Editing TOPEX/Poseidon (section)

== 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]].