The TOPEX/Poseidon GPS is based on a network of 21 military satellites in circular orbits 20,200 kilometers
above the earth. These satellites broadcast signals that allow the GPS receiver aboard TOPEX/Poseidon to
calculate the spacecraft's position in orbit. With augmented ground processing, this position is accurate
to within 3 centimeters.
The Global Positioning System antenna shown here is connected to a GPS Demonstration Receiver housed
within the Instrument Module. The GPS receiver was included as an experiment because its use in
Precision Orbit Determination for a spacecraft had never been demonstrated.
The GPS receiver aboard TOPEX/Poseidon normally does not require commanding or special attention from
the ground, and is always tracking. In the event that the receiver loses contact with the GPS satellites,
ground-initiated commands are necessary to restart the on-board processor to enable it to re-acquire the
GPS signal.
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