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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Effect of signal jitter on the spectrum of rotor impulsive noise

The effect of randomness or jitter of the acoustic waveform on the spectrum of rotor impulsive noise is studied because of its importance for data interpretation. An acoustic waveform train is modelled representing rotor impulsive noise. The amplitude, shape, and period between occurrences of individual pulses are allowed to be randomized assuming normal probability distributions. Results, in terms of the standard deviations of the variable quantities, are given for the autospectrum as well as special processed spectra designed to separate harmonic and broadband rotor noise components. Consideration is given to the effect of accuracy in triggering or keying to a rotor one per revolution signal. An example is given showing the resultant spectral smearing at the high frequencies due to the pulse signal period variability.

Brooks, Thomas F.↗
NASA NTRS1987

Synthetic aperture radar signal processing on the MPP

Satellite-borne Synthetic Aperture Radars (SAR) sense areas of several thousand square kilometers in seconds and transmit phase history signal data several tens of megabits per second. The Shuttle Imaging Radar-B (SIR-B) has a variable swath of 20 to 50 km and acquired data over 100 kms along track in about 13 seconds. With the simplification of separability of the reference function, the processing still requires considerable resources; high speed I/O, large memory and fast computation. Processing systems with regular hardware take hours to process one Seasat image and about one hour for a SIR-B image. Bringing this processing time closer to acquisition times requires an end-to-end system solution. For the purpose of demonstration, software was implemented on the present Massively Parallel Processor (MPP) configuration for processing Seasat and SIR-B data. The software takes advantage of the high processing speed offered by the MPP, the large Staging Buffer, and the high speed I/O between the MPP array unit and the Staging Buffer. It was found that with unoptimized Parallel Pascal code, the processing time on the MPP for a 4096 x 4096 sample subset of signal data ranges between 18 and 30.2 seconds depending on options.

Ramapriyan, H. K.↗
NASA NTRS1987

SMI adaptive antenna arrays for weak interfering signals

The performance of adaptive antenna arrays is studied when a sample matrix inversion (SMI) algorithm is used to control array weights. It is shown that conventional SMI adaptive antennas, like other adaptive antennas, are unable to suppress weak interfering signals (below thermal noise) encountered in broadcasting satellite communication systems. To overcome this problem, the SMI algorithm is modified. In the modified algorithm, the covariance matrix is modified such that the effect of thermal noise on the weights of the adaptive array is reduced. Thus, the weights are dictated by relatively weak coherent signals. It is shown that the modified algorithm provides the desired interference protection. The use of defocused feeds as auxiliary elements of an SMI adaptive array is also discussed.

Gupta, I. J.↗
NASA NTRS1986

A search for narrow band signals with Serendip II - A progress report

Commensal programs for SETI, carried out concurrently with conventional radio astronomical observing programs, can be an attractive and cost-effective means of exploring the large multidimensional search space intrinsic to this effort. An automated commensal system, Serendip II, searches for 0.49-Hz signals in sequential 64,700 Hz bands of the IF of a radio telescope being used for an astronomical observation. Upon detection of a signal with power above a preset threshold, the frequency, power, time, and telescope direction are recorded for later study. The system has been tested at the Hat Creek Radio Astronomy Observatory 85-ft telescope and the NASA-JPL Deep Space Station (DSS 14) 64-m telescope.

Werthimer, D.↗
NASA NTRS1987

Optimal signal-to-noise in digital phase lock amplifiers

An expression for the optimal signal-to-noise ratio for linear phase lock amplifiers is derived. In its digital form the optimal method can be implemented by real-time signal processing techniques. The case of detector-noise-limited systems is analyzed in detail; it is found that the optimal technique can be used to eliminate effectively the detector's time constant from the set of parameters determining the overall efficiency. This has important implications for optical detection system design. Also reported is an experimental verification of the optimal method, thus establishing its practicality.

Doering, Charles R.↗
NASA NTRS1986

Simulated laser fluorosensor signals from subsurface chlorophyll distributions

A semianalytic Monte Carlo model has been used to simulate laser fluorosensor signals returned from subsurface distributions of chlorophyll. This study assumes the only constituent of the ocean medium is the common coastal zone dinoflagellate Prorocentrum minimum. The concentration is represented by Gaussian distributions in which the location of the distribution maximum and the standard deviation are variable. Most of the qualitative features observed in the fluorescence signal for total chlorophyll concentrations up to 1.0 microg/liter can be accounted for with a simple analytic solution assuming a rectangular chlorophyll distribution function.

Venable, D. D.↗
NASA NTRS1987

Effect of numerical aperture on signal level in cylindrical waveguide evanescent fluorosensors

The effect of numerical aperture on signal level from fluorescent substances or solutions in the evanescent zone of a cylindrical waveguide is analyzed. The analysis applies to the case in which the fluorescence is excited by the evanescent wave of a fiber and the fluorescence signal is that which tunnels back into the same fiber. The analysis is for two cases: bulk fluorescence and fluorescence of a thin film layer. Experimental results are also presented.

Glass, Thomas R.↗
NASA NTRS1988

Robot Gripper With Signal Processing

Single-chip computer and sensor-circuit chips preprocess sensor data. Self-contained circuitry combines sensor signals for serial digital transmission. Gripping surfaces crossed with grooves for grasping differently shaped objects. Gripper cavities house sensors and preprocessing circuitry. Sensors and preprocessing circuitry in robot gripper reduce amount of data being transmitted between robot controller and gripper. Placement in gripper reduces signal delays and vulnerability to electromagnetic interference.

Killion, Richard R.↗
NASA NTRS1987

Detection of signals by the digital integrate-and-dump filter with offset sampling

The Integrate and Dump Filter (IDF) is used as a matched filter for the detection of signals in additive white Gaussian noise. The performance of the digital integrate and dump filter is evaluated. The case considered is when symbol times are known and the sampling clock is free running at a constant rate, i.e., the sampling clock is not phase locked to the symbol clock. Degradations in the output signal to noise ratio of the digital implementation due to sampling rate, sampling offset, and finite bandwidth, resulting from the anti-aliasing low pass prefilter, are computed and compared with those of the analog counterpart. It is shown that the digital IDF performs within 0.6 dB of the ideal analog IDF whenever the prefilter bandwidth exceeds four times the symbol rate and when sampling is performed at the Nyquist rate. The loss can be reduced to 0.3 dB by doubling the sampling rate, where 0.2 dB loss results from finite bandwidth and 0.1 dB results from the digital IDF.

Sadr, R.↗
NASA NTRS1987

Detection of signals by weighted integrate-and-dump filter

A Weighted Integrate and Dump Filter (WIDF) is presented that results in reducing those losses in telemetry symbol signal to noise ratio (SNR) which occur in digital Integrate and Dump Filters (IDFs) when the samples are not phase locked to the input data symbol clock. The Minimum Mean Square Error (MMSE) criterion is used to derive a set of weights for approximating the analog integrate and dump filter, which is the matched filter for detection of signals in additive white Gaussian noise. This new digital matched filter results in considerable performance improvement compared to unweighted digital matched filters. An example is presented for a sampling rate of four times the symbol rate. As the sampling offset (or phase) varies with respect to the data symbol boundaries, the output SNR varies 1 dB for an unweighted IDF, but only 0.3 dB for the optimum WIDF, averaged over random data patterns. This improvement in performance relative to unweighted IDF means that significantly lower sampling and processing rates can be used for given telemetry symbol rates, resulting in reduced system cost.

Sadr, R.↗
NASA NTRS1988

The efficacy of using human myoelectric signals to control the limbs of robots in space

This project was designed to investigate the usefulness of the myoelectric signal as a control in robotics applications. More specifically, the neural patterns associated with human arm and hand actions were studied to determine the efficacy of using these myoelectric signals to control the manipulator arm of a robot. The advantage of this approach to robotic control was the use of well-defined and well-practiced neural patterns already available to the system, as opposed to requiring the human operator to learn new tasks and establish new neural patterns in learning to control a joystick or mechanical coupling device.

Clark, Jane E.↗
NASA NTRS1988

Electrostatic waves stimulated by coherent VLF signals propagating in and near the inner radiation belt

The excitation of electrostatic waves by whistler-mode VLF signals propagating along magnetic-field lines in and near the earth's inner radiation belt is examined on the basis of ISEE-1 observations obtained during July-December 1983. The data are presented in extensive graphs and characterized in detail. The effect is seen in about 60 percent of the ISEE-1 orbits penetrating the belt and for signals originating in both hemispheres, with an electrostatic-wave effective bandwidth roughly proportional to the local magnetic-field strength. These results are interpreted in terms of a theoretical model in which scattering of the VLF waves off magnetic-field-aligned plasma density irregularities is followed by particle pitch-angle scattering and precipitation in the radiation belt; the latter effect could then produce new irregularities via a feedback mechanism.

Bell, T. F.↗
NASA NTRS1987

Performance measurements of a diode laser optical communications link with Q = 4 PPM signaling

The theoretical and measured performance characteristics of a direct detection optical communication system are reported. The system consisted of a semiconductor diode laser, silicon avalanche photodiode detector, and used Q = 4 optical pulse position modulation signaling to transmit binary source data at a rate of 25 megabits/sec. Operation at a receiver bit error probability of 10 to the -6th was demonstrated at received signal energies corresponding to an average of 160 detected photons per PPM symbol, or 80 photons per source bit.

Sun, X.↗
NASA NTRS1988

Development of an adaptive LDV signal processor

Recent attempts at realization of a frequency domain signal processor exhibit practical drawbacks such as manual gain, filter and sampling adjustments, and simplistic adaptations of frequency domain techniques. This paper introduces an adaptive LDV signal processor, one which requires no operator intervention under any circumstances by adapting dynamically to burst characteristics, while surpassing all previous techniques in range and accuracy.

Baker, Glenn D.↗
NASA NTRS1988

PC analysis of an acousto-ultrasonic signal

An introduction is given to PC software developed to analyze a digitized signal. The specific way in which the software was implemented and the relative ease with which the same software can be implemented in different systems for various applications are discussed. The basic equations and related theory used in the software are furnished. Specifically, mention is made of signal digitization, dc biasing, Fourier analysis, moment analysis, digital filtering, and transfer functions. Examples of calculations are given to indicate the physical significance of variables calculated from the frequency domain via moment equations.

Kiernan, M. T.↗
NASA NTRS1989

Managing Data From Signal-Propagation Experiments

Computer programs generate characteristic plots from amplitudes and phases. Software system enables minicomputer to process data on amplitudes and phases of signals received during experiments in ground-mobile/satellite radio propagation. Takes advantage of file-handling capabilities of UNIX operating system and C programming language. Interacts with user, under whose guidance programs in FORTRAN language generate plots of spectra or other curves of types commonly used to characterize signals. FORTRAN programs used to process file-handling outputs into any of several useful forms.

Kantak, A. V.↗
NASA NTRS1989

Detector For FM Voice Or Digital Signals

Frequency-modulation (FM) detector operates with either analog audio (usually voice) signals or digital signals sent by differential minimum-shift keying (DMSK). Performance expected similar to conventional limiter/discriminator FM detectors. Detector operates at baseband, obviating need for band-pass filtering at intermediate frequency. Baseband version made in very-large-scale integrated circuit. New detector useful in mobile communications, where trend is toward integrated voice and data service.

Davarian, Faramaz↗
NASA NTRS1988

Transmission of multiplexed video signals in multimode optical fiber systems

Kennedy Space Center has the need for economical transmission of two multiplexed video signals along multimode fiberoptic systems. These systems must span unusual distances and must meet RS-250B short-haul standards after reception. Bandwidth is a major problem and studies of the installed fibers, available LEDs and PINFETs led to the choice of 100 MHz as the upper limit for the system bandwidth. Optical multiplexing and digital transmission were deemed inappropriate. Three electrical multiplexing schemes were chosen for further study. Each of the multiplexing schemes included an FM stage to help meet the stringent S/N specification. Both FM and AM frequency division multiplexing methods were investigated theoretically and these results were validated with laboratory tests. The novel application of quadrature amplitude multiplexing was also considered. Frequency division multiplexing of two wideband FM video signal appears the most promising scheme although this application requires high power highly linear LED transmitters. Futher studies are necessary to determine if LEDs of appropriate quality exist and to better quantify performance of QAM in this application.

White, Preston, III↗