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At least 235 records · Page 13

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NASA NTRS1986

Combined coding and delay-throughput analysis for fading channels of mobile satellite communications

This paper presents the analysis of using the punctured convolutional code with Viterbi decoding to improve communications reliability. The punctured code rate is optimized so that the average delay is minimized. The coding gain in terms of the message delay is also defined. Since using punctured convolutional code with interleaving is still inadequate to combat the severe fading for short packets, the use of multiple copies of assignment and acknowledgment packets is suggested. The performance on the average end-to-end delay of this protocol is analyzed. It is shown that a replication of three copies for both assignment packets and acknowledgment packets is optimum for the cases considered.

Wang, C. C.↗

NASA NTRS1987

The carbon dioxide chaperon efficiency for the reaction H + O2 + M yields HO2 + M from ignition delay times behind reflected shock waves

Ignition delay times for stoichiometric hydrogen-oxygen in argon with and without carbon dioxide were measured behind reflected shock waves. A 20-reaction kinetic mechanism models the measured hydrogen-oxygen delay times over the temperature range 950 to 1300 K. The chaperon efficiency for carbon dioxide determined for the hydrogen-oxygen carbon dioxide mixture was 7.0. This value is in agreement with literature values but much less than a recent value obtained from flow tube experiments. Delay times measured behind a reflected shock wave were about 20% longer than those measured behind incident shock waves. The kinetic mechanism successfully modeled the high-pressure data of Skinner and the hydrogen-air data of Stack. It is suggested that the lowest temperature points for the hydrogen-air data of Slack are unreliable and that the 0.27-atm data may illustrate a case where vibrational relaxation of nitrogen is important. The reaction pathway HO2 yields H2O2 yields OH yields H was required to model the high-pressure data of Skinner. The successful modeling of the stoichiometric hydrogen-air data demonstrates the appropriateness of deriving kinetic models from data for gas mixtures highly diluted with argon. The technique of reducing a detailed kinetic mechanism to only the important reactions for a limited range of experimental data may render the mechanism useless for other test conditions.

Brabbs, Theodore A.↗

NASA NTRS1988

Transport delay compensation for computer-generated imagery systems

In the problem of pure transport delay in a low-pass system, a trade-off exists with respect to performance within and beyond a frequency bandwidth. When activity beyond the band is attenuated because of other considerations, this trade-off may be used to improve the performance within the band. Specifically, transport delay in computer-generated imagery systems is reduced to a manageable problem by recognizing frequency limits in vehicle activity and manual-control capacity. Based on these limits, a compensation algorithm has been developed for use in aircraft simulation at NASA Ames Research Center. For direct measurement of transport delays, a beam-splitter experiment is presented that accounts for the complete flight simulation environment. Values determined by this experiment are appropriate for use in the compensation algorithm. The algorithm extends the bandwidth of high-frequency flight simulation to well beyond that of normal pilot inputs. Within this bandwidth, the visual scene presentation manifests negligible gain distortion and phase lag. After a year of utilization, two minor exceptions to universal simulation applicability have been identified and subsequently resolved.

Mcfarland, Richard E.↗

NASA NTRS1987

Delayed entry of solar protons (E/p/ over 0.3 MeV) into the earth's magnetotail

The delayed entry of solar protons into the earth's magnetotail is studied using simultaneous observations from three earth-orbiting spacecraft of a number of interplanetary proton events which exhibited abrupt and relatively large discontinuities in intensity associated with magnetic field discontinuities. It is found that such protons have immediate access to the magnetosheath and that delay times increase discontinuously from essentially zero in the magnetosheath to tens of minutes as an observing point moves inward across the magnetopause. Well-defined delay times from 30 to 160 min occur for points interior to the magnetotail. The inferred entry points lie at downstream distances of 80-500 R(E). No discernible relationship is found between the direction and sense of the interplanetary magnetic field and the entry times in the northern and southern lobes of the magnetotail or the entry tiems at various distances from its central axis. These results favor some form of an open model of the magnetotail.

Van Allen, J. A.↗

NASA NTRS1988

Finite-dimensional modeling of network-induced delays for real-time control systems

In integrated control systems (ICS), a feedback loop is closed by the common communication channel, which multiplexes digital data from the sensor to the controller and from the controller to the actuator along with the data traffic from other control loops and management functions. Due to asynchronous time-division multiplexing in the network access protocols, time-varying delays are introduced in the control loop, which degrade the system dynamic performance and are a potential source of instability. The delayed control system is represented by a finite-dimensional, time-varying, discrete-time model which is less complex than the existing continuous-time models for time-varying delays; this approach allows for simpler schemes for analysis and simulation of the ICS.

Ray, Asok↗

NASA NTRS1989

Algorithm Estimates Microwave Water-Vapor Delay

Accuracy equals or exceeds conventional linear algorithms. "Profile" algorithm improved algorithm using water-vapor-radiometer data to produce estimates of microwave delays caused by water vapor in troposphere. Does not require site-specific and weather-dependent empirical parameters other than standard meteorological data, latitude, and altitude for use in conjunction with published standard atmospheric data. Basic premise of profile algorithm, wet-path delay approximated closely by solution to simplified version of nonlinear delay problem and generated numerically from each radiometer observation and simultaneous meteorological data.

Robinson, Steven E.↗

NASA NTRS1988

Assessment of satellite communications quality study. Addendum 1: Impact of propagation delay on data transmission

The single factor that irrevocably distinguishes geostationary satellite telephony transmission from terrestrial transmission is the greater propagation delay over satellite links. This difference has always provoked vigorous debate over the impact of delay on the subscribers using services incorporating satellite links. The issue is addressed from a variety of directions including human factors studies, laboratory subjective tests that evaluate delay with and without echo, and field tests that obtain data on the opinion of subscribers regarding the quality of service of operational circuits in both national U.S. domestic and international trans-Atlantic network. The tests involved the use of both echo suppressors and echo cancellers.

Campanella, S. J.↗

NASA NTRS1988

Transmission delays in hardware clock synchronization

Various methods, both with software and hardware, have been proposed to synchronize a set of physical clocks in a system. Software methods are very flexible and economical but suffer an excessive time overhead, whereas hardware methods require no time overhead but are unable to handle transmission delays in clock signals. The effects of nonzero transmission delays in synchronization have been studied extensively in the communication area in the absence of malicious or Byzantine faults. The authors show that it is easy to incorporate the ideas from the communication area into the existing hardware clock synchronization algorithms to take into account the presence of both malicious faults and nonzero transmission delays.

Shin, Kang G.↗

NASA NTRS1988

Transport delay compensation for computer-generated imagery systems

Mcfarland, Richard E.↗

NASA NTRS1990

Information content of a single pass of phase-delay data from a short baseline connected element interferometer

An analytic development of the information array obtained with a single tracking pass of phase-delay measurements made from a short baseline interferometer is presented. Phase-delay observations can be made with great precision from two antennas using a single, common distributed frequency standard, hence the name connected element. With the information array, closed-form expressions are developed for the error covariance in declination and right ascension. These equations serve as useful tools for analyzing the relative merits of candidate station locations for connected element interferometry (CEI). The navigation performance of a short baseline interferometer located at the Deep Space Network's (DSN's) Goldstone intracomplex is compared with that which is presently achievable using Very Long Baseline Interferometry (VLBI) over intercontinental baselines. The performance of an intracomplex pair of short baselines formed by three stations is also investigated, along with the use of a single baseline in conjunction with conventional two-way Doppler data. The phase-delay measurement accuracy and data rate used in the analysis are based on the expected performance of an experimental connected element system presently under construction at Goldstone. The results indicate that the VLBI system that will be used during the Galileo mission can determine the declination and right ascension of a distant spacecraft to an accuracy of 20 to 25 nrad, while the CEI triad system and the combination of CEI-Doppler system are both capable of 30 to 70 nrad performance.

Thurman, S. W.↗

NASA NTRS1990

Vertex implications of stability for a class of delay-differential interval system

Although vertex implication in parameter space does not hold for stability of general delay-differential interval systems, it is shown that in an important special case when the characteristic equation of the system is describable by certain quasi-polynomials the vertex implication for stability does hold. It is also pointed out that vertex implication for stability does not hold in general for delay-differential systems with interval delay.

Kim, K. D.↗

NASA NTRS1990

Analyzing time delays in a flight simulation environment

Transport delay in a multirate flight simulation environment is examined. An equivalent systems model is developed that quantifies the contributions of individual components and their sampled-data interactions. Mathematical algorithms used in the discrete implementation are also considered, because they are important elements of a flight simulation system. The equivalent systems model was used to demonstrate the consistency and accuracy of data obtained in the flight simulation facility at Ames Research Center. It showed that effective time delays in simulation models, including delays in scene presentation to the pilot, are considerably less than might be assumed by casual examination of raw data obtained from component-level experiments.

Mcfarland, R. E.↗

NASA NTRS1990

Cable delay compensator for microwave signal distribution over optical fibers

The basic principles of microwave fiber-optic systems are outlined with emphasis on fiber-optic cable delay compensators (CDC). Degradation of frequency and phase stability is considered, and it is pointed out that the long-term stability of a fiber-optic link is degraded by group delay variations due to temperature fluctuations in the optical fiber and low-frequency noise characteristics of the laser. A CDC employing a voltage-controlled oscillator to correct for phase variations in the optical fiber is presented, and the static as well as dynamic closed-loop analyses of the fiber-optic CDC are discussed. A constructed narrow-band fiber-optic CDC is shown to reduce phase variations caused by temperature fluctuations by a factor of 400. A wide-band CDC utilizing a temperature-controlled coil of fiber to compensate for phase delay is also proposed.

Primas, Lori E.↗

NASA NTRS1990

Delay compensation in integrated communication and control systems. II - Implementation and verification

The implementation and verification of the delay-compensation algorithm are addressed. The delay compensator has been experimentally verified at an IEEE 802.4 network testbed for velocity control of a DC servomotor. The performance of the delay-compensation algorithm was also examined by combined discrete-event and continuous-time simulation of the flight control system of an advanced aircraft that uses the SAE (Society of Automotive Engineers) linear token passing bus for data communications.

Luck, Rogelio↗

NASA NTRS1992

Predictive Display For Teleoperation With Delay

Computer-graphical simulator helps operator control robotic manipulator when controlling and monitoring signals delayed in transmission. Displays phantom image of robot superimposed on delayed "real" monitoring image. Phantom responds to control signals immediately - motion predicts that of robot. After delay, real image follows motion of phantom. System includes high-fidelity, real-time computer-graphical display which gives depth, perspective, and lighting cues improving control.

Bejczy, Antal K.↗

NASA NTRS1991

Analysis of delay reducing and fuel saving sequencing and spacing algorithms for arrival traffic

The air traffic control subsystem that performs sequencing and spacing is discussed. The function of the sequencing and spacing algorithms is to automatically plan the most efficient landing order and to assign optimally spaced landing times to all arrivals. Several algorithms are described and their statistical performance is examined. Sequencing brings order to an arrival sequence for aircraft. First-come-first-served sequencing (FCFS) establishes a fair order, based on estimated times of arrival, and determines proper separations. Because of the randomness of the arriving traffic, gaps will remain in the sequence of aircraft. Delays are reduced by time-advancing the leading aircraft of each group while still preserving the FCFS order. Tightly spaced groups of aircraft remain with a mix of heavy and large aircraft. Spacing requirements differ for different types of aircraft trailing each other. Traffic is reordered slightly to take advantage of this spacing criterion, thus shortening the groups and reducing average delays. For heavy traffic, delays for different traffic samples vary widely, even when the same set of statistical parameters is used to produce each sample. This report supersedes NASA TM-102795 on the same subject. It includes a new method of time-advance as well as an efficient method of sequencing and spacing for two dependent runways.

Neuman, Frank↗

NASA NTRS1992

Reducing transport delay through improvements in real-time program flow

This paper describes the process of measuring and reducing software-transport delays through careful analysis, and modifications of real-time programs. A 737 program is analyzed and modified to improve the simulation overall transport delay by approximately 30 percent. The transport delay was improved through modification to the real-time program flow and the implementation of quaternions in the calculation of the math model.

Smith, R. M.↗

NASA NTRS1992

Use of the VLBI delay observable for orbit determination of Earth-orbiting VLBI satellites

Very long-baseline interferometry (VLBI) observations using a radio telescope in Earth orbit were performed first in the 1980s. Two spacecraft dedicated to VLBI are scheduled for launch in 1995; the primary scientific goals of these missions will be astrophysical in nature. This article addresses the use of space VLBI delay data for the additional purpose of improving the orbit determination of the Earth-orbiting spacecraft. In an idealized case of quasi-simultaneous observations of three radio sources in orthogonal directions, analytical expressions are found for the instantaneous spacecraft position and its error. The typical position error is at least as large as the distance corresponding to the delay measurement accuracy but can be much greater for some geometries. A number of practical considerations, such as system noise and imperfect calibrations, set bounds on the orbit-determination accuracy realistically achievable using space VLBI delay data. These effects limit the spacecraft position accuracy to at least 35 cm (and probably 3 m or more) for the first generation of dedicated space VLBI experiments. Even a 35-cm orbital accuracy would fail to provide global VLBI astrometry as accurate as ground-only VLBI. Recommended charges in future space VLBI missions are unlikely to make space VLBI competitive with ground-only VLBI in global astrometric measurements.

Ulvestad, J. S.↗