4.6 Radiometric Calibration
The final step in the calibration of the data is the radiometric calibration. The retrieval of trace gases usually uses the reflectance, the ratio of Earth radiance and solar irradiance. The solar irradiance is measured with on-board diffusers in-flight. The reflectance can be written as (equ. 4-9)
with and as the Earth and sun intensity and measured signal, as the cosine of the solar zenith angle, as the instrument radiometric response for limb (L) and nadir (N) and as the instrument radiometric responses for sun diffuser measurements, as calculated from the Mueller matrix for the scanner and the polarisation sensitivity of the OBM. Because all matrix elements in equ. 4-9 relate to the detectors, we left the superscript ?D? out for better legibility. For a proper calibration the instrument responses have to be determined as a function of wavelength and incidence angle . All polarisation properties are calculated relative to the element of the polarisation sensitivity vectors, so the only remaining term needed for radiometric calibration is the term which depends on , and for any limb, nadir or solar viewing geometry summarised here as:
where identifies the first element in vector . and are provided in the calibration key data as determined from on-ground TV measurements, and is calculated with the scanner model using as input the actual viewing geometry of the instrument, the complex index of refraction of the mirror material as determined during on-ground ambient calibration, and optionally any contamination built-up on the scan mirrors since launch.