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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

VLBI observations of 3C 345 and NRAO 512 in right and left circular polarization

In October 1975, the radio telescopes of the Haystack, National Radio Astronomy, and Owens Valley Radio Observatories were used as an interferometer to monitor, at 8 GHz, the right and left circularly polarized radiation emitted by the quasars 3C 345 and NRAO 512. The data for each polarization are used separately to estimate several parameters describing a model of the fine structure of the radio brightness of 3C 345 and, subsequently, the angular separation between 3C 345 and NRAO 512. The results for the two polarizations are in approximate agreement, indicating that to the limit of resolution about 0.5 milliarcsec), the fourth Stokes parameter, V, is not significantly different from zero within the compact components of these radio sources. The corresponding quantitative limits on the degree of circular polarization are 0.08 + or - 0.07 for 3C 345 and 0.01 + or - 0.08 for NRAO 512.

Menyuk, C. R.↗
NASA NTRS1979

Connection and validation of narrow-band delta VLBI phase observations

Narrow-band differential very long baseline interferometry required phase connections between consecutive scans. An efficient computer aided scheme for this purpose is presented. This scheme is an iteration process alternating between a grand fit on many scans and integer quantization of the phase shift cycles to be assigned to the scans. Only linear simultaneous equations of a few unknowns need to be solved. A simulation analysis indicates that faultless phase connection can be expected when there is no localized systematic noise. When systematic noise of moderate level exists, the possible incorrect phase connection can be detected and connected for by comparing the connected phases from the two alternating observations, after removing the residual diurnal effects.

Wu, S. C.↗
NASA NTRS1980

Application of Laser Ranging and VLBI Data to a Study of Plate Tectonic Driving Forces

The conditions under which changes in plate driving or resistive forces associated with plate boundary earthquakes are measurable with laser ranging or very long base interferometry were investigated. Aspects of plate forces that can be characterized by such measurements were identified. Analytic solutions for two dimensional stress diffusion in a viscoelastic plate following earthquake faulting on a finite fault, finite element solutions for three dimensional stress diffusion in a viscoelastic Earth following earthquake faulting, and quantitative constraints from modeling of global intraplate stress on the magnitude of deviatoric stress in the lithosphere are among the topics discussed.

Solomon, S. C.↗
NASA NTRS1980

Polar motion and UT1: Comparison of VLBI, lunar laser, satellite laser, satellite Doppler, and conventional astrometric determinations

Very long baseline interferometry observations made with a 3900 km baseline interferometer (Haystack Observatory in Massachusetts to Owens Valley Observation in California) were used to estimate changes in the X-component of the position of the Earth's pole and in UT1. These estimates are compared with corresponding ones from lunar laser ranging, satellite laser ranging, satellite Doppler, and stellar observations.

Robertson, D. S.↗
NASA NTRS1980

Phase coherent VLBI using a satellite link

The results from experiments using the Hermes (formerly CTS) communications satellite to provide a local oscillator and data link between two antennas are presented. The techniques used to compensate for the satellite motion and translation oscillator are described. Plans for a series of three station experiments to measure UT and polar motion using the ANIK-B satellite to synchronize the local oscillators are discussed.

Waltman, W. B.↗
NASA NTRS1980

VLBI measurements of radio source positions at the Jet Propulsion Laboratory

The results of approximately 1300 observations of 67 radio sources are presented. Most of the measurements were made at the stations of the Deep Space Network in California, Spain, and Australia at wavelengths of 13.1 and 3.6 cm, between 1971 and 1978. The formal errors in the derived source positions are generally in the neighborhood of 0.01 seconds of arc and the positions agree fairly well with those previously published.

Purcell, G. H., Jr.↗
NASA NTRS1980

Phase and group delay calibration of a very long baseline interferometer by East Coast VLBI Group

Precisely timed pulses injected into the input of each receiver are used to calibrate the phase and group delay through each interferometer terminal. The short duration pulses are generated at a 1 MHz rate directly from the output of the frequency standard. The pulses are injected into the receiver at a level low enough to produce less than one percent increase in system temperature, yet can be extracted during processing with a high enough signal to noise ratio to determine the phase of the calibration rails within 1 degree in 1 second of integration. The calibration system also includes precise cable measurement electronics and a pulse echo for clock synchronization.

Rogers, A. E. E.↗
NASA NTRS1980

Mark 3 VLBI system: Tropospheric calibration subsystems

Tropospheric delay calibrations are implemented in the Mark 3 system with two subsystems. Estimates of the dry component of tropospheric delay are provided by accurate barometric data from a subsystem of surface meteorological sensors (SMS). An estimate of the wet component of tropospheric delay is provided by a water vapor radiometer (WVR). Both subsystems interface directly to the ASCII Transceiver bus of the Mark 3 system and are operated by the control computer. Seven WVR's under construction are designed to operate in proximity to a radio telescope and can be commanded to point along the line-of-sight to a radio source. They should provide a delay estimate that is accurate to the + or - 2 cm level.

Resch, G. M.↗
NASA NTRS1980

Phase versus delay in geodetic VLBI

The conditions for the applicability of the phase observable in very long baseline interferometry are discussed and examples of adequate observation programs are given. These considerations apply in particular to the Mark 2 system, which is already in operation at many observatories.

Campbell, J.↗
NASA NTRS1980

Backpack VLBI terminal with subscentimeter capability

Backpack portable equipment was developed to measure vector baseline from approximately 1 km to 100 km in length with subcentimeter to few centimeter accuracy. The equipment design features as well as the instrumentation specifications are discussed. It is shown that the unit has the following advantages: it is simple in concept; it is reliable in unattended operation; and it is inexpensive (less than $15,000 per unit).

Connselman, C. C., III↗
NASA NTRS1980

NASA geodetic applications of the Mark 3 VLBI system

The Mark 3 very long baseline interferometer system was used in monitoring the following geodetic phenomena: the regional deformation and strain accumulation related to large earthquakes in the plate boundary region of the western United States; contemporary relative plate tectonic motions of the North American, Pacific, South American, Eurasian, and Australian Plates; the internal deformation of continental and oceanic lithospheric plates with particular emphasis on North America and the Pacific; the rotational dynamics of the Earth and their possible correlation to earthquakes, plate motions, and other geophysical phenomena; and motions and deformation occurring in regions of high earthquake activity.

Coates, R.↗
NASA NTRS1980

On the VLBI-satellite laser ranging Iron Triangle intercomparison experiment

Simulations were performed to determine the suitability of the proposed station locations. The criterion was a comparison among the possible station configuration of the standard deviations of baseline and Earth rotation parameters estimated from a least squares covariance analysis. Only the relative magnitudes of the standard deviations were addressed in the analysis. Only random errors were assumed and no provision was made for systematic effects. The Iron Triangle, consisting of the stations at Westford (Haystack), Massachusetts, Owens Valley, California, and Ft. Davis, Texas, was regarded as the structure of the proposed network with options to incorporate either the Goldstone, California, or Green Bank, West Virginia, station or both. It was decided to include the Richmond, Florida, station in the analysis since it offered more North-South separation and therefore could strengthen the geometry of the network especially in the recovery of Earth rotation parameters.

Bock, Y.↗
NASA NTRS1980

The Jet Propulsion Laboratory/California Institute of Technology block 2 VLBI processor

A multistation, multichannel cross correlation processor using the 250 k bit to 4 M bit recording format is discussed. The design is modular, using bit sliced microprocessors to perform the routine calculations for phase and delay on a per station basis, as well as for fractional bit shift correction and Fourier transformation of the correlation coefficients on a per baseline basis.

Rogtad, D. H.↗
NASA NTRS1979

VLBI observations of galactic nuclei at 18 centimeters - NGC 1052, NGC 4278, M82, and M104

Compact radio sources about a light year in size have been detected in the nuclei of the galaxies NGC 1052, NGC 3034 (M82), NGC 4278, and NGC 4594 (M104) at a wavelength of 18 cm. The compact nucleus detected in M81 at 6 cm was not seen at 18 cm. The compact source in M82 is unique among extragalactic sources in its size-spectrum relationship. It is either broadened by scattering within M82 or it lies behind, and is absorbed by, an H II region. In these galaxies, the size of the nuclear radio source at 18 cm is larger than it is at higher frequencies. The nucleus of the giant radio galaxy DA 240 was not detected.

Shaffer, D. B.↗
NASA NTRS1981

Developing new architectures for the Block 2 VLBI correlator system

The overall LSI (large-scale integrated circuits) architecture design and current status of the VBLI (very long baseline interferometry) block 2 correlator is addressed. The VBLI correlator algorithms demand a computing system that provides a throughput of hundreds of millions of instructions per second to perform cross-correlation detection for six baselines. The LSI technology lights the way for the computation of complex parallel process and is raising the upper bound of computerization.

Peterson, J. C.↗
NASA NTRS1981

VLBI observations of SS 433 at 3.6 and 13 centimeters

SS 433 was detected and partially resolved at 2290 MHz on baselines with fringe spacings of 1.4, 0.1, and 0.003 arcsec. It was also detected at 8420 MHz on a baseline with a fringe spacing of 0.009 arcsec. Simple models of the source, consistent with the limited data, have elongated structures greater than 0.1 arcsec in size with position angles in 1979 May that were within about 10 deg of the position angle of the apparent bulges of the supernova remnant W50. The data also imply that the source contains a core less than 0.002 arcsec in size with a brightness temperature greater than 10 to the 9th K. The bright core and aligned structures that seem to be present in SS 433 and W50 resemble the structures seen in powerful extragalactic radio sources which are many orders of magnitude larger.

Walker, R. C.↗
NASA NTRS1981

VLBI observations of the nucleus of the radio galaxy Cygnus A

The central component of the radio galaxy Cygnus A has been observed in several (very long baseline interferometry) experiments between 1975 and 1979, and the results have been combined to derive a model for the brightness distribution. Some 65% of the nuclear emission appears to come from a compact core. The remaining flux density comes from a more extended region (or regions) up to 4 or 5 mas away lying along a position angle near 100 deg, which is close to that of the extended lobes. Evidence that compact central components reflect the collimation and transport of energy to the outer lobes of radio galaxies is discussed, and several interpretations of the observed asymmetries are considered.

Kellermann, K. I.↗