Testing the capabilities of GPS receivers for time transfer

Based on a carrier phase analysis of GPS measurements, we investigate how a geodetic GPS receiver reproduces the variations of the frequency delivered to him by an external clock. For this purpose, two PolaRx2 GPS receivers have been connected to the same antenna, and were driven by two separate hydrogen masers. The time and frequency transfer between these two masers is computed through a combined code and carrier phase GPS data analysis, and the results are compared with the measurements delivered by a phase comparator. The differences between both comparison techniques are of the order of tens of picoseconds for the short-term comparisons. On the long term, 3 weeks in our case, the maximum differences reach 180 picoseconds peak-to-peak (after correction for the day boundary jumps)