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The U.S. Navy launched GEOSAT, or the Geodetic Satellite, in 1985. GEOSAT was designed to collect closely spaced tracks for precise mapping of the Earth's geoid over the ocean. GEOSAT provided global wind speed and significant wave height derived from radar altimeter, collected during the Geodetic Mission (GM), March 1985-September 1986 and the Exact Repeat Mission (ERM), November 1986-December 1989. The GEOSAT mission concluded in 1990.
GEOSAT supported a radar altimeter that collected elevation data from the Earth's ocean surfaces, as well as associated satellite tracking beacons and a C-band transponder system. It was placed in a circular orbit of 800 kilometers, with an inclination of 108 degrees. The orbit had a repeat period of 72 days for the GM and 17 days for the ERM. The sampling period was every second, which equates to every 7 kilometers along the ground track.
The GEOSAT mission was originally managed by the Office of Naval Research, then was transferred to the Naval Electronics Systems Command (now the Space and Naval Warfare Systems Command). GEOSAT's contractor, the Applied Physics Laboratory, performed spacecraft command and control operations and collected satellite data.
GEOSAT, launched in 1985, exceeded its three-year mission, continuing to collect data through 1989. Its mission was to obtain a high-resolution image of the marine geoid. GEOSAT provided global wind speed and significant wave height derived from radar altimeter, collected during the GM and the ERM.
GEOSAT was launched on March 12, 1985 from Vandenberg Air Force Base, California. Its mass: 635 kilograms; and principal components: 20-ft scissors boom with 100-lb end mass, redundant momentum wheels for roll and yaw stiffness, and pitch and roll attitude control thrusters.
GEOSAT contained a cold gas subsystem, which used 84 pounds of Freon 14R as a propellant. The fuel was stored in six tanks, each of which was initially pressurized to 2700 pounds per square inch. This was reduced to 15 pounds per square inch by pressure regulators.
GEOSAT was placed in a circular orbit of 800 kilometers, with an inclination of 108 degrees. The orbit had a repeat period of 72 days for the GM and 17 days for the ERM. The sampling period was every second, which equates to every 7 kilometers along the ground track.
GEOSAT was equipped with one thruster pointing forward and one pointing aft for velocity control. Four additional thrusters produced both positive and negative pitch and roll torques. The system could provide a velocity change of 77 feet per second.
Attitude capture and stabilization involved a sequence of operations and maneuvers. First, a double yo-yo system was used to despin the spacecraft from the 90 revolutions per minute that resulted from spin stabilization of the launch-vehicle orbit-insertion stage. This involved the use of two pairs of yo-yo despin cables for both system despin and solar panel restraint during launch.
Next, three-axis local-vertical stabilization was achieved with this attitude capture scenario:
The GEOSAT subsystem received the digital data stream from the radar altimeter at 8.5 kilobits per second and combined it with housekeeping data at 1.5 kilobits per second collected from the spacecraft subsystems. The data are formatted into a single time-annotated frame and transmitted to the ground station via the S-band link.
Spacecraft command was achieved through a VHF uplink from the APL ground station. The microprocessor-based command subsystem received, verified, and executed commands for spacecraft configuration control on a real-time or delayed basis. Commands consisted of relay commands, pulse commands, and data commands.
S-band transmitter and tape recorder operations were managed through commands stored for delayed execution. These commands were transmitted to GEOSAT in one command block during a pass. Following this transmission, the command subsystem dumped the memory into the downlinked telemetry stream, where it was compared to a ground image of the intended command sequence.
The GEOSAT telemetry subsystem transferred consolidated radar altimeter information and spacecraft subsystem performance data to the ground station. The subsystem consisted of a redundant telemetry processor, two S-band transmitters, two tape recorders, and two encryption units. The telemetry processor's digital circuitry employed complementary metal oxide semiconductor chips.
APL's Satellite Tracking Facility acquired, preprocessed, and distributed the data from GEOSAT, as well as commanded and controlled the satellite and monitored its performance. For this mission, this ground station was in operation 24 hour per day.
Two Odetics (5 X 108) dual-track high-density tape recorders independently recorded GEOSAT's 10.205-kilobit-per-second telemetry stream and replayed it at 844.5 kilobits per second for transmission to the ground. Tape recorder management was accomplished through spacecraft-delayed commands.
The ground station acquired altimetry data from the satellite during GEOSAT's cluster passes, which occurred about every 12 hours. During the recorders' playback (dump) pass, GEOSAT replayed data from its onboard tape recorder using the 2207.5-Mhz S-band downlink. Each recorder contained 450 megabits of encrypted data accumulated at 10.205 kilobits per second during the period since the last data dump. Playback at the 833.4-kilobit-per-second downlink dump rate required about 10 minutes.
The encrypted and unencrypted data streams were separated by filters, then bit-synchronized and recorded on separate tracks of an analog tape. Time was also recorded on the tape, which is useful for archiving purposes. These tapes were to be stored at APL for an extended time after the GEOSAT mission.
Two to three satellite passes ("a cluster"), which were about 100 minutes apart, occurred in view of the ground station about every 12 hours. These passes occurred at various hours of the day over the mission's life and were the only opportunities for the ground station to acquire altimetry data, transmit commands, and monitor real-time telemetry.
Committee on Earth Observing Satellites International Directory Network. Geodetic satellite. Internet page. http://neonet.nlr.nl/ceos-idn/sources/GEOSAT.html.
Frain, William E., Michael H. Barbagallo, and Raymond J. Harvey. 1987. The design and operation of Geosat. Johns Hopkins APL Technical Digest 8(2): 184-9.
Jensen, Jack J., and Francis R. Wooldridge. 1987. The Navy Geosat mission: An introduction. Johns Hopkins Technical Digest 8(2): 169.
Jones, Samuel C., Barry E. Tossman, and Leo M. Dubois. 1987. The Geosat ground station. Johns Hopkins APL Technical Digest 8(2): 190-6.
Petroscan AB. 1997. Altimeter data. Marine Geoid, A Brief Description of Satellite Gravity, Internet page. http://www.petroscan.se/descrpt.html.
NASA/Jet Propulsion Laboratory. Geosat: Geodetic Satellite. Mission and Spacecraft Library, Internet page. http://msl.jpl.nasa.gov/QuickLooks/geosatQL.html.
Paras. Satellite data. Oceanography Group/Database, Internet page. http://www.paras.co.uk/satellit.htm.
February 4, 1998
February 3, 1998