Magellan Global Topographic Data Record (GTDR) Description

Extracted from the PDS Data Set Catalog File.

  Data Set Description 
  ==================== 

   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 resampled 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 <METERS>) - 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 <METERS> * 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 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.
                                                                              
  References
  ==========
  
    Ford, P. G., Global Altimetry and Radiometry Data Record, Project Magellan
    Software Interface Specification Document MIT-MGN- GxDR, 1991 Version 2.3,
    40p."

    Ford, P.G. and G.H. Pettengill, Venus Topography and Kilometer-Scale Slopes,
    JGR, V. 97, pp. 13103-13114, 1992.

    Saunders, R.S., G.H. Pettengill, R.E. Arvidson, W.L. Sjogren, W.T.K.
    Johnson, L. Pieri, The Magellan Venus Radar Mapping Mission, JGR, V. 95, pp.
    8339-8355, 1990.