COMPASS uses the Geostationary Earth Orbit (GEO) and Inclined Geosynchronous Satellite Orbit (IGSO) satellites, which are more suitable for regional services. Its constellation is composed of 14 satellites, including 5 GEO, 5 IGSO satellites and 4 Medium Earth Orbit (MEO) satellites. As of 2012, 13 satellites have been launched. Except for G2 (unusable) and M1 (testing only), the remaining 11 satellites, including 4GEO+5IGSO+2MEO, have successfully transmitted signals and broadcasted navigational messages, to prepare for full operation starting in 2013.
Positioning accuracy assessment for the 4GEO/5IGSO/2MEO constellation of COMPASS is published in Science China-Phys Mech Astron, 2012, 55(12). The accuracy of precise point positioning (PPP) and real-time positioning is assessed for the adopted dataset from the 4GEO/5IGSO/2MEO constellation. This paper is co-authored by Zhou Shanshi, Cao Yueling from Shanghai Astronomical Observatory and Zhou Jianhua from Beijing Global Information Application and Development Center.
Two positioning services can be provided by the satellite navigation system, post-processing PPP and real-time single point positioning. Current positioning accuracy has reached millimeter levels for GPS static points, and meter levels for real-time navigation users. Additional satellite-based or ground-based augmentation systems are built to improve the accuracy and integrity of the GNSS service. One unique design of the COMPASS system is that it simultaneously provides open service and authorized service using the same control and operation segment. To expand the COMPASS satellite navigation system, it is necessary to assess the accuracy of both PPP and real-time positioning.
This study describes the challenges of orbit determination (OD) for the COMPASS constellation and the OD strategy. For geodetic users the PPP strategy and accuracy are discussed. To evaluate the PPP accuracy, coordinate time series repeatability and discrepancies with GPS precise positioning, are computed. Experiments show that the COMPASS PPP repeatability for the East, North and Up components of a receiver within mainland China are better than 2 cm, 2 cm and 5 cm (see Figure 1). Apparent systematic offsets of several centimeters exist between COMPASS precise positioning and GPS precise positioning, indicating errors remain in the treatment of COMPASS measurement and dynamic models, and reference frame differences exist between the two systems.
For common positioning users, COMPASS provides both open and authorized services with rapid differential corrections and integrity information available to authorized users. Our assessment shows that in open service, positioning accuracy for dual-frequency and single-frequency users is about 5 m and 6 m (RMS), which may be improved to about 3 m and 4 m (RMS) with the addition of differential correction (see Figure 2). In addition, less accurate Signal In Space User Ranging Error (SIS URE) and Geometric Dilution of Precision (GDOP), contribute to the relatively inferior accuracy of COMPASS compared to GPS.
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