Previous work has shown that interannual variations in total sea ice extent may provide a sensitive indicator of global climate change. Data from passive microwave instruments have allowed mapping of global sea ice extents from 1973--76 and from 1978 up to September 1987. [Gloerson and Campbell, 1988>. In this paper data from another microwave instrument, the Geosat radar altimeter, have been used to map the Antarctic sea ice extent for the period November 1986 to January 1989. Comparison with total Antarctic sea ice extents derived from the Scanning Multichannel Microwave Radiometer (SMMR) show excellent agreement during the freeze up period but show significant differences during the late part of the melt period. ¿ American Geophysical Union 1990