24 GPS satellites in orbit, and the current level of 30 operational satellites, by keeping a “ghost fleet” of older, partially mission-capable satellites in backup mode. “Currently, three vehicles are held in residual status and are returned to the constellation every six months to ensure operational capability.” He stated that added life also is being squeezed from the satellites by reducing power to or turning off equipment for secondary missions aboard the satellites.

    Karen Van Dyke, acting director for Positioning, Navigation and Timing in the U.S. Department of Transportation’s Research and Innovative Technology Administration (RITA), told the Congressional committee that “GPS is vulnerable to interference that can be reduced, but not eliminated.” Citing the 2001 Volpe Report for which she was a key author, she stated that there has long been “an awareness within the transportation community of risks associated with use of GPS as a primary means for position determination and precision timing. Due to the reliance of transportation on GPS signals, it is essential that threats be mitigated and alternative back-ups be available, and the system be hardened for critical applications. DOT has determined that sufficient alternative navigation aids currently exist in the event of a loss of GPS-based services.”

    Nearly simultaneously with the GAO report and congressional hearings, the long-withheld Independent Assessment Team report on eLoran as a GPS backup has just been released.

    F. Michael Swiek, Executive Director, U.S. GPS Industry Council and a member of GPS World’s Editorial Advisory Board, reminded Congress of the dual-use nature of the system, saying “The U.S. Government has promoted and encouraged [GPS] development by establishing, maintaining and reinforcing a stable policy framework that has consistently received farsighted and bipartisan support. It has been a true partnership of shared visions, discussions and debates, cooperation, and coordination. This has been possible through the open dialogue that has taken place since the early days of GPS, some 25-plus years ago, between civilian and military, industry, and government on technical and policy issues as the technology, system, and applications have evolved.”

    Swiek made his recommendation that “successful adoption of modernized civilian GPS signals will occur if the installed user base can continue to trust the consistent and stable policy framework that the U.S. government has provided for GPS for two decades. The new signals will need to sustain the legacy of accuracy, availability, and reliability established over the past 20 years.”

    Chet Huber, president of OnStar, a wholly owned subsidiary of General Motors Corporation, and at nearly 6 million active subscribers probably the largest single group of civil GPS users, offered three recommendations:

    “First, we must address the health of the current constellation. We are concerned that a recent report shows eight of the current satellites are one component from total failure. Loss of signal will likely immediately affect GPS accuracy and availability (geographic coverage).

    “Second, as the GPS system is modernized, it is imperative that the U.S. government formally commit to preserving the L1C/A signal and to ensuring backward compatibility for legacy applications with no loss of performance from current levels.  . . . Any modernization initiative that degrades backward compatible performance — such as reducing the number of satellites making up the constellation — would likely adversely impact the provision of services by OnStar, including the quality of location information we provide to public safety, thereby potentially increasing the response time of public safety personnel to crash victims and others in need of emergency services.

    “Our third recommendation — and this is also important to legacy applications — is that we commit to maintaining the current PRN code (or satellite signature structure) for the primary orbital slots, as satellites in those slots are replaced. Legacy hardware is not capable of being expanded to accommodate more than 32 slots so renumbering above 32 will likely affect performance of legacy applications.”