Acquisition - ERS

The ERS AMI (SAR mode) operates by sending out rapid radar pulses while orbiting overhead and is able through signal processing to simulate a large multi-antenna array to achieve fine resolution.

Radar and Ground Interaction

The antenna points to the side to enhance terrain variations and for technical signal processing reasons. Radar pulses are transmitted and the targets' radar backscattering received by the same antenna.

The time it takes for a transmitted signal to be backscattered to and then received by the satellite determines the distance (range) between the satellite and the sensed object. The integrated return signal, composed of numerous individual backscattered signals, is brought to a more manageable frequency before it is compared to both a reference and a quadrature signal.

The reference signal was also used in generating the transmitted pulse and is regulated by a stable oscillator. The quadrature signal is simply the reference shifted by 90 degrees.

The results of these two comparisons are sampled and then downlinked (along with a host of engineering data) digitally as the return signal's cosine and sine components. The complex chirp signal structure permits the various backscattered returns to be discriminated from each other so a high level of range (x-direction) resolution can be achieved. Each location is pulsed many times while within the SAR's view, and analysis of these slightly different (Doppler shifted) returns allows a fine azimuth resolution to be achieved.

Further Information

For more detailed information on radar/ground interactions, see the ASF Scientific SAR User Guide or SAR FAQ. For information on how the downlinked data is processed at ASF, see ERS-1 and ERS-2 SAR Images Document.