THE EFFECT OF TOPOGRAPHY ON SAR CALIBRATION

During normal synthetic aperture radar (SAR) processing, a Rat earth i s assumed when performing radiometric corrections such as antenna patt ern and scattering area removal. Here we examine the effects of topogr aphic variations on these corrections. Local slopes will cause the act ual scattering area to be different from that calculated using the fla t earth assumption. It is shown that this effect, which is present for both airborne and spaceborne SAR data, may easily cause calibration e rrors larger than a decibel. Ignoring the topography during antenna pa ttern removal mag also introduce errors of several decibels in the cas e of airborne systems. The effect of topography on antenna pattern rem oval is expected to be negligible for spaceborne SAR's. In this paper we show how these effects can be taken into account if a digital eleva tion model is available for the imaged area. The errors are quantified for two different types of terrain, a moderate relief area near Tombs tone, AZ, and a high relief area near Oetztal in the Austrian Alps. We show errors for two well-known radar systems, the C-band ERS-1 spaceb orne radar system and the three frequency NASA/JPL airborne SAR system (AIRSAR). For the moderate relief area, it is shown that the scatteri ng area removal leads to errors that are smaller than 1 dB in most of the image, although the errors for the spaceborne case are larger than those of the airborne case, mainly because of the smaller incidence a ngle. In the case of the high relief area, most of the leading slopes show errors on the order of 5 dB or larger, even in the airborne case. In the spaceborne example of the Oetztal area, most of the image show s errors exceeding 1 dB, with large areas showing errors exceeding 5 d B in magnitude. Using the AIRSAR C-band antenna pattern, it is shown t hat extremely large errors, some well larger than 10 dB can be expecte d in the near range of the Oetztal image. It is also shown that not ta king the topography into account during antenna pattern removal introd uces polarimetric calibration errors.