NODC Standard Product: US Navy Geosat Enhanced JGM-3 Geophysical Data Records (GDR) from the Geodetic and Exact Repeat Missions (NCEI Accession 0053056)

From March 1985 until January 1990, the U.S. Navy satellite Geosat generated a new dataset with unprecedented spatial and temporal coverage of the global oceans. Designed and built at the Johns Hopkins University Applied Physics Laboratory, Geosat carried a radar altimeter which produced profiles of sea level, wave height, and wind speed along the satellite ground track. Such records have applications in many areas of geodesy, ocean dynamics, and global climate research. Experience with GEOS-3 and Seasat in the 1970s had
demonstrated the enormous potential of altimetry, but neither mission provided such complete long-term global coverage.

1. Geodetic Mission (GM)
Geosat was launched on March 12, 1985, and altimeter data collection began on March 30. The acronym Geosat was derived from geodetic satellite, because its primary mission was to obtain a high-resolution description of the marine geoid up to latitudes of 72 degrees. This goal was achieved during the first 18 months, known as the geodetic mission. During this time the ground track had a near-repeat period of about 23 days (330 revolutions in 23.07 days; average orbital period of 6039.84 sec). The drifting orbit resulted in a dense, global network of sea level profiles separated by about 4 km at the equator. Because of the military significance of this unique
set of observations, the GM data were initially classified but in 1995 were released to NOAA in their entirety for public distribution.

2. Transition to the Exact Repeat Mission (ERM)
At the conclusion of the GM on September 30, 1986, the satellite orbit was changed, and the exact repeat mission began on November 8, 1986. This produced sea level profiles along tracks
that repeated themselves within 1-2 km at intervals of about 17 days (244 revolutions in 17.05 days; average orbital period of 6037.55 sec). The ERM covered 62 complete 17-day cycles before
tape recorder failure in October 1989 terminated the global data set. A limited amount of data was subsequently collected by direct broadcast in the North Atlantic and Gulf of Mexico. By January
1990 continued degradation of the altimeter output power finally ended the Geosat mission

3. Upgraded JGM-3 Geophysical Data Records (GDRs)

The JGM-3 GDRs for Geosat represent the culmination of a decade of progress in refining the dataset first distributed by NOAA in 1987. The previous release of the ERM data, referred to as the T2 GDRs, is documented by Cheney et al. (1991). The initial full release of the GM data in 1995 was based on the original NSWC orbits. The 1997 release comprises the entirety of the GM + ERM data, with consistent JGM-3 orbits and enhanced geophysical corrections.

Most of the improvement has come in the area of orbit determination. For example, the initial satellite orbits for the ERM contained radial errors of 2-3 m. These were reduced to the 50-cm range in the T2 release in 1991. The present ephemeris, computed by the NASA Goddard Space Flight Center, is accurate at the level of 10 cm for most of the mission (the exception being the last year of the ERM mission, see Chapter 5 Precautions).

This remarkable achievement was made possible by (1) the 1993 release of the complete archive of Doppler tracking data and (2) development of the Joint Gravity Model-3 (JGM-3) (Tapley et al., 1996). Derivation of the JGM-3 orbit for Geosat, and the improvements gained from it, are discussed by Williamson and Nerem (1994). In the process of replacing the old NSWC (GM) and T2 (ERM) orbits, a timing bias correction of 5 msec was also applied to the GDRs. The time tags on the altimeter data were increased by a constant 5 msec over the
duration of the mission, and the JGM-3 orbits were interpolated at the
corrected GDR times. This timing bias correction greatly reduces the
'twice-per-rev' orbit errors present in earlier releases of the Geosat data.

In addition to the improved orbits, nearly all of the geophysical
corrections have been upgraded:

- ocean and load tides from the CSR 3.0 model (Eanes and Bettadpur, 1995)
- solid tide from an updated version of the Cartwright and Edden (1973) tide model
- wet and dry troposphere corrections based on the NCEP/NCAR reanalysis model (Kalnay et al., 1996)
- climatological wet troposphere correction based on the NASA NVAP dataset (Randel et al., 1996)
- ionosphere correction from the IRI95 model (Bilitza, 1997)
- wind speed from the Freilich and Challenor (1994) model
- a new sea state bias algorithm dependent on wind speed, wave height, and satellite attitude (Gaspar, Ogor, and Hamdaoui, 1996)
- the Ohio State 1995 mean sea surface (Yi, 1995)

References

Bilitza, D., International reference ionosphere - status 1995/96. Adv. Space Res.,20, #9, 1751-1754, 1997.
Cartwright, D.E., and A.C. Edden, Corrected tables of tidal harmonics. Geophys. J. Roy. Soc., 23, 253-264, 1973.
Cheney, R.E. et al., The complete Geosat altimeter handbook. NOAA Manual NOS NGS 7,Rockville, MD, 79 pp, 1991.
Eanes, R. and S. Bettadpur, The CSR 3.0 global ocean tide model. Center for Space Research, Univ. of Texas, Tech. Mem. CSR-TM-95-06, Austin, TX, 1995.
Freilich, M.H., and P.G. Challenor, A new approach for determining fully empirical altimeter wind speed model functions. J. Geophys. Res., 99, 25051-25062, 1994.
Gaspar, P., F. Ogor, and M. Hamdaoui, Analysis and estimation of the Geosat sea state bias. Hughes/STX Report No. 96-3056-K1196, Greenbelt, MD, 18 pp, 1996.
Kalnay, E. et al., The NCEP/NCAR 40-year reanalysis project. Bull. AMS, 77 (3), 437-471,1996.
Randel, D.L. et al., A new global water vapor dataset. Bull. AMS, 77 (6), 1233-1246, 1996.
Tapley, B.D. et al., The JGM-3 gravity model. J. Geophys. Res., 101 (B12), 28029-28049, 1996.
Williamson, R.G., and R.S. Nerem, Improved orbit computations for the Geosat mission: benefits for oceanographic and geodynamic studies. EOS Trans. AGU , 75(44) Spring Meet. Suppl.,S155, 1994.
Yi, Y., Determination of gridded mean sea surface from altimeter data of TOPEX, ERS-1, and GEOSAT. Ph.D. Thesis. Dept. Geodetic Science and Surveying, Ohio State Univ., Columbus,OH, 1995.

In this accession, NCEI has archived multiple versions of these data. The latest (and best) version of these data has the largest version number.