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 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 1B21 calculates the received power at the PR receiver input point
                                    from the Level-0 count value which is linearly proportional to the
                                    logarithm of the PR receiver output power. To convert the count value
                                    to the input power, extensive internal calibrations are applied, which
                                    are mainly based upon the system model, temperature dependence of
                                    model parameters and many temperature sensors attached at various
                                    locations of the PR. Periodically the input-output characteristics are
                                    measured using an internal calibration loop for the IF unit and later
                                    receiver stages. To make an absolute calibration, an Active Radar
                                    Calibrator (ARC) is placed at Kansai Branch of CRL and overall system
                                    gain of the PR is being measured every 2 months.  Using the transfer
                                    function based on the above internal and external calibrations, the PR
                                    received power is obtained. Note that the value assumes that the
                                    signal follows the Rayleigh fading, so if the fading characteristics
                                    of a scatter is different, a small bias error may occur (within 1 or 2
                                    dB).
                                    
                                    The data are stored in the Hierarchical Data Format (HDF), which
                                    includes both core and product specific metadata applicable to the PR
                                    measurements.  A file contains a single orbit of data with a file size
                                    of about 139 MB (uncompressed). The HDF-EOS &quot;swath&quot; structure is used
                                    to accommodate the actual geophysical data arrays. There are 16 files
                                    each of PR 1B21 and 1C21 data produced per day.
                                    
                                    Spatial coverage is between 38 degrees North and 38 degrees South,
                                    owing to the 35 degree inclination of the TRMM satellite. This orbit
                                    provides extensive coverage in the tropics and allows each location to
                                    be covered at a different local time each day, enabling the analysis
                                    of the diurnal cycle of precipitation. There are, in general, 9150
                                    scans along the orbit, with each scan consisting of 49 rays.  The scan
                                    width is about 220 km.