The Comparison on the Positioning Performance Between BeiDou and GPS

Y. Xu, S. Ji, W. Chen, D. Weng, Y.Xu, W. Chen, D. Weng, S. Ji

Abstract:	BeiDou is a new global satellite navigation system developed by China, which will consists of 35 satellites when the full global service is available in 2020. By the end of 2012, there are five Geostationary Earth Orbit (GEO) satellites, four Medium Earth Orbit (MEO) satellites and five Inclined Geosynchronous Satellite Orbit (IGSO) satellites in operation for regional coverage. BeiDou signals are code division multiple access (CDMA) signals the same as GPS and Galileo. BeiDou uses BeiDou navigation satellite system Time (BDT), which has a difference of 14 seconds from GPST (i.e. GPST = BDT + 14). BeiDou employs China Geodetic Coordinate System 2000 (CGCS2000), which is geocentric and referred to ITRF97 (International Terrestrial Reference Frame 1997) at the epoch of 2000.0(BeiDou navigation satellite system signal in space interface control document). The earth ellipsoid defined of CGCS2000 is slightly different from WGS84 (World Geodetic System 1984) and the difference can be ignored even for precise positioning. The purpose of this study is to make an investigation of Beidou system performance and compare it with that of GPS, to monitor the integrity of GPS/BeiDou constellation and to improve the positioning precision of combined GPS/BeiDou. To study the BeiDou positioning performance, Single Point Positioning (SPP) with both broadcast and precise ephemeris, DGPS and single epoch double difference (DD) ambiguity resolution & position performances of BeiDou are researched and compared with GPS. In addition, SPP positioning error caused by the differences of broadcast and precise orbit and satellite clock offsets are discussed. Integrity relates to the level of trust that can be placed in the information provided by the navigation system. Is the integrity of combined GPS/BeiDou constellation better than that of BeiDou only? Receiver Autonomous Integrity Monitoring (RAIM) is used to figure out this question. In addition, both internal and external reliability are the most important indications of RAIM performance, which are also be analyzed. Compared with single GNSS system, the integrated system has more error source which would reduce the positioning precision of GPS/BeiDou. At the same time the different weight matrix and weight ratio affect the positioning results heavily, and the initial weight matrix is not the best usually. Reasonable determination of weight matrix is the key to build an available integrated system. In this research, Hermert Variance Component Estimation (VCE) is employed to adjust the variance-ratio between GPS and BeiDou and the weight matrix is updated to improve the positioning precision of integrated GPS/BeiDou. The experimental data are collected on December 12, 2012 with two receivers in Shanghai, China. Dual-frequency observations of both GPS and BeiDou are observed and the observation time is from 05:49:20 to 08:11:40 with a sample interval of 5 seconds. The distance between the two stations is about 5 km. From all the experiments of this research, the conclusions can be drawn as below: 1) The SPP performance of BeiDou in 3D is obviously worse than that of GPS. The poor quality of broadcast orbit and satellite clock offsets result in the worse SPP precision of BeiDou especially the latter. Furthermore, the positioning errors due to the broadcast orbit in direction of horizontal and vertical are 4 m and 10 m, and the errors reach to 10 m and 20m due to the satellites clock offsets error. 2) When the precise ephemeris is adopted, the performance of SPP is much better. Moreover, the precisions of BeiDou are slightly better than that of GPS in the directions of horizontal (1.5m for GPS and 1.0m for BeiDou), but worse in vertical (3.5m for GPS and 5.0m for BeiDou). 3) The DGPS performance of BeiDou is similar with that of GPS (external accord precisions: 0.8m in horizontal direction and 2.2m in vertical). 4) BeiDou single epoch ambiguity resolution performance is better than that of GPS considering the percentages of ratio more than 1.5, which result from the more visible BeiDou satellites. The DD positioning precisions of BeiDou and GPS are very similar (external accord precisions: 15mm in horizontal direction and 16mm in vertical), which also point out that the phase measurement of both BeiDou and GPS basically have the same high accuracy. 5) The internal and external reliability of RAIM of BeiDou/GPS constellations are much better than that of BeiDou alone. Both MDB (Marginal Detectable Bias) values and the positioning error with BeiDou/GPS constellations are smaller than that with BeiDou alone, which demonstrates the advantage of the integrated GPS/BeiDou constellation significantly. 6) Helmert VCE is used to update the weight matrix of GPS/BeiDou. The SPP positioning error of integrated system (external accord precisions of GPS/BeiDou: 1.8m in horizontal direction and 2.7m in vertical direction) fells off remarkably and its precision are much better than that of BeiDou and GPS/BeiDou without VCE in 3D. Moreover there are nearly the same precisions in GPS system and VCE GPS/BeiDou system.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	369 - 382
Cite this article:	Xu, Y., Ji, S., Chen, W., Weng, D., Y.Xu,, Chen, W., Weng, D., Ji, S., "The Comparison on the Positioning Performance Between BeiDou and GPS," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 369-382.
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