Parent Topic: <A href="http://www.pcigeomatics.com/cgi-bin/pcihlp/AVHRRAD|DETAILS">DETAILS</a><hr>
<H4>RADIOMETRIC CORRECTION (SOLAR ZENITH ANGLE CORRECTION)</H4>
The solar zenith angle along a scanline can vary significantly for
AVHRR data. This is due to the large scan angle of the AVHRR sensor.
This difference in solar angle value along a scanline means that the
corresponding ground objects represented by the AVHRR image are
receiving unequal amounts of solar radiation. To account for this
effect, a radiometric or solar zenith angle correction can be
applied to the visible channels (1 and 2).<p>
In order to perform a solar zenith angle correction, AVHRRAD needs
to set up transformation equations between the input image pixel
coordinates and a Lat/Long coordinate system. This requires certain
information in the text segment such as the satellite name (which is
used to select the proper TLE orbital element data file), GCP
values, and approximate starting scan date/time of the input image.
Please see the DETAILS section of AVHRCOR for a more complete
description of the process for setting up these transformation
equations.<p>
AVHRRAD performs a solar zenith angle correction by computing the
value of each output pixel as follows:
<P>
<PRE>
 PixelOut = PixelIn / cos(z)
</PRE>
where z is the solar zenith angle corresponding to the center of the
input pixel. The solar zenith angle, z, is a function of
(Pixel,Line) coordinates in the input image. The output pixel is
still a digital count value (just like the input pixel). Hence the
output of a solar zenith angle correction can be supplied as input
for a subsequent visible channel calibration.<p>
Pixels in the input image which have a solar zenith angle greater
than 85 degrees are considered invalid. Such pixels are not
corrected. A report is given at the end of the correction process
which indicates how many pixels were invalid and the bounding box
encompassing the invalid pixels.
<P>
<hr>
Parent Topic: <A href="http://www.pcigeomatics.com/cgi-bin/pcihlp/AVHRRAD|DETAILS">DETAILS</a><br>
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