Twelve unfolded cubes provided by the 12 sub-channels organized as sub-matrices of a larger matrix X with an almost block-diagonal structure. The wavelength range of the consecutive blocks overlaps for a few rows, corresponding to spectral bands observed by consecutive sub-channels. Blue blocks represent channel 1, green ones channel 2, yellow ones channel 3, and red ones channel 4. White and light blue parts correspond to unobserved coefficients. The light blue area corresponds to the area we are specifically interested in reconstructing.