| Summary: | The comparison of different thermal infrared (TIR) nadir sounders is quite important for assessing their capabilities to retrieve reliable information on the concentration of greenhouse gases. However, the present satellite instruments able to provide such information are not many. While IASI (on MetOp) and AIRS (on Aqua) have been compared in the Simultaneous Nadir Observation (or SNO) configuration, this has not been attempted for IASI and TANSO‐FTS (on GOSAT). We have selected two periods from mid‐July to mid‐September in the high latitude polar region during polar summer when it is possible to observe almost coincident or superimposed footprints (IFOV) of the two infrared sounders considered in this study. These coincidences could be called Simultaneous Off‐Nadir Observations (or SONO). This has been performed for two years i.e. 2010 and 2013. This latter year has been selected because IASI CH4 level 2 (L2) products for this period are now available and also because both IASI‐A and IASI‐B were operational in 2013. The summer period was chosen because of ice free waters (in the latitude band 70N to 80N) over which retrieval are less sensitive to surface inhomogeneities over land. The inversion configuration will be described and the XCH4 and XCO2 (column average mixing ratio) will be compared in addition to the sea surface temperature Tsurf used as a control variable. The retrieved values will be compared to the L2 products available from Eumetsat for CH4 and NIES for CH4 and CO2. The retrieved sea surface temperature will also provide an additional check of the performances of the retrievals. The possibility to retrieve information on other greenhouse species (N2O, CFCs) will be discussed. These results are interesting starting points for preparing the future mission IASI‐NG on MetOP‐SG as well as GOSAT‐2.
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