The Radar Altimeter (RA) is a Ku-band (13.8 GHz) nadir-pointing active microwave sensor designed to measure the return trip time for echoes from ocean and ice surfaces. Functioning in one of two operational modes (ocean or ice) the Radar Altimeter provides information on significant wave height, surface wind speed, sea surface elevation (relating to ocean currents, the surface geoid and tides) and various parameters over sea ice and ice sheets.

Function

The Radar Altimeter is a Ku-band (13.8 GHz) nadir-pointing active microwave sensor designed to measure the time return echoes from ocean and ice surfaces. Functioning in one of two operational modes (ocean or ice) the Radar Altimeter provides information on significant wave height; surface wind speed; sea surface elevation, which relates to ocean currents, the surface geoid and tides; and various parameters over sea ice and ice sheets.

 

Principle

The altimeter emits a radar beam that is reflected back to the antenna from the Earth’s surface. RA operates at a single frequency (13.6 GHz in the Ku-band).

The Radar Altimeter operates by timing the two-way delay for a short duration radio frequency pulse, transmitted vertically downwards. The level of accuracy required for range measurement (greater than 10 cm) calls for a pulse compression (full deramp) technique. In ocean mode a chirped pulse of 20 microseconds duration is generated with a bandwidth of 330 MHz. For tracking in ice mode an increased dynamic range is used, obtained by reducing the chirp bandwidth by a factor of four to 82.5 MHz, though this also results in a coarser resolution (see how altimetry works for details).

Technical data

Emitted Frequency (GHz) Single-frequency (Ku) – 13.8
Pulse Repetition Frequency (Hz) 1020
Pulse duration (microseconds) 20
Bandwidth (MHz) 330 and 82.5
Antenna diameter (m) 1.2
Antenna beamwidth (degrees) 1.3
Power (W) 50
Redundancy Yes
Specific features 2 bandwidths for ocean and ice measurements

Further information on RA instrument (ESA website)