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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 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

All-YBa2Cu3O(7-x) edge-geometry weak links

High quality YBa2Cu3O(7-x)/normal-metal/YBa2Cu3O(7-x) edge-geometry weak links are fabricated using nonsuperconducting Y-Ba-Cu-O barrier layers deposited by laser ablation at reduced growth temperatures. Devices incorporating 25-100-A-thick barrier layers exhibit current-voltage characteristics consistent with the resistively-shunted-junction model, as well as ac Josephson steps. Magnetic field modulation of the critical currents shows Fraunhofer-like behavior with a periodicity which scales with device area. The critical currents vary exponentially with barrier thickness indicating an effective normal metal coherence length of 20 A. The average resistances scale linearly with Y-Ba-Cu-O interlayer thickness and device area. The scaling behavior of the device parameters combined with the well-behaved magnetic field modulation of the critical currents demonstrates good barrier-layer uniformity with low pinhole densities. Near the transition temperature, the critical currents exhibit a temperature dependence proportional to (1 - T/Tc) squared.

Hunt, B. D.↗
NASA NTRS1992

Space Communication Artificial Intelligence for Link Evaluation Terminal (SCAILET)

A software application to assist end-users of the high burst rate (HBR) link evaluation terminal (LET) for satellite communications is being developed. The HBR LET system developed at NASA Lewis Research Center is an element of the Advanced Communications Technology Satellite (ACTS) Project. The HBR LET is divided into seven major subsystems, each with its own expert. Programming scripts, test procedures defined by design engineers, set up the HBR LET system. These programming scripts are cryptic, hard to maintain and require a steep learning curve. These scripts were developed by the system engineers who will not be available for the end-users of the system. To increase end-user productivity a friendly interface needs to be added to the system. One possible solution is to provide the user with adequate documentation to perform the needed tasks. With the complexity of this system the vast amount of documentation needed would be overwhelming and the information would be hard to retrieve. With limited resources, maintenance is another reason for not using this form of documentation. An advanced form of interaction is being explored using current computer techniques. This application, which incorporates a combination of multimedia and artificial intelligence (AI) techniques to provided end-users with an intelligent interface to the HBR LET system, is comprised of an intelligent assistant, intelligent tutoring, and hypermedia documentation. The intelligent assistant and tutoring systems address the critical programming needs of the end-user.

Shahidi, Anoosh K.↗
NASA NTRS1991

A vision-based end-point control for a two-link flexible manipulator

The measurement and control of the end-effector position of a large two-link flexible manipulator are investigated. The system implementation is described and an initial algorithm for static end-point positioning is discussed. Most existing robots are controlled through independent joint controllers, while the end-effector position is estimated from the joint positions using a kinematic relation. End-point position feedback can be used to compensate for uncertainty and structural deflections. Such feedback is especially important for flexible robots. Computer vision is utilized to obtain end-point position measurements. A look-and-move control structure alleviates the disadvantages of the slow and variable computer vision sampling frequency. This control structure consists of an inner joint-based loop and an outer vision-based loop. A static positioning algorithm was implemented and experimentally verified. This algorithm utilizes the manipulator Jacobian to transform a tip position error to a joint error. The joint error is then used to give a new reference input to the joint controller. The convergence of the algorithm is demonstrated experimentally under payload variation. A Landmark Tracking System (Dickerson, et al 1990) is used for vision-based end-point measurements. This system was modified and tested. A real-time control system was implemented on a PC and interfaced with the vision system and the robot.

Obergfell, Klaus↗
NASA NTRS1992

Advanced Communications Technology Satellite high burst rate link evaluation terminal experiment control and monitor software maintenance manual, version 1.0

The Experiment Control and Monitor (EC&M) software was developed at NASA Lewis Research Center to support the Advanced Communications Technology Satellite (ACTS) High Burst Rate Link Evaluation Terminal (HBR-LET). The HBR-LET is an experimenter's terminal to communicate with the ACTS for various investigations by government agencies, universities, and industry. The EC&M software is one segment of the Control and Performance Monitoring (C&PM) software system of the HBR-LET. The EC&M software allows users to initialize, control, and monitor the instrumentation within the HBR-LET using a predefined sequence of commands. Besides instrument control, the C&PM software system is also responsible for computer communication between the HBR-LET and the ACTS NASA Ground Station and for uplink power control of the HBR-LET to demonstrate power augmentation during rain fade events. The EC&M Software User's Guide, Version 1.0 (NASA-CR-189160) outlines the commands required to install and operate the EC&M software. Input and output file descriptions, operator commands, and error recovery procedures are discussed in the document. The EC&M Software Maintenance Manual, Version 1.0 (NASA-CR-189161) is a programmer's guide that describes current implementation of the EC&M software from a technical perspective. An overview of the EC&M software, computer algorithms, format representation, and computer hardware configuration are included in the manual.

Reinhart, Richard C.↗
NASA NTRS1992

Advanced communications technology satellite high burst rate link evaluation terminal experiment control and monitor software user's guide, version 1.0

The Experiment Control and Monitor (EC&M) software was developed at NASA Lewis Research Center to support the Advanced Communications Technology Satellite (ACTS) High Burst Rate Link Evaluation Terminal (HBR-LET). The HBR-LET is an experimenter's terminal to communicate with the ACTS for various investigations by government agencies, universities, and industry. The EC&M software is one segment of the Control and Performance Monitoring (C&PM) software system of the HBR-LET. The EC&M software allows users to initialize, control, and monitor the instrumentation within the HBR-LET using a predefined sequence of commands. Besides instrument control, the C&PM software system is also responsible for computer communication between the HBR-LET and the ACTS NASA Ground Station and for uplink power control of the HBR-LET to demonstrate power augmentation during rain fade events. The EC&M Software User's Guide, Version 1.0 (NASA-CR-189160) outlines the commands required to install and operate the EC&M software. Input and output file descriptions, operator commands, and error recovery procedures are discussed in the document.

Reinhart, Richard C.↗
NASA NTRS1992

Single link flexible beam testbed project

This thesis describes the single link flexible beam testbed at the CLaMS laboratory in terms of its hardware, software, and linear model, and presents two controllers, each including a hub angle proportional-derivative (PD) feedback compensator and one augmented by a second static gain full state feedback loop, based upon a synthesized strictly positive real (SPR) output, that increases specific flexible mode pole damping ratios w.r.t the PD only case and hence reduces unwanted residual oscillation effects. Restricting full state feedback gains so as to produce a SPR open loop transfer function ensures that the associated compensator has an infinite gain margin and a phase margin of at least (-90, 90) degrees. Both experimental and simulation data are evaluated in order to compare some different observer performance when applied to the real testbed and to the linear model when uncompensated flexible modes are included.

Hughes, Declan↗
NASA NTRS1993

Advanced communications technology satellite high burst rate link evaluation terminal communication protocol software user's guide, version 1.0

The Communication Protocol Software was developed at the NASA Lewis Research Center to support the Advanced Communications Technology Satellite High Burst Rate Link Evaluation Terminal (ACTS HBR-LET). The HBR-LET is an experimenters terminal to communicate with the ACTS for various experiments by government, university, and industry agencies. The Communication Protocol Software is one segment of the Control and Performance Monitor (C&PM) Software system of the HBR-LET. The Communication Protocol Software allows users to control and configure the Intermediate Frequency Switch Matrix (IFSM) on board the ACTS to yield a desired path through the spacecraft payload. Besides IFSM control, the C&PM Software System is also responsible for instrument control during HBR-LET experiments, uplink power control of the HBR-LET to demonstrate power augmentation during signal fade events, and data display. The Communication Protocol Software User's Guide, Version 1.0 (NASA CR-189162) outlines the commands and procedures to install and operate the Communication Protocol Software. Configuration files used to control the IFSM, operator commands, and error recovery procedures are discussed. The Communication Protocol Software Maintenance Manual, Version 1.0 (NASA CR-189163, to be published) is a programmer's guide to the Communication Protocol Software. This manual details the current implementation of the software from a technical perspective. Included is an overview of the Communication Protocol Software, computer algorithms, format representations, and computer hardware configuration. The Communication Protocol Software Test Plan (NASA CR-189164, to be published) provides a step-by-step procedure to verify the operation of the software. Included in the Test Plan is command transmission, telemetry reception, error detection, and error recovery procedures.

Reinhart, Richard C.↗
NASA NTRS1993

Advanced communications technology satellite high burst rate link evaluation terminal power control and rain fade software test plan, version 1.0

The Power Control and Rain Fade Software was developed at the NASA Lewis Research Center to support the Advanced Communications Technology Satellite High Burst Rate Link Evaluation Terminal (ACTS HBR-LET). The HBR-LET is an experimenters terminal to communicate with the ACTS for various experiments by government, university, and industry agencies. The Power Control and Rain Fade Software is one segment of the Control and Performance Monitor (C&PM) Software system of the HBR-LET. The Power Control and Rain Fade Software automatically controls the LET uplink power to compensate for signal fades. Besides power augmentation, the C&PM Software system is also responsible for instrument control during HBR-LET experiments, control of the Intermediate Frequency Switch Matrix on board the ACTS to yield a desired path through the spacecraft payload, and data display. The Power Control and Rain Fade Software User's Guide, Version 1.0 outlines the commands and procedures to install and operate the Power Control and Rain Fade Software. The Power Control and Rain Fade Software Maintenance Manual, Version 1.0 is a programmer's guide to the Power Control and Rain Fade Software. This manual details the current implementation of the software from a technical perspective. Included is an overview of the Power Control and Rain Fade Software, computer algorithms, format representations, and computer hardware configuration. The Power Control and Rain Fade Test Plan provides a step-by-step procedure to verify the operation of the software using a predetermined signal fade event. The Test Plan also provides a means to demonstrate the capability of the software.

Reinhart, Richard C.↗
NASA NTRS1993

Modulation and coding for throughput-efficient optical free-space links

Optical direct-detection systems are currently being considered for some high-speed inter-satellite links, where data-rates of a few hundred megabits per second are evisioned under power and pulsewidth constraints. In this paper we investigate the capacity, cutoff-rate and error-probability performance of uncoded and trellis-coded systems for various modulation schemes and under various throughput and power constraints. Modulation schemes considered are on-off keying (OOK), pulse-position modulation (PPM), overlapping PPM (OPPM) and multi-pulse (combinatorial) PPM (MPPM).

Georghiades, Costas N.↗
NASA NTRS1991

The Space Station end-to-end data system - Linking architecture and operations

The paper describes tha data system designed for linking and integrating the Space Station Freedom (SSF) space and ground architecture and for providing instrument data to the user and discusses the operations of the system. Particular attention is given to the objectives, the organization, and the schedule of the SSF Program, the infrastructure elements of the end-to-end data system, the key active functional areas of the system, the types of data, the user scenario, and the SSF's space, space-to-ground, and ground infrastructures as they are active in support of the stipulated user scenario.

Fishkind, Stanley↗
NASA NTRS1993

A two cable, six link boom crane for lunar construction

This paper presents the conceptual design and analysis of a modified crane boom and cable suspension which provide contro1 over all six degrees of freedom of a payload. Two cables pass around pulleys to form six links between the payload and boom. A linearization of the pulley mechanics was derived to create finite element models of the system. The models were experimentally verified and used to explore variations of the suspension geometry. Several crane concepts which use the suspension are discussed and illustrated.

Taylor, Robert M.↗
NASA NTRS1992

High-precision VLBI astrometry of the radio-emitting star Sigma CrB - A step in linking the Hipparcos and extragalactic reference frames

VLBI observations of the optically bright radio-emitting star Sigma Coronae Borealis (Sigma CrB) have yielded its position relative to an angularly nearby quasar, its annual proper motion and its trigonometric parallax with formal uncertainties slightly better than 0.2 milliarcsec. This is the first result of the VLBI astrometric program we are conducting on 11 radio stars similar to Sigma CrB. We plan to obtain comparable results for all stars when enough epochs of observations are acquired. These stars will be used to astrometrically link the Hipparcos and VLBI extragalactic reference frames.

Lestrade, Jean-Francois↗
NASA NTRS1993

Estimating the path-average rainwater content and updraft speed along a microwave link

There is a scarcity of methods for accurately estimating the mass of rainwater rather than its flux. A recently proposed technique uses the difference between the observed rates of attenuation A with increasing distance at 38 and 25 GHz, A(38-25), to estimate the rainwater content W. Unfortunately, this approach is still somewhat sensitive to the form of the drop-size distribution. An alternative proposed here uses the ratio A38/A25 to estimate the mass-weighted average raindrop size Dm. Rainwater content is then estimated from measurements of polarization propagation differential phase shift (Phi-DP) divided by (1-R), where R is the mass-weighted mean axis ratio of the raindrops computed from Dm. This paper investigates these two water-content estimators using results from a numerical simulation of observations along a microwave link. From these calculations, it appears that the combination (R, Phi-DP) produces more accurate estimates of W than does A38-25. In addition, by combining microwave estimates of W and the rate of rainfall in still air with the mass-weighted mean terminal fall speed derived using A38/A25, it is possible to detect the potential influence of vertical air motion on the raingage-microwave rainfall comparisons.

Jameson, Arthur R.↗
NASA NTRS1993

PTTI-aided ephemeris calculation and rapid data link acquisition for manned space flight

Complexity of future manned space flight mission control can be significantly reduced by integrating GPS, the PTTI source, into telemetry, tracking, and command (TT&C). Future telecommunications, space tracking electronic intelligence, metrology, navigation, and data acquisition will thereby be served, including: on-board ephemeris determination, reduced synchronization time for time division multiple access (TDMA) links, and in-flight clock calibration, increasing on-board autonomy and reducing ground support costs. Manned space transportation through the first quarter of the 21st century will probably depend on a mix of vehicles, including the Advanced Manned Launch System (AMLS), the Personnel Launch System (PLS), and continued use of the Shuttle Fleet. Precise Ephemeris is important on-board for mission success, status monitoring, also for rendezvous and docking. Use of GPS can eliminate ground based tracking/processing, enhancing autonomy and reducing communications bandwidth. GPS time can simplify complicated functions used in bandwidth efficient time division multiple access (TDMA) communications, such as: precise and realtime synchronization of receive reference timing, transmit-timing and acquisition control, unique synchronization word (UW) detection, and elastic buffering. High clock accuracy provides increased signal-to-noise (S/N) ratio during acquisition, permitting narrower acquisition frequency and time windows. Spaceborne systems requirements to provide capabilities such as: refinement of the GEM-72 gravity model based on satellite tracking observations from ATS-6 to GEOS-3, relativistic clock experiments, NASA crustal dynamics program for developing space geodetic techniques to study the earth's crust, its gravity field, and earthquake mechanisms, and multi-disciplinary space geodetic tracking for studying global climatic changes are also reviewed.

Anderman, Alfred↗
NASA NTRS1993

Coding performance of the Probe-Orbiter-Earth communication link

The coding performance of the Probe-Orbiter-Earth communication link is analyzed and compared for several cases. It is assumed that the coding system consists of a convolutional code at the Probe, a quantizer and another convolutional code at the Orbiter, and two cascaded Viterbi decoders or a combined decoder on the ground.

Divsalar, D.↗
NASA NTRS1993

Asynchronous transfer mode link performance over ground networks

The results of an experiment to determine the feasibility of using asynchronous transfer mode (ATM) technology to support advanced spacecraft missions that require high-rate ground communications and, in particular, full-motion video are reported. Potential nodes in such a ground network include Deep Space Network (DSN) antenna stations, the Jet Propulsion Laboratory, and a set of national and international end users. The experiment simulated a lunar microrover, lunar lander, the DSN ground communications system, and distributed science users. The users were equipped with video-capable workstations. A key feature was an optical fiber link between two high-performance workstations equipped with ATM interfaces. Video was also transmitted through JPL's institutional network to a user 8 km from the experiment. Variations in video depending on the networks and computers were observed, the results are reported.

Chow, E. T.↗