RA
type: Concept
in scheme: instruments
definition: The Radar Altimeter (RA) is one of the instruments carried on-board of the European Remote Sensing Satellites (ERS-1 and ERS-2) launched by the European Space Agency on 17 July 1991 and 20 April 1995. ERS-1 and ERS-2, operating in a sun-synchronous orbit, were conceived as an orbiting platform that would be capable of measuring the Earth's atmospheric and surface properties with a high degree of accuracy on a global scale. The RA is a nadir-pointing active microwave sensor designed to make precise measurements of the echoes from ocean and ice surfaces. It provides information on: - significant wave height - surface wind speed - sea surface elevation, which relates to ocean currents, the surface geoid and tides - various parameters over sea ice and ice sheets The RA is relatively simple in concept, but depends on electronic precision and a sophisticated data processor to achieve its performance. The RA operates by timing the two-way delay for a short duration radio frequency (RF) pulse, transmitted vertically downwards. The required level of accuracy in range measurements (better than 10 cm) calls for a pulse length of a few nsec, therefore, in order to reduce the RF power requirements a pulse compression (chirp) technique is used. Instrument characteristics: ------------------------------------------------------------ Frequency: 13.8 GHz Bandwidth: 330 MHz (ocean mode); 82.5 MHz (ice mode) Beamwidth: 1.3 degrees at 3 dB Footprint: up to 20 km (depending on sea state) Mass: 96 kg Antenna diameter: 1.2 m Pulse length: 20 ms chirp RF transmit power: 50 W Pulse repetition frequency: 1020 Hz ------------------------------------------------------------ The RA operates in three modes: - acquisition mode, during which the radar finds the approximate distance to the surface and then switches to one of the tracking modes - ocean tracking mode - ice tracking mode where an increased dynamic range is used, obtained by reducing the chirp bandwidth by a factor of four to 82.5 MHz,…

Radar Altimeters
type: Concept