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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Time-dependent modulation of galactic cosmic rays by merged interaction regions

Models that solve the one-dimensional, solar modulation equation have reproduced the 11-year galactic cosmic ray using functional representations of global merged interaction regions (MIRs). This study extends those results to the solution of the modulation equation with explicit time dependence. The magnetometers on Voyagers 1 and 2 provide local magnetic field intensities at regular intervals, from which one calculates the ratio of the field intensity to the average local field. These ratios in turn are inverted to form diffusion coefficients. Strung together in radius and time, these coefficents then fall and rise with the strength of the interplanetary magnetic field, becoming representations of MIRs. These diffusion coefficients, calculated locally, propagate unchanged from approx. 10 AU to the outer boundary (120 AU). Inside 10 AU, all parameters, including the diffusion coefficient are assumed constant in time and space. The model reproduces the time-intensity profiles of Voyager 2 and Pioneer 10. Radial gradient data from 1982-1990 between Pioneer 10 and Voyager 2 are about the same magnitude as those calculated in the model. It is also shows agreement in rough magnitude with the radial gradient between Pioneer 10 and 1 AU. When coupled with enhanced, time-dependent solar wind speed at the probe's high latitude, as measured by independent observers, the model also follows Voyager 1's time-intensity profile reasonably well, providing a natural source the model also follows Voyager 1's time-intensity profile reasonably well, providing a natural source for the observed negative latitudinal gradients. The model exhibits the 11-year cyclical cosmic ray intensity behavior at all radii, including 1 AU, not just at the location of the spacecraft where the magnetic fields are measured. In addition, the model's point of cosmic ray maximum correctly travels at the solar wind speed, illustrating the well-known propagation of modulation. Finally, at least in the inner heliosphere this model accounts for the delay experienced by lower-rigidity protons in reaching their time-intensity peak. The actual delays in this model, however, are somewhat smaller than the data. In the outer heliosphere the models sees no delays, and the data are ambiguous as to their existence. It appears that strong magnetic field compression regions (merged interaction regions) that are 3-4 times the average field strength can, at least in a helioequatorial band, disrupt effects, such as drifts, that could dominate in quieter magnetic fields. The question remains: Is the heliosphere ever quiet enough to allow such effects to be unambiguously measured, at least in the midlatitudes?

Perko, J. S.↗
NASA NTRS1998

Spacecraft Doppler Tracking as a Narrow-Band Detector of Gravitational Radiation

We discuss a filtering technique for reducing the frequency fluctuations due to the troposphere, ionosphere, and mechanical vibrations of the ground antenna in spacecraft Doppler tracking searches for gravitational radiation. This method takes advantage of the sinusoidal behavior of the transfer function to the Doppler observable of these noise sources, which displays sharp nulls at selected Fourier components.

gravitational radiation doppler tracking↗
NASA NTRS2005

Data management in the mission data system

As spacecraft evolve from simple embedded devices to become more sophisticated computing platforms with complex behaviors it is increasingly necessary to model and manage the flow of data, and to provide uniform models for managing data that promote adaptability, yet pay heed to the physical limitations of the embedded and space environments.

mission data system (MDS)↗
NASA NTRS2007

Gravity-Dependent Combustion and Fluids Research - From Drop Towers to Aircraft to the ISS

Driven by the need for knowledge related to the low-gravity environment behavior of fluids in liquid fuels management, thermal control systems and fire safety for spacecraft, NASA embarked on a decades long research program to understand, accommodate and utilize the relevant phenomena. Beginning in the 1950s, and continuing through to today, drop towers and aircraft were used to conduct an ever broadening and increasingly sophisticated suite of experiments designed to elucidate the underlying gravity-dependent physics that drive these processes. But the drop towers and aircraft afford only short time periods of continuous low gravity. Some of the earliest rocket test flights and manned space missions hosted longer duration experiments. The relatively longer duration low-g times available on the space shuttle during the 1980s and 1990s enabled many specialized experiments that provided unique data for a wide range of science and engineering disciplines. Indeed, a number of STS-based Spacelab missions were dedicated solely to basic and applied microgravity research in the biological, life and physical sciences. Between 1980 and 2000, NASA implemented a vigorous Microgravity Science Program wherein combustion science and fluid physics were major components. The current era of space stations from the MIR to the International Space Station have opened up a broad range of opportunities and facilities that are now available to support both applied research for technologies that will help to enable the future exploration missions and for a continuation of the non-exploration basic research that began over fifty years ago. The ISS-based facilities of particular value to the fluid physics and combustion/fire safety communities are the Fluids and Combustion Facility Combustion Integrated Rack and the Fluids Integrated Rack.

Urban, David L.↗
NASA NTRS2003

Applications of Ergodic Theory to Coverage Analysis

The study of differential equations, or dynamical systems in general, has two fundamentally different approaches. We are most familiar with the construction of solutions to differential equations. Another approach is to study the statistical behavior of the solutions. Ergodic Theory is one of the most developed methods to study the statistical behavior of the solutions of differential equations. In the theory of satellite orbits, the statistical behavior of the orbits is used to produce 'Coverage Analysis' or how often a spacecraft is in view of a site on the ground. In this paper, we consider the use of Ergodic Theory for Coverage Analysis. This allows us to greatly simplify the computation of quantities such as the total time for which a ground station can see a satellite without ever integrating the trajectory, see Lo 1,2. More over, for any quantity which is an integrable function of the ground track, its average may be computed similarly without the integration of the trajectory. For example, the data rate for a simple telecom system is a function of the distance between the satellite and the ground station. We show that such a function may be averaged using the Ergodic Theorem.

Ergodic Theory↗
NASA NTRS2021

Timing Calibration of the NuSTAR X-Ray Telescope

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission is the first focusing X-ray telescope in the hard X-ray (3–79 keV) band. Among the phenomena that can be studied in this energy band, some require high time resolution and stability: rotation-powered and accreting millisecond pulsars, fast variability from black holes and neutron stars, X-ray bursts, and more. Moreover, a good alignment of the timestamps of X-ray photons to UTC is key for multi-instrument studies of fast astrophysical processes. In this paper, we describe the timing calibration of the NuSTAR mission. In particular, we present a method to correct the temperature-dependent frequency response of the on-board temperature-compensated crystal oscillator. Together with measurements of the spacecraft clock offsets obtained during downlinks passes, this allows a precise characterization of the behavior of the oscillator. The calibrated NuSTAR event timestamps for a typical observation are shown to be accurate to a precision of ∼65 μs.

Matteo Bachetti↗
NASA NTRS2015

Projection-Based Reduced Order Modeling for Spacecraft Thermal Analysis

This paper presents a mathematically rigorous, subspace projection-based reduced order modeling (ROM) methodology and an integrated framework to automatically generate reduced order models for spacecraft thermal analysis. Two key steps in the reduced order modeling procedure are described: (1) the acquisition of a full-scale spacecraft model in the ordinary differential equation (ODE) and differential algebraic equation (DAE) form to resolve its dynamic thermal behavior; and (2) the ROM to markedly reduce the dimension of the full-scale model. Specifically, proper orthogonal decomposition (POD) in conjunction with discrete empirical interpolation method (DEIM) and trajectory piece-wise linear (TPWL) methods are developed to address the strong nonlinear thermal effects due to coupled conductive and radiative heat transfer in the spacecraft environment. Case studies using NASA-relevant satellite models are undertaken to verify the capability and to assess the computational performance of the ROM technique in terms of speed-up and error relative to the full-scale model. ROM exhibits excellent agreement in spatiotemporal thermal profiles (<0.5% relative error in pertinent time scales) along with salient computational acceleration (up to two orders of magnitude speed-up) over the full-scale analysis. These findings establish the feasibility of ROM to perform rational and computationally affordable thermal analysis, develop reliable thermal control strategies for spacecraft, and greatly reduce the development cycle times and costs.

0000↗
NASA NTRS2020

Demonstrating Assurance of Model-Based Fault Diagnosis Systems on an Operational Mission

Developers of robotic scientific and commercial spacecraft are trending towards use of onboard autonomous capabilities for responding quickly to dynamic environments and rapidly changing situations. These capabilities need to know the state of the spacecraft’s health. Model-based fault diagnosis (MBFD) is an approach to estimating health by continuously verifying accurate behavior and diagnosing off-nominal behavior. Proper functioning of MBFD depends on 1) the quality of the diagnostic system model that is analyzed and compared to commands and onboard measurements to estimate a system’s health state, and 2) the correct functionality of the diagnosis engine interrogating the model and comparing its analyses to observed system behavior. Our goal is to develop Verification and Validation (V&V) techniques for MBFD to provide future missions sufficient confidence in its functionality and performance to deploy it on the systems they develop. Our work has been focused on infusing the techniques we developed earlier to an operational mission. First, we are constructing diagnostic models of a spacecraft attitude control system and updating our diagnostic engine so they can be demonstrated aboard the Arcsecond Space Telescope Enabling Research in Astrophysics (ASTERIA) mission, an operational spacecraft for which experiments in autonomy are being planned and executed, using the V&V techniques we have previously developed to assure they are both correct and complete. Since it is nearing the end of its life, ASTERIA provides a unique opportunity to demonstrate MBFD since the monitored components are expected to fail. Our demonstration will give system developers additional confidence to make timely, informed MBFD deployment decisions. Second, we will be completing performance assessments of the diagnostic engine/diagnostic model ensemble both on the flight system and ground-based testbeds to gain confidence in MBFD’s ability to run successfully in a spacecraft’s resource-constrained environment without adversely affecting other on-board activities. Finally, we are capturing our experience in preparing this demonstration in a set of checklists and guidance documents. Current practice includes high-level institutional guidance documents and standards, but at a high level of abstraction that does not necessarily address specific MBFD concerns. The purpose of the new checklists is to provide future mission developers clear, unambiguous, procedure-oriented guidance on assuring MBFD. This paper describes our work in these areas. For the first area, we describe the diagnostic models and updated diagnostic engine that will be used for the on-board demonstration. We describe how the V&V techniques we developed earlier are used to assure model and engine correctness and completeness. For the second area, we identify the performance measurement and assessment techniques used to characterize the diagnostic engine and diagnostic models, and discuss the effect of measured performance on overall mission operation. Finally, we present the checklist and guidance documents and describe how they meet the goals of providing system developers with clear, unambiguous, procedure-oriented guidance on MBFD assurance. We show how the techniques we have developed map into those artifacts.

Nikora, Allen↗
NASA NTRS2010

Template Matching Approach to Signal Prediction

A new approach to signal prediction and prognostic assessment of spacecraft health resolves an inherent difficulty in fusing sensor data with simulated data. This technique builds upon previous work that demonstrated the importance of physics-based transient models to accurate prediction of signal dynamics and system performance. While models can greatly improve predictive accuracy, they are difficult to apply in general because of variations in model type, accuracy, or intended purpose. However, virtually any flight project will have at least some modeling capability at its disposal, whether a full-blown simulation, partial physics models, dynamic look-up tables, a brassboard analogue system, or simple hand-driven calculation by a team of experts. Many models can be used to develop a predict, or an estimate of the next day s or next cycle s behavior, which is typically used for planning purposes. The fidelity of a predict varies from one project to another, depending on the complexity of the simulation (i.e. linearized or full differential equations) and the level of detail in anticipated system operation, but typically any predict cannot be adapted to changing conditions or adjusted spacecraft command execution. Applying a predict blindly, without adapting the predict to current conditions, produces mixed results at best, primarily due to mismatches between assumed execution of spacecraft activities and actual times of execution. This results in the predict becoming useless during periods of complicated behavior, exactly when the predict would be most valuable. Each spacecraft operation tends to show up as a transient in the data, and if the transients are misaligned, using the predict can actually harm forecasting performance. To address this problem, the approach here expresses the predict in terms of a baseline function superposed with one or more transient functions. These transients serve as signal templates, which can be relocated in time and space against the signal background. One then has the ability to reconstruct a signal regardless of the precise timing of the transients. During operation, one applies the actual start times of spacecraft activities as they occur, and produces a reconstructed, accurate predict in real-time. This general approach is valid under two important conditions. First, the transients themselves must be time-invariant. Second, the transients must be reasonably consistent with respect to different operating points. Both of these assumptions are generally valid, but in the case of a complicated system with numerous types of overlapping transients, this approach may not be effective. Fortunately, there tend to be few transients in spacecraft telemetry of sensor quantities or low-level health and status information because these signals rarely reflect multiple different types of operation. Furthermore, if the predict is at least reasonably close to actual operation, the shift either in time, or in the operating point when the transient occurs - is likely to be small. The proposed approach considers three ways to recognize a transient. The first and most reliable is to use a different signal that identifies operating mode - often the transient will be correlated to a change in operating mode, and this change can usually be detected positively from discrete signals in spacecraft telemetry. If there is no useful mode signal, the second option is to identify the transient template by hand, and detect the onset of the transient using a curve-fitting approach. Finally, if one elects not to choose by hand, there is an option for automatic selection. This technique has been applied to sensor data from several JPL missions and industrial applications, demonstrating an improvement in accurate prediction of future behavior and early detection of system problems. This technique is applicable to practically any time-varying, quantitative sensor measurement.

Mackey, Ryan↗
NASA NTRS1972

Results from OSO-IV - The long term behavior of X-ray emitting regions.

Analysis of images of the sun obtained with the aid of a grazing incidence X-ray telescope on board the OSO IV spacecraft in the 2.5 to 12-A waveband nearly continuously from Oct. 27, 1967, to May 12, 1968. The instrument had sufficient spatial resolution (one and four arc minutes) and temporal resolution (5 to 20 min) to estimate the spatial characteristics of X-ray emitting regions and to monitor the temporal behavior of individual active regions. Variations in the absence of flares of as much as a factor of 10 in the X-ray output of individual regions were observed, with typical durations ranging from several hours to several days. The X-ray time variations are related to observations at optical and radio wavelengths. The results are interpreted under the assumption that the X-ray time variations are caused by temperature changes in the coronal portions of active regions. The contribution of radiative losses to the energy budget of the coronal active region is estimated.

Krieger, A.↗
NASA NTRS1998

Test of a flexible spacecraft dynamics simulator

There are a number of approaches one can take to modeling the dynamics of a flexible body. While one can attempt to capture the full dynamical behavior subject to disturbances from actuators and environmental torques, such a detailed description often is unnecessary. Simplification is possible either by limiting the amplitude of motion to permit linearization of the dynamics equations or by restricting the types of allowed motion. In this work, we study the nonlinear dynamics of bending deformations of wire booms on spinning spacecraft. The theory allows for large amplitude excursions from equilibrium while enforcing constraints on the dynamics to prohibit those modes that are physically less relevant or are expected to damp out fast. These constraints explicitly remove the acoustic modes (i.e., longitudinal sound waves and shear waves) while allowing for arbitrary bending and twisting, motions which typically are of lower frequency. As a test case, a spin axis reorientation maneuver by the Polar Plasma Laboratory (POLAR) spacecraft has been simulated. POLAR was chosen as a representative spacecraft because it has flexible wire antennas that extend to a length of 65 meters. Bending deformations in these antennas could be quite large and have a significant effect on the attitude dynamics of the spacecraft body. Summary results from the simulation are presented along, with a comparison with POLAR flight data.

Dichmann, Donald↗
NASA NTRS1977

Tests and evaluation of multihundred watt thermoelectric generators at JPL

The multihundred watt (MHW) thermoelectric generator, based on silicon-germanium thermoelectric technology, delivers a nominal power output of 150 watts with an efficiency of about 6%. The two Voyager space probes each use three such generators assembled in tandem on a boom. A total of seven MHW type thermoelectric generators were tested at JPL in support of the Voyager project. The tests consisted of: (1) parametric evaluation of the electrical characteristics of the devices over a wide range of output voltage for different values of input power, different operating ambients (air, vacuum), and different internal environments (argon, helium, xenon, mixture of these gases, and vacuum) at different pressures to allow evaluation of the influences of both gas and pressure on the performance of the generator; (2) tests to determine the transient behavior of the generators; and (3) operation of the generator in conjunction with the Voyager spacecraft.

Rouklove, P.↗
NASA NTRS1984

The Origins of Plasmas in the Earth's Neighborhood (OPEN) program

The nature and objectives of the OPEN program are overviewed. The Origins of Plasmas in the Earth's Neighborhood program was conceived in 1979 and proposed as a major new initiative to study the energetics of the earth's space environment by the end of the 1980s. The objectives of OPEN have been integrated into the Global Geospace Study (GGS) segment to the International Solar-Terrestrial Physics (ISTP) program now being planned jointly by NASA, ESA, and Japan. The goals will be to develop a global understanding of the flow of energy from the sun through the earth's space environment above the neutral atmosphere and to define the cause and effect relationships between the plasma physics processes that link different regions of this dynamic environment. A network of four spacecraft will be used, each one carrying an instrument complement to characterize the composition and behavior of the upstream solar wind, the high-altitude polar magnetosphere, the equatorial magnetosphere, and the comet-like geomagnetic tail. Multispectral cameras will also be carried to image polar auroras at ultraviolet, visible and X-ray wavelengths. Experimentalists and theorists on the international team will participate.

Alexander, J. K.↗
NASA NTRS1993

Effect of baffle on the asymmetric gravity jitter excited slosh waves in microgravity

The paper investigates the dynamical behavior of fluids affected by asymmetric gravity jitter oscillations, in particular, the effect of surface tension on partially filled rotating fluids in a full-scale Gravity Probe-B Spacecraft dewar tank, with and without the installation of the baffle-board, imposed by time-dependent directions of background gravity. The time-sequence evolution of the 3D behavior of the liquid-vapor interface oscillations resulting in the excitation of large amplitude slosh waves exerted by gravity jitter disturbances are examined. It is shown that the damping effect provided by baffle reduces the amplitude of slosh wave excitation and lowers the degree of asymmetry in the liquid-vapor distribution.

Hung, R. J.↗
NASA NTRS1997

Extinguishment of a Diffusion Flame Over a PMMA Cylinder by Depressurization in Low-Gravity

The behavior of flames in low-speed flows in low-gravity is relevant to spacecraft fire safety. Previous work has shown that flames in the presence of low-speed forced flows in low-gravity may be more flammable than in a forced flow of the same magnitude in normal gravity. Additionally, fire suppression plans for the International Space Station include the use of venting (depressurization) as an emergency option for extinguishing fires. This procedure would induce flows in the affected compartment that could temporarily intensify the fire, as was observed in flammability tests of solids conducted on board Skylab. Despite a general Understanding, current knowledge of the combined effects of reduced pressure and forced flow on a burning solid in low-gravity is inadequate for the design of a venting extinguishment system. Previous studies in low-g have examined flammability limits for thermally thin solids. However, there are differences when burning thick materials because the interior solid-phase temperature continuously changes, which affects the percentage of gas-phase heat feedback to the solid-phase. Changes in the heat feedback to the solid-phase can affect the flammability characteristics of the material. In the current work, the extinction of a diffusion flame burning over PMMA (polymethyl methacrylate) cylinders during depressurization with a low-speed cross flow was examined experimentally and via numerical simulations.

Goldmeer, Jeffrey S.↗
NASA NTRS2011

Creep Measurement Video Extensometer

Understanding material behavior under load is critical to the efficient and accurate design of advanced aircraft and spacecraft. Technologies such as the one disclosed here allow accurate creep measurements to be taken automatically, reducing error. The goal was to develop a non-contact, automated system capable of capturing images that could subsequently be processed to obtain the strain characteristics of these materials during deformation, while maintaining adequate resolution to capture the true deformation response of the material. The measurement system comprises a high-resolution digital camera, computer, and software that work collectively to interpret the image.

Jaster, Mark↗
NASA NTRS2013

Alternating Twist Along an Erupting Prominence

Triangulation measurements using observations from the two Solar Terrestrial Relations Observatory (STEREO) spacecraft, combined with observations from the Solar Dynamics Observatory (SDO), are used to characterize the behavior of a prominence involved in two successive coronal mass ejections 6-7 December 2010. The STEREO separation at the time was 171.6 degrees, which was functionally equivalent to a separation of 8.4 degrees, and thus very favorable for feature co-identification above the limb. The first eruption at approximately 14:16 UT (Universal Time) on 6 December of the middle branch of the prominence starts off a series of magnetic reconfigurations in the right branch, which itself erupts at approximately 2:06 UT the next day, about 12 hours after the first eruption. The cool prominence material seen at 304 angstroms drains back down to the surface, but a flux-rope-like magnetic structure is seen to erupt in both 195 angstroms by the STEREO/Extreme Ultraviolet Imager (EUVI), and in white light by the STEREO/COR1 (Coronagraph 1) inner coronagraph. In between the two eruptions, two different signs of helicity are seen in the measured twist of the right branch. This is interpreted to be caused by the overall prominence channel being composed of different segments with alternating helicity signs. The erupting parts on 6 and 7 December both show positive twist, but negative twist is seen in between these positive sections. Negative twist is consistent with the dextral chirality signs seen in the He II line at 304 angstroms prior to both eruptions. However, during the period between the eruptions, a region of positive twist grows and replaces the region of negative twist, and finally erupts. We interpret these observations in the light of models that predict that helicity cancellation can be an important factor in the triggering of flares and coronal mass ejections.

Prominences↗