Diffraction effects on the Meteosat Third Generation Infrared Sounder (MTG IRS)

Geostationary Infrared Sounding missions offer good temporal coverage, however due to the large distance to the observed Earth targets the effect of diffraction is increased compared to sounding from a low Earth orbit (LEO). Because of the wave-length dependence of diffraction, the spectral channels do not sample the same volume of air, as in general assumed by retrieval algorithms for LEO IR (Infrared) sounder data. This additional error introduced in the retrieval by diffraction limited instruments is called ´pseudo noise´ throughout the paper. One such diffraction-limited geostationary system is the candidate Infrared Sounding (IRS) mission on EUMETSAT Meteosat Third Generation (MTG) satellites with a planned need date in 2015, due to the expected lifetime of the current Meteosat Second Generation (MSG) satellites. A simplified Point Spread Function (PSF) is applied. To represent the channels within natural conditions, measured spectra from the low-earth orbit Atmospheric Infrared Sounder (AIRS) at DeltaX = 15 km, as well as the Moderate Resolution Imaging Spectroradiometer (MODIS) at DeltaX = 4 km are used as an underlying scene when integrating over the PSF. The results show that the pseudo noise is highly depending on wavelength, and highest in the window and CO2 regions (within the broader 700-1200 cm-1, or 8.3-14.3 mum, region). The worst case pseudo noise values are approximately 1 K for these regions. The broad-band AIRS results are confirmed by a direct comparison to MODIS data at 4 km resolution, where the worst case pseudo noise is appx. 0.5 K, while the best case pseudo noise is comparable to AIRS data.