ERS-1 and 2 - Sensor Characteristics
Full specifications of all aspects of any of the instruments can be supplied, direct from the ESA documentation library. A summary of the most relevant information is supplied here for each sensor, with the emphasis placed on the Active Microwave Instrument in SAR Image Mode, the data which are expected to be of most interest to most customers.

Active Microwave Instrument (AMI)
The AMI comprises two separate radars: the Synthetic Aperture Radar (SAR), which can be operated in Image or in Wave Modes, and a Wind Scatterometer.

SAR Image Mode
In Image Mode, the SAR obtains strips of high-resolution (10-30 m) imagery, 100 km in width, to the right of the satellite track. The 10 m long antenna, aligned parallel to the flight track, directs a narrow radar beam onto the Earth's surface over the swath. Imagery is built up from the time delay and strength of the return signals, which depend primarily on the roughness and dielectric properties of the surface and its range from the satellite. The SAR is a C-Band radar (5.66 cm, 5.3 GHz) with a bandwidth of 15.55 MHz, and operates at an incidence angle of 23°, utilising LV polarisation. The radiometric resolution of the SAR in Image Mode is 2.5 dB at sigma 0 = -18 dB, and the quantisation of each signal component is to 5 or 6 bits depending on the applied range compression. The noise equivalent is sigma 0 = -25 dB.

Operation in Image Mode excludes the operation of the other AMI modes, and power considerations limit operating times to a maximum of 12 minutes per orbit. The data rate of 105 Mbit/s is too high to allow on-board storage, and so images are only acquired within the reception zones of suitably equipped ground stations.

SAR Wave Mode
Wave Mode operation of the SAR measures the changes in radar reflectivity of the sea surface due to the surface waves, and provides 5 km x 5 km images (`imagettes') at intervals of 200 km along track. These imagettes are transformed into spectra providing information about the wavelength and the direction of wave systems. Series of power spectra can be used to determine the evolution of swell wave systems.