On-Orbit Frequency Stability Analysis of GPS NAVSTAR Cesium and Rubidium Clocks

摘要

Naval Research Laboratory (NRL) on-orbit analysis of GPS NAVSTAR cesium and rubidium clocks has been performed using a three-year database that extended from 1 Jan., 1986 to 31 Dec., 1988. The NAVSTAR clock offset measurements were computed from pseudo-range data observed using a single frequency GPS receiver. Time and frequency inputs were derived from the U.S. Naval Observatory (USNO) time ensemble. Orbital data was obtained from the NAVSTAR broadcast ephemeris. A key feature of the NRL NAVSTAR clock analysis is the capability to analyze phase and frequency discontinuties, solve for the discontinuity, and correct the clock data. This feature was developed primarily as a means to solve and correct for the NAVSTAR time or frequency adjustments that are required to keep NAVSTAR clock time close to EPS time. Discontinuties are analyzed to find both the amount and the probable cause for the break. This feature makes possible the use of sample times of 100-days or more, and the analysis of data to identify and model long term clock, system, and environmental effects. Results for this NAVSTAR clock analysis will include the presentation of clock offset, frequency offset, and aging as a function of time. The NAVSTAR eclipse cycles will be superimposed on selected plots to demonstrate temperature sensitivity on several rubidium clocks. Clock performance in the time domain will be characterized using frequency stability profiles with sample times that vary from 1 to 100-days. Included in this analysis is the impact of clock aging on NAVSTAR frequency stability performance. It is demonstrated that uncorrected aging on the order of 1 x 10- 13/day has a measurable effect on NAVSTAR one-day frequency stability. It is further demonstrated that uncorrected aging on the order of 1 x 10(-15) day has negligible impact on one-day frequency stability and only a small effect on 100- day frequency stability.