Evaluating the Allan Variance in the Presence of On-Orbit GPS III Phase Steps Modeled by a Compound Poisson Process

Picosecond-level phase steps are observed in the publicly available GPS clock solution products provided by the National Geospatial-Intelligence Agency (NGA) and the International GNSS Service (IGS). In this paper, we evaluate the Allan variance in the presence of picosecond-level phase steps as modeled by a compound Poisson process. First, we introduce the phase steps observed in the NGA and IGS phase data, correlate the steps to establish the presence of phase steps in GPS III SVs, and compute statistics to characterize the steps. Next, we use a compound Poisson process with the step statistics to simulate GPS III SV phase data with phase steps and verify the simulation with the performance measured in the IGS phase data. Finally, we evaluate the impact of phase steps on the Allan variance by comparing the results of simulated phase steps with different compound Poisson process parameters in the overlapping Allan deviation. Our results show that the phase steps appear as an increase in white frequency noise in the Allan variance. This agrees with the actual carrier phase stabilities for regions with and without phase steps.