Determined to become a new member of the well-established GNSS family,COMPASS(or BeiDou-2) is developing its capabilities to provide high accuracy positioning services.Two positioning modes are investigated in this study to assess the positioning accuracy of COMPASS' 4GEO/5IGSO/2MEO constellation.Precise Point Positioning(PPP) for geodetic users and real-time positioning for common navigation users are utilized.To evaluate PPP accuracy,coordinate time series repeatability and discrepancies with GPS' precise positioning are computed.Experiments show that COMPASS PPP repeatability for the east,north and up components of a receiver within China's Mainland is better than 2 cm,2 cm and 5 cm,respectively.Apparent systematic offsets of several centimeters exist between COMPASS precise positioning and GPS precise positioning,indicating errors remaining in the treatments of COMPASS measurement and dynamic models and reference frame differences existing between 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 of dual-frequency and single-frequency users is about 5 m and 6 m(RMS),respectively,which may be improved to about 3 m and 4 m(RMS) with the addition of differential corrections.Less accurate Signal In Space User Ranging Error(SIS URE) and Geometric Dilution of Precision(GDOP) contribute to the relatively inferior accuracy of COMPASS as compared to GPS.Since the deployment of the remaining 1 GEO and 2 MEO is not able to significantly improve GDOP,the performance gap could only be overcome either by the use of differential corrections or improvement of the SIS URE,or both.
The BeiDou Navigation Satellite System(BDS) provides Radio Navigation Service System(RNSS) as well as Radio Determination Service System(RDSS).RDSS users can obtain positioning by responding the Master Control Center(MCC) inquiries to signal transmitted via GEO satellite transponder.The positioning result can be calculated with elevation constraint by MCC.The primary error sources affecting the RDSS positioning accuracy are the RDSS signal transceiver delay,atmospheric trans-mission delay and GEO satellite position error.During GEO orbit maneuver,poor orbit forecast accuracy significantly impacts RDSS services.A real-time 3-D orbital correction method based on wide-area differential technique is raised to correct the orbital error.Results from the observation shows that the method can successfully improve positioning precision during orbital maneuver,independent from the RDSS reference station.This improvement can reach 50% in maximum.Accurate calibration of the RDSS signal transceiver delay precision and digital elevation map may have a critical role in high precise RDSS positioning services.
Geostationary(GEO) satellites form an indispensable component of the constellation of Beidou navigation system(BDS). The ephemerides, or predicted orbits of these GEO satellites(GEOs), are broadcast to positioning, navigation, and timing users. User equivalent ranging error(UERE) based on broadcast message is better than 1.5 m(root formal errors: RMS) for GEO satellites. However, monitoring of UERE indicates that the orbital prediction precision is significantly degraded when the Sun is close to the Earth's equatorial plane(or near spring or autumn Equinox). Error source analysis shows that the complicated solar radiation pressure on satellite buses and the simple box-wing model maybe the major contributor to the deterioration of orbital precision. With the aid of BDS' two-way frequency and time transfer between the GEOs and Beidou time(BDT, that is maintained at the master control station), we propose a new orbit determination strategy, namely three-step approach of the multi-satellite precise orbit determination(MPOD). Pseudo-range(carrier phase) data are transformed to geometric range(biased geometric range) data without clock offsets; and reasonable empirical acceleration parameters are estimated along with orbital elements to account for the error in solar radiation pressure modeling. Experiments with Beidou data show that using the proposed approach, the GEOs' UERE when near the autumn Equinox of 2012 can be improved to 1.3 m from 2.5 m(RMS), and the probability of user equivalent range error(UERE)<2.0 m can be improved from 50% to above 85%.
LI XiaoJieZHOU JianHuaHU XiaoGongLIU LiGUO RuiZHOU ShanShi
