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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Frame synchronization for a channel with different data rates

In some communication links, different data rates are used at different times on the same channel and each is assigned a separate frame synchronization (sync) marker. The frame search is completed within a finite number of frame intervals once both phase-lock and bit-sync are achieved because the incoming data rate can change. The probability of failure to frame-lock, and an upper bound of probability of false frame-lock in a frame search mode are derived for a communication link with a 10 to the -5th bit error probability. The probability of failure to detect data-rate change and the probability of false detection of data-rate change in a frame-lock mode are discussed. In a frame-lock mode, the probability of failure to detect data-rate change and probability detection of data-rate change an equal to 4.96 x 10 to the -12th. A frame-lock device can be used for the Space Station Freedom Ku-band return link.

Kwon, Hyuck M.↗
NASA NTRS1992

Synchronous parallel system for emulation and discrete event simulation

A synchronous parallel system for emulation and discrete event simulation having parallel nodes responds to received messages at each node by generating event objects having individual time stamps, stores only the changes to state variables of the simulation object attributable to the event object, and produces corresponding messages. The system refrains from transmitting the messages and changing the state variables while it determines whether the changes are superseded, and then stores the unchanged state variables in the event object for later restoral to the simulation object if called for. This determination preferably includes sensing the time stamp of each new event object and determining which new event object has the earliest time stamp as the local event horizon, determining the earliest local event horizon of the nodes as the global event horizon, and ignoring the events whose time stamps are less than the global event horizon. Host processing between the system and external terminals enables such a terminal to query, monitor, command or participate with a simulation object during the simulation process.

Steinman, Jeffrey S.↗
NASA NTRS1990

Synchronization techniques for all digital 16-ary QAM receivers operating over land mobile satellite links

The performance of a low bit rate (64 Kb/s) all digital 16-ary Differentially Encoded Quadrature Amplitude Modulation (16-DEQAM) demodulator operating over a mobile satellite channel, is considered. The synchronization and detection techniques employed to overcome the Rician channel impairments, are described. The acquisition and steady state performance of this modem, are evaluated by computer simulation over AWGN and RICIAN channels. The results verify the suitability of the 16-DEQAM transmission over slowly faded and/or mildly faded channels.

Fines, P.↗
NASA NTRS1992

Node synchronization schemes for the Big Viterbi Decoder

The Big Viterbi Decoder (BVD), currently under development for the DSN, includes three separate algorithms to acquire and maintain node and frame synchronization. The first measures the number of decoded bits between two consecutive renormalization operations (renorm rate), the second detects the presence of the frame marker in the decoded bit stream (bit correlation), while the third searches for an encoded version of the frame marker in the encoded input stream (symbol correlation). A detailed account of the operation is given, as well as performance comparison, of the three methods.

Cheung, K.-M.↗
NASA NTRS1992

Verifying the interactive convergence clock synchronization algorithm using the Boyer-Moore theorem prover

The application of formal methods to the analysis of computing systems promises to provide higher and higher levels of assurance as the sophistication of our tools and techniques increases. Improvements in tools and techniques come about as we pit the current state of the art against new and challenging problems. A promising area for the application of formal methods is in real-time and distributed computing. Some of the algorithms in this area are both subtle and important. In response to this challenge and as part of an ongoing attempt to verify an implementation of the Interactive Convergence Clock Synchronization Algorithm (ICCSA), we decided to undertake a proof of the correctness of the algorithm using the Boyer-Moore theorem prover. This paper describes our approach to proving the ICCSA using the Boyer-Moore prover.

Young, William D.↗
NASA NTRS1992

A decision support tool for synchronizing technology advances with strategic mission objectives

Successful accomplishment of the objectives of many long-range future missions in areas such as space systems, land-use planning, and natural resource management requires significant technology developments. This paper describes the development of a decision-support data-derived tool called MisTec for helping strategic planners to determine technology development alternatives and to synchronize the technology development schedules with the performance schedules of future long-term missions. Special attention is given to the operations, concept, design, and functional capabilities of the MisTec. The MisTec was initially designed for manned Mars mission, but can be adapted to support other high-technology long-range strategic planning situations, making it possible for a mission analyst, planner, or manager to describe a mission scenario, determine the technology alternatives for making the mission achievable, and to plan the R&D activity necessary to achieve the required technology advances.

Hornstein, Rhoda S.↗
NASA NTRS1993

Synchronizing A Television Camera With An External Reference

Improvement in genlock subsystem consists in incorporation of controllable delay circuit into path of composite synchronization signal obtained from external video source. Delay circuit helps to eliminate potential jitter in video display and ensures setup requirements for digital timing circuits of video camera satisfied.

Rentsch, Edward M.↗
NASA NTRS1992

Tracking performance and cycle slipping in the all-digital symbol synchronizer loop of the block 5 receiver

Computer simulated noise performance of the symbol synchronizer loop (SSL) in the Block 5 receiver is compared with the theoretical noise performance. Good agreement is seen at the higher loop SNR's (SNR(sub L)'s), with gradual degradation as the SNR(sub L) is decreased. For the different cases simulated, cycle slipping is observed (within the simulation time of 10(exp 4) seconds) at SNR(sub L)'s below different thresholds, ranging from 6 to 8.5 dB, comparable to that of a classical phase-locked loop. An important point, however, is that to achieve the desired loop SNR above the seemingly low threshold to avoid cycle slipping, a large data-to-loop-noise power ratio, P(sub D)/(N(sub 0)B(sub L)), is necessary (at least 13 dB larger than the desired SNR(sub L) in the optimum case and larger otherwise). This is due to the large squaring loss (greater than or equal to 13 dB) inherent in the SSL. For the special case of symbol rates approximately equaling the loop update rate, a more accurate equivalent model accounting for an extra loop update period delay (characteristic of the SSL phase detector design) is derived. This model results in a more accurate estimation of the noise-equivalent bandwidth of the loop.

Aung, M.↗
NASA NTRS1992

Power transmission by laser beam from lunar-synchronous satellites to a lunar rover

This study addresses the possibility of beaming laser power from synchronous lunar orbits (L1 and L2 LaGrange points) to a manned long-range lunar rover. The rover and two versions of a satellite system (one powered by a nuclear reactor; the other by photovoltaics) are described in terms of their masses, geometry, power needs, mission and technological capabilities. Laser beam power is generated by a laser diode array in the satellite and converted to 30 kW of electrical power at the rover. Present technological capabilities, with some extrapolation to near future capabilities, are used in the descriptions. The advantages of the two satellite/rover systems over other such systems and over rovers with on-board power are discussed along with the possibility of enabling other missions.

Williams, M. D.↗
NASA NTRS1992

Methodology for the passive control of orbital inclination and mean local time to meet sun-synchronous orbit requirements

The mean local time (MLT) of equatorial crossing of a sun-synchronous Earth-observing spacecraft orbit drifts with inclination; therefore, in order to maintain the MLT, the inclination must be controlled. Inclination may be maintained actively by costly out-of-plane maneuvers or passively by using the perturbing forces due to the sun and moon. This paper examines the passive control approach using the Earth Observing System (EOS) as a basis for the discussion. Applications to Landsat and National Oceanic and Atmospheric Administration (NOAA) spacecraft are presented for comparison. This technique is especially beneficial to spacecraft lacking propulsion systems. The results indicate that passive inclination control appears to be the preferable maintenance method when spacecraft weight restrictions, operational considerations, and scientific requirements apply.

Folta, David↗
NASA NTRS1922

The Synchronization of NACA Flight Records

A method is described for synchronizing the different instrument records taken during flight testing. This method has been in use for sometime at the Langley Memorial Aeronautical Laboratory and has proved very valuable in connection with the study of controllability and other complicated problems of flight.

Brown, W G↗
NASA NTRS1993

Power transmission by laser beam from lunar-synchronous satellite

The possibility of beaming power from synchronous lunar orbits (the L1 and L2 Lagrange points) to a manned long-range lunar rover is addressed. The rover and two versions of a satellite system (one powered by a nuclear reactor, the other by photovoltaics) are described in terms of their masses, geometries, power needs, missions, and technological capabilities. Laser beam power is generated by a laser diode array in the satellite and converted to 30 kW of electrical power at the rover. Present technological capabilities, with some extrapolation to near future capabilities, are used in the descriptions. The advantages of the two satellite/rover systems over other such systems and over rovers with onboard power are discussed along with the possibility of enabling other missions.

Williams, M. D.↗
NASA NTRS1992

On the decomposition of synchronous state mechines using sequence invariant state machines

This paper presents a few techniques for the decomposition of Synchronous State Machines of medium to large sizes into smaller component machines. The methods are based on the nature of the transitions and sequences of states in the machine and on the number and variety of inputs to the machine. The results of the decomposition, and of using the Sequence Invariant State Machine (SISM) Design Technique for generating the component machines, include great ease and quickness in the design and implementation processes. Furthermore, there is increased flexibility in making modifications to the original design leading to negligible re-design time.

Hebbalalu, K.↗
NASA NTRS1994

Time synchronized video systems

The idea of synchronizing multiple video recordings to some type of 'range' time has been tried to varying degrees of success in the past. Combining this requirement with existing time code standards (SMPTE) and the new innovations in desktop multimedia however, have afforded an opportunity to increase the flexibility and usefulness of such efforts without adding costs over the traditional data recording and reduction systems. The concept described can use IRIG, GPS or a battery backed internal clock as the master time source. By converting that time source to Vertical Interval Time Code or Longitudinal Time Code, both in accordance with the SMPTE standards, the user will obtain a tape that contains machine/computer readable time code suitable for use with editing equipment that is available off-the-shelf. Accuracy on playback is then determined by the playback system chosen by the user. Accuracies of +/- 2 frames are common among inexpensive systems and complete frame accuracy is more a matter of the users' budget than the capability of the recording system.

Burnett, Ron↗
NASA NTRS1994

Closed-loop carrier phase synchronization techniques motivated by likelihood functions

This article reexamines the notion of closed-loop carrier phase synchronization motivated by the theory of maximum a posteriori phase estimation with emphasis on the development of new structures based on both maximum-likelihood and average-likelihood functions. The criterion of performance used for comparison of all the closed-loop structures discussed is the mean-squared phase error for a fixed-loop bandwidth.

Tsou, H.↗
NASA NTRS1992

ULF wave electric fields at synchronous orbit during substorms

Characteristics of the wave electric fields in the frequency range 3 to 22 mHz in the equatorial plane (on GEOS 2) during four substorms were investigated. During intervals of about ten min duration after the substorm onset, large electric fields appear at synchronous orbit. In particular, 3 to 4 spectral peaks in the wave electric field data can be discerned. The dominant models occur at approximately 6 and approximately 10 mHz. Additional peaks exist around 15 and 20 mHz. Although the sense of polarization of these modes varies, these modes tend to have right handed polarization with a typical ellipticity of 0.2. The major axis usually points to the east, lying at an angle of 0 to 50 deg with the azimuthal direction. Mapped into the ionosphere, this corresponds to a north-west orientation. New wave activity appears some 30 to 40 min after the onset lasting about 10 min. The wave properties of these events are relatively close to those events occurring near the substorm onset, except for the orientation angle of the major axis which is now 140 to 180 deg. Because the magnetometer data is not available, the generation mechanism of the waves studied is hard to identify. There is some evidence that these waves are a superposition of compressional waves, produced by the collapse of the magnetotail, and the field line resonances, triggered by these compressional waves.

Laakso, Harri↗
NASA NTRS1992

Synchronous dynamics of a coupled shaft/bearing/housing system with auxiliary support from a clearance bearing: Analysis and experiment

This study examines the response of a flexible rotor supported by load sharing between linear bearings and an auxiliary clearance bearing. The objective of the work is to develop a better understanding of the dynamical behavior of a magnetic bearing supported rotor system interacting with auxiliary bearings during a critical operating condition. Of particular interest is the effect of coupling between the bearing/housing and shaft vibration on the rotordynamical responses. A simulation model is developed and a number of studies are performed for various parametric configurations. An experimental investigation is also conducted to compare and verify the rotordynamic behavior predicted by the simulation studies. A strategy for reducing synchronous shaft vibration through appropriate design of coupled shaft/bearing/housing vibration modes is identified. The results are presented and discussed.

Lawen, James, Jr.↗