The Geosat (GEOdetic SATellite) 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. On 8 November 1986,
the satellite was moved into an Exact Repeat Mission (ERM) orbit with a repeat cycle of
17.05 days. 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. 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.
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.
Geosat operated until January 1990. The mission was continued by the GFO satellite, which was launched in 1998.