The altimeter antenna is used by the NASA and the CNES radar altimeters carried in TOPEX/Poseidon's Instrument Module (IM). The antenna is a 1.5 meter diameter parabolic dish and is fixed on the nadir face of the IM surrounded by the Laser Retroreflector Array.
The dual-frequency NASA radar altimeter is the primary instrument aboard the spacecraft. It works 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 can calculate the height of the sea surface to within 4.3 centimeters.
The single-frequency CNES altimeter, like the GPS receiver, is classified as an experimental sensor because TOPEX/Poseidon is the first flight to utilize this technology. The CNES altimeter is a solid-state, low-power, low-mass sensor which works in much the same way as the NASA altimeter. Its precision is also comparable with that of the NASA altimeter.
