Comment on 'Synergetic use of IASI profile and TROPOMI total-column level 2 methane retrieval products' by Schneider et al. (2022)

A great interest is growing about methods that combine measurements from two or more instruments that observe the same species either in different spectral regions or with different geometries. Recently, a method based on the Kalman filter has been proposed to combine IASI (Infrared Atmospheric Sounding Interferometer) and TROPOMI (TROPOspheric Monitoring Instrument) methane products. We show that this method is equivalent to the Complete Data Fusion method. Therefore, the choice between these two methods is driven only by the advantages of the different implementations. From the comparison of the two methods, a generalization of the Complete Data Fusion formula, which is valid also in the case that the noise error covariance matrices of the fused products are singular, is derived. This comment uses several equations reported in the preprint version of Schneider et al. (2022); therefore, reference is made to the preprint version of this paper.