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<H4>VISIBLE CHANNEL CALIBRATION</H4>
AVHRR visible channels 1 and 2 are calibrated using the procedure
described in the NOAA Polar Orbiter Data User's Guide. Each output
(or calibrated) pixel value is computed as follows:
<P>
<PRE>
 PO = S(c) * PI + I(c)

 where:

       PI   is the input pixel value
       PO   is the output (calibrated) pixel value
       S(c) is the slope coefficient for channel c
       I(c) is the intercept coefficient for channel c

 units:
       PI   is a pixel count value (usually 0-1023 for AVHRR)
       PO   is percent reflectance
       S(c) is percent reflectance/count
       I(c) is percent reflectance
</PRE>
AVHRRAD looks for the channel slope and intercept coefficients in
the AVHRRSEG text segment. If no such coefficients are found, then
default, pre-launch values are used.<p>
If a solar zenith angle correction is to be performed, it should be
done before running a visible channel calibration. The reason is
that the output of a solar zenith angle correction is still a
digital count value, which can be supplied as input for a subsequent
visible channel calibration.
<P>
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