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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Measured and computed performance of a microstrip filter composed of semi-insulating GaAs on a fused quartz substrate

The performance of a microstrip hammerhead filter that has been fabricated on an electrically thin layer of semiinsulating GaAs backed by a fused quartz substrate was measured and compared to results of a three-dimensional finite-difference time-domain (FD-TD) program used to calculate the response of the filter both with and without the GaAs layer. The program, presented by Sheen et al. (1990), discretizes the entire structure and then simulates the propagation of a Gaussian pulse through the filter. The microstrip filter is intended for applications involving ultrathin lifted-off or etched-back GaAs containing both active devices and passive microstrip circuitry backed by a much thicker mechanically rigid low-loss, low-dielectric-constant substrate. The low-pass characteristics of the hammerhead filter with the intermediate GaAs layer are compared with those of the same filter on quartz alone. Both the measured and computed data show a significant shift in cutoff frequency (about 10 percent at the 3 dB points) for a GaAs layer that is 0.007 wavelengths thick at 4 GHz.

Siegel, Peter H.↗
NASA NTRS1990

Rapid traversal of an image database using binary synthetic discriminant filters

Pattern recognition invariant to image rotations of up to 75 deg, using a single filter, has been demonstrated for binary synthetic discriminant function (BSDF) optical filters, suggesting their use in directed graph-arranged data bases which can be rapidly traversed by means of a filter-plane programmable spatial light modulator (SLM). The filter data base is arranged as a tree structure in which the root node filters are invariant to over 60 deg rotation, while the leaves are 5-deg invariant. Results are presented from experiments with BSDFs designed to recognize in-plane-rotated views of a Space Shuttle Orbiter. Using a magnetooptic SLM that is driven at 350 Hz in the filter plane, orientation identifications requiring less than 30 msec have been achieved after sequencing through only 10 BSDFs.

Reid, Max B.↗
NASA NTRS1990

Optical calculation of correlation filters

A method is presented for designing optical correlation filters based on measuring intensity patterns in the Fourier plane. The method can be used in general to design any type of frequency plane filter but is especially attractive in the case of a binary phase only filter (BPOF). The method can produce a filter that is well matched to both the object, its transforming optical system and the spatial light modulator used in the correlator input plane. A working filter was produced using the technique, but the filter response was weak. The suspected causes of the weak output are imprecise Fourier plane positioning, nonlinearities in the recording process, and/or aliasing effects.

Knopp, Jerome↗
NASA NTRS1991

Boundary implications for frequency response of interval FIR and IIR filters

It is shown that vertex implication results in parameter space apply to interval trigonometric polynomials. Subsequently, it is shown that the frequency responses of both interval FIR and IIR filters are bounded by the frequency responses of certain extreme filters. The results apply directly in the evaluation of properties of designed filters, especially because it is more realistic to bound the filter coefficients from above and below instead of determining those with infinite precision because of finite arithmetic effects. Illustrative examples are provided to show how the extreme filters might be easily derived in any specific interval FIR or IIR filter design problem.

Bose, N. K.↗
NASA NTRS1993

Evaluation of multichannel Wiener filters applied to fine resolution passive microwave images of first-year sea ice

The effectivity of multichannel Wiener filters to improve images obtained with passive microwave systems was investigated by applying Wiener filters to passive microwave images of first-year sea ice. Four major parameters which define the filter were varied: the lag or pixel offset between the original and the desired scenes, filter length, the number of lines in the filter, and the weight applied to the empirical correlation functions. The effect of each variable on the image quality was assessed by visually comparing the results. It was found that the application of multichannel Wiener theory to passive microwave images of first-year sea ice resulted in visually sharper images with enhanced textural features and less high-frequency noise. However, Wiener filters induced a slight blocky grain to the image and could produce a type of ringing along scan lines traversing sharp intensity contrasts.

Full, William E.↗
NASA NTRS1991

A 3-D nonlinear recursive digital filter for video image processing

This paper introduces a recursive 3-D nonlinear digital filter, which is capable of performing noise suppression without degrading important image information such as edges in space or time. It also has the property of unnoticeable bandwidth reduction immediately after a scene change, which makes the filter an attractive preprocessor to many interframe compression algorithms. The filter consists of a nonlinear 2-D spatial subfilter and a 1-D temporal filter. In order to achieve the required computational speed and increase the flexibility of the filter, all of the linear shift-variant filter modules are of the IIR type.

Bauer, P. H.↗
NASA NTRS1993

Experiences with thin film filter development for the Extreme Ultraviolet Explorer

The design, development, and optimization of the thin film filters used on the Extreme Ultraviolet Explorer (EUVE) Satellite to define the EUV wavelength bandpasses of the individual instruments was a complicated task. The bandpasses had to be optimized for the astrophysical goals of the EUVE mission and constrained by the strong geocoronal EUV background emission. Materials with optical constants that met these requirements had to be found and tested. In many cases these materials were not compatible or were not strong enough to survive the intense vibrations of a rocket launch. Other effects, such as photoelectron 'halo' produced in the filters, were not discovered until flight qualification. The final set of flight filters included: lexan/boron, aluminum/carbon, titanium/antimony/aluminum, and tin/silicon monoxide. This paper discusses the lessons learned in the development of these filters, including the optimization process, material interactions and problems, calibration techniques, vibration susceptibility, thermal tests, and photoelectron emission. We feel the experiences gained over the last 10 years creating the filter sets for EUVE will be invaluable for future missions that use thin film filters.

Vallerga, J. V.↗
NASA NTRS1993

Order-sorting filter transmittance measured with an array detector

The simultaneous measurement of the spectrally and spatially variant transmittance of a linear variable order-sorting filter in a manner that closely resembles its conditions of actual use is described. The transmittance of a prototype order-sorting filter was measured in the 400- to 880-nm wavelength region by illuminating it with the output beam of a spectrophotometer while the filter was attached to the front of a 30 x 32 pixel silicon array detector. The filter was designed to be used in the output beam of a grating spectrometer to prevent the dispersal of higher diffracted orders onto an array detector. Areas of the filter that were spatially matched to the corresponding detector pixel column had measured peak transmittances of about 90 percent that were uniform to within +/- 1.5 percent along a given column. Transmittances for incident wavelengths shorter than the desired bandpass, corresponding to the order overlap region, were measured in the 0.003 range. Line spread function measurements made with the array detector indicated no significant beam spreading caused by inserting the filter into the beam.

Heaney, James B.↗
NASA NTRS1993

Multirate and event-driven Kalman filters for helicopter flight

A vision-based obstacle detection system that provides information about objects as a function of azimuth and elevation is discussed. The range map is computed using a sequence of images from a passive sensor, and an extended Kalman filter is used to estimate range to obstacles. The magnitude of the optical flow that provides measurements for each Kalman filter varies significantly over the image depending on the helicopter motion and object location. In a standard Kalman filter, the measurement update takes place at fixed intervals. It may be necessary to use a different measurement update rate in different parts of the image in order to maintain the same signal to noise ratio in the optical flow calculations. A range estimation scheme that accepts the measurement only under certain conditions is presented. The estimation results from the standard Kalman filter are compared with results from a multirate Kalman filter and an event-driven Kalman filter for a sequence of helicopter flight images.

Sridhar, Banavar↗
NASA NTRS1993

Spacelab J air filter debris analysis

Filter debris from the Spacelab module SLJ of STS-49 was analyzed for microbial contamination. Debris for cabin and avionics filters was collected by Kennedy Space Center personnel on 1 Oct. 1992, approximately 5 days postflight. The concentration of microorganisms found was similar to previous Spacelab missions averaging 7.4E+4 CFU/mL for avionics filter debris and 4.5E+6 CFU/mL for the cabin filter debris. A similar diversity of bacterial types was found in the two filters. Of the 13 different bacterial types identified from the cabin and avionics samples, 6 were common to both filters. The overall analysis of these samples as compared to those of previous missions shows no significant differences.

Obenhuber, Donald C.↗
NASA NTRS1992

Recursive time-varying filter banks for subband image coding

Filter banks and wavelet decompositions that employ recursive filters have been considered previously and are recognized for their efficiency in partitioning the frequency spectrum. This paper presents an analysis of a new infinite impulse response (IIR) filter bank in which these computationally efficient filters may be changed adaptively in response to the input. The filter bank is presented and discussed in the context of finite-support signals with the intended application in subband image coding. In the absence of quantization errors, exact reconstruction can be achieved and by the proper choice of an adaptation scheme, it is shown that IIR time-varying filter banks can yield improvement over conventional ones.

Smith, Mark J. T.↗
NASA NTRS1993

Insertion loss and noise-temperature contribution of high-temperature superconducting bandpass filters centered at 2.3 and 8.45 GHz

Two superconducting Tl-Ca-Cu-Ba-O bandpass filters were fabricated for JPL by Superconductor Technologies Incorporated, Santa Barbara, California. The filters were designed to operate at 2.3 GHz (S-band) with a 0.5-dB bandwidth of 60 MHz and at 8.45 GHz (X-band) with a 0.5-dB bandwidth of 150 MHz. The structure selected for both filters incorporates half-wavelength thin-film resonators in a stripline configuration. The S-band filter uses an edge-coupled interdigital design and the X-band filter uses an end-coupled design. The insertion loss and the noise-temperature contribution were measured at 12 K for both filters.

Prater, J. L.↗
NASA NTRS1994

Progress in navigation filter estimate fusion and its application to spacecraft rendezvous

A new derivation of an algorithm which fuses the outputs of two Kalman filters is presented within the context of previous research in this field. Unlike other works, this derivation clearly shows the combination of estimates to be optimal, minimizing the trace of the fused covariance matrix. The algorithm assumes that the filters use identical models, and are stable and operating optimally with respect to their own local measurements. Evidence is presented which indicates that the error ellipsoid derived from the covariance of the optimally fused estimate is contained within the intersections of the error ellipsoids of the two filters being fused. Modifications which reduce the algorithm's data transmission requirements are also presented, including a scalar gain approximation, a cross-covariance update formula which employs only the two contributing filters' autocovariances, and a form of the algorithm which can be used to reinitialize the two Kalman filters. A sufficient condition for using the optimally fused estimates to periodically reinitialize the Kalman filters in this fashion is presented and proved as a theorem. When these results are applied to an optimal spacecraft rendezvous problem, simulated performance results indicate that the use of optimally fused data leads to significantly improved robustness to initial target vehicle state errors. The following applications of estimate fusion methods to spacecraft rendezvous are also described: state vector differencing, and redundancy management.

Carpenter, J. Russell↗
NASA NTRS1994

Groundspeed filtering for CTAS

Ground speed is one of the radar observables which is obtained along with position and heading from NASA Ames Center radar. Within the Center TRACON Automation System (CTAS), groundspeed is converted into airspeed using the wind speeds which CTAS obtains from the NOAA weather grid. This airspeed is then used in the trajectory synthesis logic which computes the trajectory for each individual aircraft. The time history of the typical radar groundspeed data is generally quite noisy, with high frequency variations on the order of five knots, and occasional 'outliers' which can be significantly different from the probable true speed. To try to smooth out these speeds and make the ETA estimate less erratic, filtering of the ground speed is done within CTAS. In its base form, the CTAS filter is a 'moving average' filter which averages the last ten radar values. In addition, there is separate logic to detect and correct for 'outliers', and acceleration logic which limits the groundspeed change in adjacent time samples. As will be shown, these additional modifications do cause significant changes in the actual groundspeed filter output. The conclusion is that the current ground speed filter logic is unable to track accurately the speed variations observed on many aircraft. The Kalman filter logic however, appears to be an improvement to the current algorithm used to smooth ground speed variations, while being simpler and more efficient to implement. Additional logic which can test for true 'outliers' can easily be added by looking at the difference in the a priori and post priori Kalman estimates, and not updating if the difference in these quantities is too large.

Slater, Gary L.↗
NASA NTRS1996

Canonical Signed Digit Study: FIR Digital Filter Simulation Results - Part 2

Finite Impulse Response digital filter using Canonical Signed-Digit (CSD) number representation for the coefficients has been studied and its computer simulation results are presented here. Minimum Mean Square Error (MMSE) criterion is employed to optimize filter coefficients into the corresponding CSD numbers. To further improve coefficients optimization process, an extra non-zero bit is added for any filter coefficients exceeding 1/2. This technique improves frequency response of filter without increasing filter complexity almost at all. The simulation results show outstanding performance in bit-error-rate (BER) curve for all CSD implemented digital filters included in this presentation material.

Kim, Heechul↗
NASA NTRS2001

Fixed-frequency and Frequency-agile (au, HTS) Microstrip Bandstop Filters for L-band Applications

In this work, we report on the performance of a highly selective, compact 1.83 x 2.08 cm(exp 2) (approx. 0.72 x 0.82 in(exp 2) microstrip line bandstop filter of YBa2CU3O(7-delta) (YBCO) on LaAlO3 (LAO) substrate. The filter is designed for a center frequency of 1.623 GHz for a bandwidth at 3 dB from reference baseline of less than 5.15 MHz, and a bandstop rejection of 30 dB or better. The design and optimization of the filter was performed using Zeland's IE3D circuit simulator. The optimized design was used to fabricate gold (Au) and High-Temperature Superconductor (HTS) versions of the filter. We have also studied an electronically tunable version of the same filter. Tunability of the bandstop characteristics is achieved by the integration of a thin film conductor (Au or HTS) and the nonlinear dielectric ferroelectric SrTiO3 in a conductor/ferroelectric/dielectric modified microstrip configuration. The performance of these filters and comparison with the simulated data will be presented.

Saenz, Eileen M.↗
NASA NTRS1993

Water Filters

The Aquaspace H2OME Guardian Water Filter, available through Western Water International, Inc., reduces lead in water supplies. The filter is mounted on the faucet and the filter cartridge is placed in the "dead space" between sink and wall. This filter is one of several new filtration devices using the Aquaspace compound filter media, which combines company developed and NASA technology. Aquaspace filters are used in industrial, commercial, residential, and recreational environments as well as by developing nations where water is highly contaminated.

Source record↗
NASA NTRS1997

Relative Navigation In Elliptical Orbits Using An Iterative Nonlinear Filter

The two step filter is applied to process intersatellite radar measurements to determine the motion of one satellite relative to another in close elliptical orbits. This filter breaks a nonlinear estimation problem into two state vectors. The 'first step' state is chosen so as to have a linear measurement equation. This is nonlinearly related to the 'second step' state which describes the dynamics. Two different forms are used. In one, the first step state is the second step state vector augmented by the measurement equation. In the other, the first step and second step state vectors are of equal dimension. The two step filter is compared against an iterated extended Kalman filter and a Kalman filter using a change of variables. Analytical differences between the two step estimator and these conventional filters are highlighted. Special concerns for initializing the first step state covariance matrix and handling the possibility of numerically rank deficient covariance matrices are addressed. Numerical simulations are performed which show that the Two Step estimator produces a lower estimation bias under two circumstances; large apriori initial error; and small dimension observation vectors which require a longer arc of measurements to generate observability of the state.

Garrison, James L.↗