INSTRUMENT: PHOTOPOLARIMETER SUBSYSTEM
    SPACECRAFT: VOYAGER 2
 
 
  Instrument Overview
  ===================
    The Voyager Photopolarimeter Experiment (PPS) utilizes a general
    purpose filter photometer/polarimeter optimized for the encounter
    phase of the mission.
 
      Instrument Id                  : PPS
      Instrument Host Id             : VG2
      Pi Pds User Id                 : ALLANE
      Instrument Name                : PHOTOPOLARIMETER SUBSYSTEM
      Instrument Type                : PHOTOPOLARIMETER
      Build Date                     : NULL
      Instrument Mass                : 2.55
      Instrument Length              : NULL
      Instrument Width               : NULL
      Instrument Height              : NULL
      Instrument Serial Number       : NULL
      Instrument Manufacturer Name   : NULL
 
 
  Scientific Objectives
  =====================
    From [LILLIEETAL1977], pp. 159-160.
 
    The primary scientific objectives of the photopolarimeter
    investigation on the Voyager mission were divided into three
    categories---studies of atmospheres, satellite surfaces, and
    rings.
 
 
    Atmospheric Objectives
    ----------------------
      Specific objectives associated with the atmospheres were:
 
      (1) to use intensity and polarization measurements as a
      function of viewing angle and wavelength to determine the
      macrostructure (vertical distribution of atmospheric aerosols)
      and microstructure (particle size, shape, and probable
      composition) of atmospheres;
 
      (2) to use polarization measurements at large phase angles to
      constrain the particle shapes and compositions within clouds;
 
      (3) to search for dark side auroral emissions.
 
 
    Satellite Surface Objectives
    ----------------------------
      Specific scientific objectives associated with the surfaces of
      satellites were:
 
      (1) to measure or set upper limits on the density of their
      atmospheres;
 
      (2) to determine the texture and probable composition of their
      surfaces;
 
      (3) to determine the bond albedo.
 
      (4) to map the distribution of sodium vapor in the vicinity of
      Io and in Jupiter's magnetosphere.
 
 
    Planetary Ring Objectives
    -------------------------
      Specific scientific objectives for the study of planetary rings
      were:
 
      (1) to use intensity and polarization measurements of scattered
      light as a function of wavelength and viewing angle to provide
      information on the size, shape, and probable composition of the
      ring particles, as well as their density and radial
      distribution;
 
      (2) to use observations of the extinction and scattering of
      starlight to give additional information on particle size and
      ring optical depth.
 
 
  Optics and Detectors
  ====================
    From [LILLIEETAL1977], pp. 160-161.
 
    The instrument consists of the following components:
 
    (1) a 6-inch f/1.4 Dahl-Kirkham type Cassegrain telescope;
 
    (2) a four-position aperture wheel providing circular fields of
    view (FOVs) with diameters of 1/16, 1/4, 1, and 3.5 degrees;
 
    (3) an eight-position analyzer wheel with open, dark and
    calibration positions, plus with five polacoat analyzers with
    transmitting axes oriented at 0, 60, 120, 45, and 135 degrees
    rotation;
 
    (4) an eight-position filter wheel holding thin interference
    filters (see below);
 
    (5) an EMR 510-E-06 photomultiplier tube (PMT) with a tri-alkali
    (S-20) photocathode.
 
    Individual photon events in the PMT are detected with pulse
    counting electronics.
 
    The wide dynamical range required by the mission (4 to 6.e11
    photons /cm^2/s/Angstrom/ster) could be accommodated by FOV
    changes and an electronic gain change in the PMT capable of
    reducing the instrumental sensitivity by a factor of ~50.
 
    A shadow caster prevents direct sunlight from entering the
    aperture for phase angles < 160 degrees.  A solar sensor turns
    off the high voltage if the PPS is pointed within 20 degrees of
    the Sun.
 
 
  Filters
  =======
    The effective wavelength of each filter, its nominal band width,
    typical instrumental sensitivities, and the particular atomic and
    molecular species to which it is sensitive are listed below (From
    [LILLIEETAL1977], Table I, p. 164).
 
        Position  Effective   Half-power   Nominal    Spectral
         number   wavelength  bandwidth  sensitivity* features
                  (Angstrom)  (Angstrom)
        ------------------------------------------------------------
           0         5900        100         30       Sodium D
           1         4900        100         50       H beta
           2         3900        100         45       He I, Ca II
           3         3100        300         40       OH Emission
           4         2630        300         25       O3, Mg II,
                                                      Chromophore
           5         2350        300         20       Si I, Rayleigh
                                                      scattering
           6         7500        300          8       K I, Aerosol
                                                      scattering
           7         7270        100          4       CH4 absorption
                                                      band
 
    *For a point source in counts accumulated during an 0.4 second
    integration per incident photon /cm^2/s/Angstrom.
 
 
  Operational Modes
  =================
    From [LILLIEETAL1977], pp. 161-162.
 
    The planned normal operational (encounter) mode of the PPS was to
    step through a programmed sequence of 40 filter/analyzer wheel
    combinations once every 24 sec.  Each measurement was to consist
    of a 400 millisecond integration period followed by a 200 ms
    period during which the next filter or analyzer would be stepped
    into place.  A full measurement set would thus consist of
    readings in the open, 0 degrees, 60 degrees, 120 degrees, and
    dark positions of the analyzer wheel for each of the eight
    filters.
 
    NOTE: Equipment failures and improved understanding of instrument
    usage considerations, however, caused substantial changes in this
    plan.  See the section on operational considerations below for
    further details.
 
    For stellar occultation and satellite eclipse measurements, the
    experiment was operated with filters and analyzers fixed in
    position and a 10-ms integration period.  This provided rapid
    measurements in order to resolve spikes in the light curves due
    to turbulence in the occulting planet's atmosphere, as seen in
    Earth-based observations.
 
    For ring observations, stellar occultations were observed using
    filter #4 (2650 Angstroms) and polarizer #7 (45 degrees).  A
    10-ms integration time was used to obtain maximum time resolution
    and the FOV set to 1 degree.
 
 
  Measured Parameters
  ===================
    From [LILLIEETAL1977], pp. 164-165.
 
    PPS raw values represent the number of photons events in the PMT
    counted by the detector during the given integration time.  Based
    on FOV, filter, and gain settings, this count could then be
    converted to an intensity.
 
    Four Stokes' parameters, I, Q, U, and V, completely specify the
    state of polarization of a quasi-monochromatic wave.  The
    advantages of measuring and using Stokes' parameters are:
 
      (1) They all have the same dimension of intensity;
      (2) They are additive;
      (3) From the Stokes' parameters it is possible to generate the
          degree and plane of polarization and ellipticity.
 
    Since the value of V is generally small and its measurement
    requires a much more complex instrument, only I, Q, and U are
    determined.
 
    Total intensity I can be measured either with no polarizer in the
    optical train, or by summing polarizers with transmission axes at
    0, 60 and 120 degrees:
 
          I = 2[I(0) + I(60) + I(120)] / 3 .
 
    The Stokes' parameter Q is given by
 
          Q = 2[2I(0) - I(60) - I(120)] / 3 .
 
    Similarly, U can be determined from
 
          U = 2[I(60) - I(120)] / sqrt(3) .
 
    The PPS data readout consists of a 30-bit digital word, of which
    20 bits provided the data count accumulated during the
    integration period, and 10 bits indicated instrument status.  In
    order to reduce the telemetry rate, data count bits were log
    compressed in the spacecraft FDS to 14 bits (a 10 bit mantissa
    and 4 bit exponent).  Log compression was removed from the FDS
    for the PPS occultation modes.  The nominal data rate was thus 40
    bps, with a maximum of 1023 1/2 bps and a minimum of 0.6 bps.
 
    NOTE: Data obtained during stellar occultation observations were
    not subjected to compression.
 
 
  Calibration
  ===========
    From [LILLIEETAL1977], p. 161.
 
    In-flight calibration was accomplished by observing a set of
    standard stars of known brightness and polarization, the sunlight
    scattered by an on-board calibration target (unpolarized light),
    and the light from stars and the planets reflected into the PPS
    from a mirror tilted to the Brewster angle (yielding 100%
    polarized light).  An internal Cerenkov radiation source mounted
    on the analyzer wheel provided a short term measure of the
    instrument's stability but was not used because comparison
    pre-flight calibration data was lost.
 
    The instrument is capable of measuring the polarization of
    reflected light from the planets and their satellites with a
    precision of +/- 0.5%, and their relative brightness with an
    accuracy of +/- 0.5 to 1%.  Absolute calibration is known to +/-
    3% in the visible and infrared, and to +/- 10% in the UV.  For
    measurements of low surface brightnesses the instrument's
    sensitivity ranged from ~140 counts/Rayleigh in the visible and
    UV to ~20 counts/Rayleigh in the infrared.
 
    Further discussions of intensity and polarization measurements
    can be found in, for example, [WESTETAL1981], [WESTETAL1983], and
    [PRYOR&HORD1991].
 
 
  Operational Considerations
  ==========================
    The PPS aboard Voyager 1 suffered extreme sensitivity loss before
    and during Jupiter encounter.  This was deemed to be irreparable
    and the instrument was turned off before Saturn encounter.
    Voyager 1 data were never analyzed or archived.
 
    The PPS instrument on board Voyager 2 suffered two hardware
    failures that affected the ability to access wheel positions.  A
    spacecraft decoder failure affected the analyzer and a PPS
    internal chip failure affected the available filter positions.
    At Jupiter, filter positions 0, 2, 4, and 6 were used.
    Afterwards, only three positions, 2, 4, and 6, were used.  Four
    of the eight analyzer wheel positions were available.  Of these,
    135 and 45 degree orientations at wheel positions 6 and 7 were
    used to acquire polarization information.  Before closest
    approach at Jupiter, data taken are unreliable due to scattered
    light.
 
    Measurement sequences could be modified by changing the PPS
    look-up-table (LUT) in the spacecraft's Flight Data System (FDS).
    This controlled the filter and analyzer wheel positioning.  The
    changes were predominantly a result of instrument electronic
    failures and PMT usage issues more fully understood as the
    mission progressed.  Knowledge of which FDS load was in effect
    during each data observation is therefore necessary for proper
    analysis.