Characterization of Periodic Variations in the GPS Satellite Clocks

The clock products of the International Global Navigation Satellite Systems GNSS Service IGS are used to characterize the timing performance of the GPS satellites. Using 5-minute and 30-second observational samples and focusing only on the sub-daily regime, approximate power-law stochastic processes are found. The Block IIA Rb and Cs clocks obey predominately random walk phase or white frequency noise processes. The Rb clocks are up to nearly an order of magnitude more stable and show a flicker phase noise component over intervals shorter than about 100 s. Due to the onboard Time Keeping System in the newer Block IIR and IIR-M satellites, their Rb clocks behave in a more complex way as an apparent random walk phase process up to about 100 s then changing to flicker phase up to a few thousand seconds. Superposed on this random background, periodic signals have been detected in all clock types at four harmonic frequencies, n x 2.0029 - 0.0005 cycles per day 24 UTC, for n1, 2, 3 and 4. The equivalent fundamental period is 11.9826 - 0.0030 hours, which surprisingly differs from the reported mean GPS orbital period of 11.9659 - 0.0007 hours by 60 - 11s. We cannot account for this apparent discrepancy but note that a clean relationship between the periodic signals and the orbital dynamics is evidenced for some satellites by modulations of the spectral amplitudes with eclipse season. All four harmonics are much smaller for the IR and IR-M satellites than for the older blocks. Awareness of the periodic variations can be used to improve the clock modeling, including for interpolation of tabulated IGS products for higher-rate GPS positioning and for predictions in real-time applications. This is especially true for high-accuracy uses, but could also benefit the standard GPS operational products. The observed stochastic properties of each satellite clock