Photonirvachak Journal of the Indian Society of Remote Sensing, Vol. 28, No. 1, 2000 Wave Directional Spectrum From SAR Imagery A.A. FERNANDES 1 . Y.V.B. SARMA 1 , H.B. MENON 2 and P. VETHAMONY 1 1 National Institute of Oceanography, Dona Paula. Goa 403004 2 Goa University. Taleigao Plateau, Goa 403 206 ABSTRACT Gaussian smoothed SAR image spectra have been evaluated from 512 x 512 pixel sub- scenes of image mode ERS-1 SAR scenes off Goa, Visakhapatnam, Paradeep and Portugal. The two recently acquired scenes off Portugal showed the signature of swell of wavelength 200m and internal waves of wavelength > 400m. Only internal waves of wavelength > 400m were seen in the scene off Goa observed on 11 March 1992. The scenes off Visakhapatnam and Paradeep did not show any wave like features, the latter appearing to be of "white noise" nature. There exists a 180° ambiguity in wave direction observed from radar imageries. Based on the method of Atanassov et al (1985), a computer program has been developed for the removal of the 180° ambiguity by using two images of the same area separated by a time interval which is small compared to the period of the dominant waves. The computer program has been successfully tested with computer simulated images. Introduction The first Synthetic Aperture Radar (SAR) was flown aboard the short lived SEASAT. the first satellite dedicated to the study of the oceans. A review of SEASAT performance is given in the book "Satellite microwave remote sensing" by Allan (ED) (1983). Allan's article therein reports that the SEASAT SAR imaged ocean waves, internal waves, slicks and streaks. Articles by D W S Lodge and NH Kenyon also Reed. 22 Feb. '99; in final form 21 Feb. 2000 therein reported that SEASAT SAR imaged large scale marine bedforms like sand banks and sand waves and was useful in identifying the direction of current flows and in indicating whether the flood or ebb current is dominant. One of SEASAT SAR mission's objectives was to detect sea and fresh water ice and to map snow cover. Johannessen et al. (1994) reported that ERS-1 SAR has imaged horizontal roll vortices in the atmosphere, atmospheric gravity waves, wind and current fronts, wind direction, rain cells, eddies, sea ice, internal waves, slicks and surface wind waves, especially swell. SAR is also useful for ship detection Ocean waves are weakly