APEX: Radiometry under spectral shift conditions

The APEX airborne imaging spectrometer has been shown to exhibit spectral shifts during in-flight conditions, linked to changes in the nitrogen gas density within the APEX optical subunit. These shifts lead to features in the recorded spectra caused by the dichroic coating used as a beam splitter between VNIR and SWIR channels. Consequently dichroic features are no longer compensated for by the radiometric calibration coefficients obtained under laboratory conditions. This paper presents results of a numerical simulation that can model the impact of spectral shifts on radiometry. As a consequence the APEX sensor model has been upgraded and according correction functions have been implemented in the APEX level 1 processor to compensate for shift dependent changes in radiometry due to the dichroic coating.