Multifrequency scatterometer measurements of Baltic Sea ice during EMAC-95

As part of the European Multi-Sensor Airborne Campaign on Snow and Ice 1995 (EMAC-95), the radar signatures of smooth low-salinity level ice in the Gu lf of Bothnia were measured by means of a ship mounted scatterometer. The s catterometer was operated at 1.25, 2.6, 5.4 and 10 GHz (L-, S-, C-, and X-b and) at incidence angles from 20 degrees to 60 degrees. Various in situ sno w and ice parameters were acquired during the field work. Utilising the ran ging capabilities of the scatterometer system, the travel times of the rada r returns are analysed at the different frequencies in order to locate the dominant scattering contributions as a function of depth in the ice. Model predictions which are based on the in situ snow and ice properties are comp ared with the measured backscattering coefficients in order to determine th e scattering mechanisms of the dominant radar returns. Experimental and mod el results show that the backscattering responses at X-band are mainly caus ed by volume inhomogeneities in the upper most part of the ice, and at C- a nd S-band by the roughness of the snow-ice interface. At L-band, however, d ominant radar returns can originate also from the interior of the ice, prob ably from the ice-water interface, which is substantiated at smaller incide nce angles. Implications of the presented results are discussed.