TOPEX/Poseidon
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The TOPEX/Poseidon satellite altimeter was a science project to measure the ocean surface topography. It had (near) global coverage from 66°N to 66°S (almost all of the ice-free oceans) from late 1992 to the end of 2005. The accuracy of TOPEX/POSEIDON data allowed global average sea level to be estimated to a precision of several millimetres every 10 days. It stopped providing science data in October 2005, after a momentum wheel malfunctioned and was turned off on January 18, 2006.
It was a joint project between the CNES (France) and NASA (United States) space agencies and was named after the ocean TOPography EXperiment and the Greek god Poseidon.
The satellite 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 NASA built Nadir pointing Radar Altimeter using C band (5.3 GHz) and Ku band (13.6 GHz) for measuring height above sea surface.
- Posideon: The CNES built solid state Nadir pointing Radar Altimeter using Ku band (13.65 GHz).
In addition to the altimeters, the TOPEX Microwave Radiometer [1] (TMR) operating at 18, 21, and 37 GHz was used to correct for atmospheric wet path delay.
The successor of TOPEX/Poseidon is Jason 1.
Data from TOPEX/Poseidon indicate that the average sea level has risen at about 3 mm/yr since 1992.
Historical data from this satellite were also used to show that the northern gyre of the Gulf Stream is slowing.
[edit] Use of information
A number of satellites (See links) use exotic dual-band radar altimeters to measure height from a spacecraft. That measurement, coupled with orbital 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.
Over water, detailed satellite altitude information has proven amazingly useful. Humps in the water indicate gravitational concentrations, permitting a computer program to construct a maps of undersea features such as mountains. The altimeters can also measure wave heights, wave directions, and wave spectra. This information permits computer programs to measure the speed of ocean currents and produce detailed maps of wind speeds and directions at the surface, even in extremely stormy conditions.
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