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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 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Residual mode filters and adaptive control in large space structures

One of the most difficult problems in controlling large systems and structures is compensating for the destructive interaction which can occur between the reduced-order model (ROM) of the plant, which is used by the controller, and the unmodeled dynamics of the plant, often called the residual modes. The problem is more significant in the case of large space structures because their naturally light damping and high performance requirements lead to more frequent, destructive residual mode interaction (RMI). Using the design/compensation technique of residual mode filters (RMF's), effective compensation of RMI can be accomplished in a straightforward manner when using linear controllers. The use of RMF's has been shown to be effective for a variety of large structures, including a space-based laser and infinite dimensional systems. However, the dynamics of space structures is often uncertain and may even change over time due to on-orbit erosion from space debris and corrosive chemicals in the upper atmosphere. In this case, adaptive control can be extremely beneficial in meeting the performance requirements of the structure. Adaptive control for large structures is also based on ROM's and so destructive RMI may occur. Unfortunately, adaptive control is inherently nonlinear, and therefore the known results of RMF's cannot be applied. The purpose is to present the results of new research showing the effects of RMI when using adaptive control and the work which will hopefully lead to RMF compensation of this problem.

Davidson, Roger A.↗
NASA NTRS1973

Image analysis techniques associated with automatic data base generation.

This paper considers some basic problems relating to automatic data base generation from imagery, the primary emphasis being on fast and efficient automatic extraction of relevant pictorial information. Among the techniques discussed are recursive implementations of some particular types of filters which are much faster than FFT implementations, a 'sequential similarity detection' technique of implementing matched filters, and sequential linear classification of multispectral imagery. Several applications of the above techniques are presented including enhancement of underwater, aerial and radiographic imagery, detection and reconstruction of particular types of features in images, automatic picture registration and classification of multiband aerial photographs to generate thematic land use maps.

Bond, A. D.↗
NASA NTRS1992

Iterative deconvolution of x ray and optical SNR images

Blind Iterative Deconvolution (BID) is a technique which was originally developed to correct the degrading effects of atmospheric turbulence on astronomical images from single short exposure, high signal-to-noise-ratio frames. At the Center for Astro physics, we have implemented a version of BID following the general approach of Ayers and Dainty (1988), but extending the technique to use Wiener filtering, and developed it for application to high energy images from Einstein and ROSAT. In the optical, the point spread function (PSF) that degrades the images is due to a combination of telescope and atmospheric aberrations. At high energies, the degrading function is the instrument response function, which is known to be time and energy level unstable. In both cases, the PSF is poorly known, so BID can be used to extract the PSF from the image and then deconvolve the blurred image to produce a sharpened image. Other aspects of this technique are discussed.

Nisenson, Peter↗
NASA NTRS1985

High-latitude filtering in a global grid-point model using model normal modes

A normal modes expansion technique is applied to perform high latitude filtering in the GLAS fourth order global shallow water model with orography. The maximum permissible time step in the solution code is controlled by the frequency of the fastest propagating mode, which can be a gravity wave. Numerical methods are defined for filtering the data to identify the number of gravity modes to be included in the computations in order to obtain the appropriate zonal wavenumbers. The performances of the model with and without the filter, and with a time tendency and a prognostic field filter are tested with simulations of the Northern Hemisphere winter. The normal modes expansion technique is shown to leave the Rossby modes intact and permit 3-5 day predictions, a range not possible with the other high-latitude filters.

Takacs, L. L.↗
NASA NTRS2015

Desensitized Optimal Filtering and Sensor Fusion Toolkit

Analytical Mechanics Associates, Inc., has developed a software toolkit that filters and processes navigational data from multiple sensor sources. A key component of the toolkit is a trajectory optimization technique that reduces the sensitivity of Kalman filters with respect to model parameter uncertainties. The sensor fusion toolkit also integrates recent advances in adaptive Kalman and sigma-point filters for non-Gaussian problems with error statistics. This Phase II effort provides new filtering and sensor fusion techniques in a convenient package that can be used as a stand-alone application for ground support and/or onboard use. Its modular architecture enables ready integration with existing tools. A suite of sensor models and noise distribution as well as Monte Carlo analysis capability are included to enable statistical performance evaluations.

Karlgaard, Christopher D.↗
NASA NTRS1975

Separation of man-made and natural patterns in high-altitude imagery of agricultural areas

A nonstationary linear digital filter is designed and implemented which extracts the natural features from high-altitude imagery of agricultural areas. Essentially, from an original image a new image is created which displays information related to soil properties, drainage patterns, crop disease, and other natural phenomena, and contains no information about crop type or row spacing. A model is developed to express the recorded brightness in a narrow-band image in terms of man-made and natural contributions and which describes statistically the spatial properties of each. The form of the minimum mean-square error linear filter for estimation of the natural component of the scene is derived and a suboptimal filter is implemented. Nonstationarity of the two-dimensional random processes contained in the model requires a unique technique for deriving the optimum filter. Finally, the filter depends on knowledge of field boundaries. An algorithm for boundary location is proposed, discussed, and implemented.

Samulon, A. S.↗
NASA NTRS1974

Separation of surface and bulk minority-carrier lifetimes in silicon

A photoexcitation technique is suggested which is direct and convenient, requiring little special equipment. No change in the dimensions of the crystal sample is necessary. Although related to the technique of interposing a filter between the light source and the sample, it differs in that the filter is part of the crystal itself. Basically, one of the electrodes is split into two electrodes, one collecting charge from carriers excited in the bulk. This phenomenon in which the carriers follow very closely the electric field lines has been used in position-sensitive semiconductor radiation detectors (Norbeck and Carlson, 1963) but has not been applied to lifetime measurements. In a sense, the split electrode creates a sample without a surface.

Carroll, K. A.↗
NASA NTRS1975

Computer-aided analysis of Skylab multispectral scanner data in mountainous terrain for land use, forestry, water resource, and geologic applications

The author has identified the following significant results. One of the most significant results of this Skylab research involved the geometric correction and overlay of the Skylab multispectral scanner data with the LANDSAT multispectral scanner data, and also with a set of topographic data, including elevation, slope, and aspect. The Skylab S192 multispectral scanner data had distinct differences in noise level of the data in the various wavelength bands. Results of the temporal evaluation of the SL-2 and SL-3 photography were found to be particularly important for proper interpretation of the computer-aided analysis of the SL-2 and SL-3 multispectral scanner data. There was a quality problem involving the ringing effect introduced by digital filtering. The modified clustering technique was found valuable when working with multispectral scanner data involving many wavelength bands and covering large geographic areas. Analysis of the SL-2 scanner data involved classification of major cover types and also forest cover types. Comparison of the results obtained wth Skylab MSS data and LANDSAT MSS data indicated that the improved spectral resolution of the Skylab scanner system enabled a higher classification accuracy to be obtained for forest cover types, although the classification performance for major cover types was not significantly different.

Hoffer, R. M.↗
NASA NTRS1976

Computer simulation and evaluation of edge detection algorithms and their application to automatic path selection

The construction of an autonomous roving vehicle requires the development of complex data-acquisition and processing systems, which determine the path along which the vehicle travels. Thus, a vehicle must possess algorithms which can (1) reliably detect obstacles by processing sensor data, (2) maintain a constantly updated model of its surroundings, and (3) direct its immediate actions to further a long range plan. The first function consisted of obstacle recognition. Obstacles may be identified by the use of edge detection techniques. Therefore, the Kalman Filter was implemented as part of a large scale computer simulation of the Mars Rover. The second function consisted of modeling the environment. The obstacle must be reconstructed from its edges, and the vast amount of data must be organized in a readily retrievable form. Therefore, a Terrain Modeller was developed which assembled and maintained a rectangular grid map of the planet. The third function consisted of directing the vehicle's actions.

Longendorfer, B. A.↗
NASA NTRS1979

Evaluation of the navigation performance of shipboard-VTOL-landing guidance systems

The objective of this study was to explore the performance of a VTOL aircraft landing approach navigation system that receives data (1) from either a microwave scanning beam (MSB) or a radar-transponder (R-T) landing guidance system, and (2) information data-linked from an aviation facility ship. State-of-the-art low-cost-aided inertial techniques and variable gain filters were used in the assumed navigation system. Compensation for ship motion was accomplished by a landing pad deviation vector concept that is a measure of the landing pad's deviation from its calm sea location. The results show that the landing guidance concepts were successful in meeting all of the current Navy navigation error specifications, provided that vector magnitude of the allowable error, rather than the error in each axis, is a permissible interpretation of acceptable performance. The success of these concepts, however, is strongly dependent on the distance measuring equipment bias. In addition, the 'best possible' closed-loop tracking performance achievable with the assumed point-mass VTOL aircraft guidance concept is demonstrated.

Mcgee, L. A.↗
NASA NTRS1981

Algorithms for onboard orbit estimation with Tracking and Data Relay Satellite System data

An investigation is carried out to determine an appropriate estimation technique for onboard orbit determination using simulated Tracking and Data Relay Satellite System (TDRSS) data. Results are presented from studies done using three estimation techniques: an extended Kalman filter, a consider filter, and a sliding batch differential corrector. These estimators are evaluated when used with both baseline and worst-case TDRSS measurement errors and with tracking configurations for two types of orbits: a high-inclination, near-circular orbit with an altitude of 700 km and a moderately inclined, lower altitude orbit. Each estimation result is evaluated by comparing the true with the estimated ephemeris.

Dunham, J. B.↗
NASA NTRS1982

Methane flux in the Great Dismal Swamp

The paper reports measurements made over a 17-month period of the methane flux in the Great Dismal Swamp of Virginia in light of the potential implications of variations in atmospheric methane concentrations. Gas flux measurements were made by a technique combining a gas filter correlation IR absorption analyzer with improved sampling chambers that enclose a soil area under conditions ranging from totally flooded soils to dry soils resulting from drought conditions. Methane emissions are found to range from 0.0013 g CH4/sq m per day to 0.019 g CH4/sq m per day, depending on temperature and season, when the soil is in a waterlogged state. During drought conditions, the peat soils in the swamp were a sink for atmospheric methane, with fluxes from less than 0.001 to 0.005 g CH4/sq m per day and decreasing with decreasing temperature. Results illustrate the potential complexity of the processes which regulate the net flux of methane between wetland soils and the atmosphere.

Harriss, R. C.↗
NASA NTRS1981

Experimental measurements of the Space Shuttle main engine fuel and oxygen turbopump vibration characteristics

The vibration characteristics of the SSME (Space Shuttle main engine) are dealt with. The Space Shuttle engine consists of a main rocket nozzle and attached to it are high pressure fuel and oxygen pumps. Various vibration problems have been encountered with both the hydrogen and oxygen pumps. The vibration spectrum of the hydrogen and oxygen pumps has been analyzed by various techniques using synchronous tracking filters and FFT analyzers. The experimental data has been correlated to theoretical predictions of resonance frequencies.

Gunter, E. J.↗
NASA NTRS1984

Dewatering Peat With Activated Carbon

Proposed process produces enough gas and carbon to sustain itself. In proposed process peat slurry is dewatered to approximately 40 percent moisture content by mixing slurry with activated carbon and filtering with solid/liquid separation techniques.

Rohatgi, N. K.↗
NASA NTRS1982

Space Shuttle Orbiter onboard rendezvous navigation

Much of the work that the Space Shuttle will perform requires a capability to rendezvous with other orbiting objects. The formulation and design philosophy behind the Space Shuttle Orbiter rendezvous navigation system are the subjects of this paper. The current rendezvous navigation design incorporates a Kalman filter to estimate the relative position and velocity. The filter is augmented with state elements for the estimation of colored measurement noise and acceleration modeling errors. Several measurement types are available to the filter: a manual optical sighting device, an automatic optical tracking instrument, and a rendezvous radar which provides range and range rate measurements as well as the line of sight direction. The filter also includes a technique for detecting highly improbable measurements and excluding them from Kalman processing.

Little, M. J.↗
NASA NTRS1986

International Halley Watch: Discipline specialists for photometry and polarimetry

The last of the 75 sets of standard filters were distributed, including re-allocation of several sets from north to south at the beginning of 1986. Observations of the photometric standard stars from several observatories were collected and synthesized into a standard system of magnitudes. Transformation equations were derived to yield absolute fluxes in various cometary emission bands and in the continuum. These transformations turned out to vary with the comet's radial velocity for some filters and with the filter's temperature for some filters, and from one set of filters to another, all effects were quantitatively assessed. Non-linearities in the extinction, observed through the OH filter, were modelled and techniques for linearizing the extinction were developed. Although not among the official goals of the IHW, NASA Headquarters chose to work through the IHW to develop two CCD camera systems. Two iterations of an archive format were performed and a third is in the works.

Ahearn, M. F.↗
NASA NTRS1987

Results of using the global positioning system to maintain the time and frequency synchronization in the Deep Space Network

There are two hydrogen maser clocks located at each signal processing center (SPC) in the DSN. Close coordination of the time and frequency of the SPC clocks is needed to navigate spacecraft to the outer planets. A recent example was the Voyager spacecraft's encounter with Uranus in January 1986. The clocks were adjusted with the goal of minimizing time and frequency offsets between the SPCs at encounter. How time and frequency at each SPC is estimated using data acquired from the Global Positioning System Timing Receivers operating on the NBS-BIH (National Bureau of Standards-Bureau International de l'Heure) tracking schedule is described. These data are combined with other available timing receiver data to calculate the time offset estimates. The adjustment of the clocks is described. It was determined that long range hydrogen maser drift is quite predictable and adjustable within limits. This enables one to minimize time and frequency differences between the three SPCs for many months by matching the drift rates of the three standards. Data acquisition and processing techniques using a Kalman filter to make estimates of time and frequency offsets between the clocks at the SPCs and UTC(NBS) (Coordinated Universal Time realized at NBS) are described.

Clements, P. A.↗
NASA NTRS1987

Development of data processing, interpretation and analysis system for the remote sensing of trace atmospheric gas species

The major emphasis is on the advancement of remote sensing technology. In particular, the gas filter correlation radiometer (GFCR) technique was applied to the measurement of trace gas species, such as carbon monoxide (CO), from airborne and Earth orbiting platforms. Through a series of low altitude aircraft flights, high altitude aircraft flights, and orbiting space platform flights, data were collected and analyzed, culminating in the first global map of carbon monoxide concentration in the middle troposphere and stratosphere. The four major areas of this remote sensing program, known as the Measurement of Air Pollution from Satellites (MAPS) experiment, are: (1) data acquisition, (2) data processing, analysis, and interpretation algorithms, (3) data display techniques, and (4) information processing.

Casas, Joseph C.↗