Sp-461 ers-envisat Symposium

Global Monitoring Mode

This is a special case since the spatial resolution of 1000x1000m makes the normal use of the transponders unfeasible.

For a reasonable calibration to be made (3 sigma value of +/- 0.5dB), a signal to clutter ratio of better than 30dB is

required. If the clutter at the calibration sites typically has a sigma nought of –6dB then this would require a transponder

RCS greater than 84dBm

2

. This would inevitably saturate the receiver invalidating the calibration. As a result, it is

necessary to come up with an alternative scenario for calibrating this mode.

The first option which will be pursued is to use the other modes (namely Wide Swath and Image) to calibrate GM mode

by means of the Amazonian Rainforest. Since the sigma nought of the rainforest is stable to within 0.3dB it will be

possible to use the sigma nought value obtained from a previous (or subsequent) pass in WS or Image mode to calibrate

GM mode. In addition, other relatively stable distributed targets may be used such as the ice caps and specific desert

regions (Gibson, Gobi etc).

The second option will allow direct calibration using a special global monitoring mode setting and a modified

calibration transponder. The intention is to use the ASAR’s digital chirp generator to offset the centre frequency by

5MHz, which is possible since the chirp bandwidth in GM mode is only around 1MHz. In the calibration transponder,

the received signal is shifted back by 5MHz allowing it to be received by the ASAR  As the clutter return will all be

outside the filter range, only the transponder response will be seen in the processed image against a background of

noise. The result of this operation is to provide a transponder signal to clutter ratio of between 25 and 30dB allowing for

good calibration of Global Monitoring Mode.