Data Set Overview
    =================
      This data set consists of groups of measurements made by the
      Radar Mapper experiment on the Pioneer Venus Orbiter
      spacecraft.  The measurements were made during each
      approximately 12-second spacecraft rotation period for a period
      of 32 minutes on either side of periapsis, when the range to
      the surface was less than about 4500 kilometers.  During those
      12 seconds, up to 8 measurements were made: (a) cold-sky
      received calibration when the antenna pointed to the zenith,
      (b) 'early' imaging, when the antenna pointer at an angle of
      between 10 and 40 degrees from nadir, (c) planetary thermal
      noise temperature when the antenna pointed to the nadir (d) one
      altimetry measurement, or (d-g) four altimetry measurements,
      and (h) 'late' imaging, when the antenna again pointed at an
      angle of between 10 and 40 degrees from nadir, but on the other
      side from 'early' imaging.
 
      The decision to make one or four altimetry measurements
      depended on the spacecraft altitude -- one measurement was made
      above 700 kilometers, four below.  Also, the imaging modes were
      only enabled below 700 kilometers and only generated useful
      results below 550 kilometers.  This data set contains results
      from the altimetry and radiometry measurements.  Each record
      describes the data collected during a single 12-second roll
      period.  If 4 altimetry measurements were made at that time,
      the individual results have been averaged.
 
 
      Processing Level Id            : 4
      Software Flag                  : N
      Processing Start Time          : 1988-12-01
      Processing Stop Time           : 1988-12-01
 
 
    Parameters
    ==========
 
      Data Set Parameter 'BRIGHTNESS TEMPERATURE'
      -------------------------------------------
        The kinetic temperature of an ideal blackbody which, if
        observed with the spectral response of the instrument
        spectral channel in question would yield the observed
        radiance.
 
        Data Set Parameter Name        : BRIGHTNESS TEMPERATURE
        Data Set Parameter Unit        : KELVIN
        Sampling Parameter Name        : TIME
        Sampling Parameter Unit        : SECOND
        Minimum Sampling Parameter     : N/A
        Maximum Sampling Parameter     : N/A
        Sampling Parameter Resolution  : 0.001
        Sampling Parameter Interval    : 12
        Minimum Available Sampling Int : 12
        Noise Level                    : N/A
 
 
      Data Set Parameter 'PIONEER-VENUS FRESNEL REFLECTIVITY CORR'
      ------------------------------------------------------------
        For the Pioneer-Venus Orbiter Radar (ORAD) measurements, a
        correction to the Fresnel reflectivity that accounts for the
        fraction of the incident radar beam that is reflected by
        sub-wavelength-sized scatterers.  This component was
        estimated, where possible, from the ORAD side looking radar
        imaging mode, and modeled by the phenomenological function
        g*a*Rf*cos(t)**1.5, where t is the scattering angle, g = 2.69
        is a fitted geometrical factor, and a is the fraction of the
        surface covered by diffusely-scattering material.  Rf is the
        true Fresnel reflectivity, and may be reconstructed from
        Fresnel reflectivity + Fresnel reflectivity correction.  This
        correction is only available for that portion of the
        altimetric dataset that is also covered by side-looking
        imaging data, i.e.  from about 15 degrees south to 45 degrees
        north latitude.
 
        Data Set Parameter Name        : PIONEER-VENUS FRESNEL
                                           REFLECTIVITY CORR
        Data Set Parameter Unit        : DIMENSIONLESS
        Sampling Parameter Name        : TIME
        Sampling Parameter Unit        : SECOND
        Minimum Sampling Parameter     : N/A
        Maximum Sampling Parameter     : N/A
        Sampling Parameter Resolution  : 0.001
        Sampling Parameter Interval    : 12
        Minimum Available Sampling Int : 12
        Noise Level                    : N/A
 
 
      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        : KILOMETER
        Sampling Parameter Name        : TIME
        Sampling Parameter Unit        : SECOND
        Minimum Sampling Parameter     : N/A
        Maximum Sampling Parameter     : N/A
        Sampling Parameter Resolution  : 0.001
        Sampling Parameter Interval    : 12
        Minimum Available Sampling Int : 12
        Noise Level                    : N/A
 
 
      Data Set Parameter 'RADAR-DERIVED FRESNEL REFLECTIVITY'
      -------------------------------------------------------
        The Fresnel reflectivity of a surface derived by modeling
        backscatter cross-section as a function of surface RMS
        slope and Fresnel reflectivity.  This value is related to
        the (complex) surface bulk dielectric constant E by
        ((sqrt(E)-1)/(sqrt(E)+1))**2.
 
        Data Set Parameter Name        : RADAR-DERIVED FRESNEL
                                           REFLECTIVITY
        Data Set Parameter Unit        : DIMENSIONLESS
        Sampling Parameter Name        : TIME
        Sampling Parameter Unit        : SECOND
        Minimum Sampling Parameter     : N/A
        Maximum Sampling Parameter     : N/A
        Sampling Parameter Resolution  : 0.001
        Sampling Parameter Interval    : 12
        Minimum Available Sampling Int : 12
        Noise Level                    : N/A
 
 
      Data Set Parameter 'RADAR-DERIVED RMS SLOPE'
      --------------------------------------------
        The RMS average surface slope derived from radar
        measurements.  Horizontal length scales that correspond to
        RMS slope estimates vary from many times the radar wavelength
        at small (220 deg) radar incidence angle observations (so
        called quasi-specular regime) to approximately the radar
        wavelength at larger incidence angles.
 
        Data Set Parameter Name        : RADAR-DERIVED RMS SLOPE
        Data Set Parameter Unit        : RADIAN
        Sampling Parameter Name        : TIME
        Sampling Parameter Unit        : SECOND
        Minimum Sampling Parameter     : N/A
        Maximum Sampling Parameter     : N/A
        Sampling Parameter Resolution  : 0.001
        Sampling Parameter Interval    : 12
        Minimum Available Sampling Int : 12
        Noise Level                    : N/A
 
 
    Source Instrument Parameters
    ============================
      Instrument Host ID              : P12
      Data Set Parameter Name         : BRIGHTNESS TEMPERATURE
      Instrument Parameter Name       : RADIANCE
      Important Instrument Parameters : 1
 
      Instrument Host ID              : P12
      Data Set Parameter Name         : PLANETARY RADIUS
      Instrument Parameter Name       : RADAR ECHO POWER
      Important Instrument Parameters : 1
 
      Instrument Host ID              : P12
      Data Set Parameter Name         : RADAR-DERIVED RMS SLOPE
      Instrument Parameter Name       : RADAR ECHO POWER
      Important Instrument Parameters : 1
 
      Instrument Host ID              : P12
      Data Set Parameter Name         : RADAR-DERIVED FRESNEL
                                          REFLECTIVITY
      Instrument Parameter Name       : RADAR ECHO POWER
      Important Instrument Parameters : 1
 
      Instrument Host ID              : P12
      Data Set Parameter Name         : PIONEER-VENUS FRESNEL
                                          REFLECTIVITY CORR
      Instrument Parameter Name       : RADAR ECHO POWER
      Important Instrument Parameters : 2
 
 
    Processing
    ==========
      N/A

Overview
    ========
      This data set includes corrections for timing errors, orbit
      determination errors, and for atmospheric delay and
      attenuation.  There remain severe systematic errors in the
      planetary radius and Fresnel reflectivity parameters, the
      former due to a combination of spacecraft timing and orbit
      determination errors, the latter to spacecraft timing errors
      alone.