Engineering topics

Whitney, A. R.

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

At least 19 records

NASA NTRS1995

Proper Motion of Components in 4C 39.25

From a series of simultaneous 8.4 and 2.3 GHz VLBI observations of the quasar 4C 39.25 phase referenced to the radio source 0920+390, carried out in 1990-1992, we have measured the proper motion of component b in 4C 39.25: mu(sub alpha) = 90 +/- 43 (mu)as/yr, mu(sub beta) = 7 +/- 68 (mu)as/yr, where the quoted uncertainties account for the contribution of the statistical standard deviation and the errors assumed for the parameters related to the geometry of the interferometric array, the atmosphere, and the source structure. This proper motion is consistent with earlier interpretations of VLBI hybrid mapping results, which showed an internal motion of this component with respect to other structural components. Our differential astrometry analyses show component b to be the one in motion. Our results thus further constrain models of this quasar.

Guirado, J. C.↗

NASA NTRS1992

Stronger Source As Reference For Weaker Source In VLBI

Observation of weak astronomical radio source by very-long-baseline interferometry (VLBI) enhanced by using relatively strong astronomical radio source within small angular distance from weak source. Strong source and weak source observed alternately during measurement session. Direct detection of strong source provides reference delays and rates of change of delays used to integrate radio visibilities of weak source. Integrated visibilities added coherently over entire measurement session, using observed phases of strong source as references.

Lestrade, Jean-Francois↗

NASA NTRS1990

Phase-referenced VLBI observations of weak radio sources - Milliarcsecond position of Algol

The Fourier inversion of all the delay/rate referenced visibilities of Algol was used to form a 'dirty' map which has dramatically improved VLBI array sensitivity. The Algol system, in a very low state of activity with a total flux density of only 3 mJy, was observed alternatively with the reference source 0309 + 411, about 1 deg away, throughout the 7 hr VLBI experiment. By combining all the individual VLBI observations, the resulting coherent integration time for Algol is 4.2 hr and the corresponding SNR is 12. Internal consistency checks and an analysis of possible systematic astrometric errors indicate that the accuracy of the relative VLBI position of Algol is + or - 0.0005 arcsec in both coordinates and that unknown systematic effects are presently limiting this astrometric measurement.

Lestrade, J.-F.↗

NASA NTRS1988

Supernova 1987A - A radiosphere resolved with VLBI five days after the neutrino burst

The results of VLBI observations of SN1987A are reported. No emission from the supernova above a level of about 20 percent of the supernova's total flux density was detected, although signals were detected from two calibrator sources with amplitudes roughly equal to those determined in earlier VLBI observations. It is inferred that the supernova's radiosphere was resolved, and a lower bound on the radiosphere's radius of 2.2 mas is estimated from an epoch 5.2 days after the neutrino burst. Given the photometric data from the supernova, a distance to the LMC of 50 + or - 5 kpc, and an apparent expansion velocity that varied systematically with time from 18,000-16,000 km/s, as estimated from the blue-shifted H-alpha absorption lines on the days preceding the observations, it is concluded that 5.2 days after the neutrino burst the supernova's radiosphere was at least 2.5 times larger than the inferred blackbody photosphere, and at least as large as the H-alpha line-forming region.

Jauncey, D. L.↗

NASA NTRS1988

The Mark III VLBI System

Geodetic measurements have errors in centimeter range. Collection of three reports describes both equipment and results of some measurements taken with Mark III very-long-baseline interferometry (VLBI) system. Has demonstrated high accuracy over short baselines, where phase-delay measurements used. Advanced hardware, called Mark III A, developed to improve system performance and efficiency. Original Mark III hardware and III A subsystem upgrades developed as part of NASA Crustal Dynamics Project at Haystack Observatory.

Rogers, A. E. E.↗

NASA NTRS1988

Radio observations of SN 1987A

The study of radio emission from SN 1987A after the neutrino burst is reviewed. The observed maximum flux density of the burst at radio wavelengths was about 140 mJy and occurred at 1.4 GHz. The radio light curves for SN 1987A at four frequencies are compared with data for SN 1979C and SN 1980K. VLBI observations of SN 1987A at 2.3 GHz and other radio monitoring programs are examined.

Bartel, N.↗

NASA NTRS1983

Very-long-baseline radio interferometry - The Mark III system for geodesy, astrometry, and aperture synthesis

Up to 112 megabit/sec from each radio telescope of an interferometric array can be recorded and processed by the Mark III VLBI system. Sample results are given for baseline lengths between three antennas in the U.S. and three in Europe, as well as for the arc lengths between the positions of six extragalactic radio sources. No significant change is detected in any of these quantities. Signals of a given polarization or of pairs of orthogonal polarizations may be recorded in up to 28 contiguous bands, each nearly 2 MHz wide, for mapping the brightness distribution of compact radio sources. The demonstrated ability to record large bandwidths, and to link many large radio telescopes, allows compact sources with flux densities below 1 millijansky to be detected and studied.

Rogers, A. E. E.↗

NASA NTRS1983

Detection of a compact radio source near the center of a gravitational lens - Quasar image or galactic core

By use of a new, very sensitive interferometric system, a faint, compact radio source has been detected near the center of the galaxy that acts as the main part of a gravitational lens. This lens forms two previously discovered images of the quasar Q0957+561, which lies in the direction of the constellation Ursa Major. The newly detected source has a core smaller than 0.002 arc second in diameter with a flux density of 0.6 + or - 0.1 millijansky at the 13-centimeter wavelength of the radio observations. This source could be the predicted third image of the transparent gravitational lens, the central core of the galaxy, or some combination of the two. It is not yet possible to choose reliably between these alternatives.

Gorenstein, M. V.↗

NASA NTRS1983

Accuracy of relative positioning by interferometry with GPS Double-blind test results

MITES (Miniature Interferometer Terminals for Earth Surveying) observations conducted on December 17 and 29, 1980, are analyzed. It is noted that the time span of the observations used on each day was 78 minutes, during which five satellites were always above 20 deg elevation. The observations are analyzed to determine the intersite position vectors by means of the algorithm described by Couselman and Gourevitch (1981). The average of the MITES results from the two days is presented. The rms differences between the two determinations of the components of the three vectors, which were about 65, 92, and 124 m long, were 8 mm for the north, 3 mm for the east, and 6 mm for the vertical. It is concluded that, at least for short distances, relative positioning by interferometry with GPS can be done reliably with subcentimeter accuracy.

Counselman, C. C., III↗

NASA NTRS1982

The nucleus of M81 - Simultaneous 2.3 and 8.3 GHz Mark III VLBI observations

Mark III very long baseline interferometry (VLBI) observations have been made of the nucleus in the normal galaxy M81 (NGC 3031) simultaneously at 2.3 and 8.3 GHz, and it was found that nearly 100% of the flux density of the nuclear region originates in an elongated radio core with linear dimensions 1000-4000 AU, dependent on frequency. This galactic nucleus is the most compact observed in any extragalactic source. The position coincides with that of the nucleus at optical and X-ray wavelengths within the larger uncertainties of the latter two. The position angle of the major axis of the M81 nucleus is within 3 deg of 75 deg at 2.3 GHz and within 6 deg of 50 deg at 8.3 GHz and is frequency dependent at the 4 sigma level. These values straddle the 62 deg position angle of the projection on the sky of the rotation axis of the galaxy. The spectrum of the core is slightly inverted, and the length of its major axis decreases with frequency. These results are consistent with the emission's being incoherent electron synchrotron radiation from an inhomogeneous region with an optical depth varying along the rotation axis of the galaxy.

Bartel, N.↗

NASA NTRS1980

Automation of the Mark 3 field system

The software and hardware components which will enable hands off operation are described. The operation of the field system begins with the scheduling of observations. An interactive program, SKED, provides displays of mutual visibility, automatic calculation of telescope slewing times, and the ability to list and edit the schedule. The output of SKED is a schedule file containing commands in the Standard Notation for Astronomy Procedures (SNAP) language which the field system uses for controlling events during the experiment. The most important features of SNAP include sophisticated time sequencing of events, automatic logging of all commands and responses, and the ability to define often used sequences of commands as procedures. A control program, BOSS, running in an HP 1000 minicomputer, reads the SNAP commands from a schedule file and interprets them in terms of commands and requests to devices. Interactive command input is possible through the operator's display terminal. Communication with all of the Mark 3 electronics modules is done via a small general purpose interface board (a microprocessor based ASCII transceiver) which has been installed in each module. Additional devices are controlled and monitored using the IEEE 488 General Purpose Interface Bus.

Vandenbere, N. R.↗

NASA NTRS1980

Mark 3 correlator hardware and software

The Mark 3 correlator system is described in some detail. The correlator system is based on a modular philosophy. Each correlator module independently processes the data from one track pair. Therefore, 28 modules are necessary to complete a full one baseline processor and 84 modules for a full 3 baseline processor. Each correlator module has two interfaces: (1) data and clock from each of the two tracks to be correlated and (2) Computer Automated Measurement and Control (CAMAC) dataway interface to the computer. The processor is organized around the IEEE CAMAC standard architecture, housing 15 correlator modules in each of 6 crates. This allows one pass processing of a full 3 baseline 28 track observation or a 6 baseline (4 station) 14 track observation. The correlator architecture allows easy expansion for up to 8 stations. The computer system is an HP 1000 system utilizing a 16 bit minicomputer with disc and tape peripherals. The processing software is also organized in a modular fashion with many independent but cooperative programs controlling the operation of the Mark 3 processor. Processing time through the correlator is normally real time or faster, with graphics displays providing real time monitor and control of the processing operation.

Whitney, A. R.↗

NASA NTRS1980

Mark 3 real-time fringe detection system

A RAM memory built into the Mark 3 decoder module allows the capture of 1 Megabit of data. Data may be collected either in real time or from a pre-recorded tape. Once collected, the data may be retrieved using a standard EIA serial data link. The data may be transmitted to a remote computer for cross correlation processing with similar data from other stations to verify fringes in real time. The data may also be analyzed by a local computer to verify phase calibration, bandpass, format, etc., during a Mark 3 observing session.

Levine, J. I.↗

NASA NTRS1980

Time-dependent radio fine structure of the compact sources NRAO 150 and 4C 39.25

Very long baseline interferometer observations at 7.85 GHz have been used to probe the milliarcsecond structure of the unidentified, very compact radio source NRAO 150 and QSO 4 C 39.25. NRAO 150 exhibited no structural variations from 1972 to the end of 1974. A model with two circular Gaussian components fits the data well. NRAO 150 had a flux density of 7.6 plus or minus 0.5 Jy in the compact component; 4 C 39.25 showed a two-component structure, the components having a separation of (2.02 plus or minus 0.05 arc sec) x 10 to the -3rd power. The upper bound on the speed of transverse separation is 0.0001 arc sec per year or less than 2.7 c. From the spectrum there are also indications of a third, larger component.

Baath, L. B.↗

NASA NTRS1979

Submilliarcsecond astrometry via VLBI. I - Relative position of the radio sources 3C 345 and NRAO 512

The relative position and relative proper motion of the radio sources 3C 345 and NRAO 512 are estimated from four sets of VLBI observations spread out over the period from October 1971 to May 1974. The use of phase-connection techniques yields the separation, in 1950.0 coordinates, of the centers of brightness of the compact components of the two sources. An upper bound of 0.0005 arcsec/yr is placed on the relative proper motion (70% estimated confidence limits). Bounds that can be placed on the distances to the two sources are considered, prospects for improvement in the determination of relative position and proper motion of these sources are discussed, and other possible applications of the basic technique are described.

Shapiro, I. I.↗

NASA NTRS1979

3C 279 - The case for 'superluminal' expansion

The compact extragalactic radio source 3C 279 was observed with the Haystack-Goldstone interferometer (wavelength approximately 3.8 cm) during six separate sessions spread between October 1970 and April 1972. The fringe amplitudes from each of these observation sessions were consistent with a two-component model of the brightness distribution of the source. The position angle of the line joining the components remained at 38 + or - 2 deg, while the angular separation between the components increased nearly linearly at the rate of 0.5 + or 0.1 milliarcsec/yr during this period. The corresponding apparent expansion speed is (21 + or - 4)c, for H = 50 km/s per Mpc and q = 0.05

Cotton, W. D.↗

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 NTRS1978

Geodesy by radio interferometry - Determination of a 1.24-km base line vector with approximately 5-mm repeatability

The paper describes a new method for determining the base line vector from X band radio interferometric observations of extragalactic sources. The procedure utilizes the precision inherent in fringe phase measurements. Eleven separate experiments were conducted to measure the 1.24-km base line vector between the two antennas of the Haystack Observatory in Westford, Mass. The repeatability, scatter, and level of accuracy are discussed.

Rogers, A. E. E.↗