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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
NASA NTRS1986

Diffusive electron acceleration at SNR shock fronts and the observed SNR radio spectral indices

The radio synchrotron emission from relativistic electrons in shell supernova remnants (SNRs) provides a unique opportunity to probe the energy distribution of energetic electrons at their acceleration site (SNR shock fronts). This information provides insight into the acceleration mechanism(s). The implications of these observations for the diffusive (first-order Fermi) acceleration of electrons at the SNR shock fronts are discussed.

Bogdan, T. J.↗
NASA NTRS2015

Late-Time Evolution of Composite Supernova Remnants: Deep Chandra Observations and Hydrodynamical Modeling of a Crushed Pulsar Wind Nebula in SNR G327.1-1.1

In an effort to better understand the evolution of composite supernova remnants (SNRs) and the eventual fate of relativistic particles injected by their pulsars, we present a multifaceted investigation of the interaction between a pulsar wind nebula (PWN) and its host SNR G327.1-1.1. Our 350 ks Chandra X-ray observations of SNR G327.1-1.1 reveal a highly complex morphology; a cometary structure resembling a bow shock, prong-like features extending into large arcs in the SNR interior, and thermal emission from the SNR shell. Spectral analysis of the non-thermal emission offers clues about the origin of the PWN structures, while enhanced abundances in the PWN region provide evidence for mixing of supernova ejecta with PWN material. The overall morphology and spectral properties of the SNR suggest that the PWN has undergone an asymmetric interaction with the SNR reverse shock(RS) that can occur as a result of a density gradient in the ambient medium and or a moving pulsar that displaces the PWN from the center of the remnant. We present hydrodynamical simulations of G327.1-1.1 that show that its morphology and evolution can be described by a approx. 17,000 yr old composite SNR that expanded into a density gradient with an orientation perpendicular to the pulsar's motion. We also show that the RSPWN interaction scenario can reproduce the broadband spectrum of the PWN from radio to gamma-ray wavelengths. The analysis and modeling presented in this work have important implications for our general understanding of the structure and evolution of composite SNRs.

Evolution↗
NASA NTRS1986

SNR estimation for the baseband assembly

The expected value and the variance of the Baseband Assembly symbol signal-to-noise ratio (SNR) estimation algorithm are derived. The SNR algorithm treated here is designated as the Split Symbol Moments Estimator (SSME). It consists of averaging the first two moments of the integrated half symbols. The SSME is a biased, consistent estimator. The SNR degradation factor due to the jitter in the subcarrier demodulation and symbol synchronization loops is taken into account. Curves of the expected value of the SNR estimator versus the actual SNR are presented.

Simon, M. K.↗
NASA NTRS2003

ASCA and Bepposax Observations of the Newly Discovered SNR RX J0852-4622

The NASA Grant supported analysis of X-ray data from the supernova remnant (SNR) G266.2-1.2, a newly discovered SNR in the direction of Vela. Our observations reveal that the X-ray emission from this remnant is nonthermal, making this the third shell-type SNR for which the X-ray emission is dominated by synchrotron processes. The interpretation is that the SNR is an efficient accelerator of cosmic rays, with direct evidence of electrons at energies approaching 100 TeV. Our spectral analysis indicates a higher absorption than for the Vela SNR, indicating that G266.2-1.2 is more distant. However, we conclude that the lack of high amounts of absorption place the remnant closer than the Vela molecular ridge at a distance of 1-2 kpc. We observe a compact X-ray source at the remnant center, surrounded by diffuse emission, which may be an associated neutron star.

Oliversen, Ronald J.↗
NASA NTRS1985

High-rate convolutional code construction with the minimum required SNR criterion

New short constraint length, high-rate convolutional codes which minimize the required signal to noise ratios (SNR) are found and tabulated for rates 2/3, 3/4, and 4/5, and for constraint length K up to 10. When compared with previously reported codes, most of the new codes reduce the required SNR only slightly. However, there are some pairs of K and code rate for which the new codes require considerably less SNR. The most significant one is the new K = 8, ate 4/5 code which requires 1.2 dB less SNR than the known code with the same parameters, for a desired bit error rate of 0.000001.

Lee, P. J.↗
NASA NTRS1980

X-ray spectrum of Kepler's SNR

Observations made with the solid state spectrometer aboard the Einstein Observatory confirm Kepler's SNR as an X-ray source with an intensity between 1-3 KeV of 7.2 x 10 to the-11th power ergs/sq cm-s. The X-ray spectrum is similar to those of Cas A and Tycho, with strong line emission from the helium-like species of Si, S, and Ar. Direct comparisons to Tycho's SNR suggest a distance of Kepler's SNR of greater than or equal to 5 kpc.

Becker, R. H.↗
NASA NTRS1993

Optimal filters - A unified approach for SNR and PCE

A unified approach for a general metric that encompasses both the signal-to-noise ratio (SNR) and the peak-to-correlation (PCE) ratio in optical correlators is described. In this approach, the connection between optimizing SNR and optimizing PCE is achieved by considering a metric in which the central correlation irradiance is divided by the total energy of the correlation plane. The peak-to-total energy (PTE) is shown to be optimized similarly to SNR and PCE. Since PTE is a function of the search values G and beta, the optimal filter is determined with only a two-dimensional search.

Juday, Richard D.↗
NASA NTRS1995

Towards optimum demodulation of bandwidth-limited and low SNR square-wave subcarrier signals

The optimum phase detector is presented for tracking square-wave subcarriers that have been bandwidth limited to a finite number of harmonics. The phase detector is optimum in the sense that the loop signal-to-noise ratio (SNR) is maximized and, hence, the rms phase tracking error is minimized. The optimum phase detector is easy to implement and achieves substantial improvement. Also presented are the optimum weights to combine the signals demodulated from each of the harmonics. The optimum weighting provides SNR improvement of 0.1 to 0.15 dB when the subcarrier loop SNR is low (15 dB) and the number of harmonics is high (8 to 16).

Feria, Y.↗
NASA NTRS1999

HRI Observations of Balmer Dominated Filaments in the SNR RCW86

The objective of this project was to use the ROSAT HRI to study the recently discovered optical Balmer-dominated filaments in the young SNR R-CW86. The observations provide complete high-resolution X-ray coverage of the shell of the SNR. These X-ray observations are combined with new optical observations (both imaging and spectroscopic), and new high resolution radio observations to provide a better overall understanding of the state of the remnant (Sedov or reverse shock), its history (as the possible SNR of SN 185 AD), and the physics of non-radiative shocks.

Long, Knox S.↗
NASA NTRS2001

Galactic SNR Candidates in the ROSAT All-Sky Survey

Identified radio supernova remnants (SNRS) in the Galaxy comprise an incomplete sample of the SNR population due to various selection effects. ROSAT performed the first all-sky survey with an imaging X-ray telescope, and thus provides another window for finding SNRS and compact objects that may reside within them. Performing a search for extended X-ray sources in the ROSAT all-sky survey database about 350 objects were identified as SNR candidates in recent years. Continuing this systematic search, we have reanalyzed the ROSAT all-sky survey (BASS) data of these candidates and correlated the results with radio surveys like NVSS, ATNF, Molonglo, and Effelsberg. A further correlation with SIMBAD and NED were used for subsequent identification purpose. About 50 of the 350 candidates turned out to be likely galaxies or clusters of galaxies. We found 14 RASS sources which are very promising SNR candidates and are currently subject of further follow-up studies. We will provide the details of the identification campaign and present first results.

Schaudel, Daniel↗
NASA NTRS2003

ASCA and Bepposax Observations of the Newly Discovered SNR RX J0852-4622

This grant supports an investigation of the supernova remnant RX J0852-4622 (G266.1-1.2), a large-diameter, nearby SNR for which the X-ray emission is dominated by nonthermal processes. As only the third such SNR discovered, for which there is direct evidence of cosmic-ray acceleration dominating the SNR dynamics, this is an exceptionally important object. Our progress on this study to date has been good. We have published the results of a spectral mapping of the XNR as well as the identification of a compact X-ray source which may be the associated neutron star. Our final work involves modeling of the emission using limits from the radio flux in order to estimate the inverse-Compton flux that might be observable in the TeV gamma-ray regime. We have just completed similar modeling of the emission from G347.3-0.5 (Lazendic et al. - submitted to ApJ), and the results have drawn considerable interest in the gamma-ray community (Slane 2002; astro-ph/0212353). Extensive observations of G266.1--1.2 with the CANGAROO TeV gamma-ray telescope have been carried out, and our modeling of the broad-band emission characteristic are of importance for such studies.

Slane, Patrick↗
NASA NTRS2007

Real-time Signal-to-noise Ratio (SNR) Estimation for BPSK and QPSK Modulation Using the Active Communications Channel

Method and apparatus for estimating signal-to-noise ratio (SNR) gamma of a composite input signal e(t) on a phase modulated (e.g., BPSK) communications link. A first demodulator receives the composite input signal and a stable carrier signal and outputs an in-phase output signal; a second demodulator receives the composite input signal and a phase-shifted version of the carrier signal and outputs a quadrature-phase output signal; and phase error theta(sub E)(t) contained within the composite input signal e(t) is calculated from the outputs of the first and second demodulators. A time series of statistically independent phase error measurements theta(sub E)(t(sub 1)), theta (sub E)(t(sub 2)),..., theta (sub E)(t(sub k)) is obtained from the composite input signal subtending a time interval delta t = t(sub k) - t(sub 1) whose value is small enough such that gamma(t) and sigma(t) can be taken to be constant in delta t. A biased estimate gamma(sup *) for the signal-to-noise ratio (SNR) gamma if the composite input signal is calculated using maximum likelihood (ML) estimation techniques, and an unbiased estimate gamma(sup ^) for the signal-to-noise ratio (SNR) gamma of the composite input signal is determined from the biased estimate gamma(sup *), such as by use of a look-up table.

Manning, Robert M.↗
NASA NTRSDate not supplied

A Modeling and Simulation Study for the GeoXO Atmospheric Composition Instrument (ACX): System Level SO2 and O3 Retrieval Performance as a Function of SNR

NOAA’s Geostationary Extended Observations (GeoXO) program is planning to include a hyperspectral UV/visible atmospheric composition instrument in geostationary orbit slated for operations by the early 2030s. Trace gas retrievals provide critical information about our Earth’s system as it relates to both natural and anthropogenic activity. This work focused on the impacts of the signal-to-noise ratio (SNR) on the retrieval uncertainty of SO2 and O3. An end-to-end physics-based imaging system and scene modeling and simulation framework was developed to assess instrument performance considerations in support of the planned GeoXO atmospheric composition instrument (ACX). The framework is composed of three primary components: 1) simulated at-sensor radiance via radiative transfer models of a known atmospheric composition, 2) a noise model driven by previous and planned sensor specifications, and 3) a physical retrieval algorithm. In this analysis, the tropospheric concentrations of SO2 and O3 in the atmospheric column were varied in the boundary layer (0-3 km) and at an elevation of 5 km with a thickness of 1-2 km; the uncertainty in retrieval was studied for various SNR levels for multiple viewing geometries and solar zenith angles. This effort supports the development of the GeoXO ACX instrument by providing a simulated ACX performance assessment of the system-level retrieval performance as a function of SNR, particularly with respect to the wavelength range of interest (approximately 305 – 340 nm) for SO2 and O3 trace gas retrievals.

Monica Cook↗
NASA NTRS1972

Reciprocal - MSE and SNR as distinct performance measures in below-threshold communication.

The possible advantages of using algorithms that are minimum mean-square-error (MSE) optimal, or other averaging-type demodulators with frequency position modulation (FPM), are investigated. An attempt is made to describe the performance of the optimal and various suboptimal demodulators in terms of three parameters which depend on SNR and bandwidth expansion. The performance measures investigated are the reciprocal of the normalized mean-square error and the output SNR, which are different in the below-threshold region. The former is compared to the rate-distortion bound. The possible advantage of averaging-type discriminators from the point of view of the threshold is discussed.

Bar-David, I.↗
NASA NTRS1977

A model of SNR degradation during solar conjunction

The downlink signal from spacecraft in solar conjunction phases suffers a drastic reduction in signal-to-noise ratio (SNR). Responsible in large part for this effect is the increase in system noise temperature (SNT) in the ground antenna-receiver system. An empirical model is presented of SNR degradation due to increasing SNT during solar conjunction phases.

Rockwell, S. T.↗
NASA NTRS1978

A soft X-ray image of the Cygnus Loop SNR

The first true X-ray image of a galactic source is presented. X-ray emission from the Cygnus Loop SNR was recorded with a rocket-borne soft X-ray imaging system having an angular resolution of approximately 10 arcmin (0.1-1.0 keV). The image suggests X-ray production arising from an asymmetrical limb-brightened shell of hot plasma, resulting from a blast wave expanding into the interstellar medium, the inherent inhomogeneity of which is indicated by spatial structure in the image, observed on scale sizes down to approximately 0.25 deg. No evidence for a central, unresolved X-ray source is found and no significant detection made of any of the unresolved radio sources which may be associated with the SNR. Significant spectral variations are observed across the image.

Kayat, M. A.↗
NASA NTRS1981

Tracking performance of the polarity-type costas loop at low SNR for UQPSK signal

Carrier tracking performance of the polarity type costas loop is analyzed for unbalanced quadriphase-shift-keyed (UQPSK) signals at low SNR. Squaring losses for various SNR, IF bandwidth, and data rate ratios are presented. The RMS phase jitter for a particular loop is computed for various I and Q channel power and data rate ratios. Experimental results using a breadboard costas loop are also included.

Park, Y. H.↗
NASA NTRS1983

A large molecular cloud toward the SNR W50 and SS 433

The CO 1 yields 0 transition at 115 GHz has been mapped over an area of more than 6 deg sq toward W50, the extended SNR surrounding the peculiar object SS 433. W50 is found to lie at the end of a filamentary molecular cloud 4 deg long and 1 deg wide that closely matches a conspicuous dust lane on the Palomar Sky Survey; the cloud's kinematic distance is within 0.7 of 2.2 kpc, in agreement with the distance 2-3.3 kpc estimated for the SNR, and its mass is about 120,000 solar masses. Though there is little evidence of star formation in the molecular filament, or evidence of an interaction between the filament and W50 (or SS 433), the positional coincidence of the two, their similar distance, and their displacement nearly two molecular scale heights from the plane all suggest a physical relationship or common origin. One possibility is that both the molecular filament and the stellar progenitor of W50 were ejected from a large active molecular complex lying in the plane at l = 40 deg.

Huang, Y.-L.↗