Synthetic aperture radar (SAR) images of the ocean surface can show evidence of the sheltering elTects of barriers, such as islands, on the incoming long ocean waves. In the presence of high background noise in SAR images, these elTects arc not always observed by visual inspection. As shown in this paper, they can be determined most effectively in the spectral domain. From the ERS–1 SAR data of 17 October 1991, we noticed that two separate swell components almost perpendicular to each other were incident on the Semidi Islands in the Gulf of Alaska. By spectral analysis, the shadow zone associated with each wave component can be identified. The distribution of an equivalent dilTraetion coefficient has been estimated at the Ice of the Chowiet Island which has a sharp tip. It has been compared to that obtained from the Sommerfeld solution of dilTraction for a semi-infinite barrier. The SAR data shows a similar dilTraction pattern to the model solution but has a slower fall-off into the geometric shadow zone.