SEARCH · Search NASA

Results for “filter”

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 307 records · Page 17

All sources NASA NTRS DOE OSTI

NASA NTRS1973

Properties of multilayer filters

New methods were investigated of using optical interference coatings to produce bandpass filters for the spectral region 110 nm to 200 nm. The types of filter are: triple cavity metal dielectric filters; all dielectric reflection filters; and all dielectric Fabry Perot type filters. The latter two types use thorium fluoride and either cryolite films or magnesium fluoride films in the stacks. The optical properties of the thorium fluoride were also measured.

Baumeister, P. W.↗

NASA NTRS1973

Video-signal improvement using comb filtering techniques.

Significant improvement in the signal-to-noise performance of television signals has been obtained through the application of comb filtering techniques. This improvement is achieved by removing the inherent redundancy in the television signal through linear prediction and by utilizing the unique noise-rejection characteristics of the receiver comb filter. Theoretical and experimental results describe the signal-to-noise ratio and picture-quality improvement obtained through the use of baseband comb filters and the implementation of a comb network as the loop filter in a phase-lock-loop demodulator. Attention is given to the fact that noise becomes correlated when processed by the receiver comb filter.

Arndt, G. D.↗

NASA NTRS1974

A motion-constraint logic for moving-base simulators based on variable filter parameters

A motion-constraint logic for moving-base simulators has been developed that is a modification to the linear second-order filters generally employed in conventional constraints. In the modified constraint logic, the filter parameters are not constant but vary with the instantaneous motion-base position to increase the constraint as the system approaches the positional limits. With the modified constraint logic, accelerations larger than originally expected are limited while conventional linear filters would result in automatic shutdown of the motion base. In addition, the modified washout logic has frequency-response characteristics that are an improvement over conventional linear filters with braking for low-frequency pilot inputs. During simulated landing approaches of an externally blown flap short take-off and landing (STOL) transport using decoupled longitudinal controls, the pilots were unable to detect much difference between the modified constraint logic and the logic based on linear filters with braking.

Miller, G. K., Jr.↗

NASA NTRS1975

Physical correction filter for improving the optical quality of an image

A family of physical correction filters is described. Each filter is designed to correct image content of a photographed scene of limited resolution and includes a first filter element with a pinhole through which light passes to a differential amplifier. A second filter element through which light passes through one or more openings, whose geometric configuration is a function of the cause of the resolution loss included. The light, passing through the second filter element, is also supplied to the differential amplifier whose output is used to activate an optical display or recorder to reproduce a photograph or display of the scene in the original photograph or display of the scene in the original photograph with resolution which is significantly greater than that characterizing the original photograph.

Lee, S. Y.↗

NASA NTRS1974

Drift in interference filters. II - Radiation effects

Studies of peak transmission drift in narrow-band interference filters have shown that there exist two mechanisms that cause drift toward shorter wavelengths. One is dependent on the thermal history of the filter and is discussed in Part 1 of this paper. The other is dependent on the exposure of the filter to radiation. For ZnS-cryolite filters of particular design, it is experimentally demonstrated that the filters are most sensitive to radiation in a 100-A band centered at approximately 3900 A. The drift rate in the focal plane of an f/20 solar image is approximately 3 A/100 hr of exposure. Further, it is also shown by model calculations that the observed radiation-induced drift is consistent with the hypothesis that the optical thickness of ZnS decreases in proportion to the radiant energy absorbed.

Title, A. M.↗

NASA NTRS1976

Performance of the square root information filter for navigation of the Mariner 10 spacecraft

The use is described of a sequential least squares filter in the orbit determination for the Mariner Venus-Mercury (Mariner 10) spacecraft. The orbit determination strategy outlining the use of both the sequential filter and a conventional batch filter is given. Highlighted are the mission events from launch to the first Mercury encounter with emphasis on the sequential filter performance. Advantages to the mission derived from the sequential filter are pointed out.

Christensen, C. S.↗

NASA NTRS1976

Filtering device

An electrical filter for removing noise from a voice communications signal is reported; seven sample values of the signal are obtained continuously, updated and subjected to filtering. Filtering is accomplished by adding balanced, with respect to a mid-point sample, spaced pairs of the sampled values, and then multiplying each pair by a selected filter constant. The signal products thus obtained are summed to provide a filtered version of the original signal.

Edwards, T. R.↗

NASA NTRS1977

Problems in the use of interference filters for spectrophotometric determination of total ozone

An analysis of the use of ultraviolet narrow-band interference filters for total ozone determination is given with reference to the New Zealand filter spectrophotometer under the headings of filter monochromaticity, temperature dependence, orientation dependence, aging, and specification tolerances and nonuniformity. Quantitative details of each problem are given, together with the means used to overcome them in the New Zealand instrument. The tuning of the instrument's filter center wavelengths to a common set of values by tilting the filters is also described, along with a simple calibration method used to adjust and set these center wavelengths.

Basher, R. E.↗

NASA NTRS1978

Tracking performance of unbalanced QPSK demodulators. I - Biphase Costas loop with passive arm filters

Unbalanced quadriphase-shift-keying (QPSK) is an attractive means for transmitting two digital data streams which in general have different average powers, data rates, and data formats. Previous analyses of the tracking performance of Costas loop demodulators of unbalanced QPSK have accounted only for the filtering effect produced by the loop's two arm filters on the equivalent additive noise perturbing the loop. When the bandwidth of these filters is selected on the basis of the order of the data rate, as is typical of optimum Costas loop design, the filtering degradations of the data modulations themselves and the cross-modulation noise produced by their multiplication in the loop often cannot be neglected. The purpose of this paper is to incorporate these additional filtering effects into the analysis. Many of the results obtained herein are in the form of closed-form expressions which can easily be evaluated numerically for design and performance prediction purposes.

Simon, M. K.↗

NASA NTRS1979

Microprocessor realizations of range rate filters

The performance of five digital range rate filters is evaluated. A range rate filter receives an input of range data from a radar unit and produces an output of smoothed range data and its estimated derivative range rate. The filters are compared through simulation on an IBM 370. Two of the filter designs are implemented on a 6800 microprocessor-based system. Comparisons are made on the bases of noise variance reduction ratios and convergence times of the filters in response to simulated range signals.

Source record↗

NASA NTRS1979

Filtering technique based on high-frequency plant modeling for high-gain control

This invention was an improvement in aircraft control systems that utilized feedback motion sensors to generate a signal to control the aircraft. The improvement consisted essentially of a complementary filter comprising a simplified model of the aircraft, a high pass filter, a low pass filter and a summing amplifier. The control signal was applied to the simplified model of the aircraft which attempted to compute the vehicle response to the signal. This computed response was then fed into the high pass filter to eliminate long term errors in the calculated response, with the result that a good estimate of the high frequency content of the aircraft motion was obtained. In order to obtain a good estimate of the low frequency content of the motion, a rate gyro signal was fed through the low pass filter that eliminates all of the offending noise.

Niessen, F. R.↗

NASA NTRS1980

Scale effects: HCMM data simulation. Usage of filtering techniques for scaling-up simulations

Image reduction used to simulate increase in altitude of an acquisition platform is equivalent to data smoothing, and can be achieved either by neighborhood averaging or by filtering techniques. The averaging approach is limited for accurate simulation. A filtering method is described which was based on the hypothesis that all changes due to altitude increase can be represented by a point spread function. Determination of the scale function and factor are discussed as well as implementation of the filtering. Filtering can be either in the spatial or frequency domain. In the spatial domain, filtering consists of the convolution of the image with the weights mask, and then of the declination of the points according to the appropriates scale factor. A simulation of an aircraft day image in the infrared channel is examined.

Digennaro, V.↗

NASA NTRS1980

UDU/T/ covariance factorization for Kalman filtering

There has been strong motivation to produce numerically stable formulations of the Kalman filter algorithms because it has long been known that the original discrete-time Kalman formulas are numerically unreliable. Numerical instability can be avoided by propagating certain factors of the estimate error covariance matrix rather than the covariance matrix itself. This paper documents filter algorithms that correspond to the covariance factorization P = UDU(T), where U is a unit upper triangular matrix and D is diagonal. Emphasis is on computational efficiency and numerical stability, since these properties are of key importance in real-time filter applications. The history of square-root and U-D covariance filters is reviewed. Simple examples are given to illustrate the numerical inadequacy of the Kalman covariance filter algorithms; these examples show how factorization techniques can give improved computational reliability.

Thornton, C. L.↗

NASA NTRS1980

Inversion of multiwavelength radiometer measurements by three-dimensional filtering

Remote sensing data from satellites typically have three dimensions: scan position, spacecraft position, and wavelength. Inversion of the radiometric data to infer geophysical parameters is a filtering problem in which the dimension of wavelength (or channel number) is transformed into a dimension of geophysical parameters, and the most general solution is a three-dimensional filter. Linear filters have the advantages of computational speed and easily described transfer functions; but often the measurements are nonlinear functions of the parameters to be inferred. To the extent that the nonlinear inversion problem is overdetermined, it can be modeled by a critically determined linear problem. As an example, inversion of Scanning Multichannel Microwave Radiometer (SMMR) data by means of a three-dimensional Wiener Filter is described. Atmospheric water vapor content, rain liquid water content, surface wind speed and surface temperature are the parameters inferred from the measurements. Nonprecipitating liquid water and water vapor scale height are also modeled but not retrieved. The a priori statistics on which the filter is trained have the effect of governing the selection of a trade-off point of noise as a function of resolution (in all three retrieval dimensions).

Rosenkranz, P. W.↗

NASA NTRS1981

On the effect of high latitude filtering in global grid point models

The application of various Fourier filtering techniques in a coarse resolution grid point model and the poor simulation of the nonlinear effects responsible for change in the mean zonal flow is discussed. The poor simulation results in spurious energy and momentum transfers. An operational filter, a second order filter on a staggered grid, a fourth order filter on a nonstaggered grid, and a biharmonic diffusion filter were considered.

Takacs, L. L.↗

NASA NTRS1982

Evaluation of spatial filtering on the accuracy of wheat area estimate

A 3 x 3 pixel spatial filter for postclassification was used for wheat classification to evaluate the effects of this procedure on the accuracy of area estimation using LANDSAT digital data obtained from a single pass. Quantitative analyses were carried out in five test sites (approx 40 sq km each) and t tests showed that filtering with threshold values significantly decreased errors of commission and omission. In area estimation filtering improved the overestimate of 4.5% to 2.7% and the root-mean-square error decreased from 126.18 ha to 107.02 ha. Extrapolating the same procedure of automatic classification using spatial filtering for postclassification to the whole study area, the accuracy in area estimate was improved from the overestimate of 10.9% to 9.7%. It is concluded that when single pass LANDSAT data is used for crop identification and area estimation the postclassification procedure using a spatial filter provides a more accurate area estimate by reducing classification errors.

Dejesusparada, N.↗

NASA NTRS1981

An optimal numerical filter for wide-field-of-view measurements of earth-emitted radiation

A technique is described in which all data points along an arc of the orbit may be used in an optimal numerical filter for wide-field-of-view measurements of earth emitted radiation. The statistical filter design is derived whereby the filter is required to give a minimum variance estimate of the radiative exitance at discrete points along the ground track of the satellite. An equation for the optimal numerical filter is given by minimizing the estimate error variance equation with respect to the filter weights, resulting in a discrete form of the Wiener-Hopf equation. Finally, variances of the errors in the radiant exitance can be computed along the ground track and in the cross track directions.

Smith, G. L.↗

NASA NTRS1982

A model for radar images and its application to adaptive digital filtering of multiplicative noise

Standard image processing techniques which are used to enhance noncoherent optically produced images are not applicable to radar images due to the coherent nature of the radar imaging process. A model for the radar imaging process is derived in this paper and a method for smoothing noisy radar images is also presented. The imaging model shows that the radar image is corrupted by multiplicative noise. The model leads to the functional form of an optimum (minimum MSE) filter for smoothing radar images. By using locally estimated parameter values the filter is made adaptive so that it provides minimum MSE estimates inside homogeneous areas of an image while preserving the edge structure. It is shown that the filter can be easily implemented in the spatial domain and is computationally efficient. The performance of the adaptive filter is compared (qualitatively and quantitatively) with several standard filters using real and simulated radar images.

Frost, V. S.↗