Data Set Overview
    =================
      This data set is a digital archive of images acquired by the
      Viking Orbiter 1 and 2 spacecraft.  The archive will include
      the best available Experiment Data Record (EDR) versions of
      images acquired by the Viking Orbiter Visual Imaging Subsystems
      (VIS).  The archive is stored on compact read-only optical disk
      media (CD-ROM) for distribution.  The EDR image data are stored
      on CD-ROM in a compressed format that allows exact
      reconstruction of the original image.  The images were
      compressed in order to reduce the number of CD-ROMs required
      for the archive.  The average Viking Orbiter image is
      compressed by a factor of about 3.5.  Each CD-ROM in the
      archive also includes documentation about the organization and
      contents of the disk.  Software is included to provide
      programmers with tools to decompress image files.  The CD-ROM
      has index files containing information about all the images
      stored in the archive.  These index files can be loaded into a
      database management system to help the user locate images of
      interest.
 
      The EDR images in this archive have not been processed in any
      form other than organizing the original telemetry into raster
      formatted files and compressing the image data using a Huffman
      encoding algorithm.  The reason for distributing unprocessed
      data rather than processed versions of the images is that the
      calibration files and processing procedures will continue to
      evolve.  This is due to improvements, not only in the
      procedures, but also in the radiometric calibration data and in
      the geometric information concerning spacecraft position and
      pointing.  By providing unprocessed images, improved image
      processing capabilities can be reapplied to the original
      unprocessed data.
 
      To make full scientific use of the image collection, it is
      necessary to understand the radiometric and geometric
      properties of the Viking Orbiter imaging systems and to perform
      corrections to the EDR data.  A number of image processing
      systems are available that provide radiometric and geometric
      corrections, display capabilities, and analysis tools for
      planetary images.  Further information on processing Viking
      Orbiter image data and the necessary radiometric and geometric
      calibration data is available through the PDS Image Node at the
      U.S.  Geological Survey in Flagstaff, Arizona.
 
 
      Processing Level Id            : 2
      Software Flag                  : N
      Processing Start Time          : 1990-11-01
      Processing Stop Time           : UNK
 
 
    Parameters
    ==========
 
      Description
      -----------
        Data Number is an integer expressing the digital value of an
        experiment's telemetry data.
 
        Sampling Parameter Name        : PIXEL
        Data Set Parameter Name        : DATA NUMBER
        Data Set Parameter Unit        : DIMENSIONLESS
        Noise Level                    : UNK
 
 
    Source Instrument Parameters
    ============================
      Instrument Host ID              : VO1
      Data Set Parameter Name         : DATA NUMBER
      Instrument Parameter Name       : RADIANCE
      Important Instrument Parameters : 1
 
      Instrument Host ID              : VO2
      Data Set Parameter Name         : DATA NUMBER
      Instrument Parameter Name       : RADIANCE
      Important Instrument Parameters : 1
 
 
    Processing
    ==========
 
      Processing History
      ------------------
        Source Data Set ID          : MASTER DATA RECORD (MDR)
        Software                    : UNK
        Product Data Set ID         : VO1/VO2-M-VIS-2-EDR-V2.0
 
        Source Data Set ID          : VO1/VO2-M-VIS-2-EDR-V1.0
        Software                    : UNK
        Product Data Set ID         : VO1/VO2-M-VIS-2-EDR-V2.0

Overview
    ========
      In the process of preparing the EDR images for inclusion in
      this data set, several steps were taken to insure that the best
      available image data and label information were archived.  The
      primary digital data source was a collection of magnetic tapes
      produced by the Planetary Image Conversion Task.  These tapes,
      also known as the Planetary Archive tapes, constitute PDS
      Version 1 of the Viking Orbiter image data set (PDS data set id
      'VO1/VO2-M-VIS-2-EDR-V1.0').  The data on the Planetary Archive
      tapes were derived from EDR tapes.  The quality of the image
      data on the Planetary Archive tapes was checked for problems
      such as missing lines, segments of data from other images, and
      mismatched images and labels.  Some of the images received on
      Earth were not available on these Planetary Archive tapes.  In
      addition, some Viking Orbiter images were never transferred to
      EDR tapes.  For images with data problems and for images not
      available from either tape source, attempts were made to
      retrieve the data from the Viking Master Data Record (MDR)
      tapes.  The accuracy of the image description keywords in the
      image label was verified by comparing data from several
      sources.  These sources included the PDS Central Node image
      catalog, image labels on the Planetary Archive tapes, and
      Mission Test and Imaging System (MTIS) photoproducts.  In cases
      where there were discrepancies among the sources, label values
      were chosen that occurred most frequently among the sources,
      maintained consistency among images within a sequence, and
      agreed with the VIS team planning notes.  For example, a
      discrepancy in exposure duration for an image might be resolved
      by choosing the value of exposure duration for other images in
      the same sequence.  If a discrepancy could not be resolved,
      that keyword was assigned the value 'UNKNOWN'.  As a check on
      the data compression procedure, each image was decompressed and
      compared to the original image.  The compressed image file was
      accepted only if the results of decompression exactly matched
      the original image data.
 
      Many Viking Orbiter images are missing data and contain some
      amount of noise.  A common pattern of missing data is a series
      of vertical bars with zero value pixels spaced at an interval
      of 7 samples.  The 7-sample interval results from the raw data
      being stored on the spacecraft and transmitted to Earth in
      packets that contained every seventh pixel.  In addition, data
      for a few horizontal image lines may be missing and such lines
      are filled with zero values.  The types of noise found in
      Viking Orbiter images include single-pixel random noise and
      several sources of coherent noise.  The random noise is usually
      due to telemetry errors.  The coherent noise arises from
      shuttering of the adjacent camera, filter wheel stepping, and
      scan platform movements.  If the amount of missing data and
      noise is large, noise removal procedures may be needed to make
      the image viewable.  Box filtering techniques that fill in zero
      values or average the bright and dark spikes of random noise
      are often successful at noise removal.
 
      It should be noted that the START_TIME and the STOP_TIME in the
      PDS catalog, with values 1976-06-22T17:59:00.469Z and
      1980-07-30T22:14:59.469Z respectively, are the times associated
      with the version 1 data set VO1/VO2-M-VIS-2-EDR-V1.0.  These
      times are approximately correct for the current data set but
      will be known more accurately only after data processing has
      been completed.  PDS labels embedded in each image file and the
      index tables on the CD-ROM contain the exact times associated
      with each image.