GPS Receiver Clock Modelling for Kinematic-Based Precise Orbit Determination of Low Earth Orbiters

This paper describes a GPS receiver clock model that can improve the accuracy of kinematic orbit determination for spacecraft in low earth orbit. The receiver clock error is commonly estimated on an epoch-by-epoch basis, along with the satellite's position. However, due to the high correlation between the spacecraft orbit altitude and the receiver clock parameters, estimates of the radial component are degraded in the kinematic-based approach. Using clocks with high stability, e.g., ultra-stable oscillators, the highly predictable frequency behaviour of the receiver oscillator can be fully exploited to improve the positioning accuracy, especially for the radial component. A simple two-state model is used to describe the deterministic and stochastic property of the receiver clock. In particular, the clock parameters are estimated as time offset and frequency offset. Additionally, residual non-deterministic random errors such as frequency white noise, flicker noise and random-walk noise are modelled. The test results of GRACE flight data indicate that the positioning accuracy could be improved significantly. In particular, the radial component error was reduced by over 60 %.