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At least 523 records · Page 29

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NASA NTRS1976

The fast decoding of Reed-Solomon codes using high-radix fermat theoretic transforms

Fourier-like transforms over GF(F sub n), where F sub n = 2(2n) + 1 is a Fermat prime, are applied in decoding Reed-Solomon codes. It is shown that such transforms can be computed using high-radix fast Fourier transform (FFT) algorithms requiring considerably fewer multiplications than the more usual radix 2 FFT algorithm. A special 256-symbol, 16-symbol-error-correcting, Reed-Solomon (RS) code for space communication-link applications can be encoded and decoded using this high-radix FFT algorithm over GF(F sub 3).

Liu, K. Y.↗

NASA NTRS1977

Multiple rate digital command detection system with range clean-up capability

A multi-rate digital command system is disclosed which uses the composite signal of a mu-type ranging system as a subcarrier to transmit range codes and data from a station to a receiver where the range codes are sequentially phase modulated on a subcarrier of frequency by one of its own subharmonics and data is phase modulated on a selected ranging component. A range cleanup loop in a spacecraft locks the phase of a locally generated reference component to a received ranging component and retransmits the component to a ground station. When the inverse phase of a ranging component is received and detected, the cleanup loop is modified to demodulate phase modulated command symbols while continuing tracking the same ranging component. The command symbol rate is coherently related to the ranging signal component bit rate.

Lesh, J. R.↗

NASA NTRS1977

Proceedings of the 1977 MACSYMA users' conference (NASA)

The MACSYMA program for symbolic and algebraic manipulation enables exact, symbolic mathematical computations to be performed on a computer. This program is rather large, and various approaches to the hardware and software problems are examined.

Source record↗

NASA NTRS1977

Observations on approximate integrations

A class of integration strategies is explored that fall in between the two extremes of symbolic integration and numerical quadrature, which are, respectively, aimed at the computer generation of answers in the form of exact expressions and numerical values. The theoretical advances in symbolic integration are discussed. Three major contexts of applications with attendant case studies, four possible types of strategies for approximate integration are examined. The feasibility and adequacy (or inadequancy) of MACSYMA for implementing these strategies are considered.

Ng, E. W.↗

NASA NTRS1979

Cockpit displays of traffic information: Airline pilots opinions about content, symbology, and format

A number of candidate computer-generated cockpit displays of traffic information (CDTI) displays and display options were simulated statically and were shown to 23 airline pilots who were asked to respond to more than 250 questions about them. The pilots indicated that the amount and complexity of navigation information displayed should increase with altitude and map scale. Terrain information should appear automatically if a pilot's own aircraft descends below the minimum safe altitude and should include only those obstruction within 2,000 ft or less. Few pilots that weather information should be displayed on a CDTI, but if it was, it should be at pilot request only. A chevron-shaped symbol, located so that the majority of the map area was ahead was preferred. The position, altitude, ground speed, ground track, weight class, and flightpath history of other aircraft should be presented graphically by coding the shape of the symbol for other aircraft or presented digitally in data tags displayed at pilot request. All pilots thought that color coding was necessary to recognize different categories of information quickly and accurately. The majority of pilots felt that a CDTI would provide useful information even though its presence might increase their workload somewhat particularly during its introductory stages.

Hart, S. G.↗

NASA NTRS1978

Trends in computerized structural analysis and synthesis; Proceedings of the Symposium, Washington, D.C., October 30-November 1, 1978

The subjects considered are related to future directions of structural applications and potential of new computing systems, advances and trends in data management and engineering software development, advances in applied mathematics and symbolic computing, computer-aided instruction and interactive computer graphics, nonlinear analysis, dynamic analysis and transient response, structural synthesis, structural analysis and design systems, advanced structural applications, supercomputers, numerical analysis, and trends in software systems. Attention is given to the reliability and optimality of the finite element method, computerized symbolic manipulation in structural mechanics, a standard computer graphics subroutine package, and a drag method as a finite element mesh generation scheme.

Noor, A. K.↗

NASA NTRS1981

High speed front end of the multimegabit telemetry demodulator detector

The high speed front end (HSFE) portion of the Multimegabit Telemetry Demodulator-Detector is discussed. The HSFE accepts in-phase and quadrature analog signals from the input signal conditioner. It samples them with analog-to-digital converters at maximum rate of 64 megasamples per second. The samples are processed digitally and the integrals of I, Q, and of their crossproduct over one symbol period are supplied at symbol rate to the rest of the system.

Wechsler, E. R.↗

NASA NTRS1980

Evaluation of the potential format and content of a cockpit display of traffic information

The types and formats of information most suitable to be displayed in a cockpit display of traffic information (CDTI) are investigated. Twenty three airline pilots and 13 instrumentated general aviation pilots were asked to select from sets of symbols of various complexities incorporating various levels of information that would contain all information necessary for monitoring the traffic situation, detecting errors, maintaining separation and merging. Display features selected by a significant number of pilots were then evaluated for their capabilities in helping pilots to assess the lateral or vertical separation between their own and another aircraft in a dynamic simulation. It is found that while some of the features initially chosen by the pilots, such as flightpath predictors, aided the pilots in perceiving the traffic situation correctly, others, such as ground speed and climb/descend arrows and relative altitude encoding of symbols for other aircraft, did not contribute to improved performance speed or accuracy.

Hart, S. G.↗

NASA NTRS1981

Computational vision

The range of fundamental computational principles underlying human vision that equally apply to artificial and natural systems is surveyed. There emerges from research a view of the structuring of vision systems as a sequence of levels of representation, with the initial levels being primarily iconic (edges, regions, gradients) and the highest symbolic (surfaces, objects, scenes). Intermediate levels are constrained by information made available by preceding levels and information required by subsequent levels. In particular, it appears that physical and three-dimensional surface characteristics provide a critical transition from iconic to symbolic representations. A plausible vision system design incorporating these principles is outlined, and its key computational processes are elaborated.

Barrow, H. G.↗

NASA NTRS1981

Erasure decoding in burst-error channels

A proven means of communicating reliably in a burst-error channel is the code interleaving scheme. Code symbols from a number of component codes are interleaved before being sent through the channel. This method effectively distributes the error detection and correction burden among the component codes and makes errors occurring in a codeword from each component code more or less independent. Erasure decoding techniques allow further refinement on the code interleaving concept. Their application leads to improved overall code performance when the symbol depth of the lead code is shallow compared to the average error-burst length of the channel. Theoretical formulations derived for predicting the performance of separate decoding and erasure decoding schemes are valuable in providing reasonably good estimates on redundancy requirements of the component codes.

Leung, K. S.↗

NASA NTRS1981

Predictor symbology in computer-generated perspective displays

An advanced display format for the four dimensional commercial aircraft approach-to-landing is evaluated. The desired curved and descending approach path is presented by displaying the perspective image of a tunnel. Attention is focussed on the predictor symbology, superimposed on the tunnel image. A perspective three dimensional predictor symbol, providing future position, as well as future attitude information, is compared with a flat two dimensional version, which only provides the future position. In addition to this, the predictor displays the actual airspeed as well as the desired airspeed, prescribed by the four dimensional path. Results show that the three dimensional predictor symbol outperforms the two dimensional predictor in following the trajectory in a moderate-to-heavy turbulent environment, which is manifested in accurate manual true airspeed control was obtained without affecting the main task performance significantly.

Grunwald, A. J.↗

NASA NTRS1982

Capacity limit of the noiseless, energy-efficient optical PPM channel

It is shown that even though the capacity per photon can be made to increase without bound, the capacity per channel use (for best power efficiency) is always less than 2 nats per symbol. Furthermore, it approaches 2 nats per symbol as the bandwidth expansion factor goes to infinity.

Lesh, J. R.↗

NASA NTRS1978

NASA Pioneer: Venus reverse playback telemetry program TR 78-2

During the entry of the Pioneer Venus Atmospheric Probes into the Venus atmosphere, there were several events (RF blackout and data rate changes) which caused the ground receiving equipment to lose lock on the signal. This caused periods of data loss immediately following each one of these disturbing events which lasted until all the ground receiving units (receiver, subcarrier demodulator, symbol synchronizer, and sequential decoder) acquired lock once more. A scheme to recover these data by off-line data processing was implemented. This scheme consisted of receiving the S band signals from the probes with an open loop reciever (requiring no lock up on the signal) in parallel with the closed loop receivers of the real time receiving equipment, down converting the signals to baseband, and recording them on an analog recorder. The off-line processing consisted of playing the analog recording in the reverse direction (starting with the end of the tape) up, converting the signal to S-band, feeding the signal into the "real time" receiving system and recording on digital tape, the soft decisions from the symbol synchronizer.

Modestino, J. W.↗

NASA NTRS1982

Survey of reader preferences concerning the format of NASA technical reports

A survey was conducted to determine the opinions of readers concerning the format (organization) of NASA technical reports and usage of technical report components. A survey questionnaire was sent to 513 LaRC engineers and scientists and 600 engineers and scientists from three (3) professional/technical societies. The response rates were 74 and 85 percent, respectively. The questionnaire included the order in which users read report components, the components reviewed or read to determine whether to read a report, report components which could be deleted, the desirability of a table of contents, the desirability of both a summary and abstract, the location of the symbols list and glossary, the integration of illustrative material, the preferred format for reference citations, column layout and right margin treatment, and person/voice. The results of the reader preference survey indicated that the conclusion was the component most often ready by survey respondents. The summary, conclusion, abstract, title page, and introduction were the components used most frequently to determine if a report would actually be read. Respondents indicated that a summary as well as an abstract should be included, that the definition of symbols and glossary of terms should be located in the front of the report, and that illustrative material should be integrated with the text rather than grouped at the end of the report. Citation by number was the preferred format for references. A one-column, ragged right margin was preferred. Third person, passive voice was the style of writing preferred by the respondents.

Pinelli, T. E.↗

NASA NTRS1982

Performance of concatenated Reed-Solomon/Viterbi channel coding

The concatenated Reed-Solomon (RS)/Viterbi coding system is reviewed. The performance of the system is analyzed and results are derived with a new simple approach. A functional model for the input RS symbol error probability is presented. Based on this new functional model, we compute the performance of a concatenated system in terms of RS word error probability, output RS symbol error probability, bit error probability due to decoding failure, and bit error probability due to decoding error. Finally we analyze the effects of the noisy carrier reference and the slow fading on the system performance.

Divsalar, D.↗

NASA NTRS1983

Capacity and cutoff rate of (M+1)-ary decision rules for noisy M-ary optical PPM channel

The channel capacity C and the cutoff rate R sub o of two (M + 1)-ary decision rules for noisy M slots/symbol optical pulse position modulation (PPM) with ideal photon counting are computed and compared. Also the values of the optimum thresholds needed to minimize the signal requirements are given. With a minor increase in hardware complexity, the symbol by symbol threshold decision rule is shown to be superior to the slot by slot threshold detection and decision rule in two aspects: first, it saves more than 0.5 dB in signal energy for the very noisy cases of more than one noise photon per slot (for low noise cases it also saves signal energy, but a negligibly small amount). Second, it is more robust to variations in the noise level.

Lee, P. J.↗

NASA NTRS1981

Mitigation of pulsed RFI via automatic gain control

Three automatic gain control (AGC) strategies and their potential capabilities for mitigation of pulsed radio frequency interference (RFI) over a coded, nonlinear channel are examined. The first updates the AGC control voltage once each symbol interval in accordance with a prespecified criterion. The second implementation is designed to first detect high level RFI and then appropriately update its gain. The final implementation is a wideband device which updates its gain once each symbol interval by reducing the gain in direct proportion to a suitable power measurement. Computed results treat pulsed noise and pulsed CW RFI and compare performances in terms of bit error rate (BER) at the output of a Viterbi decoder.

Chang, R. W.-S.↗

NASA NTRS1983

Capacity limit of the noiseless, energy-efficient optical PPM channel

Lesh, J. R.↗