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A function of Advanced Microwave Sounding Unit-A (AMSU-A) measurements at 50.3 and 52.8 GHz is defined, which has the property of being sensitive to the angular distribution of surface-scattered atmospheric thermal emission but insensitive to the surface reflectivity, assuming reflectivity to be equal at the two frequencies. A scattering model consisting of a linear combination of specular reflection and Lambertian scattering is fitted to observed values of the function over Antarctica. The Lambertian fraction typically lies in the range of 0.7-1.0 with a mean of 0.84, and it is negatively correlated with polarization of the surface reflectivity, as determined from Special Sensor Microwave/Imager data. Dual-polarized 37-GHz measurements, combined with a separate measurement of the surface temperature, could be used as a predictor of the diffuseness of surface scattering when the atmospheric temperature profile is retrieved from measurements made by AMSU-A or a similar instrument, or when the radiances are assimilated for numerical weather prediction.