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The GEOdetic SATellite (GEOSAT) was a dedicated US Navy military oceanographic satellite consisting of a radar altimeter designed to obtain closely spaced, precise mapping of the earth's geoid over the ocean. GEOSAT featured an altimeter designed by the John Hopkins University Applied Physics Laboratory (JHU/APL) to measure the marine gravity field as accurately as possible. The Geodetic Mission lasted from March 1985 to September 1986, but because of the importance of the mission to the U.S. Navy, the first 18 months were classified and unavailable to the public.

The Navy declassified the first set of Geosat data in 1990 that covered a doughnut-shaped area of ocean that surrounds Antarctica between 60 and 72 degrees south latitude. In 1992, the U.S. Navy declassified all Geodetic Mission (GM) radar altimeter data acquired by the Geosat satellite over oceanic regions south of 30o South. The Geosat data for the entire global sea surface was declassified in July, 1995.

geosat_map Following the Primary Mission, the Exact Repeat Mission was initiated in November 1986 to produce more accurate sea surface profiles with a modified orbit. The new orbit had a repeat groundtrack period of 17.05 days. Since the groundtrack for the ERM was very close to the SEASAT altimeter data tracks, the new data is unclassified. An arrangement was made with the National Ocean Service of NOAA to obtain the classified GEOSAT geophysical data records (GDR) providing wind, wave and sea-level products and made available to the user community. NASA obtained GEOSAT data for extensive waveform modeling and ice sheet research.

Under Agreement with the U.S. Navy, NOAA/National Ocean Service produced the ERM Geophysical Data Records (GDR's) which is distributed by the NODC. GEOSAT is supplying the research community with the first long-term global observations of sea level, wind speed, wave height, and ice topography.

The GEOSAT mission was originally managed by the Office of Naval Research (ONR). During the development phase, the program responsibility was transferred to the Naval Electronics Systems Command, now called the Space and Naval Warfare Systems Command (SPAWAR) in Washington, D.C. The Applied Physics Laboratory (APL) was the prime contractor for the spacecraft and radar altimeter and performed spacecraft command and control operations and collected the satellite data. The data was distributed to the Naval Surface Weapons Center (NSWC), the Naval Ocean Research and Development Activity (NORDA), and NOAA.

The basic structure of the GEOSAT is similar to the GEOS-3 satellite. The design consists of a conical structure below the core for the structural attachment of the velocity control system. The GEOSAT attitude control subsystem was designed to point the radar altimeter to within 1 degree of nadir 98 percent of the time. The system components were a 20-foot scissors boom with 100-pound end mass, redundant momentum wheels for roll and yaw stiffness, and pitch and roll attitude control thrusters. Attitude sensing was provided through the use of three digital sun-attitude detectors and a three-axis vector magnetometer. Spacecraft command was accomplished via a VHF uplink from the APL ground station. The telemetry subsystem consisted of two S-band transmitters, two tape recorders, and two encryption units. The GEOSAT was equipped with two Odetics dual-track high-density tape recorders that independently recorded the 10.205 kbps telemetry stream and played it back at 833 kbps for transmission to the ground. The GEOSAT also included redundant Doppler beacons for continuous tracking by a network of ground stations within the Defense Mapping Agency (DMA) and for a source of accurate timing to the radar altimeter and the telemetry subsystem. A C-band transponder was also included on GEOSAT.

        Launch Date and Time :  1985-03-12
        Apogee (km or AU):      814.
        Perigee (km or AU):     757.
        Inclination (degree) :  108.1


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