TOPEX/Poseidon flew two onboard altimeters sharing the same antenna, but only one altimeter was operated at any time, with TOPEX given preference (on average 9 in 10 cycles during the first 10 years of the mission).
TOPEX: The NASA-built Nadir pointing Radar Altimeter using C band (5.3 GHz) and Ku band (13.6 GHz) for measuring height above sea surface.
Poseidon: The CNES-built solid state Nadir pointing Radar Altimeter using Ku band (13.65 GHz).
Poseidon.graphic.jpg
In addition to the altimeters, the TOPEX Microwave Radiometer [1] (TMR) operating at 18, 21, and 37 GHz was used to correct for atmospheric wet path delay.
The satellite was also equipped with instruments to accurately pinpoint its location. Precise orbit determination is crucial because errors in locating the spacecraft would distort the sea level measurement calculated from the altimeter readings.