An experimental GPS receiver, built by Surrey Satellite Technology Limited (SSTL), has successfully achieved a GPS position fix at 23,300 kilometers altitude – the first position fix above the GPS constellation on a civilian satellite. The SGR-GEO receiver is collecting data that could help SSTL to develop a receiver to navigate spacecraft in geostationary orbit (GEO) or even in deep space.
GPS is routinely used on Low Earth Orbit (LEO) satellites to provide the orbital position and offer a source of time to the satellite. Spacecraft in orbits higher than the 20,000 km of the GPS constellation, however, can only receive a few of the signals that “spill over” from the far side of the Earth, meaning that the signals are much weaker and a position fix cannot always be secured.
With the support of the European Space Agency (ESA) and the ARTES 4 program, SSTL included the SGR-GEO receiver on the GIOVE-A satellite to prove that a receiver could achieve a position fix from a higher orbit. The SGR-GEO is adapted from SSTL’s SGR range of receivers and incorporates a high-gain antenna and a precise oven-controlled clock. It will demonstrate special algorithms to allow reception of weak signals and an orbit estimator intended to allow a near continuous position fix throughout orbit.
“The results from the SGR-GEO receiver are really encouraging,” said Martin Unwin, principal GNSS engineer at SSTL. “We’re getting higher signal strengths than anticipated and also acquiring side lobes from the GPS transmit antennas, which improves the availability of the usable signals for navigation. With the success of the SGR-GEO receiver, GPS, in combination with Galileo and GLONASS, could soon be helping navigate spacecraft much further away from Earth.”
The experimental GPS receiver onboard GIOVE-A has been inactive for six years while the satellite has been used for its primary purpose of transmitting prototype Galileo signals. GIOVE-A’s retirement in June 2012 has allowed the commissioning of the experiment and is now providing valuable data to SSTL and ESA in support of the future use of spaceborne GNSS receivers at GEO altitudes. Engineers at SSTL will continue operations, testing out, tuning and improving the receiver software onboard GIOVE-A to achieve the best possible performance.
The BeiDou-2/Compass geostationary satellite, G3, was moved between about November 7 and 22 from an orbital longitude of 84 degrees east to 110.5 degrees east.
The 110.5 degree east longitude slot had been previously used by BeiDou 1C, one of the demonstration or BeiDou-1 satellites, which was initially shifted to about 85 degrees east between about June 2 and July 7, 2012. BeiDou 1C has drifted slightly since and is currently at 84 degrees east.
According to orbital data supplied by the U.S. Joint Space Operations Center, once BeiDou 1C was shifted to about 85 degrees east longitude, station keeping seems to have been no longer applied. This may imply that the satellite is no longer in use but this has not yet been confirmed.
The Galileo In-orbit Validation (IOV) satellites launched on October 12 (Flight Model 3 and 4), have now been positioned in their designated orbits, according to tracking data from the U.S. Joint Space Operations Center. A plot of the IOV constellation is now available.
The four IOV satellites are in two orbital planes separated by about 120 degrees. Within each plane, the satellites are separated by about 40 degrees. This orbital arrangement will allow the four satellites to be simultaneously tracked for periods of time by GNSS monitoring stations, permitting positioning tests using only IOV data to be carried out. However, no signals from FM3 or FM4 have yet been detected by stations of the International GNSS Service.
The Lockheed Martin team developing the U.S. Air Force’s next generation Global Positioning System III satellites has completed thermal vacuum testing for the Navigation Payload Element (NPE) of the GPS III Non-Flight Satellite Testbed (GNST). The milestone is one of several environmental tests verifying the navigation payload’s quality of workmanship and increased performance compared to the current generation of satellites, the company said.
The GPS III program will affordably replace aging GPS satellites, while improving capability to meet the evolving demands of military, commercial and civilian users. GPS III satellites are expected to deliver better accuracy and improved anti-jamming power while enhancing the spacecraft’s design life and adding a new civil signal designed to be interoperable with international global navigation satellite systems.
“GPS III satellites have the most advanced navigation payloads ever manufactured. This milestone is a key indicator that we have a solid design and are on track to provide unprecedented position, navigation, and timing capability for GPS users worldwide,” said Lt. Col. Todd Caldwell, the U.S. Air Force’s GPS III program manager.
During thermal vacuum testing, the navigation payload’s performance was proven in a vacuum environment at the extreme hot and cold temperatures it will experience on orbit to ensure it will operate as planned once in space. Following the test, the NPE will now be integrated with the GNST for final satellite level testing.
The GNST is a full-sized prototype of a GPS III satellite used to identify and solve development issues prior to integration and test of the first space vehicle. The approach significantly reduces risk, improves production predictability, increases mission assurance and lowers overall program costs. Following integration and test at Lockheed Martin’s GPS Processing Facility (GPF) near Denver, the GNST will be shipped to Cape Canaveral Air Force Station, Fla., for risk reduction activities at the launch site.
“The completion of thermal vacuum testing on our first navigation payload is a critical milestone for our program that demonstrates we are on a solid path to meet our commitments,” said Keoki Jackson, vice president of Lockheed Martin’s Navigation Systems mission area. “The Air Force’s early investment in our GPS III pathfinder is now paying off and will enable highly efficient and affordable satellite production going forward.”
Lockheed Martin is on contract to deliver the first four GPS III satellites for launch. The Air Force plans to purchase up to 32 GPS III satellites.
The GPS III team is led by the Global Positioning Systems Directorate at the U.S. Air Force Space and Missile Systems Center. Lockheed Martin is the GPS III prime contractor with teammates ITT Exelis, General Dynamics, Infinity Systems Engineering, Honeywell, ATK and other subcontractors. Air Force Space Command’s 2nd Space Operations Squadron (2SOPS), based at Schriever Air Force Base, Colo., manages and operates the GPS constellation for both civil and military users.
General Designer of Russia’s GLONASS satellite navigation system Yuri Urlichich has been dismissed from his post in the wake of an embezzlement scandal, a spokesperson for RF Deputy Prime Minister Dmitry Rogozin, who is in charge of the military-industrial complex, told Itar-Tass.
Urlichich still holds the position of Director General of Russian Space Systems (RSS), but is no longer the chief designer of Russia’s GLONASS system.
The personnel decision is apparently related to a scandal involving embezzlement of 6.5 billion rubles ($200 million) of the GLONASS programs funds at RSS, Deputy Prime Minister Dmitry Rogozin told the RIA Novosti news service on Sunday. Rogozin heads the government’s military-industrial commission.
According to Igor Bozhkov, head of the Moscow Metro Internal Affairs Department, RSS’ initial contract with Russian space agency Roscosmos allowed the company several avenues for embezzlement.
No charges were reported against Urlichich or other GLONASS makers as of late Sunday.
The Washington Post is reporting that President Vladimir Putin’s chief of staff was aware of alleged embezzlement of state funds earmarked for GLONASS. Sergei Ivanov said he discussed the probe with police officials but didn’t speak publicly about it for several years, to prevent the culprits from covering up their deeds. Ivanov, a KGB veteran like Putin, said years in the spy service taught him to be sly with the enemy. As a former cabinet member, Ivanov previously oversaw the development of the GLONASS system.
Firing on all cylinders — to use a slightly outmoded technological metaphor — GNSS moved forward on virtually every front in the past month. GPS made major advances both on the ground and in space, Galileo took a giant step, Compass continued on its roll, GLONASS has good news pending in only a day or two (knock on wood), and GAGAN is settling into space. But the best news of all is a very quiet, indeed somewhat hidden item: the UK patent applications against the interoperative GPS/Galileo signal design appear to have been dropped.
Let’s eat dessert first, since life is uncertain.
Patent Dispute Evaporates
Vague rumblings emerged throughout spring and summer this year that two British technologists, backed by the U.K. Ministry Defense, had filed patents on the future interoperable GPS and Galileo binary-offset carrier signal designs. If granted and enforced, the patents would have severely disrupted modernization plans for both systems and levied unexpected costs upon receiver manufacturers. And in fact a company called Ploughshare Innovations Ltd. Started dialing up said manufacturers and asking for payment of royalties, based on the patent filings.
After significant uproar and negotiations before and behind the scenes, it now appears that the initiative has been quietly scuttled. The file on application number 11/774,412, Modulation Signals for a Satellite Navigation System, on the U.S. Patent Office’s website, now reads “Expressly Abandoned — During Examination.” The status is dated September 16, 2012, some time ago, but that I’m aware of, no parties involved, whether as filers or negotiators, ever made any kind of announcement about it.
Checking the European Patent Office and its registry — which by the way is no trivial task of website navigation — I found a note under the docket for EP1830199, Modulations Signals for a Satellite Navigation System stating “Patent surrendered.” Dated September 24, 2012. A few days later, another note: “Lapsed in a contracting state announced via postgrant inform. From Nat. Office to EPO,” with further information to the effect of “lapse because of failure to submit a translation or the description or to pay the fee within the prescribed time limit.” And for good measure, a final docket not on October 3, “Lapsed due to resignation by the proprietor.”
However abstruse and arcane, we’ll take good news however we find it. Another bullet dodged.
GPS Ground Segment Benchmark
The GPS Directorate announced on October 26 that the U.S. Air Force and Raytheon have successfully met all requirements to enter into the engineering and manufacturing development phase of the Next-Generation Operational Control System (OCX). OCX will replace the current GPS operational control segment in managing the satellite constellation and providing command and control for all modernized signals.
OCX is being developed and fielded in blocks of GPS capability, to align with GPS III and military equipment deliveries.
OCX Block 0, also known as the Launch and Checkout System, scheduled to be available in the fourth quarter of Fiscal Year 2014, will allow OCX to support the launch of GPS III satellites.
OCX Block 1, scheduled to transition to operations in the first quarter of 2016, will deliver the operational capability to command and control the entire GPS constellation including GPS II and GPS III satellites. This block will also control the legacy civil and military signals, as well as two modernized civil and military signals, L2C and L5.
OCX Block 2 will specifically support advanced capabilities for civilian and military signals, the international civil signal, L1C, and the military signal, M-Code. OCX Block 2 is currently synchronized with modernized signal broadcast and timing.
GPS Block IIF-3 satellite.
GPS Block IIF Satellite Rises, Reaches Station, and Transmits
On October 11, The L5 transmitter aboard GPS Block IIF-3 satellite SVN65/PRN24 was switched on, transmitting the civilian safety-of-life GPS signal, designed to meet demanding requirements for safety-of-life transportation and other high-performance applications.
A day earlier, SVN65 began transmitting L1 and L2 signals as PRN24 on October 8. A number of stations of the International GNSS Service are tracking the satellite. As of press date for this magazine (October 25) the satellite is included in broadcast almanacs although it is set unhealthy and will continue to be so until satellite commissioning is completed. The satellite is drifting towards its designated orbital position of Slot 1 in Plane A.
The launch of the GPS Block IIF-3 satellite took place as scheduled October 4, aboard a United Launch Alliance Delta IV rocket from Cape Canaveral, Florida.
Galileo Turns Four. Validation Satellites, That Is.
The Galileo control room.
On October 12, a Soyuz launcher carrying two Galileo In-Orbit Validation (IOV) satellites deployed its twins into orbit within four hours after take-off, at close to 23,200 kilometers altitude. They join two earlier IOV spacecraft launched in October 2011. Once all four are operational in space, they will provide the minimum number of satellites required for navigational fixes — enabling system validation testing when all are visible in the sky.
A week after the dual liftoff from Kourou, French Guiana, the two satellites completed the critical Launch and Early Orbit Phase on October 19-20.
Satellites FM3 and FM4 satellites were handed over from the joint ESA/CNES Launch and Early Orbit Phase (LEOP) team in Toulouse, France, to the Galileo Control Centre, Oberpfaffenhofen, Germany, from where Spaceopal will manage operations of the Galileo constellation.
Three orbit maneuvers were conducted for each satellite to start them on drift orbits towards their operational positions, where they are expected to arrive on November 10 (FM3) and November 12 (FM4) after a series of drift-stop and fine-positioning movements.
The satellites were configured into a secure mode shortly after handover. While underway to their final positions, they will also undergo a series of tests to confirm the performance of their subsystems before switching on the payload.
The satellites were built by a consortium led by the Astrium division of EADS, which produced the platforms and has responsibility for the payloads, while Thales Alenia Space handled assembly and testing.
Compass up to Eleven
The two BeiDou-2/Compass satellites launched on September 18 reached their circular medium-Earth orbits on October 1 and started transmitting navigation signals. Several stations participating in the International GNSS Service’s Multi-GNSS Experiment as well as some in the Cooperative Network for GNSS Observation started tracking the satellites on September 26.
Although semi-official rumors had circulated that China was preparing for the Compass G6 (G2R) satellite launch on October 25, we have not found any announcement that the event has occurred.
The November issue of GPS World will appear in a few weeks’ time, with a cover story on “What Is Achievable with the Current Compass Constellation?” The technical article by Chinese researchers gives data from a 12-station tracking network distributed through China, the Pacific region, Europe, and Africa. It demonstrates the capacity of Compass with a constellation comprising four geostationary Earth-orbit (GEO) satellites and five inclined geosynchronous orbit (IGSO) satellites in operation. The regional system will be completed around the end of 2012 with a constellation of five GEOs, five IGSOs, and four medium-Earth orbit (MEO) satellites. By 2020 it will be extended into a global system.
GLONASS News in a Day or Two
As we go to e-press with this e-newsletter on October 30, we look forward to a Russian rocket rising on November 2 with a Luch data-relay satellite payload to service the the Russian satnav system. The second of a set of three geostationary satellites launched to reactivate Roscosmos’s Luch Multifunctional Space Relay System, it will also carry transponders for the System for Differential Correction and Monitoring (SDCM), Russia’s satellite-based augmentation system. The transponders will broadcast GNSS corrections on the standard GPS L1 frequency using C/A PRN codes assigned by the GPS Directorate. According to the most recent announcement, it will be positioned at 16 degrees West longitude, joining Luch-5A, already in an orbital slot at 95 degrees East longitude.
GAGAN Unfolding
The Indian Space Research Organization announced on October 3 that orbit-raising maneuvers placed the GSAT-10 satellite, launched September 30, in an orbit with 35,000-kilometer high orbit, with an orbit period of 23 hours 50 minutes, and a designated location of 83 degree East. GSAT-10 contains a payload to support the Indian GPS and GEO Augmented Navigation (GAGAN) satellite-based augmentation system. The satellite will likely use PRN code 128.
Another Dispute Headed for Resolution?
Finally, another pink dawn on the horizon. The European Union (EU) and China will reportedly meet in December in Paris to discuss overlapping radio frequencies both plan to use for their future encrypted government/military satellite navigation services.
The meeting will be conducted under what the Joint Statement on Space Technology Cooperation specifies as the ITU Framework. ITU is the International Telecommunication Union of Geneva, a United Nations affiliate that regulates satellite orbital slots and frequencies.
The statement was signed as an annex to a broader EU-China summit held September 20 in Brussels. The two sides continue collaboration on satellite navigation despite the signal conflict, which has been a subject of debate for at least two years.
The 27-nation EU and China have agreed to continue the China-Europe GNSS Technology Training and Cooperation Center.
Figure 1. Distribution of the GPS+COMPASS tracking network established by the GNSS Research Center at Wuhan University and used as test network in this study.
Data from a tracking network with 12 stations in China, the Pacific region, Europe, and Africa demonstrates the capacity of Compass with a constellation comprising four geostationary Earth-orbit (GEO) satellites and five inclined geosynchronous orbit (IGSO) satellites in operation. The regional system will be completed around the end of 2012 with a constellation of five GEOs, five IGSOs, and four medium-Earth orbit (MEO) satellites. By 2020 it will be extended into a global system.
By Maorong Ge, Hongping Zhang, Xiaolin Jia, Shuli Song, and Jens Wickert
China’s satellite navigation system Compass, also known as BeiDou, has been in deveopment for more than a decade. According to the China National Space Administration, the development is scheduled in three steps: experimental system, regional system, and global system.
The experimental system was established as the BeiDou-1 system, with a constellation comprising three satellites in geostationary orbit (GEO), providing operational positioning and short-message communication. The follow-up BeiDou-2 system is planned to be built first as a regional system with a constellation of five GEO satellites, five in inclined geosynchronous orbit (IGSO), and four in medium-Earth orbit (MEO), and then to be extended to a global system consisting of five GEO, three IGSO, and 27 MEO satellites. The regional system is expected to provide operational service for China and its surroundings by the end of 2012, and the global system to be completed by the end of 2020.
The Compass system will provide two levels of services. The open service is free to civilian users with positioning accuracy of 10 meters, timing accuracy of 20 nanoseconds (ns) and velocity accuracy of 0.2 meters/second (m/s). The authorized service ensures more precise and reliable uses even in complex situations and probably includes short-message communications.
The fulfillment of the regional-system phase is approaching, and the scheduled constellation is nearly completed. Besides the standard services and the precise relative positioning, a detailed investigation on the real-time precise positioning service of the Compass regional system is certainly of great interest.
With data collected in May 2012 at a regional tracking network deployed by Wuhan University, we investigate the performance of precise orbit and clock determination, which is the base of all the precise positioning service, using Compass data only. We furthermore demonstrate the capability of Compass precise positioning service by means of precise point positioning (PPP) in post-processing and simulated real-time mode.
After a short description of the data set, we introduce the EPOS-RT software package, which is used for all the data processing. Then we explain the processing strategies for the various investigations, and finally present the results and discuss them in detail.
Tracking Data
The GNSS research center at Wuhan University is deploying its own global GNSS network for scientific purposes, focusing on the study of Compass, as there are already plenty of data on the GPS and GLONASS systems. At this point there are more than 15 stations in China and its neighboring regions.
Two weeks of tracking data from days 122 to 135 in 2012 is made available for the study by the GNSS Research Center at Wuhan University, with the permission of the Compass authorities. The tracking stations are equipped with UR240 dual-frequency receivers and UA240 antennas, which can receive both GPS and Compass signals, and are developed by the UNICORE company in China. For this study, 12 stations are employed. Among them are seven stations located in China: Chengdu (chdu), Harbin (hrbn), HongKong (hktu), Lhasa (lasa), Shanghai (sha1), Wuhan (cent) and Xi’an (xian); and five more in Singapore (sigp), Australia (peth), the United Arab Emirates (dhab), Europa (leid) and Africa (joha). Figure 1 shows the distribution of the stations, while Table 1 shows the data availability of each station during the selected test period.
Table 1. Data availability of the stations in the test network.
There were 11 satellites in operation: four GEOs (C01, C03, C04, C05), five IGSOs (C06, C07, C08, C09, C10), and two MEOs (C11, C12). During the test time, two maneuvers were detected, on satellite C01 on day 123 and on C06 on day 130. The two MEOs are not included in the processing because they were still in their test phase.
Software Packages
The EPOS-RT software was designed for both post-mission and real-time processing of observations from multi-techniques, such as GNSS and satellite laser ranging (SLR) and possibly very-long-baseline interferometry (VLBI), for various applications in Earth and space sciences. It has been developed at the German Research Centre for Geosciences (GFZ), primarily for real-time applications, and has been running operationally for several years for global PPP service and its augmentation. Recently the post-processing functions have been developed to support precise orbit determinations of GNSS and LEOs for several ongoing projects.
We have adapted the software package for Compass data for this study. As the Compass signal is very similar to those of GPS and Galileo, the adaption is straight-forward thanks to the new structure of the software package. The only difference to GPS and Galileo is that recently there are mainly GEOs and IGSOs in the Compass system, instead of only MEOs. Therefore, most of the satellites can only be tracked by a regional network; thus, the observation geometry for precise orbit determination and for positioning are rather different from current GPS and GLONASS.
Figure 2 shows the structure of the software package. It includes the following basic modules: preprocessing, orbit integration, parameter estimation and data editing, and ambiguity-fixing. We have developed a least-square estimator for post-mission data processing and a square-root information filter estimator for real-time processing.
Figure 2. Structure of the EPOS-RT software.
GPS Data Processing
To assess Compass-derived products, we need their so-called true values. The simplest way is to estimate the values using the GPS data provided by the same receivers.
First of all, PPP is employed to process GPS data using International GNSS Service (IGS) final products. PPP is carried out for the stations over the test period on a daily basis, with receiver clocks, station coordinates, and zenith tropospheric delays (ZTD) as parameters. The repeatability of the daily solutions confirms a position accuracy of better than 1 centimeter (cm), which is good enough for Compass data processing. The station clock corrections and the ZTD are also obtained as by-products.
The daily solutions are combined to get the final station coordinates. These coordinates will be fixed as ground truth in Compass precise orbit and clock determination. Compass and GPS do not usually have the same antenna phase centers, and the antenna is not yet calibrated, thus the corresponding corrections are not yet available. However, this difference could be ignored in this study, as antennas of the same type are used for all the stations.
Orbit and Clock Determination
For Compass, a three-day solution is employed for precise orbit and clock estimation, to improve the solution strength because of the weak geometry of a regional tracking network. The orbits and clocks are estimated fully independent from the GPS observations and their derived results, except the station coordinates, which are used as known values.
The estimated products are validated by checking the orbit differences of the overlapped time span between two adjacent three-day solutions. As shown in Figure 3, orbit of the last day in a three-day solution is compared with that over the middle day of the next three-day solution. The root-mean-square (RMS) deviation of the orbit difference is used as index to qualify the estimated orbit.
Figure 3. Three-day solution and orbit overlap. The last day of a three-day solution is compared with the middle day of the next three-day solution.
In each three-day solution, the observation models and parameters used in the processing are listed in Table 2, which are similar to the operational IGS data processing at GFZ except that the antenna phase center offset (PCO) and phase center variation (PCV) are set to zero for both receivers and satellites because they are not yet available.
Satellite force models are also similar to those we use for GPS and GLONASS in our routine IGS data processing and are listed in Table 2. There is also no information about the attitude control of the Compass satellites. We assume that the nominal attitude is defined the same as GPS satellite of Block IIR.
Table 2. Observation and force models and parameters used in the processing.
Satellite Orbits. Figure 4 shows the statistics of the overlapped orbit comparison for each individual satellite. The averaged RMS in along- and cross-track and radial directions and 3D-RMS as well are plotted. GEOs are on the left side, and IGSOs on the right side; the averaged RMS of the two groups are indicated as (GEO) and (IGSO) respectively. The RMS values are also listed in Table 3.
As expected, GEO satellites have much larger RMS than IGSOs. On average, GEOs have an accuracy measured by 3D-RMS of 288 cm, whereas that of IGSOs is about 21 cm.
As usual, the along-track component of the estimated orbit has poorer quality than the others in precise orbit determination; this is evident from Figure 4 and Table 3. However, the large 3D-RMS of GEOs is dominated by the along-track component, which is several tens of times larger than those of the others, whereas IGSO shows only a very slight degradation in along-track against the cross-track and radial. The major reason is that IGSO has much stronger geometry due to its significant movement with respect to the regional ground-tracking network than GEO.
Figure 4. Averaged daily RMS of all 12 three-day solutions. GEOs are on the left side and IGSOs on the right. Their averages are indicated with (GEO) and (IGSO), respectively.Table 3. RMS of overlapped orbits (unit, centimeters).
If we check the time series of the orbit differences, we notice that the large RMS in along-track direction is actually due to a constant disagreement of the two overlapped orbits. Figure 5 plots the time series of orbit differences for C05 and C06 as examples of GEO and IGSO satellites, respectively. For both satellites, the difference in along-track is almost a constant and it approaches –5 meters for C05.
Note that GEO shows a similar overlapping agreement in cross-track and radial directions as IGSO.
Figure 5. Time series of orbit differences of satellite C05 and C06 on the day 124 2012. A large constant bias is in along-track, especially for GEO C05.
Satellite Clocks. Figure 6 compares the satellite clocks derived from two adjacent three-day solutions, as was done for the satellite orbits. Satellite C10 is selected as reference for eliminating the epoch-wise systematic bias. The averaged RMS is about 0.56 ns (17 cm) and the averaged standard deviation (STD) is 0.23 ns (7 cm). Satellite C01 has a significant larger bias than any of the others, which might be correlated with its orbits.
From the orbit and clock comparison, both orbit and clock can hardly fulfill the requirement of PPP of cm-level accuracy. However, the biases in orbit and clock are usually compensatable to each other in observation modeling. Moreover, the constant along-track biases produce an almost constant bias in observation modeling because of the slightly changed geometry for GEOs. This constant bias will not affect the phase observations due to the estimation of ambiguity parameters. Its effect on ranges can be reduced by down-weighting them properly. Therefore, instead of comparing orbit and clock separately, user range accuracy should be investigated as usual. In this study, the quality of the estimated orbits and clocks is assessed by the repeatability of the station coordinates derived by PPP using those products.
Figure 6. Statistics of the overlap differences of the estimated receiver and satellite clocks. Satellite C10 is selected as the reference clock.
Precise Point Positioning
With these estimates of satellite orbits and clocks, PPP in static and kinematic mode are carried out for a user station that is not involved in the orbit and clock estimation, to demonstrate the accuracy of the Compass PPP service.
In the PPP processing, ionosphere-free phase and range are used with proper weight. Satellite orbits and clocks are fixed to the abovementioned estimates. Receiver clock is estimated epoch-wise, remaining tropospheric delay after an a priori model correction is parameterized with a random-walk process. Carrier-phase ambiguities are estimated but not fixed to integer. Station coordinates are estimated according to the positioning mode: as determined parameters for static mode or as epoch-wise independent parameters for kinematic mode.
Data from days 123 to 135 at station CHDU in Chengdu, which is not involved in the orbit and clock determination, is selected as user station in the PPP processing. The estimated station coordinates and ZTD are compared to those estimated with GPS data, respectively.
Static PPP. In the static test, PPP is performed with session length of 2 hours, 6 hours, 12 hours, and 24 hours. Figure 7 and Table 4 show the statistics of the position differences of the static solutions with various session lengths over days 123 to 125.
The accuracy of the PPP-derived positions with 2 hours data is about 5 cm, 3 cm, and 10 cm in east, north, and vertical, compared to the GPS daily solution. Accuracy improves with session lengths. If data of 6 hours or longer are involved in the processing, position accuracy is about 1 cm in east and north and 4 cm in vertical. From Table 4, the accuracy is improved to a few millimeters in horizontal and 2 cm in vertical with observations of 12 to 24 hours. The larger RMS in vertical might be caused by the different PCO and PCV of the receiver antenna for GPS and Compass, which is not yet available.
Figure 7. Position differences of static PPP solutions with session length of 2 hours, 6 hours, 12 hours, and 24 hours compared to the estimates using daily GPS data for station CHDU.Table 4. RMS of PPP position with different session length.
Kinematic PPP. Kinematic PPP is applied to the CHDU station using the same orbit and clock products as for the static positioning for days 123 to 125 in 2012.
The result of day 125 is presented here as example. The positions are estimated by means of the sequential least-squares adjustment with a very loose constraint of 1 meter to positions at two adjacent epochs. The result estimated with backward smoothing is shown in Figure 8. The differences are related to the daily Compass static solution. The bias and STD of the differences in east, north, and vertical are listed in Table 5. The bias is about 16 mm, 13 mm, and 1 mm, and the STD is 10 mm, 14 mm and 55 mm, in east, north, and vertical, respectively.
Figure 8. Position differences of the kinematic PPP and the daily static solution, and number of satellites observed.Table 5. Statistics of the position differences of the kinematic PPP in post-processing mode and the daily solution. (m)
Compass-Derived ZTD. ZTD is a very important product that can be derived from GNSS observations besides the precise orbits and clocks and positions. It plays a crucial role in meteorological study and weather forecasting.
ZTD at the CHDU station is estimated as a stochastic process with a power density of 5 mm √hour by fixing satellite orbits, clocks, and station coordinates to their precisely estimated values, as is usually done for GPS data.
The same processing procedure is also applied to the GPS data collected at the station, but with IGS final orbits and clocks. The ZTD time series derived independently from Compass and GPS observations over days 123 to 125 in 2012 and their differences are shown on Figure 9.
Figure 9. Comparison of ZTD derived independently from GPS and COMPASS observations. The offset of the two time series is about -14 mm (GPS – COMPASS) and the STD is about 5 mm.
Obviously, the disagreement is mainly caused by Compass, because GPS-derived ZTD is confirmed of a much better quality by observations from other techniques. However, this disagreement could be reduced by applying corrected PCO and PCV corrections of the receiver antennas, and of course it will be significantly improved with more satellites in operation.
Simulated Real-Time PPP Service
Global real-time PPP service promises to be a very precise positioning service system. Hence we tried to investigate the capability of a Compass real-time PPP service by implementing a simulated real-time service system and testing with the available data set.
We used estimates of a three-day solution as a basis to predict the orbits of the next 12 hours. The predicted orbits are compared with the estimated ones from the three-day solution. The statistics of the predicted orbit differences for the first 12 hours on day 125 in 2012 are shown on Figure 10.
From Figure 10, GEOs and IGSOs have very similar STDs of about 30 cm on average. Thus, the significantly large RMS, up to 6 meters for C04 and C05, implies large constant difference in this direction. The large constant shift in the along-track direction is a major problem of the current Compass precise orbit determination. Fortunately, this constant bias does not affect the positioning quality very much, because in a regional system the effects of such bias on observations are very similar.
Figure 10. RMS (left) and STD (right) of the differences between predicted and estimated orbits.
With the predicted orbit hold fixed, satellite clocks are estimated epoch-by-epoch with fixed station coordinates. The estimated clocks are compared with the clocks of the three-day solution, and they agree within 0.5 ns in STD. As the separated comparison of orbits and clocks usually does not tell the truth of the accuracy of the real-time positioning service, simulated real-time positioning using the estimated orbits and clocks is performed to reveal the capability of Compass real-time positioning service.
Figure 11 presents the position differences of the simulated real-time PPP service and the ground truth from the static daily solution. Comparing the real-time PPP result in Figure 11 and the post-processing result in Figure 8, a convergence time of about a half-hour is needed for real-time PPP to get positions of 10-cm accuracy. Afterward, the accuracy stays within ±20 cm and gets better with time. The performance is very similar to that of GPS because at least six satellites were observed and on average seven satellites are involved in the positioning. No predicted orbit for C01 is available due to its maneuver on the day before. Comparing the constellation in the study and that planned for the regional system, there are still one GEO and four MEOs to be deployed in the operational regional system. Therefore, with the full constellation, accuracy of 1 decimeter or even of cm-level is achievable for the real-time precise positioning service using Compass only.
Figure 11. Position differences of the simulated real-time PPP and the static daily PPP. The number of observed satellites is also plotted.
Summary
The three-day precise orbit and clock estimation shows an orbit accuracy, measured by overlap 3D-RMS, of better than 288 cm for GEOs and 21 cm for IGSOs, and the accuracy of satellite clocks of 0.23 ns in STD and 0.56 in RMS. The largest orbit difference occurs in along-track direction which is almost a constant shift, while differences in the others are rather small.
The static PPP shows an accuracy of about 5 cm, 3 cm, and 10 cm in east, north, and vertical with two hours observations. With six hours or longer data, accuracy can reach to 1 cm in horizontal and better than 4 cm in vertical. The post-mission kinematic PPP can provide position accuracy of 2 cm, 2 cm, and 5 cm in east, north, and vertical. The high quality of PPP results suggests that the orbit biases, especially the large constant bias in along-track, can be compensated by the estimated satellite clocks and/or absorbed by ambiguity parameters due to the almost unchanged geometry for GEOs.
The simulated real-time PPP service also confirms that real-time positioning services of accuracy at 1 decimeter-level and even cm–level is achievable with the Compass constellation of only nine satellites. The accuracy will improve with completion of the regional system.
This is a preliminary achievement, accomplished in a short time. We look forward to results from other colleagues for comparison. Further studies will be conducted to validate new strategies for improving accuracy, reliability, and availability. We are also working on the integrated processing of data from Compass and other GNSSs. We expect that more Compass data, especially real-time data, can be made available for future investigation.
UA240 OEM card made by Unicore company and used in Compass reference stations.
Acknowledgments
We thank the GNSS research center at Wuhan University and the Compass authorities for making the data available for this study.
The material in this article was first presented at the ION-GNSS 2012 conference.
Maorong Ge received his Ph.D. in geodesy at Wuhan University, China. He is now a senior scientist and head of the GNSS real-time software group at the German Research Centre for Geosciences (GFZ Potsdam).
Hongping Zhang is an associate professor of the State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing at Wuhan University, and holds a Ph.D. in GNSS applications from Shanghai Astronomical Observatory. He designed the processing system of ionospheric modeling and prediction for the Compass system.
Xiaolin Jia is a senior engineer at Xian Research Institute of Surveying and Mapping. He received his Ph.D. from the Surveying and Mapping College of Zhengzhou Information Engineering University.
Shuli Song is an associate research fellow. She obtained her Ph.D. from the Shanghai Astronomical Observatory, Chinese Academy of sciences.
Jens Wickert obtained his doctor’s degree from Karl-Franzens-University Graz in geophysics/meteorology. He is acting head of the GPS/Galileo Earth Observation section at the German Research Center for Geosciences GFZ at Potsdam.
October 12: A Soyuz launcher carrying two Galileo In-Orbit Validation (IOV) satellites deployed its twins into orbit within 4 hours after take-off, at close to 23 200 km altitude. They join two earlier IOV spacecraft launched in October 2011. Once all four are operational in space, they will provide the minimum number of satellites required for navigational fixes — enabling system validation testing when all are visible in the sky.
The satellites were built by a consortium led by the Astrium division of EADS, which produced the platforms and has responsibility for the payloads, while Thales Alenia Space handled assembly and testing.
October 11: The L5 transmitter aboard GPS Block IIF-3 satellite SVN65/PRN24 was switched on, transmitting the civilian safety-of-life GPS signal, designed to meet demanding requirements for safety-of-life transportation and other high-performance applications.
A day earlier, SVN65 began transmitting L1 and L2 signals as PRN24 on October 8. A number of stations of the International GNSS Service are tracking the satellite. As of press date for this magazine (October 25) the satellite is included in broadcast almanacs although it is set unhealthy and will continue to be so until satellite commissioning is completed. The satellite is drifting towards its designated orbital position of Slot 1 in Plane A.
The launch of the GPS Block IIF-3 satellite took place as scheduled October 4, aboard a United Launch Alliance Delta IV rocket from Cape Canaveral, Florida.
October 1: The two BeiDou-2/Compass satellites launched on September 18 reached their circular medium-Earth orbits and started transmitting navigation signals. Several stations participating in the International GNSS Service’s Multi-GNSS Experiment as well as some in the Cooperative Network for GNSS Observation started tracking the satellites on September 26.
Meanwhile, China is in the final stage of preparations for Compass G6 (G2R) satellite launch, scheduled to occur on October 25 from Xichang Launch Center.
October 3: The Indian Space Research Organization announced that the orbit-raising maneuvers of GSAT-10 satellite have been successfully completed from ISRO’s Master Control Facility. The maneuvers placed the GSAT-10, launched September 30, in an orbit with 35,000-kilometer high orbit, with an orbit period of 23 hours 50 minutes, and a designated location of 83 degree East. GSAT-10 contains a payload to support the Indian GPS and GEO Augmented Navigation (GAGAN) satellite-based augmentation system. The satellite will likely use PRN code 128.
Looking forward:
November 2: A Russian rocket carrying a Luch data-relay satellite with a payload to service the the Russian satnav system is due to launch on this day, postponed from earlier dates in August and October. The second of a set of three geostationary satellites launched to reactivate Roscosmos’s Luch Multifunctional Space Relay System, it will also carry transponders for the System for Differential Correction and Monitoring (SDCM), Russia’s satellite-based augmentation system. The transponders will broadcast GNSS corrections on the standard GPS L1 frequency using C/A PRN codes assigned by DoD’s Global Positioning Systems Directorate. According to the most recent announcement, it will be positioned at 16 degrees West longitude, joining Luch-5A, already in an orbital slot at 95 degrees East longitude.
Protecting GNSS Presentation at ION and INTERGEO
How to test receivers, how to monitor interference, and how to report interference formed the focus of “Protecting GNSS,” a presentation given at ION-GNSS in September and at the INTERGEO exhibition in Germany in October. GPS World hopes to present a video of the talk and its presentation slides at env-gpsworld-integration.kinsta.cloud/video in the near future.
In his talk, CEO Javad Ashjaee of JAVAD GNSS discusses the differences between out-of-band interference (“easy to deal with”) and in-band interference (“more difficult to deal with”). For the latter case, he offers a 64th-order adaptive filter for narrow-band carrier-wave (CW) interference, known as J-Shield, that is incorporated in current JAVAD GNSS receivers, for example the Triumph-VS and Victor-VS. This feature implements, he states, embedded real-time monitoring at the touch of two buttons on the receiver.
Users can then view, in the radio-frequency analysis stage, five different aspects of interference detection and monitoring:
◾ Spectrum shape
◾ Average automatic gain control (AGC)
◾ AGC variations
◾ carrier-to-noise (C/N0) losses
◾ Real-time cycle slips.
In subsequent digital analysis, after digital processing of the signals, Ashjaee showed interference detected by the company’s receivers operating from its San Jose office (see slide, interference with L2) and Moscow office, regarding both GPS and GLONASS signals.
Reporting. TRIUMPH-VS and Victor-VS can send interference reports to FTP sites and authorized persons can view them via browsers (computers, iPhones, and so on). The receivers can also email reports to intended people.
Ashjaee advocated for GNSS receivers in all reference stations to have such interference monitoring and reporting features. In this way, users could monitor interference in their area before performing tasks, just as pilots check the weather before take-off.
China, Europe to Negotiate Spectrum
The European Union (EU) and China will reportedly meet in December in Paris to discuss overlapping radio frequencies both plan to use for their future encrypted government/military satellite navigation services.
The meeting will be conducted under what the Joint Statement on Space Technology Cooperation specifies as the ITU Framework. ITU is the International Telecommunication Union of Geneva, a United Nations affiliate that regulates satellite orbital slots and frequencies.
The statement was signed as an annex to a broader EU-China summit held September 20 in Brussels. The two sides continue collaboration on satellite navigation despite the signal conflict, which has been a subject of debate for at least two years.
The 27-nation EU and China have agreed to continue the China-Europe GNSS Technology Training and Cooperation Center.
Contract for 37 New GLONASS Birds
A federal target program, approved by the Russian Government, has provided measures to maintain and develop the GLONASS system. The Reshetnev Company from 2012 to 2020 will manufacture 15 GLONASS-M satellites and 22 GLONASS-K. The work in this direction is taking place at ISS at full speed. Now the company is making space apparatus GLONASS-M No. 50 (likely to be known as 750 once launched) and has signed contracts with related enterprises for the supply of equipment for a few more satellites in this series. ISS has already completed the manufacture of satellites GLONASS-M No. 47, No. 48, and No. 49. Routine tests confirmed compliance characteristics of the design and with operational documentation. The space vehicles have been put in the assembly shop for safekeeping. ISS has sent a next-generation navigation satellite GLONASS-K No. 12L to the spaceport. A decision on the launch date of the navigation satellites will be made by Roscosmos after an analysis of the state of the GLONASS constellation.
Leadership Awards, Directions 2013 Next Month
The following individuals received GPS World 2012 Leadership Awards in September in Nashville, Tennessee. The magazine’s Leadership Dinner and awards were sponsored by Lockheed Martin and Deimos Space.
Satellites: Martin Unwin, Surrey Satellite Technology Limited. One of the driving forces behind the GIOVE-A satellite (recently retired) and the Galileo IOV satellites.
Signals: Todd Humphrey, Radionavigation Laboratory , University of Texas at Austin. Leader of several seminal studies on spoofing and jamming; testified this summer before Congress on the subject.
Services: Waldemar Kunysz, NextNav LLC. Work on WAPS (Widea Area Positioning System) design and implementation in the continental USA. He spent the previous 16 years with NovAtel on various research projects and novel antenna designs.
Products: Robert Lutwak, Symmetricom. Practical advances to overcome the intrinsic physical barriers to affordable chip-scale atomic clocks, enabling precision time and time transfer in mobile GNSS and communications systems.
Remarks by the award winners on the future of GNSS will appear as Directions 2013 in December issue.
It was thirty years ago today, Cheremisin taught the band to play. They’ve been going in and out of style, but they’re guaranteed to raise a smile. So may I introduce to you the constellation here for years, Vladimir Putin’s GLObal NAv Sat System!
While in our booth at INTERGEO in Hanover last month, I heard Andrey Kupriyanov say it was GLONASS’s 30th birthday today, that particular today being October 12. “First satellites launched,” he recalled.
“Then it is the 30th birthday of GNSS as well,” I replied. “First GPS, then GLONASS. One plus one equals two: GNSS.” Andrey Kupriyanov nodded agreement, then told me a bit about his involvement in the program back then.
After graduating from the Moscow State University of Geodesy and Cartography in 1972, he obtained a Ph.D. in geodetic astronomy, taught for a while, then worked in the U.S.S.R. Ministry of the Merchant Marine, taking part in the development, testing, and application of new operational equipment for mid-Earth orbit satellites.
We’re Vladimir Putin’s GLObal NAv Sat System, we hope that you enjoy our show. We’re Vladimir Putin’s GLObal NAv Sat System, sit back and let PNT flow.
GLONASS achieved full operational status with 24 satellites in 1995, a year after GPS hit that milestone. The constellation subsequently declined to six operational satellites in 2001.
Andrey Kuypriyanov kept busy, representing Ashtech, Magellan, and Thales Navigation in Russia, and participating in research involving GPS and GLONASS monitoring, interaction, and eventual interoperability.
A recovering economy early this century enabled Russia to invest significantly in satnav again. Renewed launches and new spacecraft designs with longer lifetimes restored the constellation to full operational capability, with worldwide availability and greater accuracy.
Vladimir Putin’s global, Vladimir Putin’s global, Vladimir Putin’s GLObal NAv Sat System!
Andrey Kupriyanov is no longer the young man he once was (who among us is, really?) but he stays involved as executive director of the GLONASS-GNSS Forum and as NovAtel’s regional manager for Russia and the Commonwealth of Independent States.
It’s wonderful to be here, it’s certainly a thrill. You’re such a lovely user group, we’d like to take you home with us, we’d love to take you home.
Andrey Kupriyanov Olkgovich is of course only one of many, many long-laboring soldiers in the international GNSS brigade: engineers who made devices, product managers who carried them forth to market, users who embraced them. But on this 30th birthday of GNSS — we’re only just now hitting our stride, entering our golden years — let’s give him, and all of us, a rousing chorus.
I don’t really want to stop the show, but I thought you might like to know, that the singer’s going to sing a song, and he wants you all to sing along. So let me introduce to you the one and only Kupriyanov, and Vladimir Putin’s GLObal NAv Sat System!
The U.S. Air Force is investing to improve the Global Positioning System (GPS) used worldwide for military and civilian purposes.
Between Sept. 28 and Oct. 1, the Air Force announced four new GPS contracts.
Three were in the $30 million range, including contracts to Rockwell Collins and L-3 Communications to test and engineer new GPS technology, while Raytheon was awarded just under $30 million to develop receiver cards for GPS systems. Honeywell International also received a $14 million contract for engineering services related to GPS.
Maintained by the Air Force, the GPS is used in everything from civilian car navigation to targeting for military weapon systems. The only competition for the American GPS is the Russian GLONASS system, although the European Union is currently developing its own system, nicknamed Galileo.
The contracts were announced days before the Oct. 4 launch that put the first new GPS satellite of 2012 into orbit. That satellite, a Boeing-designed GPS IFF, improves on navigational accuracy, provides a more secure military signal and has a longer design life than older satellite models. It should deploy fully in about three months.
The winners of GPS World’s2012 Leadership Awards will be featured in November webinar “The Future of GNSS Research & Development.” The webinar will be held Thursday, November 15, at 10 a.m. PDT / 1 p.m. ET / 5 p.m. GMT. Registration is free.
The winners are expected to discuss with moderator and editor-in-chief Alan Cameron their significant recent achievement in four fields, as well as the future directions of their research or significant research that they think should be undertaken by others in the GNSS community.
The invited speakers are:
Martin Unwin, Surrey Satellite Technology Limited. One of the driving forces behind the GIOVE-A satellite (recently retired) and the Galileo IOV satellites.
Todd Humphrey, Radionavigation Laboratory, University of Texas at Austin. Received the GPSW Signals Leadership Award. Leader of several seminal studies on spoofing and jamming; testified this summer before Congress on the subject.
Waldemar Kunysz, NextNav LLC. Received the GPSW Services Leadership Award for his work on WAPS (Widea Area Positioning System) design and implementation in the continental USA. He spent the previous 16 years with NovAtel on various research projects and novel antenna designs.
Robert Lutwak, Symmetricom. Received the GPSW Products Leadership Award for practical advances to overcome the intrinsic physical barriers to affordable chip-scale atomic clocks, enabling precision time and time transfer in mobile GNSS and communications systems.
A week after the dual liftoff from Kourou, French Guiana, the two latest Galileo satellites completed the critical Launch and Early Orbit Phase on October 19-20. The satellites are expected to reach their assigned orbits November 10 and 12.
The FM3 and FM4 satellites were handed over from the joint ESA/CNES Launch and Early Orbit Phase (LEOP) team in Toulouse, France, to the Galileo Control Centre, Oberpfaffenhofen, Germany, from where Spaceopal will manage the operations of the Galileo constellation.
Following liftoff at 18:15 GMT on October 12, the intensive LEOP activities began upon separation of the satellites from the Fregat upper stage of their Soyuz launcher, with the first signals being received from the pair almost four hours later, according to the European Space Agency.
The handovers took place at 06:00 GMT on October 19 for FM4 and at 18:10 GMT on October 20 for FM3. During the week, LEOP operations proceeded according to the planned sequence.
Three orbit manoeuvres were conducted for each satellite to start them on drift orbits towards their operational positions, where they are expected to arrive on November 10 (FM3) and November 12 (FM4) after a series of drift-stop and fine-positioning manoeuvres.
The satellites were configured into a secure mode shortly after handover. While underway to their final positions, they will also undergo a series of tests to confirm the performance of their subsystems before switching on the payload.
