Swiss-based u‑blox, a provider of GPS/GNSS and wireless semiconductors, has achieved successful satellite positioning using China’s BeiDou Navigation Satellite System. According to u-blox, the technical achievement establishes u-blox as the first GNSS component vendor to demonstrate compatibility with all globally deployed positioning systems: GPS, GLONASS, Galileo, QZSS and now BeiDou.
However, NovAtel has also announced support for the BeiDou Navigation Satellite System on its OEM6 family and select OEMStar GNSS receivers.
Customer demonstration of the u-blox technology will begin during Q1 2013.
“We are thrilled to have achieved this milestone only three weeks after the BeiDou specification was published,” said Thomas Seiler, u-blox CEO. “China will become the world’s most important single market for devices relying on embedded satellite navigation, and u-blox plans to be a major player in this market.”
BeiDou-2 currently has 15 satellites in orbit, offering navigation and positioning services to users in China and Southeast Asia. It will ultimately consist of 35 satellites providing worldwide positioning capability over its open service to within 10 meters accuracy.
u-blox will be demonstrating BeiDou compatibility with their latest GNSS platform at embeddedworld 2013 February 26-28 in Nuremberg, Germany, stand 4A-325.
NovAtel announces support for the BeiDou Navigation Satellite System on its OEM6 family and select OEMStar GNSS receivers.
The long-anticipated BeiDou Navigation Satellite System (BDS) Interface Control Document (ICD) release is a significant milestone that facilitates global acceptance of BeiDou into the growing range of satellite-based positioning applications, NovAtel said.
NovAtel has a long-standing partnership with several Chinese GNSS system manufacturers. This partnership has allowed NovAtel to verify B1 and B2 signal tracking on its latest generation receivers. The company has been supplying GNSS receivers that include the BeiDou constellation since Q4 2010.
“We are excited to see what performance improvements BeiDou will provide to our AdVance RTK, GL1DE and SPAN GNSS/INS positioning algorithms,” said Pat Fenton, NovAtel CTO.
BeiDou positioning has been available through NovAtel’s Chinese partners utilizing the receiver Application Programming Interface (API) feature. With the BeiDou ICD made available to the public, NovAtel is now able to offer BeiDou positioning on its receiver products directly.
Firmware updates for the OEM6 and OEMStar receivers will enable tracking of the BeiDou signal in conjunction with GPS, GLONASS, Galileo and QZSS signals that are currently supported. Over the coming months NovAtel will be working with early-adopter customers to optimize their receiver positioning engines to support the BeiDou signals.
Customers interested in trialing BeiDou functionality on their receivers should contact NovAtel Customer Support at [email protected].
Spirent started shipping BeiDou-ready test systems to customers in 2012. The solution will now be upgraded to full-BeiDou capability using the information from the recently released first full issue of the BeiDou-2 Signal-In-Space Interface Control Document (ICD).
“Spirent has successfully demonstrated BeiDou-2 in simulation systems at its offices in Beijing, China,” said Stuart Smith, product manager for Spirent’s positioning and navigation group. “Prior to the ICD release we used recorded navigation data to enable our systems to drive a full BeiDou receiver and qualify the implementation. With the release of ICD information, navigation data is generated automatically, as with the other constellations that the system simulates.”
Spirent’s BeiDou-2 system includes testing for GPS, GLONASS and Galileo, as well as IRNSS, QZSS and SBAS along with options such as interference generation, MEMS sensor simulation and systems targeted at transport segments.
Background on BeiDou. The BeiDou navigation system, sometimes known as Compass, is a project by China that is being deployed in three phases. BeiDou-2 (the second phase) supports regional operation from a network of geostationary, medium earth orbit and inclined orbit satellites. BeiDou-2 adds to the benefits from “Multi-GNSS” where increased accuracy, availability and integrity are possible from using separate, but interoperable GNSS systems.
As with any other GNSS, systems using BeiDou require testing. As well as testing the BeiDou stand-alone operation, Spirent’s systems enable testing of interoperability and co-existence testing with other navigation systems and sensors.
Septentrio announced on January 7 that it has successfully implemented BeiDou support in the company’s high-precision receiver software, taking advantage of the recent official release of BeiDou’s Interface Control Document (ICD) to including the Chinese satellite navigation signals into its position-velocity-time (PVT) solution.
According to the Belgian GNSS receiver manufacturer, its engineers “are currently processing further data sets to finalize the implementation of full BeiDou support. Although the BeiDou constellation is still being deployed, the data analysis already shows promising results.”
The top panel of Figure 1 compares the height from a stand-alone solution of GPS-only with a GPS+GLONASS solution and a third (in light blue) including BeiDou. “The value added by BeiDou is more than what was expected from a constellation that is still being deployed,” according to Septentrio business development manager Laurent Le Thuaut. “Although the solution is not aided by differential corrections, the position shows an increase in accuracy when sufficient BeiDou satellites are included.”
The bottom panel of Figure 1 shows that, even with the current BeiDou constellation (15 satellites total, of which five are geostationary over China, five in full mid-Earth orbit similar to GPS and GLONASS, and five in inclined geosynchronous orbit over Asia), the total number of satellites used over the European region reached 26 for a short moment.
Figure 2 shows the L1 pseudorange residuals for all constellations individually. This comparison highlights the advantage of the GPS constellation, which builds on two decades of real-time orbit prediction. The BeiDou orbits are “quite accurate for a relatively young constellation, but show typical meter-level jumps when ephemerides are updated,” according to Septentrio.
Septentrio says that the new feature will soon become available on selected company platforms. Users of its multi-constellation receivers will then benefit from improvements in urban availability and signal integrity, thanks to the augmented signal coverage.
On December 29, two days after the Compass Interface Control Document (ICD) was made publicly available, JAVAD GNSS announced that it had tracked “B3 signal from all launched Compass satellites, using TRE-G3T-E E6-band capable receiver. Graphs shows SNR and ‘code-minus-phase’ combination of GEO svn #5 (sat #215 on graph), IGSO svn #8 (sat #218) and MEO svn #14 (sat #224). ‘C/A’ stands for B1, ‘L5’ for B2, ‘CL2’ for B3.”
News compiled with assistance of CANSPACE Listserv.
Beidou
The interface control document (ICD) describing the details of the BeiDou B1I open service signal on 1561.098 MHz was released December 27 at a news conference held in Beijing by the Chinese State Council Information Office. Download the English version here. The ICD specifies the relations of the signal in space interface between BeiDou Navigation Satellite System and users’ terminal receivers. It is the essential technical document to develop and make receivers and chips.
Anyone who has questions about the ICD is invited to submit them to this email: [email protected]
The document, BeiDou Navigation Satellite System Signal In Space Interface Control Document — Open Service Signal B1I (Version 1.0), includes a system introduction, signal standards and navigation message, which defines the related contents of the open-service signal B1I between the BeiDou Navigation Satellite System and users’ terminals.
In a previous presentation given at the Seventh Meeting of the International Committee on Global Navigation Satellite Systems (ICG) held in Beijing November 5-9, 2012, BeiDou officials stated that by 2020 there will be five GEO and 30 non-GEO satellites. The number of IGSO and MEO satellites isn’t stated, but previous presentations have said three IGSOs and 27 MEOs. This is also stated in the official ICD.
Goodbye, Compass. At the news conference, Ran Chengqi, the director of the China Satellite Navigation Office, announced that the English name of the system is henceforth the BeiDou Navigation Satellite System. A new, slightly modified logo for the system was also introduced by Ran. The new version drops the parenthetical “Compass” translation of BeiDou.
Beating up the backstretch neck and neck, tied for third in the GNSS race, Galileo and Compass today offer some signals and some satellites to GNSS users — as long as those users are researchers. Galileo has more going for it in the way of signals, while Compass holds an edge in the number of satellites. Without an interface control document (ICD) to guide user/researchers and most importantly manufacturers in the employment of its signals, Compass satellites, however they may increase, are practically useless to anyone outside China. A Compass ICD has been rumored before and is now rumored again. Wait and see before placing your bets.
The fourth Galileo in-orbit validation (IOV) satellite, Flight Model 4 (FM4), began transmitting signals on December 12, joining its co-launched confrère FM3, which began airing navigation signals on December 1. The FM4 spacecraft uses PRN code E20. As of this writing, FM3 is broadcasting E1, E5, and E6 signals, and FM4 is broadcasting E1 and E5 signals; we don’t know if and when FM4 E6 signals start(ed) until ESA tells us.
GPS World authors Oliver Montenbruck (German Space Operations Center) and Richard Langley (University of New Brunswick) have written an early analysis of the signals from FM3; this account will appear in the January issue of the magazine. A few selected excerpts from that article, and one figure:
“Anyone with commonly available GNSS receivers can presently access the open signals in the E1, E5a, and E5b frequency bands as well as the wide-band E5 AltBOC signal.
Figure 1: Pseudorange errors of IOV-3 tracking at Tanegashima, Japan, using the E1 BOC(1,1) signal (top) and the E5 AltBOC signal (center). The elevation angle over time is shown in the bottom panel.
“According to an ESA statement, FM3will continue to use binary offset carrier modulation — specifically BOC(1,1) — on the E1 Open Service signals for the time being. In contrast to this, the first pair of IOV satellites has already started to use composite binary offset carrier modulation, which offers better multipath suppression in the received signal.
“The E5 AltBOC pseudorange measurements in particular exhibit an exceptionally low noise and multipath level of better than 10 centimeters at mid- and high-elevation angles.”
After discussing and displaying some carrier-phase measurements of the Galileo FM3 E1, E5, and E6 signals, Montenbruck and Langley conclude; “This level of performance highlights the potential benefit of Galileo signals in advanced triple-frequency techniques such as undifferenced ambiguity resolution and ionospheric monitoring.”
Theoretically, the total of four Galileo IOV satellites now in medium-Earth orbit yield the minimum number needed to perform a 3D navigation fix, although no statement of initial — or even sketchy — operating capability has been issued by the European Space Agency (ESA), nor is one expected.
Antonio Tajani, vice-president of the European Commission (EC) and head of the EC directorate-general responsible for industry and entrepreneurship, continues to publicly maintain a “political objective [of] the delivery of the first services before the end of 2014,” based on 18 orbiting satellites. In a December speech, he revised the basis for that position slightly to say the civil Open Service (OS) could be declared operational with as few as 12 satellites.
The system operators had announced three dual-satellite launches in 2013, two dual-satellite launches and one four-satellite launch in 2014, hypothetically producing an operable constellation of 18 satellites by the end of the promised 2014. However, unconfirmed reports from Europe suggest that problems with manufacture of the next set of 14 Galileo satellites mean that no launches at all will take place until Q4 of 2013. Whether this will push out the service delivery date beyond 2014 or not remains open to conjecture.
Compass
Another matter open to conjecture and much speculation is whether the world will soon — or ever — see an interface control document (ICD) for China’s Compass system. More than a year ago, I wrote that “The ICD has been rumored to be available previously to receiver manufacturers within China, creating some disgruntlement among companies outside the country . . . GPS/Compass chips and receivers are being actively developed by many Chinese manufacturers and research institutes.” Indeed, conference presentations, leading to a published article in this magazine’s October issue, “What Is Achievable with the Current Compass Constellation,“ confirm that this is so.
And yet, the rest of the world neither has nor holds a Compass ICD.
The end-of-year rumor mill has kicked into gear again, though. A GNSS industry representative stationed in Shanghai, China sent this message recently to a U.S. colleague: “Latest unofficial news said that the Compass Interface Control Document (ICD) will be released on 27th this month, and will be available on the internet on 28th.”
A GNSS industry representative stationed in Shanghai, China sent this message recently to a U.S. colleague: “Latest unofficial news said that the Compass Interface Control Document (ICD) will be released on 27th this month, and will be available on the internet on 28th.”
Such rumors have floated before, in late 2010, and again in late 2011. As the U.S. colleague noted in passing on this light intelligence, “There was a lot of hand-wringing at ICG [Seventh Meeting of the International Committee on Global Navigation Satellite Systems (ICG), organized by the Government of China, Beijing, China, 5 – 9 November 2012] around the Chinese keeping their promise for 2012 release of the ICD. Maybe they are just going to slip it under the wire.”
In an October, 2011 newsletter column, the GPS World editor wrote: “The long-awaited signal interface control document (ICD) for China’s growing GNSS will appear this month, according to representatives of the system who spoke in a “Compass: Progress, Status, and Future Outlook” workshop as part of ION GNSS and the CGSIC meetings in Portland in September [2011].
“The ICD has been rumored to be available previously to receiver manufacturers within China, creating some disgruntlement among companies outside the country. One of the workshop panelists affirmed that GPS/Compass chips and receivers are being actively developed by many Chinese manufacturers and research institutes.”
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.
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.
The European Union (EU) and China will be meeting in December in Paris to discuss overlapping radio frequencies both plan to use for their future encrypted government/military satellite navigation services, according to a joint statement from both parties, reports Space News.
The December 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. As Space News reports, the two sides are continuing 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.
The two BeiDou-2/Compass satellites launched on 18 September are now in their circular medium Earth orbits and have 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 26 September.
From NORAD/JSpOC, we have the following orbits for the new satellites:
Satellite M5 is using PRN code 13 and M6 is using PRN code 14.
A plot showing the argument of latitude vs. longitude of ascending node for the BeiDou-2/Compass MEO satellites, including the M1/C30 test satellite, can be downloaded.
The plane spacing for the operational satellites is about 120 degrees. The slot spacings seem to be about 45 degrees.
