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 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. Gallery See also French space program 1992 in spaceflight 2004 Indian Ocean earthquake Jason 1 Ocean Surface Topography Mission/Jason-2 Seasat Sea level rise References External links Official site at CNES Official site at NASA TOPEX/Poseidon technical page at NASA (archived) 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/. 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