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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 181 records · Page 10
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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.↗
NASA NTRS1994

A complex symbol signal-to-noise ratio estimator and its performance

This article presents an algorithm for estimating the signal-to-noise ratio (SNR) of signals that contain data on a downconverted suppressed carrier or the first harmonic of a square-wave subcarrier. This algorithm can be used to determine the performance of the full-spectrum combiner for the Galileo S-band (2.2- to 2.3-GHz) mission by measuring the input and output symbol SNR. A performance analysis of the algorithm shows that the estimator can estimate the complex symbol SNR using 10,000 symbols at a true symbol SNR of -5 dB with a mean of -4.9985 dB and a standard deviation of 0.2454 dB, and these analytical results are checked by simulations of 100 runs with a mean of -5.06 dB and a standard deviation of 0.2506 dB.

Feria, Y.↗
NASA NTRS1994

Correlator data analysis for the array feed compensation system

The real-time array feed compensation system is currently being evaluated at DSS 13. This system recovers signal-to-noise ratio (SNR) loss due to mechanical antenna deformations by using an array of seven Ka-band (33.7-GHz) horns to collect the defocused signal fields. The received signals are downconverted and digitized, in-phase and quadrature samples are generated, and combining weights are applied before the samples are recombined. It is shown that when optimum combining weights are employed, the SNR of the combined signal approaches the sum of the channel SNR's. The optimum combining weights are estimated directly from the signals in each channel by the Real-Time Block 2 (RTB2) correlator; since it was designed for very-long-baseline interferometer (VLBI) applications, it can process broadband signals as well as tones to extract the required weight estimates. The estimation algorithms for the optimum combining weights are described for tones and broadband sources. Data recorded in correlator output files can also be used off-line to estimate combiner performance by estimating the SNR in each channel, which was done for data taken during a Jupiter track at DSS 13.

Iijima, B.↗
NASA NTRS1994

The Block V Receiver fast acquisition algorithm for the Galileo S-band mission

A fast acquisition algorithm for the Galileo suppressed carrier, subcarrier, and data symbol signals under low data rate, signal-to-noise ratio (SNR) and high carrier phase-noise conditions has been developed. The algorithm employs a two-arm fast Fourier transform (FFT) method utilizing both the in-phase and quadrature-phase channels of the carrier. The use of both channels results in an improved SNR in the FFT acquisition, enabling the use of a shorter FFT period over which the carrier instability is expected to be less significant. The use of a two-arm FFT also enables subcarrier and symbol acquisition before carrier acquisition. With the subcarrier and symbol loops locked first, the carrier can be acquired from an even shorter FFT period. Two-arm tracking loops are employed to lock the subcarrier and symbol loops parameter modification to achieve the final (high) loop SNR in the shortest time possible. The fast acquisition algorithm is implemented in the Block V Receiver (BVR). This article describes the complete algorithm design, the extensive computer simulation work done for verification of the design and the analysis, implementation issues in the BVR, and the acquisition times of the algorithm. In the expected case of the Galileo spacecraft at Jupiter orbit insertion PD/No equals 14.6 dB-Hz, R(sym) equals 16 symbols per sec, and the predicted acquisition time of the algorithm (to attain a 0.2-dB degradation from each loop to the output symbol SNR) is 38 sec.

Aung, M.↗
NASA NTRS1995

Seamless data-range change using punctured convolutional codes for time-varying signal-to-noise ratios

In a time-varying signal-to-noise ration (SNR) environment, symbol rate is often changed to maximize data return. However, the symbol-rate change has some undesirable effects, such as changing the transmission bandwidth and perhaps causing the receiver symbol loop to lose lock temporarily, thus losing some data. In this article, we are proposing an alternate way of varying the data rate without changing the symbol rate and, therefore, the transmission bandwidth. The data rate change is achieved in a seamless fashion by puncturing the convolutionally encoded symbol stream to adapt to the changing SNR environment. We have also derived an exact expression to enumerate the number of distinct puncturing patterns. To demonstrate this seamless rate change capability, we searched for good puncturing patterns for the Galileo (14,1/4) convolutional code and changed the data rates by using the punctured codes to match the Galileo SNR profile of November 9, 1997. We show that this scheme reduces the symbol-rate changes from nine to two and provides a comparable data return in a day and a higher symbol SNR during most of the day.

Feria, Y.↗
NASA NTRS1993

The effect of signal noise on the remote sensing of Foliar biochemical concentration

Spectral measurements made using an imaging spectrometer contain systematic and random noise, while the former can be corrected the latter remains a source of error in the remotely sensed signal. A number of investigators have tried to determine the signal-to-noise-ratio (SNR) of the instrument, or the resultant imagery. However, the level of noise at which spectra are too noisy to be useful is not usually determined. The first attempt was by Goetz and Calvin, who suggested that the depth of the absorption feature should be at least an order of magnitude greater than the noise and more recently Dekker suggested a SNR of around 600:1 was required in visible/near infrared wavelengths to measure a 1/gl change in chlorophyll a concentration water. The wide range of applications of imaging spectroscopy make it difficult to set SNR specifications as they are dependent on a number of factors, one of the most important being reflectance of a particular target. For example, the SNR of imagery for vegetated targets is relatively low simply because vegetation has a relatively low level of reflectance. The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) is being used to estimate the concentration of biochemicals within vegetation canopies. This paper reports a study undertaken to identify first, wavebands that were highly correlated with foliar biochemical concentration and second, to determine how sensitive these correlations were to sensor noise.

Smith, Geoffrey M.↗
NASA NTRS1994

Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs

Strong x-ray emission detected in Large Magellanic Cloud (LMC) superbubbles has been explained as the result of interior supernova remnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot be found using conventional criteria. We thus investigate the possibility of using the interstellar absorption properties in the ultraviolet (UV) as a diagnostic of hidden SNR shocks. The International Ultraviolet Explorer (IUE) archives provide the database for this pilot study. They contain high-dispersion spectra of several stars in x-ray bright superbubbles. To distinguish the effects of SNR shocks from those of local stellar winds and a global hot halo around the LMC, we included control objects in different environments. We find that almost all interstellar absorption properties can be explained by the interstellar environment associated with the objects. Summarizing the two most important results of this study: (1) a large velocity shift between the high-ionization species (C IV and Si IV) and the low-ionization species (S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however, the absence of a velocity shift does not preclude the existence of SNR shocks; (2) there is no evidence that the LMC is uniformly surrounded by hot gas; hot gas is preferentially found associated with large interstellar structures like superbubbles and supergiant shells, which may extend to large distances from the plane.

Chu, You-Hua↗
NASA NTRS1998

RXTE Observation of the Tycho Supernova Remnant

SN1006 [4] and Cas A [1, 9] supernova remnants have been shown convincingly to have a hard X-ray power-law continuum. This continuum is thought to be the synchrotron radiation from accelerated electrons of approx. 100 TeV at the shock fronts. Our goal of AO2 RXTE observation is to detect the hard X-ray continuum and to determine the nature of the continuum from Tycho SNR. A detection of a power-law continuum from Tycho SNR can strongly argue for SNRs are the source of cosmic rays with the first order Fermi acceleration as the energizing process. We report the results of our AO2 RXTE 1 x 10(exp 5) sec observation of Tycho SNR. We detect two components of the X-ray spectrum from Tycho SNR both at better than 3 omega confidence. The best two component models are: bremsstrahlung (kT=2.67 +/- 0.13 keV) + bremsstrahlung (kT=7.07 +/- 2.21/1.72 keV) or bremsstrahlung (kT=2.36 +/- 0.21/0.57 keV) + power-law (gamma=2.58 +/- 0.12/0.09 ). This result is an improvement compaxed with the previous most sensitive X-ray measurements by Ginga which shows Tycho's observed X-ray continuum requires a two-component model to yield acceptable fits with the hard component parameters being highly uncertain. Our RXTE measurements constrain all parameter within 3o, ranges. However, we cannot yet distinguish between thermal and nonthermal models for the hard component. In the followings, we describe what we accomplished in the period covered by the grant proposal.

The, Lih-Sin↗
NASA NTRS2001

ROSAT HRI and ASCA Observations of the Spiral Galaxy NGC 6946 and its Northeast Complex of Luminous Supernova Remnants

Analysis of 80 ks ASCA (Advanced Satellite for Cosmology and Astrophysics) and 60 ks ROSAT HRI (High Resolution Image) observations of the face-on spiral galaxy NGC 6946 are presented. The ASCA image is the first observation of this galaxy above approximately 2 keV. Diffuse emission may be present in the inner approximately 4' extending to energies above approximately 2-3 keV. In the HRI data, 14 pointlike sources are detected, the brightest two being a source very close to the nucleus and a source to the northeast that corresponds to a luminous complex of interacting supernova remnants (SNRs). We detect a point source that lies approximately 30" west of the SNR complex but with a luminosity -1115 of the SNR complex. None of the point sources show evidence of strong variability; weak variability would escape our detection. The ASCA spectrum of the SNR complex shows evidence for an emission line at approximately 0.9 keV that could be either Ne IX at approximately 0.915 keV or a blend of ion stages of Fe L-shell emission if the continuum is fitted with a power law. However, a two-component, Raymond-Smith thermal spectrum with no lines gives an equally valid continuum fit and may be more physically plausible given the observed spectrum below 3 keV. Adopting this latter model, we derive a density for the SNR complex of 10-35 cm(exp -3), consistent with estimates inferred from optical emission-line ratios. The complex's extraordinary X-ray luminosity may be related more to the high density of the surrounding medium than to a small but intense interaction region where two of the complex's SNRs are apparently colliding.

Schlegel, E.↗
NASA NTRS2002

Real-time in Situ Signal-to-noise Ratio Estimation for the Assessment of Operational Communications Links

The work presented here formulates the rigorous statistical basis for the correct estimation of communication link SNR of a BPSK, QPSK, and for that matter, any M-ary phase-modulated digital signal from what is known about its statistical behavior at the output of the receiver demodulator. Many methods to accomplish this have been proposed and implemented in the past but all of them are based on tacit and unwarranted assumptions and are thus defective. However, the basic idea is well founded, i.e., the signal at the output of a communications demodulator has convolved within it the prevailing SNR characteristic of the link. The acquisition of the SNR characteristic is of the utmost importance to a communications system that must remain reliable in adverse propagation conditions. This work provides a correct and consistent mathematical basis for the proper statistical 'deconvolution' of the output of a demodulator to yield a measure of the SNR. The use of such techniques will alleviate the need and expense for a separate propagation link to assess the propagation conditions prevailing on the communications link. Furthermore, they are applicable for every situation involving the digital transmission of data over planetary and space communications links.

Manning, Robert M.↗
NASA NTRS2003

Functional Flight Test Report for Positive Systems' ADAR System 5500 Sensor SN8 Linear

This report describes results of the functional flight test conducted with the Positive Systems' ADAR 5500 sensor system (serial number 8, linear configuration) near Winslow, Arizona on June 30 and July 1, 1999. The in-flight test is one component of the NASA Scientific Data Purchase (SDP) Validation and Verification (V&V) process. It allows to measure characteristics of the entire sensor system affected by both performance of the sensor during a flight and post-flight image processing. The following characteristics were analyzed: changes of dark digital numbers (DN's), radiometric linearity, signal-to-noise ratio (SNR), spatial resolution, and geolocation accuracy. The measured characteristics were compared with the image product specifications defined in the Positive Systems' SDP contract. Dependence of the dark DN's on several factors was analyzed, but no significant correlation was found. However, the observed changes in dark DN's were relatively small, which justifies usage of a constant value in the dark DN subtraction procedure during post-processing. Dependence of measured at-sensor, in-band radiance (in arbitrary units) on measured in-band ground reflectance is very well described by a linear function - The sensor fulfills the linearity requirement. Measured SNR values lower than the contract specifications, but accuracy of that test was possibly affected by non-uniformity of the employed gray-scale panels. The SNR values are generally sufficiently high for most applications. SNR can also be improved during standard flights by using longer exposure times. Full width at half maximum (FWHM) of an edge response derived line spread function was used as a measure of spatial resolution. FWHM was generally smaller than twice the ground sample distance (GSD), in agreement with the contract specifications. Accuracy of the geolocation information, which is provided for the particular images in a metadata file, was found to meet contract requirements as well.

Stanley, Tom↗
NASA NTRS2008

Suzaku Observations of Thermal and Non-Thermal X-Ray Emission from the Middle-Aged Supernova Remnant G156.2+5.7

We present results from X-ray analysis of a Galactic middle-aged supernova remnant (SNR) G156.2+5.7 which is bright and largely extended in X-ray wavelengths, showing a clear circular shape (radius approx.50'). Using the Suzaku satellite, we observed this SNR in three pointings; partially covering the northwestern (NW) rim, the eastern (E) rim, and the central portion of this SNR. In the NW rim and the central portion, we confirm that the X-ray spectra consist of soft and hard-tail emission, while in the E rim we find no significant hard-tail emission. The soft emission is well fitted by either a one-component or two-component non-equilibrium ionization (NEI) model. In the NW and E rims, a one-component (the swept-up interstellar medium) NEI model well represents the soft emission. On the other hand, in the central portion, a two-component (the interstellar medium and the metal-rich ejecta) NEI model fits the soft emission better than the one-component NEI model from a statistical point of view. The relative abundances in the ejecta component suggest that G156.2+5.7 is a remnant from a core-collapse SN explosion whose progenitor mass is less than 15 Solar Mass. The origin of the hard-tail emission detected in the NW rim and the central portion of the SNR is highly likely non-thermal synchrotron emission from relativistic electrons. In the NW rim, the relativistic electrons seems to be accelerated by a forward shock with a slow velocity of APPROX.500 km/sec.

Katsuda, Satoru↗