TOPEX/Poseidon was a joint satellite altimeter mission between

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

, the U.S. space agency; and CNES, the French space agency, to map ocean surface topography. Launched on August 10, 1992, it was the first major oceanographic research satellite. TOPEX/Poseidon helped revolutionize oceanography by providing data previously impossible to obtain. Oceanographer Walter Munk described TOPEX/Poseidon as "the most successful ocean experiment of all time." A malfunction ended normal satellite operations in January 2006.

Description

Before TOPEX/Poseidon, scientists had only a brief glimpse of Earth's

ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...

as a whole from the pioneering but short-lived Seasat satellite. TOPEX/Poseidon's radar altimeter provided the first continuous global coverage of the surface topography of the oceans. From orbit 1,330 kilometers above Earth, TOPEX/Poseidon provided measurements of the surface height of 95 percent of the ice-free ocean to an accuracy of 3.3 centimeters. The satellite's measurements of the hills and valleys of the sea surface led to a fundamentally new understanding of ocean circulation and its effect on

climate Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteoro ...

Goal

The mission's most important achievement was to determine the patterns of ocean circulation - how heat stored in the ocean moves from one place to another. Since the ocean holds most of the Earth's heat from the Sun, ocean circulation is a driving force of climate. TOPEX/Poseidon made it possible for the first time to compare computer models of ocean circulation with actual global observations and use the data to improve climate predictions.

Results

While a three-year prime mission was planned, TOPEX/Poseidon delivered more than 10 years of data from orbit. In those years, the mission: * Measured

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

with an unprecedented accuracy * Mapped global

tides 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 ...

for the first time * Monitored effects of currents on global

climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

and produced the first global views of seasonal changes of currents * Monitored large-scale ocean features like Rossby and Kelvin waves and studied such phenomena as

El Niño EL, El or el may refer to: Arts and entertainment Fictional entities * El, a character from the manga series ''Shugo Chara!'' by Peach-Pit * Eleven (''Stranger Things'') (El), a fictional character in the TV series ''Stranger Things'' * El, fami ...

, La Niña, and the Pacific Decadal Oscillation * Mapped basin-wide current variations and provided global data to validate models of ocean circulation * Mapped year-to-year changes in heat stored in the upper ocean * Improved our knowledge of Earth's gravity field * Observed the temperature of the ocean and main seas for over a period of 10 years TOPEX/Poseidon was launched using an Ariane 42P expendable launch vehicle, along with Korea Institute of Technology's Kitsat-1 satellite and France's S80/T satellite . Lift-off from Kourou in

French Guiana French Guiana, or Guyane in French, is an Overseas departments and regions of France, overseas department and region of France located on the northern coast of South America in the Guianas and the West Indies. Bordered by Suriname to the west ...

took place on 1992-08-10. At lift-off the mass of the satellite was . The mission was named after the ocean TOPography EXperiment and the Greek god of the ocean

Poseidon Poseidon (; ) is one of the twelve Olympians in ancient Greek religion and mythology, presiding over the sea, storms, earthquakes and horses.Burkert 1985pp. 136–139 He was the protector of seafarers and the guardian of many Hellenic cit ...

. In October 2005 after more than 62,000 orbits, TOPEX/Poseidon stopped providing science data after a momentum wheel malfunctioned, and the satellite was turned off on January 18, 2006.

Use of results

TOPEX/Poseidon's data have been the subject of more than 2,100 research publications. Some of the areas in which the data are used include: * Climate Research * Coral Reef Research * El Niño & La Niña Forecasting * Fisheries Management * Hurricane Forecasting * Marine Mammal Research * Offshore Industries * Ship Routing

Measurements continue

TOPEX/Poseidon's follow-on mission, Jason-1, was launched in 2001 to continue the ongoing measurements of sea surface topography. The two satellites, TOPEX/Poseidon and Jason-1, flew in a tandem mission for three years providing twice the coverage of the sea surface and allowing scientists to study smaller features than could be seen by one satellite. The record of global sea surface height begun by TOPEX/Poseidon and Jason-1 continues into the future with the Ocean Surface Topography Mission on the Jason-2 satellite, which launched in June 2008. The Jason-3 mission launched January 17, 2016.

Instruments

TOPEX/Poseidon flew two onboard altimeters sharing the same antenna, but only one altimeter was operated at any time, with TOPEX given preference (on average 9 in 10 cycles during the first 10 years of the mission). * TOPEX: The

-built

Nadir The nadir is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direction opposite of the nadir is the zenith. Et ...

pointing

Radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

Altimeter using C band (5.3 GHz) and

Ku band The Ku band () is the portion of the electromagnetic spectrum in the microwave range of frequencies from 12 to 18 gigahertz (GHz). The symbol is short for "K-under" (originally ), because it is the lower part of the original NATO K ban ...

(13.6 GHz) for measuring height above

sea A sea is a large body of salt water. There are particular seas and the sea. The sea commonly refers to the ocean, the interconnected body of seawaters that spans most of Earth. Particular seas are either marginal seas, second-order section ...

surface. * Poseidon: The CNES-built solid state

pointing

Altimeter using

(13.65 GHz). In addition to the altimeters, the TOPEX Microwave

Radiometer A radiometer or roentgenometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Generally, a radiometer is an infrared radiation detector or an ultraviolet detector. Microwave radiometers operate in the micro ...

(TMR) operating at 18, 21, and 37 GHz was used to correct for atmospheric wet path delay. The satellite was also equipped with instruments to accurately pinpoint its location. Precise orbit determination is crucial because errors in locating the spacecraft would distort the sea level measurement calculated from the altimeter readings. Three independent tracking systems determined the position of the spacecraft. The first, the NASA laser retroreflector array (LRA) reflected laser beams from a network of 10 to 15 ground-based laser ranging stations under clear skies. The second, for all-weather, global tracking, was provided by the CNES Doppler Orbitography and Radiopositioning Integrated by Satellite tracking system receiver ( DORIS). This device uses microwave doppler techniques (changes in radio frequency corresponding to relative velocity) to track the spacecraft. DORIS consists of an on-board receiver and a global network of 40 to 50 ground-based transmitting stations. The third system used an on-board experimental Global Positioning System (GPS) demonstration receiver to precisely determine the satellite's position continuously by analyzing the signals received from the U.S. Air Force's GPS constellation of Earth-orbiting satellites. TOPEX/Poseidon was the first mission to demonstrate that the Global Positioning System could be used to determine a spacecraft's exact location and track it in orbit. Knowing the satellite's precise position to within 2 centimeters (less than 1 inch) in altitude was a key component in making accurate ocean height measurements possible. A number of satellites (See links) use exotic dual-band

radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

altimeters to measure height from a

spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...

. That measurement, coupled with

orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

al elements (possibly from GPS), enables determination of the

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

. The two lengths of radio waves permit the altimeter to automatically correct for varying delays in the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

Gallery

References

External links

Official site at CNES

Official site at NASA

Current technical page at NASA

University of Colorado Sea Level Change site

NASA TOPEX/Poseidon fact sheet
* TOPEX/Poseidon and Jason-1 data are available at ''http://sealevel.colorado.edu/'' and ''http://podaac.jpl.nasa.gov/''. {{DEFAULTSORT:TOPEX Poseidon Earth observation satellites of the United States Satellites of France NASA satellites 1992 in France Earth satellite radar altimeters Spacecraft launched in 1992 Spacecraft launched by Ariane rockets August 1992 in France