371 
off-boresight angle [deg] 
Figure 5: Elevation cut of the ERS-1 antenna pattern measured by ground calibration receivers. 
off-boresight angles or elevation angles. The triangles stand for our receiver measurements and the 
solid line is a sixth order polynomial fitted to receiver data. ESA’s official antenna pattern, which 
has been derived from rain forest data, is shown by the dashed line. The difference between our 
measurements and the official pattern is small at the swath center and in far range but increasing 
to almost ldB in near range. Explanations for this difference are under investigation. 
4 - RADIOMETRIC CORRECTIONS 
Knowledge of the cross-track antenna pattern and the imaging geometry allows to apply 
adequate radiometric corrections of the radar data. The sine of the local incidence single is nec 
essary to convert radar cross sections to backscattering coefficients. Terrain height variations in 
mountainous areas cause two problems in the radiometric correction of SAR images: the first be 
ing that the wrong elevation angle may be used in correcting for the radiometric variation of the 
antenna pattern; the second that the local incidence angle used in correcting the projection of the 
pixel area from slant range to ground range coordinates may vary from that given by the flat earth 
assumption. 
In [5], we proposed a novel SAR system which exploits the monopulse principle to determine 
the elevation angle and thus the height at the different parts of the image. The key element of a 
monopulse SAR is am antenna, which can be divided into a lower and upper half in elevation using 
a monopulse comparator. In addition to the usual sum pattern, the elevation difference pattern 
can be generated by a — ir phase shift on one half of the antenna (see Fig. 6). The monopulse 
SAR can be implemented using a system with two receiver channels, one recording the echo with 
the previously mentioned phase shift (difference pattern) and the other recording the same echo 
without a phase shift between the two antenna halves (sum pattern). The data would then be 
processed into two images: one radiometrically modulated with the difference, the other with the 
sum antenna pattern. These images can be ratioed to eliminate the target backscatter dependence 
(including speckle). As this ratio is only a function of the off-boresight angle, it can be inverted 
and the corresponding elevation angle for every pixel in the image can be calculated if the antenna 
depression angle is known.