Improved method to estimate undifferenced satellite fractional cycle biases using network observations to support PPP ambiguity resolution

Fixing ambiguities is beneficial to improve accuracy and convergence time of precise point positioning (PPP). In recent years, several methods have been proposed to estimate fractional cycle biases (FCBs) to realize ambiguity-fixed PPP. Existing single-difference models are mostly for GPS-only ambiguity-fixed PPP. However, the use of a single-difference model to achieve fixed ambiguities in GPS + GLONASS PPP is complicated because GLONASS uses frequency division multiple access modulation. This study presents an improved method to estimate undifferenced FCBs using network observations. The periodic feature of a trigonometric function is utilized to eliminate the integer ambiguity. Therefore, the proposed method avoids the difficulty of ambiguity fixing in undifferenced FCBs estimation at the network. Thus, it is a very simple method. GPS + GLONASS observation data of the Chinese reference station network are used to achieve ambiguity fixing for GPS-only and GPS + GLONASS PPP using the estimated FCBs. Experimental results show that the FCBs estimated with the proposed method can satisfy the accuracy requirement for ambiguity-fixed PPP. The root-mean-squares of the resolutions of GPS-only ambiguity-fixed PPP in the N, E, and U directions can reach 0.8, 0.9, and 2.5 cm, respectively, which are comparable with existing ambiguity-fixed resolutions in PPP. The resolutions of fixed ambiguity in GPS + GLONASS PPP can reach 0.7, 0.7, and 2.0 cm, which is an improvement by approximately 13, 22, and 20%, respectively, compared to the fixed solution of GPS-only PPP.