Along-track resolution loss in synthetic aperture radar ocean images is investigated using airborne C band data collected at different altitudes. A method is developed for qualifying and quantifying the amount of smearing. The smearing can be described as a low-pass-filtering process acting on the image spectra even in a nonlinear region of imaging. The dependency on the ratio of range to platform velocity and the radar incidence angle is presented. An intrinsic value for the subresolution velocity spread (i.e., the rms facet velocity) is estimated to be in the region &sgr;u=0.59--0.73 m/s for the measurements available on March 11, 1988, with significant wave height ≈4 m and wind speed ≈8 m/s. These values for &sgr;u allow a conversion of the smearing to ERS 1 geometry giving a spatial smearing &sgr;x in the range 68--84 m which corresponds to a resolution varying between 300 and 370 m in the along-track direction. ¿American Geophysical Union 1991