# Full text: International cooperation and technology transfer

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Analog aerial camera
Forward view strip
Backward view strip
Photograph with central perspective
Flight line with overlapping photographs
Fig. 4. Effect of terrain relief on the imagery
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.
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