C-BAND BACKSCATTER SIGNATURES OF OLD SEA-ICE IN THE CENTRAL ARCTIC DURING FREEZE-UP

Radar backscatter signatures of old sea ice in the central Arctic have been measured and analyzed. A ship-mounted scatterometer was used to acquire backscattering coefficients at 5.4 GHz in the four linear Polarization states and at incidence angles between 20 degrees and 60 degrees. Detailed in situ characterizations of the snow and iee were also made to enable comparison with theoretical backscatter models. Freeze-up conditions were prevalent during the experiment period with air temperatures a few degrees below the freezing point and decreasing. The average backscattering coefficient mas found to increase when the temperature of the ice surface layer decreased. The semiempirical backscatter model is used to evaluate the measurements and shows that the backscatter increase is due to an increasing penetration depth causing the volume scattering to increase. In the paper, model predictions also show that both surface and volume scattering contribute significantly for the incidence angles of the ERS-1 SAR, i.e., 20 degrees to 26 degrees. At these incidence angles, the dominating scattering mechanism changes from surface to volume scattering as the ice surface temperature decreases during freeze-up.