Multipath mitigation in GPS-based time and frequency transfer

GPS-based time and frequency transfer is presently performed either with a code-only analysis (as presently done for TAI, using C/A or P codes), or with a combined analysis of code and carrier phase measurements using geodetic analysis techniques (as used for the generation of the IGS time scale).When neglecting calibration issues, the accuracy of both solutions highly depends on the noise of the GPS codes. An important part of this code noise is caused by multipath. In this paper, we use a linear combination of GPS codes and carrier phases to mitigate the effect of multipath and noise on the code measurements, and we evaluate the influence of this code noise reduction on the time transfer results. On one hand, we get a reduction of the rms of the CGGTTS results from 0.3 to 0.1 nanosecond for particular stations. On the other hand we show that the multipath/noise mitigation does not allow reducing significantly the day boundary jumps in the time transfer solutions obtained from code and carrier phase measurements.