This dataset contains the TIROS Operational Vertical Sounder (TOVS)
                        level 3 geophysical parameters derived using the physical retrieval
                        method of Susskind et al. (1984) and processed by the Satellite Data
                        Utilization Office of the Goddard Laboratory for Atmospheres at
                        NASA/GSFC. This method, which is hydrodynamic model- and a priori
                        data-dependent, is designated as the so-called Path A scheme by the
                        TOVS Pathfinder Science Working Group. The 20 channel High resolution
                        Infrared Radiation Sounder 2 (HIRS2) and the 4 channel Microwave
                        Sounding Unit (MSU) aboard the NOAA-xx series of Polar Orbiting
                        Satellites are used to produce global fields of the 3-dimensional
                        temperature-moisture structure of the atmosphere. In addition to
                        profiles of temperature and moisture, the HIRS2/MSU data are used to
                        derive important quantities such as land and sea surface temperature,
                        outgoing longwave radiation, cloud fraction, cloudtop height, total
                        ozone overburden and precipitation estimates.
                        
                        The Path A system steps through an interactive
                        forecast-retrieval-analysis cycle. In each 6 hour synoptic period, a
                        2nd order General Circulation Model (Takacs et al., 1994) is used to
                        generate the 6 hour forecast fields of temperature and humidity. These
                        global fields are used as the first guess for all soundings occuring
                        within a 6 hour time window centered upon the forecast time. These
                        retrievals are then assimilated with all available insitu measurements
                        (such as radiosonde and ship reports) in the 6 hour interval using an
                        Optimal Interpolation (OI) analysis scheme developed by the Data
                        Assimilation Office of the Goddard Laboratory for Atmospheres. This
                        analysis is then used to specify the initial conditions for the next 6
                        hour forecast, thus completing the cycle.
                        
                        The retrieval algorithm itself is a physical method based on the
                        iterative relaxation technique originally proposed by Chahine
                        (1968). The basic approach consists of modifying the temperature
                        profile from the previous iteration by an amount proportional to the
                        difference between the observed brightness temperatures and the
                        brightness temperatures computed from the trial parameters using the
                        full radiative transfer equation applied at the observed satellite
                        zenith angle. For the case of the temperature profile, the updated
                        layer mean temperatures are given as a linear combination of
                        multichannel brightness temperature differences with the coefficients
                        given by the channel weighting functions. Constraints are imposed upon
                        the solution in order to ensure stability and convergence of the
                        iterative process. For more details see Susskind et al (1984).
                        
                        There are 6 level 3 data product files, each of which is in the HDF
                        format and each representative of a different averaging time
                        period. All files contain the same number of geophysical parameter
                        arrays stored as HDF Scientific Data Sets (SDSs). The time periods
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