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The percent reflectance in the red (a proxy for turbidity and suspended sediments) has been calculated using channels 1 and 2, with corrections for atmospheric aerosols and Rayleigh radiance and with calibrations based on the Pathfinder program to remove variation among satellites. The AVHRR reflectance algorithm was developed by Richard Stumpf. A complete description of the AVHRR water reflectance derivation can be found in Stumpf and Pennock (1989) and Stumpf and Frayer (1997; see bibliography).
For near real-time reflectance and sea surface temperature imagery, visit the NOAA Coastal Services Center, Coastal Remote Sensing homepage (/crs/composite) or the USGS eastern Gulf of Mexico homepage (http://coastal.er.usgs.gov/east_gulf/)
A scene specific image offset was subtracted from each daily reflectance image to compensate for residual drift in satellite calibration and individual scene reflectance deviations. The offset for an image was determined by averaging scene reflectance values from up to 87 locations (depending on cloud cover) in the offshore Loop Current waters. These locations were in areas deeper than 200 m. Reflectance in these clear water regions should always be close to zero. Approximately 4 percent of the images had a offset that was derived in shelf break waters (~200 m depth) due to cloud cover in the offshore locations. The final offset was taken as the average offset minus 1 standard deviation.
Filenames have the formats such as gYYMMDD_HH.ref.gif or gYYMMDD_HH.sst.tif where YY = year, MM = month, DD = day of month, HH = local standard time; '.ref' indicates a reflectance product while '.sst' indicates a sea surface temperature product; '.gif' indicates GIF image format while '.tif' indicates GeoTIFF image format.
For the GeoTIFF image format, clouds have been burned to DN value 255. See 'Entity_and_Attribute_Overview' on how to convert the DN to percent reflectance or SST. For the GIF image format clouds are grayscaled.
Where no clouds are present, SST should be within 0.5 degrees Celsius. Accuracy of reflectance product is unknown; precision where no clouds are present is 0.003 (or 0.3 %).
The imagery has been navigated to +/- one pixel (~ 1 km) of the true coastline. The navigation only removed a translation error using a landmark in the scene area. Some images may have larger errors (on the order of 2-4 pixels) somewhere in the scene, especially at the edge of an image swath. These are generally due to earth curvature and possibly changes in satellite attitude (no compensation was made for satellite roll and yaw).
For example, DN = 201 represents 28.5 degC for a SST image or 10.0% for a reflectance image.
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