Frequency Transfer Using GPS Codes and Phases: Short- and Long-Term Stability

GPS codes and carrier phases measured by multi-channel geodetic receivers can be used for accurate frequency transfer applications when using geodetic data analysis methods. At the Royal Observatory of Belgium, previous on-site studies have shown the sensibility of the frequency transfer to the temperature variations around the hardware GPS receiver, cable, and antenna. Now, some parts of the old set up have been improved the GPS receiver was moved to an environmentally controlled chamber where temperature variations are kept smaller than 0.2 deg C, and the old antenna cable has been replaced by a new cable with a low electrical length change versus temperature. We demonstrate that this upgrade improves the stability of the frequency transfer especially for sub-daily averaging times. Stabilities of 1.5 to 3.10-15 can be routinely obtained for averaging durations of only 4 hours. Characteristic for the geodetic analysis method is the simultaneous estimation of site positions, troposphere corrections, and other parameters, based on daily data sets. Basic observables for our computations are the multi-satellite common-view ionospheric-free codes and carrier phases. This daily estimation process, in addition to the presence of multipath in the code observations, introduces jumps in the estimated clock differences, which can grow up to a few ns. We focus on the minimization of the jumps between successive days to improve the long- term stability of the frequency transfer. This is possible by using overlapping data files and by carefully modeling all parameters at the day boundaries. Using IGS tracking stations, separated by 280 km, and driven by an H-maser, the results demonstrate frequency stabilities of 1.10-15 for averaging durations of 32 hours.