The case is considered of a CW microwave radar looking at the sea surface at moderate angles of incidence. This is treated using the ‘facet’ concept, in which the sea surface is considered as being covered by spatially uncorrelated facets, each a small number of Bragg wavelengths across and being transported by the particle velocities in the longer sea waves. The facets are considered to have an unknown decorrelation time. In two cases simplifications allow approximate numerical values for the coherent decorrelation time of the radar return to be calculated. These are compared with measured bandwidths and decorrelation times reported in the literature. These agree adequately with the theory and indicate that the facet decorrelation time is long. However, some properly thought-out measurements of coherent decorrelation time are badly needed.