A new estimation algorithm of rain rate by a dual-frequency radar is proposed, and its performance is evaluated by a computer simulation technique. Data on an airborne microwave rain scatterometer are analyzed by the present algorithm to infer rain rate profiles. The results are compared with those measured by a ground-based rain radar to demonstrate the potential of an airborne active sensor and the feasibility of a space-borne active sensor for remote sensing of rain. The total attenuation method for the inference of rain rates is evaluated by comparing the radar echo powers backscattered from the ocean surface with the round-trip attenuations calculated by the inferred rain rate profiles seen from the aircraft to the ocean surface. Good correlation exists between the above two quantities; however, the measured attenuations are greater than those predicted. Further investigations into the microwave scattering characteristics of the ocean surface, especially with the presence of rain, are necessary for practical applications of the method.