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At least 1,099 records · Page 61
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NASA NTRS1973

Recursive ideal observer detection of known M-ary signals in multiplicative and additive Gaussian noise.

This paper presents the derivation of the recursive algorithms necessary for real-time digital detection of M-ary known signals that are subject to independent multiplicative and additive Gaussian noises. The motivating application is minimum probability of error detection of digital data-link messages aboard civil aircraft in the earth reflection multipath environment. For each known signal, the detector contains one Kalman filter and one probability computer. The filters estimate the multipath disturbance. The estimates and the received signal drive the probability computers. Outputs of all the computers are compared in amplitude to give the signal decision. The practicality and usefulness of the detector are extensively discussed.

Painter, J. H.↗
NASA NTRS1973

Optical Signal Processing: Poisson Image Restoration and Shearing Interferometry

Optical signal processing can be performed in either digital or analog systems. Digital computers and coherent optical systems are discussed as they are used in optical signal processing. Topics include: image restoration; phase-object visualization; image contrast reversal; optical computation; image multiplexing; and fabrication of spatial filters. Digital optical data processing deals with restoration of images degraded by signal-dependent noise. When the input data of an image restoration system are the numbers of photoelectrons received from various areas of a photosensitive surface, the data are Poisson distributed with mean values proportional to the illuminance of the incoherently radiating object and background light. Optical signal processing using coherent optical systems is also discussed. Following a brief review of the pertinent details of Ronchi's diffraction grating interferometer, moire effect, carrier-frequency photography, and achromatic holography, two new shearing interferometers based on them are presented. Both interferometers can produce variable shear.

Hong, Yie-Ming↗
NASA NTRS1974

Model reference adaptive control with an augmented error signal

It is shown how globally stable model reference adaptive control systems may be designed when one has access to only the plant's input and output signals. Controllers for single input-single output, nonlinear, nonautonomous plants are developed based on Lyapunov's direct method and the Meyer-Kalman-Yacubovich lemma. Derivatives of the plant output are not required, but are replaced by filtered derivative signals. An augmented error signal replaces the error normally used, which is defined as the difference between the model and plant outputs. However, global stability is assured in the sense that the normally used error signal approaches zero asymptotically.

Monopoli, R. V.↗
NASA NTRS1974

Transmitter efficiency considerations in MAPSK signal selection

A simple model of the efficiency-power relationship for a microwave transmitter permits comparison of M-ary amplitude and phase shift keyed (MAPSK) signal array performance on the basis of average DC power in, rather than average RF signal power out. This is of interest in comparing MAPSK arrays with comparable polyphase, since the polyphase transmitter is operated at a single power level, while MAPSK varies over a range of power levels, each of differing efficiency. A constant efficiency-power curve permits comparison of signal arrays in terms of array average power, while a linear curve requires comparison on the basis of array peak power. Since typical efficiency curves will lie between these extremes, the array peak and average power levels provide bounds on array requirements for average DC input power. Examination of the performance of several MAPSK sets reveals a distinct power advantage of quadrature amplitude shift keying (QASK) over most viable signal sets.

Smith, J. G.↗
NASA NTRS1975

Graphical derivations of radar, sonar, and communication signals

The designer of a communication system often has knowledge concerning the changes in distance between transmitter and receiver as a function of time. This information can be exploited to reduce multipath interference via proper signal design. A radar or sonar may also have good a priori information about possible target trajectories. Such knowledge can again be used to reduce the receiver's response to clutter (MTI), to enhance signal-to-noise ratio, or to simplify receiver design. There are also situations in which prior knowledge about trajectories is lacking. The system should then utilize a single-filter pair which is insensitive to the effects induced by relative motion between transmitter, receiver, and reflectors. For waveforms with large time-bandwidth products, such as long pulse trains, it is possible to graphically derive signal formats for both situations (trajectory known and unknown). Although the exact form of the signal is sometimes not specified by the graphical procedure, the problem in such cases is reduced to one which has already been solved, i.e., the generation of an impulse equivalent code.

Altes, R. A.↗
NASA NTRS1976

Signal design study for shuttle/TDRSS Ku-band uplink

The adequacy of the signal design approach chosen for the TDRSS/orbiter uplink was evaluated. Critical functions and/or components associated with the baseline design were identified, and design alternatives were developed for those areas considered high risk. A detailed set of RF and signal processing performance specifications for the orbiter hardware associated with the TDRSS/orbiter Ku band uplink was analyzed. Performances of a detailed design of the PN despreader, the PSK carrier synchronization loop, and the symbol synchronizer are identified. The performance of the downlink signal by means of computer simulation to obtain a realistic determination of bit error rate degradations was studied. The three channel PM downlink signal was detailed by means of analysis and computer simulation.

Source record↗
NASA NTRS1975

Coherence properties of wideband satellite signals caused by ionospheric scintillation

Radio scintillation on satellite signals caused by small-scale irregularities in F-region ionospheric electron density can be an important limitation on earth-satellite communication and navigation systems. Scintillation imposes distortion in both amplitude and phase on wideband signals. In the present work, the shallow-modulated phase screen theory is developed in terms of coherence bandwidth including a model based on a turbulent-like power-law description of the irregularities. The model results usually show a greater coherence bandwidth in the signal phase than in the signal amplitude. Therefore, systems that require phase coherence over a large bandwidth should be less affected than those requiring amplitude coherence.

Rufenach, C. L.↗
NASA NTRS1975

Discrete model reference adaptive control with an augmented error signal

A method for designing discrete model reference adaptive control systems when one has access to only the plant's input and output signals is given. Controllers for single-input, single-output, nonlinear, nonautonomous plants are developed via Liapunov's second method. Anticipative values of the plant output are not required, but are replaced by signals easily obtained from a low-pass filter operating on the plant's output. The augmented error signal method is employed, ensuring finally that the normally used error signal also approaches zero asymptotically.

Ionescu, T.↗
NASA NTRS1976

An observational program to search for radio signals from extraterrestrial intelligence through the use of existing facilities

It is argued that a substantial portion of the capability for detecting microwave signals from extraterrestrial civilizations lies not in the application of ever larger antenna collecting areas but rather in the application of millions or billions of simultaneous frequency-channel observations combined with rapid and powerful data processing techniques. The application of these methods to existing facilities is discussed in terms of a program of modest expense and duration which will seek to discover certain classes of extraterrestrial signals of intelligent origin while defining boundaries to the search problem throughout the range of interest. This program will investigate radio-astronomical phenomena of interest and simultaneously define the background of environmental radiation in order to determine physical limitations on both the search strategies and the potential for deep-space communications. Signal parameters that must be determined are examined along with the potential of existing radio-astronomical facilities for detecting narrow-band signals. A seven-year program is described which will carry out a search for extraterrestrial intelligence over 80% of the sky and over the entire frequency range from 1 to 25 GHz with a sensitivity limit varying from 10 to the -21st power W/sq cm at the lowest frequencies to 10 to the -19th power W/sq cm at the higher frequencies.

Edelson, R. E.↗
NASA NTRS1977

SETI - A preliminary search for narrowband signals at microwave frequencies

In the search for intelligent signals of extraterrestrial origin, certain forms of signals merit immediate and special attention. Extremely narrowband signals of spectral width similar to our own television transmissions are most favored energetically and least likely to be confused with natural celestial emission. A search of selected stars has been initiated using observational and data processing techniques optimized for the detection of such signals. These techniques allow simultaneous observation of 10 to the 5th to 10 to the 6th channels within the observed spectral range. About two hundred nearby (within 80 LY) solar type stars have been observed at frequencies near the main microwave transitions of the hydroxyl radical. In addition, several molecular (hydroxyl) masers and other non-thermal sources have been observed in this way in order to uncover any possible fine spectral structure of natural origin and to investigate the potential of such an instrument for radioastronomy.

Cuzzi, J. N.↗
NASA NTRS1980

Lock detector for noise-coded signals

Circuit indicates when receiver is locked on pseudorandom-noise-coded signal. Circuit is used for reception of such digitally coded signals as scrambled voice messages or scrambled video. Circuit determines when receiver generated code is correct and synchronized with incoming signal so that receiver can track signal.

Carson, L. M.↗
NASA NTRS1979

Costas loop demodulation of suppressed carrier BPSK signals in the DSN environment: Experimental results obtained at TDL

Suppressed carrier binary phase-shift keyed (BPSK) signalling is currently being considered as a design alternative for future DSN telemetry in the multimegabit range. Carrier tracking of such signals is usually achieved by a Costas loop, as opposed to the ordinary phase lock loop. A Costas loop capable of demodulating BPSK signals with data rates up to 1 Msps was designed and constructed and its Doppler tracking performance with respect to a Block 3 receiver was tested at the Telecommunications Development Laboratory (TDL). The compatibility of suppressed carrier signalling with the current radiometric system, specifically Doppler tracking and ranging, was investigated. The experimental results obtained to-date with respect to Doppler tracking are presented.

Reasoner, R.↗
NASA NTRS1978

Signal processing in ultrasound

Signal is the term used to denote the characteristic in the time or frequency domain of the probing energy of the system. Processing of this signal in diagnostic ultrasound occurs as the signal travels through the ultrasonic and electrical sections of the apparatus. The paper discusses current signal processing methods, postreception processing, display devices, real-time imaging, and quantitative measurements in noninvasive cardiology. The possibility of using deconvolution in a single transducer system is examined, and some future developments using digital techniques are outlined.

Le Croissette, D. H.↗
NASA NTRS1980

Pilot signals for large active retro-directive arrays

It has been suggested that for large active retrodirective arrays, as in the solar power system, a two tone uplink pilot signal with frequencies symmetrically situated around the downlink frequency be used in order to reduce ionospheric biases and to lower the cost since a two tone receiver is economically much cheaper than a single tone phase locked receiver. Such a system now faces two difficulties: the Pi-ambiguity; and a large phase difference between the downlink and uplink signals. The Pi-ambiguity is shown to be easily removed by using a two tone uplink signal with both frequencies situated at one side of the downlink frequency, and the phase difference can be greatly reduced with a three tone or a four tone uplink pilot signal.

Chan, C. H.↗
NASA NTRS1980

Pilot signals for large active retrodirective arrays

It was suggested that for large active retrodirective arrays, as in the solar power system, a two tone uplink pilot signal with frequencies symmetrically situated around the downlink frequency be used in order to reduce ionospheric biases and to lower the cost. Unfortunately such a system faces the following difficulties: (1) the pi-ambiguity, and (2) a large phase difference between the downlink and uplink signals. It is shown how the pi-ambiguity can be easily removed by using a two tone uplink signal with both frequencies situated at one side of the downlink frequency, and the phase difference can be greatly reduced with a three tone or a four tone uplink pilot signal.

Chan, C. H.↗
NASA NTRS1980

A design study of a signal detection system

A system is described which can aid in the search for radio signals from extraterrestrial sources, or in other applications characterized by low signal-to-noise ratios and very high data rates. The system follows a multichannel (16 million bin) spectrum analyzer, and has critical processing, system control, and memory fuctions. The design includes a moderately rich set of algorithms to be used in parallel to detect signals of unknown form. A multi-threshold approach is used to obtain high and low signal sensitivities. Relatively compact and transportable memory systems are specified.

Healy, T. J.↗
NASA NTRS1981

A unique method to study acoustic transmission through ducts using signal synthesis and averaging of acoustic pulses

An acoustic impulse technique using a loudspeaker driver is developed to measure the acoustic properties of a duct/nozzle system. A signal synthesis method is used to generate a desired single pulse with a flat spectrum. The convolution of the desired signal and the inverse Fourier transform of the reciprocal of the driver's response are then fed to the driver. A signal averaging process eliminates the jet mixing noise from the mixture of jet noise and the internal noise, thereby allowing very low intensity signals to be measured accurately, even for high velocity jets. A theoretical analysis is carried out to predict the incident sound field; this is used to help determine the number and locations of the induct measurement points to account for the contributions due to higher order modes present in the incident tube method. The impulse technique is validated by comparing experimentally determined acoustic characteristics of a duct-nozzle system with similar results obtained by the impedance tube method. Absolute agreement in the comparisons was poor, but the overall shapes of the time histories and spectral distributions were much alike.

Salikuddin, M.↗