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A multichannel microwave propagation experimental system whose frequencies are 1.7 GHz, 11.5 GHz, 34.5 GHz, and 81.8 GHz is introduced in order to measure not only attenuation and phase shift due to rain, but also to infer a path-averaged raindrop size distribution from the measurements, in addition to 11.5 GHz, 34.5 GHz, and 81.8 GHz copolar attenuation, phase variations between the frequencies of 34.5 GHz and 11.5 GHz (with reference to 34.5 GHz) due to heavy rain are measured. These measurements, i.e., propagation constants, constitute a Fredholm integral equation of the first kind with respect to . This equation is solved numerically by several inversion techniques, and the results for > are compared with each other. It is made clear that the remote sensing of from the measurements of microwave scattering is a useful method.