Geosat

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For generic geosats, see Geosynchronous satellite.
Geosat
Geosat.jpg
Artist's interpretation of the Geosat satellite
Mission type Earth science
Operator United States Navy
COSPAR ID 1985-021A
SATCAT № 15595
Mission duration 5 Years
Spacecraft properties
Dry mass 635.0 kilometres (394.6 mi)
Start of mission
Launch date March 13, 1985, 02:00 (1985-03-13UTC02Z) UTC
Rocket Atlas E/F OIS
Launch site Vandenberg SLC-3W
Orbital parameters
Reference system Geocentric
Regime LEO
Semi-major axis 7,146.2 kilometres (4,440.4 mi)>
Eccentricity 0.00398
Perigee 757 kilometres (470 mi)
Apogee 814 kilometres (506 mi)
Inclination 108.0596 degrees
Period 100.6 minutes
RAAN 309.0682 degrees
Argument of perigee 276.8275 degrees
Mean anomaly 131.8715 degrees
Mean motion 14.37442186
Epoch 09th December 2013
Revolution number 50565

The GEOSAT (GEOdetic SATellite) was a U.S. Navy Earth observation satellite, launched on March 12, 1985 into an 800 km, 108° inclination orbit, with a nodal period of about 6040 seconds. The satellite carried a radar altimeter capable of measuring the distance from the satellite to sea surface with a relative precision of about 5 cm. The initial phase was an 18-month classified Geodetic Mission (GM) have a ground-track with a near-23-day repeat with closure to within 50 kilometers. The effect of atmospheric drag was such that by fail 1986 GEOSAT was in an almost exact 23-day repeat orbit.[1]

After the GM concluded 30 September 1986, GEOSAT's scientific Exact Repeat Mission (ERM) began on November 8, 1986 after being maneuvered into a 17.05 day, 244 pass exact repeat orbit that was more favorable for oceanographic applications. When the ERM ended in January 1990, due to failure of the two on-board tape recorders, more than three years of ERM data were collected and made available to the scientific community.

The Geosat GM goal was to provide information on the marine gravity field. If the ocean surface were at rest, and to forcing such as tides or winds were acting on it, the water surface would lie along the geoid. To first order, the Earth shape is an oblate spheroid. Subsurface features such as seamounts create a gravitational pull, and features such as ocean trenches create lower gravity areas. Spatial variations in gravity exert influence on the ocean surface and thereby cause spatial structure in the geoid. The deviations of the geoid from the first order spheroid are on the order of ± 100m. By measuring the position of the water surface above the Earth center, the geoid is observed, and the gravity field can be computed through inverse calculations.

The U.S. Navy declassified parts of the Geodetic Mission data in 1990 that covered a doughnut-shaped area of ocean that surrounds Antarctica between 60 and 72 degrees south latitude. In 1992, further parts of the Geodetic Mission data over the oceans south of 30 degrees south were released. The GEOSAT data for the entire global sea surface was declassified in July 1995 after the competition of the geodetic phase of ESA ERS-1 mission. These data were subsequently widely used to estimate ocean bathymetry along with additional satellite altimeters.[2]

Once the GM goal had been reached, the satellite still had a useful life. An opportunity existed to observe the next order physical process that affects the ocean surface. Currents, tides and wind forcing all create changes in water levels. While there are extreme waves and events (tsunamis and hurricane surge), typical ocean features would be considered large if the water level change caused by them were 1 m. Most areas of the world experience features of typical amplitude of 20 cm. The ERM established the satellite in an exact repeat orbit. The satellite would orbit the Earth, and after 17.05 days the satellite would return to the same point. The satellite was actively controlled through thrust maneuvers to maintain the exact repeat orbit to within 1 km of the predefined ground track. In this manner, the satellite could observe long term changes at the points along the ground track.

GFO[edit]

The successor to GEOSAT is the Geosat Follow-On (GFO) mission, launched 10 February 1998 by a Taurus rocket from Vandenberg AFB.[3] GFO carried a water vapor radiometer as well as a radar altimeter, and operated in the same orbit as GEOSAT's Exact Repeat Mission. In addition, GFO carried a GPS receiver (which was never used operationally), Doppler receivers, and laser retro-reflectors for orbit determination.[4] GFO was scheduled for retirement on December 31, 2008, but in late September 2008, the deteriorating state of the spacecraft resulted in a decision to accelerate the shutdown. Despite a series of system failures aboard the spacecraft, controllers were able to successfully lower GFO to a disposal orbit and shut it down on November 25, 2008.[5] A successor mission, named GFO-2, was planned for launch in 2014, and would have featured a dual-band altimeter, instead of the single-band altimeter on the previous spacecraft.[6] GFO-2 was cancelled by the U.S. Navy on 30 June 2010.[7]

External links[edit]

References[edit]

  1. ^ Born, G.H., Mitchell, J.L. and Heyler, G.A. (1987). "Design of the GEOSAT Exact Repeat Mission" (PDF). APL-APL Tech. Dig. 8. 
  2. ^ Sandwell, W. T.; Smith, W. H. F. (1997). "Marine gravity anomaly from Geosat and ERS-1 satellite altimetry" (PDF). J. Geophys. Res. 102 (B5): 10039–10054. Bibcode:1997JGR...10210039S. doi:10.1029/96JB03223. 
  3. ^ "GFO - NSSDC ID: 1998-007A". NASA NSSDC. 
  4. ^ "The GFO Spacecraft and Mission Design". Computer Sciences Corporation. 
  5. ^ Monheim, Angela L.; Pritikin, Lance; Mayer, Greg; San Juan, Gerardo; Palmer, Ryan; Miller, Kevin; Mitchell, Scott; Weiss, Michael; et al. (2009). GFO: Disposal of a Power-Challenged Satellite with an Attitude (Control) Problem (PDF). AIAA SPACE 2009 Conference & Exposition. Pasadena, California. 
  6. ^ "Ball Aerospace Wins Contract for Navy's GFO-2 Altimetry Satellite" (Press release). Ball Aerospace. April 19, 2010. 
  7. ^ "Cancelled Department of the Navy Security Classification Guide Report" (PDF). 16 March 2016.