The European Remote Sensing Satellite ERS-2 carries an Active Microwave Instrument (AMI), which combines the functions of a Synthetic Aperture Radar (SAR) and a Scatterometer. The AMI provided radar backscattering coefficient measurements over 16 years by using its three nominal operational acquisition modes: Synthetic Aperture mode (SAR mode), Scatterometer mode (wind mode) and a special combination of the two over ocean where SAR and Scatterometer mode are interleaved (wind/wave mode). The wind/wave mode consists of nominal Scatterometer operation interrupted every 30 seconds by a couple of seconds of short SAR operation in order to acquire small SAR imagettes from which the wave spectra can be derived. The main applications for data acquired in Scatterometer mode is related to the estimation of the wind vector and sea ice presence over the sea surface, soil moisture index over land surface.
The ERS Scatterometer has been designed to obtain information on wind speed and wind direction over sea surfaces. Therefore it is often called Wind Scatterometer. It uses three sideways looking antennas, one looking to the right side of the satellite track (mid beam antenna), one looking forward at 45° azimuth projection angle with respect to the mid beam (fore beam antenna), and one looking backward at 45° azimuth projection angle with respect to the mid beam (aft beam antenna). The three beams illuminate a 500 km wide swath as the satellite moves along its orbit. Thus three backscatter measurements are obtained at different viewing angles and separated by a short time delay. These ‘triplets' are used as input into mathematical models to calculate surface wind speed and direction.