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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.

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At least 307 records · Page 17

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NASA NTRS1972

Methods of computing vocabulary size for the two-parameter rank distribution

A summation method is described for computing the vocabulary size for given parameter values in the 1- and 2-parameter rank distributions. Two methods of determining the asymptotes for the family of 2-parameter rank-distribution curves are also described. Tables are computed and graphs are drawn relating paris of parameter values to the vocabulary size. The partial product formula for the Riemann zeta function is investigated as an approximation to the partial sum formula for the Riemann zeta function. An error bound is established that indicates that the partial product should not be used to approximate the partial sum in calculating the vocabulary size for the 2-parameter rank distribution.

Edmundson, H. P.↗

NASA NTRS1972

Control theory for random systems

A survey is presented of the current knowledge available for designing and predicting the effectiveness of controllers for dynamic systems which can be modeled by ordinary differential equations. A short discussion of feedback control is followed by a description of deterministic controller design and the concept of system state. The need for more realistic disturbance models led to the use of stochastic process concepts, in particular the Gauss-Markov process. A compensator controlled system, with random forcing functions, random errors in the measurements, and random initial conditions, is treated as constituting a Gauss-Markov random process; hence the mean-square behavior of the controlled system is readily predicted. As an example, a compensator is designed for a helicopter to maintain it in hover in a gusty wind over a point on the ground.

Bryson, A. E., Jr.↗

NASA NTRS1977

Imaginary-frequency polarizability and van der Waals force constants of two-electron atoms, with rigorous bounds

Variational functionals of Braunn and Rebane (1972) for the imagery-frequency polarizability (IFP) have been generalized by the method of Gramian inequalities to give rigorous upper and lower bounds, valid even when the true (but unknown) unperturbed wavefunction must be represented by a variational approximation. Using these formulas in conjunction with flexible variational trial functions, tight error bounds are computed for the IFP and the associated two- and three-body van der Waals interaction constants of the ground 1(1S) and metastable 2(1,3S) states of He and Li(+). These bounds generally establish the ground-state properties to within a fraction of a per cent and metastable properties to within a few per cent, permitting a comparative assessment of competing theoretical methods at this level of accuracy. Unlike previous 'error bounds' for these properties, the present results have a completely a priori theoretical character, with no empirical input data.

Glover, R. M.↗

NASA NTRS1977

Absolute judgment for one- and two-dimensional stimuli embedded in Gaussian noise

This study examines the effect on human performance of adding Gaussian noise or disturbance to the stimuli in absolute judgment tasks involving both one- and two-dimensional stimuli. For each selected stimulus value (both an X-value and a Y-value were generated in the two-dimensional case), 10 values (or 10 pairs of values in the two-dimensional case) were generated from a zero-mean Gaussian variate, added to the selected stimulus value and then served as the coordinate values for the 10 points that were displayed sequentially on a CRT. The results show that human performance, in terms of the information transmitted and rms error as functions of stimulus uncertainty, was significantly reduced as the noise variance increased.

Kvalseth, T. O.↗

NASA NTRS1977

The effect of a visual/motion display mismatch in a single axis compensatory tracking task

An experiment was performed to determine the effect of a performance mismatch between the visual and motion display systems on a real time piloted aircraft simulation. Pilots performed a compensatory roll tracking task with dynamics typical of medium jet transports. Between 0 and 10 rad/sec, visual and motion system responses were equivalent to either unity or a first order lag at 4.8 rad/sec. Pilot describing functions and error scores were calculated. Results show that the mismatch between visual and motion display systems has no significant effect. It is the absence of high frequency visual and/or motion cues which significantly affects pilot performance.

Shirachi, D. K.↗

NASA NTRS1979

An approach to automated longwall mining

The longwall system of mining coal, providing advantages in the areas of productivity as well as health and safety, is described, and technological developments leading to a full automation of the system are discussed. In the longwall system large blocks of coal (up to 600 feet wide and up to 5000 feet long) are developed, with each block mined out by taking successive slices across the short dimension of the block and loading the broken coal onto a conveyor. A self-advancing system supports the roof over the length of the face throughout cutting and loading, with the supports advanced with the face, and the roof allowed to collapse behind them. A double-ranging drum longwall shearer provides the system with an efficient yaw, roll, and variable-thickness vertical control. Currently two machine operators function as error detectors and controllers. It is shown that electronic sensors can lead to a fully automated vertical control system, and automatic roll control is achievable with available instruments and machine tilt actuators.

Palowitch, E. R.↗

NASA NTRS1984

Comparison of Optimum Interpolation and Cressman Analyses

The objective of this investigation is to develop a state-of-the-art optimum interpolation (O/I) objective analysis procedure for use in numerical weather prediction studies. A three-dimensional multivariate O/I analysis scheme has been developed. Some characteristics of the GLAS O/I compared with those of the NMC and ECMWF systems are summarized. Some recent enhancements of the GLAS scheme include a univariate analysis of water vapor mixing ratio, a geographically dependent model prediction error correlation function and a multivariate oceanic surface analysis.

Baker, W. E.↗

NASA NTRS1985

Comparison of Optimum Interpolation and Cressman Analyses

The development of a state of the art optimum interpolation (O/I) objective analysis procedure for use in numerical weather prediction studies was investigated. A three dimensional multivariate O/I analysis scheme was developed. Some characteristics of the GLAS O/I compared with those of the NMC and ECMWF systems are summarized. Some recent enhancements of the GLAS scheme include a univariate analysis of water vapor mixing ratio, a geographically dependent model prediction error correlation function and a multivariate oceanic surface analysis.

Baker, W. E.↗

NASA NTRS1986

Computations of atmospheric angular momentum, with emphasis on the merit period

Three-dimensional wind and pressure fields used to calculate effective angular momentum functions (EAMFs) for earth rotation/polar motion are produced by a complex 3-step system at the U.S. National Meteorological Center. The resulting series of functions have errors which can be estimated by comparisons with similar series produced by the European Center for Medium Range Weather Forecasts. For the axial wind component, related to length of day (lod) changes, the small differences between the two series of atmospheric angular momentum (AAM) are confined to the tropics and the higher latitudes of the Southern Hemisphere. In addition, differences between the seasonal components of the AAM series and those of lod are found to be significantly reduced by including the entire stratosphere's momentum. Comparisons of the EAMFs for 8 calendar years indicate substantial interannual variability exists in the annual amplitudes of these forcing terms. In particular, the axial AAM showed quite different behavior from normal in 1983 and 1984, the years of the MERIT campaign. These two years showed very high and low annual amplitudes for AAM, respectively, which can be attributed to the effect of the tropical Pacific sea surface warming (El Nino) and its aftermath.

Salstein, David A.↗

NASA NTRS1987

Measurements of surface energy budgets in the rocky mountains of Colorado

The surface energy balance of a mountain valley and a mountain peak were continuously monitored for periods of 126 and 34 days, respectively. The data yielded temporally resolved, high-quality measurements of the radiative, soil, and total atmospheric turbulent heat fluxes. Comparative data for the two sites reveal the effects of terrain setting and surface cover in modulating the heat balance. A procedure is suggested for accommodating local advective affects due to variable terrain and surface cover to allow partitioning of the total turbulent flux into values for sensible and latent heat. This procedure relies on a stability-dependent coefficient of turbulent transfer which is locally calibrated to systematically minimize advection-related errors. Scaling functions for the transfer coeffient are based on the effective blackbody temperature of the surface. Consequently, the procedure offers an opportunity for developing reliable bulk representations of heat transfer at the surface for use with remote sensing data and with models of the soil and surface cover in complex terrain.

Sheaffer, John D.↗

NASA NTRS1988

Simultaneous orbit determination

Simultaneous orbit determination is demonstrated using live range and Doppler data for the NASA/Goddard tracking configuration defined by the White Sands Ground Terminal (WSGT), the Tracking and Data Relay Satellite (TDRS), and the Earth Radiation Budget Satellite (ERBS). A physically connected sequential filter-smoother was developed for this demonstration. Rigorous necessary conditions are used to show that the state error covariance functions are realistic; and this enables the assessment of orbit estimation accuracies for both TDRS and ERBS.

Wright, J. R.↗

NASA NTRS1990

Computational efficiency of parallel combinatorial OR-tree searches

The performance of parallel combinatorial OR-tree searches is analytically evaluated. This performance depends on the complexity of the problem to be solved, the error allowance function, the dominance relation, and the search strategies. The exact performance may be difficult to predict due to the nondeterminism and anomalies of parallelism. The authors derive the performance bounds of parallel OR-tree searches with respect to the best-first, depth-first, and breadth-first strategies, and verify these bounds by simulation. They show that a near-linear speedup can be achieved with respect to a large number of processors for parallel OR-tree searches. Using the bounds developed, the authors derive sufficient conditions for assuring that parallelism will not degrade performance and necessary conditions for allowing parallelism to have a speedup greater than the ratio of the numbers of processors. These bounds and conditions provide the theoretical foundation for determining the number of processors required to assure a near-linear speedup.

Li, Guo-Jie↗

NASA NTRS1992

Open-Loop Acquisition Of Frequency In BPSK

Proposed open-loop analog/digital signal-processing system would be Costas-loop error detector functioning in closed-loop manner overall. Detector estimates difference between frequency of input signal and internal reference oscillator. Estimate used to close frequency-control loop. Precise symbol timing not necessary. Performance better than systems that effect open-loop acquisition using integrators instead of low-pass filters in arms of Costas loops and in which performance varies with symbol timing.

Shah, Biren N.↗

NASA NTRS1992

Landsat-4 (TDRSS-user) orbit determination using batch least-squares and sequential methods

TDRSS user orbit determination is analyzed using a batch least-squares method and a sequential estimation method. It was found that in the batch least-squares method analysis, the orbit determination consistency for Landsat-4, which was heavily tracked by TDRSS during January 1991, was about 4 meters in the rms overlap comparisons and about 6 meters in the maximum position differences in overlap comparisons. The consistency was about 10 to 30 meters in the 3 sigma state error covariance function in the sequential method analysis. As a measure of consistency, the first residual of each pass was within the 3 sigma bound in the residual space.

Oza, D. H.↗

NASA NTRS1993

Comparison Of Four FFT-Based Frequency-Acquisition Techniques

Report presents comparative theoretical analysis of four conceptual techniques for initial estimation of carrier frequency of suppressed-carrier, binary-phase-shift-keyed radio signal. Each technique effected by open-loop analog/digital signal-processing subsystem part of Costas-loop phase-error detector functioning in closed-loop manner overall.

Shah, Biren N.↗

NASA NTRS1988

Experimental implementation of adaptive control for flexible space structures

On-going research at The Aerospace Corporation studying the feasibility of applying adaptive control methodologies to the control of flexible space structures is described. A laboratory testbed was established to test system identification and control approaches. The laboratory set-up and controller design approach are discussed. The ARX least squares parameter estimation technique is analyzed in terms of frequency domain transfer function bias error. This analysis approach enables the determination of the effects of sampling rate, sensor type, and data prefiltering on the estimation performance. The ability to identify space structure dynamics over a range of frequencies is shown to be heavily dependent on these factors.

Mcgraw, Gary A.↗

NASA NTRS1995

Structural Damage Prediction and Analysis for Hypervelocity Impact. BUMPERII Suggestion and Problem Reports

In the course of preparing the SD_SURF space debris analysis code, several problems and possibilities for improvement of the BUMPERII code were documented and sent to MSFC. These suggestions and problem reports are included here as a part of the contract final report. This includes reducing BUMPERII memory requirements, compiling problems with BUMPERII, FORTRAN-lint analysis of BUMPERII, and error in function PRV in BUMPERII.

Source record↗

NASA NTRS1999

Temporary arterial shunts to maintain limb perfusion after arterial injury: an animal study

BACKGROUND: Temporary shunt placement can quickly restore perfusion after extremity arterial injury. This study examined the adequacy of limb blood flow with shunt use, non-heparin-bonded shunt patency over prolonged periods, and the safety of this technique. METHODS: Common iliac arteries were divided and 4.0-mm Silastic Sundt shunts placed in 16 anesthetized pigs. Eight (group I) had shunts placed immediately; eight others (group II) were shunted after an hour of limb ischemia and hemorrhagic shock. Physiologic parameters and femoral artery blood flow in both hindlimbs were continuously monitored. Limb lactic acid generation, oxygen utilization, and hematologic and metabolic effects were serially evaluated for 24 hours. RESULTS: Shunts remained patent in 13 of 16 pigs. Shunts thrombosed in two group I animals because of technical errors, but functioned well after thrombectomy and repositioning. Patency could not be maintained in one animal that died from shock. Flow in group I shunted limbs was 57 (+/-11 SD) % of control. For group II animals in shock, shunted limb flow initially averaged 46 +/- 15% of control, but 4 hours after shunt placement, the mean limb blood flow was the same as in group I. Increased oxygen extraction compensated for the lower flow. Lactic acid production was not increased in comparison to control limbs. CONCLUSION: Shunts provided adequate flow in this model of extremity trauma. Correctly placed shunts stayed patent for 24 hours, without anticoagulation, if shunt placement followed resuscitation.

Hindlimb/blood supply/injuries↗