Validation of Geolocation accuracy for millimetre and submillimetre-wave radiometers

Ice Cloud Imager (ICI) is a millimetre-wave radiometer that will be on board of the future operational EUMETSAT Polar System Second Generation (EPS-SG). ICI will cover the mm/sub-mm spectrum from 183 GHz to 664 GHz with the primary objective of providing ice cloud products for climate monitoring. At these frequencies, because of the strong water vapor absorption, the lower atmosphere is in most cases opaque to the sub-millimeter radiation. Moreover, the emissivity of water surface increases and its value becomes comparable to that of the land targets. Consequently, the brightness temperature contrast along coastlines, usually used for microwave imagery geolocation purposes, is not readily recognisable, except in very dry atmospheric conditions. This work proposes a methodology to evaluate the expected ICI geolocation error using the 183.3 ± 6.6 GHz channel of SSMIS (Special Sensor Microwave Imager Sounder) to emulate ICI observations. The idea is to extract Earth natural targets with an identifiable contour at 183.3 ± 6.6 GHz using coastal areas in the polar regions, or high altitude lakes to be compared with a reference one through a cross-correlation technique. The reference is a coastline database or synthetic aperture radar imagery. In addition, we propose mountain chains as targets to assess the geolocation at these high frequencies, because the high slope causes a large decrease in brightness temperature. At ICI frequencies, a surface target can be visible only in the winter season, so the target areas must cover both northern than southern hemisphere to have at least one target during the whole year. This work also aims to estimate the atmospheric conditions providing the sufficient visibility of the selected targets by estimating the atmospheric transmission at ICI channel frequencies with radiative transfer model simulations.