The TRMM Precipitation Radar (PR), the first of its kind in space, is
                        an electronically scanning radar, operating at 13.8 GHz that measures
                        the 3-D rainfall distribution over both land and ocean, and defines
                        the layer depth of the precipitation.
                        
                        The objectives of 2A25 is to correct for the rain attenuation in
                        measured radar reflectivity and to estimate the instantaneous
                        three-dimensional distribution of rain from the TRMM Precipitation
                        Radar (PR) data.  The estimates of attenuation-corrected radar
                        reflectivity factor and rainfall rate are given at each resolution
                        cell of the PR.  The estimated near-surface rainfall rate and average
                        rainfall rate between the two pre-defined altitudes (2 and 4 km) are
                        also calculated for each beam position.
                        
                        2A25 basically uses a hybrid of the Hitschfeld-Bordan method and the
                        surface reference method to estimate the vertical true radar
                        reflectivity (Z) profile.  (The hybrid method is described in Iguchi
                        and Meneghini (1994)).  The vertical rain profile is then calculated
                        from the estimated true Z profile by using an appropriate Z-R
                        relationship.
                        
                        The attenuation correction is, in principle, based on the surface
                        reference method. This method assumes that the decrease in the
                        apparent surface cross section (delta sigma-zero) is caused by the
                        propagation loss in rain. The coefficient a in the k-Z relationship,
                        k=a Z**b, is adjusted in such a way that the path-integrated
                        attenuation (PIA) estimated from the measured Zm-profile will match
                        the delta sigma-zero.  The attenuation correction of Z is carried out
                        by the Hitschfeld-Bordan method with the modified a.  Since a is
                        adjusted, this type of surface reference method is called the
                        a-adjustment method.  The a-adjustment method assumes that the
                        discrepancy between the PIA estimate from delta sigma-zero and that
                        from the measured Zm-profile can be attributed to the inappropriate
                        choice of a values, which may vary depending on the raindrop size
                        distribution and other conditions.  It assumes that the radar is
                        properly calibrated and that the measured Zm has no error.
                        
                        In order to avoid inaccuracies in the attenuation correction when rain
                        is weak, a hybrid of the surface reference method and the
                        Hitschfeld-Bordan method is used (Iguchi and Meneghini, 1994).  The
                        PIA is first estimated from the precipitation echo alone.  The weight
                        given by the hybrid method to the PIA estimate from the surface
                        reference increases as the attenuation estimate increases.  When rain
                        is very weak and the attenuation estimate is small, the PIA estimate
                        from the surface reference is effectively neglected.  With the
                        introduction of the hybrid method, the divergence associated with the
                        Hitschfeld-Bordan method is also prevented.
                        
                        One major difference from the method described in the above reference
                        is that, in order to deal ...