The United States and China have negotiated compatible signal characteristics that will both protect and enhance service for users of the U.S. GPS and Chinese BeiDou constellations.
The achievement resulted from several years of discussions between U.S. and Chinese GNSS experts. The consensus to make the systems compatible and interoperable at the user level will mean better service for users of both systems worldwide.
The U.S.Department of State’s Bureau of Oceans and International Environmental and Scientific Affairs Office of Space and Advanced Technology represents the United States in the ongoing U.S.-China GNSS Cooperation Dialogue, which began in May 2014 and covers various topics.
Compatibility of the Chinese BeiDou Navigation Satellite System (BDS) with GPS has been a core focus of the discussions. The U.S.-China Joint Statement on signal compatibility and interoperability is below and on the gps.gov website.
Joint Statement on Civil Signal Compatibility and Interoperability Between the Global Positioning System (GPS) and the BeiDou Navigation Satellite System (BDS)
December 4, 2017
In May 2014, China Satellite Navigation Office and the Office of Space and Advanced Technology, U.S. Department of State, jointly established the U.S.-China Civil GNSS Cooperation Dialogue, a bilateral government-to-government mechanism to promote cooperation between the U.S. Global Positioning System (GPS) and the Chinese BeiDou Navigation Satellite System (BDS).
Over the past three years, representatives and experts from both sides have studied and discussed various topics related to civil service provision and user applications, among which BDS compatibility and civil interoperability with GPS is one of the core focus areas.
Both sides have carried out extensive in-depth analysis, and have engaged in persistent discussion and coordination.
As a result, the two sides have reached the following consensus conclusions regarding compatibility and interoperability between GPS and BDS:
Consistent with the bilateral frequency compatibility coordination completed in 2010, GPS and BDS are radio frequency compatible under the framework of the International Telecommunication Union (ITU); and
The GPS L1C and BDS B1C civil signals, using two different types of multiplexed binary offset carrier (MBOC) waveforms are interoperable, which means users can receive better service by jointly using these civil signals without a significant increase in receiver cost or complexity.
Both sides agree to continue their consultations and cooperation related to compatibility and interoperability in order to provide better services for global users.
(signed by)
Ran Chengqi, China Satellite Navigation Office
David A. Turner, Office of Space and Advanced Technology, U.S. Department of State
For more information, contact [email protected]. Follow the Bureau of Oceans and International Environmental and Scientific Affairs on Twitter at @StateDeptOES.
China successfully launched a new pair of BeiDou navigation satellites on Sunday, Nov. 5, reports NASAspaceflight.com.
Beidou-3M1 (Beidou-24) and Beidou-3M2 (Beidou-25) went aloft aboard a Long March-3B/YZ-1 rocket at around 11:45 UTC from the LC2 Launch Complex of the Xichang Satellite Launch Center, Sichuan province.
The launch, previously scheduled for July, was delayed until now due to a partial launch failure with the previous launch of this rocket during the Zhongxing-9A (ChinaSat-9A) mission, which resulted in the satellite being lofted to a lower than planned orbit.
The satellites are using a new bus that features a phased array antenna for navigation signals and a laser retroreflector.
The Beidou Phase III system includes the migration of its civil Beidou 1 or B1 signal from 1561.098 MHz to a frequency centered at 1575.42 MHz — the same as the GPS L1 and Galileo E1 civil signals — and its transformation from a quadrature phase shift keying (QPSK) modulation to a multiplexed binary offset carrier (MBOC) modulation similar to the future GPS L1C and Galileo’s E1.
Sokkia’s Tammy Aalto gives GPS World a rundown on one of the company’s latest receiver, the GSX3, at Intergeo 2017 in Berlin, Germany. According to the company, improvements on this receiver include the addition of BeiDou and Galileo tracking capability, longer range base-to-rover communication, and performance in tree canopy situations.
At Intergeo 2017, Septentrio debuted the Altus NR3: a multi-frequency, quad-constellation (GPS, GLONASS, BeiDou and Galileo) RTK receiver for survey and GIS applications.
The Altus NR3 features Septentrio’s AIM+ interference mitigation and monitoring system, allowing continued operation in the presence of both intentional and non-intentional interference. According to the company, it combines advanced GNSS features with a robust communications suite in a compact, low-power and easy-to-use unit.
The Altus NR3 is configurable as either a rover or a base station. It offers one-touch logging and Septentrio’s on-board web interface so users can monitor and configure the unit as well as collect data using any Wi-Fi-capable device.
Data collection is done using either SurvCE or Septentrio’s PinPoint Data Collector with data updating to the cloud. Septentrio’s open interface and fully documented data formats are widely supported, making the Altus NR3 easy to integrate into any existing workflow, the company said.
“We’ve built on the flexibility, reliability and ease-of-use that our Altus line is famous for, and we’ve added all-in-view RTK and the most the most advanced interference mitigation system on the market today,” said Gustavo Lopez, product manager at Septentrio. “Locations with bad visibility or at risk of interference that were previously off limits can now benefit from high-precision GNSS positioning, saving both time and cost.”
Even a GNSS receiver that can supply raw pseudorange and carrier-phase measurements now costs only a few hundred dollars, and in this month’s column, a couple of researchers from Down Under pit a couple of these receivers up against a couple of survey-grade receivers. Did this cheap receiver turn out to be a good thing?
By Robert Odolinski and Peter J.G. Teunissen
ALL GOOD THINGS ARE CHEAP; ALL BAD ARE VERY DEAR. That’s what the famous American essayist (and surveyor) Henry David Thoreau wrote in his diary on March 3, 1841. He was likely referring, in part, to the cheapness of the things he came across in nature such as birdsong or the plants and trees on the shores of Walden Pond and the dearness of some luxuries and comforts of civilization, which he tended to eschew. But what has that got to do with GPS, you might ask?
When they were first introduced in the late 1970s and early 1980s, GPS receivers were very dear. Many of them sold for anywhere from $50,000 to $250,000, which would be equivalent to about twice those amounts in today’s dollars. The first civilian receivers were large bulky affairs. As I documented in this column in April 1990 (“Smaller and Smaller: The Evolution of the GPS Receiver”), the “first commercially available GPS receiver was the STI-5010 built by Stanford Telecommunications Inc. It was a dual-frequency, C/A- and P-code, slow-sequencing receiver. Cycling through four satellites took about five minutes, and the receiver unit alone required about 30 centimeters of rack space. External counters, also requiring rack space, made pseudorange measurements. An external computer controlled the receiver and computed positions.” While it could be transported in a small truck (and some were), it was not designed for portability and ease of use by surveyors or geodesists.
Then, in 1982, Texas Instruments introduced the first relatively compact civil GPS receiver, the TI 4100, also known as the Navstar Navigator. And as I also noted in that column more than 15 years ago, this “receiver could make both C/A- and P-code measurements along with carrier-phase measurements on both L1 and L2 frequencies. Its single hardware channel could track four satellites simultaneously through a multiplexing arrangement. The 37 × 45 × 21-centimeter receiver/processor had a handheld control and display unit and an optional dual-cassette data recorder for saving measurements for post-processing. The unit, although portable, weighed 25 kilograms and consumed 110 watts of power (the receiver doubled as a hand warmer). Field operation required a supply of automobile batteries.”
My, how things have changed. Beginning around 1990, receivers steadily got smaller and smaller and cheaper and cheaper. Survey-grade GNSS (not just GPS) receivers can now be purchased for well under $10,000 and consumer-grade units sell for as little as a hundred dollars or less. And, of course, the GNSS modules inside smartphones and other devices cost manufacturers only a couple of dollars or so.
But even a GNSS receiver that can supply raw pseudorange and carrier-phase measurements now costs only a few hundred dollars, and in this month’s column, a couple of researchers from Down Under pit a couple of these receivers up against a couple of survey-grade receivers. Did this cheap receiver turn out to be a good thing?
Read on to find out.
GPS has been the number-one positioning tool for a range of applications during the past few decades. The integration of the emerging global navigation satellite systems, such as the Chinese BeiDou Navigation Satellite System (BDS), can give improved precise (millimeter- to centimeter-level) real-time kinematic (RTK) positioning. When BDS is combined with GPS, about double the number of satellites are visible in the Asia-Pacific region, which can make single-frequency RTK and low-cost receiver RTK positioning possible.
In this article, we will analyze the performance of L1 GPS + B1 BDS in Dunedin, New Zealand, using low-cost receivers. We compare their performance to that of L1+L2 GPS survey-grade receivers.
First, we describe the GPS+BDS functional and stochastic models and the data used for our evaluations. Least-squares variance component estimation (LS-VCE) is used as a means to determine the code and phase (co)variances to formulate a realistic stochastic model. (An incorrect stochastic model will deteriorate the ambiguity resolution and consequently the achievable positioning precisions.)
Having correctly defined the stochastic model, we focus on the positioning performance. We investigated the ambiguity resolution and positioning performance, both formally and empirically, for customary and high-elevation cut-off angles. The high cut-off angles are used to mimic situations when low-elevation multipath is to be avoided. Lastly, we compared all our results between using low-cost and survey-grade antennas.
GPS+BDS POSITIONING MODEL
The model that we used for positioning is given as follows. Assume that sG + 1 GPS satellites are tracked on fG frequencies and sB + 1 BDS satellites on fB frequencies. As we apply system-specific double-differencing (DD), one pivot satellite is used per system. The total number of DD phase and code observations per epoch then equals 2 fGsG + 2 fBsB. We assume for now that cross-correlation between frequencies as well as code and phase is absent. The combined multi-frequency short-baseline GPS+BDS model is then defined as follows.
The system-specific DD phase and code observation vectors are denoted as φ* and p*, respectively, with * = {G, B} where G = GPS and B = BDS. The single-epoch GNSS model of the combined system is given as
(1)
and
(2)
in which
is the combined phase vector,
is the combined code vector,
is the combined integer ambiguity vector,
is the real-valued baseline vector,
is the combined phase random observation noise vector,
is the combined code random observation noise vector, and
D[.] denotes the dispersion operator.
The entries of the baseline design and wavelength matrices are given as
where is the x 1 vector of 1s, is the differencing matrix, is the unit matrix, the geometry-matrices GG and GB contain the undifferenced receiver-satellite unit direction vectors for GPS and BDS, respectively, is the wavelength of frequency , denotes the Kronecker product, and “diag” and “blkdiag” indicate diagonal and block diagonal matrices, respectively. The entries of the positive definite variance matrices are given as
(3)
where , denote the phase and code standard deviation, respectively, and the satellite elevation-angle-dependent weight.
The model in Equation 1 applies to short baselines, and thus the ionospheric and tropospheric delays are assumed absent. The broadcast ephemerides are used to obtain the satellite coordinates. Further, the Least-squares AMBiguity Decorrelation Adjustment (LAMBDA) technique is used to estimate the integer ambiguities a. The observation noise vectors εand e, respectively, are zero-mean vectors, provided that no multipath is present in Equation 1.
EXPERIMENT SETUP
The GNSS receivers we used are depicted in FIGURE 1. Firstly, two low-cost single-frequency receivers were set up to collect L1+B1 GPS+BDS data for two days. These receivers cost a few hundred U.S. dollars. Since the patch antennas we used have been shown to have less effective signal reception and multipath suppression in comparison to survey-grade antennas, the receivers that collected data for two days were additionally connected to such antennas. These antennas have a cost of slightly more than US$1,000 per antenna. To compare the low-cost solution to a survey-grade receiver-solution, two such receivers (which cost several thousand U.S. dollars) were connected to the same survey-grade antennas through splitters and collected L1+L2 GPS data. A detection, identification and adaption procedure was used to eliminate any outliers.
FIGURE 1. Low-cost single-frequency receivers collecting GPS+BDS data for single-baseline RTK, with patch antennas (left) and survey-grade antennas (right) on Jan. 4–6 and Jan. 6–8, 2016, respectively. Survey-grade dual- frequency GPS receivers were connected to the same survey-grade antennas simultaneously to truly track the same GPS constellation.
FIGURE 2 depicts the corresponding redundancy of the two receiver models (that is, the number of observations minus the number of estimated unknowns) together with the number of satellites over 48 hours (30-second epoch interval). The number of BDS satellites (magenta lines) is overall smaller than when compared to GPS (blue lines) in Dunedin. However, Figure 2 also shows that the model strength of L1+B1 GPS+BDS, as measured by its redundancy, is almost similar to that of L1+L2 GPS except for some hours at the middle of the two days. This implies that the two receiver models can potentially give competitive RTK ambiguity resolution and positioning performance. This is however only true if the receiver code and phase observation noise would be of similar magnitude between the receivers used, hence the need for an analysis of the receiver observation precision.
FIGURE 2. Redundancy (left) and number of satellites (right) of L1+B1 GPS+BDS and L1+L2 GPS during Jan. 6–8, 2016, (48 hours) for an elevation cut-off angle of 10°.
In our receiver evaluations, we determined a set of reference ambiguities by using a known baseline and treating them as time-constant parameters over the two days in a dynamic model.
LOW-COST RTK POSITIONING
The code and phase variances were estimated by LS-VCE using data independent from the data used for the following positioning analysis. The variances are needed to formulate a realistic stochastic model, whereas an incorrect stochastic model will deteriorate the ambiguity resolution and consequently the achievable positioning precisions. TABLE 1 depicts the corresponding estimated standard deviations (STDs) used for our positioning models.
TAB LE 1. Zenith-referenced undifferenced code and phase standard deviations estimated by least-squares variance component estimation.
Table 1 shows that the code precision of L1 GPS and B1 BDS improves significantly when the survey-grade antennas are used instead of patch antennas (49 centimeters STD for L1/B1 that decreases to about 30 centimeters), due to their better signal reception and multipath suppression abilities. For testing our stochastic model, we used data that is independent from the data used to estimate the code/phase precision.
Positioning Performance. The single-epoch (instantaneous) RTK positioning results for 24 hours data are shown in FIGURE 3, with ambiguity-float solutions shown at the top and ambiguity-fixed solutions at the bottom. Only the correctly fixed solutions are depicted as determined by comparing the instantaneously estimated ambiguities to the set of reference ambiguities. The 95% empirical and formal confidence ellipses and intervals are shown in green and red, respectively. They were computed from the empirical and formal position variance matrices. The empirical variance matrix was estimated from the positioning errors as obtained from comparing the estimated positions to precise benchmark coordinates. The formal variance matrix used was determined from the mean of all single-epoch formal variance matrices.
FIGURE 3. Horizontal (north (N), east (E)) position scatter and corresponding vertical (U) time series of the float (top) and correctly fixed (bottom) L1+B1 GPS+BDS single-epoch RTK solutions for an elevation cut-off angle of 10°. The 95% empirical and formal confidence ellipses and intervals are shown in green and red, respectively. The 24 hour (30 second) period is 22:00-22:00 UTC Jan. 5-6, 2016, for patch antennas in (a) and 21:48-21:48 UTC Jan. 8-9, 2016, for survey-grade antennas in (b), which are periods independent of the periods used to determine the stochastic model through the code/phase STDs in Table 1.
Figure 3 shows a good fit between the formal and empirical confidence ellipses/intervals, which thus illustrates realistic LS-VCE STDs in Table 1 that were used in the stochastic model. Note also the two-order of magnitude improvement when going from float to fixed solutions, and that the low-cost receiver plus survey-grade antenna has the most precise ambiguity-float positioning solutions.
Ambiguity Resolution and Positioning Performance for Higher Cut-Off Angles. We subsequently investigated the low-cost L1+B1 GPS+BDS performance for high elevation cut-off angles, so as to mimic situations in urban canyon environments or when low-elevation-angle multipath is present and is to be avoided. We have made comparisons to the survey-grade L1+L2 GPS results. It has been shown that a good ambiguity resolution performance does not necessarily imply a good positioning performance, so we investigated what effect this has on our positioning models.
The following integer least-squares (ILS) success rates (SRs) are thus computed based on epochs with the condition of positional dilution of precision (PDOP) ≤ 10 and averaged over all epochs over two days of data. By including and excluding epochs with large PDOPs, we can show how the positioning performance of the different models is affected by poor receiver-satellite geometries. To better understand how this exclusion of epochs with large PDOPs also influenced the empirical ambiguity-correctly-fixed positioning performance, we constructed TABLE 2, which shows the corresponding positioning STDs for two days of data. These STDs were computed by comparing the estimated positions to precise benchmark coordinates. In addition to the positioning performance, we depict in Table 2 the corresponding empirical ILS SR for full ambiguity-resolution, which is given by the ratio of the number of correctly fixed epochs to the total number of epochs.
TABLE 2. Single-epoch empirical STDs (N, E, U) of correctly fixed positions for the three positioning models together with their ILS SR for four elevation cut-off angles and 48 hours of data (Jan. 4–6 and Jan. 6–8, 2016). The empirical STDs and ILS SRs are also shown when conditioned on PDOP ≤ 10.
Table 2 shows that the L1+B1 low-cost receiver plus patch antenna combination has (as expected) smaller SRs in comparison to those when the survey-grade antenna is used. This latter combination has comparable SRs to the (PDOP-conditioned) SRs of the survey-grade L1+L2 GPS receiver for cut-off angles up to 25°.
In support of better understanding Table 2, FIGURE 4 shows typical positioning results for the different receiver and antenna combinations with elevation cut-off angles of 10° (top two rows) and 25° (bottom two rows). The first and third rows show the local horizontal (N, E) positioning scatterplots and the second and fourth rows the vertical (U) time series over two days of data. The float solutions are depicted in gray, and incorrectly and correctly fixed solutions in red and green, respectively. The zoom-in is given to better show the spread of the correctly fixed solutions with millimeter-centimeter level precisions. The formal ambiguity-float STDs are also shown under the up time series to reflect consistency between the empirical and formal positioning results.
FIGURE 4. Horizontal (N, E) scatterplots and vertical (U) time series for L1+B1 low-cost receiver with patch antenna (first column) with 99.5% (89.8%) ILS SR, L1+B1 low-cost receiver with survey-grade antenna (second column) with 100% (97.8%) ILS SR, and survey-grade L1+L2 GPS (third column) with 100% (94.1%) ILS SR, using 10° (top two rows) and 25° (bottom two rows) cut-off angles respectively (Jan. 4–6, 2016, for low-cost receiver with patch antenna and Jan. 7–8, 2016, for the low-cost and survey-grade receivers with survey-grade antennas). The SRs are conditioned on PDOP ≤ 10 and computed based on all epochs. Below the vertical time series, the ADOP is depicted in blue color, the 0.12-cycles level as red, and ambiguity-float vertical formal STDs are shown in gray.
We also depict in Figure 4 the ambiguity dilution of precision (ADOP) as an easy-to-compute scalar diagnostic to measure the intrinsic model strength for successful ambiguity resolution. The ADOP is defined as
(cycles) (4)
with n being the dimension of the ambiguity vector, the ambiguity variance matrix, and |.| denoting the determinant. ADOP gives a good approximation to the average precision of the ambiguities, and it also provides for a good approximation to the ILS SR. The rule-of-thumb is that an ADOP smaller than about 0.12 cycles corresponds to an ambiguity SR larger than 99.9%.
Figure 4 shows that more solutions are incorrectly fixed (red dots) when the ADOPs (blue lines) are larger than the 0.12 cycle level (red dashed lines). The figure also reveals that the L1+B1 low-cost receiver plus patch antenna combination achieves an ILS SR (99.5%) similar to that of the survey-grade L1+L2 GPS receiver (SR of 100%) for the cut-off angle of 10°. This ILS SR corresponds to the availability of correctly fixed solutions (green dots) with millimeter-centimeter level positioning precision over the two days. The L1+L2 GPS receiver has, moreover, large ambiguity-fixed positioning excursions at the same time as the formal STDs are large for the cut-off angle of 25° due the poor GPS-only receiver-satellite geometry for this high cut-off angle. This is also reflected by the corresponding relatively large ambiguity-fixed STDs depicted in Table 2 that are improved from decimeter- to millimeter-level when the PDOP ≤ 10 condition is applied. Figure 4 also shows that the L1+B1 low-cost receiver with the survey-grade antenna has a larger SR of 97.8% when compared to the PDOP-conditioned SR for L1+L2 GPS of 94.1% for the cut-off angle of 25° (see also Table 2), owing to the use of BDS that significantly improves the receiver-satellite geometry.
Finally, we also tested the low-cost receiver-solution (with survey-grade antennas) for a baseline length of 7 kilometers, where (small) residual slant ionospheric delays are present. It was shown that this combination still has the potential to achieve ambiguity resolution and positioning performance competitive with the survey-grade receiver-solution.
CONCLUSIONS
In this article, we evaluated a low-cost L1+B1 GPS+BDS RTK setup and compared its ambiguity resolution and positioning performance to a survey-grade L1+L2 GPS solution in Dunedin, New Zealand. The LS-VCE procedure was used to determine the variances of the low-cost receivers. The estimated variances are needed so as to formulate a realistic stochastic model, otherwise the ambiguity resolution and hence the achievable positioning precisions would deteriorate.
Since we analyzed a short baseline, the LS-VCE variances were shown to likely be affected by multipath. To mitigate multipath we connected the low-cost receivers to survey-grade antennas with better signal reception and multipath suppression abilities. It was shown that the survey-grade antennas can significantly improve the performance for the low-cost receivers so that the code/phase noise estimates more resemble that of survey-grade receivers. The LS-VCE STDs were furthermore shown to be realistically estimated for an independent time period.
We also demonstrated that the low-cost receivers can give competitive instantaneous ambiguity resolution and positioning performance to that of the survey-grade receivers. This is particularly true when the low-cost receivers are connected to survey-grade antennas.
ACKNOWLEDGMENTS
This article is based on the paper “On the Performance of a Low-cost Single-frequency GPS+BDS RTK Positioning Model” presented at the 2017 International Technical Meeting of The Institute of Navigation held Jan. 30-Feb. 1, 2017, in Monterey, California.
Ryan Cambridge at the School of Surveying, University of Otago, collected the low-cost receiver data. Author Peter J.G. Teunissen was supported by an Australian Research Council Federation Fellowship. All of this support is gratefully acknowledged.
MANUFACTURERS
The low-cost receivers used in the research were u-blox EVK-M8T receivers. The survey-grade receivers were Trimble NetRS receivers. The patch antennas were u-blox ANN-MS antennas, while the survey-grade antennas were Trimble Zephyr 2 GNSS antennas.
ROBERT ODOLINSKI conducted his Ph.D. studies at Curtin University, Perth, Australia, from 2011 to 2014. His research focus is next-generation multi-GNSS integer ambiguity resolution enabled precise positioning. In 2015, Odolinski started his position as a lecturer/research fellow in geodesy/GNSS at the School of Surveying, University of Otago, New Zealand.
PETER J.G. TEUNISSEN is a professor of geodesy and navigation and the head of the Curtin GNSS Research Centre, Curtin University. He is also with the Department of Geoscience and Remote Sensing, Delft University of Technology, Delft, The Netherlands. His research interests include multiple GNSS and the modeling of next-generation GNSS for high-precision positioning, navigation and timing applications.
FURTHER READING
• Authors’ Conference Paper
“On the Performance of a Low-cost Single-frequency GPS+BDS RTK Positioning Model” by R. Odolinski and P.J.G. Teunissen in Proceedings of the 2017 International Technical Meeting of The Institute of Navigation, Monterey, California, Jan. 30 – 1 Feb., 2017, pp. 745–753.
• Authors’ Related Work
“Single-Frequency, Dual-GNSS Versus Dual-frequency, Single-GNSS: A Low-cost and High-grade Receivers GPS-BDS RTK Analysis” by R. Odolinski and P.J.G. Teunissen in Journal of Geodesy, Vol. 90, No. 11, 2016, pp. 1255–1278, doi:10.1007/s00190-016-0921-x.
“Combined BDS, Galileo, QZSS and GPS Single-frequency RTK” by R. Odolinski, P.J.G. Teunissen and D. Odijk in GPS Solutions, Vol. 19, No. 1, 2015, pp. 151–163, doi:10.1007/s10291-014-0376-6.
“Instantaneous BeiDou+GPS RTK Positioning With High Cut-off Elevation Angles” by P.J.G. Teunissen, R. Odolinski and D. Odijk in Journal of Geodesy, Vol. 88, No. 4, 2014, pp. 335–350, doi: 10.1007/s00190-013-0686-4.
“The Future of Single-Frequency Integer Ambiguity Resolution” by S. Verhagen, P.J.G. Teunissen and D. Odijk in Proceedings of the VII Hotine-Marussi Symposium on Mathematical Geodesy, Rome, June 6–10, 2009, edited by N. Sneeuw, P. Novák, M. Crespi and F. Sanso, International Association of Geodesy Symposia, Vol. 137, 2012, pp. 33–38, doi:10.1007/978-3-642-22078-4 5.
“Centimeter-Level Positioning for UAVs and Other Mass-Market Applications” by C. Mongredien, J.-P. Doyen, M. Strom and D. Ammann in Proceedings of ION GNSS+ 2016, the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, Sept. 12–16, 2016, pp. 1441–1454.
“Initial Assessment of the COMPASS/BeiDou-2 Regional Navigation Satellite System” by O. Montenbruck, A. Hauschild, P. Steigenberger, U. Hugentobler, P.J.G. Teunissen and S. Nakamura in GPS Solutions, Vol. 17, No. 2, 2013, pp. 211–222, doi:10.1007/s10291-012-0272-x.
• LAMBDA
“On the Reliability of Integer Ambiguity Resolution” by S. Verhagen in Navigation, Vol. 52, No. 2, Summer 2005, pp. 99–110, doi: 10.1002/j.2161-4296.2005.tb01736.x.
“ADOP in Closed Form for a Hierarchy of Multi-frequency Single-baseline GNSS Models” by D. Odijk and P.J.G. Teunissen in Journal of Geodesy, Vol. 82, 2008, pp. 473–492, doi: 10.1007/s00190-007-0197-2.
Rohde & Schwarz and MediaTek have successfully completed the verification of location-based services (LBS) in the U-plane and C-plane for Assisted Beidou (A-BeiDou), China’s GNSS satellite positioning system.
The R&S TS-LBS test solution allows mobile manufacturers, chipset manufacturers, test houses and network operators to verify chipsets and mobile devices in order to obtain permission to operate them in a particular network.
The successful A-BeiDou verification of the MediaTek device under test (DUT) using the Rohde & Schwarz test system marks an important milestone in the GNSS evolution of positioning and navigation. According to Rohde & Schwarz, this was the first time that the setup could be used to validate and verify a device for A-BeiDou location-based services.
The R&S TS LBS from Rohde & Schwarz is a test system for testing GNSS and network-based LBS. It consists of an R&S CMW500 as the base station simulator and an R&S SMBV100A GNSS simulator. The R&S CMW500 provides assistance data to the DUT and the R&S SMBV100A simulates the BeiDou satellites. The R&S TS-LBS test system can be used to obtain GCF and PTCRB certification as well as network operator-specific certification for chipsets and mobile devices.
“We are delighted to collaborate with MediaTek and to contribute our test and measurement expertise to the development of A-BeiDou location based services,” said Alexander Pabst, vice president of Systems and Projects within the Rohde & Schwarz Test & Measurement Division. “Rohde & Schwarz already has a strong global footprint with testing solutions for A-GNSS, such as A-GPS or A-GLONASS, and for OTDOA/eCID. Thanks to our close cooperation with our partners, Rohde & Schwarz is committed to accompanying the evolution from existing to new satellite systems such as A-BeiDou with our innovative test and measurement solutions.”
“MediaTek is committed to developing and testing the latest mobile technologies and standards to drive the industry forward,” said TL Lee, general manager of the Wireless Communications Business Unit at MediaTek. “We have worked closely with Rohde & Schwarz to develop and validate the test solution for A-BeiDou LBS, verifying the A-BeiDou proof-of-concept trial system based on the R&S TS-LBS and MediaTek DUT. This represents an exciting step forward in the evolution of LBS technology, enabling the mobile ecosystem to verify chipsets and mobile devices on the new LBS technology.”
Here’s a panorama in broad strokes across the range of GNSSs, garnered from top system spokespersons at the Munich Satellite Navigation Summit. It’s been several years since breaking news was aired at this annual late winter/early spring event, but it’s always good for a wide-ranging update, recalibrating levels, so to speak.
GPS. With 31 operational satellites (24 is baseline) and an estimated 3 billion receivers in use worldwide, what more needs to be said about the gold standard? Its best week ever for accuracy logged a signal-in-space performance average of 45.3 centimeter. The next-generation ground control system OCX “survived quite a struggle” and has emerged from Nunn-McCurdy breach, back on track and seemingly ready for future action. Or at least for future pre-certification tests. SV1 of the GPS III generation has completed all tests and is in storage, awaiting the first GPS III launch in spring 2018. SV02 and 03 are in assembly and integration, SV04 thru 08 are in box-level assembly, and 09 and 10 are on contract. Technical challenges with payload have been resolved.
(Click to enlarge.) Galileo satellite top-level block diagram. OHB Systems AG as prime contractor and Surrey Satellite Technology (SSTL) have teamed for production of the navigation satellites. OHB is responsible for the concept, the satellite platforms and the satellite-level inegration and test. SSTL supplies the satellite payloads and supports OHB on system level. OHB also supports the customers during launch preparation and in-orbit testing. (Image courtesy OHB)
Galileo. With 18 on-orbit satellites (15 operational), the European GNSS can be termed a coming thing. Performance statistics are based on only 11 of these satellites however; the four most recently launched in November 2016 are not yet included. Nevertheless, the system is logging 80-centimeter ranging accuracy. Eight more await launch: four in 2017, and four in 2018. The constellation is broadcasting the Open Service, the Public Regulated Service, and the Search and Rescue (SAR) signal. The SAR service will officially launch in early April — on April 6, because 406 MHz is the Emergency Position Indicating Radio Beacon frequency. Galileo has improved the historic SAR location performance from 3 hours to 10 minutes. The Commercial Service is still in preparation, and will be available in 2020. Spoofing is seen as a very real threat to GNSS overall by the Galileo authorities, as exemplified by the recent bloom of amateur spoofers encouraged by Pokemon go.
GLONASS. The Russian system will undertake three or four launches this year; one of them will be a triple-satellite launch. There have been several disruptions to efforts to decrease the offset between GLONASS system time and Universal Coordinated Time but the initiative perseveres. English versions of four system interface control documents (ICDs), to include the new CDMA signal, are promised for Q2 2017; Chinese versions are coming, too. Russian-language ICDs are available at glonass.aic.ru.
BeiDou. With the addition of three new satellites in the past year, China’s system is enjoying improved system performance. Hydrogen clocks are succeeding rubidium clocks, bring an order-of-magnitude improvement in timing accuracy. A BeiDou white paper was published last June, and a revised ICD appeared in November.
In the massive Chinese mass market, 30 percent of smartphones sold in China now have BeiDou capability; that’s out of a 700–800 million total. Huawei multi-function chip LX1101 is a key driver behind this. Unistrong has released a phone with RTCM input for professional use, blurring the line between mass and professional markets.
Six to eight satellites will be launched this year, and 10 to 12 in 2018. BeiDou is in a “very ambitious and aggressive race with time to complete the global system.”
ICG. The United Nations’ International Committee on Global Navigation Satellite Systems will meet in Japan in December of this year, in China next year, and in India in 2019. This can be interpreted as vigorous international interest and “a desire to advance and promote their respective systems’ visibility” worldwide. All pertinent documents can be found at unoosa.org.
EGNOS. The European Geostationary Navigation Overlay Service has two operational geosynchronous Earth-orbit satellites (GEOs) in operation, plus one in test and one in deployment, ready to swap in. It is extending its Ranging and Integrity Monitoring Stations (RIMS) to several new countries, notably Israel and the Ukraine. EGNOS.v3 is coming and will introduce dual-frequency (L1 and L5) service, and also Galileo with GPS, for multi-constellation corrections. The new system’s qualification is planned for 2022.
QZSS. This year, Japan’s Quasi-Zenith Satellite System will launch the second and third of the figure-eight inclined geosynchronous orbit (IGSO) satellites of the Michibiki type, to become operational in 2018. A GEO bird will also be launched. A seven-satellite system is the ultimate goal.
Among other announcements of note made during the course of the Summit, although not by the GNSS operators’ spokespersons:
(Click to enlarge.) Key features of the Galileo satellites.
• OHB, the Galileo satellite manufacturer, said its customer has decided to refurbish the clocks on eight satellites in preparation. “Satellite navigation is nothing but comparison of very precise clocks.”
• Airbus announced a new concept for train positioning integrity: “virtual valises” to correct train position that will replace or augment current trackside valises that are very expensive to build and maintain.
• Munich Aerospace (munich-aerospace.de), a public-private non-profit venture between DLR, the German space agency, Bauhaus Luftfahrt and two technical universities, will mount a Ph.D-level education and research program for 70 individuals, with candidates from 27 nations. This will be located in “the Bavarian Silicon Valley.” It will also undertake a global effort with several other organizations.
• One of the above technical universities, the Federal Armed Forces University in Munich, announced that it is investigating Lidar for potential use in an asteroid mining project for future space exploration. It also has underway initiatives concerning Lidar + GNSS and inertial + GNSS for autonomous vehicles.
SBG Systems has released a new generation of its advanced and compact inertial navigation systems. The Ekinox 2 series features new accelerometers and gyroscopes, enhancing attitude accuracy by a factor of two over the original Ekinox.
Photo: SBG
The Ekinox series is a line of tactical grade MEMS-based inertial navigation systems, first released in 2013. The latest improvements come from a complete redesign of the in-house inertial measurement unit (IMU), integrating cutting-edge gyroscopes and accelerometers.
With higher accuracy for the same form factor and price level, Ekinox 2 Series is designed for industrial-grade vehicle navigation, equipment motion compensation and data georeferencing. It provides a 0.02-degree roll and pitch, 0.05-degree heading and a centimeter-level position.
Applications for the Ekinox 2 include hydrography, mobile mapping and antenna tracking. With new accelerometers, this new generation has also significantly improved its resistance to vibration. Finally, the addition of the BeiDou constellation improves signal availability in Asia.
Compact and light-weight, the Ekinox Series has been designed to simplify installation operations. Configuration is made with an intuitive embedded web interface where all parameters can be displayed and adjusted. For example, users can choose a profile (vessel, plane, car, etc.), and the 3D view will provide a visualization of settings such as the sensor position, alignment and lever arms.
The Ekinox 2 Series is ITAR Free. The product line will be available during the second quarter of 2017.
A launch schedule for future BeiDou satellites was reported on a space news discussion board. According to the schedule, the government of China is planning to launch 32 satellites through 2020.
The schedule includes nine BeiDou-3 MEO satellites launched this year, and one replacement for a BeiDou-2 satellite, which retires in January 2018.
The solution also features configurable software, allowing the addition of support for GNSS and sensing.
According to the companies, Dragonfly NB1 leverages Ceva’s long heritage of low power DSPs and modem design and Astri’s experience in RF and IC design technologies. Dragonfly NB1 has the ability to reduce the time taken to get NB-IoT products certified and also provides low-power wide-area SoC designers with a flexible, software-upgradeable platform with key benefits in terms of die size and power consumption, the companies added.
The Dragonfly NB1 solution is enabled by a Ceva-X1 IoT processor and incorporates highly power-efficient multi-standard RF with embedded PA, LNA, DC-DC and DCXO technology for NB-IoT and GNSS (GPS and BeiDou). It is specifically designed to operate with embedded flash by incorporating an optimized low latency memory subsystem with a dedicated cache controller.
“In the coming years, NB-IoT will become the dominant technology for low power wide area connectivity,” said Michael Boukaya, vice president and general manager of Ceva’s Wireless Business Unit. “For most companies, understanding how to develop this technology is a daunting task. To overcome this, we have worked relentlessly with ASTRI to develop a complete solution from the ground up, that removes the design burden and allows SoC designers to add NB-IoT connectivity to their product designs. We’re extremely excited to announce this solution and demonstrate our leadership in IP for NB-IoT.”
Ceva and ASTRI have also teamed up with GMV, a major player in navigation systems and solutions, to offer an integrated GNSS solutions for smart devices with location tracking of logistics, assets, wearables and more. According to the companies, the GNSS IP is available as an add-on software that runs on the Ceva X1 together with the NB-IoT and leverages ASTRI’s GNSS RF IP that is embedded in the solution.
Two documents of interest and importance to GNSS designers and manufacturers have been published, one from the Radio Technical Commission for Maritime Services (RTCM) and one from the U.S. Department of Homeland Security (DHS).
The latter document is the subject of a news story concerning receivers used in critical infrastructure, with an emphasis on timing receivers. It provides owners, operators, researchers, designers and manufacturers with information to improve the security and resilience of PNT equipment across the spectrum of equipment development, deployment and use. It makes specific recommendations.
The first-mentioned document is a white paper issued by the RTCM. It follows here, largely verbatim. It is titled “GNSS Community Benefit from Strong International Coordination and Cooperation,” and it addresses an important issue for GNSS receiver manufacturers and others concerning use of BeiDou signals. The authors believe that early publication and dissemination of the recommendation is needed to prevent possible confusion down the line.
GNSS Community Benefit from Strong International Coordination and Cooperation
Introduction
The ephemeris broadcast by China’s BeiDou Navigation Satellites do not directly provide unique identifiers that are similar to the GPS’s “Issue of Data, Ephemeris” (IODE) and “Issue of Data, Clock” (IODC) values. Special Committee #104 (SC-104) of the Radio Technical Commission for Maritime Services (RTCM) has been working with the China Satellite Navigation Office (CSNO) to ensure that equivalent BeiDou IODE and IODC values can be generated.
This paper presents the BeiDou IODE and IODC calculation algorithms that were developed by RTCM’s SC-104 and are being shared with the GNSS community in an effort to promote consistent BeiDou IODE and IODC computational approaches within the community.
Background
Most GNSS position and timing related algorithms need to know exactly where the satellite was at the moment the signal component of interest was transmitted. The signal sent from these satellites also contain messages, which contain parameters used to calculate the position and clock errors of that satellite for a moment of interest within the validity period of those orbital parameters. Because this validity period is relatively short (e.g., +/-4 hours of the current time), the satellites are periodically broadcasting new orbital parameters. These orbital parameters are often referred to as the satellite broadcast ephemeris. Plots from the different broadcast ephemeris for the same satellite do not directly overlay each other because there are forces acting on those satellites (such as solar wind, ionospheric drag, and gravitational anomalies) that do not permit long term exact prediction of orbits and clocks.
Many differential correction services require both the correction generator system (e.g., reference station and reference networks) and the correction consumer (e.g., GNSS rover receivers) know and use the exact same orbital parameters. That is, the consumer of the corrections needs to apply those corrections using the exact same orbital parameters as those used to create the corrections. Failure to do so results in errors and biases for reasons earlier described. In such correction services, the correction message contains information enabling the consumer to uniquely identify the orbital parameters used by the generator.
Correction services need a mechanism to uniquely identify the orbit parameters used by the correction generator system. The GPS Broadcast ephemeris messages are uniquely identified for a certain period of time by what are known as the “Issue Of Data, Ephemeris” (IODE) and the “Issue of Data, Clock” (IODC). Other GNSS constellations have similar concepts, or at least other parameters that can be used for similar purposes. Unfortunately, the 2011, 2012 and 2013 BeiDou Signal-In-Space Interface Control Documents (BDS-SIS-ICD) have offered no information enabling one to develop some mechanism for such a unique identification.
In 2013 RTCM SC-104 created the BeiDou Working Group (BDS WG). Since then, the BDS WG has worked closely with the China Satellite Navigation Office (CSNO) to ensure proper inclusion of BeiDou in RTCM standards and recommendations. As part of this effort, RTCM SC-104 and the CSNO explored several avenues concerning equivalent BeiDou values of IODE and IODC. Ultimately an approach was selected by the CSNO. The selected approach stems from a ground-segment based approach which does not require a change to the BeiDou broadcast message format. However, it does then require that the users of BeiDou needing IODE and/or IODC values ensure that they employ the exact same algorithm to compute those values from the data available in the broadcast ephemeris.
In May 2016, Kendall Ferguson (RTCM SC-104 Chair), Shaowei Han (Wuhan Navigation and LBS, Ltd. and Chair of the RTCM SC-104 BDS WG), and Dr. Hui Liu (Wuhan University /Wuhan Navigation and LBS, Ltd. and co-Chair of the RTCM SC-104 BDS WG) met with the Deputy Director of the CSNO. In that meeting, the CSNO Deputy Director indicated that a soon to be release BDS-SIS-ICD would provide information that would enable calculation of equivalent BeiDou IODE and IODC values. In November 2016, the CSNO released the BDS-SIS-ICD, Version 2.1, and that ICD contains the needed information.
The language in the new BDS-SIS-ICD indicates that the normal ephemeris update (i.e., with new ephemeris parameters) will occur every hour on the hour when everything is normal. If new parameters are needed for whatever reason, they will occur on 12 minute slots within the hour. Any parameter that is changed in a broadcast ephemeris that is related to toc will result in a new toc (coincident with the 12-minute slot of the hour). Likewise, any parameter that is changed in a broadcast ephemeris that is related to toe will result in a new toe (coincident with the 12-minute slot of the hour). Whenever toc changes so will toe. There will be no repeated toc or toe values within a week.
On February 3, 2017, RTCM SC-104 formally approved algorithms for BeiDou ephemeris unique identifiers that can be computed by both message generators and message consumers. The reason for announcing this approval is to proactively prevent a wide variety of BeiDou IODE/IODC algorithms from emerging throughout the GNSS community.
These RTCM BeiDou IODE and IODC algorithms are:
BDS IODC=mod (toc / 720, 240)
BDS IODE=mod (toe / 720, 240)
The modulo 240 gives an 8-bit IODE (and an 8-bit IODC) that provides 2 days of uniqueness and which offers the smaller bit size needed for correction messages. The values from 240 to 255 thus offer some future expansion should additional cases be needed.
Unlike the relationship between the GPS IODE and GPS IODC, the BDS IODC may not be equal to the BDS IODE. The BDS IODC may be updated much more often than BDS IODE. However, whenever the BDS IODE is changed, the BDS IODC is also changed at the same time. Thus, RTCM will be using the BDS IODC as the unique ephemeris identifier in its messages.
Conclusions
Special Committee #104 (SC-104) of the Radio Technical Commission for Maritime Services (RTCM) has been working with the China Satellite Navigation Office (CSNO) seeking methods where by BeiDou equivalents of the GPS IODE and IODC might become available. The BDS-SIS-ICD, Version 2.1, released November 2016, provides information about the constellation allowing computation of IODE and IODC values from its broadcast ephemeris. In February 2017, RTCM SC-104 approved the algorithms it will use to compute unique ephemeris identifiers that will be contained in its messages, thus allowing the recipients of RTCM BeiDou related messages to identify the ephemeris used by the sender of such messages. RTCM is announcing these algorithms in an effort to prevent a variety of such algorithms from emerging and thus causing community confusion.
The report examines the objectives behind Beijing’s decision to develop the system as an alternative to GPS, its efforts to build an industry around the system, and the effects this might have in security, economic and diplomatic terms for the U.S.
“The system’s primary purpose is to end China’s military reliance on GPS, although China’s associated industrial policies will likely affect U.S. firms operating in China’s market. Industry professionals assess there are no inherent risks to products such as smartphones receiving data from BeiDou.”
China’s BeiDou is projected to achieve global coverage by 2020.
The commission was created through a congressional mandate in October 2000, and is responsible for monitoring and investigating national security and trade issues between the United States and People’s Republic of China.
Beidou constellation
Key Findings
China has sought to field its own satellite navigation system in order to (1) address national security requirements by ending military reliance on GPS; (2) build a commercial downstream satellite navigation industry to take advantage of the quickly expanding market; and (3) achieve domestic and international prestige by fielding one of only four such systems yet developed, cementing China’s status as a leading space power and opening the door to international cooperation opportunities.
Industry professionals assess there are no inherent risks to products such as smartphones receiving data from Beidou. While concerns have been raised that malware in devices could allow China’s government to track users, experts (1) are not aware of ways to feasibly transmit malware through a navigation signal and (2) assess that manufacturers will be unlikely to include Beidou’s unique messaging function due to cost factors. Restrictions on technology purchases from China by U.S. government and military users can help guard against malware being physically installed.
Beidou is of foremost importance in allowing China’s military to employ precision-guided conventional strike weapons—a central feature of Beijing’s efforts to counter a U.S. intervention in a potential contingency—if access to GPS is denied.
GPS and Beidou signals are both provided for free and are not in “competition” for market share. Also, the satellite navigation industry is trending toward “multi-constellation” receivers that work with all systems. This means that the U.S. firms that currently dominate the downstream satellite navigation industry will likely be able to incorporate Beidou functionality and continue to compete, although prospects in the China market may narrow.
China plans to expand Beidou coverage to most of the countries covered in its “One Belt, One Road” initiative by 2018, indicating it sees the system as playing a role in its economic diplomacy efforts. China has also sought to incentivize countries in Southeast Asia and the Middle East to begin using Beidou, and seeks to build a network of ground stations throughout Asia to improve the system’s accuracy.
In response to these developments, the United States can consider allowing government and military users to take advantage of multi-constellation devices, while continuing to monitor the industry to assure that security threats do not materialize; promote interoperability to ensure its firms remain competitive; and continue to invest in maintaining its leadership in space.
Current coverage of BeiDou constellation (from report).
