The European Remote Sensing Satellite ERS-1 carries an Active Microwave Instrument (AMI), which combines the functions of a Synthetic Aperture Radar (SAR) and a Wind Scatterometer. This instrument is operated in either SAR or Wind mode. Another possible working mode is the Wind/wave mode which consist of nominal Scatterometer operations (Wind mode) interrupted every 30 seconds (roughly 200 Km) by a couple of seconds of a short SAR operation in order to acquire small SAR imagettes of roughly 6 x 6 Km from which the wave spectra can be derived.
Wind Scatterometer Mode
The Wind Scatterometer uses three sideways looking antennae, one pointing normal to the satellite flight path, one pointing 450 forward and the third pointing 450 backwards. The antenna beams illuminate a swath 500 km wide as the satellite advances along its orbit and each provides measurements of radar backscatter from the sea surface for overlapping 50 km resolution cells using 25km grid-spacing. The result is three independent backscatter measurements relating to cell centre nodes on a 25 km-grid, which have been obtained using the three different viewing directions and are separated by only a very short time delay.