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 Pola rization states and at incidence angles between 20 degrees and 60 degr ees. 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 tem peratures a few degrees below the freezing point and decreasing. The a verage backscattering coefficient mas found to increase when the tempe rature of the ice surface layer decreased. The semiempirical backscatt er model is used to evaluate the measurements and shows that the backs catter increase is due to an increasing penetration depth causing the volume scattering to increase. In the paper, model predictions also sh ow that both surface and volume scattering contribute significantly fo r the incidence angles of the ERS-1 SAR, i.e., 20 degrees to 26 degree s. At these incidence angles, the dominating scattering mechanism chan ges from surface to volume scattering as the ice surface temperature d ecreases during freeze-up.