Continuous time and frequency transfer using robust GPS PPP integer ambiguity resolution method

GPS precise point positioning (PPP) technique has been widely used in time and frequency transfer. The performance of PPP can be improved if the phase ambiguities are fixed to the correct integer number. In this study, GPS PPP integer ambiguity resolution (PPP AR) with CNES/CLS products is applied in time and frequency transfer. To eliminate the day boundary discontinuities and achieve continuous time and frequency transfer with GPS PPP AR, a method of modified satellites carrier phase continuity is proposed when multi-day GPS PPP AR solutions are calculated. In addition, to guarantee the robustness of time transfer results, a three-segment factor is used to adjust the variance of integer ambiguities constraints. Test results in comparison of PPP AR with an optical time link show that standard deviation is deduced about 40% and the frequency stability is improved about 50% for 3.5d averaging than BIPM PPP method. Besides, GPS PPP AR shows a better short-term frequency stability than BIPM IPPP while they show similar STD value and long-term frequency stability. Moreover, test results with zero-baseline, short-baseline and long-baseline time transfer links show that the average frequency stability improvement rates are about 40% compared with PPP float solutions, and about 30% compared with IGS final products when the averaging time is 21.3 h.