North-South Image Jitter and RISCT

21 April 1995

GOES-8 images can bounce around north-south by quite a bit if the image-making program relies on the "relative Imager scan count" (RISCT) reported for each Imager data block in the GVAR.

Despite its name, the RISCT value is not the number of scan lines from the top of the frame. It is the number of lines received and passed along, plus the start-of-frame. If the ground system misses lines, it does not increment the RISCT, and succeeding lines can be placed too far north.

For example:

        SCANLINE       FROM START      RISCT
        100            0               100
        101            1               101
        102            2               102
        103            3               103
        104 (oops)     4               -
        105 (oops)     5               -
        106            6               104
        107            7               105
        etc.
An error like this in the middle of an image looks like a "fold", especially if the SPS misses several lines in a row, which it does every day. A user looking in the southern part of an image series will see the accumulated errors cause that region to jump north-south between frames. Consequently, aminations of southern regions like Florida suffer more than those around the Great Lakes.

We fixed our own GSFC image-production software to sanity-check the RISCT value reported in each line, and our north-south image jitter decreased enormously.