Data Set Information
DATA_SET_NAME VOYAGER 1 TITAN RADIO OCCULTATION RAW DATA V1.0
DATA_SET_ID VG1-SSA-RSS-1-ROCC-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION 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.
DATA_SET_RELEASE_DATE 1999-12-31T00:00:00.000Z
START_TIME 1980-11-12T12:00:00.000Z
STOP_TIME 1980-11-12T11:59:59.000Z
MISSION_NAME VOYAGER
MISSION_START_DATE 1972-07-01T12:00:00.000Z
MISSION_STOP_DATE N/A (ongoing)
TARGET_NAME TITAN
TARGET_TYPE SATELLITE
INSTRUMENT_HOST_ID VG1
INSTRUMENT_NAME RADIO SCIENCE SUBSYSTEM
INSTRUMENT_ID RSS-VG1S
INSTRUMENT_TYPE RADIO SCIENCE
NODE_NAME planetary plasma interactions
ARCHIVE_STATUS ARCHIVED
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.
CITATION_DESCRIPTION Simpson, R.A., VG1-SSA-RSS-1-ROCC-V1.0, VOYAGER 1 TITAN RADIO OCCULTATION RAW DATA V1.0, NASA Planetary Data System, 1999.
ABSTRACT_TEXT 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.
PRODUCER_FULL_NAME RICHARD A. SIMPSON
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