Effect of GPS Orbit Accuracy on GPS-determined Topex/Poseidon Orbit

From the very successful TOPEX/POSEIDON (T/P) oceanographic satellite mission, it has been demonstrated that the GPS tracking system could provide better than 3 cm radial orbit accuracy using the GPS dual-frequency carrier phase observable. To achieve this 3 cm T/P orbit accuracy, analysis requires precise GPS ephemerides, which become one of the major sources of error for the precise T/P positioning using this data type. There are two approaches to obtain the precise GPS ephemeris: 1) solve the GPS orbit simultaneously with the T/P orbit, and 2) fix the GPS ephemeris to an independent determination, such as the International GPS Service for Geodynamics (IGS). For the simultaneous solution of T/P and the GPS constellation, two strategies were compared in terms of forming the double-differenced GPS phase measurement. If the explicit double differences were formed between T/P and each ground station, the data provides continuous tracking of the T/P orbit, while the coverage for a GPS satellite is only about 2530% of the orbit. Adding the double differences between the ground stations, even though these data do not contain T/P orbit information, allows higher coverage for GPS orbit. Also, the effect of empirical GPS parameters on the GPS orbits and the T/P orbit was investigated. For each case, the resulting GPS orbit was compared to the IGS GPS ephemerides. For the case where the GPS orbits were fixed, several GPS orbit related parameters were adjusted to minimize the effect of GPS orbit error on the T/P orbit.