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Jacobson, R. A.

Publications and source records attributed to Jacobson, R. A..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

The orbits of Triton and Nereid from spacecraft and earthbased observations

The determination of improved orbits for the Neptunian satellites Triton and Nereid is discussed. The primary results are the final set of model parameters which generate orbits that best fit both the earth-based satellite observations and data acquired by the Voyager spacecraft during the Neptune encounter. The accuracy of the improved orbits is assessed and they are compared with the orbits generated prior to the encounter. Mean elements are also provided as a geometrical representation for the orbits.

Jacobson, R. A.↗
NASA NTRS1990

The orbits of the satellites of Neptune

This article presents the results of a fit of numerically integrated Neptunian satellite orbits to earth-based astrometric observations and early Voyager spacecraft observations. Ephemerides based on these orbits were used by the Voyager project as the final pre-encounter ephemerides. As a by-product of the orbit fits, estimates of the Neptune mass, the second zonal harmonic of Neptune, and the pole orientation of Neptune were also obtained.

Jacobson, R. A.↗
NASA NTRS1990

Performance of three-way data types during Voyager's encounter with Neptune

Voyager's flyby of Neptune in August of 1989 was the most distant planetary encounter ever achieved. Round trip light travel time was more than eight hours, exceeding view periods at two of the three tracking station sites. Consequently, the majority of radiometric tracking was accomplished by transmitting the uplink from one station, and receiving the downlink at a different station. This procedure defines three-way data. Dependence on three-way data for orbit determination is one distinguishing element of Voyager's successful encounter with Neptune. This paper addresses the performance of three-way range and Doppler data supporting pre-encounter orbit determination and post-encounter orbit reconstruction. Also, calibrations which reduce systematic errors inherent to three-way data are described and analyzed.

Roth, D. C.↗
NASA NTRS1990

Voyager 2 orbit determination at Neptune

In August 1989 the Voyager 2 spacecraft encountered Neptune and Triton. Precise knowledge of the trajectory of the spacecraft relative to the Neptunian system was essential to ensure successful observations during the flyby, and to perform trajectory control. Determination of the orbit of Voyager 2 with respect to the Neptunian system was accomplished by the use of radiometric Doppler, range, and VLBI observations of the spacecraft in combination with spacecraft-based optical observations of Neptune, Triton, Nereid, and the Voyager-discovered satellite 1989N1. These data types were used in a new version of the JPL Orbit Determination Program to determine the orbit of the spacecraft as well as Neptunian system ephemerides and dynamical parameters, resulting in accurate delivery of the spacecraft to targeted conditions at Neptune and Triton.

Lewis, G. D.↗
NASA NTRS1990

Ephemerides of the major Neptunian satellites determined from earth-based astrometric and Voyager imaging observations

The Voyager project used Neptunian satellite ephemerides to support both navigation and acquisition of scientific data. The development of postencounter ephemerides for the satellites Triton, Nereid, and 1989N1 is discussed. Primary results are the final set of model parameters which generate orbits that best fit both the earth-based satellite observations and data acquired by Voyager. The ephemerides are compared with those generated preencounter, and the accuracy of the final ephemerides is assessed. Mean orbital elements are also provided as a geometrical representation for the satellite orbits.

Jacobson, R. A.↗
NASA NTRS1988

Galilean satellite ephemeris improvement using Galileo tour encounter information

Accurate navigation of the satellite tour portion of the Galileo mission requires an accurate ephemeris of the Galilean satellites. The ephemeris is updated using radiometric and optical tracking data acquired during the satellite tour. The improved accuracy of the satellite ephemeris leads to improved targeting accuracy at subsequent encounters. The Galileo mission will benefit from improved targeting accuracy through reduced propellant costs and improved pointing accuracy. The predicted error in the updated ephemeris can be less than approximations inherent in the analytical theory used for the ephemeris, so an alternate numerical representation is applied. This alternate description shows promise but also raises questions of numerical stability.

Murrow, D. W.↗
NASA NTRS1987

Radio science with Voyager 2 at Uranus - Results on masses and densities of the planet and five principal satellites

Phase-coherent Doppler data generated by the Deep Space Network with the radio communication system during the Voyager 2 encounter with Uranus in January 1986, optical navigation data generated by the Voyager Navigation Team with the Voyager 2 imaging system, and ground-based astrometric data obtained over an 8-yr period are compiled and analyzed to determine the masses and densities of Uranus and its principal satellites. The data-analysis procedures are explained in detail, and the results are presented in tables and graphs. The mean density of Uranus is found to be 1.285 + or - 0.001 g/cu cm, whereas the mean uncompressed mass of all five satellites is 1.48 + or - 0.06 g/cu cm, or 0.10 g/cu cm above the density expected for a homogeneous solar mix of rock, H2O and NH3 ice, and CH4 as clathrate hydrate. This difference is tentatively attributed to the presence of 15 mass percent of pure graphite, which would provide the thermal conductivity required to keep the satellites cold and undifferentiated.

Anderson, J. D.↗
NASA NTRS1987

GUST86 - An analytical ephemeris of the Uranian satellites

The General Uranus Satellite Theory GUST (Laskar, 1986) is used for the construction of an analytical ephemeris for the Uranian satellites. The theory is fitted against earth-based observations from 1911 to 1986, and all radio and optical data obtained during Voyager encounter with Uranus. Earth-based observations alone allow the determination of masses which are within 15 percent of the values determined by the Uranus flyby. The analysis of all the observations confirm the values of the masses obtained during the encounter (Stone and Miner, 1986) and give a complete set of dynamical parameters for the analytical theory. An analytical ephemeris, GUST86, with an estimated precision of about 100 km with respect to Uranus is obtained.

Laskar, J.↗
NASA NTRS1986

Ephemerides of the Uranian satellites determined from earth-based astrometric and Voyager imaging observations

Uranian satellite ephemerides were needed by the Voyager project to support both navigation and acquisition of scientific data. This paper outlines the mathematical modeling approach taken to generate those ephemerides and discusses their pre-encounter development, real time updating, and post-encounter refinement. The results presented include the final set of model parameters which generate the ephemerides that best fit the Voyager optical data as well as the earth based observations of the satellites.

Jacobson, R. A.↗
NASA NTRS1986

Orbit determination for the Voyager II Uranus encounter

The Voyager II flyby of Uranus in January 1986 was the most distant planetary encounter ever attempted, and presented unique challenges to the process of orbit determination. Long light-times and spacecraft receiver difficulties hampered the collection of two-way radiometric data and helped bring about the maturation of a Very Long Baseline Interferometry navigational data type during the long cruise from Saturn. Planet and satellite ephemeris uncertainties necessitated the use of the onboard spacecraft optical system for Uranus-relative navigation. During the close approach phase, these optical data were combined with radiometric data to drive the Uranus system-relative uncertainties down to the level of a few tens of kilometers. This paper contains qualitative and quantitative results and conclusions based on orbit determination experience during Uranus cruise and encounter. Topics include an overview of the navigation-related mission events and requirements, and a review of the salient orbit determination results.

Taylor, T. H.↗
NASA NTRS1984

Satellite ephemerides for the Voyager Uranus encounter

Uranian satellite ephemerides are needed by the Voyager project to support both navigation and acquisition of scientific data. This paper presents the approach being taken to develop the ephemerides and details the initial phase of the development. That phase involves the analytical modeling of the satellites' motion and the adjustment of the model to fit astronomical observations. The paper describes the model and gives the result of a fit to 71 years of observations.

Jacobson, R. A.↗
NASA NTRS1983

Comet Halley ephemeris uncertainties in 1985-1986

For the planned flyby missions to Comet Halley in March 1986, the comet's ephemeris uncertainties completely dominate the spacecraft-comet miss distance. In an effort to determine realistic Comet Halley ephemeris uncertainties, a statistical covariance analysis was conducted using the actual data in 1909-1910-1911 and simulated data in 1984-1985-1986. In 1985-1986, Comet Halley's ephemeris uncertainties are very sensitive to the comet's orbital position, the optical data noise, data schedule, and whether or not the old data is included in the orbital solutions. The comet's ephemeris uncertainties in March 1986 are relatively insensitive to reasonable center of light/center of mass offsets and also to possible radar data taken in late November 1985. Accurate Space Telescope observations made in early March 1986 might significantly improve upon the comet's position uncertainties for the various intercepting spacecraft.

Yeomans, D. K.↗
NASA NTRS1982

Performance of differenced range data types in Voyager navigation

Voyager radio navigation made use of a differenced rage data type for both Saturn encounters because of the low declination singularity of Doppler data. Nearly simultaneous two-way range from two-station baselines was explicitly differenced to produce this data type. Concurrently, a differential VLBI data type (DDOR), utilizing doubly differenced quasar-spacecraft delays, with potentially higher precision was demonstrated. Performance of these data types is investigated on the Jupiter-to-Saturn leg of Voyager 2. The statistics of performance are presented in terms of actual data noise comparisons and sample orbit estimates. Use of DDOR as a primary data type for navigation to Uranus is discussed.

Taylor, T. H.↗
NASA NTRS1982

Satellite ephemerides for Voyager Saturn encounter

Satellite ephemerides played an important role in the navigation of the Voyager 1 and 2 Saturn encounters. They were used in the models of the gravitational forces acting on the spacecraft and of the onboard optical navigation measurements. They also were used in predicting the pointing directions of the onboard instruments for satellite scientific observations. This paper describes the form of the Voyager Saturn satellite ephemerides and gives the values of the parameters used in the ephemeris generation. The update of those parameters as part of the optical navigation process is also discussed, and the best Voyager ephemerides are presented and compared to those based on earlier Saturn satellite theories.

Jacobson, R. A.↗