Data Set Overview
    =================
      This data set contains the martian surface atmospheric pressure
      readings obtained through much of the duration of the Viking
      Lander 1 and 2 missions (data are included for Viking Lander 1
      sols 0 - 2245 and Viking Lander 2 sols 0 - 1050).  The data are
      derived from the ambient pressure sensor carried onboard the
      Landers and values are presented on a point by point basis.
      The sampling rate was variable throughout the mission; the
      maximum sampling rate was one measurement per second, but
      ranged up to 65 minutes for Lander 1 and 105 minutes for Lander
      2.  For further background information on the Viking
      Meteorology Instrument System (VMIS) and results from this
      experiment, see CHAMBERLAIN_ETAL1976, HESS_ETAL_1976A,
      HESS_ETAL_1976B, and HESS_ETAL_1977.  For discussions of
      analyses of experiment data, see also TILLMAN_1977,
      HESS_ETAL_1980, and TILLMAN_1988.  An earlier version of this
      data set is archived at the NSSDC (NSSDC ID 75-075C-07D and
      75-083C-07D).
 
      This data set is composed of the following fields (listed as
      the field name followed by a description):
 
      SC_ID
       Spacecraft id
 
      SOL_LON
       Areocentric longitude of the Sun (Ls), derived for the
       local time of each measurement
 
      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 minute)
 
      LOCAL_SECOND
       Local second (Earth second)
 
      PRESSURE
       Surface atmospheric pressure
 
 
      Processing Level Id            : 3
      Software Flag                  : Y
      Processing Start Time          : UNK
      Processing Stop Time           : UNK
      Native Start Time              : 97.1 AREOCENTRIC SOL LON,
                                       EARTH YR 1976
      Native Stop Time               : 226.7 AREOCENTRIC SOL LON,
                                       EARTH YR 1982
 
 
    Parameters
    ==========
 
      Data Set Parameter 'ATMOSPHERIC PRESSURE'
      -----------------------------------------
        The force per unit area exerted by an atmosphere, measured in
        millibars (10**3 dyne cm**-2).
 
        Data Set Parameter Name        : ATMOSPHERIC PRESSURE
        Data Set Parameter Unit        : MILLIBAR
        Sampling Parameter Name        : TIME
        Sampling Parameter Unit        : SECOND
        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
    ============================
      Instrument Host ID              : VL1
      Data Set Parameter Name         : ATMOSPHERIC PRESSURE
      Instrument Parameter Name       : PRESSURE
      Important Instrument Parameters : 1
 
      Instrument Host ID              : VL2
      Data Set Parameter Name         : ATMOSPHERIC PRESSURE
      Instrument Parameter Name       : PRESSURE
      Important Instrument Parameters : 1
 
 
    Processing
    ==========
 
      Processing History
      ------------------
        Source Data Set ID          : MASTER DATA RECORD (MDR)
        Software                    : UNK
        Product Data Set ID         : VL1/VL2-M-MET-3-P-V1.0

Overview
    ========
      The pressure transducers were calibrated to approximately 0.01
      to 0.02 millibars accuracy over the ranges expected for martian
      atmospheric pressure variation.  Calibrations to accuracies
      greater than allowed by the digitizing system were accomplished
      by test support instrumentation.  Repeatability from year to
      year of the daily average pressures indicates that the
      transducers have remained stable to significantly better than
      0.04 millibars throughout the mission, possibly maintaining
      stabilities on the order of 0.01 millibars.  The pressure
      measurements were digitized into 255 approximately equal
      increments (digital numbers or DNs), giving a basic pressure
      digitization interval of 0.088 millibars.  Raw DNs between 0
      and 255 were converted to pressures by linear interpolation
      between calibration points.  Due to the limited pressure range
      on Mars, the only calibration points used are those for DNs
      100, 120, and 160.  DN 140 was left out of the calibration
      table, producing a maximum error of 0.0425 millibar for VL1 and
      0.008 millibar for VL2; corrections have been applied to the
      data set to correct for this missing calibration point.
 
      As the output of the pressure transducer is quantized in 0.088
      millibar increments, the resolution of a single measurement is
      limited to 0.088 millibar.  On average, half of the
      measurements producing a given pressure value are at least one
      half a quantum above the indicated value and that value is
      actually the lower bound of the actual value.  By adding 0.044
      millibar to each measurement, any estimate of the average
      pressure (generally having greater resolution than one DN) will
      not be biased (see HESS_ETAL_1980).  Such accuracy is important
      as features of the annual pressure cycle repeat within 0.02
      millibar or less and it is important to establish the actual
      value for comparison between years and for future missions to
      Mars.