ERS-1 & 2 have been recording data since 1991. Unlike other satellites that respond to reflected solar radiation, the ERS satellite transmits microwave pulses and receives the reflected signal. The time for the pulse to return determines where in the image the signal came from. Large vertical relief changes the surface to satellite distance giving a geometric distortion to steep slopes. Interference between reflections from within each ground cell (12.5 x 12.5m) causes the image to have a much more noisy (speckled) appearance than the near optical sensors.

The ERS satellite can record images through cloud cover and during day or night giving it an obvious advantage in tropical regions. It currently has a repeat cycle of 35 days.

The image texture depends not on the colour of the ground material but principally on the surface roughness and slope direction. The microwave pulses are sent at an angle to the ground so a rough surface or one sloping towards the satellite would generally appear bright whilst a flat smooth surface such as calm water or fine sediment would reflect the signal away from the satellite and appear dark. The satellite has proved a sensitive detector of oil slicks at sea.

Thus SAR imagery is useful for studying surface structure and provides information that is complimentary to optical data. The satellite was primarily designed for ocean applications but is providing valuable onshore imagery, especially in areas that are frequently obscured by clouds. In desert areas ERS images sometimes show very near sub-surface features not so easily visible with Landsat or SPOT.

Image coverage is 100 x 100 km with 12.5 m pixels and image frames can be shifted along track. Receiving stations for global coverage are not yet all operational and there is no tape recorder so some areas cannot be covered. Digital images cost approximately €180 (with options for volume purchase discounts) and take about 2 weeks to be delivered. NPA uses ERS and other SAR data extensively for SAR interferometry applications and projects.

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