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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

An optical filtering system for remote sensing of phytoplankton and suspended sediment

Aerial photography revealing blue-green Anacystis phytoplankton blooms in the Potomac River is presented. Data scanned were near-IR, green, and yellow nadir radiance; concurrent ground measurements are correlated with these data, including salinity, Secchi disk depth, chlorophyll a concentration, and river depth. In nonbloomed areas, R (radiance) is proportional to the reciprocal of Secchi depth for the green and yellow filters; in bloomed areas, R is constant for the green filter and random for the near-IR and yellow filters.

Bressette, W. E.↗
NASA NTRS1975

A prefiltering version of the Kalman filter with new numerical integration formulas for Riccati equations

A prefiltering version of the Kalman filter is derived for both discrete and continuous measurements. The derivation consists of determining a single discrete measurement that is equivalent to either a time segment of continuous measurements or a set of discrete measurements. This prefiltering version of the Kalman filter easily handles numerical problems associated with rapid transients and ill-conditioned Riccati matrices. Therefore, the derived technique for extrapolating the Riccati matrix from one time to the next constitutes a new set of integration formulas which alleviate ill-conditioning problems associated with continuous Riccati equations. Furthermore, since a time segment of continuous measurements is converted into a single discrete measurement, Potter's square root formulas can be used to update the state estimate and its error covariance matrix. Therefore, if having the state estimate and its error covariance matrix at discrete times is acceptable, the prefilter extends square root filtering with all its advantages, to continuous measurement problems.

Womble, M. E.↗
NASA NTRS1977

Circulating-lines digital filter

Filter has array of line segments of various lengths which may be switched into circulating line by machine operator. Design is useful in cases where filter speed is not critical; by sacrificing speed, filter can be made at lower cost.

Zohar, S.↗
NASA NTRS1977

RAM digital filter

Modification of conventional digital counting filter is designed to store all possible combinations of filter coefficients in random access memory. Filter includes analog-to-digital coverter, X shift register, memory, accumulator, and digital-to-analog converter.

Zohar, S.↗
NASA NTRS1976

Low-cost mechanical filters for OMEGA receivers

A pair of prototype low frequency mechanical filters were obtained for use as the RF front-end components of an OMEGA-VLF navigation receiver. The filter units are of interest because of very narrow bandwidths and high skirt selectivity to minimize noise and off-channel carriers in the reception of OMEGA signals. In addition, the filters have a characteristic low impedance of 75 to 5,000 ohms which results in less critical PC board circuitry compared to some previous resonators with termination resistances of 25,000 ohms to 5 megohms.

Burhans, R. W.↗
NASA NTRS1975

Fixed memory least squares filtering

Buxbaum has reported on three algorithms for computing least squares estimates that are based on fixed amounts of data. In this correspondence, the filter is arranged as a point-deleting Kalman filter concatenated with the standard point-inclusion Kalman filter. The resulting algorithm is couched in a square root framework for greater numerical stability, and special attention is given to computer implementation.

Bierman, G. J.↗
NASA NTRS1976

Linear minimum variance filters applied to carrier tracking

A new approach is taken to the problem of tracking a fixed amplitude signal with a Brownian-motion phase process. Classically, a first-order phase-lock loop (PLL) is used; here, the problem is treated via estimation of the quadrature signal components. In this space, the state dynamics are linear with white multiplicative noise. Therefore, linear minimum-variance filters, which have a particularly simple mechanization, are suggested. The resulting error dynamics are linear at any signal/noise ratio, unlike the classical PLL. During synchronization, and above threshold, this filter with constant gains degrades by 3 per cent in output rms phase error with respect to the classical loop. However, up to 80 per cent of the maximum possible noise improvement is obtained below threshold, where the classical loop is nonoptimum, as demonstrated by a Monte Carlo analysis. Filter mechanizations are presented for both carrier and baseband operation.

Gustafson, D. E.↗
NASA NTRS1977

Electrically-controlled variable-color optical filters

Optical transmission characteristics of birefringent element are changed by applying voltage to plate. Filters are used on sequential-color television cameras and in color displays. Filters are more convenient and less costly than mechanical color wheels and crystal-based filters.

Miller, A.↗
NASA NTRS1975

A summary of nonlinear filtering

The formation of nonlinear filtering problems are discussed along with approaches to solving nonlinear filtering problems. An approach to the design and evaluation of the performance of suboptimal filters is presented in terms of upper and lower bounds.

Rhodes, I. B.↗
NASA NTRS1976

Determination of the energy or pressure of a solar X-ray structure using X-ray filtergrams from a single filter

While the X-ray fluxes measured through two different broadband filters can be used to derive the temperatures and emission measures of solar plasmas, it is generally assumed that no plasma parameters can be measured quantitatively with a single filtergram alone. It is shown that the thermal energy density (or pressure) of a solar plasma can be measured with an ideal single filtergram. Calculations have been done to compare the properties of the Skylab S-054 X-ray telescope filters with those of the ideal filter.

Kahler, S.↗
NASA NTRS1978

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 that these problems can 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 possibly eliminate the instability. These filtering procedures are equivalent to a weighted average of conservation and nonconservation differencing. The entire spectrum of filtered equations retains a three point character as well as second order spatial accuracy. Burgers equation was considered as a model.

Khosla, P. K.↗
NASA NTRS1978

Requirements for Kalman filtering on the GE-701 whole word computer

The results of a study to determine scaling, storage, and word length requirements for programming the Kalman filter on the GE-701 Whole Word Computer are reported. Simulation tests are presented which indicate that the Kalman filter, using a square root formulation with process noise added, utilizing MLS, radar altimeters, and airspeed as navigation aids, may be programmed for the GE-701 computer to successfully navigate and control the Boeing B737-100 during landing approach, landing rollout, and turnoff. The report contains flow charts, equations, computer storage, scaling, and word length recommendations for the Kalman filter on the GE-701 Whole Word computer.

Pines, S.↗
NASA NTRS1978

Development and flight tests of a Kalman filter for navigation during terminal area and landing operations

A Kalman filter for aircraft terminal area and landing navigation was implemented and flight tested in the NASA Ames STOLAND avionics computer onboard a Twin Otter aircraft. This system combines navaid measurements from TACAN, MODILS, air data, radar altimeter sensors along with measurements from strap-down accelerometer and attitude angle sensors. The flight test results demonstrate that the Kalman filter provides improved estimates of the aircraft position and velocity as compared with estimates from the more standard complementary filter. The onboard computer implementation requirements to achieve this improved performance are discussed.

Schmidt, S. F.↗
NASA NTRS1977

An application of the square root information filter to large scale linear interconnected systems

It is demonstrated that use of the square root information filter (SRIF) can reduce the storage and computation required for estimation of certain classes of large-scale interconnected systems. The SRIF uses an information array that is related to the Kalman filter covariance and estimate. The SRIF algorithm, which is optimal, is a direct application of matrix partitioning to some optimal filtering algorithms described in the literature. The SRIF algorithm is able to reduce the storage requirements of a 40-subsystem 10-state problem by a full order of magnitude.

Bierman, G. J.↗
NASA NTRS1978

Performance analysis of a filtered wide field-of-view radiometer for earth radiation budget measurements

The proposed Earth Radiation Budget Satellite System (ERBSS) of the 1980's will include a wide field-of-view (WFOV) fixed axes earth radiator discriminator consisting of a shortwave channel and a total (unfiltered) channel. The broadband spectral isolation required for the shortwave channel is achieved by use of a hemispherical fused silica (Suprasil W) dome filter placed in front of a wire wound thermopile radiation detector. A description is presented of the thermal response of the single-fused silica dome filter in the ERBSS WFOV shortwave channel conceptual design and the impact of that response on the channel measurement. Results from design definition and performance analysis studies are included. Problems associated with achieving the desired levels of confidence in a high accuracy filtered, WFOV radiometer are discussed. Design approaches, ground calibration, and data reduction techniques which minimize measurement uncertainties are explained.

Cooper, J. E.↗
NASA NTRS1979

The discrete-time compensated Kalman filter

A suboptimal dynamic compensator to be used in conjunction with the ordinary discrete-time Kalman filter is derived. The resultant compensated Kalman filter has the property that steady-state bias estimation errors, resulting from modelling errors, are eliminated. The implementation of the compensated Kalman filter involves the use of accumulators in the residual channels in addition to the nominal dynamic model of the stochastic system.

Lee, W.-H.↗
NASA NTRS1980

Aliasing filter for multirate systems

Rolloff filter is inexpensive way of reducing aliasing in digital control systems. Rolloff filter operating at faster sample rate (or rates) of system with 2:1 rate ratio gives infinite attenuation at half-sample rate of fast-rate loop. Tested successfully on Space Shuttle primary flight-control systems, filter technique could be applied to other multirate sampled-data systems.

Lee, J. F. L.↗
NASA NTRS1980

Design of a nonlinear adaptive filter for suppression of shuttle pilot-induced oscillation tendencies

Analysis of a longitudinal pilot-induced oscillation (PIO) experienced just prior to touchdown on the final flight of the space shuttle's approach landing tests indicated that the source of the problem was a combination of poor basic handling qualities aggravated by time delays through the digital flight control computer and rate limiting of the elevator actuators due to high pilot gain. A nonlinear PIO suppression (PIOS) filter was designed and developed to alleviate the vehicle's PIO tendencies by reducing the gain in the command path. From analytical and simulator studies it was shown that the PIOS filter, in an adaptive fashion, can attenuate the command path gain without adding phase lag to the system. With the pitch attitude loop of a simulated shuttle model closed, the PIOS filter increased the gain margin by a factor of about two.

Smith, J. W.↗