A method to estimate the drop size distribution (DSD) parameters for dual-frequency radar observations that does not rely on the surface echoes is proposed. The method adjusts the estimates of attenuation factor at the surface iteratively until these estimates retrieve the reflectivity factors that are equal to the original measured reflectivity factors. In this iteration method, the backward-recursion equations are used as a main routine to calculate the DSD parameters for each range bin. The estimates of attenuation factor at the surface are obtained from these DSD parameters. The method is applied to the airborne radar data from the CaPE experiment in 1991. The comparison between the results derived from the surface reference technique and those derived from the proposed method is presented and discussed.