Abstract
The GPS Composite Clock defines GPS Time, the timescale used today in GPS operations. GPS Time is illuminated by examination of its role in the complete estimation and control problem relative to UTC/TAI. Simulated GPS clock phase and frequency deviations, and simulated GPS pseudo-range measurements, are used to understand GPS Time in terms of Kalman filter estimation errors.
Highlights
GPS time is created by processing GPS pseudorange measurements with the operational GPS Kalman filter
Brown [2] refers to the object created by the Kalman filter as the GPS composite clock, and to GPS time as the implicit ensemble mean phase of the GPS composite clock
GPS time is the mean phase of an ensemble of many GPS clocks, and yet the clock phase of every operational GPS clock is unobservable from GPS pseudorange observations, as demonstrated below
Summary
GPS time is created by processing GPS pseudorange measurements with the operational GPS Kalman filter. Brown [2] refers to the object created by the Kalman filter as the GPS composite clock, and to GPS time as the implicit ensemble mean phase of the GPS composite clock. I have no need to further distinguish between TAI and UTC.) I present here a quantitative analysis of the GPS composite clock, derived from detailed simulations and associated graphics. GPS clock diffusion coefficient values used here were derived from Allan deviation graphs presented by Oaks et al [12] in 1998. My interest in the GPS composite clock derives from my interest in performing real-time orbit determination for GPS NAVSTAR spacecraft from ground receiver pseudorange measurements. I use simulated GPS clock phase and frequency deviations, and simulated GPS pseudorange measurements, to study Kalman filter estimation errors. I am indebted to Charles Greenhall (JPL) for encouragement and help in this work
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