Parent Topic: <A href="http://www.pcigeomatics.com/cgi-bin/pcihlp/AVHRCOR">AVHRCOR</a><hr>
<H3>DETAILS</H3>
AVHRCOR uses the NORAD SGP (Simplified General Perturbations)
satellite orbital model. With this model, AVHRCOR performs a
geometric correction using the following steps:<p>
Step 1: Use the satellite name to locate the corresponding TLE (Two
Line Element) file in either the current working directory, or
$PCIHOME/etc. For example, the TLE file for the NOAA-12 satellite is
called, noaa-12.2le. A TLE file contains orbital element values for
a satellite at various epochs (dates).<p>
Step 2: Locate the epoch in the TLE file which is closest to the
approximate starting scan date. Extract the corresponding orbital
element values. These orbital element values are used in satellite
position/velocity prediction models. For best results, ensure that
your TLE files are kept up-to-date. Ideally, the epoch date should
not differ from the starting scan date by more than a few days.<p>
Step 3: Given one GCP and an approximate starting scan date, use the
satellite orbital model to compute the exact starting scan date.
This process is referred to as image date refinement. Note that the
GCP is only used to refine the image date. After image date
refinement is performed, the mapping between pixel-line coordinates
and geographic coordinates may still not match the GCP precisely. As
a result a pixel-line shift is applied to ensure that the GCP is
matched precisely. Note that image date refinement is only performed
if the single GCP is available in the input file's text segment.<p>
Image date refinement may fail in certain situations. If this
occurs, the geometric correction cannot proceed. Possible reasons
for image date refinement failure:
<P>
<UL>
<LI> The approximate starting scan date is too far from the true
starting scan date. In this instance, try adjusting the
approximate starting scan date (e.g. the DAY value in the
AVHRRSEG segment) and then re-run AVHRCOR with this new value.
Alternatively, try increasing the search interval through the use
of the TIMEMULT parameter. 
</UL>
<UL>
<LI> The epoch date of the TLE lines located by AVHRCOR is too far
from the starting scan date. In this instance, try to obtain a
more up-to-date TLE file for your satellite. 
</UL>
<UL>
<LI> The GCP supplied by the input file's text segment may be
incorrect or very inaccurate (e.g., off by at least a few degrees
of latitude and/or longitude). 
</UL>
Step 4: Once the starting scan date is found, the transformations
between the input image coordinates and the output image
georeferencing system are fully determined. Every pixel in the
output image is created by resampling the input image using these
transformations.
<P>
<hr>
Parent Topic: <A href="http://www.pcigeomatics.com/cgi-bin/pcihlp/AVHRCOR">AVHRCOR</a><br>
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