Some sensors generate band images that contain a dark-reference region (data acquired when the sensors were deliberately shielded from external radiation). Pixel values in the dark-reference region are a sample of an additive contribution from non-scene sources to pixel values elsewhere in the band. DRSUB may be used to subtract dark-reference values from band image values in order to obtain band image values that are more closely proportional to at-sensor radiance. DRSUB is normally applied before any other image processing operation. Even when the image data have been delivered with a dark-reference region, a dark-reference subtraction may have been performed before delivery. In this case, DRSUB should not be used.
As described at Image metadata support, the image metadata may include a band response profile for each band. These metadata would typically be extracted from in-laboratory sensor calibration results. As a sensor is exposed to mechanical stress over time, the actual band responses may depart from the calibration values. Commonly experienced changes in band response, particularly in the center wavelength value, can have an important effect on image processing and analysis. SHFTCW provides a convenient means of directly changing the center wavelength values in band response profiles. The required shift values themselves are not computed by SHFTCW. These may be determined iteratively by applying small shifts and examining image-derived reflectance spectra in the vicinity of atmospheric absorption bands.
© PCI Geomatics Enterprises, Inc.®, 2026. All rights reserved.