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 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.
