Abstract: Conventional processing method of BDS precise kinematic positioning is usually based on the experience threshold of baseline length to determine to use the raw or the ionospheric-free (IF) combination observation. However, the ionospheric delay is different at different regions and different time, it is difficult to obtain the optimal solution with the fixed threshold of baseline length, especially in kinematic positioning with the continuously change of baseline length. In this paper, we presented a new method that is flexible observation selection (FOS) for BDS real-time positioning for baselines with a variable lengths. The FOS method is based on the noise of different observations of the receiver and the size of the double-difference ionospheric residuals, and determining the optimal observation of baseline solution by the relationship of the ionospheric delay residual and the measurement noise. When the ionospheric residual is greater than the observed noise of the IF observation, the results of the IF method are better than those of the raw B1 observations, otherwise the results of the B1 observations are better. The results in the static test show that the positioning performance of the FOS method in E, N, and U directions are improved by about 9.6%, 7.5% and 11.4% with respect to the B1 method, respectively, while compared with the IF method are 8.6%, 9.9% and 12.2%. The positioning bias in kinematic test of the FOS method compared with B1 and IF methods are improved by about 11.1%, 13.7%, 17.0%, and 8.3%, 10.7% and 13.8%, respectively, and the ambiguity resolution (AR) rate are increased by 16.9% and 10.2%.

Key words: BDS, ionospheric delay residual, B1 observation, IF combination