Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “cross-correlation”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. III - Further observations of NGC 5548 at optical wavelengths

The results of the second year of an intensive ground-based spectroscopic and photometric study of variability in the bright Seyfert 1 galaxy NGC 5548 are reported in order to study the relationship between continuum and emission-line variability. Relative to the first year of the monitoring program, the nucleus of NGC 5548 was considerably fainter and the continuum variations slower during the second year, but the continuum H-beta cross-correlation results for the two years are nearly identical. The variations in the broad H-beta emission-line lag behind those in the continuum by somewhat less than 20 days, as concluded from the first year's data.

Peterson, B. M.↗
NASA NTRS1992

Structure in the COBE differential microwave radiometer first-year maps

Results of the first year of data from the differential microwave radiometers on the Cosmic Background Explorer are presented. Statistically significant structure that is well described as scale-invariant fluctuations with a Gaussian distribution is shown. The rms sky variation, smoothed to a total 10-deg FWHM Gaussian, is 30 +/-5 micro-K for Galactic latitude greater than 20-deg data with the dipole anisotropy removed. The rms cosmic quadrupole amplitude is 13 +/-4 micro-K. The angular autocorrelation of the signal in each radiometer channel and cross-correlation between channels are consistent and give a primordial fluctuation power-law spectrum with index of 1.1 +/-0.5, and an rms-quadrupole-normalized amplitude of 16 +/-4 micro-K. These features are in accord with the Harrison-Zel'dovich spectrum predicted by models of inflationary cosmology.

Smoot, G. F.↗
NASA NTRS1993

Stereoscopic Vision System For Robotic Vehicle

Distances estimated from images by cross-correlation. Two-camera stereoscopic vision system with onboard processing of image data developed for use in guiding robotic vehicle semiautonomously. Combination of semiautonomous guidance and teleoperation useful in remote and/or hazardous operations, including clean-up of toxic wastes, exploration of dangerous terrain on Earth and other planets, and delivery of materials in factories where unexpected hazards or obstacles can arise.

Matthies, Larry H.↗
NASA NTRS1993

Do galactic potential wells depend on their environment?

Using galaxies in complete samples as tracers of the galaxy density field and about 1000 galaxies with measured circular velocities as targets, we examine the cross-correlation functions between the targets and tracers as a function of galaxy circular velocities. The correlation strength does not vary with the circular velocities except for elliptical galaxies with the highest velocity dispersions, where the effect may well be due to morphological segregations in clusters of galaxies. This is contrasted with the strong dependence of the correlation functions of dark halos on their circular velocities in some models of galaxy formation.

Mo, H. J.↗
NASA NTRS1993

Surface velocity fields of Ice Streams D and E derived from repeat satellite imagery

Sequential Landsat TM images were used to map in detail the surface velocity of Ice Streams D and E by tracking small ice features in coregistered images. The majority of both ice streams have now been mapped for velocity, and in most areas, approximate strain-rates are also determined. Tracking was accomplished semiautomatically using an image-to-image cross-correlation technique. Measurement density is roughly four per sq km in regions with distinct surface features (crevasses, snow dunes, etc.), and with cloud-free image coverage.

Scambos, Ted A.↗
NASA NTRS1992

Ice tracking techniques, implementation, performance, and applications

Present techniques of ice tracking make use both of cross-correlation and of edge tracking, the former being more successful in heavy pack ice, the latter being critical for the broken ice of the pack margins. Algorithms must assume some constraints on the spatial variations of displacements to eliminate fliers, but must avoid introducing any errors into the spatial statistics of the measured displacement field. We draw our illustrations from the implementation of an automated tracking system for kinematic analyses of ERS-1 and JERS-1 SAR imagery at the University of Alaska - the Alaska SAR Facility's Geophysical Processor System. Analyses of the ice kinematic data that might have some general interest to analysts of cloud-derived wind fields are the spatial structure of the fields, and the evaluation and variability of average deformation and its invariants: divergence, vorticity and shear. Many problems in sea ice dynamics and mechanics can be addressed with the kinematic data from SAR.

Rothrock, D. A.↗
NASA NTRS1992

Application of the multiple PRF technique to resolve Doppler centroid estimation ambiguity for spaceborne SAR

Estimation of the Doppler centroid ambiguity is a necessary element of the signal processing for SAR systems with large antenna pointing errors. Without proper resolution of the Doppler centroid estimation (DCE) ambiguity, the image quality will be degraded in the system impulse response function and the geometric fidelity. Two techniques for resolution of DCE ambiguity for the spaceborne SAR are presented; they include a brief review of the range cross-correlation technique and presentation of a new technique using multiple pulse repetition frequencies (PRFs). For SAR systems, where other performance factors control selection of the PRF's, an algorithm is devised to resolve the ambiguity that uses PRF's of arbitrary numerical values. The performance of this multiple PRF technique is analyzed based on a statistical error model. An example is presented that demonstrates for the Shuttle Imaging Radar-C (SIR-C) C-band SAR, the probability of correct ambiguity resolution is higher than 95 percent for antenna attitude errors as large as 3 deg.

Chang, C. Y.↗
NASA NTRS1992

Seyfert galaxy ultraviolet emission-line intensities and variability - A self-consistent photoionization analysis applied to broad-line-emitting gas in NGC 3783

Well-constrained photoionization models for the Seyfert I galaxy NGC 3783 are developed. Both cross-correlation analyses and line variability trends with varying ionizing radiation flux require a multicomponent picture. All the data for He II 1640 A, C IV 1549 A, and semiforbidden C III 1909 A can be reasonably well reproduced by two cloud components. One has a source-cloud distance of 24 lt-days, gas density around 3 x 10 exp 10/cu cm, ionization parameter range of 0.04-0.2, and cloud thickness such that about half of the carbon is doubly ionized and about half is triply ionized. The other component is located approximately 96 lt-days from the source, is shielded from the source by the inner cloud, has a density about 3 x 10 to the 9th/cu cm, and is characterized by an ionization parameter range of 0.001-0.03, The cloud thickness is such that about 45 percent carbon is doubly ionized and about 55 percent is singly ionized.

Koratkar, Anuradha P.↗
NASA NTRS1992

Comparison of MGII and CIV emission lines in NGC5548

Results are presented of a cross-correlation analysis in which the Seyfert galaxy NGC 5548 was monitored in the UV portion of its spectrum every 4 d for an 8-mo period by an international collaborative effort using the IUE satellite. The analysis shows that the CIV 1550-A line correlates with the underlying continuum with a time delay of about 10 d. A similar analysis for the MgII 2798-A line yields a much broader peak and a delay of about 50 +/- 20 d. Thus the MgII line emitting gas appears to be about 5 times more distant from the central source. The MgII and CIV profiles in this series of spectra are compared on the same velocity scale. The two lines are quite similar on the red side of the profile, with the CIV more extended on the blue side. The overall width of the CIV is as much as 40 percent larger, but it is definitely not larger by a factor or two or more. It is concluded that the motion of the broad line emitting gas is not dominated by the gravitational influence of a central massive object.

Ptak, R.↗
NASA NTRS1993

Finite element large deflection random response of thermally buckled plates

The governing nonlinear equations of motion are derived for a rectangular isotropic plate subjected to a combined thermal-acoustic loading. The thermal loading is taken to be a steady state temperature distribution, and the acoustic loading is considered to be a stationary Gaussian pressure with zero mean and uniform magnitude and phase over the plate surface. With these types of loads, the thermal-acoustic analysis can be separated into two problems: (1) thermal post-buckling; and (2) random vibration of the thermally buckled plate. Since both analyses are nonlinear, iterative techniques are used to solve each. For the random vibration analysis, the method of equivalent linearization is used; both auto- and cross-correlation terms are included. Results obtained using the present formulation are compared with previous classical equivalent linearization results.

Locke, J. E.↗
NASA NTRS1992

Coronal Alfven waves detected by radio sounding during the solar occultations of the Helios spacecraft

The two Helios spacecraft underwent regular solar occultations during their extended missions from Dec 1974-Feb 1986 (Helios 1) and Jan 1976-Mar 1980 (Helios 2) thereby providing many opportunities for radio propagation experiments in the solar corona. On certain rare occasions over the course of these investigations, Faraday rotation measurements of the linearly polarized Helios signals could be recorded simultaneously at two widely-spaced ground stations. Many of these two-station measurement intervals display clear evidence of wave-like structures with quasi-periods of the order of a few minutes to a few hours. These structures are attributed to coronal Alfven waves. The radial propagation direction and velocity of these waves are estimated from a cross-correlation analysis of the data between the two stations. The majority of the waves appear to propagate away from the Sun, but about 30 percent of the cases indicate a propagation direction toward the Sun.

Bird, M. K.↗
NASA NTRS1993

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. IV - Intensity variations of the optical emission lines of NGC 5548

Measurements of optical emission-line flux variations based on spectra of the Seyfert galaxy NGC 5548 obtained between December 1988 and October 1989 are reported. All of the measured optical emission lines, H-alpha, H-beta, H-gamma, He I 5876, and He II 4686, exhibit the same qualitative behavior as the UV and optical continua, but with short time delays, or lags, which are different for the various lines. Cross-correlation analysis is applied to measure the lags between the various lines and the continuum. Similar lags are found with respect to the UV continuum for H-alpha and H-beta, 17 and 19 d, respectively. The lag for H-gamma is shorter (13 d), only somewhat larger than the lag measured for Ly-alpha (about 10 d). The helium lines respond to continuum variations more rapidly than the hydrogen lines, with lags of about 7 d for He II 4686 and 11 d for He I 5876.

Dietrich, M.↗
NASA NTRS1992

Concept for a lunar array for very low frequency radio astronomy

We discuss the design considerations relevant to a very low frequency array, to be deployed on the lunar near side during an early expedition. Such an array would operate in the frequency range 1-10 MHz, and would consist nominally of 20 antennas distributed over a region approximately 40 km in extent. Each antenna station would consist of a crossed-dipole antenna, together with a receiver, digitizer, solar cells, and batteries. In addition, the station will contain a UHF transmitter for relaying the digitized signal to a central station where it will be transmitted to Earth for subsequent processing, including cross-correlation with signals from other antennas. Each antenna station (including the central station) would be deployed as a self-contained unit, mounted on a miniature robotic vehicle. No fixed structures are required for the array.

Marsh, Kenneth A.↗
NASA NTRS1991

Object enhanced optical correlation

A major drawback of optical correlator systems is the poor quality of correlation signals. Background noise is one of the many sources of false correlation peaks. However, by reconstructing an input scene with the object's amplitude function, an object-enhanced scene - used as input to the optical correlators - (with respect to background) can be obtained. Thus, the optical correlation of this enhanced image improves the signal-to-noise ratio in the correlation image. Furthermore, construction of a filter plane mask that permits simultaneous scene enhancement and cross-correlation operations is described. Experimental results demonstrate the practicality of this approach.

Scholl, Marija S.↗
NASA NTRS1993

Multiwavelength monitoring of the BL Lacertae object PKS 2155-304. I - The IUE campaign

Daily monitoring of PKS 2155-304 with the IUE satellite throughout November 1991 revealed dramatic large-amplitude rapid variations in the UV flux of this BL Lac object. Many smaller, rapid flares are superposed on a general doubling of the intensity. During the five-day period when sampling was roughly continuous, the rapid flaring had an apparent quasi-periodic nature, with peaks repeating every 0.7 day. The short- and long-wavelength UV light curves are well correlated with each other, and with the optical light curve deduced from the Fine Error Sensor on IUE. The formal lag is zero, but the cross-correlation is asymmetric in the sense that the shorter wavelength emission leads the longer. The UV spectral shape varies a small but significant amount. The correlation between spectral shape and intensity is complex. The sign of the correlation is consistent with the nonthermal acceleration processes expected in relativistic plasmas, so that the present results are consistent with relativistic jet models, which can also account for quasi-periodic flaring.

Urry, C. M.↗
NASA NTRS1991

Spectroscopic binary orbits from ultraviolet radial velocities. VIII - LZ Cephei (HD 209481)

We present a study of the double-lined spectroscopic binary LZ Cep based on high-resolution IUE spectra. We determine the radial velocities by using a cross-correlation technique incorporating profile fitting; this allows measurements of velocity separations down to unusually small values. The final velocity curve has a small but significant eccentricity. By considering other constraints, we construct a model for the system, which correctly predicts the light curve; in the model, the secondary fills its critical equipotential lobe. The secondary is conspicuously overluminous for its mass, and must have reached its present configuration following a phase of case A mass transfer.

Howarth, Ian D.↗
NASA NTRS1993

Noncosmological signal contributions to the COBE DMR anisotropy maps

We examine the COBE Differential Microwave Radiometer (DMR) data for evidence of noncosmological source contributions. The DMR maps are cross-correlated with maps of rich clusters, extragalactic IRAS sources, HEAO 1 A-2 X-ray emission, and 5 GHz radio sources. We limit the rms contributions from these sources on a 7 deg angular scale to less than 10 micro-K (95 percent confidence level) in the DMR maps, although the LMC probably contributes about 50 micro-K to a limited region of the sky. Thus, our previous interpretation that the fluctuations in the COBE DMR data are most likely due to cosmic fluctuations at the surface of last scattering remains intact. The Comptonization parameter for hot electrons traced by rich clusters is limited to delta(y) less than 2 x 10 exp -6 (95 percent confidence level) averaged over the 7 deg DMR beam.

Bennett, C. L.↗
NASA NTRS1957

Near Noise Field of a Jet-Engine Exhaust

Aircraft structures located in the near noise field of a jet engine are subjected to extremely high fluctuating pressures that may cause structural fatigue. Studies of such structures have been limited by lack of knowledge of the loadings involved. The acoustic near field produced by the exhaust of a stationary turbojet engine having a high pressure ratio was measured for a single operating condition without burning. The maximum overall sound pressure without afterburning was found to be about 42 pounds per square foot along the jet boundary in the region immediately downstream of the jet-nozzle exit. With afterburning maximum sound pressure was increased by 50 percent. The sound pressures without afterburning were obtained on a constant percentage band width basis in the frequency range from 350 to 700 cps. Cross-correlation measurements with microphones were made for a range of jet velocities at locations along the jet and at a distance from the jet. In general, little change in the correlation curves was found as a function of jet velocity or frequency-band width.

Howes, Walton L↗