The GPS data acquired by the TOPEX/POSEIDON (T/P) Demonstration Receiver (DR) have been used in a dynamic orbit determination, which was based on the description of the gravitational and nongravitational forces in the equations of motion. The GPS carrier phase data were processed in a double difference mode to remove clock errors, including the effects of Selective Availability. Simultaneous estimation of the T/P orbit and GPS orbits was performed using five 10-day cycles in the interval between December (1992) and April (1993). The resulting T/P orbits have been compared with the orbits determined from Satellite Laser Ranging, the French one-way Doppler tracking system, DORIS, and with the JPL reduced dynamic orbits orbtained from the GPS/DR data.

Using similar dynamic orbit determination strategies and force models with the GPS/DR to those used with SLR/DORIS, the radial component of the T/P orbit (based on JGM-2) was found to agree to better than 30 mm (rms) and 35 mm with the JPL reduced dymamic orbit. An experimental gravity tuning was accomplished using four cycles of GPS/DR data. The resulting GPS/DR-orbits, determined by the dynamic technique with the experimental gravity field, are in better agreement with the JPL reduced dynamic orbits in both the radial component (21--25 mm) and altimeter crossover residuals than the JGM-2 orbits. ¿ American Geophysical Union 1994