5,131 research outputs found
Photoacoustic Tomography in a Rectangular Reflecting Cavity
Almost all known image reconstruction algorithms for photoacoustic and
thermoacoustic tomography assume that the acoustic waves leave the region of
interest after a finite time. This assumption is reasonable if the reflections
from the detectors and surrounding surfaces can be neglected or filtered out
(for example, by time-gating). However, when the object is surrounded by
acoustically hard detector arrays, and/or by additional acoustic mirrors, the
acoustic waves will undergo multiple reflections. (In the absence of absorption
they would bounce around in such a reverberant cavity forever). This disallows
the use of the existing free-space reconstruction techniques. This paper
proposes a fast iterative reconstruction algorithm for measurements made at the
walls of a rectangular reverberant cavity. We prove the convergence of the
iterations under a certain sufficient condition, and demonstrate the
effectiveness and efficiency of the algorithm in numerical simulations.Comment: 21 pages, 6 figure
Buying Time 2000: Television Advertising in the 2000 Federal Elections
Summarizes a study of political television advertising in the 2000 federal primaries and elections with a focus on the use of the issue ad loophole to evade campaign finance laws. Questions the standard used to differentiate issue ads from election ads
Pioneer Venus spacecraft charging model
Five environmental models were constructed to represent the solar wind and the upper, middle, and lower ionosphere of Venus. The spacecraft structure was modeled with over 140 passive electrical elements representing structural elements of the spacecraft. Electron, ion, secondary electron, and photocurrents to the spacecraft from the plasma were calculated, ignoring sheath effects. In all but one case, potentials of interest were less than 1 volt. Potential differences between widely separated points on the equipment shelf were less than 1 mV. The one area of concern is the solar panel potential when the orbiter is passing through the bowshock region
Using Transit Timing Observations to Search for Trojans of Transiting Extrasolar Planets
Theoretical studies predict that Trojans are likely a frequent byproduct of
planet formation and evolution. We examine the sensitivity of transit timing
observations for detecting Trojan companions to transiting extrasolar planets.
We demonstrate that this method offers the potential to detect terrestrial-mass
Trojans using existing ground-based observatories. We compare the transit
timing variation (TTV) method with other techniques for detecting extrasolar
Trojans and outline the future prospects for this method.Comment: submitted to ApJL, 12 pages, 2 figure
Stability of Satellites in Closely Packed Planetary Systems
We perform numerical integrations of four-body (star, planet, planet,
satellite) systems to investigate the stability of satellites in planetary
Systems with Tightly-packed Inner Planets (STIPs). We find that the majority of
closely-spaced stable two-planet systems can stably support satellites across a
range of parameter-space which is only slightly decreased compared to that seen
for the single-planet case. In particular, circular prograde satellites remain
stable out to (where is the Hill Radius) as opposed to
in the single-planet case. A similarly small restriction in the
stable parameter-space for retrograde satellites is observed, where planetary
close approaches in the range 2.5 to 4.5 mutual Hill radii destabilize most
satellites orbits only if . In very close planetary pairs (e.g.
the 12:11 resonance) the addition of a satellite frequently destabilizes the
entire system, causing extreme close-approaches and the loss of satellites over
a range of circumplanetary semi-major axes. The majority of systems
investigated stably harbored satellites over a wide parameter-space, suggesting
that STIPs can generally offer a dynamically stable home for satellites, albeit
with a slightly smaller stable parameter-space than the single-planet case. As
we demonstrate that multi-planet systems are not a priori poor candidates for
hosting satellites, future measurements of satellite occurrence rates in
multi-planet systems versus single-planet systems could be used to constrain
either satellite formation or past periods of strong dynamical interaction
between planets.Comment: 11 pages, 5 figures. Accepted for publication, ApJ
Canadian national sport organisations’ use of the web for relationship marketing in promoting sport participation
Sport participation development requires a systematic process which involves knowledge creation, dissemination and interactions between National Sport Organisations, participants, clubs and associations as well as other agencies. Using a relationship marketing approach (Grönroos, 1997, Gummesson, 2002, Olkkonen, 1999), this paper addressed the question ‘How do Canadian NSOs use the Web, in terms of functionality and services offered, to create and maintain relationships with sport participants and their sport delivery partners?’ Ten Canadian NSOs’ websites were examined: functionality was analysed using Burgess and Cooper’s (2000) eMICA model, while NSOs’ utilisation of the Internet to establish and maintain relationships with sport participants was analysed using Wang, Head and Archer’s (2000) relationship-building process model for the Web. It was found that Canadian NSOs were receptive to the use of the Web, but their information-gathering and dissemination activities, which make-up the relationship-building process, appear sparse, and in some cases are lagging behind the voluntary sector in the country
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