Characterization of Periodic Variations in the GPS Satellite Clocks

Abstract

Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. 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 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 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 hr UTC), for n =1, 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 ± 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. REPORT TYPE 1. REPORT DATE (DD-MM-YYYY) TITLE AND SUBTITLE AUTHOR(S) PERFORMING ORGANIZATION REPORT NUMBER PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S ACRONYM(S) 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S REPORT NUMBER(S)

Broadband LEO Constellations for Navigation

“…Its Allan Deviation, the clock stability as a function of averaging time, is 10 −11 s/s after 24 h . Compare this to GPS which is typically around 3 × 10 −14 s/s . The best CSAC stability occurs at an averaging time of around 100 min, where it is closer to 10 −12 s/s .…”

Leveraging Commercial Broadband LEO Constellations for Navigating

“…Its Allan Deviation, the clock stability as a function of averaging time, is 10 -11 second/second after 24 hours [26]. Compare this to GPS which is typically around 3x10 -14 second/second [27,28]. The best CSAC stability occurs at an averaging time of around 100 minutes, where it is closer to 10 -12 second/second [26].…”

Compass/BeiDou‐2 spaceborne clock performance assessment and its error detection, mitigation

“…Senior et al . investigated the periodic variation of GPS satellite clocks [4], Epstein et al . studied the long-term clock behavior of GPS IIR satellites [5], Lepek [6] and Camparo [7] researched on the characteristics of a navigation satellite spaceborne atomic clock, and Mallette et al .…”