S. M. Lichten

GPS precise tracking of TOPEX/POSEIDON: Results and implications

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.

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.

First epoch geodetic measurements with the Global Positioning System across the Northern Caribbean Plate Boundary Zone

The first geodetic survey across the northern Caribbean plate boundary zone with the Global Positioning System (GPS) was conducted in June 1986. Baseline vectors defined by the six station regional GPS network ranged from 170 to 1260 km in length. Repeatability of independent daily baseline estimates was better than 8 mm plus 1.3 parts in 108 of baseline length for horizontal components. The wet tropospheric path delay during the experiment was both high, sometimes exceeding 30 cm at zenith, and variable, sometimes exceeding 5 cm variation over several hours. Successful carrier phase cycle ambiguity resolution (“bias fixing”) could not be achieved prior to construction of a regional troposphere model. Tropospheric calibration was achieved with water vapor radiometers at selected sites, and with stochastic troposphere models and estimation techniques at remaining sites. With optimum troposphere treatment and single-day orbital arcs, we resolved most biases on baselines up to about 550 km in length. With multiday orbital arcs we resolved most biases in the network regardless of baseline length. Our results suggest that constraints on plate boundary zone deformation in the Greater Antilles, and on the North America-Caribbean relative plate motion vector, can be obtained with a series of GPS experiments spanning less than 10 and 15 years, respectively.

Preliminary determination of Pacfic‐North America relative motion in the southern Gulf of Calfornia using the Global Positioning System

Global Positioning System (GPS) data from experiments conducted in 1985 and 1989 in the southern Gulf of California, Mexico, allow a determination of relative motion between the Pacific and North American plates. The data indicate motion of Cabo San Lucas on the Pacific plate relative to North America at a rate of 47±7 mrn/yr and azimuth of 57±6° west of north (1σ errors), equivalent within uncertainties to the NUVEL-1 global plate motion model.

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>>

Geocenter location and variations in earth orientation using global positioning system measurements

We have studied the use of Global Positioning System (GPS) ground and flight tracking data to measure short-period Earth orientation variations and changes in Earths center of mass location (the geocenter). Ground-based GPS estimates of daily and subdaily changes in Earth orientation presently show centimeter-level precision. Comparisons between GPS-estimated Earth rotation variations (UT1-UTC) and those calculated from ocean tide models suggest that observed subdaily variations in Earth rotation are dominated by oceanic tidal effects. Our preliminary GPS estimates for geocenter location (from a 3-week experiment) agree with an independent satellite laser ranging estimates to 10–15 cm. Covariance analysis predicts that temporal resolution of GPS estimates for Earth orientation and geocenter improves significantly when data collected from low Earth-orbiting satellites as well as from ground sites are combined. The low-Earth GPS tracking data enhance the accuracy and resolution for measuring high-frequency global geodynamical signals over time scales less than 1 day. Low-Earth orbiters such as Topex/Poseidon carrying GPS flight receivers will provide opportunities to demonstrate these enhancements.