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
                  include daily, 5 day (pentad) and monthly, with the AM and PM portions
                  of the orbits treated separately.  All data are mapped to a 1 degree
                  longitude by 1 degree latitude global grid.