Sien-Chong Wu

Reduced-dynamic technique for precise orbit determination of low earth satellites

A reduced-dynamic technique for precise orbit determination of low earth satellites is described. This technique optimally combines the conventional dynamic technique with the nondynamic technique which uses differential GPS continuous carrier phase to define the state transition. A Kalman filter formulation for this reduced-dynamic technique is given. A covariance analysis shows that when neither the dynamic nor the nondynamic technique is clearly superior, the reduced-dynamic technique appreciably improves the orbit accuracy. Guidelines for selecting a near-optimum weighting for the combination are given. Sensitivity to suboptimal weighting is assessed.

First assessment of GPS‐based reduced dynamic orbit determination on TOPEX/Poseidon

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.

Precise tracking of remote sensing satellites with the Global Positioning System

The Global Positioning System (GPS) can be applied in a number of ways to track remote sensing satellites at altitudes below 3000 km with accuracies of better than 10 cm. All techniques use a precise global network of GPS ground receivers operating in concert with a receiver aboard the user satellite, and all estimate the user orbit, GPS orbits, and selected ground locations simultaneously. The GPS orbit solutions are always dynamic, relying on the laws of motion, while the user orbit solution can range from purely dynamic to purely kinematic (geometric). Two variations show considerable promise. The first one features an optimal synthesis of dynamics and kinematics in the user solution, while the second introduces a novel gravity model adjustment technique to exploit data from repeat ground tracks. These techniques, to be demonstrated on the TOPEX/Poseidon mission in 1992, will offer subdecimeter tracking accuracy for dynamically unpredictable satellites down to the lowest orbital altitudes. >

An optimal GPS data processing technique for precise positioning

A mathematical formula for optimally combining dual-frequency Global Positioning System (GPS) pseudorange and carrier phase data streams into a single data stream is derived in closed form. The data combination reduces the data volume and computing time in the filtering process for parameter estimation by a factor of four while preserving the full data strength for precise positioning. The resulting single data stream is that of carrier phase measurements with both data noise and bias uncertainty strictly defined. With this mathematical formula the single stream of optimally combined GPS measurements can be efficiently formed by simple numerical calculations. Carrier phase ambiguity resolution, when feasible, is strengthened due to the preserved full data strength with the optimally combined data and the resulting longer wavelength for the ambiguity to be resolved. >

GPS-based precise tracking of Earth satellites from very low to geosynchronous orbits

Various GPS (Global Positioning System)-based tracking strategies for Earth orbiting satellites are reviewed. Three different categories of user satellites are studied: low circular orbits with altitudes between a few hundred and a few thousand kilometers, highly elliptical orbits with perigees as low as a few hundred and apogees as high as tens of thousands of kilometers, and high circular orbits up to the geosynchronous altitude. Results of covariance analyses which assess the orbit determination performance in all three categories are presented. Low circular orbits can be determined to subdecimeter or even a few-centimeters accuracy using up-looking differential GPS. Highly elliptical orbits, because of wide altitude range, require both up-looking and down-looking observing scenarios for optimum tracking. Among high circular orbits, geosynchronous satellites present the most difficult tracking challenge: the information content of ground-based observations is weak due to lack of temporal change in geometry; and the users are well beyond the GPS altitude and can hardly receive GPS signals. Inverted differential GPS, which requires the user to transmit signal ground GPS receivers can observe, appears ideal for tracking geosynchronous satellites.<<ETX>>

Precise GPS Orbit Determination Results from 1985 Field Tests

Data from three different receiver types have been used to obtain precise orbits for the satellites of the Global Positioning System (GPS). The data were collected during the 1985 March-April GPS experiment to test and validate GPS techniques for precision orbit determination and geodesy. A new software package developed at the Jet Propulsion Laboratory (JPL), GIPSY (GPS Inferred Positioning SYstem), was used to process the data. To assess orbit accuracy, solutions are compared using integrated doppler data from various different receiver types, different fiducial sites, and independent data arcs, including one spanning six days. From these intercomparisons, orbit accuracy for a well-tracked GPS satellite of three meters in altitude and about five meters in each of down and cross-track components are inferred.