NODC Standard Product: US Navy Geosat altimeter T2 Geophysical Data Records for the Exact Repeat Mission (6 disc set) (NCEI Accession 0053521)

This accession contains a copy of the NODC CD-ROM product titled US Navy Geosat altimeter T2 GDRs for the Exact Repeat Mission for the time period of November 08, 1986 to December 30, 1989. The original six CD-ROMs were prepared by the NOAA National Ocean Service Satellite Altimeter Laboratory. Parameters include: time, orbit, latitude/longitude, and sea surface height. Corrections to the data include: sea level, and wave data, geoid height, tides, dry and wet troposphere, ionosphere corrections. Data are in binary format.

The US Navy Geodetic Satellite (Geosat) radar altimeter mission lasted for nearly 5 years (March 1985 to January 1990) and collected approximately 750 million measurements of sea level, wave height, and wind speed over the global oceans. During the Geodetic Mission (GM, March 1985 to September 1986), the satellite did not repeat its ground track for the duration of the mission. The orbit of the satellite was changed in early 1988 to begin the Exact Repeat Mission (ERM) which lasted from November 1988 until the satellite quit functioning in January 1990. During the ERM, the satellite repeated its ground track every 17 days.

Responsibility for producing the ERM Geophysical Data Records (GDRs) for the scientific community was given to NOAA's National Ocean Service (Cheney et al. 1987), and these data were distributed during the mission through the NOAA National Oceanographic Data Center. Because these data are based on the operational satellite orbits computed by the Naval Astronautics Group (NAG), we refer to this original release of data as the NAG GDRs. A new, more accurate satellite ephemeris has since been computed for the ERM by NASA's Goddard Space Flight Center (Haines et al. 1990). We have incorporated these new orbits, more precise by an order of magnitude, in a new set of T2 GDRs (so called because the orbits are based on the GEM-T2 gravity model of Marsh et al. 1989).

In addition to the improved orbit, the T2 GDRs contain other new fields which significantly increase the overall accuracy of the Geosat data. The most important of these is the wet troposphere correction which has been derived from satellite sensors operating during the Geosat mission. Through July 1987 water vapor was retrieved by Emery et al. (1990) from the TIROS Operational Vertical Sounder (TOVS) and thereafter by Wentz (1989) from the Special Sensor Microwave Imager (SSM/I). These new water vapor corrections represent a vast improvement (see Zimbelman and Busalacchi 1990) over the Fleet Numerical Oceanographic Center (FNOC) value provided in the NAG GDRs. An auxiliary dry troposphere correction based on the European Center for Medium-Range Weather Forecasting (ECMWF) model is also provided in the new GDRs, enabling substitution for the FNOC value. Finally, the T2 GDRs incorporate a more accurate geoid model (Rapp and Pavlis 1990) and modifications to the ocean tide model.

This disc set has been superseded by the US Navy Geosat altimeter JGM-3 enhanced altimeter data collection for the Geodetic Mission and the Exact Repeat Mission (NCEI Accession 0053056).