Artist’s rendering of GPS IIF satellite. (Image: U.S. Air Force )
Boeing has secured a 10-year, $329.3 million contract to help the U.S. Space Force engineer operational GPS Block IIF satellites, the Department of Defense announced Dec. 20.
The company will perform engineering work to support on-orbit operations of the Block IIF satellites, which were manufactured by Boeing.
Space Systems Command issued the indefinite-delivery/indefinite-quantity contract to address GPS IIF mission requirements across the military and expects work to conclude by Dec. 20, 2031.
The U.S. Air Force deployed the first Boeing-built IIF satellite in May 2010 and launched the 12th and final satellite in February 2016.
By Peter Steigenberger, André Hauschild, Steffen Thoelert and Richard B. Langley
Between Feb. 7, 05:02 UTC and Feb. 8, 12:30 UTC, 2017, all seven operational GPS Block IIR-M satellites were consecutively subject to short periods of unavailability. These official outage periods, when the satellite signals were set unhealthy and deemed unusable, were announced ahead of time through Notice Advisories to Navstar Users (NANUs). An overview of the outage periods and the corresponding NANUs for each satellite identified by their pseudorandom noise code (PRN) assignment and space vehicle number (SVN) is provided in TABLE 1.
Table 1. GPS Block IIR-M satellite outage periods and corresponding 2017 NANUs.
An analysis of the measured signal-to-noise-density ratio (C/N0) from several tracking stations of the International GNSS Service (IGS) indicates that the satellites’ transmit powers were increased during the outage periods. The effect is visible in the plots in FIGURES 1 and 2, which show C/N0 of the L1 C/A-code over time for satellite passes on the three consecutive days Feb. 6, 7 and 8, 2017.
Figure 1 shows the results for PRN 17 as tracked by a Septentrio PolaRx4TR receiver (USN8) located in Washington, DC. The pass on the outage day Feb. 7 is plotted in blue. Obviously, the receiver is configured to not track unhealthy satellites, since no observations are available during the outage period. However, a clear increase in the C/N0 is visible from about 50.5 dB-Hz before the outage to approximately 52 dB-Hz after the outage. The C/N0 level of the day before is similar to the level prior to the outage. The C/N0 level on the following day is very similar to the C/N0 after the outage, which indicates that the satellite continues to transmit with an increased power.
Figure 1. Plot of L1 C/A-code C/N0 over time for consecutive satellite passes of satellite PRN 17 (SVN 53) tracked by a Septentrio PolaRx4TR receiver located in Washington, DC, on Feb. 6–8, 2017. The satellite’s unhealthy period on Feb. 7 is indicated by the gray shaded area.
The plot in Figure 2 shows the same analysis, this time for PRN 05 and for a Leica GR10 receiver (KOUG) located in Kourou, French Guiana. This receiver continues to track the satellite during the unhealthy period. The distinct step in C/N0 is clearly visible shortly after the satellite is set unhealthy. Also, this satellite continues to transmit with increased power during the pass on the following day. The same observations as in Figure 1 and Figure 2 can also be made for all other Block IIR-M satellites and other receivers.
Figure 2. Plot of L1 C/A-code C/N0 over time for consecutive passes of satellite PRN 05 (SVN 50) tracked by a Leica GR10 receiver located in Kourou, French Guiana, on Feb. 6–8. The satellite’s unhealthy period on Feb. 7 is indicated by the gray shaded area.
The difference between the measured C/N0 before and after the unhealthy period is typically 1–2 dB-Hz depending on the receiver and the satellite (see TABLE 2). On average, a power increase of 1.5 dB with a scatter of ±0.25 dB among the various satellites is suggested by the measured data.
Furthermore, it may be noted that different receivers respond with a different change in C/N0 for a given change in transmit power. At the average 1.5 dB power increment, C/N0 changes between 1 dB and 2 dB are reported by the different types of receivers. This indicates manufacturer-specific algorithms for C/N0 estimation that impact the use of measured C/N0 as a reliable indicator of received signal power strength.
Table 2. Changes in C/N0 (dB-Hz) obtained from differences of days before and after the increase of the transmit power.
It is interesting to notice in this context that NANU 2017005 issued Jan. 19, 2017, states that “The 2d Space Operations Squadron (2 SOPS) periodically conducts configuration changes on GPS satellites to assess current capabilities, validate future capabilities and ensure continued interoperability.”
Furthermore, the Civil GPS Service Interface Committee Executive Secretariat released the following statement on Jan. 25, 2017: “Beginning 25 January 2017, Air Force Space Command (AFSPC) will conduct a limited duration test implementing an increase of the L1 C/A power level on the GPS Block IIR-M and IIF satellites (19 vehicles).”
However, no maintenance has been announced so far for any of the Block IIF satellites, and no obvious increase in the measured C/N0 could be found for these satellites. A repeated analysis for the Block IIR-M satellites on Feb. 22, 2017, confirmed that the L1 C/A-code power levels were still at their increased levels.
Measurements with the German Aerospace Center’s (DLR’s) 30-meter-diameter high-gain antenna at Weilheim, Germany, have been recorded to independently confirm the GPS Block IIR-M transmit power increase of the L1 C/A-code. FIGURE 3 shows the L1 spectral flux density for March 4, 2017 (blue line), and a previous measurement taken on Dec. 7, 2015 (red line). The sharp peak in the middle of the spectrum represents the C/A-code. A clear increase of the power in the measurement of March 2017 compared to Dec. 2015 is visible. Further analysis of the high-gain antenna data yields a power increase of about 2 dB.
Figure 3. L1 spectral flux density of PRN 29 (SVN 57) for Dec. 7, 2015 (red, normal C/A-code power level) and March 4, 2017 (blue, increased C/A-code power level).
However, the M-code flux density with main lobes near 1565 and 1585 MHz is reduced in March 2017 compared to Dec. 2015, whereas the P(Y)-code signal strength remains essentially unaltered. The total transmit power in the L1 frequency band is the same for both time periods. Therefore, the analysis reveals a redistribution of transmit power from M-code to C/A-code for the Block IIR-M satellite PRN 29 (SVN 57).
Authors Peter Steigenberger, André Hauschild and Steffen Thoelert are from the German Aerospace Center (DLR).
Richard B. Langley is from the University of New Brunswick and authors the monthly Innovation column for GPS World magazine.
The U.S. Air Force successfully launched the 12th Boeing-built GPS IIF satellite aboard a United Launch Alliance Atlas V Evolved Expendable Launch Vehicle from Space Launch Complex 41, Cape Canaveral Air Force Station, Fla., at 8:38 a.m. EST (5:38 a.m. PST) on Feb. 5.
“Today’s launch is a significant achievement in the history of GPS, as we launch the last of the GPS IIF satellites to be delivered on-orbit,” said Lt. Gen. Samuel Greaves, Space and Missile Systems Center commander and Air Force program executive officer for space. “The GPS IIF satellite performance has been exceptional and is expected to be operational for years to come.”
“This milestone is the result of the remarkable relationship between SMC, our operators within the 14th Air Force and our ULA/Boeing industry partners. Their continued tenacity and dedication to mission success ensures we continue to maintain a robust satellite constellation with modernized, more resilient GPS capabilities,” said Greaves. “A job ‘Well Done!’”
According to Greaves, this mission demonstrates the Air Force’s continued intent to deliver pre-eminent space-based positioning, navigation and timing service to users around the globe. GPS IIF is critical to U.S. national security and to sustainment of the GPS constellation for civil, commercial, and military users. GPS IIF satellites play an integral part in the modernization efforts vigorously being pursued across space, ground and user equipment to provide stronger signals and improved resiliency in the GPS constellation.
“Today’s launch marks a momentous milestone in the history of the Global Positioning System. It is the twelfth and last GPS IIF satellite and closes out nearly 27 years of launches for the GPS Block II family of satellites,” said Col. Shawn Fairhurst, 45th Space Wing vice commander, who served as the Launch Decision Authority. “As the nation’s premier gateway to space, we are proud to be part of the team providing GPS and its capabilities to the world and look forward to the future as we begin preparation for the next generation of GPS III satellites. Together with the Space and Missile Systems Center and our industry partners, we make up one team delivering assured space launch and combat capabilities for the nation.”
An Airmen-led processing team at CCAFS has processed every satellite of the series since GPS IIF-1 launched here in May 2010.
The Boeing-built GPS IIF satellites provides improved accuracy through advanced atomic clocks, a longer design life than previous GPS satellites, and a new operational third civil signal (L5) that benefits commercial aviation and safety-of-life applications. It also continues to deploy the modernized capabilities that began with the GPS IIR-M satellites, including a more robust military signal.
GPS is the United States Department of Defense’s largest satellite constellation with 31-operational satellites on orbit.
Operated by Air Force Space Command’s 50th Space Wing at Schriever Air Force Base, located east of Colorado Springs, Colo., the GPS constellation provides precise positioning, navigation and timing services worldwide as a free service provided by the Air Force, seven days a week, 24-hours a day.
Space and Missile Systems Center, located at Los Angeles Air Force Base in El Segundo, Calif., is the U.S. Air Force’s center for acquiring and developing military space systems. Its portfolio includes GPS, military satellite communications, defense meteorological satellites, space launch and range systems, satellite control networks, space-based infrared systems and space situational awareness capabilities.
On Wednesday, the GPS Directorate said further data analysis shows that a technical error affecting some Boeing GPS IIF satellites first appeared in 2011, two years earlier than originally stated, according to a Reuters report.
The error first appeared one year after the GPS IIF satellites became operational. The error affects the way the ground control system builds and uploads messages transmitted by the satellites, but does not affect the accuracy of GPS signals. It involves the ground-based software used to index messages.
Lockheed Martin runs the GPS ground control segment, which enables Air Force officials to operate all GPS satellites, including the IIF satellites built by Boeing.
A patch for the upcoming GPS satellite launch shows a stylized GPS IIF-IX formed by the Delta 4 rocket and the constellation Cygnus (Latinized Greek word for swan). The satellite is nicknamed Deneb, the brightest star in the constellation and one of the 57 stars used in celestial navigation. The patch also indicates that the satellite is SV-10 and 71. The SV-10 identifier may indicate that it is the 10th satellite off the IIF assembly line. Photo: US Air Force
The U.S. Air Force’s ninth GPS Block IIF satellite (GPS IIF-9) is set to launch Wednesday at 2:36 p.m. EDT (1836 GMT) from Space Launch Complex 37 at Cape Canaveral Air Force Station, Fla.
The GPS IIF-9 will ride aboard a United Launch Alliance (ULA) Delta IV rocket, marking the 29th Delta IV launch and the 57th operational GPS satellite to launch on a ULA or heritage launch vehicle.
GPS IIF-9 is one of the next-generation GPS satellites, incorporating various improvements to provide greater accuracy, increased signals, and enhanced performance for users.
UPDATE (April 17, 2015): The USCG Navigation Center has confirmed that SV-10 was launched instead of SV-9. The Air Force discovered a problem with SV-9, so it was pulled from the launch and SV-10 was sent into orbit in its place. SV-9 will be used in a future launch.
Constellation Changes. The Air Force Second Space Operations Squadron (2 SOPS) indicates that IIF-9, SVN-71/PRN-26, will replace SVN-35 (currently being operated in Launch, Anomaly Resolution and Disposal Operations [LADO]) in the B plane slot 1F. SVN-38/PRN-08 will be taken out of the operational constellation prior to SVN-71 payload initialization and sent to LADO. PRN-08 will be assigned to SVN-49 in May and set to test, but is tentatively scheduled for assignment to IIF-10 to launch June 16. SVN-35, launched on August 30, 1993, has been in a residual status since March 2013 in an expanded node slot in the B plane and successfully served 21.5 years, 14.0 years beyond its designed service life, due to the diligent efforts of the men and women of the U.S. Air Force. SVN-51 is still in a re-phase journey from E1 (GLAN=146 °) to an auxiliary node at E7 (GLAN=60.7 °) scheduled to arrive sometime this summer.
The U.S. Air Force is working to resolve a technical error that affected some Boeing GPS satellites, according to a report by Reuters.
The error does not affect the accuracy of GPS signals. It involves the ground-based software used to index some messages transmitted by GPS IIF satellites built by Boeing, Air Force Space Command said according to Reuters. Still, officials are investigating other possible causes.
Lockheed Martin runs the GPS ground control segment, which enables Air Force officials to operate all GPS satellites, including the IIF satellites built by Boeing.
Air Force Space Command Public Affairs released the following statement:
“A GPS message indexing issue was recently identified that affects a limited number of active GPS IIF satellites, but does not degrade the accuracy of the GPS signal received by users around the globe. The result is an occasional broadcast not in accordance with U.S. technical specifications. The issue appears to be related to the ground software that builds and uploads messages transmitted by the GPS constellation during regular system operations, although the Air Force continues to investigate all possible causes.
“Although the issue was brought to light in the last few days, a close examination of archived GPS message data reveals that the message indexing error has gone unnoticed since 2013. Air Force Space Command has implemented a workaround to prevent further message indexing violations and is taking steps to permanently correct the error.”
A quarterly meeting of the U.S. GPS Program’s interagency Civil Navigation Signals (CNAV) Tiger Team on March 5 focused on the new L2C and L5 GPS civil signals. “CNAV Message Types 10, 11, 30 and 33 are currently transmitted on seven GPS IIR-M (L2C) and eight GPS IIF satellites (L2C and L5),” wrote Rick Hamilton, CGSIC Executive Secretariat, USCG Navigation Center, in a status email to the Civil Global Positioning System Service Interface Committee (CGSIC).
“A Modernized Navigation (MODNAV) Tool integrated with the GPS ground control software (Architecture Evolution Plan or AEP) is generating the CNAV data messages,” Hamilton wrote. “Daily CNAV uploads began December 31, 2014, and the U.S. Air Force reports that signal performance of CNAV matches or slightly outperforms Legacy performance: average user range error (RMS URE) from 25 February – 3 March 2015 was 0.50 m for Legacy and 0.57 m for Modernized; best week for Modernized signals since the broadcast initiated April 2014 was 0.42 m for 6 – 13 January 2015.
The graph above, from the Coast Guard Navigation Center website, illustrates the CNAV performance.
“Users are reminded that these CNAV signals are ‘pre-operational’ and should be used with discretion until they become fully operational; the L5 message is currently set unhealthy,” Hamilton concluded.
A Close Look at GPS SVN62 Triple-Frequency Signal Combinations Finds Carrier-Phase Variations on the New L5
By Oliver Montenbruck, André Hauschild (DLR/German Space Operations Center), Peter Steigenberger (Technische Universität München), and Richard B. Langley (University of New Brunswick)
The recently launched Block IIF satellite (SVN62/PRN25) is the first of a new generation of GPS satellites designed to transmit ranging signals for civil users on three frequencies: the C/A-code on L1 at 1575.42 MHz, the L2C-code on L2 at 1227.60 MHz, and the I5/IQ codes on L5 at 1176.45 MHz. Unlike L2, the L5 signal is located inside the protected Aeronautical Radionavigation Services (ARNS) band, which makes it specifically useful for safety critical aviation applications. In combination with the legacy L1 signal, civil aviation users can now perform ionospheric corrections without referring to the L2C signal. Compared to L2C, the new L5 signal offers a much higher chipping rate (the same as the encrypted P-code signal) of 10.23 MHz, which promises a lower ranging noise and better multipath resistance. L5 signals have already been transmitted for some time by the geostationary satellites of the United States’ Wide Area Augmentation System (WAAS) and are now about to become an integral part of the GPS constellation.
Following a short test transmission on June 17, 2010, the L5 signal was continuously activated on the morning of June 28. According to GPS officials, the checkout of the satellite is proceeding nominally and all signals have been found to fully comply with specifications. This will allow the satellite to be set healthy as soon as all commissioning tasks have been completed.
Scientists have long discussed the potential of new signals for multi-frequency, multi-GNSS applications, and expresed a great interest in signal combinations, particularly those of carrier-phase measurements, involving all three frequencies simultaneously. The use of triple-frequency combinations has, for example, been demonstrated to be of great interest for ambiguity resolution in precise carrier-phase-based positioning, for receiver autonomous integrity monitoring, and for ionospheric research (see the articles in Further Reading).
In consideration of the multitude of proposed applications for triple-frequency combinations, we took a close look at the quality of the new GPS L5 carrier-phase signal. For this purpose, we made use of measurements from the COoperative Network for GIOVE Observation (CONGO), jointly established by the German Federal Agency for Cartography and Geodesy (BKG) and the German Aerospace Center (DLR). CONGO is the first network of multi-constellation, multi-frequency GNSS receivers offering worldwide tracking of the SVN62 space vehicle on all frequencies (see Table 1).
Table 1. Subset of CONGO stations used for triple-frequency tracking of the new Block IIF satellite.
As suggested by Andrew Simsky (see Further Reading), the availability of carrier-phase measurements on three frequencies offers a particularly simple way to assess carrier-phase quality and multipath effects. By forming a linear combination
(1)
of the L1, L2, and L5 carrier-phase ranges with the additional conditions ,
a geometry- and ionosphere-free measurement is obtained, which reflects a weighted sum of the carrier-phase multipath and measurement noise on the individual frequencies. Here λ i with i = 1, 2, and 5, denotes the wavelength of the L1, L2, and L5 signals, respectively. Since the above conditions determine the factors α, β, and γ only up to an arbitrary scaling factor, we furthermore impose the normalizing conditions.
The latter condition ensures that the noise of the tri-carrier combination will match that of the individual carrier phases if the measurement noise is equal on all frequencies. As a result, we obtain the coefficients
with .
Introducing the carrier wavelengths of the L1, L2, and L5 signals, the coefficients attain the values (2)
It can be recognized that the tri-carrier combination is dominated by the L2 and L5 signals due to the proximity of their respective frequencies. Noise and multipath errors of L2 and L5 measurements are thus most prominently seen in the resulting combination, whereas any L1 phase errors are strongly attenuated.
A long pass of L1, L2, and L5 code and phase measurements from the new Block IIF satellite was recorded by the O’Higgins station of the CONGO network shortly after the activation of the L5 signal generator on June 28. The SVN62 satellite was tracked for more than 6 hours and achieved a peak elevation angle of more than 75° on this date.
Figure 1 shows the resulting multipath combination computed from carrier-phase measurements of L1 C/A-code tracking, semi-codeless L2 P(Y) tracking (rather than L2C), and L5 I/Q tracking. The data have been leveled to a zero mean over the entire pass to remove the impact of the unknown carrier-phase ambiguities. Except at low elevation angles, near rise and set of the satellite where signal strengths are low, the tri-carrier combination shows a very low noise level that is consistent with the expected carrier-phase noise on all three frequencies. However, a pronounced long-term variation with a peak-to-peak amplitude of almost 20 centimeters may be recognized, which certainly comes as a big surprise and cannot be explained by local multipath. Frequency-dependent differences of the effective phase centers of the receiving or transmitting antennas can likewise be excluded, since these would result in a purely elevation-angle-dependent variation.
FIGURE 1. Triple-frequency (M=0.142·L1-0.767·L2+0.626·L5) carrier-phase multipath combination for SVN62/PRN25 tracking from the OHIX0 station on June 28.
Looking at the entire set of measurements from all available CONGO stations, we could rapidly recognize that the variation of the tri-carrier combination with time is essentially the same for all stations with a common visibility of the SVN62 space vehicle, irrespective of the employed receiver and antenna. This suggests the presence of time-varying inter-frequency biases in the L1, L2, and L5 carriers transmitted by SVN62.
Thanks to the global distribution of the CONGO stations, the SVN62 space vehicle is always tracked by one or more stations, which enables a continuous monitoring of the L1/L2/L5 carrier-phase consistency. By adjusting the unknown offset of the tri-carrier combination for individual tracking arcs in such a way as to obtain a best match of consecutive and overlapping arcs, the variation can be traced over multiple days as shown in Figure 2. The graph shows a distinct orbital (that is, 12-hour) periodicity with a superimposed twice-per-revolution harmonic. In addition, a pronounced drift can be recognized for up to one day after activation of the L5 signal generator. Both observations suggest a temperature-dependent line bias in one or more carriers as a likely cause of the observed variation in the tri-carrier combination. (A line bias is a circuitry delay common in all observed satellites and is usually absorbed in the estimated clock offset.) However, an independent analysis of SVN62 temperature data from the onboard telemetry will be required to confirm the validity of this assumption. The space vehicle is in a deep eclipse orbit right now and therefore experiences substantial changes in its thermal conditions. However, the extreme points of the carrier-phase variation in Figure 2 are slightly shifted with respect to the local space vehicle noon (at 01:30 and 13:30 UTC) and the eclipse intervals (07:00–08:00 and 19:00–20:00 UTC).
FIGURE 2. Triple-frequency carrier-phase combination (M=0.142·L1-0.767·L2+0.626·L5) for the first five days of L5 activation on SVN62. The curve has arbitrarily been shifted to obtain a near-zero mean during the final days of the entire arc.
While the tri-carrier combination provides a very sensitive measurement for the analysis of differential delays between the individual carriers, it does not allow us to uniquely attribute the observed variations to one of the three signals. We therefore made use of code measurements (pseudoranges) to further investigate the consistency of specific sets of measurements. Since the observed variation of the tri-carrier combination exhibits an amplitude comparable to the noise level of the code measurements, a suitably chosen code-carrier combination can indeed help to identify which signal or signals are affected by line-bias variations. To this end, we consider a generalized form
of the well-known code-multipath combination, in which we difference the code measurement Pi at frequency i with an ionosphere-corrected combination of carrier-phase ranges Lj and Lk at frequencies j and k. In so doing, we remove geometric contributions along with clock and atmospheric variations, leaving primarily code multipath, receiver noise, and any signal perturbation that is not coherent on the involved frequencies. In the traditional case of dual-frequency tracking, the frequency of one of the involved carrier-phase measurements is necessarily identical to that of the code measurements. With triple-frequency tracking, in contrast, we are free to consider a larger variety of combinations. For the analysis of the SVN62 signals, we have specifically evaluated the L5 code-multipath combination using (a) the L5 and L1 carrier phases
and (b) the L2 and L1 carrier-phase measurements
The results shown in FIGURE 3 reveal a dramatic difference, which clearly hints at the L5 carrier as the main source of the observed carrier-phase variations.
FIGURE 3. L5 code-multipath combination formed with L1/L5 carrier-phase measurements (top) and with L1/L2 carrier-phase measurements (bottom). The figure is based on SVN62 tracking from the O’Higgins station and covers the same arc as considered in FIGURE 1.
In the first case, a variation close to that of Figure 1 is obtained, albeit with a 5–6 times larger amplitude that reflects the different weighting of the L5 carrier phase in the corresponding measurement combinations. A good consistency, in contrast, is obtained for the L5 code measurements when differenced against the ionosphere-corrected combination of L1 and L2 carrier-phase measurements.
Overall, we may conclude that the L5 carrier of the SVN62 space vehicle exhibits quasi-periodic line-bias variations with an amplitude of about 10 centimeters in relation to the L1 and L2 carriers. The L5 code measurements, in contrast, appear to be consistent with both the code and phase measurements on L1 and L2 at the respective noise levels. Further observations at a later time will be required to see whether the observed amplitude of the L5 phase variation is specific to the current eclipse orbit and whether it will possibly become lower when a higher angle of the Sun with respect to the orbital plane (the so-called beta-angle) is achieved.
What are the possible consequences of the L5 phase-bias variations for users of the new L5 signal? Evidently, new positioning services building on the L5 code measurements (and possible combinations) will not at all be affected! Even in the case of carrier-phase smoothing, the smoothing time scale will be much shorter than the periodicity of the carrier-phase bias variation. The L5 code measurement quality itself is well within the system specification and no concerns exist that would prevent the satellite from soon being declared healthy.
With respect to carrier-phase-based positioning applications, it is important to note that the L5 line bias acts like an additional frequency-specific satellite-clock offset. This has, for example, been confirmed in preliminary tests of SVN62 orbit determination conducted by the Technische Universität München. Orbit solutions using L1 and L5 measurements from the CONGO network differed by typically 15 centimeters (3D root-mean-square error) from reference orbits obtained by the Center for Orbit Determination in Europe analysis center using the IGS L1/L2 receiver network. At the same time, however, the L1/L5-based clock solutions showed a periodic offset from the L1/L2-based values that reflects the same variations as the tri-carrier combination discussed above.
As a common error for all receivers, the L5 line bias fully cancels in differential processing. Care must be taken though, that satellite clock offsets derived from L1/L2 carrier-phase observations cannot be employed for precise point positioning using L1/L5 measurements without explicit consideration of the inter-frequency carrier-phase bias. Likewise, efforts to correct second order ionospheric effects through the use of triple-frequency measurements are likely to suffer from an imperfect knowledge of the L5 bias and its variation with time.
Whereas some of the proposed ideas for triple-frequency processing may be difficult to materialize at present, a better characterization of the SVN62 L5 signal will certainly help to exploit the available benefits of the new signal and to establish refined processing schemes for scientific and other demanding applications. A continued monitoring of the L5 line bias and its variation with time is therefore deemed necessary and should be supported by a large number of suitably equipped tri-band GNSS monitoring stations.
— Oliver Montenbruck, Andre Hauschild (DLR/German Space Operations Center), Peter Steigenberger (Technische Universität München)
Richard B. Langley (University of New Brunswick)
Acknowledgment
The authors are grateful to Tom Stansell and Col. David Goldstein from the GPS Wing for early discussions and their independent assessment and interpretation of the SVN62 triple-frequency carrier-phase data.
Equipment
The CONGO network makes use of JavadTriumph Delta-G2T/G3TH and LeicaGRX1200+GNSS GNSS receivers for tracking GPS signals on the L1, L2, and L5 frequencies. The stations are equipped with TrimbleZephyr Geodetic II or LeicaAX1203+GNSS and AR25R3 antennas.
Further Reading
“The WAAS L5 Signal: An Assessment of Its Behavior and Potential End Use,” by H. Rho and R.B. Langley in GPS World, Vol. 20, No. 5, May 2009, pp. 42–50.
“Using Multi-Frequency for GPS Positioning and Receiver Autonomous Integrity Monitoring” by Y.-H. Tsai, F.-R. Chang, W.-C. Yang, and C.-L. Ma in Proceedings of the 2004 IEEE International Conference on Control Applications, Taipei, Taiwan, September 2–4, 2004, pp. 205–210.
“Triple Frequency Ambiguity Resolution Using GPS/Galileo” by O. Julien, M.E. Cannon, P. Alves, and G. Lachapelle in European Journal of Navigation, Vol. 2, No. 2, May 2004, pp. 51–57.
“Three’s the Charm — Triple Frequency Combinations in Future GNSS” by A. Simsky in Inside GNSS, Vol. 1, No. 5, July/August 2006, pp. 38–41.
“Total Electron Content Monitoring Using Triple Frequency GNSS Data: A Three-Step Approach” by J. Spits and R. Warnant in Journal of Atmospheric and Solar-Terrestrial Physics, Vo. 70, No. 15, December 2008, pp. 1885–1893, doi:10.1016/j.jastp.2008.03.007.
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.”
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.
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.”
