Algorithms for On-Board Orbit Determination using GPS OBODE-GPS

Abstract

In the framework of GPS-based onboard orbit determination in real-time, a prototype implementation of an orbit determination software (OBODE-GPS) has been developed. The main goal of the OBODE-GPS aims to achieve an accuracy of 10 m along with minimum computational cost and usage of a taylored force model. For this sake, only the GPS code pseudo-range at the L1 frequency is applied, considering the effects of the clock offsets of the GPS and user satellites. The dynamic model considers a complex geopotential, while the state covariance matrix is propagated based on a simplified model. The Cowell`s method is used to propagate the orbit state vector; and a simple 4th order Runge-Kuta (RK 4) algorithm is used. Based on a conventional extended Kalman Filter, the spacecraft position and velocity state vectors as well as the user clock offset is extimated. To validate this model, real data from Topex/Poseidon satellite, which makes use of a dual frequency GPS receiver onboard, are apllied, and the results are compared with the Topex/Poseidon Precision Orbit Eqhemeris (POE) generated by NASA/JPL. In the analysis, emphasis is given to the determintion of an appropriate order of the gravity field and the step-size of the RK4 integrator. Furthermore, the consequences of including or excluding the J2 effects in the computation of the transition matrix are evaluated. Finally, the GPS broadcast ephemeris are comared with the GPS precise ephemeris.