A Novel Method for Calibrating the ATSR-2 1.6-μm Channel Using Simultaneous Measurements Made in the 3.7-μm Channel in Sun Glint

Abstract Reflection of the sun’s radiation from the sea surface can give rise to abnormally high brightness temperatures in the 3.7-μm channel of the Along-Track Scanning Radiometer (ATSR-2). Using a radiative transfer model, this effect is used to calibrate the instrument’s 1.6-μm channel by first characterizing the atmosphere with the aid of 11- and 12-μm measurements and then calculating the surface’s effective reflectivity for radiation in the 3.7-μm channel. Scaling of this reflectivity by a factor deduced from the refractive indices of water at 1.6 and 3.7 μm allows, in principle, the calculation of the 1.6-μm channel reflectivity. This can be used directly to give a calibration factor for the 1.6-μm ATSR-2 channel. The polarization sensitivity of the ATSR-2 instrument requires a small correction to be made, and this can be derived by using measurements made in sun-glint areas in both the nadir and the forward views.