PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "
R. SIMPSON 1999-08-23;
D. Kazden removed Zs from START_TIME and STOP_TIME, added
ABSTRACT_DESC, CITATION_DESC, DATA_SET_TERSE_DESC and
DATA_SET_MISSION 2011-05-12;
D. Kazden, updated REFERENCE_KEY_IDs, 2011-10-25;
D. Kazden, updated ABSTRACT_DESC and CITATION_DESC, 2020-02-27;"
RECORD_TYPE = FIXED_LENGTH
RECORD_BYTES = 72
OBJECT = DATA_SET
DATA_SET_ID = "VG1-SSA-RSS-1-ROCC-V1.0"
OBJECT = DATA_SET_INFORMATION
DATA_SET_NAME = "VOYAGER 1 TITAN RADIO
OCCULTATION RAW DATA V1.0"
DATA_SET_COLLECTION_MEMBER_FLG = "N"
DATA_OBJECT_TYPE = TABLE
START_TIME = 1980-11-12T00:00:00
STOP_TIME = 1980-11-12T23:59:59
DATA_SET_RELEASE_DATE = 1999-12-31
PRODUCER_FULL_NAME = "RICHARD A. SIMPSON"
DETAILED_CATALOG_FLAG = "N"
ABSTRACT_DESC = "This data set consists of raw
data collected during the Titan radio occultation of Voyager 1 in
November 1980 plus 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 occultations. The data have been reduced to
give profiles of atmospheric temperature and pressure as a function
of height above the surface on both the ingress and egress sides of
Titan and to make a marginal detection of an ionosphere."
CITATION_DESC = "Simpson, R.A., VG1-SSA-RSS-1-ROCC-V1.0,
VOYAGER 1 TITAN RADIO OCCULTATION RAW DATA V1.0,
NASA Planetary Data System, 1999."
DATA_SET_TERSE_DESC = "This data set consists of raw
data collected during the Titan radio occultation of Voyager 1 in
November 1980 plus ancillary files that might be useful in analysis
of those data."
DATA_SET_DESC = "
Data Set Overview
=================
This data set consists of raw data collected during the Titan
radio occultation of Voyager 1 in November 1980 plus 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 occultations. The data have been reduced to give profiles
of atmospheric temperature and pressure as a function of
height above the surface on both the ingress and egress sides
of Titan [LINDALETAL1983] and to make a marginal detection of
an ionosphere [BIRDETAL1997].
During the Titan occultation, the Voyager 1 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 of Titan. As the spacecraft moved on its
trajectory, the radio signal probed different levels in the
atmosphere. An hour later the signals were received by
antennas of the NASA Deep Space Network (DSN) on Earth.
Because the density of Titan's atmosphere was so poorly known
prior to the Voyager encounter, experiment planners did not
know how much refractive bending to expect during the
observations. Models predicted a range of behaviors from very
little bending to so much that the narrow beam from the
spacecraft high-gain antenna (HGA) would be deflected away from
Earth and the surface occultation would not be seen. Timing
uncertainties in the motion of the spacecraft with respect to
Titan only complicated the problem. The experiment was
implemented with a very small (0.11 deg) fixed HGA offset
during the ingress occultation and a large (2.36 deg) offset
during egress. These choices, in retrospect, were very good
given the atmosphere that was found.
Parameters
==========
The output of the 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.
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,
Titan, 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 and separated onto computer
compatible tapes (CCTs) with S-band and X-band data on
different sets 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 Titan 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 Titan data from
Recorder A were numbered VJ6281 through VJ6360; tapes
from Recorder B were numbered VJ6361 through VJ6380. Test
and calibration data after the Titan encounter were collected
on Recorder A and have numbers VJ6589 through VJ6594.
The original tape numbering has been preserved in the current
file names, which have the form VJnnnnCC.ODR. On tapes where
one or more records could not be read, the original has been
separated into two or more files. The character 'C' 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 VJnnnnCC.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 Titan CRS file has been converted to ASCII and
is archived as file CRS009AA.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 VG1_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. The original file is included in the GEOMETRY
directory under the name VH009.DAT; an ASCII table containing
the same information is stored in VH009T.TAB.
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 VG1TPOC1.DAT file in the CALIB directory contains binary
POCA data; file VG1TPOC2.TAB holds the same data in ASCII
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 both the ingress and egress Voyager 1
radio occultations at Titan.
Quality issues include (1) the occasional loss of digital
samples resulting from use of analog recording technology
(dropouts), (2) the loss of records on VJ6363 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. (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. The buffering determined by the
Voyager Radio Science Team has not been recorded.
Limitations
===========
None."
END_OBJECT = DATA_SET_INFORMATION
OBJECT = DATA_SET_TARGET
TARGET_NAME = TITAN
END_OBJECT = DATA_SET_TARGET
OBJECT = DATA_SET_MISSION
MISSION_NAME = "VOYAGER 1"
END_OBJECT = DATA_SET_MISSION
OBJECT = DATA_SET_HOST
INSTRUMENT_HOST_ID = VG1
INSTRUMENT_ID = "RSS-VG1S"
END_OBJECT = DATA_SET_HOST
OBJECT = DATA_SET_REFERENCE_INFORMATION
REFERENCE_KEY_ID = "BIRDETAL1997"
END_OBJECT = DATA_SET_REFERENCE_INFORMATION
OBJECT = DATA_SET_REFERENCE_INFORMATION
REFERENCE_KEY_ID = "LINDALETAL1983"
END_OBJECT = DATA_SET_REFERENCE_INFORMATION
OBJECT = DATA_SET_REFERENCE_INFORMATION
REFERENCE_KEY_ID = "TYLERETAL1981B"
END_OBJECT = DATA_SET_REFERENCE_INFORMATION
END_OBJECT = DATA_SET
END
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