Instrument Host Overview  ========================    For most Voyager experiments, data were collected by    instruments on the spacecraft.  Those data were then relayed    via the telemetry system to stations of the NASA Deep Space    Network (DSN) on Earth.  Radio Science experiments (such as    radio occultations) required that DSN hardware also participate    in data acquisition.  The following sections provide an    overview first of the spacecraft and then of the DSN ground    system as both supported Voyager science activities.  Instrument Host Overview - Spacecraft  =====================================    The Voyager 1 and Voyager 2 spacecraft were identical and were    built by the Jet Propulsion Laboratory (JPL).  With a mass of    815 kilograms, each carried its own power, propulsion, and    communications systems and its own science instruments.    Spacecraft electrical power was supplied by Radioisotope    Thermoelectric Generators (RTGs) that produced about 400 watts.    The Attitude and Articulation Control Subsystem (AACS),    Computer Command Subsystem (CCS), and Flight Data Subsystem    (FDS) managed spacecraft operations.  Thrusters and gyros    provided physical propulsion and attitude control.    Communications between the spacecraft and Earth were carried    out via a high-gain radio antenna using both S-band and X-band    frequencies at data rates as high as 115.2 kilobits per second.    A Digital Tape Recorder (DTR) could save up to 500 million bits    when no Earth station was available for real-time data    transmission.  Voyager control systems could record sets of    several thousand instructions, allowing autonomous operation    for days or weeks at a time.  More information on the    spacecraft can be found in [MORRISON1982], [KOHLHASE1989], and    [JPLPD618-128].    The spacecraft itself was built around its 'bus' -- a decagonal    prism, which was about 2 meters in diameter and about 60 cm    deep.  Each of the ten sides of the bus was associated with a    'bay' containing engineering systems or science instrument    electronics.  Bay 1, for example, contained the radio    transmitter.  The High-Gain Antenna (HGA) was mounted to the    end of the bus facing Earth.  The bays were numbered 1 through    10 in a clockwise direction when viewed from Earth.  Extending    away from the bus were three booms: a science boom and scan    platform to which most instruments were mounted, a magnetometer    boom, and a boom to which the RTGs were mounted.    Spacecraft Coordinate System    ----------------------------      The centerline of the bus was the roll axis of the      spacecraft; it also served as the z-axis of the spacecraft      coordinate system with the high-gain antenna (HGA) boresight      defining the negative z-direction.  The HGA boresight was      also defined as cone angle 0 degrees and as azimuth 180      degrees, elevation 7 degrees.  The science boom, supporting      the scan platform, extended in the general direction of      positive y; this boom was also defined as being at cone angle      90 degrees, clock angle 215 degrees and at azimuth 180      degrees, elevation 97 degrees.  A boom supporting the RTGs      was mounted on the bus in generally the negative y direction.      The positive y-axis (yaw axis) of the spacecraft coordinate      system passed through Bay 3; the negative y-axis passed      through Bay 8.  The x-axis (pitch axis) was in a direction      which defined a right-handed rectangular coordinate system.      The positive x-axis was at cone angle 90 degrees, clock angle      305 degrees (azimuth 270 degrees, elevation 90 degrees).    Telecommunications Subsystem    ----------------------------      The high-gain antenna was mounted to the spacecraft bus,      pointing in the negative z-direction.  It was a parabolic      reflector 3.7 meters in diameter with a feed that permitted      simultaneous operation at both S-band (13 cm wavelength) and      X-band (3.6 cm).  The half-power full-width of the antenna      beam was 0.6 degrees at X-band and 2.3 degrees at S-band.      The Low-Gain Antenna (LGA) was mounted on the feed structure      of the HGA and radiated approximately uniformly over the      hemisphere into which the HGA pointed.      The Telecommunications Subsystem (TCS) electronics included a      redundant pair of transponders, meaning that a failed      functional unit in one transponder could be bypassed by      swapping to the redundant unit.  The TCS could transmit      science data on the X-band link at rates between 4.8 and      115.2 kilobits per second and engineering data on the S-band      link at 40 bits per second.  It could receive instructions      sent (uplinked) from ground stations at a rate of 16 bits per      second.  Commands were extracted from the uplink signal by      the Command Detector Unit (CDU) and were then sent to the      Computer Command Subsystem (CCS).      Spacecraft receivers were designed to lock to the uplink      signal.  Without locking, Doppler effects -- resulting from      relative motion of the spacecraft and ground station -- could      result in loss of the radio link as the frequency of the      received signal drifted.  Unfortunately, a series of failures      in the Voyager 2 receivers left that transponder unable to      track the uplink signal.  Beginning in April 1978, Doppler      shifts were predicted and the uplink carrier was tuned so      that Voyager 2 would see what appeared to be a signal at      constant frequency (to an accuracy of 100 Hz).    Attitude and Articulation Control Subsystem    -------------------------------------------      The Attitude and Articulation Control Subsystem (AACS)      provided three-axis-stabilized control so that the spacecraft      could maintain a fixed orientation in space.  Attitude      control was accomplished using gyroscopes or by celestial      reference.  The AACS also controlled motion of the scan      platform, upon which the four 'remote sensing' instruments      were mounted.      Gyro control was used in special situations (e.g., trajectory      corrections and solar conjunctions) for periods of up to      several days.  The inertial reference unit operated with      tuned rotor gyros having an uncalibrated drift rate of less      than 0.5 degrees per hour and a calibrated drift rate of less      than 0.05 degrees per hour.      Celestial control was based on viewing the Sun (through a      sensor mounted on the high-gain antenna) and a single bright      star (through a second sensor named the Canopus Star Tracker,      after the star used most frequently as the reference).  When      the spacecraft attitude drifted by more than a small amount      from the reference objects, the AACS fired small thrusters      which returned the spacecraft to the proper orientation.  The      Sun sensor was an optical potentiometer with a cadmium      sulfide detector; its error was less than 0.01 degrees and      its limit cycle was +/-0.05 degrees.  The Canopus Star      Tracker was an image dissector tube with a cesium detector,      an error of less than 0.01 degree, and a limit cycle of      +/-0.05 degrees.      Redundant (backup) sun sensors, star trackers, and computers      were also part of the AACS.  The non-redundant portions of      the AACS were those controlling the pointing of the      instrument scan platform, which had two degrees of freedom --      elevation and azimuth (see below).    Propulsion Subsystem    --------------------      The propulsion system was part of the AACS and consisted of      16 hydrazine thrusters.  These thrusters were also used to      control the three-axis stabilization of the spacecraft.  Two      thrusters on opposite sides of the spacecraft were used to      perform positive roll turns around the +Z axis.  Two      oppositely pointed thrusters were used to perform negative      roll turns.  One thruster was used to perform positive yaw      turns (around the +Y axis) and one was used to perform      negative yaw turns.  One thruster was used to perform      positive pitch turns (around the +X axis) and one was used to      perform negative pitch turns.  A backup hydrazine system was      connected to a redundant set of eight thrusters.    Power Subsystem    ---------------      Spacecraft power was provided by three Radioisotope      Thermoelectric Generators (RTGs) mounted on a boom in the      negative y-direction.  At Launch the three RTGs converted      7000 watts of heat into 475 watts of electrical power.  RTG      electrical output decreased by about 7 watts per year because      of decay of the plutonium dioxide fissionable material and      degradation of the silicon-germanium thermocouples.  The      difference between available electrical power and the power      required to operate spacecraft subsystems was called the      'power margin.' Voyager Project guidelines required a power      margin of at least 12 watts to guard against electrical      transients and miscalculations; excess electrical power was      dissipated as heat in a shunt radiator.    Data Storage Subsystem    ----------------------      The Digital Tape Recorder (DTR) was used to store data when      real-time communications with Earth were either not possible      or not scheduled.  The DTR recorded data on eight tracks;      rates were 115.2 kilobits per second (record only), 21.6      kilobits per second (playback only), and 7.2 kilobits per      second (record and playback).  Capacity of each track was 12      images or equivalent.    Computer Command Subsystem    --------------------------      The Computer Command Subsystem (CCS) consisted of two      identical computer processors, their software algorithms, and      associated electronic hardware.  The CCS was the central      controller of the spacecraft.  During most of the Voyager      mission the two CCS computers on each spacecraft were used      non-redundantly to increase the command and processing      capability of the spacecraft.    Flight Data Subsystem    ---------------------      The Flight Data Subsystem (FDS) consisted of two      reprogrammable digital computers and associated encoding      hardware.  The FDS collected and formatted science and      engineering telemetry data for transmission to Earth.      Convolutional coding was imposed on all data transmitted from      the spacecraft.  Additionally, both Golay encoding and Reed-      Solomon encoding were available for use on spacecraft data.      Data compression was also performed within the FDS.    Science Boom    ------------      The Voyager science instrument boom carried the plasma      detector, cosmic ray detector and the low energy charged      particle detector.  The scan platform was mounted on the      science boom.    Scan Platform    -------------      Four instruments (Imaging, PhotoPolarimeter, Infra-Red      Interferometric Spectrometer, and Ultra Violet Spectrometer)      were mounted on the scan platform, which could be slewed by      motors and gears (called actuators).  Elevation of the scan      platform was measured with respect to a plane slightly offset      (by approximately 7 degrees) from the spacecraft x-z plane;      the spacecraft positive y-axis was at 97 degrees elevation      (see Spacecraft Coordinate System above).  The scan platform      azimuth reference was defined by the y-z plane, with zero      azimuth being in the negative z-direction.  Drive actuators      were controlled by fine feedback potentiometers; the error of      each was less than 0.03 degrees, and the final pointing error      of the scan platform was nominally +/-0.1 degrees (2-sigma      per axis).  Subsequent analysis by the Navigation and      Ancillary Information Facility (NAIF) at JPL has shown larger      errors during at least the Jupiter and Saturn encounters.      High rate slews of 1 deg/sec were discontinued after the      azimuth drive mechanism on Voyager 2 temporarily froze a      short time after Saturn closest approach.  The medium slew      rate was 0.33 deg/sec, and the low slew rate was 0.08      deg/sec.    Magnetometer Boom    -----------------      Two low-field magnetometers were mounted on a 13-meter-long      boom that was unfurled and extended automatically after      Launch.  One low-field magnetometer was mounted at the end of      the boom and a second was mounted about 3 meters from the      end.  Two high-field magnetometers were mounted at the base      of the boom.    Science Sensors    ---------------      Each Voyager spacecraft carried instrumentation to support      eleven science investigations.  Target body (or remote      sensing) instruments included:      (1) Imaging Science Subsystem (ISS)      (2) Photopolarimeter Subsystem (PPS)      (3) Infrared Radiometer Interferometer Spectrometer (IRIS)      (4) Ultraviolet Spectrometer (UVS)      Fields, waves, and particles (or in situ) sensors included:      (1) Plasma Subsystem (PLS)      (2) Low-Energy Charged Particle (LECP)      (3) Cosmic-Ray Subsystem (CRS)      (4) Magnetic Fields (MAG)      (5) Plasma Wave Subsystem (PWS)      (6) Planetary Radio Astronomy (PRA)      The Radio Science (RSS) investigation was carried out using      the on-board and ground elements of the Telecommunications      Subsystem (TCS).  More information on instrumentation for      each of the science investigations can be found elsewhere.  Instrument Host Overview - DSN  ==============================    Voyager Radio Science investigations utilized instrumentation    with elements on both the spacecraft and at ground stations of    the NASA Deep Space Network (DSN).  Much of this was shared    equipment, used for routine telecommunications as well as for    Radio Science.    The DSN is a telecommunications facility managed by the Jet    Propulsion Laboratory of the California Institute of Technology    for NASA.  The primary function of the DSN is to provide    two-way communications between the Earth and spacecraft    exploring the solar system.  To carry out this function the DSN    is equipped with high-power transmitters, low-noise amplifiers    and receivers, and appropriate monitoring and control systems.    During the Voyager era the DSN consisted of three complexes    situated at approximately equally spaced longitudinal intervals    around the globe at Goldstone (near Barstow, California),    Robledo (near Madrid, Spain), and Tidbinbilla (near Canberra,    Australia).  Two of the complexes are located in the northern    hemisphere while the third is in the southern hemisphere.    The network comprised several subnets, each of which included    one antenna at each complex.  The subnets were defined    according to the properties of their respective antennas.  Over    the course of the Voyager Mission, those antennas were expanded    and improved.  Nominal dimensions at the end (and beginning) of    the Voyager Mission were: 70-m diameter (initially 64-m),    standard 34-m diameter (initially 26-m), and high-efficiency    34-m diameter (did not exist at beginning of Voyager).    Additional ground equipment was provided by the Commonwealth    Scientific and Industrial Research Organization (CSIRO) in    Australia, the Institute of Space and Astronautical Science    (ISAS) in Japan, and the National Radio Astronomy Observatory    (NRAO) in the United States.  For the Voyager 2 encounters with    Uranus and Neptune, the CSIRO 64-m diameter radio astronomy    antenna near Parkes (Australia) was included in the receiving    system for both telemetry and Radio Science.  For the Voyager 2    encounter with Neptune, the ISAS 64-m diameter antenna near    Usuda (Japan) was added for Radio Science and the NRAO Very    Large Array (VLA) near Socorro (New Mexico) was added for    telemetry.  The VLA consisted of 27 25-m antennas.  Parkes,    Usuda, and the VLA were integrated with the permanent stations    at Goldstone, Robledo, and Tidbinbilla by DSN personnel.  Acronyms and Abbreviations  ==========================    AACS       Attitude and Articulation Control Subsystem    CCS        Computer Command Subsystem    CDU        Command Detector Unit    CRS        Cosmic Ray (investigation) Subsystem    CSIRO      Commonwealth Scientific and Industrial Research                 Organization    DSN        Deep Space Network    DTR        Digital Tape Recorder    FDS        Flight Data Subsystem    HGA        High-Gain Antenna    IRIS       Infra-Red Interferometric Spectrometer    ISAS       Institute for Space and Astronautical Science    ISS        Imaging Science Subsystem    JPL        Jet Propulsion Laboratory    kbps       kilobits per second    LECP       Low-Energy Charged Particle (investigation subsystem)    LGA        Low-Gain Antenna    MAG        Magnetometer (subsystem)    NAIF       Navigation and Ancillary Information Facility    NASA       National Aeronautics and Space Administration    NRAO       National Radio Astronomy Observatory    PLS        Plasma (science investigation) Subsystem    PPS        PhotoPolarimeter Subsystem    PRA        Planetary Radio Astronomy (investigation subsystem)    PWS        Plasma Wave (investigation) Subsystem    RSS        Radio Science Subsystem    RTG        Radioisotopic Thermoelectric Generator    TCS        TeleCommunications Subsystem    UVS        Ultra-Violet Spectrometer    VLA        Very Large Array