Abstract: The ionospheric delay parameters in the precise point positioning (PPP) using raw observations must be considered.The estimation of the ionospheric delay parameters is strengthened by priori ionospheric correction models. But it is difficult to determine the relative weight ratios between the priori ionospheric corrections and the observations. A weight factor searching algorithm based on ionospheric delay constraint for multi-GNSS PPP using raw observations is proposed in which weight factors are utilized to adjust the variances of priori ionospheric corrections. According to the principle thatthe sum of the quadratic forms of weighted residuals be the minimum, the optimal weight factors are searched for and the variances of priori ionospheric corrections are adjusted dynamically in order to promote the positioning results. Data sets collected from 8 stations of the multi-GNSS experiment (MGEX) tracking network are processed to validate the proposed algorithm. The results of PPP in static mode show that the RMS values of the positioning errors are improved from 3.96 cm to 3.40 cm andthe convergence time is reduced from 76.3 min to 59.9 min whenthe searching algorithm is used, in comparison with the traditional priori ionospheric constraint.

Key words: multi-constellation, precise point positioning, raw observations, global ionospheric map, ionospheric delay constraint