The dual-frequency NASA radar altimeter, NRA, was the primary instrument aboard the spacecraft. It worked by sending radio pulses at 13.6 GHz and 5.3 GHz toward the earth and measuring the characteristics of the echo. By combining this measurement with data from the microwave radiometer and with other information from the spacecraft and the ground, scientists were able to calculate the height of the sea surface to within 4.3 centimeters.
The single-frequency CNES altimeter, Poseidon-1, like the GPS receiver, is classified as an experimental sensor because TOPEX/Poseidon was the first flight to utilize this technology. The CNES altimeter is a solid-state, low-power, low-mass sensor which worked in much the same way as the NASA altimeter. It shared the same antenna as the NRA; thus only one altimeter operated at any given time.
The TOPEX/Poseidon microwave radiometer was a three-frequency sensor used to estimate the atmospheric water vapor content in the nadir column through which the altimeter signal is traveling. Since water vapor distorts the altimeter's reading, the water vapor content is measured to correct the altimetry measurement. The sensor uses one frequency to make the measurement, and two frequencies to remove the effects of wind speed and cloud cover. There is a backup receiver for the measurement frequency.
The DORIS antenna was connected to the DORIS receiver inside the Instrument Module. The function of the receiver was to listen for signals from ground stations. By using the Doppler shift of these signals the satellite's position was determined.
Currently, the CNES DORIS system has about 60 ground beacons distributed evenly around the globe. DORIS measurements are taken about every ten seconds.
Combining the DORIS information with that of the Laser Ranging System gave TOPEX/Poseidon an unprecedented precision orbit determination capability.
Laser Retroreflector Array (LRA)
The TOPEX/Poseidon LRA, was a set of 192 quartz corner-cubes mounted in two concentric rings around the altimeter antenna.
The array was a completely passive unit which reflects laser beams originating from one of about thirty ground-based laser tracking stations positioned around the earth. By measuring the length of time the laser beam takes to travel to the spacecraft and back, scientists were able to calculate TOPEX/Poseidon's orbital radial position to within 3 centimeters.
Global Positioning System (GPS)
The TOPEX/Poseidon GPS was based on a network of 24 military satellites in circular orbits 20,200 kilometers above the earth. These satellites broadcast signals that allowed the GPS receiver aboard TOPEX/Poseidon to calculate the spacecraft's position in orbit. With augmented ground processing, this position was 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.