Data Records derived from GEOSAT Geodetic Mission (GM) and Exact Repeat Mission (ERM) data from 30 March 1985 to 31 December 1989

This collection contains Sensor Data Records (SDRs), Geodetic Data Records (GDRs), Waveform Data Records (WDRs), and Crossover Difference data Records (XDRs) from the GEOSAT Geodetic Mission (GM) and Exact Repeat Mission (ERM) for the time period of March 30, 1985 to December 31, 1989. Parameters include: time, orbit, latitude/longitude, and sea surface height. Corrections to the data include: sea level, wind and wave data, tides, dry and wet troposphere, ionosphere, height bias, altimeter crossover differences, altimeter sea surface measurement, and spacecraft orientation. These data were submitted by the U.S. Navy via National Ocean Service.

This collection also contains complete copies of four NODC Standard Products, including US Navy Geosat altimeter geophysical data records (GDRs) for the Geodetic Mission (4 CD-ROMs), US Navy Geosat wind/wave data (WWDR) from the Geodetic Mission (4 CD-ROMs), US Navy Geosat Enhanced JGM-3 Geophysical Data Records (GDR) from the Geodetic and Exact Repeat Missions (4 CD-ROMs), and US Navy Geosat altimeter Crossover Differences (XDRs) for the Geodetic Mission (8 CD-ROMs).

The US Navy Geosat (Geodetic Satellite) 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 (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 October 1986 to begin the Exact Repeat Mission (ERM) which lasted from November 8, 1986 until the satellite quit functioning in January 1990. During the ERM, the satellite repeated its ground track every 17 days. Data were initially processed by the Johns Hopkins University Applied Physics Laboratory in Laurel, MD in coordination with the NOAA/NOS Satellite Altimeter Group. Data products were transferred to the NOAA National Oceanographic Data Center (NODC) for distribution. Data products are archived at the NODC.

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.

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 (GM). 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.

At the conclusion of the GM on September 30, 1986, the satellite orbit was changed, and the Exact Repeat Mission (ERM) 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 dataset. 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.