The precise satellite ephemerides for TOPEX/POSEIDON are identical to those used in the second generation of the T/P Geophysical Data Records. These ephemerides are of unprecedented accuracy, and they are, in large part, responsible for the enhanced accuracy of TOPEX/POSEIDON over any other historical or present-day satellite-altimeter mission. The ephemerides are computed by the Space Geodesy Branch at NASA Goddard Space Flight Center. They are computed for nominal 10-day arc segments, corresponding to the groundtrack repeat period of the satellite. They are based on a combination of laser and doppler tracking data and on force models derived from the JGM-3 earth gravity model, a T/P-based ocean-tide model, and a `box-wing' satellite model for drag and radiation forces. The first-generation version of these orbits was described by Nerem et al. [1993] and Tapley et al. [1994]; those papers give many further details on the ephemeris calculations. The present version of the orbits is described by Marshall et al. [1995], who examine in detail the error budget while presenting strong evidence for a radial orbit accuracy of about 2 to 3 cm rms.
The JGM-3 (Joint Gravity Model 3) is an update to the JGM-1 and JGM-2 models developed at NASA/GSFC and the University of Texas and described by Nerem et al. [1994]. The JGM-1 model was developed before the launch of TOPEX/POSEIDON and was the result of a multi-year effort to improve the earth's gravity model by a new inversion of tracking data on over 30 satellites, altimeter data from Seasat and Geosat, and direct gravity measurements on the earth's surface (land and marine gravimetry). The JGM-2 model was a `tuning' of JGM-1 after the launch of TOPEX/POSEIDON by inclusion of 150 days of T/P tracking observations. The JGM-2 model was used for computing the first-generation orbits on the original T/P GDRs. The JGM-3 model represents a further tuning of JGM-1 by inclusion of more tracking data on T/P, and especially the inclusion of about 40 days of GPS tracking. The JGM-3 model is described in detail by Tapley et al. [1996].
The tidal perturbations on the satellite are deduced from a new ocean-tide model that was itself produced primarily from TOPEX/POSEIDON altimetry. This model was developed by Ray et al. [1994]; polewards of T/P's maximum 66-deg latitude, this model was supplemented by Schwiderski's older hydrodynamic model. Spherical harmonics through degree and order 15 are used from this ocean model to compute satellite perturbations, except for the resonant terms (degree-2, order-2 semidiurnals; degree-2, order-1 diurnals) which are taken from the tracking-based GEM-T3S model of Lerch et al. [1992]. In addition to the ocean tide, of course, the earth tide also produces satellite perturbations; as is standard, this earth model is the one developed by Wahr [1981]. Any error in the earth tide will be absorbed, and hence adequately compensated for, by the GEM-T3 ocean resonance terms.
The box-wing model which is used for modeling forces arising from atmospheric drag, solar and earth radiation, and spacecraft radiation has been described in detail by Marshall & Luthcke [1994a; 1994b].
Four different tracking systems are available on TOPEX/POSEIDON: the French DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) system, SLR (Satellite Laser Ranging), satellite-satellite tracking via the TDRSS satellites, and the GPS (Global Positioning System). For these computed orbits, only the two `operational' tracking methods (DORIS and SLR) are used. (The TDRSS and GPS data have been used by GSFC and other groups for orbital accuracy assessments and for other studies.) During the force-model integrations and the fitting to the tracking data, one-per-revolution acceleration adjustments are also included to handle any remaining modeling errors; the theory for this adjustment is described by Colombo [1986] and Cretaux et al. [1994].
Many other details concerning these orbits, including critical details concerning the geodetic reference frame and tracking-station positioning, are described in the references.
REFERENCES:
Colombo, O L, Bull. Geod., 60, 64-84, 1986.
Cretaux, J F, F Nouel, C Valorge, P Janniere, Manuscripta Geod., 19, 135-156, 1994.
Lerch, F J, R S Nerem, B Putney, S Klosko, G Patel, R Williamson, D Chinn, J Chan, K Rachlin, N Chandler, J McCarthy, J A Marshall, S Luthcke, D Pavlis, J Robbins, S Kapoor, E Pavlis, NASA Technical Memo. 104555, 118 pp., 1992.
Marshall, J A and S B Luthcke, J. Spacecraft & Rockets, 31, 89-105, 1994a.
Marshall, J A and S B Luthcke, J. Astronaut. Sci., 42, 229-246, 1994b.
Marshall, J A, N P Zelensky, S M Klosko, D S Chinn, S B Luthcke, K E Rachlin, R G Williamson, J. Geophys. Res., 100(C12), 25331-25352, 1995.
Nerem, R S, B Putney, J A Marshall, F Lerch, E Pavlis, S Klosko, S Luthcke, G Patel, R Williamson, N Zelensky, IEEE Trans. Geoscience & Remote Sens., 31, 333-354, 1993.
Nerem, R S, F Lerch, J A Marshall, E Pavlis, B Putney, B Tapley, R Eanes, J Ries, B Schutz, C K Shum, M Watkins, S Klosko, J Chan, S Luthcke, G Patel, N Pavlis, R Williamson, R Rapp, R Biancale, F Nouel, J. Geophys. Research, 99, 24421-24448, 1994.
Ray, R D, B Sanchez, D E Cartwright, (abstract), EOS, 75(16), Spring Meeting Suppl., 108, 1994.
Tapley, B, J Ries, G Davis, R Eanes, B Schutz, C K Shum, M Watkins, J A Marshall, R S Nerem, B Putney, S Klosko, S Luthcke, D Pavlis, R Williamson, N Zelensky, J. Geophys. Research, 99, 24383-24404, 1994.
Tapley, B, M Watkins, J Ries, G Davis, R Eanes, S Poole, H Rim, B Schutz, C K Shum, R S Nerem, F Lerch, J A Marshall, S Klosko, N Pavlis, R Williamson, J. Geophys. Research, 101, 28029-28049, 1996.