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 (NAIF) Node at: http://pds.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 Rings 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 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.