Data Set Overview
    =================
      Viking Lander camera images of the Sun were used to compute
      total normal atmospheric optical depth at the two landing sites
      over a period of about 900 Mars days.  The data set contains
      1044 measurements of optical depth and associated error
      estimates.  The optical depths were derived from Sun diode
      images, which were obtained by the Lander cameras at a
      wavelength of 0.67 micrometers.  Note that no images of Phobos
      were used in deriving this data set.  Studies of diurnal,
      seasonal, and annual variations in optical depth, surface-based
      observations of great dust storms, and analyses of the
      contributions of ice haze and dust particles to the observed
      optical depth, have been carried out using these data.  See
      COLBURN_ETAL_1989, POLLACK_ETAL_1977, and POLLACK_ETAL_1977 for
      further information on the derivation and interpretation of
      this data set.
 
      This data set is composed of the following fields (listed as
      the field name followed by a description):
 
      SC_ID
       Spacecraft id
 
      CAMERA_ID
       Viking Lander camera id
 
      EARTH_DAY
       The fractional 1976 day number (Universal Time)
 
      SOL_LON
       Areocentric longitude of the Sun (Ls)
 
      VIKING_YEAR
       Viking mission year, starting at 1 when the Viking
       spacecraft reached Mars, and incremented at Ls = 0
       every martian year
 
      MARTIAN_DAY
       The martian solar day (sol), starting at day 0 when
       each Lander touched down
 
      LOCAL_HOUR
       Local hour (Earth hour), beginning at midnight (hr 0)
 
      LOCAL_MINUTE
       Local minute (Earth minutes)
 
      OPACITY
       Atmospheric optical depth
 
      OPACITY_FLAG
       Opacity limitation flag, identifying upper and lower
       bound estimates
 
      ERR_DIGITIZE
       The uncertainty in optical depth due to digitization
       error in the brightness signal
 
      ERR_DEVIATE
       The uncertainty in optical depth due to the deviation
       in the multiple measurements averaged to determine
       the observed solar intensity at the surface
 
      ERR_VIGNET
       The uncertainty in optical depth due to brightness loss
       in the Lander cameras resulting from vignetting
 
      ERR_INTEN_0
       The uncertainty in optical depth due to uncertainty in
       the determination of the observed solar intensity at
       the surface
 
      ERR_TOTAL
       The total uncertainty in optical depth, taken as the
       root mean square of the relevant errors (ERR_DEVIATE,
       ERR_VIGNET, and ERR_INTEN_0).  ERR_DIGITIZE is
       excluded as it is implicit in ERR_DEVIATE.
 
 
      Processing Level Id            : 5
      Software Flag                  : Y
      Processing Start Time          : UNK
      Processing Stop Time           : UNK
      Native Start Time              : 99.7 AREOCENTRIC SOL LON,
                                       EARTH YR 1976
      Native Stop Time               : 234.7 AREOCENTRIC SOL LON,
                                       EARTH YR 1979
 
 
    Parameters
    ==========
 
      Data Set Parameter 'OPTICAL DEPTH'
      ----------------------------------
        Optical depth is a dimensionless number that expresses the
        amount of radiation that is absorbed or scattered over a
        given path length through an atmosphere.  Given an optical
        depth of 1, the amount of radiation reaching the surface is
        1/e times the amount incident at the top of the atmosphere.
 
        Data Set Parameter Name        : OPTICAL DEPTH
        Data Set Parameter Unit        : N/A
        Sampling Parameter Name        : TIME
        Sampling Parameter Unit        : MINUTE
        Minimum Sampling Parameter     : N/A
        Maximum Sampling Parameter     : N/A
        Sampling Parameter Resolution  : N/A
        Sampling Parameter Interval    : N/A
        Minimum Available Sampling Int : 1.000000
        Noise Level                    : N/A
 
 
    Source Instrument Parameters
    ============================
      N/A
 
 
    Measurement Information
    =======================
      N/A
 
 
    Processing
    ==========
 
      Processing History
      ------------------
        Source Data Set ID        : VIKING LANDER SUN DIODE IMAGES
        Software                  : UNK
        Product Data Set ID       :
                           VL1/VL2-M-LCS-5-ATMOS-OPTICAL-DEPTH-V1.0

Overview
    ========
      An error analysis of the derivation technique indicates errors
      on the order of 0.02 to 0.03 can be ascribed to the optical
      depth measurements contained in this data set.  See
      COLBURN_ETAL_1989 for a full discussion of the error analysis,
      derivation, and interpretation of this data set.
 
      The data set contains the following fields documenting the
 
      ERR_DIGITIZE
       The uncertainty in optical depth due to digitization
       error in the brightness signal
 
      ERR_DEVIATE
       The uncertainty in optical depth due to the deviation
       in the multiple measurements averaged to determine
       the observed solar intensity at the surface
 
      ERR_VIGNET
       The uncertainty in optical depth due to brightness loss
       in the Lander cameras resulting from vignetting
 
      ERR_INTEN_0
       The uncertainty in optical depth due to uncertainty in
       the determination of the observed solar intensity at
       the surface
 
      ERR_TOTAL
       The total uncertainty in optical depth, taken as the
       root mean square of the relevant errors (ERR_DEVIATE,
       ERR_VIGNET, and ERR_INTEN_0).  ERR_DIGITIZE is
       excluded as it is implicit in ERR_DEVIATE.