Sp-461 ers-envisat Symposium

Table 1: Number of ACFs to be calculated for each mode and polarization of ASAR

Polarization

Mode

HH

VV

HHVV

HHHV

VVVH

Total

Image

7

7

14

Wave

7

7

Wide Swath

1

1

2

Global Monitoring

1

1

2

Alternating Polarization

7

7

7

21

Techniques

The method to be used to determine the ACFs, as was the case for ERS, is to image a target of known radar cross-

section, integrate the power in its Impulse Response Function (IRF) corrected for the associated background (clutter)

power and hence calculate the correction (the ACF) which must be applied to

the image values in order to arrive at the same cross-section for that target.

For this purpose, precision calibration transponders are deployed in the

Netherlands. The radar cross-section of these transponders is known to within

±0.13dB and they are stable to 0.08dB [1]. It is necessary to use active radar

calibrators (transponders) as opposed to passive ones (e.g. corner reflectors)

since the ratio of signal to clutter determines the accuracy to which the

calibration can be made.

Once the ACF for a particular configuration has been calculated it will be

possible to make a direct comparison with the on-ground measurements of the

end-to-end system gain carried out during FM testing.

For the narrow swath modes the method described will be used exclusively.

However, for the ScanSAR modes there will be a combined use of the narrow

swath calibration together with dedicated calibration processing.