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At least 379 records · Page 21
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NASA NTRS1980

Geodetic long baseline interferometry research in Canada

The objectives and results of several studies using the Canadian long baseline interferometry system (LBI) are presented. The precision of measurements from radio telescopes at the Algonquin Radio Observatory (ARO), Lake Traverse, Ontario; the Owens Valley Radio Observatory (OVRO), Big Pine, California; and the Chilbolton Observatory (CHIL), Chilbolton, England, is discussed. Also, since LBI is insensitive to the uncertainty in the geocentric gravitational constant, it is a very useful technique for determining the scales of the coordinate systems used by other precise techniques. Beginning in May 1977, a number of LBI observing sessions were accompanied by simultaneous satellite Doppler observations. The baseline components obtained from the satellite Doppler observations were compared to the LBI values. The weighted mean scale bias of the NSWC 9Z-2 satellite Doppler coordinate system relative to the LBI system was found to be 0.42 + or - 0.05 PPM. The weighted mean difference in the origin of longitude was found to be 0.87 sec + or - 0.01 while the difference in declination origin was found to be 0.06 sec + or - 0.01.

Langley, R. B.↗
NASA NTRS1980

Crustal deformation at very long baseline interferometry sites due to seasonal air-mass and ground water variations

The seasonal deformation normal to the Earth's surface was calculated at stations involved or interested in very long baseline interferometry (VLBI) geodesy and at hypothetical sites in Australia and Brazil using global atmospheric pressure data, values for groundwater storage, and load Love numbers deduced from current Earth models. It was found that the annual range of deformation approached the centimeter level measuring potential of the VLBI technqiue at Greenbank, Haystack, and the Brazil site.

Stolz, A.↗
NASA NTRS1980

Geophysical and astronomical models applied in the analysis of very long baseline interferometry

Very long baseline interferometry presents an opportunity to measure at the centimeter level such geodetic parameters as baseline length and instantaneous pole position. In order to achieve such precision, the geophysical and astronomical models used in data analysis must be as accurate as possible. The Mark-3 interactive data analysis system includes a number of refinements beyond conventional practice in modeling precession, nutation, diurnal polar motion, UT1, solid Earth tides, relativistic light deflection, and reduction to solar system barycentric coordinates. The algorithms and their effects on the recovered geodetic, geophysical, and astrometric parameters are discussed.

Ma, C.↗
NASA NTRS1980

A very long baseline interferometry sky survey

A systematic very long baseline interferometry (VLBI) sky survey, undertaken to find a suitable set of compact celestial radio sources from which a more complete VLBI reference frame can be constructed, discussed. The survey was conducted by searching known celestial radio sources for compact components by means of VLBI observations. Baseline lengths were about 7 x 10 to the 7th power RF wavelengths (lambda = 13.1 cm), so the spatial wavelengths being sampled by the interferometer were generally on the order of a few milliarcseconds. Hence, the radio sources detected have a measurable portion of their total flux density contained in components that are no more than a few milliarcseconds in angular extent. Existing information of radio sources were used as clues to source size.

Preston, R. A.↗
NASA NTRS1980

The Deep Space Network capabilities for radio interferometry

The very long baseline interferometry capability as well as the planned capability of the deep space network (DSN) are described. Major emphasis is placed on the following: VLBI development; operational VLBI for DSN calibration; and VLBI for spacecraft navigation.

Fanselow, J. L.↗
NASA NTRS1979

Time transfer via satellite-link radio interferometry

Very long baseline interferometry using natural radio sources was investigated as a possible time transfer method. Antennas were linked using a synchronous communications satellite instead of the customary independent frequency standards and tape recorders. A successful preliminary time transfer was performed using a wideband data link that was accurate at the 100 nanosecond level, and frequency standards were compared to a part in 10 to the minus thirteenth power over a 24 hour period using a phase coherent satellite link. The narrow band phase coherent link method is potentially capable of timing accuracy of 10 picoseconds, and frequency comparison accuracy of 10 to the minus sixteenth power, and is in addition economical of spectrum usage.

Knowles, S. H.↗
NASA NTRS1979

Source structure errors in the synchronization of clocks by radio interferometry

Radio interferometry has the potential of synchronizing clocks across intercontinental distances with accuracies better than one nanosecond. One of the potential error sources in such determinations is the spatial structure of the natural radio sources that provide the reference signals. Due to their extent, the effective position of these sources can vary as a function of the length and orientation of the baseline vector joining the two antennas. If they are not corrected, such variations can lead to errors in clock synchronization. The theory of structure corrections is presented and specific examples are given to illustrate the nature and size of the effect.

Thomas, J. B.↗
NASA NTRS1980

Thermal expansion of composites using Moire interferometry

An experimental technique for precise measurement of the thermal response of fiber-reinforced composite materials uses moire interferometry with fringe multiplication which yield a sensitivity of 833 nm (32.8 mu in.) per fringe. Results from the technique are compared with those obtained from electrical resistance strain gages, and also those predicted from classical lamination theory. Temperature dependent coefficients of thermal expansion for composite materials subjected to thermal cycling in the temperature range of 297 K (75 F) to 422 K (300 F) were determined for four laminate configurations (0, 90, 0/ + or - 45/90 sub s and 0/90/ + or - 45 sub s) of T300/5208 graphite epoxy, and ranged from -0.107 mu epsilon K/1 (-0.059 mu epsilon deg F/-) for the 0 laminate to 32.18 mu epsilon K/1 (17.88 mu epsilon F/1) for the 90 laminate. Moisture was found to greatly influence the thermal response of a quasi-isotropic laminate, resulting in hysteresis and residual compressive strain as the moisture content was reduced. Comparisons between moire and strain gage measurements were inconclusive with both techniques giving consistent but systematically different results. Differences of as much as 29% were observed.

Bowles, D. E.↗
NASA NTRS1980

Digital recording acquisition subsystem for radio interferometry applications implementation plan

The digital recording acquisition subsystem (DRAS) is a wideband digital recording system that can accommodate data sample rates from 0.125 to 8 MHz in binary increments. The implementation of the DRAS for radio interferometry applications in support of the Crustal Dynamics Project is described. The functional and performance characteristics of the equipment and the key aspects of the implementation plan are discussed.

Kapoor, V. B.↗
NASA NTRS1980

Differential Very Long Baseline Interferometry (delta VLBI) spacecraft tracking system demonstration. Part 2: Data acquisition and processing

A set of experiments in the use of Differential Very Long Baseline Interferometry (delta VLBI) for spacecraft navigation were completed. Data using both Voyager spacecraft and a single quasar were acquired during the Jupiter encounter time period. The data were processed and analyzed to assess the navigation accuracy of delta VLBI. The data reduction and techniques for assessing data quality and consistency are discussed.

Christensen, C. S.↗
NASA NTRS1980

Water vapor - The wet blanket of microwave interferometry

The various techniques that utilize microwave interferometry could be employed to determine distances of several thousand kilometers with an accuracy of 1 cm or 2 cm. Such measurements would be useful to obtain new knowledge of earth dynamics, greater insight into fundamental astronomical constants, and the ability to accurately navigate a spacecraft in interplanetary flight. There is, however, a basic problem, related to the presence of tropospheric water vapor, which has to be overcome before such measurements can be realized. Differing amounts of water vapor over the interferometer stations cause errors in the differential time of arrival which is the principal observable quantity. Approaches for overcoming this problem are considered, taking into account requirements for water vapor calibration to support interferometric techniques.

Resch, G. M.↗
NASA NTRS1981

Detection of acoustic emission in composites by optical fiber interferometry

Acoustic emission in prestressed composite panels using embedded single mode optical fiber was detected. Static loading of the composite matrix produces acoustic emission events, pulsed ultrasonic waves, which mechanically modulate the embedded fiber geometry and phase modulate the transmitted optical field. The modulation is detected by optical interferometry and Fourier optical processing techniques to produce an electronic signal proportional to acoustic field amplitude integrated along the length of the fiber in the specimen. Static and dynamic calibration of the detection system is discussed and frequency domain characterization of the acoustic field interaction with the fiber is presented.

Wade, J. C.↗
NASA NTRS1981

Holographic interferometry of transparent media with reflection from imbedded test objects

In applying holographic interferometry, opaque objects blocking a portion of the optical beam used to form the interferogram give rise to incomplete data for standard computer tomography algorithms. An experimental technique for circumventing the problem of data blocked by opaque objects is presented. The missing data are completed by forming an interferogram using light backscattered from the opaque object, which is assumed to be diffuse. The problem of fringe localization is considered.

Prikryl, I.↗
NASA NTRS1982

Fringe localization requirements for three-dimensional flow visualization of shock waves in diffuse-illumination double-pulse holographic interferometry

A theory of fringe localization in rapid-double-exposure, diffuse-illumination holographic interferometry was developed. The theory was then applied to compare holographic measurements with laser anemometer measurements of shock locations in a transonic axial-flow compressor rotor. The computed fringe localization error was found to agree well with the measured localization error. It is shown how the view orientation and the curvature and positional variation of the strength of a shock wave are used to determine the localization error and to minimize it. In particular, it is suggested that the view direction not deviate from tangency at the shock surface by more than 30 degrees.

Decker, A. J.↗
NASA NTRS1982

Flow field studies using holographic interferometry at Langley

Some of the uses of holographic interferometry at Langley Research Center both for flow visualization and for density field determinations are described and tests in cryogenic flows at the Langley 0.3-Meter Transonic Cryogenic Tunnel are discussed. Experimental and theoretical fringe shift data are compared.

Burner, A. W.↗
NASA NTRS1981

Moire interferometry for thermal expansion of composites

Moire interferometry by reflection has been demonstrated using a real reference grating of 1200 lines/mm. The method is shown to be well adapted to thermal environments. Thermal expansion coefficients of graphite-epoxy composites have been measured with high precision over a wide range from nearly zero to 3300 microstrains in the temperature range 297-422 K. Random errors characterized by one standard deviation can be as small as one microstrain.

Bowles, D. E.↗
NASA NTRS1982

Interferometry and holography in a low-gravity environment

The groundwork for the use of advanced optical measurement techniques in Space Shuttle materials processing in space (MPS) experiments is being laid in tests, conducted aboard a NASA KC-135 aircraft flying a parabolic trajectory, involving a Mach-Zehnder interferometer and a sideband holographic unit. The two experiments described are (1) the observation of flow during solidification, in which fluid concentration and temperature profiles were measured during unidirectional solidification of a saturated NH4Cl-H2O solution, and (2) the observation of electrodeposition flow, in which interferometry is used to provide quantitative data required in the understanding of electrochemical process transport properties. The free-floating holographic unit was operated in the microgravity environment to both test the practicality of optical systems in such conditions and test the shock and vibration characteristics of the package.

Owen, R. B.↗