Characterization of periodic variations in the GPS satellite clocks |
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| Authors: | Kenneth L Senior Jim R Ray Ronald L Beard |
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| Institution: | (1) US Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375, USA;(2) NOAA/National Geodetic Survey, 1315 East–West Highway, Silver Spring, MD 20910, USA |
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| Abstract: | 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-min and 30-s 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 predominantly 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
and 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 × (2.0029 ± 0.0005) cycles per day (24 h coordinated universal time or UTC), for n = 1, 2, 3, and 4. The equivalent fundamental period is 11.9826 ± 0.0030 h, which surprisingly differs from the reported mean
GPS orbital period of 11.9659 ± 0.0007 h by 60 ± 11 s. We cannot account for this apparent discrepancy but note that a clear
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 IIR and IIR-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 type are used to estimate the growth of interpolation and prediction errors with time interval. |
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| Keywords: | IGS GPS satellite clocks Harmonic analysis Oblateness (J 2) Relativistic corrections |
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