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NASA NTRS1996

Global and Local Distortion Inference During Embedded Zerotree Wavelet Decompression

This paper presents algorithms for inferring global and spatially local estimates of the squared-error distortion measures for the Embedded Zerotree Wavelet (EZW) image compression algorithm. All distortion estimates are obtained at the decoder without significantly compromising EZW's rate-distortion performance. Two methods are given for propagating distortion estimates from the wavelet domain to the spatial domain, thus giving individual estimates of distortion for each pixel of the decompressed image. These local distortion estimates seem to provide only slight improvement in the statistical characterization of EZW compression error relative to the global measure, unless actual squared errors are propagated. However, they provide qualitative information about the asymptotic nature of the error that may be helpful in wavelet filter selection for low bit rate applications.

Huber, A. Kris↗

NASA NTRS1986

Automated matching of pairs of SIR-B images for elevation mapping

During the SIR-B mission in October 1984, a significant number of overlapping synthetic aperture radar (SAR) images of various ground areas was collected. This has offered the first opportunity to perform stereo analyses on images from space that cover large ground areas to determine elevation information. This paper presents the preliminary results of an investigation to obtain elevation data from stereo pairs of SIR-B images. First, the accuracy with which elevation information can be derived from SIR-B image pairs is evaluated theoretically. It is shown that elevation accuracy is a function of the slant range resolution, the incidence angles with which the stereo pair is obtained, the accuracies in spacecraft state estimation, and determination of corresponding pixels in the stereo pair. Next, a hierarchical method is developed to match the corresponding pixels. This method involves iterative removal of local distortions and correlations of pairs of local neighborhoods in the two images. Since it is necessary to perform the matching at every pixel in the image, it is very computationally intensive. Therefore, it has been implemented on the Massively Parallel Processor (MPP) at the Goddard Space Flight Center (GSFC). The MPP's speed permits two iterations of this technique to operate on a pair of 512 x 512 images within 7 s. Results of applying this algorithm of SIR-B images of Mount Shasta, CA, are shown. The matching algorithm performs well in regions of the image with significant features. An approximate elevation image derived from the matching process corresponds to published topographic map data, except for certain obvious discontinuities.

Ramapriyan, H. K.↗

NASA NTRS1991

Detection of reflector surface from near field phase measurements

The deviation of a reflector antenna surface from a perfect parabolic shape causes degradation of the performance of the antenna. The problem of determining the shape of the reflector surface in a reflector antenna using near field phase measurements is not a new one. A recent issue of the IEEE tansactions on Antennas and Propagation (June 1988) contained numerous descriptions of the use of these measurements: holographic reconstruction or inverse Fourier transform. Holographic reconstruction makes use of measurement of the far field of the reflector and then applies the Fourier transform relationship between the far field and the current distribution on the reflector surface. Inverse Fourier transformation uses the phase measurements to determine the far field pattern using the method of Kerns. After the far field pattern is established, an inverse Fourier transform is used to determine the phases in a plane between the reflector surface and the plane in which the near field measurements were taken. These calculations are time consuming since they involve a relatively large number of operations. A much faster method can be used to determine the position of the reflector. This method makes use of simple geometric optics to determine the path length of the ray from the feed to the reflector and from the reflector to the measurement point. For small physical objects and low frequencies, diffraction effects have a major effect on the error, and the algorithm provides incorrect results. It is believed that the effect is less noticeable for large distortions such as antenna warping, and more noticeable for small, localized distortions such as bumps and depressions such as might be caused by impact damage.

Ida, Nathan↗

NASA NTRS1993

Aircraft measurement of electric field - Self-calibration

Aircraft measurement of electric fields is difficult as the electrically conducting surface of the aircraft distorts the electric field. Calibration requires determining the relations between the undistorted electric field in the absence of the vehicle and the signals from electric field meters that sense the local distorted fields in their immediate vicinity. This paper describes a generalization of a calibration method which uses pitch and roll maneuvers. The technique determines both the calibration coefficients and the direction of the electric vector. The calibration of individual electric field meters and the elimination of the aircraft's self-charge are described. Linear combinations of field mill signals are examined and absolute calibration and error analysis are discussed. The calibration method was applied to data obtained during a flight near thunderstorms.

Winn, W. P.↗

NASA NTRS1996

Interactions Between Klebanoff Modes and TS Waves

The Harmonic Point Source (HPS) is the simplest possible form of 3D disturbance yet even this case is not fully understood. One of the characteristics of HPS generated Tolmien Schlichting (TS) waves is that the maximum rms amplitude downstream of the source occurs away from the centerline. As the waves fan out in the spanwise direction they can encounter Klebanoff modes (weak streamwise vortices) which may be present as background disturbances within the layer. The vortices originate near the leading edge and they appear to be caused by amplification of almost immeasurably small nonuniformities in the free stream. The vortices can be steady but they often appear as very low frequency background unsteadiness (as observed by Klebanoff). The vortices locally distort the mean flow and therefore (by definition) they are nonlinear phenomena e.g. the growth rate of the TS waves is altered. Kendall (private communication) has demonstrated total suppression of TS waves by deliberately introducing strong vortices generated by a delta wing. For the very weak vortices that are often encountered in experiments, their effect is characterized by a local reduction in the rms TS magnitude. The laminar wake behind a fine wire is used to generate Klebanoff modes and the interactions with HPS generated TS waves are explored in a controlled manner. It is shown that a proportion of the reduction in the observed TS magnitude can be attributed to washout of the phase-averaged signals owing to phase jitter.

Watmuff, Jonathan↗

NASA NTRS1972

Measured performance of a 1089 K (1500 deg F) heat storage device for sun-shade orbital missions

Tubes designed for a solar heat receiver to serve as an energy source for a Brayton power system were tested for 2002 hours and 1251 sun-shade cycles. The tubes were designed to transfer a constant thermal input to the Brayton system during an orbit. Excess solar energy during a sun period is stored as heat of fusion of lithium fluoride. The niobium - 1% zirconium tubes accommodate the 23 percent volume decrease of LiF during freezing. Test results showed slight, local distortions. The gas discharge temperature varied from 16 K (29 F) below to 28 K (50 F) above the nominal value of 1089 K (1500 F). The tube surface temperatures ranged from 1039 K (1410 F) to 1183 K (1670 F).

Namkoong, D.↗

NASA NTRS1977

Topography, structure, and mare ridges in southern Mare Imbrium and northern Oceanus Procellarum

The gross topography in southern Mare Imbrium and northern Oceanus Procellarum correlates with the buried structure and deposits of the Imbrium Basin and its rim, and many of the mare slopes may be depositional and reflect the pre-existing major features of the basin. Post-depositional, local distortion of the mare surface, however, is present and in many places associated with mare ridges. Many mare ridges are concentric to the Imbrium Basin suggesting that they are influenced by basin structures. Morphologic diversity of mare ridges suggests that not all have the same origin, and volcanic activity took place along some of them. Most mare ridges are associated with faults. The faults could be reverse and associated with a shrinking of the moon, however, some ridges parallel slopes and could be interpreted as decollement thrust faults along gliding horizons or as vertical faults caused by differential gravitational settling along lines that separate areas that are thinly flooded on topographic highs from areas that are thickly flooded in adjacent lows.

Lucchitta, B. K.↗

NASA NTRS1987

Loran-C approach guidance project current status

There are four areas of work in the Loran-C flight test project. Current results provide performance data on the effects of Signal Noise Ratio (SNR) on the dynamic performance of the receiver filters for Loran-C data, and data on Loran-C grid deformation at a microscale of 100 meters. The Loran-C receiver provides a line of position (LOP) Master and Slave transmitter at an angle 0 to magnetic north. No transformation to latitude-longitude reference frame is required since this is the major source of Loran-C navigation errors. A local coordinate frame is established centered at touchdown point on the runway with directions along and across the runway. A Loran-C data collection system was set up. The Loran-C data are sent directly to an Apple II computer with a 12 inch monitor. The effect of SNR on Loran-C precision is shown for two receiver filters of different frequency response. A set of ground level static readings of touchdown was taken around Hanscom Field and transferred to an accurate detailed layout drawing; this showed local distortions of the average touchdown values.

Elias, A. L.↗

NASA NTRS1988

A streamwise vortex embedded in a plane mixing layer

Experimental results are presented on the interaction of a single streamwise vortex with a plane turbulent mixing layer. In the present setup, the vortex is generated by a half-delta wing mounted in the settling chamber of a blower-driven wind tunnel. Initially, the vortex is shown to ride just below the mixing layer, locally distorting the mean velocity and turbulence Reynolds stress distributions in the lower part of the mixing layer. Once the vortex becomes embedded within the mixing layer, the distortions are found to spread through the whole width of the layer, and the induced extra strain rates in the mixing layer result in the generation of additional, relatively large, Reynolds normal and shear stresses, mainly in the regions above the vortex.

Bell, James H.↗

NASA NTRS1989

Quasi-conformal remapping for compensation of human visual field defects - Advances in image remapping for human field defects

Image coordinate transformations are investigated for possible use in a low vision aid for human patients. These patients typically have field defects with localized retinal dysfunction predominately central (age related maculopathy) or peripheral (retinitis pigmentosa). Previously simple eccentricity-only remappings which do not maintain conformality were shown. Initial attempts on developing images which hold quasi-conformality after remapping are presented. Although the quasi-conformal images may have less local distortion, there are discontinuities in the image which may counterindicate this type of transformation for the low vision application.

Juday, Richard D.↗

NASA NTRS1992

Multi-disciplinary optimization of aeroservoelastic systems

The purpose of the research project was to continue the development of new methods for efficient aeroservoelastic analysis and optimization. The main targets were as follows: to complete the development of analytical tools for the investigation of flutter with large stiffness changes; to continue the work on efficient continuous gust response and sensitivity derivatives; and to advance the techniques of calculating dynamic loads with control and unsteady aerodynamic effects. An efficient and highly accurate mathematical model for time-domain analysis of flutter during which large structural changes occur was developed in cooperation with Carol D. Wieseman of NASA LaRC. The model was based on the second-year work 'Modal Coordinates for Aeroelastic Analysis with Large Local Structural Variations'. The work on continuous gust response was completed. An abstract of the paper 'Continuous Gust Response and Sensitivity Derivatives Using State-Space Models' was submitted for presentation in the 33rd Israel Annual Conference on Aviation and Astronautics, Feb. 1993. The abstract is given in Appendix A. The work extends the optimization model to deal with continuous gust objectives in a way that facilitates their inclusion in the efficient multi-disciplinary optimization scheme. Currently under development is a work designed to extend the analysis and optimization capabilities to loads and stress considerations. The work is on aircraft dynamic loads in response to impulsive and non-impulsive excitation. The work extends the formulations of the mode-displacement and summation-of-forces methods to include modes with significant local distortions, and load modes. An abstract of the paper,'Structural Dynamic Loads in Response to Impulsive Excitation' is given in appendix B. Another work performed this year under the Grant was 'Size-Reduction Techniques for the Determination of Efficient Aeroservoelastic Models' given in Appendix C.

Karpel, Mardechay↗

NASA NTRS1993

Equatorward and poleward expansion of the auroras during auroral substorms

The formation of the auroral bulge is investigated on the basis of all-sky TV auroral data with high spatial and temporal resolution. Ways in which the discrete auroral structures within the poleward expanding bulge develop systematically toward the west, the east, and also equatorward from a localized breakup region are shown. Auroral structure at the western end of the bulge (a surge) develops with clockwise rotation as viewed along the magnetic field direction. At the eastern part of the bulge, thin auroral features propagate eastward from the breakup region. Around the central meridian of the bulge, auroral features expand equatorward and become north-south aligned (the N-S aurora). The N-S aurora and the eastward propagating aurora develop into diffuse and pulsating aurora after the expansion. It is suggested that these discrete auroral structures in the bulge develop along the plasma streamlines in a localized distorted two-cell equipotential distribution.

Nakamura, Rumi↗

NASA NTRS1928

Air force and moment for N-20 wing with certain cut-outs

The airplane designer often finds it necessary, in meeting the requirements of visibility, to remove area or to otherwise locally distort the plan or section of an airplane wing. This report, prepared for the Bureau of Aeronautics January 15, 1925, contains the experimental results of tests on six 5 by 30 inch N-20 wing models, cut out or distorted in different ways, which were conducted in the 8 by 8 foot wind tunnel of the Navy Aerodynamical Laboratory in Washington in 1924. The measured and derived results are given without correction for vl/v for wall effect and for standard air density, p=0.00237 slug per cubic foot.

Smith, R H↗

NASA NTRS1995

Acoustic receptivity due to weak surface inhomogeneities in adverse pressure gradient boundary layers

The boundary layer receptivity to free-stream acoustic waves in the presence of localized surface disturbances is studied for the case of incompressible Falkner-Skan flows with adverse pressure gradients. These boundary layers are unstable to both viscous and inviscid (i.e., inflectional) modes, and the finite Reynolds number extension of the Goldstein-Ruban theory provides a convenient method to compare the efficiency of the localized receptivity processes in these two cases. The value of the efficiency function related to the receptivity caused by localized distortions in surface geometry is relatively insensitive to the type of instability mechanism, provided that the same reference length scale is used to normalize the efficiency function for each type of instability. In contrast, when the receptivity is induced by variations in wall suction velocity or in wall admittance distribution, the magnitudes of the related efficiency functions, as well as the resulting coupling coefficients, are smaller for inflectional (i.e., Rayleigh) modes than for the viscous Tollmien-Schlichting waves. The reduced levels of receptivity can be attributed mainly to the shorter wavelengths and higher frequencies of the inflectional modes. Because the most critical band of frequencies shifts toward higher values, the overall efficiency of the wall suction- and the wall admittance-induced receptivity decreases with an increase in the adverse pressure gradient.

Choudhari, Meelan↗

NASA NTRS1994

Aircraft electric field measurements: Calibration and ambient field retrieval

An aircraft locally distorts the ambient thundercloud electric field. In order to determine the field in the absence of the aircraft, an aircraft calibration is required. In this work a matrix inversion method is introduced for calibrating an aircraft equipped with four or more electric field sensors and a high-voltage corona point that is capable of charging the aircraft. An analytic, closed form solution for the estimate of a (3 x 3) aircraft calibration matrix is derived, and an absolute calibration experiment is used to improve the relative magnitudes of the elements of this matrix. To demonstrate the calibration procedure, we analyze actual calibration date derived from a Lear jet 28/29 that was equipped with five shutter-type field mill sensors (each with sensitivities of better than 1 V/m) located on the top, bottom, port, starboard, and aft positions. As a test of the calibration method, we analyze computer-simulated calibration data (derived from known aircraft and ambient fields) and explicitly determine the errors involved in deriving the variety of calibration matrices. We extend our formalism to arrive at an analytic solution for the ambient field, and again carry all errors explicitly.

Koshak, William J.↗

NASA NTRS2011

Multi-Modal Image Registration and Matching for Localization of a Balloon on Titan

A solution was developed that matches visible/IR imagery aboard a balloon in Saturn's moon Titan's atmosphere to SAR (synthetic aperture radar) and visible/IR data acquired from orbit. A balloon in Titan's atmosphere must be able to localize itself autonomously both globally and with respect to local terrain. The orbital data is used to provide the balloon imagery with global context. The work is novel in applying mutual information (MI) to orbital vs. aerial data. There are unique challenges in this setting. Image offsets are much higher than in medical imaging, there is local distortion due to 3D terrain relief, and the fields of regard from orbit and from the air are quite different.

Ansar, Adnan I.↗

NASA NTRS1993

Local isotropy in distorted turbulent boundary layers at high Reynolds number

This is a report on the continuation of our experimental investigations of the hypothesis of local isotropy in shear flows. This hypothesis, which states that at sufficiently high Reynolds numbers the small-scale structures of turbulent motions are independent of large-scale structures and mean deformations, has been used in theoretical studies of turbulence and computational methods such as large-eddy simulation. Since Kolmogorov proposed his theory, there have been many experiments, conducted in wakes, jets, mixing layers, a tidal channel, and atmospheric and laboratory boundary layers, in which attempts have been made to verify - or refute - the local-isotropy hypothesis. However, a review of the literature over the last five decades indicated that, despite all these experiments in shear flows, there was no consensus in the scientific community regarding this hypothesis, and, therefore, it seemed worthwhile to undertake a fresh experimental investigation into this question.

Saddoughi, Seyed G.↗

NASA NTRS1993

A local instability of tidally distorted accretion disks

Hydrodynamic accretion disks in binary star systems have local, three-dimensional, approximately incompressible instabilities whose growth rate is linearly proportional to the tidal field of the secondary star. The instability occurs throughout the disk but is most rapid in the outer parts. The free energy for the instability derives from the tidal distortion of the disk rather than its differential rotation. The instability will excite m-parallel 1 internal waves of the type discussed by Vishniac et al. (1992), and it should facilitate return of angular momentum from the disk to the secondary star. It is closely related to a recently discovered instability that has been suggested as a mechanism for transition to turbulence in terrestrial and laboratory flows, but whether it will lead to strong turbulence in disks is as yet unclear.

Goodman, Jeremy↗