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We are experimenting with three different techniques of rain rate estimation to assess their appropriateness by analyzing the data on simultaneous rain measurements with a C band weather radar and a dual-wavelength (3- and 0.86-cm) radar scatterometer. These techniques are (1) a conventional single-wavelength technique based on an empirical relation between a reflectivity factor and a rain rate; (2) a dual-wavelength technique based on radar wave attenuation at one wavelength, where it is assumed that the raindrops sampled are Rayleigh scatterers at both wavelengths; and (3) an alternative dual-wavelength technique using the method of least squares to minimize the difference between measurement-derived quantities and their corresponding quantities derived from a radar equation. The rain rate estimates from the above three techniques are compared with each other, and the causes of their differences are discussed. The rain rate estimates obtained with each technique and gauge rain rates measured near the radar site are also compared to evaluate their absolute accuracy. The advantages and disadvantages of each rain rate estimation technique are discussed for specific situations and applications. As a result of investigation the following conclusions are obtained: The single-wavelength technique gives fairly good rain rate estimates if an external radar calibration is made accurately. The dual-wavelength technique based on radar wave attenuation needs verification of the estimates by auxiliary means to avoid unrealistic estimates, and the dual-wavelength technique by means of a least squares method is useful for an airborne or a spaceborne application where an external calibration is difficult to perform.