View source for TOPEX/Poseidon

{{short description|Satellite mission to map ocean surface topography}}
{{Infobox spaceflight
| name                  = TOPEX/Poseidon
| image                 = TOPEX;Poseidon.jpg
| image_size            = 275px
| image_caption         = Artist's rendering of the TOPEX/Poseidon satellite.

| mission_type          = [[Remote sensing]]
| operator              = [[NASA]] and [[CNES]]
| website               = 
| COSPAR_ID             = 1992-052A
| SATCAT                = 22076
| mission_duration      = Achieved: {{time interval|10 August 1992|January 2006|show=ymd|sep=,}}<br/> In Orbit: {{time interval|August 10, 1992|show=ymd|sep=,}} 

| spacecraft_bus        = 
| manufacturer          = 
| dimensions            = 
| dry_mass              = 
| launch_mass           = {{convert|2400|kg}}
| power                 =

| launch_date           = {{start-date|10 August 1992}}
| launch_rocket         = [[Ariane (rocket)|Ariane]] 42P
| launch_site           = ''[[Guiana Space Centre]]'', [[Kourou]]
| launch_contractor     =

| declared              = {{end-date|January 18, 2006}}
| deactivated           = October 2005

| decay_date            =

| orbit_reference       = [[geocentric orbit|Geocentric]]
| orbit_regime          = Non [[Sun-synchronous orbit|Sun-synchronous]]
| orbit_periapsis       = {{convert|1340|km|mi|sp=us}}<ref name="IRL">{{Cite web|url=https://ilrs.cddis.eosdis.nasa.gov/missions/satellite_missions/past_missions/topx_general.html|title=TOPEX/Poseidon|website=International Laser Ranging Service}}</ref>
| orbit_apoapsis        = {{convert|1340|km|mi|sp=us}}<ref name="IRL" />
| orbit_inclination     = 66&nbsp;degrees<ref name="IRL" />
| orbit_eccentricity    = 0.000<ref name="IRL" />
| orbit_period          = 112&nbsp;minutes<ref name="IRL" />
| apsis                 = gee

| insignia              = 
| insignia_size         = 
| insignia_caption      = 
}}

'''TOPEX/Poseidon''' was a joint [[satellite altimeter]] mission between [[NASA]], 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."<ref>Munk.W.: Testimony Before the U.S. Commission on Ocean Policy, April 2002, http://govinfo.library.unt.edu/oceancommission/meetings/apr18_19_02/munk_statement.pdf</ref> A malfunction ended normal satellite operations in January 2006.<ref name="NASA" />

==Description==

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

=== Goal ===
The mission's most important achievement was to determine the patterns of ocean circulation - how [[ocean heat content|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.<ref>{{cite web | url = http://www.jpl.nasa.gov/news/features.cfm?feature=973| title = Topex/Poseidon Sails Off Into the Sunset | publisher = NASA/JPL}}</ref>

== Results ==
While a three-year prime mission was planned, TOPEX/Poseidon delivered more than 10 years of data from orbit.<ref>{{cite web | url = http://topex-www.jpl.nasa.gov/mission/topex.html | archive-url = https://web.archive.org/web/20011023012406/http://topex-www.jpl.nasa.gov/mission/topex.html | url-status = dead | archive-date = 2001-10-23 | title = Ocean Surface Topography from Space | publisher = NASA/JPL }}</ref> In those years, the mission:

* Measured [[sea level]] with an unprecedented accuracy
* Mapped global [[tides]] for the first time
* Monitored effects of currents on global [[climate change]] 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 (rocket)|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]] took place on 1992-08-10. At lift-off the mass of the satellite was {{convert|2402|kg|lb}}.<ref name=nssdc>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1992-052A |title=Topex/Poseidon - NSSDC ID: 1992-052A |publisher=NASA}}</ref> The mission was named after the ocean TOPography EXperiment and the [[Ancient Greece|Greek]] god of the ocean [[Poseidon]].

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.<ref name="NASA">{{cite web | url = http://www.nasa.gov/home/hqnews/2006/jan/HQ_06001_TOPEX_update.html| title = NASA's Topex/Poseidon Oceanography Mission Ends | publisher = NASA}}</ref>

=== Use of results ===
TOPEX/Poseidon's data have been the subject of more than 2,100 research publications.<ref name="NASA" />
Some of the areas in which the data are used include:<ref>{{cite web|title=Ocean Surface Topography from Space|url=http://topex-www.jpl.nasa.gov/overview/benefits.html|url-status=dead|archive-url=https://web.archive.org/web/20011230142820/http://topex-www.jpl.nasa.gov/overview/benefits.html|archive-date=2001-12-30|publisher=NASA/JPL}}</ref>

* Climate Research
* Coral Reef Research
* [[El Niño Southern Oscillation|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]],<ref>{{cite web | url = http://topex-www.jpl.nasa.gov/mission/jason-1.html | archive-url = https://web.archive.org/web/20011011234534/http://topex-www.jpl.nasa.gov/mission/jason-1.html | url-status = dead | archive-date = 2001-10-11 | title = Ocean Surface Topography from Space | publisher = NASA/JPL }}</ref> 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.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/ostm/news/ostm-20080620.html |title=NASA Launches Ocean Satellite to Keep a Weather, Climate Eye Open |publisher=NASA}}</ref> The [[Jason-3]] mission launched January 17, 2016.<ref>{{cite web|url=http://www.eumetsat.int/idcplg?IdcService=GET_FILE&dDocName=PDF_OST_WORKSHOP_CONCLUSIONS&RevisionSelectionMethod=LatestReleased |title=CEOS Ocean Surface Topography (OST) Constellation Strategic Workshop |publisher=Eumetsat |url-status=dead |archive-url=https://web.archive.org/web/20081114124055/http://www.eumetsat.int/idcplg?IdcService=GET_FILE&dDocName=PDF_OST_WORKSHOP_CONCLUSIONS&RevisionSelectionMethod=LatestReleased |archive-date=November 14, 2008 }}</ref><ref>{{cite web |url=https://sealevel.jpl.nasa.gov/missions/jason3/ |archive-url=https://web.archive.org/web/20110813232116/http://sealevel.jpl.nasa.gov/missions/jason3/ |url-status=dead |archive-date=2011-08-13 |title=Jason-3 |publisher=NASA JPL }}</ref>

== Instruments ==
TOPEX/Poseidon flew two onboard [[altimeter]]s sharing the same [[antenna (radio)|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 [[NASA]]-built [[Nadir]] pointing [[Radar]] [[Altimeter]] using [[C band (IEEE)|C band]] (5.3&nbsp;GHz) and [[Ku band]] (13.6&nbsp;GHz) for measuring height above [[sea]] surface.
* Poseidon: The [[CNES]]-built solid state [[Nadir]] pointing [[Radar]] [[Altimeter]] using [[Ku band]] (13.65&nbsp;GHz).

[[Image:Poseidon.graphic.jpg|thumb|The accurate determination of the ocean height is made by first characterizing the precise height of the spacecraft above the center of the Earth.]]In addition to the altimeters, the TOPEX Microwave [[Radiometer]] (TMR) operating at 18, 21, and 37&nbsp;GHz was used to correct for atmospheric wet path delay.{{clarify|date=October 2017}}

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 (geodesy)|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&nbsp;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]] altimeters to measure height from a [[spacecraft]]. That measurement, coupled with [[orbit]]al elements (possibly from [[GPS]]), enables determination of the [[topography]].  The two lengths of radio waves permit the altimeter to automatically correct for varying delays in the [[ionosphere]].

== Gallery ==
{{Gallery
|width=160 | height=170
|align=center
|File:Ariane42P rocket.png
 |The Ariane 4 rocket, with TOPEX/Poseidon on board.
|Image:1997 El Nino TOPEX.jpg
 |The [[1997–98 El Niño event|1997-98 El Nino]]
|Image:TOPEX-JasonSeries2008.jpg 
 |Jason-1 continued the same sea surface measurements begun by TOPEX/Poseidon, later succeeded by the Ocean Surface Topography Mission on Jason-2, and then Jason-3.
|File:TOPEX-Poseidon-drawing.png
 |Diagram of the various systems on board.
}}

{{Clear}}

==See also==
{{Portal|Spaceflight}}
* [[French space program]]
* [[1992 in spaceflight]]
* [[2004 Indian Ocean earthquake#Energy of the earthquake|2004 Indian Ocean earthquake]]
* [[Jason 1]]
* [[Ocean Surface Topography Mission/Jason-2]]
* [[Seasat]]
* [[Sea level rise]]

==References ==
{{reflist|2}}

==External links==
{{Commons category|TOPEX/Poseidon}}
* [https://web.archive.org/web/20090503223608/http://www.cnes.fr/web/CNES-en/1461-topexposeidon.php Official site at CNES]
* [http://sealevel.jpl.nasa.gov/ Official site at NASA]
* [https://web.archive.org/web/20090730205052/http://sealevel.jpl.nasa.gov/technology/technology.html TOPEX/Poseidon technical page at NASA (archived)]
* [http://sealevel.jpl.nasa.gov/technology/ Current technical page at NASA]
* [http://sealevel.colorado.edu/ University of Colorado Sea Level Change site]
* [https://web.archive.org/web/20110813220106/http://sealevel.jpl.nasa.gov/missions/topex/topexfactsheet/ NASA TOPEX/Poseidon fact sheet]
* TOPEX/Poseidon and Jason-1  data are available at ''http://sealevel.colorado.edu/'' and ''http://podaac.jpl.nasa.gov/''.

{{Jet Propulsion Laboratory}}
{{Orbital launches in 1992}}

{{DEFAULTSORT:TOPEX Poseidon}}
[[Category:Earth observation satellites of the United States]]
[[Category:Satellites of France]]
[[Category:NASA satellites]]
[[Category:1992 in France]]
[[Category:Earth satellite radar altimeters]]
[[Category:Spacecraft launched in 1992]]
[[Category:Spacecraft launched by Ariane rockets]]
[[Category:August 1992 in France]]