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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
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NASA NTRS1974

Precise time and time interval (PTTI), an overview

Present applications of precise time and frequency (T/F) technology can be grouped as follows: (1) Communications systems which require T/F for time division multiplexing and for using spread spectrum techniques. (2) Navigation systems which need T/F for position fixing using a timed signal. (3) Scientific-Metrological applications which use T/F as the most precisely reproducible standard of measurement. (4) Astronomical-Space applications which cover a variety of the most demanding applications such as pulsar research, Very Long Baseline Interferometry (VLBI) and laser/radar ranging. In particular, pulsar time-of-arrival measurements require submicrosecond precision over a period of one-half year referred to an extraterrestrial inertial system, and constitute the most stringent requirements for uniform timekeeping to date.

Winkler, G. M. R.↗
NASA NTRS1974

Application of very long baseline interferometry to Astrometry and Geodesy: effects of frequency standard instability on accuracy

The accuracy of geodetic and astrometric information obtained from very long baseline interferometry (VLBI) observations is dependent upon the stability of the frequency standard, or clock, used at each site of VLBI array. The sensitivities of two hydrogen maser frequency standards of different design to pressure, temperature, and magnetic field variations were measured; and, for one of the standards, sensitivity was found to be severe enough to degrade the information content of VLBI measurements. However, the effect on the geometric and astrometric information of such clock instabilities, with time scales of hours or greater, can be sharply reduced through the use of differencing techniques.

Clark, T. A.↗
NASA NTRS1974

Frequency standards requirements of the NASA deep space network to support outer planet missions

Navigation of Mariner spacecraft to Jupiter and beyond will require greater accuracy of positional determination than heretofore obtained if the full experimental capabilities of this type of spacecraft are to be utilized. Advanced navigational techniques which will be available by 1977 include Very Long Baseline Interferometry (VLBI), three-way Doppler tracking (sometimes called quasi-VLBI), and two-way Doppler tracking. It is shown that VLBI and quasi-VLBI methods depend on the same basic concept, and that they impose nearly the same requirements on the stability of frequency standards at the tracking stations. It is also shown how a realistic modelling of spacecraft navigational errors prevents overspecifying the requirements to frequency stability.

Fliegel, H. F.↗
NASA NTRS1976

Radio source positions from very-long-baseline interferometry observations

Accurate positions of compact radio sources have been determined from very-long-baseline interferometry (VLBI) observations based on the bandwidth-synthesis technique. The coordinates for 18 extragalactic sources were obtained from sets of observations spread over the period from April 1972 to January 1975; the scatter among the independent determinations of the source coordinates from the separate sets of observations is about 0.05 arcsec, except for the declinations of near-equatorial sources, where the scatter is about 0.15 arcsec. Comparison of these positions with those determined with the Cambridge 5-km radio interferometer shows the rms scatter about the mean difference to be about 0.04 arcsec in each coordinate (no sources of low declination were in common). A similar comparison of the present results with those obtained by the Jet Propulsion Laboratory from separate VLBI observations yields a slightly larger rms scatter, after exlusion of the declinations of the near-equatorial sources. A position is also obtained for the galactic object Beta Persei (Algol), which agrees well with the position given in the FK 4 catalogue.

Clark, T. A.↗
NASA NTRS1976

Magnitude of 64-m elevation axis movements due to alidade temperature changes

In projected very-long baseline interferometry (VLBI) work a reference point for 64-m antennas is the intersection of the elevation and azimuth axes. A minimum-level effort at DSS 14 to determine the magnitude of the effect of diurnal changes in the temperature of the alidade legs on the height of the elevation axis is described. The thermal expansion between the lowest recorded temperature -3 C (27 F) and the highest, 36 C (97 F), over the period covered was 8.9 mm (0.35 in.).

Hung, N. T.↗
NASA NTRS1975

Geodetic and Astrometric Measurements with Very-Long-Baseline Interferometry

The use of very-long-baseline interferometry (VLBI) observations for the estimation of geodetic and astrometric parameters is discussed. Analytic models for the dependence of delay and delay rate on these parameters are developed and used for parameter estimation by the method of weighted least squares. Results are presented from approximately 15,000 delay and delay-rate observations, obtained in a series of nineteen VLBI experiments involving a total of five stations on two continents. The closure of baseline triangles is investigated and found to be consistent with the scatter of the various baseline-component results. Estimates are made of the wobble of the earth's pole and of the irregularities in the earth's rotation rate. Estimates are also made of the precession constant and of the vertical Love number, for which a value of 0.55 + or - 0.05 was obtained.

Robertson, D. S.↗
NASA NTRS1977

Study of lunar librations using differential very-long-baseline observations of ALSEPs

Observations of signals from the ALSEP transmitters via differential very-long-baseline interferometry (VLBI) are being made to determine the relative selenocentric coordinates of the ALSEPs and the motion of the moon about its center of mass. By VLBI the relative positions of the ALSEPs can be determined with uncertainties less than 5 meters, and the orientation of the moon within 0.5 second of selenocentric arc.

King, R. W.↗
NASA NTRS1978

Mission feasibility study of a very long baseline interferometer utilizing the space shuttle

An introductory overview of very long baseline interferometry (VLBI) as it exists and is used today is given and the scientific advances that have been achieved with this technique in the past decade are described. The report briefly reviews developments now in progress that will improve ground station VLBI in the next few years, and the limitations that still will exist. The advantages and the scientific return on investment that may be expected from a VLBI terminal in space are described. Practical problems that have to be faced range from system design through hardware implementation, to data recovery and analysis.

Burke, B. F.↗
NASA NTRS1978

VLBI/Laser Intercomparison Project: Session 2

Recent experiments are described which directly intercompare very long baseline interferometry (VLBI) with laser ranging to artificial satellites by measuring the vector lengths of the same intercontinental baselines. These experiments are part of the VLBI/Laser-Intercomparison Project, a continuing task to assess the suitability of VLBI to various geophysical applications.

Fliegel, H. F.↗
NASA NTRS1978

Algorithms for isolating worst case systematic data errors

Two separate algorithms are derived for testing filter sensitivity to systematic data errors. One algorithm provides the absolute minimum Euclidean norm data error for a given estimate component error. The second algorithm can be used to find the minimum norm data error which can be generated by restricted degree Legendre polynomials. A specific very long baseline interferometry (VLBI) baseline estimation is analyzed with the algorithm. It is found that the local vertical is the most sensitive component to error in the data space. The efficiency of a data error sequence linear in elevation angle is within 7% that of the absolute worst case sequence. Elevation angle dependent errors are explored and the special case of a mismodeled troposphere is treated.

Curkendall, D. W.↗
NASA NTRS1978

Earth rotation and network geometry optimization for very long baseline interferometers

The accuracy of direction measurements is substantially improved by Very Long Baseline Interferometry (VLBI). The technique is geometric in that it operates independently of the earth's gravity field, and dynamic in that observations of polar motion, nutation, length of day variations, and precession are also possible. Attention is given to a VLBI study of earth rotation within a framework of network geometry parameters.

Dermanis, A.↗
NASA NTRS1979

PTTI applications to deep space navigation

Radio metric deep space navigation relies nearly exclusively upon coherent, two way, Doppler and ranging for all precise applications. These data types and the navigational accuracies they can produce are reviewed. The deployment of hydrogen maser frequency standards and the development of Very Long Baseline Interferometry (VLBI) systems within the Deep Space Network are used in the development of non-coherent, one way data forms that promise much greater inherent navigational accuracy. The underlying structure between each data class and clock performance is charted. VLBI observations of the natural radio sources are the planned instrument for the synchronization task. This method and a navigational scheme using differential measurements between the spacecraft and nearby quasars are described.

Curkendall, D. W.↗
NASA NTRS1979

Tectonic motion site survey of the National Radio Astronomy Observatory, Green Bank, West Virginia

A geological and geophysical site survey was made of the area around the National Radio Astronomy Observatory (NRAO) to determine whether there are at present local tectonic movements that could introduce significant errors to Very Long Baseline Interferometry (VLBI) geodetic measurements. The site survey consisted of a literature search, photogeologic mapping with Landsat and Skylab photographs, a field reconnaissance, and installation of a seismometer at the NRAO. It is concluded that local tectonic movement will not contribute significantly to VLBI errors. It is recommended that similar site surveys be made of all locations used for VLBI or laser ranging.

Webster, W. J., Jr.↗
NASA NTRS1978

Estimability and simple dynamical analyses of range (range-rate range-difference) observations to artificial satellites

Non-Bayesian statistics were used in simulation studies centered around laser range observations to LAGEOS. The capabilities of satellite laser ranging especially in connection with relative station positioning are evaluated. The satellite measurement system under investigation may fall short in precise determinations of the earth's orientation (precession and nutation) and earth's rotation as opposed to systems as very long baseline interferometry (VLBI) and lunar laser ranging (LLR). Relative station positioning, determination of (differential) polar motion, positioning of stations with respect to the earth's center of mass and determination of the earth's gravity field should be easily realized by satellite laser ranging (SLR). The last two features should be considered as best (or solely) determinable by SLR in contrast to VLBI and LLR.

Vangelder, B. H. W.↗
NASA NTRS1979

Satellite emission radio interferometric earth surveying series - GPS geodetic system

A concept called SERIES (satellite emissions radio interferometric earth surveying) which makes use of GPS (global positioning system) radio transmissions without any satellite modifications, is described. Through the use of very long baseline interferometry (VLBI) and its calibration methods, 0.5 to 3 cm three dimensional baseline accuracy can be achieved over distances of 2 to 200 km respectively, with only 2 hours of on-site data acquisition. Attention is given to such areas as: the radio flux equivalent of GPS transmissions, synthesized delay precision, transmission and frequency subsystem requirements, tropospheric and ionospheric errors. Applications covered include geodesy and seismic tectonics.

Macdoran, P. F.↗
NASA NTRS1979

Determination of UT1 and polar motion by the deep space network using very long baseline interferometry

Measurements of UT1 with a formal accuracy as low as 0.6 msec with only 6 hr of data and construction of a radio astrometric catalog of approximately 45 sources whose positions are known to better than 0.05 arcsec are described which were made possible by very long-baseline interferometry (VLBI) with the Deep Space Network (DSN). The characteristics and anticipated performance of the complete VLBI system being implemented within the DSN are discussed. It is noted that one of the capabilities of this system is the measurement of UT1 and polar motion at weekly intervals and that the determinations should be obtained with decimeter accuracy.

Fanselow, J. L.↗
NASA NTRS1980

Long baseline vector determinations and intercomparisons

The East Coast very long baseline interferometry (VLBI) group has performed about 60 experiments over the last 10 years in the process of developing and exploiting advanced VLBI techniques. Of those, the most recent 14 experiments are briefly described.

Knight, C. A.↗
NASA NTRS1980

Crustal deformation at very long baseline interferometry sites due to seasonal air-mass and ground water variations

The seasonal deformation normal to the Earth's surface was calculated at stations involved or interested in very long baseline interferometry (VLBI) geodesy and at hypothetical sites in Australia and Brazil using global atmospheric pressure data, values for groundwater storage, and load Love numbers deduced from current Earth models. It was found that the annual range of deformation approached the centimeter level measuring potential of the VLBI technqiue at Greenbank, Haystack, and the Brazil site.

Stolz, A.↗