4,416 research outputs found
Ranging performance of satellite laser altimeters
Topographic mapping of the earth, moon and planets can be accomplished with high resolution and accuracy using satellite laser altimeters. These systems employ nanosecond laser pulses and microradian beam divergences to achieve submeter vertical range resolution from orbital altitudes of several hundred kilometers. Here, we develop detailed expressions for the range and pulse width measurement accuracies and use the results to evaluate the ranging performances of several satellite laser altimeters currently under development by NASA for launch during the next decade. Our analysis includes the effects of the target surface characteristics, spacecraft pointing jitter and waveform digitizer characteristics. The results show that ranging accuracy is critically dependent on the pointing accuracy and stability of the altimeter especially over high relief terrain where surface slopes are large. At typical orbital altitudes of several hundred kilometers, single-shot accuracies of a few centimeters can be achieved only when the pointing jitter is on the order of 10 mu rad or less
Towards Bootstrapping QED
We initiate the conformal bootstrap study of Quantum Electrodynamics in
space-time dimensions (QED) with flavors of charged fermions by
focusing on the 4-point function of four monopole operators with the lowest
unit of topological charge. We obtain upper bounds on the scaling dimension of
the doubly-charged monopole operator, with and without assuming other gaps in
the operator spectrum. Intriguingly, we find a (gap-dependent) kink in these
bounds that comes reasonably close to the large extrapolation of the
scaling dimensions of the singly-charged and doubly-charged monopole operators
down to and .Comment: 29 pages plus an appendix, 5 figures, v2 minor improvements, refs
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Urban heat stress vulnerability in the U.S. Southwest: The role of sociotechnical systems
Heat vulnerability of urban populations is becoming a major issue of concern with climate change, particularly in the cities of the Southwest United States. In this article we discuss the importance of understanding coupled social and technical systems, how they constitute one another, and how they form the conditions and circumstances in which people experience heat. We discuss the particular situation of Los Angeles and Maricopa Counties, their urban form and the electric grid. We show how vulnerable populations are created by virtue of the age and construction of buildings, the morphology of roads and distribution of buildings on the landscape. Further, the regulatory infrastructure of electricity generation and distribution also contributes to creating differential vulnerability. We contribute to a better understanding of the importance of sociotechnical systems. Social infrastructure includes codes, conventions, rules and regulations; technical systems are the hard systems of pipes, wires, buildings, roads, and power plants. These interact to create lock-in that is an obstacle to addressing issues such as urban heat stress in a novel and equitable manner
Twin CWG systems Final report
Construction, operation, and maintenance of twin control moment gyroscope system for space vehicle motion simulato
Heat pipe cooling of power processing magnetics
A heat pipe cooled transformer and input filter were developed for the 2.4 kW beam supply of a 30 cm ion thruster system. This development yielded a mass reduction of 40% (1.76 kg) and lower mean winding temperature (20 C lower). While these improvements are significant, preliminary designs predict even greater benefits to be realized at higher power. The design details are presented along with the results of thermal vacuum operation and the component performance in a 3 kW breadboard power processor
Bootstrapping Vector Models in
We use the conformal bootstrap to study conformal field theories with
global symmetry in and spacetime dimensions that have a scalar
operator transforming as an vector. The crossing symmetry of
the four-point function of this vector operator, along with unitarity
assumptions, determine constraints on the scaling dimensions of conformal
primary operators in the OPE. Imposing a lower bound on
the second smallest scaling dimension of such an -singlet conformal
primary, and varying the scaling dimension of the lowest one, we obtain an
allowed region that exhibits a kink located very close to the interacting
-symmetric CFT conjectured to exist recently by Fei, Giombi, and
Klebanov. Under reasonable assumptions on the dimension of the second lowest
singlet in the OPE, we observe that this kink
disappears in for small enough , suggesting that in this case an
interacting CFT may cease to exist for below a certain critical
value.Comment: 24 pages, 5 figures; v2 minor improvement
A New Duality Between Superconformal Field Theories in Three Dimensions
We propose a new duality between two 3d superconformal
Chern-Simons-matter theories: the ABJM theory and a
theory consisting of the product between the BLG theory and a free theory of
eight real scalars and eight Majorana fermions. As evidence supporting this
duality, we show that the moduli spaces, superconformal indices,
partition functions, and certain OPE coefficients of BPS operators in the two
theories agree.Comment: 29 pages, 2 figure
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