A Fitting Algorithm and Positioning Analysis of BDS WADS Zone Correction

The BeiDou Navigation Satellite System (BDS) provides wide area differential service (WADS) for users in China territory by broadcasting equivalent satellite clock, orbit correction, ionospheric grid information and zone correction through Geostationary Earth Orbit (GEO) satellites. By using WADS corrections correctly, users with code observations can get meter-level positioning accuracy while carrier phase users can get decimeter-level result. However, the imperfection and discontinuity of zone correction bring a negative influence upon positioning. Aiming at solving the precision deterioration and positioning re-convergence caused by the discontinuity of zone correction, a fitting algorithm is proposed for zone correction generation and user's positioning. The fitting algorithm is established on the basis of the stability of double-differenced zone corrections. By fixing the zone correction of one reference satellite and the double-differenced bias between multiple zones and satellites, the correction gap can be filled up successfully. Then, the validity of the algorithm and the performance of fitting zone correction was evaluated by differential positioning experiments using observations from six Multi-GNSS Experiment (MGEX) stations within zone correction service range under kinematic PPP mode. Experiment results show that although the positioning precision of fitting zone corrections, which is 0.20 m and 0.44 m for horizontal and vertical components respectively, is slightly inferior to the result derived by using the original corrections. In the discontinuous periods of zone correction, the positioning results are much more stable and continuous, and without positioning re-convergence. With no additional information used in correction fitting, the algorithm can be used for the zone correction generation period and position, which will improve the service integrity and positioning reliability.