| DATA_SET_DESCRIPTION |
Data Set Overview
================
This data set consists of raw data collected during the Saturn
radio occultation of Voyager 2 on 26 August 1981, ring
scattering data collected during the same time period, test
and calibration data collected about 8 hours earlier, and
ancillary files that might be useful in analysis of those data.
The raw data are sampled voltage outputs from receivers tuned to
the Voyager carrier frequencies at both S-band and X-band during
the occultation. The data have been reduced to give profiles
of temperature and pressure as a function of height in the
atmosphere [LINDALETAL1985] and to infer magnetic field
orientations in the upper ionosphere [HINSON1984].
During the Saturn occultation, the Voyager 2 spacecraft
provided a coherent, dual-frequency microwave radio signal
source. The signal frequency was derived from a precision,
onboard Ultra-Stable Oscillator (USO). The spacecraft
high-gain antenna (HGA) beamed that signal through the
atmosphere and rings of Saturn. As the spacecraft moved on
its trajectory, the radio signal probed the atmosphere and
rings at different radial positions from the center of mass.
An hour later the signals were received using a 64-meter
antenna of the NASA Deep Space Network (DSN) near Canberra
(Australia).
To keep the refracted radio beam aimed toward the DSN antenna
during the atmospheric occultation, the spacecraft attitude
was adjusted so that the high-gain antenna (HGA) was pointed
at the virtual image of Earth on Saturn's limb. This ensured
that maximum signal strength would be available from the
deepest probing. During the deepest part of the atmospheric
occultation, the HGA was pointed toward the rings to determine
whether signals forward scattered by the ring particles could
be detected on Earth.
Related data sets of possible interest include:
DATA_SET_ID Description
----------------------- --------------------------------
VG1-SSA-RSS-1-ROCC-V1.0 Titan radio occultation raw data
VG1-S-RSS-1-REDR-V1.0* Saturn Voyager 1 ingress radio
occultation raw data
VG1-S-RSS-1-ROCC-V1.0 Saturn Voyager 1 egress radio
occultation and ring
scattering raw data
*tentative DATA_SET_ID assignment
Parameters
==========
The output of each S-band receiver was a sinusoidal carrier
signal embedded in noise -- a voltage with bandwidth
approximately 50 kHz and sampled at 300000 samples per second.
The X-band receiver output was similar; but, because of greater
potential for Doppler drift and prediction uncertainty, its
bandwidth was 150 kHz and sampling rate was 300000 samples per
second. Voltages typically were in the range +/- 10 volts;
but the absolute levels were not calibrated. In fact, they
are generally not needed since it is the frequency (or phase)
of the signal (rather than amplitude) that is most useful in
inferring properties of a neutral atmosphere or ionosphere,
and amplitude calibration for ring observations can be
obtained by referencing signals to the background radiothermal
noise in the data stream.
The frequency of the USO was known from monitoring during the
Jupiter-Saturn cruise (and from post-Saturn observations).
Doppler contributions from motions of the spacecraft, Earth,
Saturn, and other bodies of the solar system were determined
jointly with the Voyager Navigation Team. Relativistic
Doppler contributions could be estimated from proximity to
large masses. Receiver tuning was recorded in POCA
(Programmable Oscillator Control Assembly) files, which are
included with this archive.
Processing
==========
No processing per se has been carried out on these data.
However, because of the high sampling rate, the 8-bit samples
were recorded originally on wide-bandwidth analog video tape.
The analog tapes were then replayed later at slower speeds and
the digital data were extracted onto computer compatible tapes
(CCTs) with each receiver channel on a separate set of tapes.
Because the S-band data had been oversampled originally
(300 ksps for a 50 kHz bandwidth), only one of every three
samples was saved during the transfer of S-band data to CCTs.
This process, known as 'decimation', meant that 300 seconds
of data could be stored on an S-band CCT whereas only
100 seconds of X-band data would fit.
Because analog recording technology was required to save the
high data rate digital samples, there are occasional dropouts
in the sample stream. These can be detected by paying special
attention to counter fields in data record headers.
Two analog recorders (A and B) were available at each DSN
complex. Because a single recorder could not capture the
entire set of Saturn occultation activities, the two were run
in parallel with staggered start/stop times. Most data were
collected using Recorder A; but Recorder B was used to
capture the samples while Recorder A was being reloaded.
Data
====
Primary data were delivered to Voyager Radio Science Team
members in the form of 30 megabyte (MB) CCTs covering
300 s (S-band) or 100 s (X-band). Each tape had 6000
records of 5056 bytes (56 bytes of header information
and 5000 8-bit samples of receiver output voltage). Tapes
were numbered sequentially as CCTs were generated from the
high density video originals. Tapes with Saturn data from
Recorders A and B were numbered as follows:
1981-08-26 UTC Tape Numbers Recorder
----------------- ------------- --------
03:45:00-04:20:00 C0BP01-C0BP21 B
04:15:00-04:55:00 C1BP01-C1BP24 A
04:50:00-05:40:00 C2BP01-C2BP30 B
05:10:00-06:05:00 C3BP01-C3BP30 A
06:15:00-07:05:00 C4BP01-C4BP30 B
06:20:00-07:15:00 C5BP01-C5BP33 A
07:15:00-07:40:00 C6BP01-C6BP15 B
07:20:00-08:00:00 C7BP01-C7BP24 A
Test and calibration data were collected before the Saturn
encounter on Recorder A (tapes C8BP01-C8BP19) and Recorder B
(tapes C9BP01-C9BP19). 'B' in the tape names stands for the
frequency band (S or X); 'P' stands for the circular
polarization (L or R). Not all band/polarization
combinations were collected at any given time.
The original tape numbering has been preserved in the current
file names, which have the form CnBPmmZZ.ODR. On tapes where
one or more records could not be read, the original has been
separated into two or more files. The character 'Z' indicates
the ordering of these file fragments with 'A' being first (and
the default with no tape reading errors), 'B' next, etc.
Each Original Data Record (ODR) file is accompanied by a
minimal PDS label briefly describing the contents and referring
the user to detailed documentation on file format. The label
file has name CnBPmmZZ.LBL.
Ancillary Data
==============
Geometry Data - The raw radio data were originally reduced by
the Voyager Radio Science Team using ephemerides in Celestial
Reference Set (CRS) format -- state vectors at regular
intervals. The Saturn CRS file has been converted to ASCII and
is archived as file CRS32_1A.CRS in the GEOMETRY directory.
The current version of the NAIF SPK file for the Voyager Saturn
encounter is also provided; it is in the NAIF 'transfer' format
and is archived under the name VG2_SAT.SPK in the GEOMETRY
directory.
HGA Pointing Data - High-Gain Antenna (HGA) pointing information
was delivered to the Voyager Radio Science Team as binary files
on computer tape. Two original binary files and an equal
number of ASCII translations are included in the GEOMETRY
directory. File VU005.DAT and its ASCII translation VU005T.TAB
are assumed to be the best files for analysis, but only because
of the apparently higher file number (records documenting the
history of these files have been lost).
POCA Data - The Programmable Oscillator Control Assembly (POCA)
set the (tunable) local oscillator in the DSN receivers. To
recover Doppler shifts resulting from gravitational forces on
the spacecraft or propagation through media with varying index
of refraction, the receiver tuning must be known precisely.
The VG2SPOC1.DAT file in the CALIB directory contains binary
POCA data; file VG2SPOC2.TAB holds the same frequency data in
an ASCII table format.
Coordinate System
=================
Original files (CRS files and HGA pointing files) were defined
using the EME-1950 coordinates system. NAIF files, accessed
with NAIF Toolkit software, allow extractions of positions and
velocities in many coordinate systems. The basic radio data
(ODR files) are independent of coordinate system.
Software
========
The following main programs are included in the archive. All
were written in FORTRAN 77; all have been tested and used on
a Sun ULTRA-5 running Solaris 2.5.1. The SOFTWARE directory
includes source code for these programs (and their subroutines)
and a Unix Makefile which can be used to generate binary
executables. For non-Sun/Solaris systems, the listings
may serve as a starting point for versions which will run on
the local machine.
RDHDR: Reads and displays contents of individual ODR record
headers.
UNPK: Separates header and data components of records in
an ODR file.
CRS2ASC.F: Converts Univac binary CRS file to ASCII. Included
only for historical purposes since the binary CRS file
has not been included with this archive.
CRS2LBL.F: Extracts information from ASCII CRS file useful in
creating a PDS label. Included only for historical
purposes.
PREP_11_6.F: Program reformats Saturn data for use with
Stanford quick-look and other processing software.
Converts 8-bit samples in 5056-byte records to 16-bit
samples in 1024-byte records, optionally corrects for
missing or extra samples (analog tape dropout
artifacts), and creates header record.
Media/Format
============
The archival data set is written on CD-WO media using GEAR
software and a Yamaha writer. The CD-WO volumes conform to
ISO 9660 standards.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
This is a good data set. Known problems are minor.
Review
======
This archival data set was examined by a peer review panel
prior to its acceptance by the Planetary Data System (PDS).
The peer review was conducted in accordance with PDS
procedures.
Data Coverage and Quality
=========================
This data set covers the Voyager 2 Saturn radio occultation,
the ring scattering experiment, and a post-experiment
mini-ASCAL (HGA calibration) sequence.
Quality issues include (1) the occasional loss of digital
samples resulting from use of analog recording technology
(dropouts), (2) the loss of records on a few tapes from
deterioration of the CCT, (3) less than 8-bit accuracy in
the analog to digital conversion, and (4) spurs and other
anomalous signals in addition to the carrier. Items (1)
and (2) have been discussed above; items (3) and (4) are
simply limitations in working at the technological
frontier.
Buffering: There were several points where buffering of data
(momentary storage) may be important to the user. (1) Time
tags in ODR data records were buffered. The first sample
in an ODR file was taken on an integer second, but the time
tag is slightly (a few microseconds) earlier. (2) The
data samples themselves were buffered, by an amount between
1 and 2 sample periods (less than 6 microseconds).
(3) POCA frequencies were buffered, so that the frequency
in the POCA file should be associated with the previous
second. (4) The frequency rate in the POCA was double
buffered (offset by 2 seconds). The POCA buffering varied
among data acquisition systems; the amounts indicated by
(3) and (4) are the extremes. Users should conduct
self-consistency checks to determine the correct POCA
buffering correction. The buffering determined by the
Voyager Radio Science Team has not been recorded.
Polarization: Right-circularly polarized (RCP) signals were
transmitted from the spacecraft at both S-band and X-band.
Both right- and left-circular polarizations (LCP) were
captured at the ground station. The LCP data were collected
primarily to detect depolarized signal during the ring
scattering phases of these observations. Neither
transmitting nor receiving antenna was capable of
providing purely circular polarization; the end-to-end
cross-polarized signal was probably somewhere between 20 and
30 dB below the same-polarized signal. A brief period after
the ionospheric occultation and before a roll turn (07:04-
07:07) may be useful for determining the actual level of
cross-polarization. No Saturn-related cross-polarization
was reported by the Voyager Radio Science Team.
Limitations
===========
None.
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