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At least 91 records · Page 5

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NASA NTRS1978

The classification of LANDSAT data for the Orlando, Florida, urban fringe area

Procedures used to map residential land cover on the Orlando, Florida, Urban fringe zone are detailed. The NASA Bureau of the Census Applications Systems Verification and Transfer project and the test site are described as well as the LANDSAT data used as the land cover information sources. Both single-date LANDSAT data processing and multitemporal principal components LANDSAT data processing are described. A summary of significant findings is included.

Walthall, C. L.↗

NASA NTRS1984

Optimization of fringe-type laser anemometers for turbine engine component testing

The fringe type laser anemometer is analyzed using the Cramer-Rao bound for the variance of the estimate of the Doppler frequency as a figure of merit. Mie scattering theory is used to calculate the Doppler signal wherein both the amplitude and phase of the scattered light are taken into account. The noise from wall scatter is calculated using the wall bidirectional reflectivity and the irradiance of the incident beams. A procedure is described to determine the optimum aperture mask for the probe volume located a given distance from a wall. The expected performance of counter type processors is also discussed in relation to the Cramer-Rao bound. Numerical examples are presented for a coaxial backscatter anemometer.

Seasholtz, R. G.↗

NASA NTRS1984

Optimization of fringe-type laser anemometers for turbine engine component testing

Seasholtz, R. G.↗

NASA NTRS1986

Combined fringe and Fabry-Perot laser anemometer for three component velocity measurements in turbine stator cascade facility

A laser anemometer is described that was developed for use in a 508 mm diameter annular turbine stator cascade facility. All three velocity components are measured through a single restricted optical port, both within the stator vane row and downstream of the vanes. The measurements are made through a cylindrical window in the casing that matches the tip radius of the cascade. The stator tested has a contoured hub endwall that results in a large radial flow near the hub. The anemometer uses a standard fringe configuration (LFA) with a fluorescent aerosol seed to measure the axial and circumferential velocity components. The radial component is measured with a confocal Fabry-Perot interferometer. The two configurations are combined in a single optical system and can operate simultaneously. Data are presented to illustrate the capabilities of the system.

Seasholtz, R. G.↗

NASA NTRS1987

A computer controlled signal preprocessor for laser fringe anemometer applications

The operation of most commercially available laser fringe anemometer (LFA) counter-processors assumes that adjustments are made to the signal processing independent of the computer used for reducing the data acquired. Not only does the researcher desire a record of these parameters attached to the data acquired, but changes in flow conditions generally require that these settings be changed to improve data quality. Because of this limitation, on-line modification of the data acquisition parameters can be difficult and time consuming. A computer-controlled signal preprocessor has been developed which makes possible this optimization of the photomultiplier signal as a normal part of the data acquisition process. It allows computer control of the filter selection, signal gain, and photo-multiplier voltage. The raw signal from the photomultiplier tube is input to the preprocessor which, under the control of a digital computer, filters the signal and amplifies it to an acceptable level. The counter-processor used at Lewis Research Center generates the particle interarrival times, as well as the time-of-flight of the particle through the probe volume. The signal preprocessor allows computer control of the acquisition of these data.Through the preprocessor, the computer also can control the hand shaking signals for the interface between itself and the counter-processor. Finally, the signal preprocessor splits the pedestal from the signal before filtering, and monitors the photo-multiplier dc current, sends a signal proportional to this current to the computer through an analog to digital converter, and provides an alarm if the current exceeds a predefined maximum. Complete drawings and explanations are provided in the text as well as a sample interface program for use with the data acquisition software.

Oberle, Lawrence G.↗

NASA NTRS1986

Combined fringe and Fabry-Perot laser anemometer for 3 component velocity measurements in turbine stator cascade facility

Seasholtz, Richard G.↗

NASA NTRS1987

Enhancement of surface-atmosphere fluxes by desert-fringe vegetation through reduction of surface albedo and of soil heat flux

Under the arid conditions prevailing at the end of the dry season in the western Negev/northern Sinai region, vegetation causes a sharp increase relative to bare soil in the daytime sensible heat flux from the surface to the atmosphere. Two mechanisms are involved: the increase in the surface absorptivity and a decrease in the surface heat flux. By increasing the sensible heat flux to the atmosphere through the albedo and the soil heat flux reductions, the desert-fringe vegetation increases the daytime convection and the growth of the planetary boundary layer. Removal of vegetation by overgrazing, by reducing the sensible heat flux, tends to reduce daytime convective precipitation, producing higher probabilities of drought conditions. This assessment of overgrazing is based on observations in the Sinai/Negev, where the soil albedo is high and where overgrazing produces an essential bare soil. Even if the assessment for the Sinai/Negev does not quantitatively apply throughout Africa, the current practice in many African countries of maintaining a large population of grazing animals, can contribute through the mesoscale mechanisms described to reduce daytime convective precipitation, perpetuating higher probabilities of drought. Time-of-day analysis of precipitation in Africa appears worthwhile, to better assess the role of the surface conditions in contributing to drought.

Otterman, J.↗

NASA NTRS1989

Signal Preprocessor For Laser-Fringe Anemometers

Signal-preprocessing unit contains digital and analog circuitry added to existing equipment in laser-fringe anemometer system to filter raw anemometer signal and relieve researcher of some tedious and difficult control functions. Preprocessor operates under control of same digital computer that acquires measurement data. Both automatically and in response to control settings applied manually by researcher, preprocessor helps system to establish conditions that produce signals of highest quality.

Oberle, Lawrence G.↗

NASA NTRS1989

Tracing Rays In Laser-Fringe Anemometers

"OPTMAIN" is simple ray-tracing computer code developed to quantify refractive effects that result when laser-fringe anemometer used to observe flows through window. Code calculates changes for four different types of windows: flat-plate windows, simple cylindrical windows, "general" axisymmetric windows, and smooth general-surface windows. Written in FORTRAN IV.

Owen, Karl↗

NASA NTRS1988

Global fringe fitting for space interferometry

Global fringe fitting for VLBI has been incorporated into the NRAO Astronomical Image Processing System as the VBFIT program. A generalized program, called VBGFF, has been developed for use with VLBI arrays that include orbiting radio telescopes. Various aspects of these programs are discussed.

Jones, Dayton L.↗

NASA NTRS1992

White-Light, Dispersed-Fringe Interferometric Keratometer

Proposed keratometer based on scheme involving spectral dispersal of white-light interference fringes. Instrument operates in "snapshot" mode: no scanning necessary, not necessary to immobilize patient's eye. Insensitive to vibration, involves no phase shifting, and has variable sensitivity. Intended primarily for use in medical assessments of human corneas, also used to measure shapes of animal corneas, lenses, and other aspherical or spherical reflective or partly reflective surfaces.

Hochberg, Eric B.↗

NASA NTRS1992

Projected-Fringe, Phase-Stepping Profilometer

Projected-fringe, phase-stepping profilometer is system of optical, electronic, optoelectronic, and electro-optical equipment measuring surface profiles of objects. Projects optical interference pattern onto object and steps phase of pattern to extract phases of interfering beams where they intersect object. Data obtained within seconds. Noncontact, full-field surface profilometer has applications, including inspection of machined and ceramic parts, printed circuit boards, and optical components. Rugged and compact, measures surfaces with variety of sizes and finishes.

Mercer, Carolyn↗

NASA NTRS1991

Topography estimation with interferometric synthetic aperture radar using fringe detection

Methods are presented for using Synthetic Aperture Radar (SAR) interferometry data to estimate surface topography. An expression is given to relate the elevation of a ground point to the phase difference of SAR images received from two spatially separated antennas. An iterative algorithm which solves for the position and elevation of each point in the image simultaneously is developed. One of the critical issues that determines the accuracy of the terrain mapping is the phase unwrapping. An approach to the problem by fringe line detection is proposed. The algorithms are tested with two Seasat SAR images of terrain near Yellowstone National Park. The resultant elevation map is compared with a USGS terrain elevation model. The error of the SAR elevation with respect to the digital terrain map is about 8.2 percent of the total terrain variation.

Lin, Qian↗

NASA NTRS1992

A deconvolution technique for Hubble Space Telescope FGS fringe analysis

A technique has been developed for directly transforming interferometer fringe visibility functions ('S curves') from the Hubble Space Telescope (HST) fine guidance sensors (FGSs) into intensity profiles of the program object. In a process analogous to Fourier transform image deconvolution, an S curve from a double star yields a pair of narrow profiles containing information on the separation (in one coordinate), and relative brightness of the two stars. The procedure has yielded high internal precision for the separation and relative brightness in tests with HST data. Simulations indicate that it can also deconvolve S curves from multiple stars or continuous intensity distributions such as resolvable stellar or extragalactic objects. Similar deconvolution analyses may be useful in other types of interferometry.

Hershey, John L.↗

NASA NTRS1993

Geometric moire methods with enhanced sensitivity by optical/digital fringe multiplication

A robust fringe-shifting and image processing scheme is applied to geometric moire experiments. High quality contour maps of U, V and W displacement fields are obtained with sensitivity enhanced by a factor of ten. The method is compatible with complicated intensity distributions, variable bar-to-space ratios of gratings, and optical noise. It is applicable to large fields for both in-plane moire and shadow moire measurements.

Han, B.↗

NASA NTRS1996

Digital Fringe Counting for Intrinsic Fabry-Perot Interferometers

A digital dual-wavelength method of demodulating long (17cm) Intrinsic Fabry-Perot Interferometers (IFPI) is presented. The method builds upon previous techniques using small shifts in the lasing wavelength of a laser diode induced by changes in the driving current. The intensity of the light reflected from the IFPI is sampled at two carefully selected values of laser diode drive current. Digital logic is then used to count fringes based upon these two sampled values. Experimental verification of the method is described.

Froggatt, Mark↗

NASA NTRS1998

Comparison of Computational, Model and Experimental, Example Trained Neural Networks for Processing Speckled Fringe Patterns

The responses of artificial neural networks to experimental and model-generated inputs are compared for detection of damage in twisted fan blades using electronic holography. The training-set inputs, for this work, are experimentally generated characteristic patterns of the vibrating blades. The outputs are damage-flag indicators or second derivatives of the sensitivity-vector-projected displacement vectors from a finite element model. Artificial neural networks have been trained in the past with computational-model- generated training sets. This approach avoids the difficult inverse calculations traditionally used to compare interference fringes with the models. But the high modeling standards are hard to achieve, even with fan-blade finite-element models.

Decker, A. J.↗

NASA NTRS1998

Comparison of Computational-Model and Experimental-Example Trained Neural Networks for Processing Speckled Fringe Patterns

The responses of artificial neural networks to experimental and model-generated inputs are compared for detection of damage in twisted fan blades using electronic holography. The training-set inputs, for this work, are experimentally generated characteristic patterns of the vibrating blades. The outputs are damage-flag indicators or second derivatives of the sensitivity-vector-projected displacement vectors from a finite element model. Artificial neural networks have been trained in the past with computational-model-generated training sets. This approach avoids the difficult inverse calculations traditionally used to compare interference fringes with the models. But the high modeling standards are hard to achieve, even with fan-blade finite-element models.

Decker, A. J.↗