63 
Analog aerial camera 
Forward view strip 
Nadir view strip 
Backward view strip 
Photograph with central perspective 
Flight line with overlapping photographs 
Fig. 4. Effect of terrain relief on the imagery 
4. RADIOMETRIC CONSIDERATIONS 
The best possible signal to noise ratio (SNR) is a 
precondition for signal processing, digitising, data 
compression and data transfer with little 
interference. The signal to noise ratio of the 
elements of a CCD are given by: 
SNR = —= (1) 
ycfv +°nm +°7/> 
where n s : signal electron count 
07 : variance of signal electron count 
o; ms : variance of the time dependant noise 
a) : variance of local sensitivity differences 
(fixed pattern noise). 
The signal electron count is directly proportional to 
the number of arriving photons (within a defined 
narrow wavelength interval). The noise of the signal 
electrons therefore is subject to the Poisson statistics 
of photon noise: 
o s = (2) 
The time dependent noise of the CCD and of the 
analogue channel (rms noise) contains: 
• temporary dark signal noise (Poisson statistic) 
• reset-noise and on-chip-amplifier noise (“kTC- 
noise”) 
• transfer noise 
• other electronic noise (1/f - noise, thermal noise). 
For estimation purposes the following calculations 
are based on a noise electron count of 
The fixed pattern noise has two sources 
• photo response non-uniformity (PRNU) of the 
CCD elements 
• shading of the light intensity in the focal plane of 
a wide-angle optics. 
Observing the behaviour of only one CCD element 
by ignoring the PRNU (photo response non 
uniformity), e.g. the fixed pattern noise, we find the 
conditions shown in Figure 5, if we take into 
account a saturation electron count of >500,000. 
The SNR amounts to 8 or 9 bits (SNR = 250 ... 670) 
for an electron count >100,000. 
If we now look at the real conditions in the focal 
plane of a wide-angle lens, we obtain the diagram of 
the signals at the outlet of a CCD line, as shown in 
Figure 6: flat field illumination creates in the focal 
plane of wide-angle optics a CCD signal including 
the effects of shading due to optics and PRNU.