Positioning with BeiDou Navigation Satellite System

Shanshi Zhou, Yueling Cao, Xiaogong Hu, Junping Chen, Bin Wu, Xiuqiang Gong, Chengpan Tang

Abstract:	To develop an independent navigation satellite system, China has for more than a decade pursued the build-up of the BeiDou system, which is designed as a Global Navigation Satellite System (GNSS). With the deployment of new satellites, Beidou is now approaching its capabilities to provide high accuracy regional positioning services. The official start of operational positioning, navigation, and timing (PNT) services was 27 December 2011, providing services to China and surrounding areas. Aiming to become a new member of the well-established GNSS family, BeiDou is developing its capabilities to provide high accuracy positioning services, even at the phase of its constellation deployment. This paper presents orbit determination strategy and positioning algorithm. Positioning results are also discussed using huge amount of observations from Beidou receivers under two different scenarios: the post-processing static positioning and real-time point positioning. Different from GPS, orbital and clock information of Beidou are generated by control segment based on data collected with a domestic monitoring network. Several challenges are facing Beidou precise orbit determination. This study applied the multi-satellite orbit determination strategy to compute orbit and clock errors, which may be briefly outlined as using 3-day data arc of ionosphere-free carrier phase and psudo-range combination. For precise orbit determination, the estimates are initial state vectors for all satellites, solar radiation pressure parameters (scale factor and y-bias) for each satellite, empirical accelerations on both orbital trace (T) and normal (N) directions, zenith tropospheric delay factor every 8 hours for each site, phase ambiguities of the ionospheric-free combination and epoch by epoch clocks for all satellites and receivers after fixing one. The processing of orbit and clocks estimation is performed in hourly batches with a data interval of 30 s. The orbital overlap comparisons, which show the internal consistency of orbit products, present decimeter-level repetition in tangential and normal directions and centimeter-level in radial direction. Beidou is unique among all working GNSS systems to incorporate a two-way time synchronization mechanism between all satellite clocks and system time standard, which is able to provide direct measurements of satellite clocks with accuracy better than 0.1ns. The discrepancies between estimated satellite clocks and the measured satellite clocks “ground truth” , or errors of clock estimates, are actually useful indicators of the accuracy of precise orbit determination, with smaller discrepancy corresponds to better orbital accuracy. This study applied TS data to evaluate orbit accuracy. Results show that clock estimations accuracy is about 1.7 ns and IGSO satellite clock errors accuracy is better than others which are the same with orbit accuracy. Post-processing mode is also named as Precise Point Positioning (PPP). In this scenario, precise station daily coordinates were derived by PPP technique. Beidou satellite orbits and clocks were derived by the precise orbit determination facility (PODF) using the pseudo-range and phase data of the Beidou regional monitor network. In PPP positioning, the precise satellite orbits and clocks were fixed and the position and clocks of user receiver were estimated. Daily observations of several stations in mainland China with sampling of 60 seconds were analyzed in PPP mode. We compared the estimated coordinates to the coordinates derived using GPS observations for the same station, and observe systematic coordinate biases at few centimeters for each coordinate component. This study reports centimeter-level accuracy of post-processing PPP with the Beidou regional satellite navigation system of the full satellite constellation (5GEO+5IGSO+4MEO).As one of the major error sources, the orbit impact on PPP accuracy was also analyzed in the paper. Results show that within mainland China, the COMPASS positioning repeatability of East, North and Up components of receiver positions are better than 2 cm, 2cm and 5cm respectively. Several centimeters of systematic errors exist between Beidou positioning and GPS positioning. One of the major differences of Beidou system compared to the existing GNSS is that it is designed to simultaneously provide open service and authorized service using the same control and operation segment. Basic navigation information is provided by open service for free in the form of navigational messages and differential corrections and integrity information are provided to authorized users for the need of high precision positioning and timing. It is noteworthy that two kinds of ionospheric correction models are provided in open service, 8 parameters /14 parameters Klobuchar model. Compared with open service, authorized service offers differential corrections and integrity information to users, aiming at high precise navigation, positioning and timing. This study describes the wide-area differential algorithm, which include ionospheric grid and equivalent satellite clock error. Experimenting with real data, user real-time positioning errors with pseudo range datum in both open and authorized services are estimated. The results show that in open service the positioning errors of dual-frequency and single-frequency are respectively 5m and 6m in terms of Root-Mean-Square (RMS), while in authorized service the positioning errors of dual-frequency and single-frequency are improved to about 3m and 4m (RMS). Of the three position components, the largest error is found to occur in the height component to reach 4~5 meters in open service, which is caused by the COMPASS constellation geometry. However, differential corrections supplied by authorized service are able to reduce the positioning errors especially for the height component.
Published in:	Proceedings of the ION 2015 Pacific PNT Meeting April 20 - 23, 2015 Marriott Waikiki Beach Resort & Spa Honolulu, Hawaii
Pages:	890 - 900
Cite this article:	Zhou, Shanshi, Cao, Yueling, Hu, Xiaogong, Chen, Junping, Wu, Bin, Gong, Xiuqiang, Tang, Chengpan, "Positioning with BeiDou Navigation Satellite System," Proceedings of the ION 2015 Pacific PNT Meeting, Honolulu, Hawaii, April 2015, pp. 890-900.
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