1992https://ntrs.nasa.gov/search.jsp?R=19920019100 2020-08-05T14:56:30+00:00ZThe results of these simulations and their implication on the ability to precisely model the TOPEX/Poseidon orbit will be discussed. Mission/ScienceOverview The seasonal variations of the world's ocean circulations, and how they influence the Earth's climate will soon be investigated with a high-precision Earth orbiting altimetric satellite. This project, referred to as the Ocean TOPography EXperiment/Poseidon Mission (T/P), is a joint venture between the U.S.'s National Aeronautics and Space Administration (NASA) and the French Centre National d'Etudes Spatiales (CNES). The TOPEX/Poseidon spacecraft will be equipped with two radar altimeters (1 US, and 1 French), which will measure the ocean surface topography. By measuring the height of a satellite above the ocean surface with the use of a radar altimeter and subtracting the satellite's height in geocentric coordinates, the sea surface topography in geocentric coordinates is calculated. Subtracting the geoid height and accounting for Earth/ocean tides yields a measure of the sub-satellite ocean dynamic topography. Knowledge of the dynamic topography over time is very important in that it consists mainly of surface geostrophic currents and ocean thermal responses which drive global weather patterns and their changes.
We have conducted an extensive investigation of orbit determination strategies for the Geosat ExactRepeat Mission (ERM). The goal of our studies is to establish optimum geodetic parameters and procedures for the computation of the most accurate Geosat orbits possible and to apply these procedures for the routine computation of Geosat orbits during the ERM for the following purposes: (!) to enhance the value of' the Geosat oceanographic investigations by providing the user community with improved ephemerides, (2) to develop orbit determination techniques for the upcoming altimetric mission TOPEX/POSEIDON, and (3) to assess the radial orbit accuracy obtainable with recently developed gravity models. To this end, ephemerides for the entire first year of the ERM have been computed using the GEODYN II orbit program on the Cyber 205 supercomputer system at the Goddard Space Flight Center. The GEM-T1 gravity solution was used in developing solutions in 25 separate 17-day arcs. Estimated radial orbit errors have been reduced from a level of 3-m rootmean-square (rms) with the operational orbits to about the 85-cm level. Both the operational and precision ephemerides were produced from the same tracking data collected by the U.S. Navy's Operational Network, so most of this improvement is due to improved force and measurement modeling. Recently, a more accurate gravity field, GEM-T2, has become available. Preliminary orbit tests performed with the GEM-T2 gravity model, along with Geosat TRANET-2 Doppler data that have recently been acquired, suggest that radial orbit accuracies of about 35 cm rms can be achieved for Geosat. !. INTRODUCTIONThe U.S. Navy Geodetic Satellite (Geosat) was launched into a retrograde orbit by an Atlas Agena from the Western Test Range on March 12, 1985. It carried a Seasat class radar altimeter and a Doppler beacon. Geosat's primary mission was to provide a dense global altimeter data base for the determination of the marine geoid with a spatial resolution of !5 km. Because the satellite ground tracks were nonrepeating during the primary (geodetic) mission, the altimeter data are classified; thus this phase of the mission was not well suited for determining sea level variability.The long lifetime of the Geosat altimeter and the maneuverability of the spacecraft enabled a secondary mission subsequent to the 18-month primary mission. For this secondary mission, the satellite was maneuvered into a 17nodal-day repeat and frozen orbit [Born et al., 1987]. This orbit, modeled after the 17-day near-repeat orbit for Seasat, is well suited for monitoring the variability of the sea surface. For Geosat the ground track repeats to within 1 km every 17 nodal days. (A nodal day is one revolution of the Earth with respect to the line of nodes of the Geosat orbit.) An exact repeat orbit allows for the direct computation of sea level variability by examining an ensemble of repeating ground tracks. No reference geoid is necessary, since the geoid height is common to the repeating tracks. "Frozen" implies that becau...
The orbit accuracies needed to support the global crustal dynamics project and recent satellite altimeter missions have placed unique demands on both the data precision and the analysis systems. These include requirements for accurate and well‐distributed observations, improved computational techniques and substantial enhancements in the force models which describe the satellite's motion. For example, the satellite altimeter mission (TOPEX), whose objectives will be (1) to measure the time variable ocean surface topography and (2) to demonstrate the ability to map the general ocean circulation, requires that the radial component of the satellite's orbit be known with a root‐sum‐square absolute error of 13 cm for the three‐year mission lifetime. The primary force model error, which limits the contemporary orbit computation accuracy, is the inaccuracy in the values assigned to the spherical harmonic coefficients used to model the earth's gravity field. This investigation describes the effects of gravity model errors on previous satellite altimeter missions and looks at the projected effects of current geopotential model errors on future satellite altimeter missions. The investigation demonstrates that (1) the radial orbit error, represented in a geographic sense, will contain a regionally dependent mean and variability, (2) historically, it has not been possible to use conventional ground‐based tracking to remove these effects, and (3) the regionally correlated component can lead to bias in mean ocean surfaces created by minimizing altimeter differences at ground track crossover points. This last effect has been a primary limitation in using satellite altimetry to obtain the long‐wavelength components of the ocean surface topography. The magnitude of the regionally correlated error can be reduced only by improvement in the knowledge of the gravity field. The conclusion reached in this investigation is that an order of magnitude improvement in the accuracy of the earth's gravity field model is required to realize the full potential of the proposed TOPEX altimeter measurements.
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