Geosat
Encyclopedia
The GEOSAT was a U.S. Navy
Earth observation satellite
, launched on March 12, 1985 into an 800 km, 108° inclination orbit, with an orbital period of 23.07 days and a 330 pass orbit. 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 GM mission concluded September 30, 1986.
After a classified Geodetic Mission (GM) for the Navy, GEOSAT's scientific Exact Repeat Mission (ERM) began on November 8, 1986 after being maneuvered into a 17.05 day, 244 pass repeat orbit. 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 seamount
s 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.
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
, tide
s and wind forcing all create changes in water levels. While there are extreme waves and events (tsunami
s 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.
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. 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. The next satellite in the series, named GFO-2, is planned for launch in 2014, and will feature a dual-band altimeter, instead of the single-band altimeter on the previous spacecraft.
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...
Earth observation satellite
Earth observation satellite
Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc....
, launched on March 12, 1985 into an 800 km, 108° inclination orbit, with an orbital period of 23.07 days and a 330 pass orbit. The satellite carried a radar altimeter
Radar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...
capable of measuring the distance from the satellite to sea surface with a relative precision of about 5 cm. The GM mission concluded September 30, 1986.
After a classified Geodetic Mission (GM) for the Navy, GEOSAT's scientific Exact Repeat Mission (ERM) began on November 8, 1986 after being maneuvered into a 17.05 day, 244 pass repeat orbit. 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
Geoid
The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...
. To first order, the Earth shape is an oblate spheroid. Subsurface features such as seamount
Seamount
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of depth. They are defined by oceanographers as...
s create a gravitational pull, and features such as ocean trenches
Oceanic trench
The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....
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
60th parallel south
The 60th parallel south is a circle of latitude that is 60 degrees south of the Earth's equatorial plane. No land lies on the parallel—it crosses nothing but ocean...
and 72 degrees south latitude. In 1992, further parts of the Geodetic Mission data over the oceans south of 30 degrees south
30th parallel south
The 30th parallel south is a circle of latitude that is 30 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America....
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
European Remote-Sensing Satellite
European remote sensing satellite was the European Space Agency's first Earth-observing satellite. It was launched on July 17, 1991 into a Sun-synchronous polar orbit at a height of 782–785 km.-Instruments:...
mission. These data were subsequently widely used to estimate ocean bathymetry along with additional satellite altimeters.
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
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
, tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
s and wind forcing all create changes in water levels. While there are extreme waves and events (tsunami
Tsunami
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...
s and hurricane surge
Storm surge
A storm surge is an offshore rise of water associated with a low pressure weather system, typically tropical cyclones and strong extratropical cyclones. Storm surges are caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea...
), 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
The successor to GEOSAT is the Geosat Follow-On (GFO) mission, launched 1998-02-10 by a Taurus rocket from Vandenberg AFB. GFO carried a water vapor radiometerRadiometer
A radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term radiometer denotes an infrared radiation detector, yet it also includes detectors operating on any electromagnetic wavelength....
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. 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. The next satellite in the series, named GFO-2, is planned for launch in 2014, and will feature a dual-band altimeter, instead of the single-band altimeter on the previous spacecraft.