We found that the STRAYLIGHT flag logic on coastal areas is not the same for all sensors:
- SeaWiFS: there is always a band of STRAYLIGHT pixels along the coastline (1 GAC or 3 MLAC pixels), and LAND pixels are always fully flagged STRAYLIGHT
- VIIRSN/J1: the STRAYLIGHT on coastal areas follow the same logic than on other areas: it's a margin around cloud pixels
- MODISA/T: there is always a band of STRAYLIGHT pixels along the coastline but only on the water side (and on both sides if there are cloudy pixels)
Is this normal?
Thanks in advance,
Yes, it is normal. As I mentioned earlier, for SeaWiFS there is a specific function to correct for straylight - that function sets the
For MODIS and VIIRS (most sensors in fact) the
STRAYLIGHTflag is effectively a dilation of the
The band used in cloud detection for MODIS (869nm) saturates over bright targets. Land is typically bright enough. The coastline, therefore, typically is
of high enough signal to set either the
HILTflag, and thus the
Keep in mind, this flag is set just as a warning that the data may be contaminated. It should not be used in isolation from the
Now I understand the differences we observed, they come from the HILT flag: it is always raised on lands for MODIST/A and SeaWiFS, but for VIIRSN/J1 this flag seems not used because I did not found any HILT pixels on products, am I right?
For SeaWiFS the HILT flag is set when data are "above the knee"...SeaWiFS has a bi-linear gain to prevent saturation. However, this creates a "knee" in radiance space for the DN-to-radiance conversion.
VIIRS doesn't saturate like MODIS and while some bands have dual gains, it's not the same as SeaWiFS. Yes, HILT is less likely (if ever) to be set in VIIRS.