Data Set Overview
    =================
      Global Topographic Data Record (GTDR).  The range to surface is
      derived by fitting altimeter echoes from the fan-beam altimetry
      antenna as a function of time to Hagfors' radar backscatter
      model templates.  The ranges are subtracted from the spacecraft
      radial coordinate (derived from Doppler tracking), yielding
      measurements of planetary radius.  The data are filtered to
      remove low-frequency errors in the in-plane elements of each
      spacecraft orbit, and then re-sampled into sinusoidal,
      Mercator, and north and south polar stereographic projections
      at a resolution of about 5x5 kilometers per pixel.
 
      A fifth image contains estimates of the absolute accuracy of
      the radius values in each pixel of the sinusoidal projection.
      This is derived by taking the root mean square of (a) the
      statistical accuracy anticipated from the template fitting and
      (b) the systematic errors estimated from the in-plane element
      filtration.  Note that this is the absolute topographic
      accuracy -- relative errors are much smaller, about 10-15
      meters in relatively flat areas.
 
      Except in the radius error image, each GTDR pixel is
      represented by a 16-bit integer.  In sinusoidal images, pixels
      lying outside the domain of the projection are set to zero.  In
      all images, pixels with unknown or unmeasured values are also
      set to zero.  The remaining values are related to the planetary
      radius by the following equation:
 
         DN = (RADIUS ) - 6039999.
 
      In the radius error image, each pixel is represented by an
      8-bit integer.  Pixels lying outside the domain of the
      projection are set to zero, as are all pixels with unknown or
      unmeasured values.  The remaining values are related to the
      radius error by the following equation:
 
         DN = (RADIUS-ERROR  * 0.2) + 1.
 
 
      See: Software Interface Specification Document (Peter G.
           Ford, 1991, Global Altimetry and Radiometry Data
           Records, MIT-MGN-GxDR SIS Version 2.3, 40 p.) that
           is included on the GxDR CD-ROMs.
 
      Note: There exist photoproducts that are identified as GxDRP
            (where x is E (Emissivity), S (Slope), RE (Reflectivity),
            or T (Topography)) however, they are not generated from
            the digital GxDR products.  They are instead produced
            from the Magellan ARCDR (Altimetry and Radiometry
            Composite Data Record) data files.
 
      Processing Level Id            : 5
      Software Flag                  : N
      Processing Start Time          : 1991-06-24T20:11:00.000
      Processing Stop Time           : 1992-12-04T00:55:49.000
 
 
    Parameters
    ==========
 
      Data Set Parameter 'PLANETARY RADIUS'
      -------------------------------------
        Radial distance from center of planet or satellite to given
        location on the surface.
 
        Data Set Parameter Name        : PLANETARY RADIUS
        Data Set Parameter Unit        : METER
        Sampling Parameter Name        : PIXEL
        Sampling Parameter Unit        : METER
        Minimum Sampling Parameter     : N/A
        Maximum Sampling Parameter     : N/A
        Sampling Parameter Resolution  : 9282
        Sampling Parameter Interval    : 4641
        Minimum Available Sampling Int : N/A
        Noise Level                    : UNK
 
 
    Source Instrument Parameters
    ============================
      Instrument Host ID              : MGN
      Data Set Parameter Name         : PLANETARY RADIUS
      Instrument Parameter Name       : RADAR ECHO POWER
      Important Instrument Parameters : 1
 
 
    Processing
    ==========
 
      Processing History
      ------------------
        Source Data Set ID  : MGN-V-RDRS-5-CDR-ALT/RAD-V1.0
        Software            : MGMMAP
        Product Data Set ID : MGN-V-RDRS-5-GDR-TOPOGRAPHIC-V1.0
 
 
      Software 'MGMMAP'
      -----------------
        The MIT Altimetry and Radiometry Mapping Software (MGMMAP)
        reads Altimetry and Radiometry Composite Data Records
        (ARCDRs), and generates an image of a specified data field i
        a specified map projection.  Each footprint datum is added
        with a weight and horizontal extent proportional to the
        appropriate ARCDR elements.  This software will be released t
        Magellan Data Management Archive Team (DMAT) upon the
        completion of the nominal mission.
 
        Software Name               : MGMMAP
        Software Type               : N/A
        Software Release Date       : N/A
        Node ID                     : N/A
        Cognizant Engineer          : DR. PETER G. FORD
        Software Access Description : N/A

Overview
    ========
      UNK