High‐precision orbit determination of Global Positioning System (GPS) satellites is a key requirement for GPS‐based precise geodetic measurements and precise low‐Earth orbiter tracking. We explore different strategies for orbit determination with data from 1985 GPS field experiments. The most successful strategy uses multiday arcs for orbit determination and incorporates fine tuning of spacecraft solar pressure coefficients and stochastic station zenith tropospheric delays using the GPS data. Average rms orbit repeatabilities for five of the GPS satellites are 1.0, 1.2, and 1.7 m in altitude, cross‐track, and downtrack components, when two independent 5‐day fits are compared. Orbits predicted up to 24 hours outside a 7‐day arc show average rms component differences of 1.5–2.5 m when compared to independent solutions obtained with a separate, nonoverlapping 5‐day arc. For a 246‐km baseline, with 6‐day arc carrier phase solutions for GPS orbits, baseline repeatability is 2 parts in 108 (0.4–0.6 cm) for east, north, and length components and 8 parts in 108 for the vertical component. For a 1314‐km baseline with the same orbits, baseline repeatability is about 2 parts in 108 for the north component (2.5 cm) and 4 parts in 108 or better for east, length, and vertical components. When GPS carrier phase is combined with pseudorange, the 1314‐km baseline repeatability improves further to 5 parts in 109 for the north (0.6 cm) and 2 parts in 108 for the other components (2–3 cm).

Baseline lengths and geocentric radii have been determined from GPS data without the use of fiducial sites. Data from the first GPS experiment for the IERS and Geodynamics (GIG '91) have been analyzed with a no‐fiducial strategy. A baseline length daily repeatability of 2 mm + 4 parts per billion was obtained for baselines in the northern hemisphere. Comparison of baseline lengths from GPS and the global VLBI solution GLB659 (Caprette et al. 1990) show rms agreement of 2.1 parts per billion. The geocentric radius mean daily repeatability for all sites was 15 cm. Comparison of geocentric radii from GPS and SV5 (Murray et al. 1990) show rms agreement of 3.8 cm. Given n globally distributed stations, the n(n ‐ 1)/2 baseline lengths and n geocentric radii uniquely define a rigid closed polyhedron with a well‐defined center of mass. Geodetic information can be obtained by examining the structure of the polyhedron and its change with time.

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A reduced dynamic filtering strategy that exploits the unique geometric strength of the Global Positioning System(GPS) to minimize the effects of force model errors has yielded orbit solutions for TOPEX/POSEIDON which appear accurate to better than 3 cm (1 σ) in the radial component. Reduction of force model error also reduces the geographic correlation of the orbit error. With a traditional dynamic approach, GPS yields radial orbit accuracies of 4–5 cm, comparable to the accuracy delivered by satellite laser ranging and the Doppler orbitography and radio positioning integrated by satellite (DORIS) tracking system. A portion of the dynamic orbit error is in the Joint Gravity Model‐2 (JGM‐2); GPS data from TOPEX/POSEIDON can readily reveal that error and have been used to improve the gravity model.

The reduced dynamic GPS tracking technique has been applied for the first time as part of the GPS experiment on TOPEX/Poseidon. This technique employs local geometric position corrections to reduce orbit errors caused by the mismodeling of satellite forces. Results for a 29‐day interval in early 1993 are evaluated through postfit residuals and formal errors, comparison with GPS and laser/DORIS dynamic solutions, comparisons on 6‐hr overlaps of adjacent 30‐hr data arcs, altimetry closure and crossover analysis. Reduced dynamic orbits yield slightly better crossover agreement than other techniques and appear to be accurate in altitude to about 3 cm RMS.