Engineering topics

Clark, T. A.

Publications and source records attributed to Clark, T. A..

At least 37 records · Page 2

NASA NTRS1985

Precision geodesy using the Mark-III very-long-baseline interferometer system

Very-long-baseline interferometry (VLBI) has been used to make precise measurements of the vector separation between widely separated antennas. The system for acquiring and processing VLBI data known as Mark-III is described. Tests of the system show it to have millimeter-level accuracy on short baselines; measurements of baselines longer than a few hundred kilometers suggest that accuracy is limited by the uncertainty in the calibration of tropospheric path delay to the level of a few centimeters. VLBI experiments conducted between 1976 and 1983 have demonstrated the stability of the North American plate by showing that there is no change in the distance between eastern California and Massachusetts at the level of a few millimeters per year or greater. Experiments made from 1980 to 1984 indicate that the distance from Massachusetts to Sweden is increasing by 1.7 + or - 1 cm/year where the quoted standard deviation includes the estimated effects of systematic errors.

Clark, T. A.↗

NASA NTRS1985

Quasar 4C39.25 is not contracting

A map has been made of the quasar 4C39.25, which has been described as consisting of two components whose angular separation remains constant at about 2 marc-sec, using VLBI observations made at 3.6 cm during 1983. From the map it is concluded that Shaffer's (1984) suggestion that this source may have been contracting superluminally during 1979-82 is not correct. Three distinct components are found in the quasar structure, two separated by 2.0 marc-sec and a third, presumably new and not previously reported, situated between the other two. It is possible either that the third component is stationary and that its flux density has rapidly increased to render it visible, or that it has recently been ejected from the westernmost component.

Marcaide, J. M.↗

NASA NTRS1985

The quasars 1038 + 528 A and B

The results of VLBI observations of the quasars 1038 + 528 A and B at 2.8, 3.6, 13, and 18 cm at various times between November 1979 and March 1981 are reported. The observations and data calibration are described, as are the mapping and astrometric techniques applied in the study. Both quasars are found to have 'core-jet' morphologies. The core of the A quasar dominates its morphology at centrimetric wavelengths with the brightness temperature of its 400 pc long jet being about 1/100 that of the core. By contrast, the 'jet' in the B quasar is very short (about 70 pc); the tail of this jet has the steepest spectral index found to date in extragalactic compact sources, indicating that high electron losses are responsible for the shortness of the jet. No evidence for appreciable morphological change in the B quasar was found over the time span of the study, whereas a new feature may be emerging from the A quasar core at superluminal speed.

Marcaide, J. M.↗

NASA NTRS1984

Precise time transfer using MKIII VLBI technology

It is well known that Very Long Baseline Interferometry (VLBI) is capable of precise time synchronization at subnanosecond levels. This paper deals with a demonstration of clock synchronization using the MKIII VBLI system. The results are compared with clock synchronization by traveling cesium clocks and GPS. The comparison agrees within the errors of the portable clocks (+ 5 ns) and GPS(+ or - 30 ns) systems. The MKIII technology appears to be capable of clock synchronization at subnanosecond levels and appears to be very good benchmark system against which future time synchronization systems can be evaluated.

Johnston, K. J.↗

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 NTRS1982

Recent SETI observations at Arecibo

Preliminary results are presented for a high-resolution search for narrowband signals from the direction of 210 nearby solar-type stars and five OH masers. The 305-m Arecibo radio telescope was used to seek narrowband radio emission at frequencies surrounding the 21-cm H I line and/or the 18-cm OH lines; the observations were made in right and left circular polarizations simultaneously. At the 98% level, 291 'birdies' were found, of which six sources at 18 cm and eight at 21 cm are considered potential candidates for reobservation and further study to identify the nature of the signals. It is noted that: (1) five of the 21-cm 'birdies' are probably H I clouds along the line of sight; (2) some of the 18-cm 'birdies' are associated with the extreme 13 frequency bands bordering unprotected bands above and below the established radio astronomy band; and (3) one of the 21-cm 'birdies' is distinctly narrowband and has no signal detected in the 'off' source.

Tarter, J. C.↗

NASA NTRS1982

Radio interferometric detection of a traveling ionospheric disturbance excited by the explosion of Mount St. Helens

A large-amplitude traveling ionospheric disturbance (TID) was detected over Owens Valley, California, on May 18, 1980, by a highly sensitive very long baseline interferometry (VLBI) radio astronomy experiment. This TID is interpreted as the response of the ionosphere to a gravity wave excited in the neutral atmosphere by the explosion of Mount St. Helens that took place at 1532 UT on that day. A model, invoking the point-excitation of internal gravity waves in an isothermal atmosphere, which fits observations of the TID at several other stations, leads to identification of the features observed in the VLBI data. Small-amplitude higher-frequency changes in the ionosphere were detected for several hours after the passage of the large-amplitude Mount St. Helens TID, but it is not clear whether these were excited by the passage of the gravity wave or were background fluctuations.

Roberts, D. H.↗

NASA NTRS1981

Geodesy by radio interferometry - Intercontinental distance determinations with subdecimeter precision

Analysis of very-long-baseline interferometer (VLBI) observations yielded estimates of the distances between three radio telescopes in the United States and one in Sweden, with formal standard errors of a few centimeters: Westford, Massachusetts-Onsala, Sweden: 5,599,714.66 + or - 0.03 m; Green Bank, West Virginia-Onsala, Sweden: 6,319,317.75 + or - 0.03 m; and Owens Valley, California-Onsala, Sweden: 7,914,131.19 + or - 0.04 m, where the earth-fixed reference points are defined in each case with respect to the axes of the telescopes. The actual standard errors are difficult to estimate reliably but are probably not greater than twice the formal errors.

Herring, T. A.↗

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

Mark 3 system overview

The Mark 3 very long baseline interferometry (VLBI) system, comprising a complete end to end VLBI system optimized for both high accuracy geodesy and radio astronomy, is described. The data flow, the data base handler system, and the field station component and configurations are briefly discussed. The use of mobile and transportable stations allows measurements to be taken from a large number of sites with relatively few sets of equipment. Fixed stations form a long term reference network for tying together the measurements with the mobile and transportable stations.

Clark, T. A.↗

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

The very flat radio spectrum of 0735 plus 178 - A cosmic conspiracy

Multifrequency interferometric and total flux density measurements of the radiation from 0735 plus 178 are discussed. It is shown that it is far more likely that the very flat radio spectrum of this source results from a superposition of incoherent synchrotron radiation from four distinct, homogeneous components, each with a peaked spectrum, than from radiation from a single inhomogeneous component.

Cotton, W. D.↗

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

Synchronization of clocks by very-long-baseline interferometry

Two hydrogen-maser clocks, one at Haystack Observatory and one at the National Radio Astronomy Observatory, were synchronized by means of observations of several extragalactic radio sources on March 28, and again on September 23, 1977. Observations were made sequentially in eight 360-kHz bands distributed between about 8.4 and 8.5 GHz with spacings designed to enable the group-delay difference between the signals received at the two observatories from a given source to be estimated unambiguously, within an uncertainty of less than 1 ns set by receiver noise. The epoch and the rate differences between the observatories' clocks for each experiment were estimated by analysis of observations that spanned several hours. The application of corrections for the contributions to the delays of the antennas, feeds, receiver systems, and recorders yielded absolute determinations of the clock epoch differences. During each experiment, portable cesium clocks were flown from the U.S. Naval Observatory to the observatories and back. The traveling-clock data, analyzed in each case after the VLBI synchronization had been completed, confirmed the VLBI results to within 18 and 14 ns for the first and second experiments, respectively.

Clark, T. A.↗

NASA NTRS1979

Geodetic interferometry submission for the IUGG Quadrennial Report

The present paper reviews the activities of two major VLBI (Very Long Baseline Interferometry) groups, within the United States, which have been concentrating on geodetic measurements for a number of years. The activities have resulted in the development of transportable VLBI terminals for regional surveying applications; development of VLBI techniques for obtaining station positions; establishment of relative epochs and rates of the hydrogen maser clocks at the stations; the acquisition of polar motion and earth rotation data in support of deep space tracking functions; and the development of VLBI techniques necessary for tectonic measurements on trans- and intercontinental baselines and for astrometry, polar-motion and earth-rotation observations.

Clark, T. A.↗

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

A high declination search at 8 GHz for compact radio sources

With the Haystack-NRAO interferometer (baseline length of 20 million wavelengths at 3.8 cm) 37 sources were observed whose declinations were above 50 deg. Seven of these sources have compact cores with diameters smaller than 5 milliarcsec and with correlated flux densities greater than about 0.5 Jy; the remaining sources have no cores with flux densities above about 0.3 Jy, the sensitivity limit of the interferometer. Two of the sources with detected compact cores, 4C 67.05 and 3C 418, were also observed with longer-baseline interferometers; the diameter of the core of 4C 67.05 was estimated to be smaller than 1 milliarcsec and that of 3C 418 to be smaller than 0.4 milliarcsec. All diameter estimates were based on an assumed circular Gaussian distribution of radio brightness and refer to the contour with brightness density e to the -1/2 power times that of the center. Positions for the detected sources were also obtained from the interferometric data, the uncertainty in these coordinate estimates ranging from 0.04 to 0.6 arcsec. The compact core detected in 3C 390.3 was found to lie near the center of this extended (approximately 4 arcmin in diameter) double radio source and to be coincident to within 1 arcsec with an N galaxy previously identified with 3C 390.3.

Wittels, J. J.↗