Voyager PPS TUTORIAL.TXT
Return to PPS data set page.
PDS_VERSION_ID = PDS3
RECORD_TYPE = STREAM
DATA_SET_ID = "VG2-SR/UR/NR-PPS-2/4-OCC-V1.0"
OBJECT = TEXT
PUBLICATION_DATE = 2003-09-30
AUTHOR_FULL_NAME = "MARK R. SHOWALTER"
NOTE = "Voyager PPS Ring Occultation
Data Set Tutorial"
END_OBJECT = TEXT
END
Voyager PPS Ring Occultation Data Set Tutorial
==============================================
This document provides a tutorial on how to make optimal use of this
data set. It is organized as follows:
1. Data Set Overview
2. Directory Structure
3. File Names
4. File Formats
5. PDS Labels
6. Voyager PPS Overview
7. Processing Guidelines
8. PPS Calibration
9. References
1. 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, egress.
Uranus sigma Sgr Delta, Lambda and Epsilon rings only,
ingress and egress.
Uranus beta Per All rings, ingress and egress.
Neptune sigma Sgr All rings, ingress.
------------------------------------------------------------
2. Directory Structure
======================
This volume contains a large variety of files, preserving the PPS data
at various levels of processing. Also included are ancillary files
related to the geometry and calibration of the observations.
The files in this data set fit into several general categories and are
grouped into directories as follows:
----------------------------------------------------------------
Category Directories
----------------------------------------------------------------
Derived ring profiles EASYDATA
Primary data/ancillary files EDITDATA, GEOMETRY, CALIB
Additional ancillary files VECTORS
Processing history files SORCDATA, RAWDATA, NOISDATA,
TRAJECT
Support files IMAGES, FOVMAPS, SPICE, JITTER
Documentation DOCUMENT, CATALOG, INDEX
Software tools SOFTWARE
----------------------------------------------------------------
The EASYDATA directory tree contains simple, calibrated ring profiles
that should meet the needs of most investigators interested in a
particular aspect of the ring, but not wishing to dig into the details
of the PPS data set.
The primary data files in the EDITDATA, GEOMETRY and CALIB directories
should meet the need of most remaining users. The Ring-Moon Systems Node has gone to
considerable effort to analyze, format and validate these files to make
them as reliable as possible and as simple as possible to use.
2.1 Derived Ring Profiles
-------------------------
EASYDATA - This directory contains easy-to-use derived ASCII profiles
tabulating 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, separate profiles are
provided for each ring of these planets.
For users who wish to make plots of ring opacity without worrying about
the many subtle issues relating to calibration, geometry, and
sensitivity of the PPS data, this directory is the place to look. The
data are suitable for most theoretical analyses and for comparison with
other data sets.
2.2 Edited Raw Data and Simplified Ancillary Data Files
-------------------------------------------------------
EDITDATA - We recommended that users needing access to the raw PPS
samples refer to the EDITDATA directory. This directory contains edited
versions of the raw data, converted to a common, easy-to-use binary
format. The Ring-Moon Systems Node has carefully processed these files so that they
have the following properties: (1) each data file contains a continuous,
uniformly-spaced time-series of PPS samples; and (2) invalid and missing
data samples have been flagged. This directory also contains data from a
number of star calibration experiments, in which the PPS observed a star
without any rings present.
Note that some very simple sample programs are provided in the SOFTWARE
directory to help users access the raw data in these binary files.
Programs DUMPOC1.FOR and DUMPOC1.C retrieve data from the OC-1 mode
files; programs DUMPGS3.FOR and DUMPGS3.C retrieve data from the GS-3
mode files. See section 6.2 below for more information about operating
modes.
Two other directories contain ancillary data files to support the
analysis of the edited data files. In both cases, the files contain
ASCII files that tabulate information about the occultation at times
that match the data records in the corresponding edited data file.
GEOMETRY - Files in this directory tabulate the ring intercept geometry
(ring intercept time, radius, and longitude).
CALIB - Files in this directory tabulate models for the stellar and
background photon count rates during each occultation. Using this
information, estimates of ring opacity can be derived.
Over the years, several investigators have arrived at different
solutions for the geometry and calibration during the PPS experiments.
Because of this, the Ring-Moon Systems Node has provided multiple versions of these
files for many occultations. Our own models for occultation geometry and
calibration are also included.
2.3 Additional ancillary files
------------------------------
VECTORS - Files in this directory tree tabulate 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.
2.4 Processing History Files
----------------------------
The Ring-Moon Systems Node has obtained data from a variety of sources and in a
variety of formats. Our work has involved extensive re-formatting to
simplify the use of this data set, as well as some actual editing to
identify questionable data samples. We have provided the data in less
processed formats to document our own processing, to support users who
might wish to "second guess" our processing, and also to support those
investigators who might already be familiar with the data in a different
format.
SORCDATA - This directory contains "source" data files exactly as they
were received by the Ring-Moon Systems Node. 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.
RAWDATA - Files in this directory contain essentially the same data as
are found in the SORCDATA directory, but in a simplified, uniform format
consisting of binary integers. A summary of how each of these files was
generated from its source files is found in file label.
NOISDATA - This directory contains a set of binary data files identical
in format to those found in the EDITDATA directory. However, in these
files the known rings of Uranus and Neptune have been removed. These
were generated by the Ring-Moon Systems Node and were used for fitting some
calibration models.
TRAJECT - This directory contains the Voyager trajectory model (position
relative to the planet center vs. time) used in the derivation of all
geometric parameters.
2.5 Support Files
-----------------
The Ring-Moon Systems Node has gathered a number of additional files to support more
detailed investigations of the PPS ring occultation experiments.
IMAGES - This directory contains Voyager images obtained during the ring
occultation experiments, which can be used for more detailed pointing
reconstructions.
FOVMAPS - This directory shows the result of an attempt by the PPS team
to map out the variation in the response of the PPS instrument to the
position of a star in its 1-degree field of view.
SPICE - This directory contains SPICE kernels recording the ephemeris
Voyager 2 during each encounter, along with the planet and its moons;
these files are to be used with the SPICE toolkit, available from the
PDS NAIF Node at
//pds.jpl.nasa.gov/naif.html
JITTER - This directory contains reconstructions of the instrument
pointing during each occultation experiment.
2.6 Documentation
-----------------
Several additional directories contain documentation associated with
this data set. These are directories are required by PDS standards.
DOCUMENT - This directory contains a variety of documents associated
with the PPS experiments. This directory also contains subdirectories
named CALIB, EDITDATA, FOVMAPS, GEOMETRY, JITTER, RAWDATA, SORCDATA,
which contain programs and intermediate data files developed at the
Ring-Moon Systems Node for the production of the files found in the associated
root-level directories. These files are provided for documentation
purposes only and are not otherwise supported.
CATALOG - This directory contains the PDS "Catalog Objects" providing
general information about the data set. The files are:
--------------------------------------------------------------
File name Topic
--------------------------------------------------------------
DATASET.CAT This data set
DSCOLL.CAT The data set collection (ring occultations from
Voyager)
INST.CAT The PPS instrument
INSTHOST.CAT The instrument host, Voyager 2
MISSION.CAT The overall Voyager mission
PERSON.CAT Personnel involved in the production of this
data set (Ring-Moon Systems Node staff & PPS team members)
REF.CAT Bibliographic references.
SOFTWARE.CAT Supported software tools.
--------------------------------------------------------------
INDEX - This directory contains a tabulation of all the data files in
this data set.
2.7 Software Tools
------------------
SOFTWARE - This directory contains the source code for several toolkits
supporting the processing of this (and other) ring occultation profiles.
More information about these toolkits is found below (Section 7.4).
3. File Names
=============
ISO-9660 Level 1 standards for CD-ROMs require that file names be a
maximum of eight characters, followed by an extension or type of up to
three characters.
3.1 File Extensions
-------------------
The major file types used on this volume are:
Binary files:
Images *.IMG, *.GIF
Other binary data files *.DAT
SPICE ephemeris files *.BSP
Postscript documentation files *.PS, *.EPS
Source data files (for documentation only) *.FOT, *.GS3, *.SCO,
ASCII text files:
PDS labels *.LBL
ASCII tables and indices *.TAB
PDS catalog files *.CAT
Text documentation files *.TXT, *.ASC
TeX documentation files *.TEX
Source code files *.C, *.H, *.FOR,
*.INC, *.PRO
Software build scripts *.MAK, *.COM, *.DEF
SPICE kernels *.TLS, *.TPC
SPICE transfer ephemeris files *.XSP
On this volume, a few files obtained from other sources have unique
extensions. See the associated PDS label for more information.
3.2 File Naming Conventions
---------------------------
Most of the supported data files provided on this volume are named
according to the following nomenclature:
Characters 1-3: Experiment indicator
1st character: P for PPS.
2nd character: S for Saturn; U for Uranus; N for Neptune.
3rd character: 1-8, indicating a stellar occultation sequence
number at the given planet.
Based on these rules, the following indicators have been used:
PS1 = Saturn delta Sco ring occultation data.
PS2 = Extended Saturn delta Sco occultation data.
PS3-8 = Star calibration observations (no rings).
PU1 = Uranus sigma Sgr ring occultation data.
PU2 = Uranus beta Per ring occultation data.
PU3-6 = Star calibration observations associated with Uranus (no
rings).
PN1 = Neptune sigma Sgr ring occultation data.
Character 4: File type
C = calibration models.
D = edited data.
G = geometry files.
J = jitter (instrument pointing) files.
N = noise data.
P = resampled, calibrated profiles.
R = raw data.
T = trajectory files.
Characters 5-6: Version number
A two-digit number indicating the version number of the given file
(using a leading zero if necessary). In general, the meaning of
this number depends on the experiment and file type.
Note that the vector files are exceptional in that the fifth
character specifies the coordinate system for the vectors. This
leaves only the sixth digit to use as a version number.
Characters 7-8: Supplemental
These characters are only used when needed to further identify a
file. They are used to distinguish rings, ingress vs. egress,
coordinate frames, etc. These characters are explained in the
*INFO.TXT file found in each relevant subdirectory.
These naming rules apply to the files in the CALIB, EASYDATA,
EDITDATA, GEOMETRY, JITTER, NOISDATA, RAWDATA, TRAJECT and VECTORS
directories. Non-data files (in the CATALOG, DOCUMENT, INDEX and
SOFTWARE directories) do not follow these conventions. Also, some
ancillary files (FOVMAPS, IMAGES, and SPICE) do not. Finally, the
source data files provided by other investigators (SORCDATA directory)
have been archived under their original names wherever possible.
4. File Formats
===============
4.1 ASCII Text Files
--------------------
Different popular operating systems use different standards for how
ASCII text files are formatted. On Unix systems, lines are terminated
by a >lf< (linefeed character, control-J, ASCII 10). On Macintosh
computers, lines are terminated by a >cr< (carriage return character,
control-M, ASCII 13). On PCs, lines are terminated by a >cr<>lf< pair.
PDS standards require all text files to use >cr<>lf< line termination.
If the PDS label indicates INTERCHANGE_FORMAT = ASCII or RECORD_TYPE =
STREAM, then this line termination is in use.
On occasion, users on Unix and Macintosh computers may need to change
the line termination on some text files before they can use them. This
can be handled via text editors or a variety of utilities. For example,
the Unix tr (translate) command can be used to change carriage returns
to blanks:
tr "\015" " " >oldfile.txt <newfile.txt
Note that some software tools provided with this data set could fail if
the extraneous >cr< characters are simply removed from ASCII-format data
files rather than being replaced by blanks; the reason is that removing
the >cr< changes the record lengths within the file, which could make
the file incompatible with its PDS label.
4.2 Binary Data Files
---------------------
The binary data files provided on this volume are tables and images.
As described in the labels, most binary files consist of one- and
two-byte integers. For two-byte integers, the least significant byte
appears first, followed by the most significant byte with (optional)
sign. Users of PCs and Digital workstations will find that these files
are in the native machine format. For users of Macintoshes and
workstations from Sun and Silicon Graphics, binary numbers read from
these files will need to be byte-swapped first. However, note that the
software tools included in the SOFTWARE directory take care of this
conversion automatically.
Some binary files in the SORCDATA subdirectory contain four-byte
(single precision) floating-point numbers in Vax format. These are
source data files as provided by the original investigators; the same
data can be found in integer form in the RAWDATA and EDITDATA
subdirectories.
5. PDS Labels
=============
5.1 Types of Labels
-------------------
On this volume, every file has a PDS label. Most are in the form of
detached labels, where the label corresponding to a given file has the
same name but an extension ".LBL". For example, the file PS1D01.DAT is
described by the label file PS1D01.LBL.
There are a few exceptions to this rule. Some text files (like this
one) have attached labels, meaning that the label information can be
found at the top of the actual file. These can be recognized by the
fact that the file ends with .TXT and there is no corresponding file
ending in .LBL. Information files *INFO.TXT that appear in most
directories and describe that directory's contents use this approach.
Finally, a few directories employ "combined-detached" labels, in which
a single label file describes most if not all the files in the
directory. In this case, the label file is given the same name as the
enclosing directory, with the .LBL extension. For example, the
directory DOCUMENT/EDITDATA/ contains a single combined-detached label
file EDITDATA.LBL.
5.2 PDS Label Structure
-----------------------
PDS labels contain nothing but ASCII text and can be viewed using any
editor or word processor software. However labels have a very specific
format that can be read by humans and also (relatively) easily parsed by
computers. The PDS has developed toolkits called the Label Library (L3)
and the Object Access Library (OAL), which makes it easy to read,
manipulate and write PDS labels and the data files that they describe,
using programs written in C or FORTRAN. The directory SOFTWARE/OAL on
this volume contains the source code for both of these libraries.
A PDS label consists of a sequence of expressions of the form "keyword
= value". These keywords are sometimes nested inside data objects,
indicated by "OBJECT = xxx" and terminated by "END_OBJECT = xxx".
These data objects describe specific components of the corresponding
data files. PDS objects can be nested; for example a PDS TABLE object
generally contains multiple COLUMN objects. Keywords inside the TABLE
object describe the overall properties of the table, and afterward the
keywords inside each COLUMN object describe one particular column.
Here is an annotated example, excerpted from file
EASYDATA/KM010/PS1P01.LBL, which describes the derived profile of
Saturn's ring system sampled at 10 km intervals. The file begins with
information about the structure of the data file:
PDS_VERSION_ID = PDS3
RECORD_TYPE = FIXED_LENGTH
RECORD_BYTES = 50
FILE_RECORDS = 7051
^SERIES = "PS1P01.TAB"
This indicates that file PS1P01.TAB is contains 7051 fixed-length
records, each of which is 50 characters long (including the terminal
>cr<>lf< pair). Next come some cataloging parameters related to the
file:
DATA_SET_ID = "VG2-SR/UR/NR-PPS-2/4-OCC-V1.0"
RING_OBSERVATION_ID = "S/OCC/VG2/PPS/1981-08-25/DELTA_SCO"
PRODUCT_ID = "KM010/PS1P01.TAB"
PRODUCT_TYPE = RING_PROFILE
PRODUCT_CREATION_TIME = 2000-06-01T16:00:00
SOURCE_PRODUCT_ID = {"PS1D01.DAT",
"PS1G02.TAB",
"PS1C01.TAB"}
SPACECRAFT_NAME = "VOYAGER 2"
SPACECRAFT_ID = VG2
INSTRUMENT_NAME = "PHOTOPOLARIMETER SUBSYSTEM"
INSTRUMENT_ID = PPS
TARGET_NAME = "S RINGS"
START_TIME = 1981-08-26T00:00:39.342
STOP_TIME = 1981-08-26T01:55:00.793
SPACECRAFT_CLOCK_START_COUNT = "44001:22:666"
SPACECRAFT_CLOCK_STOP_COUNT = "44003:45:624"
OBSERVATION_TYPE = OCCULTATION_PROFILE
OCCULTATION_TYPE = STELLAR
STAR_NAME = "DELTA SCO"
FEATURE_NAME = "RING SYSTEM"
RING_OCCULTATION_DIRECTION = EGRESS
PLANETARY_OCCULTATION_FLAG = N
WAVELENGTH = 0.264 /* microns */
INSTRUMENT_MODE_ID = OC1
RING_EVENT_START_TIME = 1981-08-26T00:00:38.420
RING_EVENT_STOP_TIME = 1981-08-26T01:55:00.193
MINIMUM_RING_RADIUS = 72000.
MAXIMUM_RING_RADIUS = 142500.
RADIAL_RESOLUTION = 10.
INCIDENCE_ANGLE = 61.30638
This label describes a data file containing derived ring opacity vs.
radius. It is formatted as a table with columns. This is now described
by a SERIES object:
OBJECT = SERIES
NAME = OCCULTATION_PROFILE
INTERCHANGE_FORMAT = ASCII
ROWS = 7051
COLUMNS = 6
ROW_BYTES = 50
SAMPLING_PARAMETER_NAME = RING_INTERCEPT_RADIUS
SAMPLING_PARAMETER_UNIT = KILOMETER
MINIMUM_SAMPLING_PARAMETER = 72000.
MAXIMUM_SAMPLING_PARAMETER = 142500.
SAMPLING_PARAMETER_INTERVAL = 10.
DESCRIPTION = "This is a radial profile of ...."
Most of the data products on this volume are structured as either a
table or a series. Both of these have the same logical structure---a
set of rows structured identically, each containing a sequence of column
values. The difference is that a series has rows that are uniformly
spaced in a particular "sampling parameter", as indicated by the
SAMPLING_PARAMETER keywords shown above. They indicate that this file
is sampled uniformly in ring intercept radius, with one record every 10
km, starting at 72,000 km and ending at 142,500 km.
Next come a set of six COLUMN objects, each describing one column in the
table. For example, the fourth column is described this way:
OBJECT = COLUMN
NAME = MEDIAN_NORMAL_OPACITY
DATA_TYPE = ASCII_REAL
START_BYTE = 26
BYTES = 7
FORMAT = "F7.4"
MAXIMUM = 99.
UNIT = 'N/A'
DESCRIPTION = "Estimate of the mean normal
opacity for a band of ring material centered at the given radius.
For opaque rings, this value is set to 99. For unconstrained
segments of the data, this value is set to 0."
END_OBJECT = COLUMN
It indicates that characters 26-32 in each row contain the median normal
opacity of the ring, in F7.4 format. After the last column object, the
file ends as follows:
END_OBJECT = SERIES
END
The first line here marks the end of the SERIES object description, and
the second marks the end of the label itself.
Here are the first three lines of the data file:
72000.00, 55.469, 0.777,-0.0199,-0.0290,-0.0106
72010.00, 54.832, 0.774,-0.0124,-0.0216,-0.0031
72020.00, 54.425, 0.770,-0.0076,-0.0169, 0.0018
_______
The six columns are, in order, named RING_INTERCEPT_RADIUS, MEAN_SIGNAL,
MEAN_SIGNAL_UNCERTAINTY, MEDIAN_NORMAL_OPACITY,
NORMAL_OPACITY_LOWER_LIMIT, and NORMAL_OPACITY_UPPER_LIMIT. The column
described by the MEDIAN_NORMAL_OPACITY column is underlined.
The detailed definition of every PDS keyword appearing in the label of a
data file can be found in the file DOCUMENT/PDSDD.TXT. These
definitions have been extracted from the PDS Data Dictionary.
See the PDS Standards Reference (JPL D-7669) for a complete description
of PDS label formats. This document can also be found on line at
//pds.jpl.nasa.gov/stdref/
6. Voyager PPS Overview
=======================
6.1 Voyager FDS Time Tags
-------------------------
Each record in each binary data file has a time tag generated by
Voyager's "Flight Data System" (FDS) clock. FDS clock counts consist
of three integers, typically called the "FDS hours", "FDS minutes",
and "FDS seconds". An FDS hour has a duration of 48 true minutes; an
FDS minute has a duration of 48 true seconds; an FDS second has a
duration of 60 milliseconds. Hence, there are 800 FDS seconds per
minute (numbered 1 through 800) and 60 FDS minutes per hour (numbered
0 through 59). The FDS hour value is typically a five-digit number.
In this data set, FDS clock counts are typically formatted
"hhhhh:mm:sss".
6.2 PPS Operating Modes
-----------------------
The PPS occultation data were acquired in two different instrument
modes, called OC-1 and GS-3. OC-1 mode was used for the actual ring
occultations and for some star calibration experiments. Here the
photons counts were sampled every 10 ms (7.5 ms integration plus 2.5
ms readout, according to Graps and Lane (1986). OC-1 data samples are
grouped into records of 600 samples, corresponding to 100 FDS seconds
(6 true seconds). GS-3 is a slower counting mode used for additional
star calibration experiments. In this case the photon counts were
sampled at 600 ms intervals (400 ms integration plus 200 ms readout
according to Graps and Lane). GS-3 samples are grouped into records
of 80 samples, corresponding to one FDS minute (48 true seconds).
For all of these experiments, the 0.264 micron filter was used,
along with the 1-degree aperture. See CATALOG/INST.CAT for more
information about the PPS instrument.
7. Processing Guidelines
========================
As noted above, many users will find that files in the EASYDATA
directories meet their needs for PPS data. These contain tabulations of
ring opacity and its uncertainty, resampled to be uniformly spaced in
ring radius.
In this section I discuss some of the technical details behind the
generation of these calibrated profiles.
7.1 Edited Data
---------------
The primary input file for one of these profiles is an edited data file,
as are found in the EDITDATA directory. The key property of these files
is that they are uniformly spaced in time; there are no gaps. In
addition, all invalid or missing samples have been flagged, meaning that
they have been replaced a particular numeric value.
The edited occultation data files (OC-1 mode) are organized in 612-byte
binary records. Each record consists of 600 unsigned, single-byte
samples, followed by an FDS time tag consisting of three four-byte
integers (FDS hours, minutes and seconds). The time tag refers to the
beginning of the corresponding data record. Sample values are the
integral number of photons counted by the PPS during its 10-ms
integration interval (7.5 ms of integration followed by 2.5 ms dead time
during readout). Thus, each record contains six true seconds (or 100
FDS seconds) of data. Missing samples are indicated by a value of 255;
invalid samples are indicated by a value of 254.
Note that some edited data files are from star calibration experiments
obtained in GS-3 mode, where the PPS sampling rate is slower. These
files are organized in 172-byte binary records, consisting of 80 signed,
two-byte integer samples followed by the FDS time tag. Sample values
are photons counted during its 600-ms integration interval (400 ms of
integration followed by 200 ms for readout). Thus, each record contains
48 true seconds (or 1 FDS minute) of data. Missing samples are
indicated by a value of -1; invalid samples are indicated by a value of
-2.
The editing that has been performed on each file is summarized in the
file's label, and also in the file EDITDATA/DATAINFO.TXT. The programs
developed at the Ring-Moon Systems Node to perform the editing are found in the
DOCUMENT/EDITDATA subdirectory (although these files are provided for
documentation purposes only and are not supported). Users who have
reason to can revisit this process and generate their own edit of the
data.
Because the missing and invalid samples in an edited data file contain
specific values, they can be easily recognized. Users who write
software to read the edited data files should test all samples and
ignore any that have one of these values.
As noted above, the edited data files are continuous. As necessary,
missing records have been filled in by empty records (containing nothing
but the missing flag, 255). As a result, it is relatively easy to index
directly into an edited data file to find a particular record.
7.2 Geometry Files
------------------
The edited data files are labeled as SERIES objects, in which the
SAMPLING_PARAMETER is called RECORD_INDEX. Associated geometry files
are indexed using exactly the same sampling parameter, so that it is
relatively straightforward to find the values of geometry parameters
associated with the beginning of a given record. Geometry parameters
can be interpolated to generate the value associated with a particular
sample.
Files in the GEOMETRY directory tabulate the location in the ring plane
sampled by the PPS instrument at the beginning of a given record. In
addition to the RECORD_INDEX, columns in these files include
RING_INTERCEPT_TIME (corrected for light travel time),
RING_INTERCEPT_RADIUS (in km), RING_INTERCEPT_LONGITUDE (measured from
the ring plane's ascending node on the Earth's equator of J2000),
B1950_RING_INTERCEPT_LONGITUDE (same as previous but measured using the
Earth's equator of B1950), and SPACECRAFT_EVENT_TIME (in seconds). The
RING_INTERCEPT_TIME and SPACECRAFT_EVENT_TIME are measured in seconds
relative to a REFERENCE_TIME specified in the corresponding COLUMN
object.
Values in these files are generally provided at much greater precision
than can be considered reliable. The additional precision makes it
easier to interpolate the models smoothly.
For the rings of Saturn, two geometry solutions are provided, PS1G01 and
PS1G02. The former only contains only RING_INTERCEPT_RADIUS and is the
result of an early polynomial fit to the Saturn ring geometry. It is
included only for users who might wish to compare new results with old
ones obtained using that particular solution. For all other purposes,
solution 02 is recommended.
Because many rings of Uranus are inclined, the ring intercept geometry
depends on the specific inclination and orientation of a given ring. As
a result, it was necessary to provide a unique geometry file for each
ring. The naming convention is as follows:
PUnG01rd.TAB/.LBL
where
n = occultation index: 1 = sigma Sgr; 2 = beta Per.
r = ring identifier: 6 = Ring 6;
5 = Ring 5;
4 = Ring 4;
A = Ring alpha;
B = Ring beta;
N = Ring eta;
G = Ring gamma;
D = Ring delta;
L = Ring lambda;
E = Ring epsilon.
d = occultation direction: I = ingress; E = egress.
In addition, the files PUnG01.TAB/.LBL (with no ring identifier or
occultation direction specified) contain ring intercept geometry for a
hypothetical, purely equatorial ring system.
For the rings of Neptune, two geometry solutions are provided. PN1G01
describes the intercept for a hypothetical equatorial ring system using
the planetary pole inferred by Jacobson et al. (1991). PN1G02 describes
a hypothetical ring system using the pole inferred by Porco (1991) for
the Adams Ring; this file is likely to more accurate for the Adams Ring
and maybe for other rings as well.
The highest-numbered version of each geometry file was generated by the
Ring-Moon Systems Node for this data set. The files are believed to be extremely
accurate, although refinements to the Voyager trajectories, poles or
ring inclinations could change results slightly. The software used to
generate these files was written by Philip Nicholson of Cornell and is
archived in the DOCUMENT/GEOMETRY/PDN subdirectory, although these
programs are not supported.
For users who might wish to revisit the geometry solutions, note that
the spacecraft trajectory solutions used by Nicholson are archived in
the GEOMETRY directory under the names PxxT01*.TAB/.LBL. The files
tabulate the position of Voyager relative to the center of the planet at
times corresponding to the beginning of individual PPS records.
Alternatively, users can consult the SPICE ephemeris files in the SPICE
directory.
7.3 Calibration Files
---------------------
Like the geometry files, calibration files are labeled as SERIES objects
and use the same SAMPLING_PARAMETER as the corresponding edited data
files.
To convert a measured photon rate to ring opacity "tau", one requires
some additional information. The relationship between measured counts
and tau is:
measured_counts = background_counts + stellar_counts*exp(-tau/mu)
Here
measured_counts = the number of measured photons (typically
averaged to reduce noise).
background_counts = the expected number of photons entering the PPS
optics that did not come from the star.
stellar_counts = the expected number of photons that would be
received from the star if no ring were obscuring
it.
mu = cosine of the incidence angle, i.e. the angle
between the ring plane normal and the direction
to the star.
The calibration file contains columns called BACKGROUND_SIGNAL and
STELLAR_SIGNAL that provide models of the expected count values during
the occultations. They also contain the TOTAL_SIGNAL, defined by
TOTAL_SIGNAL = BACKGROUND_SIGNAL + STELLAR_SIGNAL
This quantity is included simply because it can be directly measured in
the vicinity of isolated rings.
These tabulations also contain estimates of the uncertainty in each of
these parameters, plus an estimate of the correlation between the
uncertainties. (Note that, because the TOTAL_SIGNAL is often directly
measurable, BACKGROUND_SIGNAL and STELLAR_SIGNAL can be strongly
anti-correlated). If one wishes, one can use these parameters to
estimate the calibration uncertainty associated with a particular
value of tau.
The value for mu is constant for an occultation so it is not tabulated
in the files. Instead, it can be derived from the value for the
INCIDENCE_ANGLE keyword in the label of each calibration and geometry
file. Note that, in general, the value found in the geometry files is
likely to be slightly more accurate.
Over the years, investigators have made many different assumptions about
the calibration of the PPS data for the different ring systems and, in
some cases, for individual rings. For this reason, many different
calibration models are provided. For Uranus, the file naming echoes
that for the geometry files:
PUnC0vrd.TAB/.LBL
where
n = occultation index: 1 = sigma Sgr; 2 = beta Per.
v = calibration model version: 1-4.
r = ring identifier: 6 = Ring 6;
5 = Ring 5;
4 = Ring 4;
A = Ring alpha;
B = Ring beta;
N = Ring eta;
G = Ring gamma;
D = Ring delta;
L = Ring lambda;
E = Ring epsilon.
d = occultation direction: I = ingress; E = egress.
For models that apply to the entire ring system rather than to a
specific ring, the last two characters are omitted.
A similar file naming scheme is used for Neptune:
PN1C0vr.TAB/.LBL
where
v = calibration model version: 1-2.
r = ring identifier: A = Adams Ring; L = Leverrier Ring.
Again, for models that apply to the entire ring system rather than to a
specific ring, the last character is omitted.
The label of each calibration file contains some information about the
origin of the given file, including bibliographic references. For
Saturn, only one calibration model has ever been derived. For Uranus
and Neptune, the highest-numbered version of each calibration file was
generated by the Ring-Moon Systems Node for this data set. Files were generated by
editing out the known rings of Uranus and Neptune from the raw data
files and then fitting continuous, linear segments to the remaining
data. (The NOISDATA directory contains the edited files with known rings
removed). The result is a reliable model for TOTAL_SIGNAL and its
uncertainty throughout each of these occultations. STELLAR_SIGNAL was
then held fixed based on the results of previous investigations.
Finally, BACKGROUND_SIGNAL was derived from the difference. See Section
8 below for further discussions of PPS calibration.
7.4 Profile Generation Procedure
--------------------------------
The files in the EASYDATA directory have been generated by the Rings
Node using program PPSRESAM.FOR. This is a sample program packaged with
the Ring-Moon Systems Node's Profile Toolkit; see the SOFTWARE subdirectory. It
should be possible for computer-savvy users to build this program and
run it locally if they wish. The label of each derived EASYDATA profile
includes a summary of the input parameters used by the program.
The program requires three input files: edited data, geometry, and
calibration. These must refer to the same ring occultation and must
overlap, as indicated by a common range of SAMPLING_PARAMETER values.
Using the selected 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. The weighting function is derived from the known weighting
of the raw data (boxcar-averages with 7.5 ms duration) and a target
point-spread function (PSF). The target PSF is usually another boxcar
function, but the user is free to chose alternative PSFs.
Invalid and missing samples are zero-weighted in this averaging step.
Based on the other weighting factors, it is then possible to derive the
relationship between the uncertainty in the weighted sum and the
uncertainties in the original raw samples. For this purpose, raw
samples are assumed to be Poisson-distributed, which is valid for most
purposes. (However, see Showalter and Nicholson, 1990).
Next, the resampled photon count values are converted to ring opacity
using the calibration model and the formula above. By adding or
subtracting the derived uncertainty from the measured count value and
then solving the formula above again, one can obtain the +/- 1-sigma
confidence interval for tau.
Clearly, other resampling methods are possible. Although this one can
be rather slow, it was chosen because it supports a rigorous estimate of
the confidence interval on tau.
8. PPS Calibration
==================
One of the goals of the Ring-Moon Systems Node during the preparation of this data
set was to understand better the calibration properties of the PPS
occultation experiments. For example, between the known rings of Uranus
and and Neptune, the raw occultation data show distinct variations that
cannot be attributed to ring material.
Several documents archived on this volume discuss some of the questions
about absolute calibration of the PPS instrument. Coleman (1987a,
1987b, 1987c, 1988) wrote several internal reports studying the
temperature-dependence of the PPS sensitivity and also exploring
possible intrinsic variations in the brightness of delta Sco.
Transcriptions of these reports can be found in the DOCUMENT directory
under the names C1987A*.*, C1987B.*, C1987C.*, and C1988.*.
In addition, the Ring-Moon Systems Node commissioned the report (Black and
Nicholson, 1995), which employed spacecraft telemetry and support
imaging to reconstruct the movement of the target stars within the PPS
instrument's field of view during each occultation experiment. This
report can be found in the DOCUMENT directory under BN1995*.*. The
DOCUMENT/BN1995 subdirectory contains programs and data files that Black
& Nicholson developed for their study. The occultation support images
obtained by Voyager are found in the IMAGES directory and a map of the
PPS's sensitivity is found in FOVMAPS. Finally, the JITTER directory
contains tabulations of the instrument pointing during each occultation
experiment, re-sampled to match the timing of the corresponding edited
data, geometry and calibration files.
On this volume we have also included data from a number of star
calibration experiments. During these experiments, PPS observed a star
for an extended period during the cruise phases of the mission, without
any intervening ring material to occult it. These files are found in
the RAWDATA and EDITDATA directories with names beginning PS3-PS8 and
PU3-PU5. Other star calibration experiments were performed (including
some for Neptune) but the files could not be obtained for this data set.
The primary conclusions from this assembly of information are as
follows.
(1) According to Black and Nicholson (1995), the target stars never
wandered very far from the center of the PPS field of view during an
occultation. Overall variations in the effective value of the stellar
count rate are probably at the level of a few percent.
(2) Nevertheless, variations in the total count rate during the Uranus
and Neptune occultations correlate closely with the small changes in
instrument pointing. We now believe that these are variations in the
background count rate, caused by changes in the number of off-axis
photons from the nearby, bright planet finding their way into the
instrument. It would be hopeless to try to model this variation
quantitatively.
Therefore, we simply have to live with the uncertainty. This means that
broad, subtle features in a derived ring profile should always be taken
with a grain of salt, because changes in the background count rate could
be involved. If in doubt, one could to check the jitter files to
determine if the PPS pointing was changing during the period when a
broad feature is observed. This can at least provide a clue as to
whether the variation is real, although one could never be certain using
this data set alone.
9. References
=============
Black, G., and P. Nicholson 1995. Voyager 2 pointing during stellar
ring occultations. Report to the Planetary Data System Ring-Moon Systems Node.
Coleman, L. 1987a. Voyager 2 electronics temperature calibration.
Report to the Voyager PPS Team.
Coleman, L. 1987b. A statistical test of the variability of Delta
Scorpii. Report to the Voyager PPS Team.
Coleman, L. 1987c. A Fourier Transform search for periodicities in
Delta Scorpii. Report to the Voyager PPS Team.
Coleman, L. 1988. A search for variability in Voyager 2 occultation
stars. Report to the Voyager PPS Team.
Graps, A.L., and A.L. Lane 1986. Voyager 2 Photopolarimeter experiment:
Evidence for tenuous outer ring material at Saturn. Icarus 67, 205-210.
Jacobson, R. A., J. E. Riedel, and A. H. Taylor 1991. The orbits of
Triton and Nereid from spacecraft and Earth-based observations. Astron.
Astrophys. 247, 565-575.
Porco, C. C. 1991. An explanation for Neptune's ring arcs. Science
253, 995-1001.
Showalter, M. R., and P. D. Nicholson 1990. Saturn's Rings through a
microscope: Particle size constraints from the Voyager PPS scan. Icarus
87, 285-306.
Return to PPS data set page.