The Measurement of Precipitation with Synthetic Aperture Radar

Abstract

The radar equation for the measurement of precipitation by SAR isidentical to that for a conventional radar. The achievable syntheticbeamwidth, ?s, is proportional to ?v/U, the ratio of the spread ofthe precipitation Doppler spectrum to the platform velocity. Thus,a small ?s can be achieved only with small ?v or from a fast-movingvehicle such as a spacecraft. Also, the along-track resolution isvariable with ?v and is not known. Nevertheless, the reflectivityis measured correctly. A possible approach to the measurement of?v 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 0.5mm h?1 at nadir when the beam is filled. Because the cross-trackbeam dimension is about 20 km wide, we suggest use of a high-resolutionmicrowave radiometer to correct for the unfilled beam and the variationof gain across it. Alternatively, the cross-track dimension shouldbe decreased to no more than about 5 km by increasing the antennawidth and/or decreasing the wavelength