Data Set Information
DATA_SET_NAME VOYAGER 2 TRITON RADIO OCCULTATION REDUCED DATA V1.0
DATA_SET_ID VG2-NSA-RSS-5-ROCC-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : This data set consists of several tables and supporting documentation from final analysis of the Voyager 2 radio occultation by Triton. The data set is based on a Ph.D. dissertation by Eric M. Gurrola of Stanford University [GURROLA1995]. The tabulated data were derived from raw radio science observations, which are being archived separately. General principles for conducting these types of experiments have been described by [TYLER1987]; early results of the Triton analysis were published by [TYLERETAL1989]. During the Triton 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 of Triton. As the spacecraft moved on its trajectory, the radio signal probed different levels in the atmosphere. Four hours later the signals were received at stations of the NASA Deep Space Network (DSN) on Earth. Because Triton's atmosphere is so tenuous, changes in the signal resulting from its passage through the atmosphere were very subtle. Nevertheless, a phase change of about 2 radians at X-band was observed and can be attributed to retardation by a neutral atmosphere. Because oscillator instabilities on the spacecraft were comparable to the atmospheric effects being measured, more than one physically plausible model atmosphere is consistent with the data. [GURROLA1995] used several techniques to model the oscillator instability, define a phase 'baseline,' and extract an atmospheric profile. The data archived here are in the form of six tables giving measured radio phase, residual phase, and results of the atmospheric modeling. Parameters : Six tables are included in the data set. In all cases, tables are constructed with altitude above the surface as the free parameter -- in steps of 0.5 km. In four tables phase, relative phase, or residual phase (in radians) is the dependent parameter. In one table temperature (in Kelvins) is the dependent parameter. The sixth table is an autocorrelation, so the dependent variable is the dimensionless autocorrelation value, with a potential range of +/-1.0. Processing : The spacecraft transmitted coherent signals with right circular polarization along the boresight of the high-gain antenna at both S-band (13 cm, or 2298 MHz) and X-band (3.5 cm, or 8415 MHz) wavelengths. Signals were received coherently at both wavelengths in right-circular polarization using the 70-meter DSN antenna near Canberra, Australia. The microwave frequencies were converted to baseband (0-25 kHz) using fixed and tunable local oscillators; the output analog signals corresponding to both S- and X-band inputs were converted to digital representations (8-bit samples) and stored for later processing. At Stanford, the sampled signal was filtered digitally and all deterministic corrections for frequency/phase shifts were applied. A residual phase shift in the S- and X-band signals was observed during both the ingress and egress occultation periods. Standard methods were used to determine the contribution from plasma effects. The remaining phase shift was attributed to Triton's neutral atmosphere; it is the subject of Gurrola's dissertation and the focus of this data set. Analysis of radio occultation data does not require knowledge of target body radius. Rather, the occultation time is taken to be the time when the signal voltage has decreased to half of its free space value. The time may then be translated to a radius value for a specific spacecraft trajectory. Trajectory uncertainties were large enough during this analysis that no attempt was made to derive a new radius. Heights are defined relative to the occultation point; they are not sensitive to small differences in trajectory. Structure of the atmosphere (over hundreds of vertical kilometers) is somewhat sensitive to radius through assumptions of hydrostatic equilibrium. Data : The six tables included here are, in the order they appear in the dissertation: VGRPHASE.TAB: The 3.6-cm residual radio phase for Triton ingress and egress, corrected for free space propagation and ionospheric effects; this table contains the basic observational data. BASELINE.TAB: A comparison of the best fit model to Triton 3.6-cm egress data with several different estimates of USO drift (baselines). ATM_MODL.TAB: Four theoretical temperature profiles for Triton; each is consistent with the data, but each involves different assumptions. USO_FLUC.TAB: The residual USO phase fluctuations after the measured values in VGRPHASE.TAB have been adjusted by an estimate of the slowly-varying long-term USO drift. AUTOCORR.TAB: The autocorrelation of phase noise for lags up to 65 km. PH_RESID.TAB: Residual phase from 3.6-cm egress data for the best fit isothermal model atmosphere and for a thermal inversion model. Each table is accompanied by a full PDS detached label with the same name except for suffix *.LBL. The PDS label completely describes the format and contents of the table. Ancillary Data : The current Voyager 2 ephemeris for the Neptune encounter was obtained from the JPL/PDS Navigation Ancillary Information Facility (NAIF) and is included in the GEOMETRY directory. This is not the file used by Gurrola for his analysis, but it may allow users to reconstruct the observing configuration in a way that facilitates understanding of the results. Coordinate System : Tables are given in terms of height above Triton's surface, which is taken to be approximately 1350 km from its center of mass. The assumed radius is not important for this data set; the occultation data themselves define the 'zero' height level, which is the point where the received signal power is one-fourth (voltage is one-half) of its free space value. When used in conjunction with a specific spacecraft trajectory, the zero- height level can be used to derive a radius for Triton. When a specific number was required, Gurrola himself used 1352.5 km. The published value of radius from the radio occultation is 1355+/-7 km [TYLERETAL1989]. Software : There is no software in this data set. Media/Format : The archival data set is written on CD-WO media using the Young Minds CD Studio authoring system. The CD-WO volumes conform to ISO 9660 standards.
DATA_SET_RELEASE_DATE 1997-12-31T00:00:00.000Z
START_TIME 1989-08-25T12:00:00.000Z
STOP_TIME 1989-08-25T12:00:00.000Z
MISSION_NAME COMET SL9/JUPITER COLLISION
VOYAGER
MISSION_START_DATE 1993-01-01T12:00:00.000Z
1972-07-01T12:00:00.000Z
MISSION_STOP_DATE 1996-01-01T12:00:00.000Z
N/A (ongoing)
TARGET_NAME TRITON
TARGET_TYPE SATELLITE
INSTRUMENT_HOST_ID VG2
INSTRUMENT_NAME RADIO SCIENCE SUBSYSTEM
INSTRUMENT_ID RSS
INSTRUMENT_TYPE RADIO SCIENCE
NODE_NAME Planetary Atmospheres
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Confidence Level Overview : Derivation of an atmospheric profile for Triton depends on the accuracy with which USO drift on short time scales can be determined. For these observations, the effects of drift and the effects of Triton's atmosphere are comparable. 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. Prior to creation of the final version of the archival data set, key elements of the archive were distributed for preliminary review. These included electronic versions of example PDS labels, CATALOG files, and Software Interface Specifications. These materials were distributed to PDS personnel, the experiment investigator, and others, as appropriate. Data Coverage and Quality : This data set covers both the ingress and egress Voyager 2 radio occultations at Triton. X-band plots illustrate the effects; S-band results would have been identical, except for scaling by the 3/11 ratio of the two frequencies. Egress data were somewhat more amenable to the types of modeling developed by Gurrola and are emphasized in the results; but both ingress and egress phase measurements are included in the data set. Estimates of occultation radius are not included in this data set; instead, an empirical determination of the 'zero' height level was made from the data for both ingress and egress (see also, Processing, above). The uncertainties in the observations are comparable to the effects being measured. Limitations : The limitations in this data set follow from the quality of the execution, which is described above under Data Coverage and Quality. Gurrola assumed a radius of 1352.5 km when he needed a specific value; he believes the height parameter uncertainty is on the order of 100 m.
CITATION_DESCRIPTION Simpson, R.A., VOYAGER 2 TRITON RADIO OCCULTATION REDUCED DATA V1.0, VG2-NSA-RSS-5-ROCC-V1.0, NASA Planetary Data System, 1997
ABSTRACT_TEXT This data set consists of several tables and supporting documentation from final analysis of the Voyager 2 radio occultation by Triton. The data set is based on a Ph.D. dissertation by Eric M. Gurrola of Stanford University [GURROLA1995]. The tabulated data were derived from raw radio science observations, which are being archived separately. General principles for conducting these types of experiments have been described by [TYLER1987] results of the Triton analysis were published by [TYLERETAL1989].
PRODUCER_FULL_NAME RICHARD A. SIMPSON
SEARCH/ACCESS DATA
  • Atmospheres Online Archives
    		•