TECHNOLOGY (continued)
To verify the accuracy of measurements independently, CNES and NASA each established
a verification site along the ground track of the satellite. The CNES verification
site is on the French island of Corsica in the Mediterranean Sea; the NASA site is
on Platform Harvest, an Arguello Inc. oil rig at the eastern entrance to the Santa
Barbara Channel, off the coast of central California. Each site is equipped with
tracking systems (e.g., GPS, SLR) that enable accurate surveying of the stations
into the reference frame that underlies the satellite orbit computations (POD). This
survey information is combined with local measurements of the water level
(from "tide gauges") to derive an independent estimate of the ocean height relative
to the center of the Earth. Every 10 days the TOPEX/Poseidon satellite flies directly
overhead, and the two independently derived measurements of the instantaneous sea level
(satellite vs. "ground truth") are compared in an analysis called "closure." With
sufficiently redundant instrumentation at the experiment site, along with careful
monitoring of potential systematic errors, any unexpected mismatch between the satellite
and "ground truth" measurements can be attributed to an error in the satellite
measurements. The "closure" data from Corsica and Harvest, along with similar
information from other verification sites and observing programs, are used to
continuously monitor the performance of the TOPEX/Poseidon measurement system.
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Configuration in Corsica
Image Credit: OCA-CERGA/CNES
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Platform Harvest
Image Credit: Daniel G. Kubitschek (JPL)
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