In the figure above, the viewing angle to the cloud feature for the lower satellite A is greater than the angle for satellite B, so the displacement error of the cloud due to parallax is greater for satellite A. Note that the apparent footprint of the cloud feature is larger with satellite A. This illustrates that there are limits to the width of good polar data in a single pass because of the viewing angle. Nwer polar satellites are able to significantly reduce image degradation near the edge of the scan by “oversampling”, which will be discussed in a later blog.
The graph below is a plot of the Normalized Cloud Offset (NCO) as a function of distance. This plot is similar to the geostationary version in the previous post, but for polar satellites with units of kilometers on the x-axis rather than degrees of latitude. The blue line is the polar NCO with respect to distance from the nadir, and for comparison the dashed grey line shows the NCO for geostationary data at the same distance.
As mentioned in the previous post, the NCO is a ratio of the cloud displacement away from the nadir to the height of the cloud (in equivalent units). So, at a distance of 1300 kilometers from the nadir the NCO is approximately 2.0. For a cloud top at 20,000 ft (3.8 miles) at this distance you could expect a displacement error of around 7.6 miles away from the center of the satellite track (2.0 * 3.8). Note that for surface-based features such flooded rivers, burn scars, and open leads in the sea ice there would be negligible parallax error regardless of the distance from the nadir (NCO is near zero).
To see an example of cloud displacement at the swath edge, two overlapping passes from VIIRS and MODIS were selected that were only a few minutes apart (image below). Two boxes outline areas that are near the center of one pass and the edge of another. Box A is near the center of the MODIS pass and the edge of VIIRS, Box B is near the center of VIIRS and the edge of MODIS.