Search NASA⌕ Search

SEARCH · Search NASA

Results for “SNR”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

The effects of bandpass limiters on n-phase tracking systems

The combination of a bandpass limiter and an n(th) power tracking loop is considered. It is shown that the performance of the combination does not depend on the form of the nonlinear function used to create the n(th) harmonic signal so long as some energy in that zone is produced. Furthermore, the order in which the limiter and n(th) power nonlinearity occur is unimportant so that the results apply equally well to the n-phase Costas type of implementation of the n(th) power loops. Closed form expressions for the signal suppression factors are obtained and it is shown that the coherent and noncoherent SNR's out of the n(th) power-limiter combination can be expressed in terms of two of these suppression factors. This later point enables one to estimate the loop SNR of an n(th) power-limiter phase locked loop by simply calculating the suppression factors for n(th) and 2n(th) power loops. Finally, n(th) power phase locked loops without limiters are compared with similar loops with limiters where it is shown that the presence of the limiter can actually enhance the output SNR at moderate to large values of input SNR.

Butman, S. A.↗
NASA NTRS1981

Symbol error rate performance evaluation of the LM37 multimegabit telemetry modulator-demodulator unit

The LM37 multimegabit telemetry modulator-demodulator unit was tested for evaluation of its symbol error rate (SER) performance. Using an automated test setup, the SER tests were carried out at various symbol rates and signal-to-noise ratios (SNR), ranging from +10 to -10 dB. With the aid of a specially designed error detector and a stabilized signal and noise summation unit, measurement of the SER at low SNR was possible. The results of the tests show that at symbol rates below 20 megasymbols per second (MS)s) and input SNR above -6 dB, the SER performance of the modem is within the specified 0.65 to 1.5 dB of the theoretical error curve. At symbol rates above 20 MS/s, the specification is met at SNR's down to -2 dB. The results of the SER tests are presented with the description of the test setup and the measurement procedure.

Malek, H.↗
NASA NTRS1982

An X-ray image of Tycho's supernova remnant

An image of Tycho's SNR has been obtained using the imaging proportional counter aboard the Einstein Observatory, revealing a nearly circular, 8 arcmin, open shell of emission. The X-ray and radio shells are spatially coincident, although there is no correlation in intensity between the two wavelengths. Adopting a distance of 3 kpc, a 0.15-4.5 keV luminosity of 5 x 10 to the 36th ergs/sec is inferred. The mass of the X-ray emitting material is 7-15 solar masses if the SNR is in the adiabatic stage of its evolution. This is consistent with the interpretation that the SNR was the result of a Type I explosion and that the distance to Tycho's SNR is between 2.2 and 4.3 kpc.

Reid, P. B.↗
NASA NTRS1983

The extraordinary extragalactic supernova remnant in NGC 4449. II - X-ray and optical investigations

Einstein Observatory X-ray observations indicate that the SNR in NGC 4449 is the most luminous X-ray, as well as optical and radio, SNR known. New optical spectroscopy, together with the X-ray data, suggest this exceptional luminosity to be due to chemically enriched material at unusually high densities. A model is proposed in which this SNR resulted from the explosion of a massive star in a medium of nearly 25/cu cm density about 100 years ago. The chemistry in question further suggests that the SNR in NGC 4449 resulted from the explosion of a star whose mass was greater than 25 solar masses.

Blair, W. P.↗
NASA NTRS1984

Far-infrared sources in the vicinity of the supernova remnant W28

The W28 supernova remnant molecular cloud complex has been surveyed in the 40-250 micron spectral range with a three-sigma sensitivity of 100-165 Jy per beam. Only two sources have been found. One, located near the center of the SNR, has a luminosity of 54,000 solar luminosities and can be identified with the thermal radio source G6.6-0.1. The energizing source is probably an O-type ZAMS star. The second source, which is near the boundary of the SNR at a sharp gradient of OH absorption, has a luminosity of 6000 solar luminosities and has no radio counterpart. It is best explained as a B1 ZAMS star or a pre-main sequence object presumably formed from the impact of an expanding SNR with a molecular cloud. No far-IR emission is observed at the position of the CO molecular cloud, which seems to have been impacted by the SNR.

Odenwald, S. F.↗
NASA NTRS1984

X-ray synchrotron nebulae and the origin of neutron stars

Einstein Observatory (EO) X ray data on supernova remnants (SNR) and radio pulsars are examined for evidence supporting the concept that neutron stars arise after supernova (SN) events. Recent data reviews have revealed that only 13 of the 140 known galactic SNR exhibit characteristics, e.g., radio pulsars, X ray binaries or synchrotron nebulae, that link neutron stars with SNR. An EO survey of radio pulsars 1 kyr-1 Myr old found eight objects, all embedded in X ray emitting nebulae. The data indicated that all the SNR and pulsar objects had about 1 percent of their spin down energy converted into synchrotron X rays. The majority of SNRs did not have X ray emitting nebulae. It is therefore concluded that only some SN events result in a neutron star, while the event may leave no remnant at all.

Helfand, D. J.↗
NASA NTRS1983

X-ray observations of Crab-like supernova remnants

Distinguishing radio and morphological characteristics of Crab Nebula-like supernova remnants (SNR) are described. Radio features comprise a flat spectrum, a filled center brightness distribution, and a centrally located pulsar. The radio signals are linearly polarized and suggest a synchrotron emission mechanism. Known objects with those characteristics include the Crab Nebula, Vela X, 3C58, G21.5 - 0.9 and G74.9 + 1.2. Only the Crab Nebula exhibits pulsations, while all have unresolved X-ray sources and nonthermal X-ray spectra. Although the Crab-like SNR are distinctly different from shell-like SNR, the SNR CTB80, G326.3 - 1.8, W28 and G29.7 0.3 display characteristics of both, particularly shell-like structures and flat spectra. X-ray spectra from compact sources have also been detected from 3C58, CTB80, W28 and MSH 15 - 52 and exhibit nonthermal power law features. The X-ray spectra could be used as a measure of the evolutionary stage of the source.

Becker, R. H.↗
NASA NTRS1983

The X-ray properties of supernova remnants in the Large Magellanic Cloud

Einstein Observatory instruments were used to survey the generic properties of LMC supernova remnants (SNR). The study covered the diffuse optical bar, the 30 Dor nebula, the surrounding H I cloud, and a high fraction of the spiral arms. Twelve SNRs were identified as X-ray sources and two SNR, N157B and 0540 - 69.3, were located from radio emissions. Filamentary H-alpha emissions also revealed five other objects. X-ray intensity data indicated that density levels of the matter surrounding SNR decrease with SNR diameter. The data suggest that Sedov models for SN may not be accurate for LMC SN, which exhibit too great a luminosity scatter in the surrounding interstellar material. The emissions may be accounted for by ejecta from the SN or by multiphase interstellar material.

Long, K. S.↗
NASA NTRS1985

Onion-shell model for cosmic ray electrons and radio synchrotron emission in supernova remnants

The spectrum of cosmic ray electrons, accelerated in the shock front of a supernova remnant (SNR), is calculated in the test-particle approximation using an onion-shell model. Particle diffusion within the evolving remnant is explicity taken into account. The particle spectrum becomes steeper with increasing radius as well as SNR age. Simple models of the magnetic field distribution allow a prediction of the intensity and spectrum of radio synchrotron emission and their radial variation. The agreement with existing observations is satisfactory in several SNR's but fails in other cases. Radiative cooling may be an important effect, especially in SNR's exploding in a dense interstellar medium.

Beck, R.↗
NASA NTRS1984

Nuclear-electric power in space

Prospective missions requiring large power supplies that might be satisfied with space nuclear reactors (SNR) are discussed, along with design concepts and problems and other potential high-power space systems. Having a minimum economic output of 10 kWe, SNR seem well-suited as the power sources for DBS systems, space-based ATC systems manned planetary missions, an expanding Space Station, materials processing, and outer planets missions. SNR avoid the large area problems of solar cell arrays, short lifetimes of thermionic converters, and vibration and heat control in Stirling engines. Design problems exist for SNR in the heat transfer and rejection systems, radioactive emissions and degradation of reactor materials, and size. The latter is a function of Shuttle payload constaints and raises the possibility of having to load the fuel while in orbit. The earliest operational date of SNRs is projected for the early 1990s, if progress is good in the current SP-100 program.

Truscello, V. C.↗
NASA NTRS1985

Coherent and incoherent lidar - An overview

Characteristics of coherent and incoherent lidars are described. The carrier-to-noise ratio (CNR), SNR relationship, coherence diameter, speckle averaging, multiple pulse averaging, and the possibility of single-frequency pulse generation from the laser transmitter for the two lidar types are compared. Diagrams of the incoherent lidar's direct detection system and the coherent lidar's heterodyne detection system are presented. The transverse coherence diameter is discussed and equations for backscatter from a distributed medium, transverse coherence length, and return path refractive index variations are provided. The importance of transverse coherence length in the coherence lidar system design is considered. The effectiveness of the lidar systems is evaluated in terms of CNR and SNR. The differences in generation of noise-equivalent power are studied and it is observed that the coherent lidar system achieves higher CNR values. The relation between signal fluctuation due to speckle effects and SNR is analyzed; equations for SNR in coherent and incoherent lidar systems are given.

Menzies, R. T.↗
NASA NTRS1987

Filter distortion effects on telemetry signal-to-noise ratio

The effect of filtering on the Signal-to-Noise Ratio (SNR) of a coherently demodulated band-limited signal is determined in the presence of worse-case amplitude ripple. The problem is formulated mathematically as an optimization problem in the L2-Hilbert space. The form of the worst-cast amplitude ripple is specified, and the degradation in the SNR is derived in a closed form expression. It is shown that when the maximum passband amplitude ripple is 2 delta (peak to peak), the SNR is degraded by at most (1 - delta squared), even when the ripple is unknown or uncompensated. For example, an SNR loss of less than 0.01 dB due to amplitude ripple can be assured by keeping the amplitude ripple to under 0.42 dB.

Sadr, R.↗
NASA NTRS1989

The Crab-like supernova remnants

Recent observational and theoretical investigations of Galactic and extragalactic SNRs suspected of having neutron central stars are reviewed, with an emphasis on Einstein X-ray observations. Sections are devoted to the Crab Nebula and its twin SNR 0540-69.3, SNR with observationally well determined and indistinct central X-ray nebulae, SNR with weak and strong unresolved internal X-ray sources, binary systems, and the relationship between X-ray and radio luminosity and pulsar energy loss. Consideration is given to radio pulsars, SNR with filled-center X-ray emission attributed to thermal processes, radio plerions and composite SNRs, MSH 15-56, and SNRs in the LMC. Extensive contour maps, sample optical images, and tables of numerical data are included.

Seward, Frederick D.↗
NASA NTRS1989

Degradation of signal-to-noise ratio due to amplitude distortion

The effect of filtering on the signal-to-noise ratio (SNR) of a coherently demodulated band-limited signal is determined in the presence of worst-case amplitude ripple. The problem is formulated as an optimizaton in the Hilbert space L2. The form of the worst-case amplitude ripple is specified, and the degradation in the SNR is derived in closed form. It is shown that, when the maximum passband amplitude ripple is 2Delta (peak-to-peak), the SNR is degraded by at most (1-Delta-squared), even when the ripple is unknown or uncompensated. For example, an SNR loss of less than 0.01 dB due to amplitude ripple can be assured by keeping the amplitude ripple under 0.42 dB.

Sadr, Ramin↗
NASA NTRS1991

IRAS 15099 - 5856 and the composite supernova remnant G320.4 - 1.2

The supernova remnant (SNR) G320.4 - 1.2 is a composite SNR, containing both a shock-dominated shell, and a pulsar-dominated plerion. G320.4 - 1.2 is one of the few SNRs in which a pulsar has been detected. This paper reports on IRAS observations of this SNR. A pointlike source (IRAS 15099 - 5856) near the position of the pulsar appears quite prominently at 25 microns. The region appears more complex at 60 and 100 microns. Three models that may account for the IR emission of IRAS 15099 - 5856 are examined; none of these is entirely satisfactory. Some IR emission is detected from the brightest portion of the shell of this SNR, but this emission is nearly hidden by other confusing sources.

Arendt, Richard G.↗
NASA NTRS1991

A new look at the 'jet' in the CTB 37A/B supernova remnant complex

Very Large Array observations of the unusual southern Galactic supernova remnant (SNR) complex near l = 348 deg, b = 0 deg at wavelengths of 6, 20, and 90 cm are presented. Derived continuum spectra and observed morphologies indicate that G348.5 + 0.1 (CTB 37A) does not have a 'jet' as reported by previous observers but is instead superposed on a second, previously unidentified SNR lying along the line of sight. This new SNR is designated G348.5 - 0.0 according to the usual convention. Other observers have also noted a faint, flat spectrum bridge of emission possibly connecting the G348.5 + 0.1/G348.5 - 0.0 superposition with a third nearby remnant G348.7 + 0.3 (CTB 37B). However, a connection appears unlikely, and it is suggested that the 'bridge' merely consists of faint emission which has 'leaked' from the southeastern side of G348.7 + 0.3 and has no relation to the G348.5 + 0.1/G348.5 - 0.0 superposition. These new data also reveal a remarkable region of 'blown-out' emission from the southwestern part of G348.5 + 0.1 which most likely reflects the presence of large-scale density inhomogeneities in the interstellar medium into which the SNR shell is expanding.

Kassim, Namir E.↗
NASA NTRS1991

Emission processes and dynamics of hot gases in astrophysics

A detailed model was developed for Kepler's supernova remnant (SNR). Observations of the SNR revealed a strong interaction with the surrounding circumstellar medium, which was studied through both analytical and numerical calculations. Effects were studied of electron thermal conduction on the structure of radiative interstellar shock waves. An explanation is sought for the observed line emission from metal rich ejecta in SNR, incorporating atomic data. Light echoes around SN 1987A was also studied. Analysis of infrared and scattered circumstellar light echoes was accomplished with early observations to set limits on the mass of circumstellar dust. Work was completed on the emission from heavy element gas ejected in the supernova explosion of massive stars. It was assumed that a radioactive energy source was present and calculated the detailed heating and ionization of the gas. The evolution was studied of SNR in the very high pressure environment of a starburst galaxy.

Chevalier, Roger A.↗
NASA NTRS1992

Diversity imaging techniques in lidar

Diversity imaging techniques have been successfully employed in conventional microwave range-Doppler imaging radars to obtain high resolution images of both natural and man-made targets. These techniques allow microwave radars to achieve image resolution which would otherwise require excessively large antennas. Recent advances in coherent laser radar techniques and signal processing have led to the development of range-Doppler imaging laser radars. While much of the theory and signal processing techniques used in microwave radars can be brought to bear on laser radars, the significant difference in wavelength results in issues peculiar to laser radar systems. Both the fundamental concepts and specific applications of diversity imaging techniques applied to laser radar imaging systems will be discussed. Angle, frequency, and bistatic angle degrees of freedom can be employed in a coherent laser radar imaging system to achieve image resolution which exceeds the traditional Rayleigh criterion associated with the receive aperture. In diversity imaging, angle and frequency degrees of freedom can be used to synthesize an effective aperture providing range and Doppler target information. The ability to vary the bistatic angle provides an additional means of synthesizing an effective aperture. Both simulated and experimentally obtained laser radar images of spinning and/or tumbling objects utilizing both angular and frequency diversity will be presented. In coherent laser radar systems, image quality can be dominated by laser speckle effects. In particular, the signal-to-noise ratio (SNR) of a coherent laser radar image is at most unity in the presence of fully developed speckle. Diversity techniques can be utilized to improve the image SNR; simple incoherent averaging of images utilizing temporal and polarization degrees of freedom can significantly improve image SNR. Both the SNR and image resolution (as defined by the synthetic aperture) contribute to image quality. The relationship between the available degrees of freedom and image quality are discussed.

Schultz, K. I.↗