The radar equation for the measurement of precipitation by SAR is identical to that for a conventional radar. The achievable synthetic beamwidth, βs, is proportional to σv/U, the ratio of the spread of the precipitation Doppler spectrum to the platform velocity. Thus, a small βs can be achieved only with small σv or from a fast-moving vehicle such as a spacecraft. Also, the along-track resolution is variable with σv and is not known. Nevertheless, the reflectivity is measured correctly. A possible approach to the measurement of σv is noted. The C-band SAR proposed for the Shuttle Imaging Radar-C (SIR-C) mission is capable of detecting a rain rate as small as 0.5 mm h−1 at nadir when the beam is filled. Because the cross-track beam dimension is about 20 km wide, we suggest use of a high-resolution microwave radiometer to correct for the unfilled beam and the variation of gain across it. Alternatively, the cross-track dimension should be decreased to no more than about 5 km by increasing the antenna width and/or decreasing the wavelength.