Voyager PPS DATASET.CAT
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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 =
"VG2 SR/UR/NR PPS EDITED/RESAMPLED RING OCCULTATION V1.0"
ARCHIVE_STATUS = ARCHIVED
DATA_SET_COLLECTION_MEMBER_FLG = Y
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
PRODUCER_FULL_NAME = "MARK R. SHOWALTER"
DETAILED_CATALOG_FLAG = N
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
are:
-----------------------------------------------------------
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.
Parameters
==========
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.
Processing
==========
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
obtained.
(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
rings.
(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.
Data
====
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
purposes.
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 Facility
(NAIF) Node at:
//pds.jpl.nasa.gov/naif.html
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.
Software
========
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.
Media/Format
============
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_NOTE = "
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
OBJECT = DATA_SET_REFERENCE_INFORMATION
REFERENCE_KEY_ID = "N/A"
END_OBJECT = DATA_SET_REFERENCE_INFORMATION
END_OBJECT = DATA_SET
END
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