OVERVIEW

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Altimeter(s)

OSTM/Jason-2
The Poseidon-3 radar altimeter is the main instrument on the Jason-2 mission. Derived from the Poseidon-1 altimeter on Topex/Poseidon and the Poseidon-2 on Jason-1, it measures sea level, wave heights and wind speed. The Poseidon-3 altimeter emits pulses at two frequencies 13.6 and 5.3 GHz to measure the distance from the satellite to the surface (range). Free electrons in the atmosphere can delay the signal's return, affecting the measurement accuracy. The delay is directly related to the radar frequency, so the difference between the two measurements can be used to determine atmospheric electron content. Poseidon-3 is coupled with Doris/Diode, to improve measurements over coastal areas, inland waters and ice. Additional information on the Poseidon-3 instrument is available on the AVISO site.
Jason-1
The POSEIDON-2 altimeter is the main instrument on the Jason-1 mission. Derived from the POSEIDON-1 altimeter on TOPEX/Poseidon, it measures sea level, wave heights and wind speed. It operates at two frequencies and is also able to estimate atmospheric electron content. The altimeter emits a radar beam that is reflected back to the antenna from the Earth's surface. POSEIDON-2 operates at two frequencies (13.6 GHz in the Ku-band, 5.3 GHz in the C-band) to determine atmospheric electron content, which affects the radar signal path delay. These two frequencies also serve to measure the amount of rain in the atmosphere. Additional information on the Poseidon-2 instrument is available on the AVISO site.
TOPEX/Poseidon
The dual-frequency NASA radar altimeter, NRA, 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, POSEIDON-1, 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. It shares the same antenna as the NRA; thus only one altimeter operates at any given time. It operates about 10% of the time, or one cycle over ten. Its precision is also comparable with that of the NASA altimeter.