Data Set Overview
    =================
      This data set contains summary pressure data obtained from the
      Viking Meteorology Instrument System (VMIS) through the
      duration of the Viking Lander 1 and 2 missions.  The data are
      derived from the ambient pressure sensor carried onboard the
      Landers.  The data set consists of the daily average pressure
      values and relevant statistics presented on a sol by sol basis.
      For further background information on the VMIS and results from
      this experiment, see CHAMBERLAIN_ETAL1976, HESS_ETAL_1977,
      TILLMAN_ETAL_1979, HESS_ETAL_1980, and SHARMAN_RYAN_1980.  An
      earlier version of this data set, including descriptive
      documents and plots on microfiche, is archived at the NSSDC
      (NSSDC ID 75-075C-07I, 75-075C-07J, 75-083C-07I, and
      75-083C-07J).
 
      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
       beginning of each local day (sol)
 
      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
 
      PRESS_MEAN
       Daily average atmospheric pressure
 
      PRESS_MIN
       Minimum daily atmospheric pressure
 
      PRESS_RANGE
       Maximum variation in pressure during each day
 
      PRESS_SIGMA
       Standard deviation in the daily average pressure
 
      MAX_TIME_GAP
       Length of maximum time gap between two
       measurements (hours)
 
      BEG_GAP_HOUR
       Local hour of maximum time gap start
 
      BEG_GAP_MIN
       Local minute of maximum time gap start
 
      BEG_GAP_SEC
       Local second of maximum time gap start
 
      END_GAP_HOUR
       Local hour at which maximum time gap ended
 
      END_GAP_MIN
       Local minute at which maximum time gap ended
 
      END_GAP_SEC
       Local second at which maximum time gap ended
 
      BIT_ERR_RATE
       Indicates quality of spacecraft to ground communications
 
      WARNING_FLAG
       Flags indicating number and length of time gaps
 
 
      Processing Level Id            : 4
      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        : MARS SOLAR DAY
        Minimum Sampling Parameter     : N/A
        Maximum Sampling Parameter     : N/A
        Sampling Parameter Resolution  : 1
        Sampling Parameter Interval    : 1
        Minimum Available Sampling Int : 1
        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          : VL1/VL2-M-MET-3-P-V1.0
        Software                    : UNK
        Product Data Set ID         :
                            VL1/VL2-M-MET-4-DAILY-AVG-PRESSURE-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.
 
      The pressure measurements are presented in the form of sol
      averages and sol range.  The sol average is obtained by
      weighting each measurement according to the period that it
      represents.  The sol range is determined by subtracting the
      minimum from the maximum pressure on each sol.  With sampling
      intervals of less than approximately two hours, the average
      very closely approximates the true average.  Each sol is
      checked for the maximum period between samples and when this
      value exceeds approximately three hours, the raw data are
      manually edited.  The editing determines the validity of the
      data for each of the several functions which are separately
      flagged.  For example, the data may be adequate to form a daily
      average but not a daily range.  Since the pressure quantization
      is 0.088 millibar, the daily range has discrete jumps and
      fluctuates by one quantum.  Gaps in the data indicate missing
      sols or sols for which the sampling is too infrequent to obtain
      a meaningful value.  In addition, bit error rate information is
      tabulated on a sol by sol basis so that errors on the
      communication link can be quantified.
 
      The data set contains the following fields related to
      confidence level (listed as the field name followed by a
 
      MAX_TIME_GAP
       Length of maximum time gap between two
       measurements (hours)
 
      BEG_GAP_HOUR
       Local hour of maximum time gap start
 
      BEG_GAP_MIN
       Local minute of maximum time gap start
 
      BEG_GAP_SEC
       Local second of maximum time gap start
 
      END_GAP_HOUR
       Local hour at which maximum time gap ended
 
      END_GAP_MIN
       Local minute at which maximum time gap ended
 
      END_GAP_SEC
       Local second at which maximum time gap ended
 
      BIT_ERR_RATE
       Indicates quality of spacecraft to ground communications
 
      WARNING_FLAG
       Flags indicating number and length of time gaps