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PDS_VERSION_ID                    = PDS3
LABEL_REVISION_NOTE               = "M.R. SHOWALTER, 2003-09-23"
RECORD_TYPE                       = STREAM

OBJECT                            = DATA_SET
  DATA_SET_ID                     = "VG2-SR/UR/NR-PPS-2/4-OCC-V1.0"

  OBJECT                          = DATA_SET_INFORMATION
  DATA_SET_NAME                   =

  ARCHIVE_STATUS                  = ARCHIVED
  DATA_OBJECT_TYPE                = SERIES
  START_TIME                      = 1981-08-01T00:00:00
  STOP_TIME                       = 1989-12-31T00:00:00
  DATA_SET_RELEASE_DATE           = 2003-10-01

  DATA_SET_TERSE_DESC             = "
    This data set contains stellar occultation data from the Voyager 2
    Photopolarimeter. It includes ring profiles from Saturn, Uranus
    and Neptune at several levels of processing, plus geometry and
    calibration files, software and documentation."

  DATA_SET_DESC                   = "

  Data Set Overview
    This data set contains stellar occultation data obtained by the
    Voyager 2 Photopolarimeter (PPS). It includes ring occultation
    profiles from Saturn, Uranus and Neptune, plus a selection of star
    calibration files. It does not include other types of data
    obtained by the Voyager PPS.

    The specific ring occultation experiments carried out by PPS

      Planet    Star         Comments
      Saturn    delta Sco    Entire ring system
      Uranus    sigma Sgr    Delta, Lambda and Epsilon rings only
      Uranus    beta Per     All rings
      Neptune   sigma Sgr    All rings

    Ring profiles are archived in a variety of formats, from
    completely raw data to calibrated and resampled optical depth
    profiles. Processing software, geometry solutions and calibration
    models are also provided.

    Raw data files contain the integer number of photons counted by
    the PPS within each integration interval. Samples are time-tagged
    by Voyager's internal clock, the Flight Data System (FDS) count.
    Sampling intervals are 10 ms in OC-1 mode and 600 ms in GS-3 mode.

    Data are archived at a variety of levels of processing. The fully
    processed, resampled and calibrated ring profiles have been
    generated as follows:

    (1) Raw data files are edited to flag missing and invalid data and
    are padded as necessary to yield a continuous, uniformly-spaced
    time series of data.

    (2) A geometry model is used to match up each raw sample in the
    edited data file with the corresponding intercept point in the
    ring plane. The GEOMETRY directory contains these files; in some
    cases a geometry file comes in several versions to record the
    differing geometry solutions that various investigators have

    (3) A calibration model is used to determine the number of
    background photons and the number of stellar photons corresponding
    to a given sample. The CALIB directory contains these files. As
    with geometry, multiple calibration models are provided for many

    (4) Using the geometry model, the time-series of raw samples is
    converted to a uniformly-spaced series of radial samples, in which
    each new radial sample is calculated via a weighted average of the
    relevant raw samples. Invalid and missing samples are
    zero-weighted at this step.

    (5) Using the weighting function described above and the known
    noise properties of the raw data, the uncertainty in each
    resampled value is calculated.

    (6) Resampled photon count values are converted to ring opacity
    tau using the calibration model, via the following formula:

    measured_counts = background_counts + stellar_counts*exp(-tau/mu)

    where mu is the cosine of the emission angle (listed in the labels
    of the geometry files).

    (7) By adding or subtracting the derived uncertainty from the
    measured count value and then solving the formula above for tau,
    the confidence interval on the ring opacity is derived.

    A variety of data files are included on this volume.

    Source data (SORCDATA directory)
      The (typically) binary data files as contributed to the Rings
      Node by several investigators. Although file formats are fully
      documented, they vary significantly from investigator to
      investigator and from file to file. These files are provided
      for documentation purposes and are not otherwise supported.

    Raw data (RAWDATA directory)
      These files contain essentially the same data as are found in
      the SORCDATA directory, but in a simplified format consisting of
      binary integers.

    Edited data (EDITDATA directory)
      Edited versions of the raw data, padded with empty records to
      represent continuous, uniformly-spaced time-series of PPS
      samples. Invalid and missing data samples are flagged. These
      are the easiest binary files to use and are recommended for most

    Derived profiles (EASYDATA directory)
      Easy-to-use ASCII data files containing ring opacity and its
      uncertainty vs. radial location in a ring system. Files are
      included at a variety of radial resolutions: 0.1, 0.2, 0.5, 1,
      2, 5, 10, 20 and 50 km per sample. Because the rings of Uranus
      and Neptune are narrow and inclined, there are separate profiles
      for each ring of these planets.

  Ancillary Data
    Many different types of ancillary data files are provided to
    document the PPS experiments and to describe the geometry and
    calibration during each occultation.

    Geometry models (GEOMETRY directory)
      ASCII data files, containing a description of the ring intercept
      'footprint' geometry at times matching the data records in each
      corresponding edited data file. Because the rings of Uranus and
      Neptune are inclined, there are separate geometry files for each
      ring of these planets.

    Vector models (VECTORS directory)
      ASCII data files, containing the time and vector positions of
      the ring intercept point and of Voyager during each occultation
      experiment. These files are provided in a variety of useful
      coordinate systems, including celestial (B1950 and J2000) and in
      planet and ring-centered frames. These can be used to derive
      geometry parameters that are not specifically listed in the
      GEOMETRY directory.

    Trajectory models (TRAJECT directory)
      ASCII data files, containing a position of Voyager relative to
      the center of the planet at times matching the data records in
      each corresponding edited data file.

    Calibration models (CALIB directory)
      ASCII data files, containing the modeled stellar and background
      photon count rates at times matching the data records in each
      corresponding edited data file.

    Instrument pointing reconstructions (JITTER directory)
      ASCII data files, containing the reconstructed instrument
      pointing at times matching the data records in each
      corresponding edited data file.

    Noise models (NOISDATA directory)
      Edited, binary data files in which the known rings of Uranus and
      Neptune have been removed. These are used for fitting some
      calibration models. Their formats are identical to the
      corresponding edited data files.

    Support images (IMAGES directory)
      Voyager support images obtained before, during and after the
      ring occultations.

    Field of view maps (FOVMAPS directory)
      Measured instrument response vs. star position.

    Ephemeris files (SPICE directory)
      Voyager ephemeris files for each of the planetary encounters, in
      SPICE format. For more information about SPICE data and
      toolkits, consult the PDS Navigation and Ancillary Information
      (NAIF) Node at:

  Coordinate System
    All geometric quantities appearing in the labels are in J2000
    coordinates. In this coordinate frame, the z-axis points
    northward along the Earth's J2000 rotation axis and the x-axis
    points toward the First Point of Aries. In some cases, B1950
    coordinate values are also included.

    Included on this disk is the source code for several libraries
    of software tools designed to be used with this and other ring
    occultation data sets.

    Light Label Library (SOFTWARE/OAL directory)
      A toolkit supported by the PDS Central Node for reading and
      writing PDS labels.

    Object Access Library (SOFTWARE/OAL directory)
      A toolkit supported by the PDS Central Node for reading and
      writing data objects described by PDS labels. Built atop the
      Light Label Library.

    Profile Library (SOFTWARE/PROFILE directory)
      A toolkit supported by the PDS Ring-Moon Systems Node for manipulating
      one-dimensional profiles of planetary rings. Built atop the
      Light Label Library and the Object Access Library.

    This data set is archived on compact disc (CDROM) media.
    Organization and formats are according to PDS and ISO 9660
    level 1 standards.

    Most binary data files are in least-significant-byte first, which
    is the native format for PCs and Digital workstations. Users of
    Suns and other workstations may need to swap bytes in some data
    files before use. However, note that most of the edited data
    files contain only single-byte values, so no swapping is likely to
    be needed. Also, note that the software tools provided on this
    volume swap the bytes automatically if this is necessary."


  Confidence Level Overview

    For the primary ring occultation observations, sampling rates of
    10 ms resulted in fewer than 150 photon counts per sample. Thus,
    significant smoothing of the data is needed before accurate ring
    opacities can be derived.

    Uncertainties in the absolute calibration of the PPS instrument
    remain. Different investigators have made different assumptions;
    see the labels on the calibration files for more information.

    Some variations in overall photon count are correlated with
    instrument jitter, apparently related to varying amounts of
    scattered light entering the instrument optics. As a result,
    subtle features in the data that correlate with instrument jitter
    should be regarded with some skepticism.

  Peer Review
    Peer review of this data set was completed in August 2002. The
    peer reviewers were Joshua Colwell (U. Colorado), Amara Graps (Max
    Planck Institute, Heidelberg), and Richard Simpson (Stanford). The
    data set was also reviewed at the PDS Central Node by Ron Joyner.

  Data Coverage and Quality

    The PPS experiments provided complete coverage of the ring systems
    of Saturn, Uranus and Neptune.

    A few small, isolated gaps are present. In the edited data files,
    missing data samples have been indicated via a MISSING_CONSTANT
    flag. Unreliable samples have been replaced by an
    INVALID_CONSTANT flag. Unedited versions of the same data are
    also provided."

  END_OBJECT                    = DATA_SET_INFORMATION

  OBJECT                        = DATA_SET_TARGET
    TARGET_NAME                 = "S RINGS"
  END_OBJECT                    = DATA_SET_TARGET

  OBJECT                        = DATA_SET_TARGET
    TARGET_NAME                 = "U RINGS"
  END_OBJECT                    = DATA_SET_TARGET

  OBJECT                        = DATA_SET_TARGET
    TARGET_NAME                 = "N RINGS"
  END_OBJECT                    = DATA_SET_TARGET

  OBJECT                        = DATA_SET_HOST
    INSTRUMENT_HOST_ID          = VG2
    INSTRUMENT_ID               = PPS
  END_OBJECT                    = DATA_SET_HOST

    REFERENCE_KEY_ID            = "N/A"

END_OBJECT                      = DATA_SET

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