| Satellite radio navigation technology has got rapid development with the accelerated development and modernization of GNSS (Global Navigation Satellite System). Currently there are four global navigation satellite systems: GPS, GLONASS, Galileo and BDS(BeiDou Navigation Satellite System). The requirement of positioning accuracy become more demanding because the satellite navigation technology has significantly expanded, especially for precision relative positioning application. Signal-in-space errors(ionosphere and troposphere), orbit error, clock error can be reduced, even eliminated by double difference or accurate correction model in precision relative positioning, but de-correlation error such as multipath error is difficult to be reduced. Multipath error has always been one of the main errors that restrained the precision positioning accuracy. Multipath effect can distort carrier,and ambiguity cannot be fixed successfully when the effect is serious, which affected the accuracy and reliability of carrier phase differential positioning. Therefore, this paper carries out studies aiming at characteristics of multipath error in BDS, on this basis, the study focus on the suppression method of multipath error in BDS carrier phase differential positioning.This paper first analyzes the GNSS carrier phase precision relative positioning model and main error source which degrades the positioning accuracy. In precision relative positioning, multipath effect is a main error source that affected positioning accuracy, the spatial correlation of multipath effect is too weak to eliminate by difference technology.The mathematical model of multipath error is established on the basis of multipath effect generation principle. The impact on carrier period and amplitude caused by multipath effect is analyzed. The periodic character of multipath error is proved theoretically, the period is coincides with satellite orbit period, and the spectral characteristic of multipath error is closely related to environment. Satellite motion trajectory of BDS is drawn by the method of sky view, and satellite orbit period is calculated by broadcast ephemeris. Considering that multipath error mainly include system error and random error, wavelet analysis is adopted to decomposition de-noising and reconstruction. The low-frequency part of single difference residual is extracted as multipath system error, and used in sidereal filter algorithm to reduce multipath error.Multipath is the main error source to affect carrier phase differential positioning performance. Analysis and comparisons of positioning results which adopted traditional random model are carried out, included equal weight, elevating angle and SNR(Signal to Noise Ratio) random model. It shows that adopting elevating angle and SNR weighting can inhibit the affect of multipath to a certain extent, but only little effect.A BDS short-baseline multipath error mitigation method that combined Extended Kalman Filter with Sidereal Filter(EKF/SF) is proposed to suppress simultaneously the system and random part of multipath error. The impact caused by multipath system term is weakened in single difference observation field, then combining the advantage of EKF to restrain the random error that included in residual multipath error, thus the aim of maximum inhibition of multipath error can be achieved.Experimental results of BDS short-baseline carrier phase differential positioning show that, compare with traditional SF algorithm based on single observation field and traditional EKF algorithm, The fix rate of carrier phase integer ambiguity that adopted EKF/SF is increased by 10% and 43% respectively,and the positioning accuracy is enhanced 84% and 90% respectively. In addition, cep-95 can reach centimeter-level by adopting EKF/SF algorithm. The proposed EKF/SF algorithm can effectively reduce the multipath error,improve the quality of observation data, the aim of enhancing positioning performance can be achieved. |
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