The GEOSAT (
GEOdetic
Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...
SATellite) was a
U.S. Navy
The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage o ...
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
A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it t ...
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 fall 1986 GEOSAT was in an almost exact 23-day repeat orbit.
Mission
The Geosat GM goal was to provide information on the marine gravity field. If the ocean surface were at rest, and no forces such as tides or winds were acting on it, the water surface would lie along the
geoid
The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...
. 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.
Exact Repeat Mission
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.
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
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
,
tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another.
Tide tables ...
s and wind forcing all create changes in water levels. While there are extreme waves and events (
tsunami
A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater exp ...
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
A ground track or ground trace is the path on the surface of a planet directly below an aircraft's or satellite's trajectory. In the case of satellites, it is also known as a suborbital track, and is the vertical projection of the satellite's ...
. In this manner, the satellite could observe long term changes at the points along the ground track.
Declassification
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
The 30th parallel south is a circle of latitude that is 30 degrees south of the Earth's equator. It stands one-third of the way between the equator and the South Pole and crosses Africa, the Indian Ocean, Australia, the Pacific Ocean, South Am ...
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 (ERS) was the European Space Agency's first Earth-observing satellite programme using a polar orbit. It consisted of 2 satellites, ERS-1 and ERS-2.
ERS-1
ERS-1 launched 17 July 1991 from Guiana Space Centre ...
mission. These data were subsequently widely used to estimate ocean bathymetry along with additional satellite altimeters.
GFO
The successor to GEOSAT is the Geosat Follow-On (GFO) mission, launched 10 February 1998 by a
Taurus
Taurus is Latin for 'bull' and may refer to:
* Taurus (astrology), the astrological sign
* Taurus (constellation), one of the constellations of the zodiac
* Taurus (mythology), one of two Greek mythological characters named Taurus
* '' Bos tauru ...
rocket from
Vandenberg AFB
Vandenberg Space Force Base , previously Vandenberg Air Force Base, is a United States Space Force Base in Santa Barbara County, California. Established in 1941, Vandenberg Space Force Base is a space launch base, launching spacecraft from th ...
. 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. 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. 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. GFO-2 was cancelled by the U.S. Navy on 30 June 2010.
See also
*
Gladys West
References
External links
Geosat Handbook
{{Orbital launches in 1985
Earth observation satellites of the United States
Military space program of the United States
Spacecraft launched in 1985
United States Navy
Geodetic satellites
Earth satellite radar altimeters
Satellites in low Earth orbit