On 28 July 2020, a new “Terrain Correction” was applied to SNPP and NOAA-20 VIIRS Imagery EDR geolocation thanks to work done by the VIIRS EDR Imagery Team. The terrain correction software provides consistent navigation of a given surface pixel, no matter the elevation or position within a swath. Prior to the change, high elevation pixels would appear to shift location within a scene from swath to swath as a result of their changing position within the swath relative to nadir. Examples of the change are shown below. The message from NESDIS:
First we analyze a scene over WA/OR, with the high elevation Cascade Mountain Range flanked by lower elevations to the west and east (Fig 1). Two daytime swaths of NOAA-20 VIIRS contained the scene on 27 July 2020, at 1937 UTC (western part of swath) and 2119 UTC (eastern part of swath). I1 band (0.64 um VIS) EDR imagery appears to depict a shift in the mountain range from west to east from the 1937 UTC swath to the 2119 UTC swath, while the position of low elevation areas within the scene remain relatively static.
Now viewing the same scene/imagery but on 31 July 2020, with terrain correction applied, there is very little (if any) shift in terrain (Fig 2).
A similar daytime example is shown using SNPP VIIRS I1 band imagery over south-central Alaska (Fig 3). On 14 July 2020, the position of the mountains within the scene appear to shift dramatically from 2119 UTC to 2301, while the adjacent lower elevations experience no shift at all.
The same scene on 04 August, following the terrain correction, experiences very minimal shift of the mountains from swath to swath (Fig 4).
Another example is applied to the VIIRS Day Night Band Near Constant Contrast EDR product at night (Fig 5). The first example, from 16 June 2020, is centered over northern Arizona and the active Mangum wildfire. Much of the scene is at an elevation between 4500 ft and 8000 ft, with the wildfire around 7500 ft. From 0837 UTC to 1014 UTC, illumination associated with the wildfire, and nearby towns, appear to shift from west to east.
Now viewing a similar scene over western Colorado from the late night of 04 August, after the terrain correction (Fig 6). The scene also contains a wildfire, and similar elevation range as previous. As we compare swaths, however, the light associated with the high elevation wildfire and nearby towns remain stationary.
Bill Line, NESDIS/STAR