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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Robust Power System Stability Assessment Against Adversarial Machine Learning-Based Cyberattacks via Online Purification

The increasing complexity associated with renewable generation brings more challenges to power system stability assessment (SA). Data-driven approaches based on machine learning (ML) techniques for stability assessment have received significant research interest and shown their promising performance. However, ML-based models are recognized to be vulnerable to adversarial disturbances, where a slight perturbation to power system measurements could lead to unacceptable errors. To address this issue, this paper develops a novel lightweight mitigation strategy, i.e., robust online stability assessment (ROSA), to enhance the ML-based assessment model against both white-box and the black-box adversarial disturbances (i.e., purification) in the online implementation. The ROSA involves a supervised learning-based module for the primary stability assessment and a self-supervised learning-based module. Further, the two modules are trained jointly with different objective (loss) functions and implemented in sequence. A suitable purification objective and various time-series data augmentation methods are designed for SA applications to tackle adversarial disturbances adaptively. Case studies are performed, and the comparative results have clearly illustrated the competitive, robust accuracy against various adversarial scenarios and verified the effectiveness of the proposed online purification strategy.

24 POWER TRANSMISSION AND DISTRIBUTION↗
NASA NTRS1976

A brief description of the Jameson-Caughey NYU transonic swept-wing computer program: FLO 22

A computer program for analyzing inviscid, isentropic, transonic flow past 3-D swept configurations is presented. Some basic aspects of the program are: (1) the free-stream Mach number is restricted only by the isentropic assumption; (2) weak shock waves are automatically located wherever they occur in the flow; (3) the finite-difference form of the full equation for the velocity potential is solved by the method of relaxation, after the flow exterior to the airfoil is mapped to the upper half plane; (4) the mapping procedure allows exact satisfaction of the boundary conditions and use of supersonic free stream velocities; (5) the finite difference operator is locally rotated in supersonic flow regions so as to properly account for the domain of dependence; and (6) the relaxation algorithm was stabilized using criteria from a time-like analogy.

Jameson, A.↗
NASA NTRS1981

Control technology as applied to Space Telescope

The pointing control system (PCS) of the Space Telescope is addressed. The sensors employed in the primary PCS mode are the rate gyro assembly and the fine guidance sensors, while four reaction wheel assemblies provide control torques. Stability considerations based on structural modes are discussed, noting the stability margin criteria used in the design. Acquisition methodology is considered with regard to the PCS component hardware simulators and vehicle dynamics simulation.

Dougherty, H.↗
NASA NTRS1983

Spherical accretion onto quasars

The problem of time-steady accretion of gas irradiated by a self-consistently generated quasar-like continuum is investigated. A controversy in the literature is resolved by a careful distinction between the existence of solutions and their stability. Specific criteria are found for when thermal instabilities may disrupt single-phase steady flow. As a function of initial temperature (1,000,000-100,000,000 K), the boundaries of stable flow in the luminosity are determined relative to the Eddington (L/LE)-efficiency plane. The thermal and dynamical history of these steady flows is analyzed. The observational data that define quasars and type I Seyfert galaxies are matched to find values of efficiency and L/LE required for consistent spherically symmetric steady-flow models. The efficiency is constrained to be less than 1/100, and L/LE resides between several times 1/1000-1/100.

Krolik, J. H.↗
NASA NTRS1985

Crystal Chemistry of Meteoritic Hibonites

The characteristics of cation vacancies and site occupancies of natural hibonites in meteors are discussed. Note is taken of the effect of the specific locations of the hibonites in the matrices on the crystal chemistry, electronic stabilities, and spectral features of the metal ions which replace the Al(3+). Hibonites form beta-alumina hexagonal cells with 26 cation sites. Fe cations are found in octahedral, tetrahedral, and five-fold coordinates. Fe(2+) is found in face-shared Al(3) octahedra, a siting explained in terms of ionic radius and crystal field stabilization energy criteria. Spectral colors are attributed to V(3+) and Ti(3+) field transitions at 400 and 700 nm, respectively, although the 700 nm line may arise from transitions of Ti(3+) to Ti(4+) or Fe(2+) to Fe(3+) during heating. Excesses of Mg-26 can be traced to decay of nebular Al-26.

Burns, R. G.↗
NASA NTRS1984

Crystal chemistry of meteoritic hibonites

The characteristics of cation vacancies and site occupancies of natural hibonites in meteors are discussed. Note is taken of the effect of the specific locations of the hibonites in the matrices on the crystal chemistry, electronic stabilities, and spectral features of the metal ions which replace the Al(3+). Hibonites form beta-alumina hexagonal cells with 26 cation sites. Fe cations are found in octahedral, tetrahedral, and five-fold coordinations. Fe(2+) is found in face-shared Al(3) octahedra, a siting explained in terms of ionic radius and crystal field stabilization energy criteria. Spectral colors are attributed to V(3+) and Ti(3+) field transitions at 400 and 700 nm, respectively, although the 700 nm line may arise from transitions of Ti(3+) to Ti(4+) or Fe(2+) to Fe(3+) during heating. Excesses of Mg-26 can be traced to decay of nebular Al-26.

Burns, R. G.↗
NASA NTRS1991

Results of the 1989 SAR calibration experiment at Oberpfaffenhofen

Results from the 1989 Airborne Synthetic Aperture Radar (SAR) Calibration Campaign at the DLR test site in Oberpfaffenhofen are presented. Passive corner reflectors were used to derive the receiver and transmitter distortion matrices and the absolute calibration factor of the multispectral polarimetric DC-8 SAR from the complex high resolution data. A basic requirement for cross-calibration of data from different tracks is the stability of the SAR system. The polarimetric properties of the uncalibrated data were used to describe this stability. Quality criteria for a successful polarimetric calibration were provided by the channel balance and cross-talk parameters.

Zink, M.↗
NASA NTRS1993

Cross-calibration between airborne SAR sensors

As Synthetic Aperture Radar (SAR) system performance and experience in SAR signature evaluation increase, quantitative analysis becomes more and more important. Such analyses require an absolute radiometric calibration of the complete SAR system. To keep the expenditure on calibration of future multichannel and multisensor remote sensing systems (e.g., X-SAR/SIR-C) within a tolerable level, data from different tracks and different sensors (channels) must be cross calibrated. The 1989 joint E-SAR/DC-8 SAR calibration campaign gave a first opportunity for such an experiment, including cross sensor and cross track calibration. A basic requirement for successful cross calibration is the stability of the SAR systems. The calibration parameters derived from different tracks and the polarimetric properties of the uncalibrated data are used to describe this stability. Quality criteria for a successful cross calibration are the agreement of alpha degree values and the consistency of radar cross sections of equally sized corner reflectors. Channel imbalance and cross talk provide additional quality in case of the polarimetric DC-8 SAR.

Zink, Manfred↗
NASA NTRS1997

Galactic and Solar Cosmic Ray Shielding in Deep Space

An analysis of the radiation hazards in support of NASA deep space exploration activities is presented. The emphasis is on materials required for radiation protection shielding. Aluminum has been found to be a poor shield material when dose equivalent is used with exposure limits for low Earth orbit (LEO) as a guide for shield requirements. Because the radiation issues are cost related-the parasitic shield mass has high launch costs, the use of aluminum as a basic construction material is clearly not cost-effective and alternate materials need to be developed. In this context, polyethylene is examined as a potentially useful material and demonstrates important advantages as an alternative to aluminum construction. Although polyethylene is useful as a shield material, it may not meet other design criteria (strength, stability, thermal); other polymer materials must be examined.

Wilson, John W.↗
NASA NTRS2001

Predicting Turbulent Convective Heat Transfer in Fully Developed Duct Flows

The performance of an explicit algebraic stress model (EASM) is assessed in predicting the turbulent flow and forced heat transfer in both straight and wavy ducts, with rectangular, trapezoidal and triangular cross-sections, under fully developed conditions. A comparison of secondary flow patterns. including velocity vectors and velocity and temperature contours, are shown in order to study the effect of waviness on flow dynamics, and comparisons between the hydraulic parameters. Fanning friction factor and Nusselt number, are also presented. In all cases. isothermal conditions are imposed on the duct walls, and the turbulent heat fluxes are modeled using gradient-diffusion type models. The formulation is valid for Reynolds numbers up to 10(exp 5) and this minimizes the need for wall functions that have been used with mixed success in previous studies of complex duct flows. In addition, the present formulation imposes minimal demand on the number of grid points without any convergence or stability problems. Criteria in terms of heat transfer and friction factor needed to choose the optimal wavy duct cross-section for industrial applications among the ones considered are discussed.

Rokni, Masoud↗
NASA NTRS2009

Application of Bounded Linear Stability Analysis Method for Metrics-Driven Adaptive Control

This paper presents the application of Bounded Linear Stability Analysis (BLSA) method for metrics-driven adaptive control. The bounded linear stability analysis method is used for analyzing stability of adaptive control models, without linearizing the adaptive laws. Metrics-driven adaptive control introduces a notion that adaptation should be driven by some stability metrics to achieve robustness. By the application of bounded linear stability analysis method the adaptive gain is adjusted during the adaptation in order to meet certain phase margin requirements. Analysis of metrics-driven adaptive control is evaluated for a second order system that represents a pitch attitude control of a generic transport aircraft. The analysis shows that the system with the metrics-conforming variable adaptive gain becomes more robust to unmodeled dynamics or time delay. The effect of analysis time-window for BLSA is also evaluated in order to meet the stability margin criteria.

Bakhtiari-Nejad, Maryam↗
NASA NTRS2014

All-Digital Baseband 65nm PLL/FPLL Clock Multiplier using 10-cell Library

PLLs for clock generation are essential for modern circuits, to generate specialized frequencies for many interfaces and high frequencies for chip internal operation. These circuits depend on analog circuits and careful tailoring for each new process, and making them fault tolerant is an incompletely solved problem. Until now, all digital PLLs have been restricted to sampled data DSP techniques and not available for the highest frequency baseband applications. This paper presents the design and preliminary evaluation of an all-digital baseband technique built entirely with an easily portable 10-cell digital library. The library is also described, as it aids in research and low volume design porting to new processes. The advantages of the digital approach are the wide variety of techniques available to give varying degrees of fault tolerance, and the simplicity of porting the design to new processes, even to exotic processes that may not have analog capability. The only tuning parameter is digital gate delay. An all-digital approach presents unique problems and standard analog loop stability design criteria cannot be directly used. Because of the quantization of frequency, there is effectively infinite gain for very small loop error feedback. The numerically controlled oscillator (NCO) based on a tapped delay line cannot be reliably updated while a pulse is active in the delay line, and ordinarily does not have enough frequency resolution for a low-jitter output.

Shuler, Robert L., Jr.↗
NASA NTRS2014

ALL-Digital Baseband 65nm PLL/FPLL Clock Multiplier Using 10-Cell Library

PLLs for clock generation are essential for modern circuits, to generate specialized frequencies for many interfaces and high frequencies for chip internal operation. These circuits depend on analog circuits and careful tailoring for each new process, and making them fault tolerant is an incompletely solved problem. Until now, all digital PLLs have been restricted to sampled data DSP techniques and not available for the highest frequency baseband applications. This paper presents the design and preliminary evaluation of an all-digital baseband technique built entirely with an easily portable 10-cell digital library. The library is also described, as it aids in research and low volume design porting to new processes. The advantages of the digital approach are the wide variety of techniques available to give varying degrees of fault tolerance, and the simplicity of porting the design to new processes, even to exotic processes that may not have analog capability. The only tuning parameter is digital gate delay. An all-digital approach presents unique problems and standard analog loop stability design criteria cannot be directly used. Because of the quantization of frequency, there is effectively infinite gain for very small loop error feedback. The numerically controlled oscillator (NCO) based on a tapped delay line cannot be reliably updated while a pulse is active in the delay line, and ordinarily does not have enough frequency resolution for a low-jitter output.

Schuler, Robert L., Jr.↗
NASA NTRS1993

Structural design/margin assessment

Determining structural design inputs and the structural margins following design completion is one of the major activities in space exploration. The end result is a statement of these margins as stability, safety factors on ultimate and yield stresses, fracture limits (fracture control), fatigue lifetime, reuse criteria, operational criteria and procedures, stability factors, deflections, clearance, handling criteria, etc. The process is normally called a load cycle and is time consuming, very complex, and involves much more than structures. The key to successful structural design is the proper implementation of the process. It depends on many factors: leadership and management of the process, adequate analysis and testing tools, data basing, communications, people skills, and training. This process and the various factors involved are discussed.

Ryan, R. S.↗
NASA NTRS1979

Lightman-Eardley instabilities and accretion disk thickening

After reviewing the role of Compton scattering in accretion disks around black holes, it is discussed whether Lightman-Eardley (LE) secular instabilities can trigger and maintain Pringle-Rees (PR) thermal instabilities. The radiative-transfer-equation and equation-of-state criteria for LE stability in alpha-viscosity-law disk models and dynamic viscosity criteria for more general situations is derived. On the basis of these considerations the LE instability is insufficient for inducing PR instabilities and hot thick inner regions important in accretion-disk models of compact hard X-ray sources. The density thinning due to radial velocity gradients in the accretion flow is suggested as a more likely and satisfactory mechanism.

Stoeger, W. R.↗