4,568 research outputs found
Design and Performance of the Wide-Field X-Ray Monitor on Board the High-Energy Transient Explorer 2
The Wide-field X-ray Monitor (WXM) is one of the scientific instruments
carried on the High Energy Transient Explorer 2 (HETE-2) satellite launched on
2000 October 9. HETE-2 is an international mission consisting of a small
satellite dedicated to provide broad-band observations and accurate
localizations of gamma-ray bursts (GRBs). A unique feature of this mission is
its capability to determine and transmit GRB coordinates in almost real-time
through the burst alert network. The WXM consists of three elements: four
identical Xe-filled one-dimensional position-sensitive proportional counters,
two sets of one-dimensional coded apertures, and the main electronics. The WXM
counters are sensitive to X-rays between 2 keV and 25 keV within a
field-of-view of about 1.5 sr, with a total detector area of about 350 cm.
The in-flight triggering and localization capability can produce a real-time
GRB location of several to 30 arcmin accuracy, with a limiting sensitivity of
erg cm. In this report, the details of the mechanical
structure, electronics, on-board software, ground and in-flight calibration,
and in-flight performance of the WXM are discussed.Comment: 28 pages, 24 figure
In flight performance and first results of FREGATE
The gamma-ray detector of HETE-2, called FREGATE, has been designed to detect
gamma-ray bursts in the energy range [6-400] keV. Its main task is to alert the
other instruments of the occurrence of a gamma-ray burst (GRB) and to provide
the spectral coverage of the GRB prompt emission in hard X-rays and soft
gamma-rays. FREGATE was switched on on October 16, 2000, one week after the
successful launch of HETE-2, and has been continuously working since then. We
describe here the main characteristics of the instrument, its in-flight
performance and we briefly discuss the first GRB observations.Comment: Invited lecture at the Woods Hole 2001 GRB Conference, 8 pages, 15
figure
The Burst Alert Telescope (BAT) on the Swift MIDEX Mission
The Burst Alert Telescope (BAT) is one of 3 instruments on the Swift MIDEX
spacecraft to study gamma-ray bursts (GRBs). The BAT first detects the GRB and
localizes the burst direction to an accuracy of 1-4 arcmin within 20 sec after
the start of the event. The GRB trigger initiates an autonomous spacecraft slew
to point the two narrow field-of-view (FOV) instruments at the burst location
within 20-70 sec so to make follow-up x-ray and optical observations. The BAT
is a wide-FOV, coded-aperture instrument with a CdZnTe detector plane. The
detector plane is composed of 32,768 pieces of CdZnTe (4x4x2mm), and the
coded-aperture mask is composed of approximately 52,000 pieces of lead
(5x5x1mm) with a 1-m separation between mask and detector plane. The BAT
operates over the 15-150 keV energy range with approximately 7 keV resolution,
a sensitivity of approximately 10E-8 erg*cm^-2*s^-1, and a 1.4 sr (half-coded)
FOV. We expect to detect >100 GRBs/yr for a 2-year mission. The BAT also
performs an all-sky hard x-ray survey with a sensitivity of approximately 2
mCrab (systematic limit) and it serves as a hard x-ray transient monitor.Comment: 18 Pages, 12 Figures, To be published in Space Science Review
Analysis of an On-Line Stability Monitoring Approach for DC Microgrid Power Converters
An online approach to evaluate and monitor the stability margins of dc microgrid power converters is presented in this paper. The discussed online stability monitoring technique is based on the Middlebrook's loop-gain measurement technique, adapted to the digitally controlled power converters. In this approach, a perturbation is injected into a specific digital control loop of the converter and after measuring the loop gain, its crossover frequency and phase margin are continuously evaluated and monitored. The complete analytical derivation of the model, as well as detailed design aspects, are reported. In addition, the presence of multiple power converters connected to the same dc bus, all having the stability monitoring unit, is also investigated. An experimental microgrid prototype is implemented and considered to validate the theoretical analysis and simulation results, and to evaluate the effectiveness of the digital implementation of the technique for different control loops. The obtained results confirm the expected performance of the stability monitoring tool in steady-state and transient operating conditions. The proposed method can be extended to generic control loops in power converters operating in dc microgrids
DeSyRe: on-Demand System Reliability
The DeSyRe project builds on-demand adaptive and reliable Systems-on-Chips (SoCs). As fabrication technology scales down, chips are becoming less reliable, thereby incurring increased power and performance costs for fault tolerance. To make matters worse, power density is becoming a significant limiting factor in SoC design, in general. In the face of such changes in the technological landscape, current solutions for fault tolerance are expected to introduce excessive overheads in future systems. Moreover, attempting to design and manufacture a totally defect and fault-free system, would impact heavily, even prohibitively, the design, manufacturing, and testing costs, as well as the system performance and power consumption. In this context, DeSyRe delivers a new generation of systems that are reliable by design at well-balanced power, performance, and design costs. In our attempt to reduce the overheads of fault-tolerance, only a small fraction of the chip is built to be fault-free. This fault-free part is then employed to manage the remaining fault-prone resources of the SoC. The DeSyRe framework is applied to two medical systems with high safety requirements (measured using the IEC 61508 functional safety standard) and tight power and performance constraints
Continuous harmonic analysis and power quality measurements in three-phase systems
A virtual instrument, named Power Quality
Meter, is presented for (a) measuring power consumption and
harmonics in three-phase systems, under non-sinusoidal and
imbalance conditions (b) detecting, classifying and organizes
power disturbance events. Measurement of the power
consumption follows the formulation proposed by the members
of the IEEE Working Group on Nonsinusoidal Situations
(1996). So, definitions are based on the analysis of functions in
the frequency domain, separating the fundamental terms from
the harmonic terms of the Fourier series. The virtual instrument
has been developed too for monitoring and measuring power
disturbances, which are automatically classified and organized
in a database while they are being recorded. Software tools use
the database structure to present summaries of power
disturbances and locate an event by severity or time of
occurrence. Records of actual measurements are included to
demonstrate the versatility of the instrument
- …