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At least 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Tracking performance of unbalanced QPSK demodulators. II - Biphase Costas loop with active arm filters

In a Costas loop study for biphase modulation conducted by Simon and Lindsey (1977), it was demonstrated that considerable improvement in tracking performance could be obtained by employing active arm filters of the integrate-and-dump type as opposed to passive arm filters. An investigation is conducted concerning the possibility to obtain a similar performance improvement for an unbalanced quadriphase-shift-keying (QPSK) modulation. It is found that the biphase Costas loop can be used as an efficient demodulator of QPSK in cases in which the ratio of data rates is of the same order of magnitude as the inverse of the power ratio. These cases involve approximately equal signal energies in the two channels.

Simon, M. K.↗
NASA NTRS1979

Computation-saving digital filter

Two stage digital low-pass filter circuit that averages input over given period and filters average over comparatively slow rate, reduces computation, speed, and word-length requirements. Applications include data preprocessing before entry to central processor.

Sowada, D. J.↗
NASA NTRS1980

CMOS analog switches for adaptive filters

Adaptive active low-pass filters incorporate CMOS (Complimentary Metal-Oxide Semiconductor) analog switches (such as 4066 switch) that reduce variation in switch resistance when filter is switched to any selected transfer function.

Dixon, C. E.↗
NASA NTRS1978

Partial polarizer filter

A birefringent filter module comprises, in seriatum. (1) an entrance polarizer, (2) a first birefringent crystal responsive to optical energy exiting the entrance polarizer, (3) a partial polarizer responsive to optical energy exiting the first polarizer, (4) a second birefringent crystal responsive to optical energy exiting the partial polarizer, and (5) an exit polarizer. The first and second birefringent crystals have fast axes disposed + or -45 deg from the high transmitivity direction of the partial polarizer. Preferably, the second crystal has a length 1/2 that of the first crystal and the high transmitivity direction of the partial polarizer is nine times as great as the low transmitivity direction. To provide tuning, the polarizations of the energy entering the first crystal and leaving the second crystal are varied by either rotating the entrance and exit polarizers, or by sandwiching the entrance and exit polarizers between pairs of half wave plates that are rotated relative to the polarizers. A plurality of the filter modules may be cascaded.

Title, A. M.↗
NASA NTRS1979

Pre-A/D filter and AGC requirements for multimegabit telemetry data detection

A candidate pre-Analog to Digital filter bandwidth versus data rate design is presented for the multimegabit telemetry demodulator/detector. The design is based on considerations of Analog to Digital bias, quantization errors, and automatic gain control effects, as well as the deleterious effects of filtering. Two methods of gain control of the input level to the analog to Digital converter are considered. The first method uses a particular value of gain, according to which bandwidth is selected. The second method uses a second narrowband noncoherent Automatic Gain Controls (in the LPF bandwidth) to attempt to keep the Analog to Digital input level constant. This second method reduces the bit error rate degradation slightly but appears to be more difficult to implement.

Holmes, J. K.↗
NASA NTRS1979

Effects of asymmetric passband filtering on the phase of the Costas loop's reconstructed carrier

The reconstructed carrier of a telemetry return signal is used in deriving the Doppler and range information in the radiometric systems. When suppressed carrier BPSK signalling with Costas loop demodulation is used, there are concerns on the amount of shift in the reconstructed carrier phase, when the received signal suffers asymmetric bandpass filtering through the various stages of the receiver. It is concluded that the phase shifts due to asymmetric bandpass filtering on the Costas loop's reconstructed carrier can be slightly worse than those suffered by the residual carrier loop's reconstructed carrier. However, they are well within the error budgets of the radiometric system.

Woo, K. T.↗
NASA NTRS1980

Improved particulate-sampling filter

Small surface indentations entrain larger and more representative sampling than conventional petri-dish smeared with smooth layer adhesive. Filter is assembled from perforated disk and flat backing plate with sticky surface. Due to design-created currents, particulates come in contact with surface for longer time and have greater probability of being trapped. Filter is useful in air-quality monitoring at industrial sites, in mines, and in and around nuclear power plants.

Hoffman, A. R.↗
NASA NTRS1981

Converting a digital filter to its analog equivalent

Two complementary methods for conversion are direct conversion method and inverse of Tustin's method. Required accuracy of filter is achieved using best-matched technique. Both require only direct computations and are simpler and more efficient than conventional iterative systems or methods requiring "ad hoc" filter parameter adjustment.

Lee, J. F. L.↗
NASA NTRS1980

Sulfate and nitrate collected by filter sampling near the tropopause

Filter samples collected near the tropopause with an F-106 aircraft and two Boeing 747 aircraft were analyzed for sulfate and nitrate ion content. Within the range of routine commercial flight altitudes (at or below 12.5 km), stratospheric mass mixing ratios for the winter-spring group averaged 0.26 ppbm for sulfate and 0.35 ppbm for nitrate. For the summer-fall group, stratosphere mixing ratios averaged 0.13 ppbm and 0.25 ppbm for sulfate and nitrate, respectively. Winter-spring group tropospheric mass mixing ratios averaged 0.08 ppbm for sulfate and 0.10 ppbm for nitrate, while summer-fall group tropospheric mixing ratios averaged 0.05 ppbm for sulfate and 0.08 ppbm for nitrate. Correlations of the filter data with available ozone data suggest that the sulfate and nitrate are transported from the stratosphere to the troposphere.

Humenik, F. M.↗
NASA NTRS1980

Active notch filter network with variable notch depth, width and frequency

An active notch filter having independently adjustable notch frequency, width, and depth is provided by three equal capacitors connected in series with an operational amplifier (connected in a voltage follower configuration), a potentiometer across the series connected capacitors for notch depth adjustment, and a potentiometer (for notch frequency connected across the center capacitor); with its tap connected to receive a voltage feedback signal from a variable voltage divider comprised of another potentiometer for notch width. Adjusting the voltage dividing potentiometer will independently set the notch width, and adjusting the tap on the potentiometer across the center capacitor will independently adjust the notch frequency of the filter. A second operational amplifier connected in a voltage follower configuration may be used to connect the voltage divider output to the adjustable tap of the potentiometer across the center capacitor.

Black, J. M.↗
NASA NTRS1979

Temperature profile retrievals with extended Kalman-Bucy filters

The Extended Kalman-Bucy Filter is a powerful technique for estimating non-stationary random parameters in situations where the received signal is a noisy non-linear function of those parameters. A practical causal filter for retrieving atmospheric temperature profiles from radiances observed at a single scan angle by the Scanning Microwave Spectrometer (SCAMS) carried on the Nimbus 6 satellite typically shows approximately a 10-30% reduction in rms error about the mean at almost all levels below 70 mb when compared with a regression inversion.

Ledsham, W. H.↗
NASA NTRS1980

Performance of fused silica as a filter in a wide field-of-view earth radiation budget radiometer

The thermal response of the fused silica dome filter in an earth radiation budget WFOV shortwave channel conceptual design and the impact of that response on the channel measurement, is described. Attention is given to results from design definition and performance analysis studies. Consideration is given to problems associated with achieving the desired levels of confidence in a high accuracy filtered earth radiation budget WFOV radiometer. Finally, design approaches, ground calibration, and data reduction techniques that minimize measurement uncertainties are covered.

Cooper, J. E.↗
NASA NTRS1979

Asymptotic eigenstructures of filters

This paper describes relationships between Kalman-Bucy filter design parameters and the asymptotic eigenstructures of the resulting error dynamics. Simple, explicit formulas are developed which select noise intensity matrices to achieve pre-specified eigenstructures. The results also add basic understanding of the asymptotic properties of filters, particularly with respect to recently proposed robustness recovery processes.

Stein, G.↗
NASA NTRS1980

Distributed parameter system coupled ARMA expansion identification and adaptive parallel IIR filtering - A unified problem statement

A novel interconnection of distributed parameter system (DPS) identification and adaptive filtering is presented, which culminates in a common statement of coupled autoregressive, moving-average expansion or parallel infinite impulse response configuration adaptive parameterization. The common restricted complexity filter objectives are seen as similar to the reduced-order requirements of the DPS expansion description. The interconnection presents the possibility of an exchange of problem formulations and solution approaches not yet easily addressed in the common finite dimensional lumped-parameter system context. It is concluded that the shared problems raised are nevertheless many and difficult.

Johnson, C. R., Jr.↗
NASA NTRS1981

Spatial and temporal filtering technique for processing lidar photocount data

Shot noise places a practical limit on the spatial and temporal resolution of lidar photocount data. A 2-D signal-processing technique that utilizes spatial and temporal filtering to reduce shot noise and increase resolution is described. The technique is applied to sodium lidar data collected during the fall of 1979 over Urbana, Illinois. Temporal filtering is shown to enhance the spatial resolution of the sodium profiles significantly by reducing shot noise by more than 10 dB. The signal-processing technique is applicable to a wide variety of lidar data.

Gardner, C. S.↗
NASA NTRS1981

Near millimeter wave bandpass filters

The properties of bandpass filters for broadband photometry are reported in the 3-12/cm frequency range. The filters are based on a combination of capacitive grids deposited on thick Mylar substrates and are designed to have very high out-of-band rejection. Low frequencies are blocked by a thick grill that consists of a hexagonal grid of circular holes in a thick metal plate.

Timusk, T.↗
NASA NTRS1982

Kalman filtering for spacecraft attitude estimation

Several schemes in current use for sequential estimation of spacecraft attitude using Kalman filters are examined. These differ according to their treatment of the attitude error, namely: using the complete four-component quaternion; using a truncated quaternion in which one of the components has been eliminated; or using a quaternion referred to approximate body-fixed axes. These schemes are examined for the case of a spacecraft carrying line-of-sight attitude sensors and three-axis gyros whose measurements are corrupted by noise on both the drift rate and the drift-rate ramp. The analysis of the covariance is carried out in detail. The historical development of Kalman filtering of attitude is reviewed.

Lefferts, E. J.↗