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At least 397 records · Page 22

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NASA NTRS1987

Reconfigurable tree architectures using subtree oriented fault tolerance

An approach to the design of reconfigurable tree architectures is presented in which spare processors are allocated at the leaves. The approach is unique in that spares are associated with subtrees, and sharing of spares between these subtrees can occur. The subtree-oriented fault-tolerance approach is more reliable than previous approaches capable of tolerating link and switch failures for both single-chip and multichip tree implementations while reducing redundancy in terms of both spare processors and links. VLSI layout is O(n) for binary trees and is directly extensible to N-ary trees and fault tolerance through performance degradation.

Lowrie, Matthew B.↗

NASA NTRS1987

The scatterometer for DIRBE

A scatterometer for monitoring the performance degradation of an infrared instrument in the Diffuse Infrared Background Experiment (DIRBE) due to contamination of the primary mirror is described. The scatterometer monitors the scattered light from an off-axis internal source to provide a measure of the integrity of the optical system. Preliminary results indicate that the measured BRDF of the primary mirror is two orders of magnitude worse than the specification, and thus may be unacceptable.

Bolton, John F.↗

NASA NTRS1988

Solar dynamic heat receiver thermal characteristics in low earth orbit

A simplified system model is under development for evaluating the thermal characteristics and thermal performance of a solar dynamic spacecraft energy system's heat receiver. Results based on baseline orbit, power system configuration, and operational conditions, are generated for three basic receiver concepts and three concentrator surface slope errors. Receiver thermal characteristics and thermal behavior in LEO conditions are presented. The configuration in which heat is directly transferred to the working fluid is noted to generate the best system and thermal characteristics. as well as the lowest performance degradation with increasing slope error.

Wu, Y. C.↗

NASA NTRS1987

Intermodulation interference avoidance for enhanced performance of ESA Deep Space Telecommand Systems

The performance degradation of the European Space Agency (ESA) Deep Space Telecommand receiving subsystem due to the presence of cross-modulation and ranging interference for simultaneous ranging and command operations on the Earth-to-space link is investigated. The most important result is that under imperfect carrier reference the command subcarrier and the ranging tone frequencies can be selected to suppress the cross-modulation interference and interference from the ranging channel for the specified command bit rate and ranging code component. Recommendations as to use of specific command subcarrier frequency, command bit rate, and ranging tone frequency are made for consideration in future standards systems.

Nguyen, Tien M.↗

NASA NTRS1988

Passive control plate for shock-boundary layer interactions

Computational and experimental results are presented for a passive method capable of reducing or completely eliminating the separation region in shock-boundary layer interactions of the type that can result in severe performance degradation due to flow separation in supersonic and hypersonic inlets. The method involves the placement of an embedded plate in the outer portion of the boundary layer, parallel to the wall, so that the incident shock will impinge upon, and be reflected from, its surface. This will lower the pressure gradient in the low-momentum, near-wall region. A porous control plate's effect has also been investigated.

Goodman, Wesley L.↗

NASA NTRS1988

Technique to select the optimum modulation indices for suppression of undesired signals for simultaneous range and data operations

An algorithm to search for the optimum set of modulation indices that will optimize a given simultaneous range/command/telemetry communications link is presented. This technique provides a way to suppress the ranging signal in order to limit performance degradation in the data channel due to interference from the ranging channel to a desired level, given a specified ranging accuracy. The link (when optimized) will (1) provide maximum available power to both the data and ranging channels for a specified degradation in the data channel so that it will transmit at the required data rate, (2) achieve a specified ranging accuracy over a maximum distance, under a certain set of conditions, and (3) provide adequate power for carrier tracking without degrading the data-channel thresholds. In addition, both data and ranging channels will fall below the threshold at the same point.

Nguyen, Tien Manh↗

NASA NTRS1988

Effects of switch leakages upon Nimbus-7 SMMR calibration

A calibration model for the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) is studied. This model not only removes major drawbacks of the current calibration model but also helps us understand the performance degradation of the aging instrument. The current Nimbus-7 SMMR calibration algorithm was derived without considering the interference effect between the two orthogonally polarized signals merging at a ferrite polarization selector switch. The resulting calibrated brightness temperatures, considered as a function of scan angle phi, are not symmetric around phi=0. However, neither the origin of the asymmetry nor the manner in which the two orthogonal components are mixed is fully understood. The proposed calibration model incorporates all the leakage factors associated with the ferrite switches along the signal paths. The resulting calibration equations clarify how the orthogonal components of surface brightness are coupled at radiometers. As a consequence, the origin of the asymmetry is clearly identified and explained. In addition, the feasibility of absolute calibration using in-orbit data is discussed.

Han, Daesoo↗

NASA NTRS1989

Microwave and millimeter-wave power generation in silicon carbide (SiC) IMPATT devices

There are two points that should be noted. First, in the thermal resistance calculations it is assumed that the device is operating at 773 K while the results of the room temperature simulations are used. This was done because there is not enough information to correctly predict the material parameters at 773 K. Since, in general, device performance degrades with increasing temperature, the cw results are perhaps a bit optimistic. Second, the electric field in these structures gets extremely high and there might be some possibility of tunneling. This was not incorporated into the simulation. Again, this could result in different device operating conditions.

Mehdi, I.↗

NASA NTRS1989

A new analysis of beam-waveguide antennas considering the presence of the enclosure

Beam-waveguide (BWG) antennas provide multiple frequency band operations and other operational benefits for large ground-station antennas. Present design practices use diffraction analyses that ignore the presence of the BWG enclosure and may be inaccurate at lower frequency bands for ground-station antennas operating over multiple frequency bands. Introduced here is a new analysis approach that considers the presence of the BWG enclosure. Results based on the new analysis have revealed new understandings of the performance degradation mechanisms in a BWG antenna and have provided direction for potential design improvements.

Cha, A. G.↗

NASA NTRS1988

LEO life tests on a 75 Ah bipolar nickel-hydrogen battery

The design, building, and testing of an actively cooled 10-cell 75-Ah bipolar nickel/hydrogen battery are discussed. During the last 1000 cycles, the battery has shown some evidence of elecrical performance degradation. In particular, EOC and EOD voltages have increased and decreased by several millivolts, respectively, and deep discharge capacities to a 1.0 V/cell average cutoff voltage have decreased.

Lenhart, S.↗

NASA NTRS1988

An approximation technique for computing optimal fixed-order controllers for infinite-dimensional systems

The finite-dimensional approximation of the infinite-dimensional Bernstein/Hyland optimal projection theory is investigated analytically. The approach yields fixed-finite-order controllers which are optimal with respect to high-order approximating finite-dimensional plant models. The technique is illustrated by computing a sequence of first-order controllers for a one-dimensional SISO parabolic (heat/diffusion) system using a spline-based Ritz-Galerkin finite-element approximation. The numerical studies indicate convergence of the feedback gains with less than 2-percent performance degradation over full-order LQG controllers.

Bernstein, Dennis S.↗

NASA NTRS1989

Additional software developments wanted for modeling and control of flexible space systems

Existing modeling and control software packages are either inadequate or inefficient for applications to flexible space structures. Some additional software developments are wanted for effective design and evaluation of the control systems. The following are discussed: (1) linear-quadratic optimal regulators as usual can be designed using various modern control design software packages, (2) modal dashpots are very effective output-feedback vibration controllers for flexible structures, not only effective for augmenting a small amount of active damping to a large number of vibration modes (like the so-called low-authority structural controllers), but also effective for quick suppression of large vibrations (like high-authority structural controllers), and (3) the actual performance of any control design needs to be evaluated against a faithful model of the flexible structure to be controlled. The potential of destabilization or serious performance degradation needs to be detected by numerical simulation of the structure with the control loops being closed.

Lin, Jiguan Gene↗

NASA NTRS1989

MT's algorithm: A new algorithm to search for the optimum set of modulation indices for simultaneous range, command, and telemetry

MT's algorithm was developed as an aid in the design of space telecommunications systems when utilized with simultaneous range/command/telemetry operations. This algorithm provides selection of modulation indices for: (1) suppression of undesired signals to achieve desired link performance margins and/or to allow for a specified performance degradation in the data channel (command/telemetry) due to the presence of undesired signals (interferers); and (2) optimum power division between the carrier, the range, and the data channel. A software program using this algorithm was developed for use with MathCAD software. This software program, called the MT program, provides the computation of optimum modulation indices for all possible cases that are recommended by the Consultative Committee on Space Data System (CCSDS) (with emphasis on the squarewave, NASA/JPL ranging system).

Nguyen, Tien Manh↗

NASA NTRS1990

Preliminary results from a subsonic high angle-of-attack flush airdata sensing (HI-FADS) system: Design, calibration, and flight test evaluation

A nonintrusive high angle-of-attack flush airdata sensing (HI-FADS) system was installed and flight-tested on the F-18 high alpha research flight vehicle. The system is a matrix of 25 pressure orifices in concentric circles on the nose of the vehicle. The orifices determine angles of attack and sideslip, Mach number, and pressure altitude. Pressure was transmitted from the orifices to an electronically scanned pressure module by lines of pneumatic tubing. The HI-FADS system was calibrated and demonstrated using dutch roll flight maneuvers covering large Mach, angle-of-attack, and sideslip ranges. Reference airdata for system calibration were generated by a minimum variance estimation technique blending measurements from two wingtip airdata booms with inertial velocities, aircraft angular rates and attitudes, precision radar tracking, and meteorological analyses. The pressure orifice calibration was based on identifying empirical adjustments to modified Newtonian flow on a hemisphere. Calibration results are presented. Flight test results used all 25 orifices or used a subset of 9 orifices. Under moderate maneuvering conditions, the HI-FADS system gave excellent results over the entire subsonic Mach number range up to 55 deg angle of attack. The internal pneumatic frequency response of the system is accurate to beyond 10 Hz. Aerodynamic lags in the aircraft flow field caused some performance degradation during heavy maneuvering.

Whitmore, Stephen A.↗

NASA NTRS1989

Reflector surface distortion analysis techniques (thermal distortion analysis of antennas in space)

A group of large computer programs are used to predict the farfield antenna pattern of reflector antennas in the thermal environment of space. Thermal Radiation Analysis Systems (TRASYS) is a thermal radiation analyzer that interfaces with Systems Improved Numerical Differencing Analyzer (SINDA), a finite difference thermal analysis program. The programs linked together for this analysis can now be used to predict antenna performance in the constantly changing space environment. They can be used for very complex spacecraft and antenna geometries. Performance degradation caused by methods of antenna reflector construction and materials selection are also taken into consideration. However, the principal advantage of using this program linkage is to account for distortions caused by the thermal environment of space and the hygroscopic effects of the dry-out of graphite/epoxy materials after the antenna is placed into orbit. The results of this type of analysis could ultimately be used to predict antenna reflector shape versus orbital position. A phased array antenna distortion compensation system could then use this data to make RF phase front corrections. That is, the phase front could be adjusted to account for the distortions in the antenna feed and reflector geometry for a particular orbital position.

Sharp, R.↗

NASA NTRS1989

Vector performance analysis of three supercomputers - Cray-2, Cray Y-MP, and ETA10-Q

Results are presented of a series of experiments to study the single-processor performance of three supercomputers: Cray-2, Cray Y-MP, and ETA10-Q. The main object of this study is to determine the impact of certain architectural features on the performance of modern supercomputers. Features such as clock period, memory links, memory organization, multiple functional units, and chaining are considered. A simple performance model is used to examine the impact of these features on the performance of a set of basic operations. The results of implementing this set on these machines for three vector lengths and three memory strides are presented and compared. For unit stride operations, the Cray Y-MP outperformed the Cray-2 by as much as three times and the ETA10-Q by as much as four times for these operations. Moreover, unlike the Cray-2 and ETA10-Q, even-numbered strides do not cause a major performance degradation on the Cray Y-MP. Two numerical algorithms are also used for comparison. For three problem sizes of both algorithms, the Cray Y-MP outperformed the Cray-2 by 43 percent to 68 percent and the ETA10-Q by four to eight times.

Fatoohi, Rod A.↗

NASA NTRS1990

Modification of the PARC Navier-Stokes Code to predict rocket engine nozzle performance

The PARC2D Navier-Stokes Code was modified to compute the performance parameters for rocket engine nozzles. The perfect gas code was applied to the SSME engine nozzle for inviscid, laminar and turbulent flow. Inviscid computations compare well with Rocketdyne computations. Performance degradation due to the boundary layers is very reasonable. Application of the code to nontraditional nozzle geometries and to low Reynolds nozzles is demonstrated. Modification of the code for equilibrium H2/O2 chemistry is described. Thermodynamic and equilibrium constants are determined from statistical mechanics and the transport properties from exact kinetic theory, using collison integrals determined from appropriate intermolecular potentials. The equilibrium code was used to compute the SSME flowfield. Modifications of the flowfield due to the change of composition are described.

Collins, Frank G.↗

NASA NTRS1989

A tone-aided dual vestigial sideband system for digital communications on fading channels

A spectrally efficient tone-aided dual vestigial sideband (TA/DVSB) system for digital data communications on fading channels is presented and described analytically. This PSK (phase-shift-keying) system incorporates a feed-forward, tone-aided demodulation technique to compensate for Doppler frequency shift and channel- induced, multipath fading. In contrast to other tone-in-band-type systems, receiver synchronization is derived from the complete data VSBs. Simulation results for the Rician fading channel are presented. These results demonstrate the receiver's ability to mitigate performance degradation due to fading and to obtain proper data carrier synchronization, suggesting that the proposed TA/DVSB system has promise for this application. Simulated BER (bit-error rate) data indicate that the TA/DVSB system effectively alleviates the channel distortions of the land mobile satellite application.

Hladik, Stephen M.↗