Radar Altimeters
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, resulting in a coarser resolution Echo characteristics are analysed with respect to: - time delay of return pulse, providing altitude measurements - slope of the echo leading edge, relating to wave height parameters - power level of return signal, affected by small scale surface roughness giving an indication of surface wind field parameters Over the ocean the waveform profile is sufficiently well understood to permit real-time estimates of ocean parameters to be carried out on-board the satellite. For other surfaces the waveform shape does not always conform to a simple model and further data analysis is necessary. The return echo from sea ice appears more specular than from the ocean and has a peaked trace. The information provided by the RA, in accordance with the objective of the ERS programme, is of significant importance to the commercial and scientific user communities, with marked benefits for ocean related activities, including ship routing and the design of offshore facilities. Infact the RA was designed to permit the following: - Ice mapping and monitoring - Weather and sea state forecast - Sea surface topography and ocean currents - Experimental altimetry over land Related URL: "http://earth.esa.int/ra" Reference online documentation: "http://earth.esa.int/services/esa_doc/doc_ra_.html" For any query, please refer to: ESA/ESRIN Earth Observation Help Desk "http://earth.esa.int" E-mail: eohelp@esa.int Phone: +39 06 94180777 Fax: +39 06 94180292 Address: ESA/ESRIN Via G.Galilei 00044 Frascati Italy