Evaluation of state-of-the-art geodetic GPS receivers for frequency comparisons

In this paper we show results of a test campaign, designed to investigate the stability of GPS receiver clocks locked to an external frequency source. Two types of geodetic receivers and one timing receiver have been compared. The receivers were operated in pairs in zero and short baseline setups with common reference frequency. Care was taken to minimize environmental effects. The observation data were processed using both a simple carrier phase single difference algorithm and the Bernese GPS Software 5.0 in Precise Point Positioning (PPP) mode. Some shortcomings of previous studies in terms of receiver equipment and the experimental setup of the short baseline could be avoided. It is shown that using geodetic receivers and the ionosphere-free linear combination of the carrier phase observations, frequency comparisons at a level of sigma_y(tau)=2ldr10^-16 for averaging times of one day are feasible on a zero baseline, i.e. if all environmental effects are eliminated.