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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 NTRS1981

High-stability Shuttle pointing system

It was recognized that precision pointing provided by the Orbiter's attitude control system would not be good enough for Shuttle payload scientific experiments or certain Defense department payloads. The Annular Suspension Pointing System (ASPS) is being developed to satisfy these more exacting pointing requirements. The ASPS is a modular pointing system which consists of two principal parts, including an ASPS Gimbal System (AGS) which provides three conventional ball-bearing gimbals and an ASPS Vernier System (AVS) which magnetically isolates the payload. AGS performance requirements are discussed and an AGS system description is given. The overall AGS system consists of the mechanical hardware, sensors, electronics, and software. Attention is also given to system simulation and performance prediction, and support facilities.

Van Riper, R.↗
NASA NTRS1981

Atmospheric neutral points outside of the principal plane

It is noted that the positions in the sky where the skylight is unpolarized, that is, the neutral points, are in most cases located in the vertical plane through the sun (the principal plane). Points have been observed outside the principal plane (Soret, 1888) when the plane intersected a lake or sea. Here, the neutral points were located at an azimuth of about 15 deg from the sun and near the almucantar through the sun. In order to investigate the effects of water surface and aerosols in the neutral point positions, the positions are computed for models of the earth-atmosphere system that simulate the observational conditions. The computed and measured positions are found to agree well. While previous observations provided only qualitative information on the degree of polarization, it is noted that the computations provide details concerning the polarization parameters.

Fraser, R. S.↗
NASA NTRS1981

Determination of specular points on a surface of revolution

For a given source and observation point, specular points on a conducting surface must be found in order to apply the geometrical optics theory to calculate the reflected field. An efficient method is presented for finding all the specular points for the special case in which the conducting surface is a part of a surface of revolution. Our method, which reduces the conventional two-dimensional search to a one-dimensional search also makes it economically feasible to obtain a global solution for certain classes of surface, source, and observation point geometries that do not render solutions in the more conventional two-dimensional searches.

Lee, S. W.↗
NASA NTRS1981

Pilot estimates of glidepath and aim point during simulated landing approaches

Pilot perceptions of glidepath angle and aim point were measured during simulated landings. A fixed-base cockpit simulator was used with video recordings of simulated landing approaches shown on a video projector. Pilots estimated the magnitudes of approach errors during observation without attempting to make corrections. Pilots estimated glidepath angular errors well, but had difficulty estimating aim-point errors. The data make plausible the hypothesis that pilots are little concerned with aim point during most of an approach, concentrating instead on keeping close to the nominal glidepath and trusting this technique to guide them to the proper touchdown point.

Acree, C. W., Jr.↗
NASA NTRS1982

Development of a high stability pointing mechanism for wide application

A recurrent requirement of spaceborne instruments and communications equipment is that of accurate pointing. This need is recognizable in such diverse applications as Star Sensor trimming, Momentum Wheel gimballing, in-orbit adjustment or alignment of equipment, inter-satellite communication and Antenna Pointing. A pointing mechanism of novel design having several advantages over the more conventional gimbal, centre-pivoted, or cross axis pointing concepts is described.

Brunnen, A. J. D.↗
NASA NTRS1982

Application of two-point difference schemes to the conservative Euler equations for one-dimensional flows

An implicit finite difference method is presented for obtaining steady state solutions to the time dependent, conservative Euler equations for flows containing shocks. The method used the two-point differencing approach of Keller with dissipation added at supersonic points via the retarded density concept. Application of the method to the one-dimensional nozzle flow equations for various combinations of subsonic and supersonic boundary conditions shows the method to be very efficient. Residuals are typically reduced to machine zero in approximately 35 time steps for 50 mesh points. It is shown that the scheme offers certain advantages over the more widely used three-point schemes, especially in regard to application of boundary conditions.

Wornom, S. F.↗
NASA NTRS1982

Application of two-point difference schemes to the conservative Euler equations for one-dimensional flows

An implicit finite-difference method is presented for obtaining steady-state solutions to the time-dependent, conservative Euler equations for flows containing shocks. The method uses a two-point central difference scheme with dissipation added at supersonic points via the retarded density concept. Application of the method to the one-dimensional nozzle flow equations for various combinations of subsonic and supersonic boundary conditions show the method to be very efficient. Residuals are typically reduced to machine zero in approximately 35 time steps for 50 mesh points. It is shown that the scheme offers certain advantages over the more widely-used three-point schemes, especially in regard to application of boundary conditions.

Wornom, S. F.↗
NASA NTRS1981

Computer-based classification accuracy due to the spatial resolution using per-point versus per-field classification techniques

Data sets simulating three different spatial resolutions (SR's) are computed from data with a 15-m nominal SR that were obtained with NASA's Thermatic Mapper Simulator from an altitude of about 6 km. The classification accuracies (CA's) achieved with the data of each of the four different SR's using a per-point Gaussian maximum likelihood classifier (GMLC) are intercompared. The CA's obtained using simulated 30-m SR data with the per-point GMLC are compared with the CA's achieved with a per-field classifier. It is found that: (1) the use of successively higher SR data resulted in lower overall CA's for classifications with the per-point GMLC, especially in cover classes associated with relatively higher spectral variability across adjacent pixels; (2) higher CA's were achieved using the per-field classifier with 30-m SR data than were achieved with the per-point GMLC; and (3) the largest increases in CA's were achieved with the per-field classifier in cover classes associated with relatively high levels of spectral variability across adjacent pixels.

Latty, R. S.↗
NASA NTRS1984

Interstitial Digital-Image-Point Generator

Resolution of digital imagery increased by calculating estimated values of intensity at coordinate points between original intensity-data points. Calculator implements algorithm that generates such interstitial points using four-by-four array of convolute integer coefficients. Coefficients multiplied by intensities at 16 original image points nearest each interstice to obtain weighted average intensity at each interstice.

Edwards, T. R.↗
NASA NTRS1982

Evaluation of methods for rapid determination of freezing point of aviation fuels

Methods for identification of the more promising concepts for the development of a portable instrument to rapidly determine the freezing point of aviation fuels are described. The evaluation process consisted of: (1) collection of information on techniques previously used for the determination of the freezing point, (2) screening and selection of these techniques for further evaluation of their suitability in a portable unit for rapid measurement, and (3) an extensive experimental evaluation of the selected techniques and a final selection of the most promising technique. Test apparatuses employing differential thermal analysis and the change in optical transparency during phase change were evaluated and tested. A technique similar to differential thermal analysis using no reference fuel was investigated. In this method, the freezing point was obtained by digitizing the data and locating the point of inflection. Results obtained using this technique compare well with those obtained elsewhere using different techniques. A conceptual design of a portable instrument incorporating this technique is presented.

Mathiprakasam, B.↗
NASA NTRS1982

Advanced pointing systems for space astronomy

It is noted that the requirements on image stability are increasing, often at the same time that instrument external disturbances are increasing. A prime example of this is the pointing of large diameter optics from the Shuttle bay. To achieve cost-effective advanced pointing systems in the face of these problems, a system approach must be followed, one that encompasses a realistic assessment of requirements, the best possible detector technology, and a broad look at space vehicles and pointing systems that are available. By way of example, a rocket instrument for making measurements of the interstellar gas that uses a standard pointing system to achieve a spectral resolution of 2 x 10 to the 5th is described. It is noted that the use of a free-flying instrument avoids the high-frequency components that degrade the instrument resolution.

Delamere, W. A.↗
NASA NTRS1983

A system for load isolation and precision pointing

A system capable of satisfying the accuracy and stability requirements dictated by Shuttle-borne payloads utilizing large optics has been under joint NASA/Sperry development. This device, denoted the Annular Suspension and Pointing System, employs a unique combination of conventional gimbals and magnetic bearing actuators, thereby providing for the complete isolation of the payload from its external environment, as well as for extremely accurate and stable pointing (equal to about 0.01 arcsec). This effort has been pursued through the fabrication and laboratory evaluation of engineering model hardware. Results from these tests have been instrumental in generating high fidelity computer simulations of this load isolation and precision pointing system, and in permitting confident predictions of the system's on-orbit performance. The applicability of this system to the Solar Optical Telescope mission has been examined using the computer simulation. The worst case pointing error predicted for this payload while subjected to vernier reaction control system thruster firings and crew motions aboard the Shuttle was approximately 0.006 arcsec.

Keckler, C. R.↗
NASA NTRS1983

Influence of deflocculant on the isoelectric point of refractory powders: Considerations on the action of deflocculant

Isoelectric point changes in suspensions of refractory materials vis-a-vis the role of deflocculants used in monolithic refractories were investigated by considering the mineral compositions and adsorbed ions in four kinds of clay. Three types of curves represented the relation between the isoelectric point and the deflocculant. The surface charge of clay particles in the suspensions became negative as a result of the deflocculant, since the isoelectric point of suspensions decreased as the deflocculant was added. The isoelectric point changes of calcined alumina were also compared with those of the clays, and a similar phenomenon was observed, except that the deflocculant dispersed the calcined alumina better than it did the clays. A simple model was used to analyze the results.

Naruse, Y.↗
NASA NTRS1984

High accuracy pointing for earth observation experiments

An evaluation of mission requirements for postulated earth observation orbital payload experiments has indicated that high accuracy pointing and stabilization must be provided for adequate performance. A computer simulation has been conducted to determine the capabilities of an auxiliary pointing system which employs a combination of conventional gimbals and magnetic bearing actuators. This Annular Suspension and Pointing System (ASPS) is found to be a viable means of providing the high slew rates associated with landmark tracking from low orbit, while simultaneously achieving the high degree pointing and stabilization dictated by science payload requirements.

Keckler, C. R.↗
NASA NTRS1984

Rectification of single and multiple frames of satellite scanner imagery using points and edges as control

Rectification of single and overlapping multiple scanner frames produced by such satellite-borne scanners as the LANDSAT MSS was carried out using a newly developed comprehensive parametric model. Tests with both simulated and real image data demonstrate conclusively that this model in general is superior to the widely used polynomial model, and that the simultaneous rectification of overlapping frames using least squares techniques yields a high accuracy than sngle frame rectification due to the inclusion of tie points between the image frames. Used to control, edges or lines, whic are much more likely to be found in images, can replace conventional control points and can easily be implemented into the least squares approach. An efficient algorithm for findng corresponding points in image paris was developed which can be used for determining tie points between image frames and thus increase the ecnomy of the whole rectification procedure.

Paderes, F. C., Jr.↗
NASA NTRS1984

The influence of the number of ground control points on the scene-to-map registration accuracy

The optimum number of ground control points required to rectify a full scene or a portion of a LANDSAT MSS scene was investigated using data from southeastern Louisiana/southwestern Mississippi and eastern Kansas. The ground control points utilized were randomly distributed across the partial or full scene. This work suggest that 24 round control points are more than adequate to rectify a partial or full scene of LANDSAT MSS data. The error incurred in choosing ground control points representing artificial versus natural features was also studied.

Dow, D. D.↗
NASA NTRS1985

Differentiating matrices for arbitrarily spaced grid points

Differentiating matrices allow the numerical differentiation of functions defined at points of a discrete grid. A type of differentiating matrix based on local approximation on a sequence of sliding subgrids is considered. Previous derivations of this type of matrix have been restricted to grids with uniformly spaced points, and the resulting derivative approximations have lacked precision, especially at endpoints. The new formulation allows grids which have arbitrarily space points. It is shown that high accuracy can be achieved through use of differentiating matrices on non-uniform grids which include near-boundary points. Use of the differentiating matrix as an operator to solve eigenvalue problems involving ordinary differential equations is also considered.

Lakin, W. D.↗
NASA NTRS1985

The number of stable points of an infinite-range spin glass memory

A rigorous asymptotic expression for the number of stable points of an infinite-range spin glass with independently identically distributed (i.i.d) zero-mean gaussian exchange interactions is discussed. The result also applies tot he number of stable points of a Hopfield Memory (a kind of associative memory) when the memory connections are i.i.d. zero-mean gaussians. The result is that the number of stable points is asymptotic to a constant slightly larger than 1 times 2 to a power slightly larger thann/4, where n is the number of spins in the glass, or the length of the n-tuples to be remembered by the memory. The answer is easily derived using simple asymptotic techniques from an exact expression for the probability that an arbitrary plus or minus 1 n-tuple of spins is a fixed point. This expression is obtained from the fact that any distribution of joint zero-mean gaussians of given covariances is specified solely by these covariances. This is a far shorter derivation of the result than those existing.

Posner, E. C.↗