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NJPL1I00PDS000000000 FILE_TYPE = TEXT RECORD_TYPE = STREAM END Atmospheric pressure statistics from the surface of Mars: the Viking Meteorology Experiment James E. Tillman William R. Guest Department of Atmospheric Sciences University of Washington Seattle, Washington 98195 This medium contains summary pressure data from the Viking Landed Meteorology Experiment throught the mission. These data, also available in graphical form on a video disk, consist of statistical functions and supporting measurements presented on a sol by sol basis for Viking Lander 1, the Thomas Mutch Memorial Station, and Viking Lander 2 for the complete mission. On a typical sol, pressure is periodically sampled with intervals between samples of 30 to 90 minutes. The results are presented in the form of averages of functions over a sol as well as supporting information. The parameters provided are: Flags Sol number Average pressure Length of maximum time gap Minimum pressure Pressure range Standard deviation Starting and ending times of large gaps Bit error rate A short description of each parameter is given below and greater detail will be included in future documents. DATA FORMAT This document. Header for Lander 1 50 sols of data on one page 50 sols of data on each of the subsequent pages Repetition of this structure until the end of Lander 1 data End of file. This document. Header for Lander 2 50 sols of data on one page 50 sols of data on each of the subsequent pages Repetition of this structure until the end of Lander 2 data End of file. Data and text are written in 80 character lines with each page containing 55 lines. The following sections provide descriptions of the parameters and their relation to the original data set. A more complete description will be provided with the point by point data set. EDITING Prior to inclusion in the data set, each measurement was subjected to a series of screening criteria designed to eleminate spurrious points. Subsequent to the screening, the data were plotted and examined as to meteorological consistiency. Obviously bad points were removed and some questionable points also were removed. In some instances where the communications from Mars to Earth were noisy, some data have been retained which have noise on the order of 0.088 to 0.176 millibars. These data were retained since they provide valuable daily average pressure estimates. They can usually be identified by their association with the highest bit error rates. At the present, bit error rates have not been computed for part of the Lander 1 data. However, this parameter will be generated for the remaining data during subsequent processing. FLAGS Columns 1 and 2 of the each line contain characters which indicate whether the rest of the line is to be considered text or data. If there is a C in column 1, the rest of the line is to be considered text. Any other character, including a blank character, indicates the presence of data. Valid characters are, ".", "I" and "C". These characters provide a rough indication of the length and position of time gaps in the sampling for the sol. A blank indicates that there are no gaps greater than 3 hours in the sol which might adversely affect the statistics for the sol. Other characters indicate that the MAXIMUM TIME GAP and the TIME GAP might need to be considered, depending on the application. SOL One entry is provided for each sol regardless of whether any data were received for the sol. A Martian sol is 88776.25 seconds long or approximately 24.66 terrestrial days. A line is provided for sol 0 for format consistiency and to indicate that there are data for this sol which are available in the point by point data set. SOL MEAN PRESSURE The SOL MEAN PRESSURE is a time weighted daily average pressure with certain assumptions regarding gaps longer than three hours. Depending on the location of the gaps and their length, missing data are either filled in by interpolation or extrapolation or are excluded from the time weighted statistics. For gaps greater than four hours spanning midnight, interpolation is not used. Although interpolation is used for all intervals wholly within a sol gaps longer than three hours can cause noticable errors if the gap falls during the time of the diurnal pressure minimum for sols with strong diurnal variations due to heavy atmospheric dust loading. Such instances can be excluded from analyses by using the information contained in the flags or the time gap parameters. Details of the interpolation or extrapolation will be presented in future documents. The pressure transducers were produced by the Tavis Corporation and were calibrated to aproximately 0.01 to 0.02 millibars accuracy over the ranges of 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 pressure indicates that they have remained stable to significantly better than 0.04 millibars throught the mission, possibly maintaining stabilities on the order of 0.01 millibars. Pressure data were converted from digitized to scientific units by Mission Operations software rather than Viking Meteorology Science Team software. In the operational software, the instrumental calibration was approximated by a piecewise linear approximation and one point was left out of the calibration table, producing errors of up to 0.05 millibars which are incorporated in the pressure values submitted to and distributed by NSSDC. In producing the current data, corrections were made for the missing calibration point for each lander while recomputing the correct value for each pressure datum. For approximately the first 16 sols of Lander 1 operation, the wrong calibration constants were used in reducing the pressure data by the mission operations software. These errors are contained in the NSSDC data and have been corrected prior to the calculation of these data. A complete corrected data set will be provided for NSSDC and through other channels, as appropriate. Since the basic pressure quantization interval was 0.087 to 0.088 millibars, (depending on the lander and measured pressure,) averages of the pressure would underestimate the true pressure by approximately 0.044 millibars. This is due to the fact that all pressures within the 0.088 millibar range would be registered as the lower bound of the interval even though half would generally be 0.044 millibars or more higher. To compensate for this, 0.044 millibars has been added to each individual pressure value which is reflected in all subsequent "recalibrated" data and all derivative products such as these "daily average" data. MAXIMUM TIME GAP The MAXIMUM TIME GAP parameter provides an indication of the continuity of sampling during the sol and consequently, the represeatativeness of the statistics for the sol. For example, if several hours of data are missing around 17:00 local time during one of the intense dust storms, the pressure range and minimum probably will be in error by several tenths of a millibar. MINIMUM PRESSURE The MIN. PRES. is the minimum pressure observed during the sol. Due to the quantization of the analog to digital converter, it changes by approximately 0.088 milibar increments. RANGE The RANGE is the difference between the maximum and the minimum pressure for the sol. While it can indicate the magnitude of diurnal tidal harmonics, it also is significantly influenced by large sol to sol synoptic pressure variations. STANDARD DEVIATION The STANDARD DEVIATION is computed as a time-weighted standard deviation around the daily average pressure. Although it is a better indicator of the atmospheric tides due to solar heating of dust than is the RANGE, especially at Lander 1, it also is significantly influenced by synoptic systems. TIME GAP The time gap gives the starting and ending times of the longest interval without data during the sol. Since the atmospheric tides generally produce more rapid variations during late afternoon than other times of the sol, especially during intense dust storms, it helps to indicate the degree to which the daily average statistics approxiamte those of a continuously sampled sol. BIT ERR RATE The BIT ERROR RATE is an indicator of the reliability of the Spacecraft to Earth communications which is calculated from data in each Viking Meteorology Instrument frame. Due to system design, a portion of the data transmitted with each frame consists of all 1's, providing the ability to calculate the error rate for each frame. Although the VMIS and the pressure data are contained in different formats, sampled at different times and, in some instances, averaged over different portions of the sol, the bit error rate does provide a useful indication of the data quality. For Lander 2, they have been calculated for the complete mission while for Lander 1, they are currently available for only approximately one third of the mission. However, they will soon be available for the complete mission. Values below 0.001 provide fairly good data while values below 0.0001 provide almost error free measurements with the exception of short bursts. Due to the extensive automated and manual editing, it is quite unlikely that individual pressure spikes greater than 0.176 millibars are included in the data and even 0.176 spikes are infrequent except in certain instances. In these instances, sols with high bit error rates are fairly noisy on the level of 0.088 to 0.176 millibars. These have been included to provide continuity of the daily average pressure values and the standard deviations and ranges for these sols should be viewed with caution. OTHER PRODUCTS In the near future, the complete recalibrated pressure data set will be released along with documentation and plots. Subsequent to these products, wind and temperature data sets will be released as well as a compact summary of the data. ACKNOWLEGEMENTS We would like to gratefully acknowlege the assistance of the following individuals. Charlie Strauss, Gary Selness, Karen Fitzgerald, Keith Bankston, Robert Henry, Neal Johnson and Harry Edmon. This work was accomplished with NASA funding for Viking Mission operations, Viking Science Team support, the Mars Data Analysis Program and through JPL operations support.