TOPEX/Poseidon was a joint

satellite altimeter Satellite geodesy is geodesy by means of artificial satellites—the measurement of the form and dimensions of Earth, the location of objects on its surface and the figure of the Earth's gravity field by means of artificial satellite techniques. ...

mission between

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

, the U.S. space agency; and

CNES The (CNES; French: ''Centre national d'études spatiales'') is the French government space agency (administratively, a "public administration with industrial and commercial purpose"). Its headquarters are located in central Paris and it is und ...

, the French space agency, to map

ocean surface topography Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map. These variations are exp ...

. 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 Seasat was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board one of the first spaceborne synthetic-aperture radar (SAR). The mission was designed to demonstrate the feasibility of global sa ...

satellite. TOPEX/Poseidon's radar

altimeter An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...

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 standardise ...

with an unprecedented accuracy * Mapped global tides 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 ...

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 Niño (; ; ) is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (approximately between the International Date ...

, La Niña, and the

Pacific Decadal Oscillation The Pacific decadal oscillation (PDO) is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20°N. O ...

* 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 An expendable launch system (or expendable launch vehicle/ELV) is a launch vehicle that can be launched only once, after which its components are either destroyed during reentry or discarded in space. ELVs typically consist of several rocket st ...

, along with Korea Institute of Technology's

Kitsat-1 KITSAT-1 or KITSAT-A (Korean Institute of Technology Satellite) is the first satellite to be launched for the South Korea. Once launched, the satellite was given the nickname "Our Star" ( 우리별). The KITSAT-1 is a low Earth orbit (LEO) sate ...

satellite and France's S80/T satellite . Lift-off from

Kourou Kourou () is a commune in French Guiana, an overseas region and department of France in South America. Kourou is famous for being the location of the Guiana Space Centre, the main spaceport of France and the European Space Agency (ESA). It i ...

French Guiana French Guiana ( or ; french: link=no, Guyane ; gcr, label=French Guianese Creole, Lagwiyann ) is an overseas department/region and single territorial collectivity of France on the northern Atlantic coast of South America in the Guianas. ...

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 Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

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 ...

. In October 2005 after more than 62,000 orbits, TOPEX/Poseidon stopped providing science data after a

momentum wheel A reaction wheel (RW) is used primarily by spacecraft for three-axis attitude control, and does not require rockets or external applicators of torque. They provide a high pointing accuracy, and are particularly useful when the spacecraft must be ...

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 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 s ...

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 mission launched January 17, 2016.

Instruments

TOPEX/Poseidon flew two onboard

s 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 (, ; ar, نظير, naẓīr, counterpart) 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 direc ...

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, we ...

Altimeter An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...

using C band (5.3 GHz) and Ku band (13.6 GHz) for measuring height above sea surface. * Poseidon: The

-built solid state

pointing

using Ku band (13.65 GHz). In addition to the altimeters, the TOPEX Microwave Radiometer (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, we ...

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 ...

al elements (possibly from

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

), 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 sc ...

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

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