Instrument Host Overview
  ========================
    The Viking spacecraft consisted of two orbiters, each with a
    heat-sterilized lander within a capsule.  Together with fuel
    they each weighed 3530 kg just after launch.  After landing,
    their masses were about 900 kg for the orbiters and 600 kg for
    each lander.

    The orbiters, derived from earlier Mariner designs, were
    powered by solar panels and were stabilized in three axes by
    use of solar and star sensors in conjunction with attitude
    control jets.  Three remote sensing science investigations
    resided on the two-axis scan platform: a pair of high
    resolution slow scan televison framing cameras (the Visual
    Imaging Subsystem, or VIS); a near-infrared spectrometer for
    atmospheric water detection (the Mars Atmosphere Water
    Detector, MAWD); and a visual and thermal infrared radiometer
    system (Infrared Thermal Mapper, IRTM).

    The landers each carried two facsimile cameras providing stereo
    views of the surface; a boom-mounted set of meteorology
    instruments; an X-ray fluorescence experiment for inorganic
    chemical analysis of soil; a gas chromatograph mass
    spectrometer to measure the soil and atmosphere; three biology
    experiments: the pyrolytic release, labeled release, and gas
    exchange systems; a seismometer; a magnetic properties
    experiment.  Soil physical properties experiments were carried
    out using several of the abovementioned systems.  Upper
    atmosphere measurements were performed using detectors on the
    landers during entry through the atmosphere.  Finally, radio
    science was done using transmitters and receivers on both the
    orbiters and landers.  An extensible arm and scoop system on
    each lander was programmable to acquire samples of soil and
    rock for analysis on board the landers and to affect the local
    environment for visual inspection.

  Platform Descriptions
  =====================

    SCAN PLATFORM
    -------------
      The three Viking orbiter science instruments are mounted on a
      scan platform that provides temperature control for them and
      pointing in two axes: cone and clock.  The platform can be
      rotated in cone and/or clock at rates of 0.25 or 1.0
      degree/sec.  Subroutines stored in the memory of the on-board
      computer can be called upon to move the platform in specified
      patterns.  The nominal range of motion possible for the scan
      platform is from 45 to 175 degrees cone and 80 to 310 clock;
      however, there are mechanical constraints that reduce this
      range.  See reference.