10,240 research outputs found
Nonuniform Coverage Control on the Line
This paper investigates control laws allowing mobile, autonomous agents to
optimally position themselves on the line for distributed sensing in a
nonuniform field. We show that a simple static control law, based only on local
measurements of the field by each agent, drives the agents close to the optimal
positions after the agents execute in parallel a number of
sensing/movement/computation rounds that is essentially quadratic in the number
of agents. Further, we exhibit a dynamic control law which, under slightly
stronger assumptions on the capabilities and knowledge of each agent, drives
the agents close to the optimal positions after the agents execute in parallel
a number of sensing/communication/computation/movement rounds that is
essentially linear in the number of agents. Crucially, both algorithms are
fully distributed and robust to unpredictable loss and addition of agents
Application of digital analysis of MSS data to agro-environmental studies
There are no author-identified significant results in this report
Surface contamination on LDEF exposed materials
X-ray photoelectron spectroscopy (XPS) has been used to study the surface composition and chemistry of Long Duration Exposure Facility (LDEF) exposed materials including silvered Teflon (Ag/FEP), Kapton, S13GLO paint, quartz crystal monitors (QCM's), carbon fiber/organic matrix composites, and carbon fiber/Al Alloy composites. In each set of samples, silicones were the major contributors to the molecular film accumulated on the LDEF exposed surfaces. All surfaces analyzed have been contaminated with Si, O, and C; most have low levels (less than 1 atom percent) of N, S, and F. Occasionally observed contaminants included Cl, Na, K, P, and various metals. Orange/brown discoloration observed near vent slots in some Ag/FEP blankets were higher in carbon, sulfur, and nitrogen relative to other contamination types. The source of contamination has not been identified, but amine/amide functionalities were detected. It is probable that this same source of contamination account for the low levels of sulfur and nitrogen observed on most LDEF exposed surfaces. XPS, which probes 50 to 100 A in depth, detected the major sample components underneath the contaminant film in every analysis. This probably indicates that the contaminant overlayer is patchy, with significant areas covered by less that 100 A of molecular film. Energy dispersive x-ray spectroscopy (EDS) of LDEF exposed surfaces during secondary electron microscopy (SEM) of the samples confirmed contamination of the surfaces with Si and O. In general, particulates were not observed to develop from the contaminant overlayer on the exposed LDEF material surfaces. However, many SiO2 submicron particles were seen on a masked edge of an Ag/FEP blanket. In some cases such as the carbon fiber/organic matrix composites, interpretation of the contamination data was hindered by the lack of good laboratory controls. Examination of laboratory controls for the carbon fiber/Al alloy composites showed that preflight contamination was the most significant factor for all the contaminants generally detected at less than 1 atom percent, or detected only occasionally (i.e., all but Si, O, and C). Flight control surfaces, including sample backsides not exposed to space radiation or atomic oxygen flux, have accumulated some contamination on flight (compared to laboratory controls), but experimentally, the LDEF exposed surface contamination levels are generally higher for the contaminants Si and O. For most materials analyzed, Si contamination levels were higher on the leading edge surfaces than on the trailing edge surfaces. This was true even for the composite samples where considerable atomic oxygen erosion of the leading edge surfaces was observed by SEM. It is probable that the return flux associated with atmospheric backscatter resulted in enhanced deposition of silicones and other contaminants on the leading edge flight surfaces relative to the trailing edge. Although the Si concentration data suggested greater on-flight deposition of contaminants on the leading edge surfaces, the XPS analyses did not conclusively show different relative total thicknesses of flight deposited contamination for leading and trailing edge surfaces. It is possible that atomic oxygen reactions on the leading edge resulted in greater volatilization of the carbon component of the deposited silicones, effectively 'thinning' the leading edge deposited overlayer. Unlike other materials, exposed polymers such as Kapton and FEP-type Teflon had very low contamination on the leading edge surfaces. SEM evidence showed that undercutting of the contaminant overlayer and damaged polymer layers occurred during atomic oxygen erosion, which would enhance loss of material from the exposed surface
Stent implant follow-up in intravascular optical coherence tomography images
The objectives of this article are (i) to
utilize computer methods in detection of stent struts
imaged in vivo by optical coherence tomography
(OCT) during percutaneous coronary interventions
(PCI); (ii) to provide measurements for the assessment
and monitoring of in-stent restenosis by OCT post PCI.
Thirty-nine OCT cross-sections from seven pullbacks
from seven patients presenting varying degrees of
neointimal hyperplasia (NIH) are selected, and stent
struts are detected. Stent and lumen boundaries are
reconstructed and one experienced observer analyzed
the strut detection, the lumen and stent area measurements,
as well as the NIH thickness in comparison to
manual tracing using the reviewing software provided
by the OCT manufacturer (LightLab Imaging, MA,
USA). Very good agreements were found between
the computer methods and the expert evaluations
for lumen cross-section area (mean difference =
0.11 ± 0.70 mm2; r2 = 0.98, P\ 0.0001) and the
stent cross-section area (mean difference = 0.10 ±
1.28 mm2; r2 = 0.85, P value\ 0.0001). The average
number of detected struts was 10.4 ± 2.9 per crosssection
when the expert identified 10.5 ± 2.8
(r2 = 0.78, P value\0.0001). For the given patient
dataset: lumen cross-sectional area was on the average
(6.05 ± 1.87 mm2), stent cross-sectional area was
(6.26 ± 1.63 mm2), maximum angle between struts
was on the average (85.96 ± 54.23), maximum,
average, and minimum distance between the stent
and the lumen were (0.18 ± 0.13 mm), (0.08 ±
0.06 mm), and (0.01 ± 0.02 mm), respectively, and
stent eccentricity was (0.80 ± 0.08). Low variability
between the expert and automatic method was
observed in the computations of the most important
parameters assessing the degree of neointimal tissue
growth in stents imaged by OCT pullbacks. After
further extensive validation, the presented methods
might offer a robust automated tool that will improve
the evaluation and follow-up monitoring of in-stent
restenosis in patients
Concepts for 18/30 GHz satellite communication system, volume 1
Concepts for 18/30 GHz satellite communication systems are presented. Major terminal trunking as well as direct-to-user configurations were evaluated. Critical technologies in support of millimeter wave satellite communications were determined
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