Engineering topics

Linfield, R. P.

Publications and source records attributed to Linfield, R. P..

44 records · Page 3

NASA NTRS1986

Reference frame studies at JPL/CALTECH

Studies being undertaken in order to interconnect the three principal celestial coordinate systens are reviewed. These systems are the optical frame (FK4/FK5) based on positions of Galactic stars, the planetary/lunar ephemeris frame based on the major celestial bodies of the solar system, and the radio frame constructed from observations of quasars. The optical frame is being connected to the radio frame by VLBI observations of radio-emitting stars, and the radio frame is being tied to the ephemeris frame in several ways; for example, differential VLBI measurements between quasars and planet-orbiting spacecraft. Prospects for future interconnection studies are briefly discussed.

Dickey, J. O.↗

NASA NTRS1985

Precise position measurements of Jupiter, Saturn and Uranus systems with the Very Large Array

VLBI observations of the Jovian satellites Europa, Ganymede, and Callisto obtained in 1983, of Titan obtained in 1984, and of Uranus obtained in 1985, all with the VLA at 15 GHz, are used to determine the ephemerides. The results are presented graphically, and it is found that the standard ephemeris reference frame for the outer planets is in error by about -200 marcsec in right ascension.

Muhleman, D. O.↗

NASA NTRS1985

Aperture synthesis using orbiting telescopes

A study was carried out to determine the feasibility, with current technology, of performing aperture synthesis using two telescopes orbiting the earth in coordinated orbits separated by approximately 10 m to 1 km. The objective was to determine whether there is a practical alternative to a very large, deployed, servo-controlled submillimeter telescope (i.e., the Large Deployable Reflector) for obtaining high-resolution submillimeter images of astronomical sources. It is found that suitable classes of orbits exist which can provide good UV coverage over the entire sky and the real-time correlation of wideband signals can be performed in orbit using current technology. The most difficult task appears to be the real-time determination of the orientation of the baseline vector in a stable coordinate system. A plausible scheme has been identified for the determination of an arbitrary direction to within 0.003 arcsec in an astrometric coordinate system. This scheme not only makes submillimeter interferometric image reconstruction possible but should also have numerous other applications.

Kuiper, T. B. H.↗

NASA NTRS1985

Strong source VLBI observations at 22 GHz

Twenty-six strong high-frequency radio sources have been observed at 22.3 GHz on interferometer baselines of approximately 500 million wavelengths. Twenty-five were detected, and 14 have mean visibilities over 50 percent. The strongest sources as a group are more heavily resolved than the weaker sources.

Lawrence, C. R.↗

NASA NTRS1985

Arcsecond positions for milliarcsecond VLBI nuclei of extragalactic radio sources. III - 74 sources

VLBI measurements at 2290 and 8420 MHz on baselines of 10,000 km have been used to determine the positions of the milliarcsecond nuclei in 74 extragalactic radio sources. Estimated accuracies range from 0.1 to 4.3 arcsec in both right ascension and declination with typical accuracies of about 0.3 arcsec. The observed sources are part of an all-sky VLBI catalog of milliarcsecond radio sources. Arcsecond positions have been determined for 819 sources. These positions are presently being used to identify optical counterparts in the southern hemisphere.

Morabito, D. D.↗

NASA NTRS1985

Arcsecond Positions for Milliarcsecond VLBI Nuclei of Extragalactic Radio Sources. Part 3: 74 Sources

VLBI measurements at 2290 MHz and 8420 MHz on baselines of 10,000 km between Deep Space Network stations have been used to determine the positions of the milliarcsecond nuclei in 74 extragalactic radio sources. Estimated accuracies range from 0.1 sec. to 4, 3 sec. in both right ascension and declination with typical accuracies of approx. 0.3 sec. The observed sources are part of an all-sky VLBI catalog of milliarcsecond radio sources. Arcsecond positions have now been determined for 819 sources. These positions are presently being used to identify optical counterparts in the Southern Hemisphere.

Morabito, D. D.↗

NASA NTRS1985

VLBI monitoring of the superluminal quasar 3C 273, 1977-1982

The compact ratio structure of the quasar 3C 273 has been monitored with a VLBI array at 5.0 and 10.7 GHz at six-month intervals during 1977-1982. THe VLBI monitoring observations, data reduction, calibration, and mapping techniques are described, and hybrid maps from the monitoring program are presented along with a brief discussion of the main features. Results on the source kinematics (superluminal motion of the knots and their time evolution) are given, and synchro-Compton models of the knots are presented and limits to their Doppler factors (delta) derived from X-ray measurements. The constraints on jet geometry provided by the combined measurements of delta and v/c are considered. For the core, the inhomogeneous jet model of Koenigl (1981) is compared with the available data. The relation between 3C 273 and the similar superluminal source 3C 345 is discussed.

Unwin, S. C.↗

NASA NTRS1984

A study on the imaging potential of QUASAT

The orbit, ground array, and sensitivities of the space and ground systems of the QUASAT spaceborne astronomy mission were studied. Parameters which yield images in which the noise is within a factor of three of the thermal noise limit were derived. With a very large ground array QUASAT should produce images as complex as the best of the VLA. At 22 GHz the resolution will be 0.1 milliarcsec, the field of view 15 milliarcsec, and it will be possible to make noise limited maps of complex objects. The QUASAT is essential for mapping southern declination objects. Without QUASAT, even the simple core-jet source cannot be mapped at declination minus 60 deg. It will be possible with QUASAT to make maps with observing times as short as 6 hr although the very best maps will require 48 hr observations.

Readhead, A. C. S.↗