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Topex altimeter, or NRA

The Topex altimeter, or NRA (206 kg including redundancy, 237 W), operating simultaneously at 13.6 GHz (Ku-band) and 5.3 GHz (C-band) was provided by Nasa. It is a fifth generation altimeter, whose design is based on the previous Seasat and Geosat altimeters with significant improvements including the 5.3 GHz channel for the ionospheric measurement. It was the main sensor on the Topex/Poseidon mission. The measurements made at both frequencies were combined to obtain the altimetry height of the satellite above the sea (satellite range), wind speed modulus, significant wave height and ionospheric correction.


Topex measures range (the distance from the satellite to the Earth's surface), wave height and wind speed.


The altimeter emits a radar beam that is reflected back to the antenna from the Earth's surface (see how altimetry works for details). Topex operates at two frequencies (13.6 GHz in the Ku-band and 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.

Technical data

Emitted Frequency (GHz)

Dual-frequency (Ku, C) - 13.575 and 5.3

Pulse Repetition Frequency (Hz)

4200 (Ku), 1220 (C)

Pulse duration (microseconds)

102.4 (Ku), 102.4 or 32 (C)

Bandwidth (MHz)

320 (Ku), 320 or 100 (C)

Antenna diameter (m)


Antenna beamwidth (degrees)

1.1 (Ku), 2.7 (C)

Power (W)




Specific features

Dual-frequency for ionospheric correction

Further information on Instrument Description: Altimeter(s) (Nasa/JPL website)

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