370 
Figure 3: Shape of ERS-1 transmit pulses measured by ground calibration receivers. 
Therefore we can transform the ground receiver data in a platform fixed or if precise antenna attitude 
information is available in an antenna fixed co-ordinate system. In the antenna co-ordinate system 
we then calculate azimuth and elevation angles which cure commonly used for antenna pattern 
measurements. Fig. 4 shows an example of an azimuth cut of the ERS-1 antenna pattern. The 
power received by one ground receiver is plotted versus azimuth angles. The maximum gain is in 
the direction of the normal on the antenna surface, which means that the ERS-1 SAR antenna has 
no squint. Small gain differences of about ldB can be seen between the first sidelobes. 
For radiometric corrections we need the main cut in cross-track direction. If the antenna has 
no squint the cross-track antenna pattern is given by the elevation cut at an azimuth angle 0°. For 
a squinted antenna the maximum values of the antenna gain have to be located and connected. Fig. 
5 shows an example from our ERS-1 measurements. The normalized antenna gain is plotted against 
Figure 4: Azimuth cut of the ERS-1 antenna pattern measured by ground calibration receivers.