B. H. Putney

Precision orbit determination for TOPEX/POSEIDON

The TOPEX/POSEIDON mission objective requires that the radial position of the spacecraft be determined with an accuracy better than 13 cm RMS (root mean square). This stringent requirement is an order of magnitude below the accuracy achieved for any altimeter mission prior to the definition of the TOPEX/POSEIDON mission. To satisfy this objective, the TOPEX Precision Orbit Determination (POD) Team was established as a joint effort between the NASA Goddard Space Flight Center and the University of Texas at Austin, with collaboration from the University of Colorado and the Jet Propulsion Laboratory. During the prelaunch development and the postlaunch verification phases, the POD team improved, calibrated, and validated the precision orbit determination computer software systems. The accomplishments include (1) increased accuracy of the gravity and surface force models and (2) improved performance of both the laser ranging and Doppler tracking systems. The result of these efforts led to orbit accuracies for TOPEX/POSEIDON which are significantly better than the original mission requirement. Tests based on data fits, covariance analysis, and orbit comparisons indicate that the radial component of the TOPEX/POSEIDON spacecraft is determined, relative to the Earths mass center, with an RMS error in the range of 3 to 4 cm RMS. This orbit accuracy, together with the near continuous dual-frequency altimetry from this mission, provides the means to determine the oceans dynamic topography with an unprecedented accuracy.

Gravity model development for TOPEX/POSEIDON: Joint gravity models 1 and 2

The TOPEX/POSEIDON (T/P) prelaunch Joint Gravity Model-1 (JGM-I) and the postlaunch JGM-2 Earth gravitational models have been developed to support precision orbit determination for T/P. Each of these models is complete to degree 70 in spherical harmonics and was computed from a combination of satellite tracking data, satellite altimetry, and surface gravimetry. While improved orbit determination accuracies for T/P have driven the improvements in the models, the models are general in application and also provide an improved geoid for oceanographic computations. The postlaunch model, JGM-2, which includes T/P satellite laser ranging (SLR) and Doppler orbitography and radiopositioning integrated by satellite (DORIS) tracking data, introduces radial orbit errors for T/P that are only 2 cm RMS with the commission errors of the marine geoid for terms to degree 70 being ±25 cm. Errors in modeling the nonconservative forces acting on T/P increase the total radial errors to only 3–4 cm RMS, a result much better than premission goals. While the orbit accuracy goal for T/P has been far surpassed, geoid errors still prevent the absolute determination of the ocean dynamic topography for wavelengths shorter than about 2500 km. Only a dedicated gravitational field satellite mission will likely provide the necessary improvement in the geoid.

Goddard earth models for oceanographic applications (GEM 10B and IOC)

Abstract Some important oceanographic results have been obtained with Goddard Earth Models using satellite altimetry. GEM 9 and GEM 10 have been extended through the addition of worldwide GEOS‐3 altimetry to give new solutions, GEM 10B and IOC, fields that are complete in harmonics to degree 36 and 180, respectively. GEM 9 is a field derived solely from satellite tracking observations, whereas GEM 10 is a combination solution containing surface gravimetry. The accuracy of the oceanic geoid for these models has been estimated by using independent altimeter tracks of GEOS‐3. After empirically removing long wavelength orbital errors, residuals of 1.8 m (rms) were obtained for GEM 10, 0.94 m for GEM 10B, while GEM IOC gave 0.75 m. Large discrepancies, as much as 60 mgals, were found when ocean gravimetry anomalies (5° blocks) were compared to altimetry‐derived values. Altimeter values were verified through comparison with anomalies from GEM‐9, yielding an rms difference of only 5 mgals. An estimate of the lon...

Expected orbit determination performance for the TOPEX/Poseidon mission

The research that has been conducted in the Space Geodesy Branch at NASA/Goddard Space Flight Center in preparation for meeting the 13-cm radial orbit accuracy requirement for the TOPEX/Poseidon (T/P) mission is described. New developments in modeling the Earths gravitational field and modeling the complex nonconservative forces acting on T/P are highlighted. The T/P error budget is reviewed, and a prelaunch assessment of the predicted orbit determination accuracies is summarized. >

An Improved Model of the Earth’s Gravity Field: GEM-T3

An improved model of the Earth’s gravitational field has been developed from a combination of conventional satellite tracking, satellite altimeter and surface gravimetric data (GEM-T3). This combination model represents a significant improvement in the modeling of the gravity field at half-wavelengths of 300 km and longer. GEM-T3 is complete to degree and order 50. This model gives improved performance for the computation of satellite orbital effects as well as a superior representation of the geoid from that achieved in any previous Goddard Earth Model. The GEM-T3 model uses altimeter data directly to define the orbits, geoid and dynamic height fields. Altimeter data acquired, during the GEOS-3 (1975–76), SEASAT (1978) and GEOSAT (1986–87) Missions were used to compute GEM-T3. In order to accommodate the non-gravitational signal mapped by these altimeters, spherical harmonic models of the dynamic height of the ocean surface were recovered for each mission simultaneously with the gravitational field. Herein, each of these dynamic height fields are referenced to a common geoidal model and are tied into the Conventional Terrestrial Reference System established by Satellite Laser Ranging (SLR). The tracking data utilized in the solution includes more than 1300 arcs of data encompassing 31 different satellites. The observational data base is highly dependent on SLR, but also includes TRANET Doppler, optical, S-Band average range-rate and satellite-to-satellite tracking acquired between ATS-6 and GEOS-3. The tracking data are largely the same as used to develop GEM-T2 with certain important improvements in data treatment and expanded laser coverage. The GEM-T3 model has undergone extensive error calibration. The method employed an optimal data weighting technique which insures reliable estimates of the model’s uncertainty. This method relies on statistical testing using a subset solution technique. The subset solution testing is based on the premise that the expected mean squares deviation of a subset gravity solution from the overall solution is predicted by the solution covariances. Data weights are iteratively adjusted until this condition is satisfied.