5.2. 
90 
The previous missions considered here are those which have 
contributed to the formation of European microwave user 
community: the spaceborne Seasat SAR and the airborne 
Convair-580 multifrequency, multipolarisation radars. The focus 
is on digitally recorded and processed data: optically recorded 
and/or processed data are not suitable for calibration since they 
are affected by multiple non-linearities. Therefore the SIR-A 
mission is not discussed here. 
Objectives of Calibration 
For a synthetic aperture network the objectives of 
calibration can be summarized as follows: 
1) To restitute, on an absolute scale, an accurate estimate of 
the backscatter coefficient of the sensed area or target. 
2) To restitute, on an absolute scale, an accurate estimate of 
geophysical parameters which can be derived from the radar 
data: 
Examples are: 
- Soil moisture 
- Canopy water content 
- Wave number 
- Wave direction 
- Wind speed 
3) To allow multitemporal comparisons of measurement s/ 
estimates of both the backscattering coefficient and the 
derived geophysical parameters. 
The third objective stresses the importance, from a user 
viewpoint, of designing systems which allow an easy and reliable 
relative calibration of the desired products. 
It is important to underline that the purpose of a 
calibration exercise within the framework of a mission with 
specific application objectives is not to calibrate the sensor, 
but, as stated in the above three points, to calibrate a set of 
well defined products. Calibration of the sensor itself is 
therefore a necessary step but not the final goal of the 
activity.