Instrument Host Overview
  ========================
    The Pioneer 11 spacecraft was designed to fit within the 3-meter
    diameter shroud of the Atlas-Centaur launch vehicle. To do so it had
    to be stowed with its booms retracted, and with its antenna dish
    facing forward (i.e. upward on the launch pad).
 
    The spacecraft comprises several distinct subsystems: a general
    structure, an attitude control and propulsion system, a
    communications system, thermal control system, electrical power
    system, navigation system, and, most important to the scientific
    mission, a payload of 11 onboard instruments.
 
    To communicate over long distances the spacecraft's dish-shaped
    antenna has to be pointed toward Earth. A simple and inexpensive way
    to do this is to spin stabilize the spacecraft and keep the spin
    axis pointed to Earth. So the spacecraft is stabilized by rotation.
 
 
    General Structure
    -----------------
      Each spacecraft is 2.9-meters long from its cone-shaped,
      medium-gain antenna to the adapter ring which fastened the
      spacecraft to stage three of the launch vehicle.
 
      Spacecraft structure centers around a 36-cm deep, flat, equipment
      compartment, the top and bottom of which are regular hexagons. Its
      sides are each 71-cm long. One joins to a smaller 'squashed'
      hexagonal compartment that carries most of the scientific
      experiments.
 
      The 2.74-meter diameter, 46-cm deep, parabolic, dish-shaped,
      high-gain antenna of aluminum honeycomb sandwich material is
      attached to the front of the equipment compartment. Its feed is
      topped with a medium-gain antenna on three struts which project
      about 1.2 meters forward. A low-gain, omni antenna extends about
      0.76 meters behind the equipment compartment, mounted below the
      high-gain dish.
 
      Two three-rod trusses, 120 degrees apart, project from two sides
      of the equipment compartment. At their ends, nuclear electric
      power generators are held about 3 meters from the center of the
      spacecraft. A third boom, 120 degrees from the other two, projects
      from the experiment compartment and positions a magnetometer
      sensor about 6.6 meters from the center of the spacecraft. All
      three booms are extended after launch.
 
 
    Attitude Control and Propulsion
    -------------------------------
      The spacecraft possesses a star sensor to provide a reference on
      the bright southern star Canopus, and two sun sensors to provide a
      reference to the Sun. Attitude position is calculated from the
      reference direction to the Earth and the Sun, with the known
      direction to Canopus provided as backup. Pioneer 11 's star sensor
      gain and threshold settings were modified to improve performance
      based on experience with this sensor on Pioneer 10.
 
      Three pairs of rocket thrusters located near the rim of the
      antenna dish are used to direct the spin axis of the spacecraft,
      to keep it spinning at the desired rate of 4.8 revolutions per
      minute, and to change the spacecraft's velocity. The system's six
      thruster nozzles can be fired steadily or pulsed by command.
 
      Each thruster develops its propulsive jet force from the
      decomposition of liquid hydrazine by a catalyst in a small rocket
      thrust chamber attached to the nozzle of the thrusters.
 
      Attitude and velocity changes are made by two thruster pairs
      mounted on opposite sides of the antenna dish rim. One thruster of
      each pair points forward, the other, aft. To change attitude, the
      spacecraft spin axis is rotated in the desired direction by firing
      two thrusters, one on each side of the antenna dish. One thruster
      is fired forward, one aft, in brief thrust pulses at a precise
      position in the circle of spacecraft rotation. Each thrust pulse,
      timed to the spacecraft's rotation, moves (precesses) the spin
      axis a few tenths of a degree, until the desired attitude is
      reached.
 
      To change velocity, the spin axis is precessed until it points in
      the desired direction, then two thruster nozzles, one on each side
      of the antenna dish, are fired continuously, both in the same
      direction, i.e., forward or aft to increase or decrease the flight
      path velocity.
 
      To adjust spin rate, two more pairs of thrusters, also set along
      the rim of the antenna dish, are used. These thrusters are aligned
      tangentially to the antenna rim, one pointing against the
      direction of spin and the other with it. Thus to reduce spin rate,
      two thrusters fire against spin direction.
 
 
    Communications
    --------------
      The spacecraft carries two identical receivers. The omni and
      medium-gain antennas operate together and as such are connected to
      one receiver while the high-gain is connected to the other, though
      the receivers do not operate together. The receivers can be
      interchanged by command, or, should there be a period of
      inactivity, automatically. Thus, should a receiver fail during the
      mission, the other can automatically take over.
 
      Two radio transmitters, coupled to two traveling-wave-tube power
      amplifiers, each produce 8 watts of power in S-band.
 
      The communication frequency uplink from Earth to the spacecraft is
      at 2110 MHz, the downlink to Earth at 2292 MHz. The turnaround
      ratio, downlink to uplink, is precisely controlled to be
      compatible with the Deep Space Network.
 
      The spacecraft data system turns science and engineering
      information into a specially coded stream of data bits for radio
      transmission to Earth. A convolutional encoder rearranges the data
      in a form that allows detection and correction of most errors by a
      ground computer at the receiving site of the Deep Space Network.
      There are 11 data formats divided into science and engineering
      data groups. Some science formats are optimized for interplanetary
      data, others for the Jovian encounter. Engineering data formats
      specialize in data handling, electrical, communications,
      orientation, and propulsion data. All formats are selected by
      ground command.
 
 
    Thermal Control
    ---------------
      Temperature on the spacecraft is controlled at between -23 and 38
      degrees C inside the scientific instrument compartment, and at
      various other levels elsewhere for satisfactory operation of the
      onboard equipment.
 
      The temperature control system coped with gradually decreasing
      heating as the spacecraft moved away from the Sun, and with two
      frigid periods - one when Pioneer 10 passed through Earth's shadow
      at launch; the other at the time of passage through Jupiter's
      shadow during flyby. The system also controlled the effects of
      heat from the third-stage engine, atmosphere friction, spacecraft
      nuclear electric power generators, and other equipment.
 
      The equipment compartments are insulated by multi-layered blankets
      of aluminized plastic. Temperature-responsive louvers at the
      bottom of the equipment compartment, opened by bi-metallic
      springs, allow controlled escape of excess heat. Other equipment
      has individual thermal insulation and is warmed by electric
      heaters and 12 one-watt radioisotope heaters, fueled with
      plutonium-238.
 
 
    Electrical Power
    ----------------
      Nuclear-fueled electric power for Pioneer Jupiter comes from the
      four SNAP-19 type Radioisotope Thermoelectric Generators (RTGs),
      developed by the Atomic Energy Commission (AEC), similar to those
      used to power the Nimbus-3 meteorological satellite. These units
      turn heat from plutonium-238 into electricity.
 
      The RTGs are on the opposite side of the spacecraft from the
      scientific instrument compartment to reduce the effects of their
      neutron radiation on the instruments. Mounted two each on the end
      of each boom, these four RTGs developed about 155 watts of
      electrical power at launch, which decayed to approximately 140
      watts by the time the spacecraft reached Jupiter. One hundred
      watts output is expected five years after launch. The depletion of
      power is not from the nuclear source itself but from deterioration
      of the junctions of the thermocouples which convert heat into
      electricity. The RTGs supply adequate power for the mission since
      the spacecraft needs only 100 watts to operate all systems, of
      which 26 watts are for the science instruments. Excess power from
      the RTGs over that needed by the spacecraft is radiated to space
      thermally through a shunt radiator, or charges a battery which
      automatically supplies additional power needed for short periods
      when the spacecraft demands more power than the output of the
      RTGs.
 
 
    Navigation
    ----------
      The axis of the high-gain antenna dish is slightly offset from,
      but parallel to, the spin axis of the spacecraft within close
      tolerances throughout the mission. Except during initial stages of
      the flight near Earth and for periods when alignment must be
      changed to suit course correction, the spin axis of the spacecraft
      is always pointed toward Earth within a tolerance of one degree to
      provide best communication.
 
      Analysts use the shift in frequency of the Pioneer radio signal
      and angle tracking by the antennas of the Deep Space Net to
      calculate the speed, distance, and direction of the spacecraft
      from Earth. Motion of the spacecraft away from Earth causes the
      frequency of the spacecraft's radio signals to drop and their
      wavelength to increase. Known as a Doppler shift, this effect
      allows the speed of the spacecraft to be calculated from
      measurement of the frequency change in the signal received at
      Earth.
 
      The radio beam is offset one degree from the spin axis. As a
      result, when the spin axis is not directed exactly towards Earth,
      uplink signals received at the spacecraft from Earth vary in
      intensity synchronously with rotation of the spacecraft. A system
      on the spacecraft, known as Conical Scan (CONSCAN), was originally
      intended to be automatically used to change the spacecraft's
      attitude in a direction to reduce these variations in signal
      strength, thereby returning the spin axis to the precise Earth
      point to within the threshold of 0.3 degrees. However, flight
      operations personnel developed and used a direct command technique
      that results in conserving the spacecraft's gas supply.
 
 
    Scientific Payload
    ------------------
      Investigation of interplanetary space on the way to and beyond
      Jupiter aimed to resolve a number of unknowns about the magnetic
      field in interplanetary space; cosmic rays, fast moving parts of
      atoms from both the Sun and the Galaxy; the solar wind, a flow of
      charged particles from the Sun, and its relationships with the
      interplanetary magnetic field and cosmic rays; and interplanetary
      dust concentrations, if any, in the asteroid belt.
 
  [INSTRUMENT_HOST_DESC was adapted from FIMMELETAL pp. 39-45.]