Abstract: Seaice is a sensitive indicator of climate change. The knowledge of sea ice extent and surface humidity are not only of great importance to polar region, but also very crucial to the prediction of future temperature trend and the establishment of global climate model. The tracks of ENVISAT RA2 altimeter can reach up to the high latitude of 81.4°of north and south, which provides a way of active microwave sensing to monitor sea ice. In this paper, we developed a method using backscatter coefficients (sigma0) based on the ENVISAT satellite radar altimeter RA2 to detect the monthly changes of polar sea ice extent and surface properties. Considering the variations of scattering characteristics of the sea water and ice surface, we prove that a threshold of 13db for sigma0 can distinguish the sea ice cover from sea.Except for summer times, a highly correspondence between seaice boundaries from altimeter and from radiometer released by NSIDC is presented. For lacking of satellite tracks on the central areas of arctic, we only estimated the seaice extent of antarctic zones. And the ENVASAT altimeter gives a greater value in summer months compared to the radiometer result from NSIDC, which relates to the excellent capacity of altimeter to sensing the dispersed thin ice. In other seasons with high seaice concentration, the difference of sea ice extent is pretty small as the mean difference of winter is just 0.17Mkm2.We also research the discrepancy of sea ice properties over two polar zones, and the result shows that in arctic the sea ice surface is more rough and dry in in the winter and more wet in the summer than antarctica.Based on this research, the ENVISAT radar altimeter can precisely detect seasonal evolution of sea ice cover and surface properties and is proved to be a useful tool in sea ice monitoring and polar exploration.

Key words: backscatter coefficient, satellite altimetry, sea ice cover, surface moisture