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At least 775 records · Page 43

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NASA NTRS1990

Development and applications of a multi-level strain energy method for detecting finite element modeling errors

A computational procedure is described which can be used efficiently in identifying modeling errors which may arise from development of a structural finite element model. The procedure, which is referred to as the multi-level strain energy check, is set up in the form of a set of NASTRAN DMAP alters which provide sufficient information about the modeling errors at G-Set, N-Set, and F-Set levels. This technique was applied to two NASTRAN models, namely, the AH-64A and AH-1G models. Two modeling errors were identified for the AH-1G, which were then corrected.

Hashemi-Kia, Mostafa↗

NASA NTRS1990

Computation of the factorized error covariance of the difference between correlated estimators

A state estimation problem where some of the measurements may be common to two or more data sets is considered. Two approaches for computing the error covariance of the difference between filtered estimates (for each data set) are discussed. The first algorithm is based on postprocessing of the Kalman gain profiles of two correlated estimators. It uses UD factors of the covariance of the relative error. The second algorithm uses a square root information filter applied to relative error analysis. In the absence of process noise, the square root information filter is computationally more efficient and more flexible than the Kalman gain (covariance update) method. Both the algorithms (covariance and information matrix based) are applied to a Venus orbiter simulation, and their performances are compared.

Wolff, Peter J.↗

NASA NTRS1991

Effects of atmospheric modeling errors on determinations of baseline vectors from very long baseline interferometry

An investigation is conducted in order to expose errors in the formula for the elevation angle dependence of the hydrostatic atmospheric propagation delay, called the mapping function. A series of special VLBI experiments that involve a large fraction of group delay data from very low elevation angles are performed. Twenty-two experiments, each 26 hours in duration, were performed using VLBI antennas at Goldstone, California and at Westford, Massachusetts. Elevation angle cutoff tests were used to examine the contribution of these errors to the estimates of the vertical coordinate of site position. This contribution is determined to be about 19 mm. It is also found that, for the spacing of 1-2 months between experiments, the mapping function errors do not exhibit a coherent annual signature, but appear to be random in the long term.

Davis, J. L.↗

NASA NTRS1991

Test functions for the moment method which yield the minimum mean square error

The method of moments has found extensive applications in the solution of a wide class of electromagnetic problems. Additionally, there are a number of basis functions which have found favor and with them different types of testing methods. This work examines the selection of test functions, given a selected set of basis functions, which serve to minimize the mean square error. These test functions will guarantee equal or lesser error compared with any other test function. Additionally, these test functions provide a monotonic improvement of the approximation as the increase in the order of the system. Hence while these test functions require the application of the differential or integral operator one additional time, they provide an absolute lower bound to the mean square error in the approximation and a means to systematically improve accuracy with each increase in the order of the unknowns.

Shamansky, H.↗

NASA NTRS1990

Computing maximum tracking error due to payload dynamics

The determination of maximum endpoint tracking error due to uncompensated inertial payload dynamics is investigated. The problem is formulated on the basis of well-known kinetic and kinematic relationships with constraints on maximum joint positions and velocities as well as the generalized force/speed constraint relationship for each actuator. An endpoint tracking error objective function is formed, and numerical optimization is used to determine the maximum. An example computation is presented to show the tracking error under progressively increasing payload using a PUMA 560 manipulator with its first three joints in motion.

Kelkar, Atul G.↗

NASA NTRS1991

How to observe the gyre to global-scale variability in satellite altimetry - Signal attenuation by orbit error removal

Formulas analogous to the frequency response functions for commonly used filters in orbit error removal are analytically derived to devise observational strategies for the large-scale oceanic variability and to decipher the signal contents of previous results. These include the polynomial orbit error approximations, i.e., the linear, bias-only and quadratic corrections, and the sinusoidal orbit error approximations (the purely sinusoidal correction, and the sinusoid-and-bias correction). It is shown that the frequency response function for a polynomial correction is a function of the ratio of wavelength/track length and to retain 90 percent or more of the signal at a certain wavelength, the ratio must be less than 0.65 (for the quadratic case), 0.90 (linear), and 1.54 (bias-only).

Tai, Chang-Kou↗

NASA NTRS1990

High-order neural models for error correcting code

The decoding and error-correction of data transmitted over a noisy channel is in principle equivalent to the operation of a neural network performing as a content-addressable memory. For a successful application, however, the neural network has to be capable of storing arbitrary words, and it has to be guaranteed that the stored words represent the only stable attractors of the memory. This paper presents a novel high-order neural network architecture that has these characteristics. The analog nature of the network can be used to perform soft-decision decoding with any block code. The performance in terms of postdecoding bit error rate versus SNR is demonstrated for two exemplary block codes. The comparison with a conventional decoding algorithm for a (15,5) cyclic redundancy code shows, for example, that the bit error rate at 7dB SNR can be decreased by two orders of magnitude.

Jeffries, Clark↗

NASA NTRS1991

Syntactic error modeling and scoring normalization in speech recognition

The objective was to develop the speech recognition system to be able to detect speech which is pronounced incorrectly, given that the text of the spoken speech is known to the recognizer. Research was performed in the following areas: (1) syntactic error modeling; (2) score normalization; and (3) phoneme error modeling. The study into the types of errors that a reader makes will provide the basis for creating tests which will approximate the use of the system in the real world. NASA-Johnson will develop this technology into a 'Literacy Tutor' in order to bring innovative concepts to the task of teaching adults to read.

Olorenshaw, Lex↗

NASA NTRS1990

Masking of errors in transmission of VAPC-coded speech

A subjective evaluation is provided of the bit error sensitivity of the message elements of a Vector Adaptive Predictive (VAPC) speech coder, along with an indication of the amenability of these elements to a popular error masking strategy (cross frame hold over). As expected, a wide range of bit error sensitivity was observed. The most sensitive message components were the short term spectral information and the most significant bits of the pitch and gain indices. The cross frame hold over strategy was found to be useful for pitch and gain information, but it was not beneficial for the spectral information unless severe corruption had occurred.

Cox, Neil B.↗

NASA NTRS1992

The Laser Atmospheric Wind Sounder (LAWS) Preliminary Error Budget and Performance Estimate

The Laser Atmospheric Wind Sounder (LAWS) study phase has resulted in a preliminary error budget and an estimate of the instrument performance. This paper will present the line-of-sight (LOS) Velocity Measurement Error Budget, the instrument Boresight Error Budget, and the predicted signal-to-noise ratio (SNR) performance. The measurement requirements and a preliminary design for the LAWS instrument are presented in a companion paper.

Kenyon, David L.↗

NASA NTRS1991

Monte Carlo simulation of errors in the anisotropy of magnetic susceptibility - A second-rank symmetric tensor

Monte Carlo perturbations of synthetic tensors to evaluate the Hext/Jelinek elliptical confidence regions for anisotropy of magnetic susceptibility (AMS) eigenvectors are used. When the perturbations are 33 percent of the minimum anisotropy, both the shapes and probability densities of the resulting eigenvector distributions agree with the elliptical distributions predicted by the Hext/Jelinek equations. When the perturbation size is increased to 100 percent of the minimum eigenvalue difference, the major axis of the 95 percent confidence ellipse underestimates the observed eigenvector dispersion by about 10 deg. The observed distributions of the principal susceptibilities (eigenvalues) are close to being normal, with standard errors that agree well with the calculated Hext/Jelinek errors. The Hext/Jelinek ellipses are also able to describe the AMS dispersions due to instrumental noise and provide reasonable limits for the AMS dispersions observed in two Hawaiian basaltic dikes. It is concluded that the Hext/Jelinek method provides a satisfactory description of the errors in AMS data and should be a standard part of any AMS data analysis.

Lienert, Barry R.↗

NASA NTRS1991

Validating system-level error recovery for spacecraft

The system-level software onboard a spacecraft is responsible for recovery from communication, thermal, power, and computer-health anomalies that may occur. The recovery must occur without disrupting any critical scientific or engineering activity that is executing at the time of the error. Thus, the error-recovery software may have to execute concurrently with the ongoing acquisition of scientific data or with spacecraft maneuvers. This paper provides a technique by which the rules that constrain the concurrent execution of these processes can be modeled in a graph. An algorithm is described that uses this model to validate that the constraints hold for all concurrent executions of the error-recovery softwave with the softwave that controls the science and engineering events on the spacecraft.

Lutz, Robyn R.↗

NASA NTRS1992

Surface roughness considerations for atmospheric correction of ocean color sensors. I - The Rayleigh-scattering component. II - Error in the retrieved water-leaving radiance

The first step in the Coastal Zone Color Scanner (CZCS) atmospheric-correction algorithm is the computation of the Rayleigh-scattering (RS) contribution, L sub r, to the radiance leaving the top of the atmosphere over the ocean. In the present algorithm, L sub r is computed by assuming that the ocean surface is flat. Calculations of the radiance leaving an RS atmosphere overlying a rough Fresnel-reflecting ocean are presented to evaluate the radiance error caused by the flat-ocean assumption. Simulations are carried out to evaluate the error incurred when the CZCS-type algorithm is applied to a realistic ocean in which the surface is roughened by the wind. In situations where there is no direct sun glitter, it is concluded that the error induced by ignoring the Rayleigh-aerosol interaction is usually larger than that caused by ignoring the surface roughness. This suggests that, in refining algorithms for future sensors, more effort should be focused on dealing with the Rayleigh-aerosol interaction than on the roughness of the sea surface.

Gordon, Howard R.↗

NASA NTRS1993

Errors In Graphical Analysis Of Computed Flows

Report discusses sources of error in graphical display and analysis of flow fields simulated numerically by computers. Sources of error explored by examples of graphical techniques and errors they introduce in depiction of flow fields computed from Reynolds-averaged Navier-Stokes equations. Specific graphical techniques examined: contouring, plotting arrows on pictures to represent three-dimensional vectors, and particle tracing.

Buning, Pieter↗

NASA NTRS1993

System Measures Errors Between Time-Code Signals

System measures timing errors between signals produced by three asynchronous time-code generators. Errors between 1-second clock pulses resolved to 2 microseconds. Basic principle of computation of timing errors as follows: central processing unit in microcontroller constantly monitors time data received from time-code generators for changes in 1-second time-code intervals. In response to any such change, microprocessor buffers count of 16-bit internal timer.

Cree, David↗

NASA NTRS1992

Comparison of truncation error of finite-difference and finite-volume formulations of convection terms

Judging by errors in the computational-fluid-dynamics literature in recent years, it is not generally well understood that (above first-order) there are significant differences in spatial truncation error between formulations of convection involving a finite-difference approximation of the first derivative, on the one hand, and a finite-volume model of flux differences across a control-volume cell, on the other. The difference between the two formulations involves a second-order truncation-error term (proportional to the third-derivative of the convected variable). Hence, for example, a third (or higher) order finite-difference approximation for the first-derivative convection term is only second-order accurate when written in conservative control-volume form as a finite-volume formulation, and vice versa.

Leonard, B. P.↗

NASA NTRS1992

Diffraction analysis and evaluation of several focus- and track-error detection schemes for magneto-optical disk systems

A commonly used tracking method on pre-grooved magneto-optical (MO) media is the push-pull technique, and the astigmatic method is a popular focus-error detection approach. These two methods are analyzed using DIFFRACT, a general-purpose scalar diffraction modeling program, to observe the effects on the error signals due to focusing lens misalignment, Seidel aberrations, and optical crosstalk (feedthrough) between the focusing and tracking servos. Using the results of the astigmatic/push-pull system as a basis for comparison, a novel focus/track-error detection technique that utilizes a ring toric lens is evaluated as well as the obscuration method (focus error detection only).

Bernacki, Bruce E.↗

NASA NTRS1991

An analysis of temperature-induced errors for an ultrasound distance measuring system

The presentation of research is provided in the following five chapters. Chapter 2 presents the necessary background information and definitions for general work with ultrasound and acoustics. It also discusses the basis for errors in the slant range measurements. Chapter 3 presents a method of problem solution and an analysis of the sensitivity of the equations to slant range measurement errors. It also presents various methods by which the error in the slant range measurements can be reduced to improve overall measurement accuracy. Chapter 4 provides a description of a type of experiment used to test the analytical solution and provides a discussion of its results. Chapter 5 discusses the setup of a prototype collision avoidance system, discusses its accuracy, and demonstrates various methods of improving the accuracy along with the improvements' ramifications. Finally, Chapter 6 provides a summary of the work and a discussion of conclusions drawn from it. Additionally, suggestions for further research are made to improve upon what has been presented here.

Wenger, David Paul↗