An analytic expression is derived for the spectrum of a synthetic aperture radar image in terms of the surface reflectivity covariance function. It is shown that within the severe constraints imposed by the short integration time, the third (temporal) dimension of the spectrum can be extracted by varying the processor focus parameter. The results also illustrate the dependence of the image spectrum on the scene coherence time, as well as various nonlinear effects associated with the velocity bunching mechanism. Example calculations are presented for two cases corresponding to data sets collected during the Tower Ocean Wave and Radar Dependence Experiment.