Meris O2 Calibration Using Sciamachy Measurements

A new MERIS spectral calibration method is proposed exploiting the synergy between MERIS and SCIAMACHY ENVISAT instruments. Detailed method description and preliminary results are presented in this paper. In the O2 calibration mode MERIS is measuring atmospheric radiances in the 750-780 nm spectral range in order to verify the instrument ability to reproduce position and shape of O2 band-A absorption features. The same spectral range is covered by SCIAMACHY, although at different space and spectral resolution. When operated in this mode the MERIS slit functions are characterized by a FWHM of 1.2 nm, while for SCIAMACHY the spectral resolution is 0.3 nm. Ground pixel size for MERIS and SCIAMACHY are respectively 1.1x1.3 and 30x40 km. The proposed method is based on the assumption that the two instruments are probing the same atmosphere and, hence, detected spectral features are approximately the same. Moreover, it assumes SCIAMACHY spectral calibration to be stable and very accurate. Nevertheless, this method can be used adopting other independent sources of oxygen data as reference for the inter comparison with MERIS spectra (e.g. GOME spectral measurements, HITRAN spectral database). The estimated shift refers to the entire wavelength range and not a single MERIS band. Preliminary results indicate that this simple method can efficiently detect MERIS wavelength shifts with respect to SCIAMACHY (or to any other reference spectral database) with an accuracy of 0.01 nm along the orbit, allowing for global analysis.