The Tropical Rainfall Measuring Mission (TRMM) is a joint U.S.-Japan  
                                    satellite mission to monitor tropical and subtropical precipitation   and to
                                    estimate its associated latent heating.
                                    
                                    The primary objective of the 2A21 is to compute the path integrated
                                    attenuation (PIA), using the surface reference technique (SRT).  The
                                    SRT relies on the assumption that the difference between the
                                    measurements of the normalized surface cross section within and
                                    outside the rain provides a measure of the PIA.
                                    
                                    Two types of non-rain surface cross section (sigma-zero) reference
                                    estimates are used: spatial and temporal.  In the spatial surface
                                    reference data set, the mean and standard deviation of the surface
                                    cross sections are calculated over a running window of Ns fields of
                                    view before rain is encountered.  These operations are performed
                                    separately for each of the 49+2 incidence angles of TRMM
                                    (corresponding to the cross-track scan from -17 degrees to + 17
                                    degrees with respect to nadir).  The two additional angle bins (making
                                    the total 51 rather than 49) are to account for non-zero pitch/roll
                                    angles that can shift the incidence angle with respect to nadir
                                    outside the normal range.
                                    
                                    For the temporal surface reference data set, the running mean and
                                    standard deviation are computed over a 1 degree x 1 degree (latitude,
                                    longitude) grid.  Within each 1 degree x 1 degree grid cell, the data
                                    are further categorized into incidence angle categories (26).  The
                                    number of observations in each category, Nt, are also recorded.  Note
                                    that, for the temporal reference data set, no distinction is made
                                    between the port and starboard incidence angles.  So, instead of 49
                                    incidence angles, there are only 25 + 1, where the additional bin
                                    corresponds to angles greater than the normal range.
                                    
                                    When rain is encountered, the mean and standard deviations of the
                                    reference sigma-zero values are retrieved from the spatial and
                                    temporal surface reference data sets.  To determine which reference
                                    measurement is to be used, the algorithm checks whether Nt >= Ntmin
                                    and Ns >= Nsmin, where Ntmin and Nsmin are the minimum number of
                                    samples that are needed to be considered a valid reference estimate
                                    for the temporal and spatial reference data sets, respectively.
                                    (Presently, Ntmin = 50 and Nsmin = 8).  If neither condition is
                                    satisfied, no estimate of the PIA is made and the flags are set
                                    accordingly.  If only one condition is met, then the surface reference
                                    data which corresponds to this is used.  If...