TOPEX/Poseidon is based on earlier earth-orbiting satellite missions. The earliest missions provided proof of the concept that some ocean observations can be made more economically and with better coverage from space than from the earth's surface. Each mission carried a different complement of instruments and discovered new facts that inspired successive missions and generations of spacecraft.
TOPEX/Poseidon's family tree begins with the Geodynamics Experimental Ocean Satellite, GEOS-3. Launched in 1975, it carried the first altimeter intended for long term operations, provided over three years of gravitational and oceanographic data, and produced the first maps of the marine geoid. The GEOS-3 altimetric system measured sea level with a precision of approximately 20 centimeters and an accuracy of better than 50 centimeters.
The next satellite, Seasat, was the first designed to assess and prove the value of microwave sensors for remote examination of the world's oceans. Launched in 1978, Seasat carried five instruments that measured sea surface winds and temperatures, ocean wave heights, internal waves, atmospheric water content, sea ice, topography of the ocean surface, and the shape of the marine geoid. Seasat demonstrated that sea level could be measured with a precision of 10 centimeters at an accuracy of better than 30 centimeters, a significant improvement over GEOS-3. Unfortunately, the mission ended prematurely after about 100 days of operation.
In 1985 the U.S. Navy launched the Geodesy Satellite or Geosat. Geosat made global measurements of fronts, eddies, winds, waves, sea ice extent, and surface currents and produced a complete map of the marine geoid. Geosat's accuracy and precision were comparable to Seasat. The mission lasted for approximately 4 years.
