TOPEX/Poseidon was a joint satellite altimeter mission between

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

, 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 Walter Heinrich Munk (October 19, 1917 – February 8, 2019) was an American physical oceanographer. He was one of the first scientists to bring statistical methods to the analysis of oceanographic data. His work won awards including the Nation ...

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 (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...

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.

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 average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...

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 can ...

for the first time * Monitored effects of currents on global

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

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, 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 Ariane may refer to: *Ariana (name), also Ariane, Arianne Arts * ''Ariane'' (Martinů), an opera by Bohuslav Martinů, first performed 1961 * ''Ariane'' (Massenet), an opera by Jules Massenet, first performed 1906 * ''Ariane'' (film), a 1931 ...

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 ; french: link=no, Guyane ; gcr, label=French Guianese Creole, Lagwiyann ) is an overseas departments and regions of France, overseas department/region and single territorial collectivity of France on the northern Atlantic ...

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 (; grc-gre, Ποσειδῶν) was one of the Twelve Olympians in ancient Greek religion and myth, god of the sea, storms, earthquakes and horses.Burkert 1985pp. 136–139 In pre-Olympian Bronze Age Greece, he was venerated as a ch ...

. 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 Jason-1 was a satellite altimeter oceanography mission. It sought to monitor global ocean circulation, study the ties between the ocean and the atmosphere, improve global climate forecasts and predictions, and monitor events such as El Niño ...

, 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 OSTM/Jason-2, or Ocean Surface Topography Mission/Jason-2 satellite, was an international Earth observation satellite altimeter joint mission for sea surface height measurements between NASA and CNES. It was the third satellite in a series st ...

satellite, which launched in June 2008. The

Jason-3 Jason-3 is a satellite altimeter created by a partnership of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and National Aeronautic and Space Administration (NASA), and is an international cooperative mis ...

mission launched January 17, 2016.

Instruments

TOPEX/Poseidon flew two onboard altimeters sharing the same

antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...

, 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 pointing

Radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

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 german: Kurz-unten), because it is the lower part of the ori ...

(13.6 GHz) for measuring height above

sea The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, ...

surface. * Poseidon: The CNES-built solid state Nadir 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 microwave w ...

(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 detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

altimeters to measure height from a

spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...

. That measurement, coupled with

orbit In celestial mechanics, an orbit 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 object or position in space such as a p ...

al elements (possibly from

GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...

), 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 land forms 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 an ...

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