Why SAR wave mode data of ERS and Envisat are inadequate for giving the probability of occurrence of freak waves

It would be highly desirable to have a global monitoring tool for the detection of freak or rogue ocean surface waves. Based on the SAR imaging theory of ocean surface waves developed already in the 1980's, it is argued that unfortunately, despite recent claims to the contrary, this is not possible using SAR wave mode data of the ERS and Envisat satellites. The main reason for this is that, due to the motion of the sea surface, the SAR imaging mechanism is, in general, strongly nonlinear. Thus the SAR image of the ocean wave field is a highly distorted image of the ocean wave field which bears little resemblance with the actual ocean wave field. Exceptions are perhaps cases where narrow-band swell trains propagate in range direction in the absence of a wind sea. However, these cases are rare and cannot serve as representative samples for the global distribution of possible sea states. The locations of the maxima in the SAR image intensity or the "groupiness" in the image intensity distribution do not, in general, represent the locations of maxima in the wave amplitude or "groupiness" in the actual ocean wave field. Thus it is impossible to obtain with this technique reliable information on the occurrence probability of extreme ocean waves on a global scale.