Instrument Host Overview
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      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
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      Platform 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.