The dual-frequency NASA radar altimeter (NRA) was the primary instrument aboard the joint NASA/French Space Agency (CNES) TOPEX/Poseidon spacecraft. The altimeter sent radio pulses at 13.6 GHz and 5.3 GHz toward Earth and measured the characteristics of the return echo. By combining this measurement with data from the spacecraft's 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.
