372 
0 
theta [deg] 
Figure 6: Monopulse SAR: normalized sum and difference antenna pattern in cross-track direction 
(elevation plane). 
If we know the elevation angle and the slant range to any point in the image, we can calculate 
the height of the platform above it using information on the antenna pointing and the platform 
attitude. This operation, repeated at many locations throughout the image, allows us to build up a 
topographic map of the height of the aircraft above each location. Inversion of this map, using the 
precisely determined aircraft altitude and the accurate flight path, leads to the actual topography 
of the imaged surface. The precise elevation of one point in the image could also be used to convert 
the height map to a topographic map. 
5 - ABSOLUTE RADIOMETRIC CALIBRATION 
Internal calibration can provide sufficient information to calibrate SAR image data on its 
own. It is very difficult, however, to completely tie down sill the losses and gains in any SAR 
system and often external calibration, using targets with known scattering characteristics, is used 
to complete the calibration process. In any event, external calibration should be used as a way to 
verify wether SAR data from a particular sensor is calibrated. 
Trihedral corner reflectors are the most practical devices for external calibration. We have 
some 50 high precision reflectors of different size from 70cm to 3m leg length. High precision means 
mechanical tolerances for the plate orthogonality < 0.2°, for the surface ripple < 0.5mm (RMS) 
and for the plate curvature < 0.5mm. The 3m reflectors are used for the L- and C-band spacebome 
sensors. Smaller trihedrals are suitable for airborne systems and shorter wavelengths. 
We have deployed six 3m comer reflectors on fixed spots in the area around DLR. These 
reflectors have been imaged nineteen times by the ERS-1 and three times by the JERS-1 SAR 
systems. From NASDA we have received only one data set so far. Therefore, we concentrate on 
ERS-1 results. Table 1 includes radar cross sections of these six trihedrals derived from ERS-1 
precision image products (PRI). PRI images are absolutely calibrated. The well-known integral 
approach has been applied to analyse the point target responses in the image data and to obtain 
the image power corresponding to the trihedrals RCS. This image power can then be converted to 
a RCS value by subtracting the absolute calibration factor provided by ESA. Any deviations from 
the expected theoretical RCS value of 50.3dBm 2 indicate the limits on the accuracy and stability