SEARCH · Engineering Papers

Results for “Filters”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 559 records · Page 31

All sources NASA NTRS DOE OSTI

NASA NTRS1975

Ghost suppression in image restoration filtering

An optimum image restoration filter is described in which provision is made to constrain the spatial extent of the restoration function, the noise level of the filter output and the rate of falloff of the composite system point-spread away from the origin. Experimental results show that sidelobes on the composite system point-spread function produce ghosts in the restored image near discontinuities in intensity level. By redetermining the filter using a penalty function that is zero over the main lobe of the composite point-spread function of the optimum filter and nonzero where the point-spread function departs from a smoothly decaying function in the sidelobe region, a great reduction in sidelobe level is obtained. Almost no loss in resolving power of the composite system results from this procedure. By iteratively carrying out the same procedure even further reductions in sidelobe level are obtained. Examples of original and iterated restoration functions are shown along with their effects on a test image.

Riemer, T. E.↗

NASA NTRS1976

CCD filter and transform techniques for interference excision

The theoretical and some experimental results of a study aimed at applying CCD filter and transform techniques to the problem of interference excision within communications channels were presented. Adaptive noise (interference) suppression was achieved by the modification of received signals such that they were orthogonal to the recently measured noise field. CCD techniques were examined to develop real-time noise excision processing. They were recursive filters, circulating filter banks, transversal filter banks, an optical implementation of the chirp Z transform, and a CCD analog FFT.

Borsuk, G. M.↗

NASA NTRS1977

Modeling error analysis of stationary linear discrete-time filters

The performance of Kalman-type, linear, discrete-time filters in the presence of modeling errors is considered. The discussion is limited to stationary performance, and bounds are obtained for the performance index, the mean-squared error of estimates for suboptimal and optimal (Kalman) filters. The computation of these bounds requires information on only the model matrices and the range of errors for these matrices. Consequently, a design can easily compare the performance of a suboptimal filter with that of the optimal filter, when only the range of errors in the elements of the model matrices is available.

Patel, R.↗

NASA NTRS1977

Spatial filter for Q-switched lasers

A spatial filtered Q-switched laser system is reported that prevents ionization of air in close proximity to the aperture of the spatial filter. A compound lens system having an astigmatic focus is positioned between the laser and the spatial filter for defocusing the light beam emanating from the laser in the vicinity of the aperture of the spatial filter to an intensity below that which produces ionization of air. The preferred construction of the compound lens system as viewed from the laser comprises a cylindrical lens and a pair of positive lenses.

Wuerker, R. F.↗

NASA NTRS1977

Numerical comparison of Kalman filter algorithms - Orbit determination case study

Numerical characteristics of various Kalman filter algorithms are illustrated with a realistic orbit determination study. The case study of this paper highlights the numerical deficiencies of the conventional and stabilized Kalman algorithms. Computational errors associated with these algorithms are found to be so large as to obscure important mismodeling effects and thus cause misleading estimates of filter accuracy. The positive result of this study is that the U-D covariance factorization algorithm has excellent numerical properties and is computationally efficient, having CPU costs that differ negligibly from the conventional Kalman costs. Accuracies of the U-D filter using single precision arithmetic consistently match the double precision reference results. Numerical stability of the U-D filter is further demonstrated by its insensitivity to variations in the a priori statistics.

Bierman, G. J.↗

NASA NTRS1976

Numerical comparison of discrete Kalman filter algorithms - Orbit determination case study

Bierman, G. J.↗

NASA NTRS1977

Thin aluminum filters for use on the Apollo Telescope Mount XUV spectrographs

Thin, unbacked, aluminum film filters were used on two extreme ultraviolet (XUV) instruments flown on the Apollo Telescope Mount of Skylab by the Naval Research Laboratory to transmit the XUV radiation while blocking the longer wavelength radiation that would saturate the detector. The requirements placed on these filters - large size, resistance to degradation by high acoustic and vibration fields, and low pinhole transmittances - were far more severe than those placed on any filters previously flown. Special techniques were developed for vacuum evaporation of the aluminum, removal of the films from the substrates, supporting the films and mounting them to obtain finished filters, and storing them so that no degradation took place. A description of these techniques will be given.

Schumacher, R. J.↗

NASA NTRS1976

A realization of the RAM digital filter

The digital filtering algorithm of W. D. Little, which employs a large RAM to obtain high speed, is implemented in a simple hardware configuration. The nonrecursive version of this filter is compared to the counting digital filter and found to be competitive for low-order filters up to order 7 (8 coefficients).

Zohar, S.↗

NASA NTRS1978

An extended Kalman-Bucy filter for atmospheric temperature profile retrieval with a passive microwave sounder

An extended Kalman-Bucy filter has been implemented for atmospheric temperature profile retrievals from observations made using the Scanned Microwave Spectrometer (SCAMS) instrument carried on the Nimbus 6 satellite. This filter has the advantage that it requires neither stationary statistics in the underlying processes nor linear production of the observed variables from the variables to be estimated. This extended Kalman-Bucy filter has yielded significant performance improvement relative to multiple regression retrieval methods. A multi-spot extended Kalman-Bucy filter has also been developed in which the temperature profiles at a number of scan angles in a scanning instrument are retrieved simultaneously. These multi-spot retrievals are shown to outperform the single-spot Kalman retrievals.

Ledsham, W. H.↗

NASA NTRS1978

Digital filter structures having low errors and simple hardware implementation

Sensitivity and roundoff errors can seriously limit the application of recursive digital filters in practice, particularly when the filters have poles near z = + 1. A filter structure, based on digital incremental computers is proposed, which has low sensitivity, good error characteristics, and simple hardware implementation for pole locations close to z = + 1. Expressions for the roundoff errors are derived and compared to those for conventional structures. A design procedure is suggested to implement the new filter structure given the transfer function. Simulation results are presented.

Abu-El-haija, A. I.↗

NASA NTRS1977

A structure suitable for implementing digital filters with poles near z = +1

Errors limiting the application of recursive digital filters which have poles near z = +1 are considered, and a filter structure for pole locations close to z = +1 is proposed. The filter structure, based on digital incremental computers, has low sensitivity and good error characteristics for pole locations near z = +1. Expressions for the roundoff error are derived, and errors associated with the proposed structure and a conventional structure are compared. A design procedure is suggested for implementing the new filter structure when the transfer function is given. Simulation results are presented.

Abu-El-haija, A. I.↗

NASA NTRS1979

A structure for digital notch filters

Narrow-band digital notch filters have their poles near the unit circle. As the sampling rate is increased, the poles move towards Z = +1. Implementing such filters requires long registers to overcome the sensitivity and roundoff errors. A filter structure based on digital incremental computers is proposed which has low sensitivity and round-off errors, and simple hardware implementation. The filter structure can be directly used on differentially pulse-code modulated signals. Hardware multipliers are not required as the poles approach z = +1, and excellent results can be obtained using multipliers with very short word lengths, or with small size read-only memories.

Abu-El-haija, A. I.↗

NASA NTRS1979

Filtering of non-linear instabilities

For Courant numbers larger than one and cell Reynolds numbers larger than two, oscillations and in some cases instabilities are typically found with implicit numerical solutions of the fluid dynamics equations. This behavior has sometimes been associated with the loss of diagonal dominance of the coefficient matrix. It is shown here that these problems can in fact be related to the choice of the spatial differences, with the resulting instability related to aliasing or nonlinear interaction. Appropriate 'filtering' can reduce the intensity of these oscillations and in some cases possibly eliminate the instability. These filtering procedures are equivalent to a weighted average of conservation and non-conservation differencing. The entire spectrum of filtered equations retains a three-point character as well as second-order spatial accuracy. Burgers equation has been considered as a model. Several filters are examined in detail, and smooth solutions have been obtained for extremely large cell Reynolds numbers.

Khosla, P. K.↗

NASA NTRS1980

Filter development for the extreme ultraviolet explorer

Thin film filters with extreme ultraviolet bandpasses unattainable with previously available filter materials were developed. Titanium and antimomy were identified as the most promising materials for providing the scientifically important 350-500 A bandpass heretofore unattainable with thin film filters. Titanium foils and titanium-antimony sandwich foils of the required uniformity and durability were fabricated. Transmission measurements and optimization studies indicate that the titanium-antimony combination provides an ideal bandpass for scientifically valuable, high-sensitivity observations with EUVE. Environmental testing of the mechanical strength, moisture sensitivity and aging characteristics of the newly developed filter materials demonstrate flightworthy performance.

Kimble, R.↗

NASA NTRS1980

Low-pass interference filters for submillimeter astronomy

Low-pass (long-wave transmitting) interference filters, suitable for broadband photometric observations, previously have been constructed from series of capacitive grids stretched on thin Mylar. These filters have the desired optical properties of high transmission, sharp cut-ons, and good blocking at short wavelengths. Their designs, however, do not scale from one wavelength to another and their performance can deteriorate at low temperatures due to differential contraction of the dielectric backing and the supporting structure. The deviation of these early filters from the predicted scaling was due primarily to the difference in refractive index between the backing material and the medium between the grids. In the present paper, filters are described in which dielectric spacers are used, instead of air, as the medium between the grids. This technique has improved the scaling and has reduced the distortion from differential contraction.

Whitcomb, S. E.↗

NASA NTRS1980

Microprocessor realizations of range and range-rate filters in radar systems

This paper describes the implementation of digital radar range-rate filters on a microprocessor-based system. A range-rate filter processes a digitized noisy range signal to recover smoothed range data and its derivative, range rate. Two filter designs are implemented. Considerations aiding their efficient operation on an 8-bit microprocessor are discussed. The filters are subjected to a noisy range input signal of known variance, and the associated output signals are statistically analysed to determine noise-rejection characteristics. These results are compared to analytical predictions.

Fleischer, D.↗

NASA NTRS1980

Geomagnetic field modeling by optimal recursive filtering

Five individual 5 year mini-batch geomagnetic models were generated and two computer programs were developed to process the models. The first program computes statistics (mean sigma, weighted sigma) on the changes in the first derivatives (linear terms) of the spherical harmonic coefficients between mini-batches. The program ran successfully. The statistics are intended for use in computing the state noise matrix required in the information filter. The second program is the information filter. Most subroutines used in the filter were tested, but the coefficient statistics must be analyzed before the filter is run.

Source record↗

NASA NTRS1981

Far infrared filters for the Galileo-Jupiter and other missions

Progress in the development of FIR multilayer interference filters for the net flux radiometer and photopolarizing radiometer to be carried on board the Galileo mission to Jupiter is reported. The multilayer interference technique has been extended to the region above 40 microns by the use of PbTe/II-VI materials in hard-coated combination, with the thickest layers composed of CdSe QWOT at 74 microns and PbTe QWOT. Improvements have also been obtained in filters below 20 microns on the basis of the Chebyshev stack design. A composite filter cutting on steeply at 40 microns has been designed which employs a thin crystal quartz substrate, shorter wavelength absorption in ZnS and As2S3 thin films, and supplementary multilayer interference. Finally, absorptive filters have been developed based on II-VI compounds in multilayer combination with KRS-5 (or 6) on a KRS-5 (or 6) substrate

Seeley, J. S.↗