The reflectance of dark targets (water, vegetation) gets too small or negative

[rese_support]

The reflectance of dark targets (water, vegetation) is too small or even negative in the blue-to-red spectral region. What action is necessary to obtain reasonable reflectance values?
There are several possibilities for this behavior:
1. Check the solar zenith angle: is it correct? A too small zenith angle, i.e. high sun, reduces the reflectance values.
2. Check the visibility with the SPECTRA module: the default value may not be appropriate. You have to increase the visibility, i.e. decrease the path radiance, to raise the reflectance values.
3. Try a different aerosol type (on the ATCOR main panel), e.g. switch from a maritime to a rural aerosol.
If the actions (1-3) do not lead to reasonable reflectance values then change the radiometric calibration coefficients: (a) Edit a copy of the "sensor.cal" file, e.g. "sensor_edit2002.cal" and raise the offset c0 in the appropriate channels until the reference spectrum (e.g., water or vegetation) is matched. (b) use the "Inflight Calibration" module and select some ground target(s) whose reflectance spectra serve as reference and will be matched. A new calibration file will be generated.