Advances in Multi-GNSS Time Transfer

This paper proposes a method to combine GPS and GLONASS measurements for time transfer in All-in-View, using calibration data for both GPS and GLONASS. GLONASS has to date a complete constellation, but its use for time transfer is made difficult by the existence of biases for each satellite-receiver pair. These biases contain both a receiver contribution, due to the different frequencies emitted by the different GLONASS satellites, and a satellite contribution, varying with time, and coming from the satellite clock products. Up to recently, these biases were determined with respect to the calibrated GPS solution. We propose here to determine these biases for the two stations of a time link, in a constrained least square analysis, where the constraint is given by the GLONASS calibration data obtained from a link calibration with these two stations. With these biases, it is then possible to obtain an All-in View solution combining GPS and GLONASS measurements and calibrated for the GPS as well as GLONASS signals. The method was tested on the link Brussels-Paris for which a calibration was performed for GPS and GLONASS. Using one month of data, the impact of using or not the GLONASS calibration results is in that case smaller than 2.5 ns peak to peak.