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Ocean Surface Topography from Space
T/P's "Triple Tracking"
TOPEX/Poseidon Measurement System

  • A ground-based beacon called "DORIS" sends radio signals to a satellite receiver that measures the "Doppler." The "Doppler effect" is what causes the distinctive sound pattern you hear when a car passes. The distance between your ear and the car changes as it passes and so does the frequency of the sound waves generated by the car;
  • Light from ground-based lasers is reflected off mirrors on the T/P satellite; and
  • The Global Positioning System (GPS) network is used. At times, a T/P receiver tracks several GPS satellites simultaneously and, because the position of GPS satellites is well-known, T/P's position can be calculated. This was originally only an experiment but is now a very reliable data source.

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