Specific data correction for EnMAP and DESIS

Kurzfassung

The processing chains of the upcoming hyperspectral missions DESIS (DLR Earth Sensing Imaging Spectrometer), and EnMAP (Environmental Mapping and Analysis Program) need to deal with several systematic errors. This work will present some of the existing problems and the selected correction and attenuation procedures. One common error is the smile effect, affecting push-broom hyperspectral sensors by shifting the central wavelength in the across-track direction. This spectral distortion is minimal in the center of the sensor, increasing towards the sensor edges. The smile effect is particularly noticeable on hyperspectral sensors as the band width is in the order of a few nanometers. EnMAP performs a column-wise smile-aware atmospheric correction, taking the shifted wavelengths into account and interpolating the BOA (Bottom Of Atmosphere) reflectances to the sensor's nominal wavelengths. On the other hand, DESIS addresses the smile correction interpolating TOA (Top Of Atmosphre) radiances. An even more common data error, not only to DESIS and EnMAP but to every type of imaging system, is the existence of abnormal or defective pixels. These abnormal pixels can be present due to sensor aging, errors during data acquisitions, saturation of the sensor, etc. EnMAP strategy for abnormal pixel is based on linear interpolations of the BOA reflectances in the spectral direction. In the case of DESIS, a hybrid interpolation method for abnormal pixels is used. The algorithm selects the optimum value between spectral and spatial cubic spline interpolations of the TOA radiances. The selection criterion is based on the spectral gradient difference between the interpolated pixels and spatial neighbors. Finally this work will present a DESIS specific effect which is introduced, during the data acquisition, by the imaging sensor’s shutter mode. The rolling shutter mode provides higher frame rate and better SNR (Signal to Noise Ratio) than global shutter. As a negative side effect, every individual band from any specific row in the along-track direction is acquired at slightly different time, and therefore, at slightly different position on the ground. To correct this effect, the speed of the ISS (International Space Station) and the band acquisition times are used in order to calculate the point to be interpolated via a cubic spline interpolation in the along-track direction.