TECHNOLOGY From its vantage point 1336 kilometers (830 miles) above the Earth, TOPEX/Poseidon can measure the height of the ocean surface directly underneath the satellite with an accuracy of 4-5 centimeters (better than 2 inches). Traveling in excess of 7 kilometers (4 miles) every second as it traces out its orbit, TOPEX/Poseidon covers the global oceans every 10 days (the "repeat period" of the satellite orbit).  The accurate determination of the ocean height is made by first characterizing the precise height of the spacecraft above the center of the Earth. This is achieved through a technique called "precise orbit determination" (POD), of which satellite-tracking information is the most important ingredient. The baseline tracking system for TOPEX/Poseidon is the onboard NASA retroreflector array, which serves as a target for 10-20 satellite laser ranging (SLR) stations that dot the Earth's surface. The CNES DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) system provides an important additional set of tracking data. Anchored by approximately 50 ground-based beacons, the DORIS receiver measures the Doppler shift of microwave signals to support POD. NASA's experimental GPS receiver onboard TOPEX/Poseidon provides precise, continuous tracking of the spacecraft by monitoring range and timing signals from up to 6 GPS spacecraft at the same time. In order to produce accurate estimates of the satellite orbital height, POD combines the satellite tracking information with accurate models of the forces (e.g., gravity, aerodynamic drag) that govern the satellite motion. For TOPEX/Poseidon, this process supports the determination of the satellite orbital height with an accuracy of 2-3 centimeters (1 inch).  The second component of the ocean height measurement is the range from TOPEX/Poseidon to the ocean surface. The satellite carries two radar altimeters for providing this information: the NASA TOPEX instrument beams microwaves at 13.6 and 5.3 GHz; the CNES POSEIDON instrument, at 13.65 GHz. To take a measurement, an onboard altimeter bounces microwave pulses off the ocean surface and measures the time it takes the pulses to return to the spacecraft. This measurement, multiplied by the speed of light, gives the range from the satellite to the ocean surface. After correction for atmospheric and instrumental effects, the TOPEX/Poseidon range measurements are accurate to 3-4 centimeters. The range measurements are subtracted from POD-derived estimates of the satellite orbital height, resulting in ocean height measurements that are good to 4-5 centimeters (better than 2 inches) relative to the center of the Earth.
This accuracy figure pertains to a few-kilometer spot on the ocean surface directly beneath the TOPEX/Poseidon satellite. By averaging the few-hundred thousand measurements collected by the satellite in the time it takes to cover the global oceans (10 days), global mean sea level can be determined with a precision of several millimeters.
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