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At least 253 records · Page 14
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NASA NTRS1971

Applications of laser ranging and VLBI observations for selenodetic control

The observation equations necessary to utilize lunar laser ranging and very long baseline interferometry measurements were developed for the establishment of a primary control network on the moon. The network consists of coordinates of moon points in the selenodetic Cartesian coordinate system, which is fixed to the lunar body, oriented along the three principal axes of inertia of the moon, and centered at the lunar center of mass. The observation equations derived are based on a general model in which the unknown parameters included: the selenodetic Cartesian coordinates, the geocentric coordinates of earth stations, parameters of the orientation of the selenodetic coordinate system with respect to a fixed celestial system, the parameters of the orientation of the average terrestrial coordinate system with respect to a fixed celestial coordinate system, and the geocentric coordinates of the center of mass of the moon, given by a lunar ephemeris.

Fajemirokun, F. A.↗
NASA NTRS1972

Spacecraft techniques for lunar research.

The most significant findings about the Moon obtained by spacecraft so far, have resulted from measurements of gravity, electromagnetic properties, seismicity, mechanical properties, geologic features, composition, ages, and the lunar environment. A number of major lunar questions remain to be answered. Other properties, measurable with spacecraft, which may provide data critical to answering these questions include geometrical shape, motions, and heat flow. In this paper specific measurements that should provide critical data for each of these questions are identified, with some candidate techniques. Among the suggested techniques that have not yet been used are very long baseline interferometry (Earth-Moon baseline), gravity gradiometry, elemental analysis by neutron interactions, and remotely-controlled on-moon microscopy.

Jaffe, L. D.↗
NASA NTRS1973

VLBI observations of the Crab Nebula pulsar.

Observations of the Crab Nebula pulsar at meter wavelengths using very long baseline interferometry techniques have been made. At 196.5 MHz, no resolution of the pulsar is observed, all the pulse shapes observed with the interferometers are similar to single-dish profiles, and all the power pulsates. At 111.5 MHz, in addition to the pulsing power, there is always a steady component, presumably due to multipath propagation effects. The pulsar is slightly resolved at 111.5 MHz with an apparent angular diameter of 0.07 plus or minus 0.01 sec. Fifty per cent linear polarization of the time-averaged power is observed at 196.5 MHz; at 111.5 MHz, 20% of the total time-averaged power is polarized, and 35% of the pulsing power is polarized. The steady component is unpolarized. The total flux of the steady plus pulsating component appears to remain constant, while the distribution of power between these components varies.

Vandenberg, N. R.↗
NASA NTRS1973

Long wavelength VLBI.

The application of very-long baseline interferometry (VLBI) techniques at meter and decameter wavelengths has proven to be a unique tool for studying a number of different phenomena. Because of the high angular resolution which we achieve with VLBI, candidate objects for study include the compact nuclei and 'jets' associated with quasars and radio galaxies, pulsars, small knots in galactic supernova remnants, and nonthermal planetary sources. Our investigations have been aimed at determining the emission spectra of these objects, measuring their apparent angular size, and mapping their structure. We expect that these properties will be interrelated, particularly at or below the frequencies at which their spectra peak.

Clark, T. A.↗
NASA NTRS1973

Low-noise microwave receiving systems in a worldwide network of large antennas.

The Deep-Space Network (DSN) consists of a worldwide network of 26- and 64-m antennas spaced approximately 120 deg around the world. The DSN is a precision communications system designed to communicate with and control unmanned spacecraft traveling at interplanetary ranges. Recent developments in the performance and evaluation of the antennas and maser receivers are discussed, and developments which have contributed to radio-science projects, such as planetary radar astronomy, very-long-baseline interferometry, pulsar, and other astronomical observations are described. It is also shown how these radio-science measurements have been used to evaluate the performance of the antenna-receiver systems.

Reid, M. S.↗
NASA NTRS1970

A systems plan for an earth and ocean dynamics satellite applications program

A system for investigating phenomena associated with the earth's dynamical and crustal motions, and the sea surface is described. The proposed observing systems include the following facilities: (1) A network of fundamental stations, whose locations and motions, including polar, variations in the earth's rotational rate, and crustal motions, are determined by very long baseline interferometry, relative to the basic reference system of radio stars. The sites are all NASA Goddard stations with the exception of Kashima, Japan, a facility which has participated extensively in the ATS program. (2) A system of Geopause spacecraft in nearly circular polar orbits, normal to the ecliptic at a distance of about five earth radii with a period on the order of two thirds of a day.

Siry, J. W.↗
NASA NTRS1972

Wind speeds in lower atmosphere of Venus: Status report on possible measurement via differential VLBI tracking of entry probes

The potential of very-long-baseline interferometry (VLBI) is examined for use in the determination of wind speeds in Venus' lower atmosphere via the differential tracking of entry probes. A simplified mathematical model is presented in detail. An incomplete error analysis based on this model permits an educated guess to be made: an uncertainty in wind speed determination of no more than about 100/t m/sec, where t l is the corresponding time resolution in seconds, is an achievable goal, without the use of transponders on the miniprobes. If transponders are available on all probes, there should be little difficulty in estimating wind speeds with useful precision.

Shapiro, I. I.↗
NASA NTRS1972

Proceedings of the Fourth Precise Time and Time Interval Planning Meeting

The proceedings of a conference on Precise Time and Time Interval Planning are presented. The subjects discussed include the following: (1) satellite timing techniques, precision frequency sources, and very long baseline interferometry, (2) frequency stabilities and communications, and (3) very low frequency and ultrahigh frequency propagation and use. Emphasis is placed on the accuracy of time discrimination obtained with time measuring equipment and specific applications of time measurement to military operations and civilian research projects.

Acrivos, H. N.↗
NASA NTRS1973

Time frequency requirements for radio interferometric earth physics

Two systems of VLBI (Very Long Baseline Interferometry) are now applicable to earth physics: an intercontinental baseline system using antennas of the NASA Deep Space Network, now observing at one-month intervals to determine UTI for spacecraft navigation; and a shorter baseline system called ARIES (Astronomical Radio Interferometric Earth Surveying), to be used to measure crustal movement in California for earthquake hazards estimation. On the basis of experience with the existing DSN system, a careful study has been made to estimate the time and frequency requirements of both the improved intercontinental system and of ARIES. Requirements for the two systems are compared and contrasted.

Thomas, J. B.↗
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

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 NTRS1974

Requirements and applications of marine geodesy and satellite technology to operations in the oceans

Marine geodesy is concerned with the determination of marine geographic positions, geodetic controls, and the geoid. The activities of marine geodesy are related to the utilization of satellite technology, electronic distance measurement, geodetic astronomy, gravimetric geodesy, and the potentially usable very long baseline interferometry. Assessments are made of the different accuracy and/or precision requirements involved in the various approaches. It is shown that marine geodesy is highly relevant to man's various practical and scientific operations at sea, including those on the continental shelves.

Mourad, A. G.↗
NASA NTRS1976

ALSEP: Quasar differential VLBI

A program of Apollo Lunar Surface Experiments Package (ALSEP)-Quasar Very Long Baseline Interferometry (VLBI) is being reported. These observations primarily employ a 4 antenna technique, whereby simultaneous observations with two antennas at each end of an intercontinental baseline are used to derive the differential interferometric phase between a compact extragalactic radio source (usually a quasar) and a number of ALSEP transmitters on the lunar surface. A continous ALSEP-quasar differential phase history over a few hour period leads to extremely high angular accuracy in measuring the lunar position against the quasar reference frame. This application of the 4 antenna technique has been underway for more than a year and is now producing high quality data utilizing Deep Space Network (DSN) stations in Australia, Spain, and Goldstone, California, as well as the Spaceflight Tracking and Data Network (STDN) Apollo station at Goldstone.

Slade, M. A.↗
NASA NTRS1976

Apparent 'superrelativistic' expansion of the extragalactic radio source 3C 345

The compact extragalactic radio source 3C 345 was observed by very-long-baseline interferometry (wavelength about 3.8 cm) at 12 epochs distributed over the nearly four-year period from February 1971 to October 1974. For one of these epochs, the multibaseline data were sufficient to allow the brightness distribution to be estimated in a model-independent manner; the resultant distribution was clearly dominated by two components. The remaining sets of data were also represented adequately by two-component models. The angular separation of the two components increased during this period from about 1.00 to 1.30 milliarcsec, corresponding to an apparent average speed of expansion of approximately 2.5 c at a fixed position angle of 105 (plus or minus 5) deg. These results, coupled with the fact that contraction has never been observed, seem difficult to reconcile with the so-called Christmas-tree model of the 'superrelativistic' expansion of extragalactic radio sources.

Wittels, J. J.↗
NASA NTRS1976

Meter-wavelength VLBI. III - Pulsars

Observations of pulsars, especially the Crab Nebula pulsar, made in very long baseline interferometry (VLBI) experiments are discussed. Based on a crude 144 MHz visibility curve which is consistent with a Gaussian brightness distribution, the measured visibilities at 196, 111, and 74 MHz were interpreted to yield apparent angular diameters (at half-power) of about 0.03 sec, 0.07 sec, and 0.18 sec, respectively. These sizes scale approximately as wavelength-squared, and the 74 MHz size agrees with recent observations using interplanetary scintillation techniques. The total flux densities lie on the extrapolation from higher frequencies of the pulsing flux densities. Variations in the total flux density up to 25 per cent were observed. A lack of fine structure other than the pulsar in the nebula is indicated by the simple visibility curves. The pulse shapes are similar to single-dish measurements at 196 MHz but reveal a steady, nonpulsing component at 111 MHz. The ratio of pulsing to total power was approximately equal to one-half but varied with time. It was found that four strong, low-dispersion pulsars were only slightly resolved.

Vandenberg, N. R.↗
NASA NTRS1976

New vertical geodesy

The paper contains a review of the theoretical difference between orthometric heights and heights labeled geometric which are determined through use of an extraterrestrial frame of reference. The theory is supplemented with examples which portray very long baseline interferometry as a measuring system that will provide estimates of vertical crustal motion which are radically improved in comparison with those obtained from analysis of repeated geodetic levelings. The example of the San Fernando earthquake of 1971 is used to show how much estimates of orthometric and geometric height change might differ. A comment by another author is appended which takes issue with some of the conclusions of this paper. In particular, an attempt is made in the comment to rebut the conclusion that geodetic leveling is less reliable than VLBI measurements for determining relative elevation change of points separated by more than 56 km.

Whitcomb, J. H.↗
NASA NTRS1976

Pioneer Venus 1978 mission support

The Tracking and Data Acquisition portion of the Ground Data System, which will support the Differential Long Baseline Interferometry Wind Measurement Experiment, is described.

Miller, R. B.↗
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.↗