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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

The Galactic Black Hole Transient H1743-322 During Outburst Decay Connections Between Timing Noise, State Transitions, And Radio Emission

Multiwavelength observations of Galactic black hole transients during outburst decay are instrumental for our understanding of the accretion geometry and the formation of outflows around black hole systems. H1743-322, a black hole transient observed intensely in X-rays and also covered in the radio band during its 2003 decay, provides clues about the changes in accretion geometry during state transitions and also the general properties of X-ray emission during the intermediate and low-hard states. In this work, we report on the evolution of spectral and temporal properties in X-rays and the flux in the radio band, with the goal of understanding the nature of state transitions observed in this source. We concentrate on the transition from the thermal dominant state to the intermediate state that occurs on a timescale of 1 day. We show that the state transition is associated with a sudden increase in power-law flux. We determine that the ratio of the power-law flux to the overall flux in the 3-25 keV band must exceed 0.6 for us to observe strong timing noise. Even after the state transition, once this ratio was below 0.6, the system transited back to the thermal dominant state for 1 day. We show that the emission from the compact radio core does not turn on during the transition from the thermal dominant state to the intermediate state but does turn on when the source reaches the low-hard state, as seen in 4U 1543-47 and GX 339-4. We find that the photon index correlates strongly with the QPO frequency and anticorrelates with the rms amplitude of variability. We also show that the variability is more likely to be associated with the power-law emission than the disk emission.

Kalemci, E.↗
NASA NTRS2003

Analysis of Return and Forward Links from STARS' Flight Demonstration 1

Space-based Telemetry And Range Safety (STARS) is a Kennedy Space Center (KSC) led proof-of-concept demonstration, which utilizes NASA's space network of Tracking and Data Relay Satellites (TDRS) as a pathway for launch and mission related information streams. Flight Demonstration 1 concluded on July 15,2003 with the seventh flight of a Low Power Transmitter (LPT) a Command and Data Handler (C&DH), a twelve channel GPS receiver and associated power supplies and amplifiers. The equipment flew on NASA's F-I5 aircraft at the Dryden Flight Research Center located at Edwards Air Force Base in California. During this NASA-ASEE Faculty Fellowship, the author participated in the collection and analysis of data from the seven flights comprising Flight Demonstration 1. Specifically, the author examined the forward and return links bit energy E(sub B) (in Watt-seconds) divided by the ambient radio frequency noise N(sub 0) (in Watts / Hertz). E(sub b)/N(sub 0) is commonly thought of as a signal-to-noise parameter, which characterizes a particular received radio frequency (RF) link. Outputs from the data analysis include the construction of time lines for all flights, production of graphs of range safety values for all seven flights, histograms of range safety E(sub b)/N(sub 0) values in five dB increments, calculation of associated averages and standard deviations, production of graphs of range user E(sub b)/N(sub 0) values for the all flights, production of graphs of AGC's and E(sub b)/N(sub 0) estimates for flight 1, recorded onboard, transmitted directly to the launch head and transmitted through TDRS. The data and graphs are being used to draw conclusions related to a lower than expected signal strength seen in the range safety return link.

Gering, James A.↗
NASA NTRS1974

Low noise parametric amplifiers for radio astronomy observations at 18-21 cm wavelength

The principle characteristics and use of SHF parametric amplifiers for radiometer input devices are explored. Balanced parametric amplifiers (BPA) are considered as the SHF signal amplifiers allowing production of the amplifier circuit without a special filter to achieve decoupling. Formulas to calculate the basic parameters of a BPA are given. A modulator based on coaxial lines is discussed as the input element of the SHF. Results of laboratory tests of the receiver section and long-term stability studies of the SHF sector are presented.

Kanevskiy, B. Z.↗
NASA NTRS2011

Multiple-Beam Detection of Fast Transient Radio Sources

A method has been designed for using multiple independent stations to discriminate fast transient radio sources from local anomalies, such as antenna noise or radio frequency interference (RFI). This can improve the sensitivity of incoherent detection for geographically separated stations such as the very long baseline array (VLBA), the future square kilometer array (SKA), or any other coincident observations by multiple separated receivers. The transients are short, broadband pulses of radio energy, often just a few milliseconds long, emitted by a variety of exotic astronomical phenomena. They generally represent rare, high-energy events making them of great scientific value. For RFI-robust adaptive detection of transients, using multiple stations, a family of algorithms has been developed. The technique exploits the fact that the separated stations constitute statistically independent samples of the target. This can be used to adaptively ignore RFI events for superior sensitivity. If the antenna signals are independent and identically distributed (IID), then RFI events are simply outlier data points that can be removed through robust estimation such as a trimmed or Winsorized estimator. The alternative "trimmed" estimator is considered, which excises the strongest n signals from the list of short-beamed intensities. Because local RFI is independent at each antenna, this interference is unlikely to occur at many antennas on the same step. Trimming the strongest signals provides robustness to RFI that can theoretically outperform even the detection performance of the same number of antennas at a single site. This algorithm requires sorting the signals at each time step and dispersion measure, an operation that is computationally tractable for existing array sizes. An alternative uses the various stations to form an ensemble estimate of the conditional density function (CDF) evaluated at each time step. Both methods outperform standard detection strategies on a test sequence of VLBA data, and both are efficient enough for deployment in real-time, online transient detection applications.

Thompson, David R.↗
NASA NTRS1992

Optical and radio variability in blazars

An analysis is presented of the optical (0.44 micron) and radio flux density variations (4.8, 8.0, and 14.5 GHz) of the five best observed variable AGN, of which three are BL Lac objects and two are optically violent variable (OVV) quasars. For each object the nature of the variability at individual wavelengths is determined by using a structure function analysis and the autocorrelation function as calculated from the discrete correlation technique. Both techniques yield similar results, showing that the optical variations are similar for all objects and are a combination of flicker and shot noise. The radio variability is distinct, being described by shot noise (less variation at short times), with the two OVVs exhibiting the most extreme behavior. The time lag between the variations at the three radio frequencies and the optical flux is studied with the discrete cross-correlation function and the mean variance methods. It is concluded that the optical and radio emitting regions are physically related, exist on distinct time scales, and excite their synchrotron emitting plasmas differently.

Hufnagel, Beth R.↗
NASA NTRS1966

Radio electronics

Low noise amplifiers, laser radar optics, gas lasers and holography, and microminiaturized solid state devices for radio electronics

LIDAR↗
NASA NTRS1966

Statistical communication theory

Decision theory and binary detection of signal transmission in noisy channel, and determination of noise caused by tape in magnetic recording system

DECISION THEORY↗
NASA NTRS1973

Observations of noise bands associated with the upper hybrid resonance by the Imp 6 radio astronomy experiment.

The intense noise bands occurring near the upper hybrid resonance frequency have been observed with the Imp 6 GSFC radio astronomy experiment in the plasmasphere. The identification of the upper hybrid resonance provides an accurate measure of the local electron density and allows the observed noise data to be fit to the scale of characteristic frequencies in the plasma. The data are consistent with earlier theoretical interpretations in which noise is generated between the upper hybrid and plasma frequencies and propagates to region 4 of the CMA diagram, where it is reflected at the L = 0 plasma cutoff.

Mosier, S. R.↗