5.3. Review of Two Calibration Exercises 
5.3.1.8easat SAR 
Calibration of Seasat was achieved using two aspects: 
— the pilot tone, a continuous wave signal in the data stream. 
- a set of corner reflectors in the Goldstone area, of various 
return cross-sections. 
In addition, thoughout most of the mission, a sensitivity 
time control device was employed, which applied a time varying 
gain to the analogue signal to compensate for the antenna pattern 
in range. 
The pilot tone was not originally intended for calibration, 
but its use allows compensation of variations in the gain of the 
analogue data link and of daily variations in the gain of the 
ground receiving systems (from antennas to digital recorders). 
The corner reflectors, carefully selected and distributed 
over a flat terrain area, had stated calibration objectives (to 
allow reconstruction of the antenna pattern on the ground, as a 
function of range), and they have been extensively used for 
comparison of different digital correlators (which could be 
called relative calibration to some extent). The parameters 
analysed in this comparison were related to the impulse response 
of the system (peak value, 3 dB width, shape of response, 
sidelobe presence and relative value, peak to background ratio) 
and were only indirectly useful for calibration [16]. 
Nevertheless the exercise was important: the present remote 
sensing environment allows, or forces, the user to collect his 
data sets from a number of different sources and comparison of 
data from these sources becomes a difficult exercise if proper 
parameters have not been defined. 
5.3.2.Convair 580 Experiment 
This multifrequency/multipolarization radar was flown over 
39 European test sites for a total of 190 passes in the summer of 
1981, and, at the outset, had ambitious calibration goals. 
= A calibration loop had been set up on board based on a 
calibration signal generator (CALSIG). The signal was inserted 
into the receiver, and then passed through the data handling 
chain, to be recorded at the beginning and at the end of each 
flight both digitally and optically.