Radiance lookup table generation

The RLUT is generated by running GENTP5 to create a MODTRAN4 "tape5" file, then running the MODTRAN4 program so that it reads the "tape5" file and creates a MODTRAN4 "tape7" file that contains the RLUT radiance values, and finally running GENRLUT to read the "tape7" file and and output the RLUT into the binary segment of a PCIDSK file. See either GENTP5 or GENRLUT for the description of a full RLUT generation example.

To run GENTP5, you must specify a range and number of samples within the range for each of the following nine RLUT dimensions:

aerosol optical depth or surface meteorological range
wavelength
surface reflectance
relative view-solar azimuth
view zenith angle
solar zenith angle
scene elevation
sensor altitude

You must also specify values for several parameters that are independent of the RLUT dimensions (atmospheric model, MODTRAN4 multiple scattering sub-algorithm to use, aerosol model, and aerosol profile and stratographic extinction). This requires basic knowledge of the imaging situation and sensor characteristics; see GENTP5 for a parameter specification example.

The functions VIEWZAZ and SOLARZAZ are useful for determining values for the view zenith angle and solar zenith angle interval endpoints.

The RLUT generated by GENRLUT is stored in a PCIDSK file binary segment and has the following dimensions:

sensor altitude
solar zenith angle
reflectance
surface meteorological range
atmospheric water vapor content
scene elevation
relative view-solar azimuth(solar azimuth minus view azimuth)
view zenith angle
wavelength

Two at-sensor radiance values are stored in each RLUT cell:

the at-sensor radiance received directly from the target (Lg)
the at-sensor radiance from all other sources (Lo)

The values for Lg or Lo over a wavelength range and with all other table coordinates fixed comprise an at-sensor radiance spectrum.

RLUTSP may be used to extract a set of at-sensor radiance spectra from an RLUT and write them to a spectra library file. The spectra may then be displayed using the Focus spectra plotting window to verify that the spectra vary as expected within the RLUT.

The wavelength samples in an RLUT generally do not correspond to the band center wavelengths for any sensor; they are generally more closely spaced. Functions that use an RLUT as input (GENAWVC, ATRLUT, GENCLUT) will resample the RLUT along the wavelength dimension upon reading it, to match the relative response profiles of the sensor's bands.

The execution time of GENAWVC and ATRLUT may be shortened by using a version of the RLUT that already matches the relative response profiles of the sensor's bands. This allows those functions to skip the resampling step. RESRLUT is used to generate a resampled version of the RLUT; this may significantly shorten overall production time if multiple images from the same sensor and flight are to be corrected.

The original (non-resampled) RLUT should always be used as input to GENCLUT.