Spectral stability monitoring of an imaging spectrometer by means of onboard sources

The In-Flight Characterization (IFC) facility of the Airborne Prism Experiment (APEX) is presumably the first onboard characterization unit implemented in an airborne imaging spectrometer. This study is meant to test methodologies for the retrieval of temporal relative center wavelength drifts based on IFC data. A rare Earth material filter with a set of well-known absorption features is imaged through the IFC on APEX and recorded at several time positions. The shift of the center wavelengths covered by a spectral feature is estimated by means of curve matching algorithms. Two algorithms are evaluated: in the former the shift is determined by using the correlation coefficient as merit function to determine changes of the feature shape and position, while the latter evaluates the distance between centers of gravity. These methods have demonstrated an uncertainty in the order of 6-9 % of a pixel. A test case has been designed in which the APEX system was exposed to a temperature profile with a thermal excursion of 26°C, reproducing flight conditions. Results show the spectral stability of the APEX imaging spectrometer.