Combined precise orbit determination of GPS and GLONASS with ambiguity resolution

Precise orbit products of Global Navigation Satellite Systems (GNSS) are an essential precondition for precise positioning. Ambiguity resolution (AR) can enhance the orbit accuracy in precise orbit determination (POD). To improve the quality of orbits, we propose a method of combined POD for GPS and GLONASS with AR. Firstly, GLONASS wide-lane and narrow-lane fractional cycle biases (FCBs) are daily estimated. Then, by applying the estimated FCBs, GLONASS and GPS double-differenced wide-lane and narrow-lane ambiguities are successfully resolved, even for the baselines of up to several thousand kilometers. Finally, the ambiguity-resolved solutions are achieved by introducing the constraints of the resolved ambiguities into the real-valued solutions. To prove the contribution of the AR to GPS and GLONASS POD, a network including 141 sites is processed over 2018. The results show that the receiver types and firmware versions seriously affect the stability of the daily wide-lane FCBs. The fluctuation of the inter-system biases between two adjacent days is obviously larger than a half narrow-lane wavelength, causing an irregular change of the daily narrow-lane FCBs. After FCB calibration, the success rate of GLONASS can reach up to 90% over the whole year, which is at the same level compared with that of GPS. The improvements of GLONASS and GPS orbits after AR are confirmed by the orbit comparison with the International GNSS Service final products, the orbit misclosures at day boundaries and satellite laser ranging residuals. Due to some other issues, such as the GLONASS frequency-division multiple access and the high noise of observations, the improvement of GLONASS orbit is still less obvious than that of GPS orbit.