Spectral Smile Correction of CRISM/MRO Hyperspectral Images

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) is affected by a common artifact to pushbroom-type imaging spectrometers, the so-called “spectral smile.” For this reason, the central wavelength and the width of the instrument spectral response vary according to the spatial dimension of the detector array. As a result, the spectral capabilities of CRISM get deteriorated for the off-axis detector elements while the distortions are minimal in the center of the detector array, the so-called “sweet spot.” The smile effect results in a data bias that affects hyperspectral images and whose magnitude depends on the column position (i.e., the spatial position of the corresponding detector element) and the local shape of the observed spectrum. The latter is singularly critical for images that contain chemical components having strong absorption bands, such as carbon dioxide on Mars in the gas or solid phase. The smile correction of CRISM hyperspectral images is addressed by the definition of a two-step method that aims at mimicking a smile-free spectral response for all data columns. First, the central wavelength is uniformed by resampling all spectra to the sweet-spot wavelengths. Second, the nonuniform width of the spectral response is overcome by using a spectral sharpening which aims at mimicking an increase of the spectral resolution. In this step, only spectral channels particularly suffering from the smile effect are processed. The smile correction of two CRISM images by the proposed method show remarkable results regarding the correction of the artifact effects and the preservation of the original spectra.