The SEASAT altimeter mean sea surface model

Abstract

An 18-day set of SEASAT altimeter data and precisely computed SEASAT ephemefides have been combined using accurate gfidding techniques to compute global contour maps of the mean sea surface topography. The earth gravity models specially developed for SEASAT precision ephemeris computations (PGS-S3 and PGS-S4) have been used to calculate the ephemerides over this 18-day time period. The altimeter data have an rms agreement of 111 cm with the SS3 mean sea surface computed using the PGS-S3 ephemefides and 70 cm with the SS4 mean surface computed using the PGS-S4 ephemerides. Comparisons with the GEM 10B 1 ø x 1 ø detailed gravimetfic geoid have rms differences of 2.8 m, while comparisons with a global mean sea surface computed from GEOS 3 altimeter data have rms differences of 1.3 m for the SS3 surface and 1.1 m for the SS4 surface. INTRODUCTIONOne of the principal goals of satellite altimetry is the generation of accurate mean sea surfaces. While these topographic maps have obvious application to gravimetric and solid earth studies [Rapp, 1977], they also contain information on ocean circulation dynamics. When combined with detailed geoid models, altimetric surfaces can provide descriptions of the surface current field. This information, together with subsurfac•e measurements of the density structure, may permit determination of the velocity field at all depths. In addition, statistical descriptions of the timevadable flow can be derived from altimetric surfaces alone, without the need for geoid models.The SEASAT altimeter experiment provided an important set of data for the computation of a global mean sea surface. Analyses of these data indicated that an independent, accurate mean sea surface could be computed from the partial 18day set of SEASAT altimeter data preprocessed at the Jet Propulsion Laboratory (JPL) for evaluation by the Altimeter/Precision Orbit Determination Experment Team. Such a computation was performed, and mean sea surface values were included as part of the final SEASAT altimeter geophysical data records (GDR) processed by JPL.The accuracy of the altimeter data is 10 cm or better. However, a comparable accuracy could not be achieved for the mean sea surface because of radial spacecraft ephemeris errors (primarily due to uncertainty of the earth's gravity field), tidal model errors, and tropospheric and ionospheric refraction model errors. These ephemeris errors are generally long wavelength in nature [Marsh and Williamson, 1980] (wavelengths greater than several thousand kilometers), and except for the gravity model errors the errors will generally be reduced by averaging over several passes.Gravimetric geoid information is also included as part of the altimeter GDR's processed at JPL. The gravimetric geoid adopted is based upon the Goddard Space Flight Center (GSFC) Goddard Earth Model (GEM) 10B earth gravity model augmented by a set of 1 ø x 1 ø mean free air surface gravity data. The accuracy of this geoid is about 1-2

The Topography and Surface Characteristics of the Larsen Ice Shelf, Antarctica, Using Satellite Altimetry

“…The best estimates available to us for the geoid were from the GEMIOB geoid model provided on the Seasat GDR tapes. The accuracy of this geoid model is about 1-2 m in the north-west Atlantic, although errors of several metres with wavelengths of the order of several hundred kilometres have been noted elsewhere (Marsh and Martin, 1982). in the satellite height and the corrections mentioned above, can be minimized through an analysis of elevation differences at orbit cross-overs.…”

Evaluation of GEOSAT data and application to variability of the Northeast Pacific Ocean

“…The latter output was rated at ±.5 m or 10% of the actual SWH (whichever was greater), for SWH values from 1-20 m. Range resolution was comparable to the current generation GEOSAT, being 0.1 m after 1 s of data averaging (Stewart, 1985 Wavenumber spectra of the oceanic mesoscale were computed by Fu (1983), while Marsh and Martin (1982) produced global contour maps of the mean sea surface topography. Tai and Wunsch (1983) mapped the subtropical gyre in the north Pacific; the first direct measurement of such a feature not dependent on conventional hydrography.…”

“…Tai and Wunsch (1983) mapped the subtropical gyre in the north Pacific; the first direct measurement of such a feature not dependent on conventional hydrography. Fu and Chelton (1984) November 1986, and was optimized for the study of oceanic variability (Cheney et al, 1987 (Wunsch and Gaposchkin, 1980 To a good approximation, surface currents are in geostrophic equilibrium and hydrostatic balance (Wunsch and Gaposchkin, 1980 (Wunsch and Gaposchkin [1980], Stommel [1965] (Wunsch and Gaposchkin, 1980) Cheney and Marsh (1981b), and Tai and Wunsch (1983) (1980), and on SEAS AT data by Marsh and Martin (1982) (Douglas et al, 1984 Examples of the collinear technique may be seen in the work of Douglas and Gaborski (1979), Mather et al (1980), Cheney et al (1983), and Fu (1983 The spatial and temporal sampling which result from this scheme are fairly close to the 20-day repeat and 140 km equatorial groundtrack spacing suggested by Mitchell (1983) (Cole, 1985). The main reference on the GDR is the GDR User Handbook (Cheney et al, 1987 Jones et al [1987] and Cole [1985] for further discussion).…”

On gravity from SST, geoid from Seasat, and plate age and fracture zones in the Pacific

“…Through gridding and contouring, however, these data have-been smoothed somew:,., t by using all points within a cap wiCh a radius of five degrees. More details of the procedure of computing this mean sea surface are described by Marsh and Martin (1982). there is a significant change in thickness of the lithosphere, which is 1 reflected by a sudden change in ocean lepth.…”