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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Clock Agreement Among Parallel Supercomputer Nodes

This dataset presents measurements that quantify the clock synchronization time-agreement characteristics among several high performance computers including the current world's most powerful machine for open science, the U.S. Department of Energy's Titan machine sited at Oak Ridge National Laboratory. These ultra-fast machines derive much of their computational capability from extreme node counts (over 18000 nodes in the case of the Titan machine). Time-agreement is commonly utilized by parallel programming applications and tools, distributed programming application and tools, and system software. Our time-agreement measurements detail the degree of time variance between nodes and how that variance changes over time. The dataset includes empirical measurements and the accompanying spreadsheets.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS2002

A Conceptual Design for a Reliable Optical Bus (ROBUS)

The Scalable Processor-Independent Design for Electromagnetic Resilience (SPIDER) is a new family of fault-tolerant architectures under development at NASA Langley Research Center (LaRC). The SPIDER is a general-purpose computational platform suitable for use in ultra-reliable embedded control applications. The design scales from a small configuration supporting a single aircraft function to a large distributed configuration capable of supporting several functions simultaneously. SPIDER consists of a collection of simplex processing elements communicating via a Reliable Optical Bus (ROBUS). The ROBUS is an ultra-reliable, time-division multiple access broadcast bus with strictly enforced write access (no babbling idiots) providing basic fault-tolerant services using formally verified fault-tolerance protocols including Interactive Consistency (Byzantine Agreement), Internal Clock Synchronization, and Distributed Diagnosis. The conceptual design of the ROBUS is presented in this paper including requirements, topology, protocols, and the block-level design. Verification activities, including the use of formal methods, are also discussed.

Miner, Paul S.↗
NASA NTRS2008

A Primer on Architectural Level Fault Tolerance

This paper introduces the fundamental concepts of fault tolerant computing. Key topics covered are voting, fault detection, clock synchronization, Byzantine Agreement, diagnosis, and reliability analysis. Low level mechanisms such as Hamming codes or low level communications protocols are not covered. The paper is tutorial in nature and does not cover any topic in detail. The focus is on rationale and approach rather than detailed exposition.

Butler, Ricky W.↗
NASA NTRS1989

Slot clock recovery in optical PPM communication systems with avalanche photodiode photodetectors

Slot timing recovery in a direct-detection optical PPM communication system can be achieved by processing the photodetector output waveform with a nonlinear device whose output forms the input to a phase-locked loop. The choice of a simple transition detector as the nonlinearity is shown to give satisfactory synchronization performance. The rms phase error of the recovered slot clock and the effect of slot timing jitter on the bit error probability were directly measured. The experimental system consisted of an AlGaAs laser diode (wavelength = 834 nm) and a silicon avalanche photodiode photodetector. The system used Q = 4 PPM signaling and operated at a source data rate of 25 Mbits/s. The mathematical model developed to compute the rms phase error of the recovered clock is shown to be in good agreement with results of actual measurements of phase errors. The use of the recovered slot clock in the receiver resulted in no significant degradation in receiver sensitivity compared to a system with perfect slot timing. The system achieved a bit error probability of 10 to the -6th at a received optical signal energy of 55 detected photons per information bit.

Davidson, Frederic M.↗
NASA NTRS1984

First Results of GPS Time Transfer to Australia

A global positioning system (GPS) time transfer unit was installed at Tidbinbilla Deep Space Communications Complex of the DSN in June 1983. It was used to estimate the relationship to UTC(USNO MC) of the Tidbinbilla frequency and time system TID(FTS) based on a hydrogen maser, and to estimate the performance of the Australian free-running time scale UTC(AUS). Data from the first 3 months were analyzed three ways: by two-hop common view using JPL as intermediary; by long-arc interpolation of measurements against space vehicle clocks; and by long arc interpolation of GPS-Time results. Residuals from a single quadratic fit through 3 months of UTC(USNO MC) -TID(FTS) results were white noise with standard error 15 ns, and a flying clock measurement gave 70 ns agreement. A straight line fit through results UTC(USNO MC) - UTC(AUS) gave 90 ns standard error and 120 ns agreement. It is proposed to use the GPS measurements to steer UTC(AUS) to UTC(BIH), and to rename the existing time scale TA(AUS).

Mck.luck, J.↗
NASA NTRS1985

First results of GPS time transfer to Australia

A Global Positioning System time transfer unit built by NBS under contract to JPL was installed at Tidbinbilla Deep Space Communications Complex of the NASA Deep Space Network in June 1983. It has been used to estimate the relationship to UTC(USNO MC) of the Tidbinbilla frequency and time system TID(FTS) based on a hydrogen maser, and thence to estimate the performance of the Australian free-running time scale UTC(AUS). Data from the first three months has been analyzed three ways: by two-hop common view using JPL as intermediary; by long-arc interpolation of measurements against space vehicle clocks; and by long-arc interpolation of GPS-Time results. Residuals from a single quadratic fit through three months of UTC(USNO MC) - TID (FTS) results were white noise with standard error 15 ns, and a flying clock measurement gave 70 ns agreement. A straight line fit through results UTC(USNO MC) - UTC (AUS) gave 90 ns standard error and 120 bns agreement. It is proposed to use the GPS measurements to steer UTS(AUS) to UTC(BIH), and to rename the existing time scale TA(AUS).

Luck, J. M.↗
NASA NTRS2005

ROBUS-2: A Fault-Tolerant Broadcast Communication System

The Reliable Optical Bus (ROBUS) is the core communication system of the Scalable Processor-Independent Design for Enhanced Reliability (SPIDER), a general-purpose fault-tolerant integrated modular architecture currently under development at NASA Langley Research Center. The ROBUS is a time-division multiple access (TDMA) broadcast communication system with medium access control by means of time-indexed communication schedule. ROBUS-2 is a developmental version of the ROBUS providing guaranteed fault-tolerant services to the attached processing elements (PEs), in the presence of a bounded number of faults. These services include message broadcast (Byzantine Agreement), dynamic communication schedule update, clock synchronization, and distributed diagnosis (group membership). The ROBUS also features fault-tolerant startup and restart capabilities. ROBUS-2 is tolerant to internal as well as PE faults, and incorporates a dynamic self-reconfiguration capability driven by the internal diagnostic system. This version of the ROBUS is intended for laboratory experimentation and demonstrations of the capability to reintegrate failed nodes, dynamically update the communication schedule, and tolerate and recover from correlated transient faults.

Torres-Pomales, Wilfredo↗
NASA NTRS2004

A Unified Fault-Tolerance Protocol

Davies and Wakerly show that Byzantine fault tolerance can be achieved by a cascade of broadcasts and middle value select functions. We present an extension of the Davies and Wakerly protocol, the unified protocol, and its proof of correctness. We prove that it satisfies validity and agreement properties for communication of exact values. We then introduce bounded communication error into the model. Inexact communication is inherent for clock synchronization protocols. We prove that validity and agreement properties hold for inexact communication, and that exact communication is a special case. As a running example, we illustrate the unified protocol using the SPIDER family of fault-tolerant architectures. In particular we demonstrate that the SPIDER interactive consistency, distributed diagnosis, and clock synchronization protocols are instances of the unified protocol.

Miner, Paul↗
NASA NTRS1979

T and F comparisons via broadcasting satellite and navigation technology satellite

The preliminary time/frequency dissemination experiments were made using the BSE. In order to establish the technique of the doppler shift canceling, the phase control servo including the satellite link and the precompensating frequency control using the measured values or using the orbital data of the satellite are tested. The amount of the residual doppler shift at the control station can be reduced to the order of 1 part in 10 to the 12th power or less by use of the first and the second methods. The method using the orbit data is expected to give a control capability of a few parts in 10 to 11th power. Thus, the maximum value of the doppler shift at the farther most place of the country is estimated to be + or - 2 x 10 to the -10 power without any correction. The experiment of the international time comparison via the NTS 1 was made for about one year since October 1978. The data of time difference between UTC(USNO) and UTC(RRL) are in good agreement with those via the portable clock of the USNO. By applying the correction for ionospheric delay the standard deviation of the data can be reduced to about one-half.

Saburi, Y.↗
NASA NTRS1979

Traveling clock verification of VLBI synchronization

Four experiments are described which involved measurements of clock offsets at two DSN stations. Both VLBI and traveling clock measurements were performed and the agreement between the two methods was within about 6 ns for all four comparisons.

Young, L. E.↗
NASA NTRS2005

Design of the Protocol Processor for the ROBUS-2 Communication System

The ROBUS-2 Protocol Processor (RPP) is a custom-designed hardware component implementing the functionality of the ROBUS-2 fault-tolerant communication system. The Reliable Optical Bus (ROBUS) is the core communication system of the Scalable Processor-Independent Design for Enhanced Reliability (SPIDER), a general-purpose fault tolerant integrated modular architecture currently under development at NASA Langley Research Center. ROBUS is a time-division multiple access (TDMA) broadcast communication system with medium access control by means of time-indexed communication schedule. ROBUS-2 is a developmental version of the ROBUS providing guaranteed fault-tolerant services to the attached processing elements (PEs), in the presence of a bounded number of faults. These services include message broadcast (Byzantine Agreement), dynamic communication schedule update, time reference (clock synchronization), and distributed diagnosis (group membership). ROBUS also features fault-tolerant startup and restart capabilities. ROBUS-2 tolerates internal as well as PE faults, and incorporates a dynamic self-reconfiguration capability driven by the internal diagnostic system. ROBUS consists of RPPs connected to each other by a lower-level physical communication network. The RPP has a pipelined architecture and the design is parameterized in the behavioral and structural domains. The design of the RPP enables the bus to achieve a PE-message throughput that approaches the available bandwidth at the physical layer.

Torres-Pomales, Wilfredo↗
DOE OSTI2023

High-fidelity imaging of a band insulator in a three-dimensional optical lattice clock

Here, we report on the observation of a high-density, band insulating state in a three-dimensional optical lattice clock. Filled with a nuclear-spin-polarized degenerate Fermi gas of 87 Sr, the three-dimensional (3D) lattice has one atom per site in the ground motional state, thus guarding against frequency shifts due to contact interactions. At this high density where the average distance between atoms is comparable to the probe wavelength, conventional imaging techniques at saturation intensity suffer from large systematic errors. To spatially probe frequency shifts in the clock and measure thermodynamic properties of this system, accurate imaging techniques at high optical depths are required. Using a combination of highly saturated fluorescence and absorption imaging, we confirm the density distribution in our 3D optical lattice in agreement with a single spin band insulating state. Combining our clock platform with this high filling fraction opens the door to studying new classes of long-lived, many-body states arising from dipolar interactions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1994

An algorithm for the Italian atomic time scale

During the past twenty years, the time scale at the IEN has been realized by a commercial cesium clock, selected from an ensemble of five, whose rate has been continuously steered towards UTC to maintain a long term agreement within 3 x 10(exp -13). A time scale algorithm, suitable for a small clock ensemble and capable of improving the medium and long term stability of the IEN time scale, has been recently designed taking care of reducing the effects of the seasonal variations and the sudden frequency anomalies of the single cesium clocks. The new time scale, TA(IEN), is obtained as a weighted average of the clock ensemble computed once a day from the time comparisons between the local reference UTC(IEN) and the single clocks. It is foreseen to include in the computation also ten cesium clocks maintained in other Italian laboratories to further improve its reliability and its long term stability. To implement this algorithm, a personal computer program in Quick Basic has been prepared and it has been tested at the IEN time and frequency laboratory. Results obtained using this algorithm on the real clocks data relative to a period of about two years are presented.

Cordara, F.↗
NASA NTRS1988

Optical communication with semiconductor laser diodes

Slot timing recovery in a direct detection optical PPM communication system can be achieved by processing the photodetector waveform with a nonlinear device whose output forms the input to a phase lock group. The choice of a simple transition detector as the nonlinearity is shown to give satisfactory synchronization performance. The rms phase error of the recovered slot clock and the effect of slot timing jitter on the bit error probability were directly measured. The experimental system consisted of an AlGaAs laser diode (lambda = 834 nm) and a silicon avalanche photodiode (APD) photodetector and used Q=4 PPM signaling operated at a source data rate of 25 megabits/second. The mathematical model developed to characterize system performance is shown to be in good agreement with actual performance measurements. The use of the recovered slot clock in the receiver resulted in no degradation in receiver sensitivity compared to a system with perfect slot timing. The system achieved a bit error probability of 10 to the minus 6 power at received signal energies corresponding to an average of less than 60 detected photons per information bit.

Davidson, F.↗
NASA NTRS1990

Word timing recovery in direct detection optical PPM communication systems with avalanche photodiodes using a phase lock loop

A technique for word timing recovery in a direct-detection optical PPM communication system is described. It tracks on back-to-back pulse pairs in the received random PPM data sequences with the use of a phase locked loop. The experimental system consisted of an 833-nm AlGaAs laser diode transmitter and a silicon avalanche photodiode photodetector, and it used Q = 4 PPM signaling at source data rate 25 Mb/s. The mathematical model developed to describe system performance is shown to be in good agreement with the experimental measurements. Use of this recovered PPM word clock with a slot clock recovery system caused no measurable penalty in receiver sensitivity. The completely self-synchronized receiver was capable of acquiring and maintaining both slot and word synchronizations for input optical signal levels as low as 20 average detected photons per information bit. The receiver achieved a bit error probability of 10 to the -6th at less than 60 average detected photons per information bit.

Sun, Xiaoli↗
NASA NTRS1990

MAFT: The Multicomputer Architecture for Fault-Tolerance

Multicomputer Architecture for Fault-Tolerance (MAFT) is a loosely coupled multiprocessor system designed to achieve an unreliability of less than 10(exp -10)/hr in flight-critical real time applications. The MAFT design objectives and architecture are presented. The fault-tolerance implementation of major functions in MAFT is also presented, including communication; task scheduling; reconfiguration; clock synchronization; and data handling and voting. The need for Byzantine agreement or approximate agreement in various functions is discussed. Different methods were selected to achieve agreement in various subsystems. These methods are illustrated by a more detailed description of the task scheduling and error handling subsystems.

Kieckhafer, Roger M.↗
DOE OSTI2017

An evaluation of the state of time synchronization on leadership class supercomputers

We present a detailed examination of time agreement characteristics for nodes within extreme-scale parallel computers. Using a software tool we introduce in this paper, we quantify attributes of clock skew among nodes in three representative high-performance computers sited at three national laboratories. Our measurements detail the statistical properties of time agreement among nodes and how time agreement drifts over typical application execution durations. We discuss the implications of our measurements, why the current state of the field is inadequate, and propose strategies to address observed shortcomings.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS1998

The Distant Tail at 200 R(sub E): Comparison Between Geotail Observations and the Results from a Global Magnetohydrodynamic Simulation

This paper reports a comparison between Geotail observations of plasmas and magnetic fields at 200 R(sub E) in the Earth's magnetotail with results from a time-dependent, global magnetohydrodynamic simulation of the interaction of the solar wind with the magnetosphere. The study focuses on observations from July 7, 1993, during which the Geotail spacecraft crossed the distant tail magnetospheric boundary several times while the interplanetary magnetic field (IMF) was predominantly northward and was marked by slow rotations of its clock angle. Simultaneous IMP 8 observations of solar wind ions and the IMF were used as driving input for the MHD simulation, and the resulting time series were compared directly with those from the Geotail spacecraft. The very good agreement found provided the basis for an investigation of the response of the distant tail associated with the clock angle of the IMF. Results from the simulation show that the stresses imposed by the draping of magnetosheath field lines and the asymmetric removal of magnetic flux tailward of the cusps altered considerably the shape of the distant tail as the solar wind discontinuities convected downstream of Earth. As a result, the cross section of the distant tail was considerably flattened along the direction perpendicular to the IMF clock angle, the direction of the neutral sheet following that of the IMF. The simulation also revealed that the combined action of magnetic reconnection and the slow rotation of the IMF clock angle led to a braiding of the distant tail's magnetic field lines along the axis of the tail, with the plane of the braid lying in the direction of the IMF.

Berchem, J.↗