8,164 research outputs found
Analogues of Weyl's formula for reduced enveloping algebras
In this note we study simple modules for a reduced enveloping algebra
U_chi(g) in the critical case when chi element of g^* is ``nilpotent''. Some
dimension formulas computed by Jantzen suggest modified versions of Weyl's
dimension formula, based on certain reflecting hyperplanes for the affine Weyl
group which might be associated to Kazhdan--Lusztig cells.Comment: AMS-LaTeX, 10 pages, 2 figure
Thermal fatigue performance of integrally cast automotive turbine wheels
Fluidized bed thermal fatigue testing was conducted on 16 integrally cast automotive turbine wheels for 1000-10,000 (600 sec total) thermal cycles at 935/50 C. The 16 wheels consisted of 14 IN-792 + 1% Hf and 2 gatorized AF2-1DA wheels; 6 of the IN-792 + Hf wheels contained crack arrest pockets inside the blade root flange. Temperature transients during the thermal cycling were measured in three calibration tests using either 18 or 30 thermocouples per wheel. Thermal cracking based on crack length versus accumulated cycles was greatest for unpocketed wheels developing cracks in 8-13 cycles compared to 75-250 cycles for unpocketed wheels. However, pocketed wheels survived up to 10,000 cycles with crack lengths less than 20 mm, whereas two unpocketed wheels developed 45 mm long cracks in 1000-2000 cycles
Thermal fatigue and oxidation data of TAZ-8A and M22 alloys and variations
Thermal fatigue and oxidation data were obtained on 36 specimens, representing 18 distinct variations (including the base systems) of TAZ-8A and M22 alloys. Double-edge wedge specimens for these systems were cycled between fluidized beds maintained at 1088 C and 316 C with a 180 s immersion in each bed. The systems included alloys TAZ-8A, M22, and 16 variations of these alloys. Each alloy variation consisted of a unique composition with an alternation in the percentage of carbon (C1 and C2), molydenum (M1 and M2), tungsten (W1 and W2), columbium (CB1, CB2, and CB3), tantalium (T1, T2, and T3), or boron (B1, B2, and B3) present. All of the alloys showed little weight change due to oxidation compared with other alloys previously tested in fluidized beds. Only both C1 alloy variation specimens survived 3500 cycles without cracking in the small radius, although substantial cracks were present, emanating from the end notches which were used for holding the specimens
The Price of Anarchy in Active Signal Landscape Map Building
Multiple receivers with a priori knowledge about
their own initial states are assumed to be dropped in an unknown
environment comprising multiple signals of opportunity (SOPs)
transmitters. The receivers draw pseudorange observations from
the SOPs. The receivers’ objective is to build a high-fidelity
signal landscape map of the environment, which would enable
the receivers to navigate accurately with the aid of the SOPs.
The receivers could command their own maneuvers and such
commands are computed so to maximize the information gathered
about the SOPs in a greedy fashion. Several information
fusion and decision making architectures are possible. This
paper studies the price of anarchy in building signal landscape
maps to assess the degradation in the map quality should the
receivers produce their own maps and make their own maneuver
decisions versus a completely centralized approach. In addition,
a hierarchical architecture is proposed in which the receivers
build their own maps and make their own decisions, but share
relevant information. Such architecture is shown to produce maps
of comparable quality to the completely centralized approach.Aerospace Engineering and Engineering Mechanic
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Motion Planning for Optimal Information Gathering in Opportunistic Navigation Systems
Motion planning for optimal information gathering in an opportunistic navigation (OpNav)
environment is considered. An OpNav environment can be thought of as a radio
frequency signal landscape within which a receiver locates itself in space and time by extracting
information from ambient signals of opportunity (SOPs). The receiver is assumed
to draw only pseudorange-type observations from the SOPs, and such observations are
fused through an estimator to produce an estimate of the receiver’s own states. Since
not all SOP states in the OpNav environment may be known a priori, the receiver must
estimate the unknown SOP states of interest simultaneously with its own states. In this
work, the following problem is studied. A receiver with no a priori knowledge about its
own states is dropped in an unknown, yet observable, OpNav environment. Assuming that
the receiver can prescribe its own trajectory, what motion planning strategy should the
receiver adopt in order to build a high-fidelity map of the OpNav signal landscape, while
simultaneously localizing itself within this map in space and time? To answer this question,
first, the minimum conditions under which the OpNav environment is fully observable are
established, and the need for receiver maneuvering to achieve full observability is highlighted.
Then, motivated by the fact that not all trajectories a receiver may take in the
environment are equally beneficial from an information gathering point of view, a strategy
for planning the motion of the receiver is proposed. The strategy is formulated in a
coupled estimation and optimal control framework of a gradually identified system, where
optimality is defined through various information-theoretic measures. Simulation results
are presented to illustrate the improvements gained from adopting the proposed strategy
over random and pre-defined receiver trajectories.Aerospace Engineering and Engineering Mechanic
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Considerations for Future IGS Receivers
Future IGS receivers are considered against the backdrop of GNSS signal modernization
and the IGS’s goal of further improving the accuracy of its products. The purpose of this paper is to
provide IGS members with a guide to making decisions about GNSS receivers. Modernized GNSS signals
are analyzed with a view toward IGS applications. A schedule for minimum IGS receiver requirements
is proposed. Features of idealized conceptual receivers are discussed. The prospects for standard commercial
receivers and for software-defined GNSS receivers are examined. Recommendations are given
for how the IGS should proceed in order to maximally benefit from the transformation in GNSS that
will occur over the next decade.Aerospace Engineering and Engineering Mechanic
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A Technique for Determining the Carrier Phase Differences between Independent GPS Receivers during Scintillation
A method for recovering the carrier phase differences between
pairs of independent GPS receivers has been developed
and demonstrated in truth-model simulations. This
effort is in support of a project that intends to image the disturbed
ionosphere with diffraction tomography techniques
using GPS measurements from large arrays of receivers.
Carrier phase differential GPS techniques, common in surveying
and relative navigation, are employed to determine
the phase relationships between the receivers in the imaging array. Strategies for estimating the absolute carrier phase
disturbances at each receiver are discussed. Simulation results
demonstrate that the system can rapidly detect the onset
of scintillation, identify one non-scintillating reference
signal, and recover the carrier phase differences accurate to
0.1 cycles.Aerospace Engineering and Engineering Mechanic
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Data-Driven Generalized Integer Aperture Bootstrapping for Real-Time High Integrity Applications
A new method is developed for integer ambiguity
resolution in carrier-phase differential GPS (CDGPS) positioning.
The method is novel in that it is (1) data-driven, (2) generalized
to include partial ambiguity resolution, and (3) amenable to a
full characterization of the prior and posterior distributions of
the three-dimensional baseline vector that results from CDGPS.
The technique is termed generalized integer aperture bootstrapping
(GIAB). GIAB improves the availability of integer
ambiguity resolution for high-integrity, safety-critical systems.
Current high-integrity CDGPS algorithms, such as EPIC and
GERAFS, evaluate the prior risk of position domain biases due to
incorrect integer ambiguity resolution without further validation
of the chosen solution. This model-driven approach introduces
conservatism which tends to reduce solution availability. Common
data-driven ambiguity validation methods, such as the ratio test,
control the risk of incorrect ambiguity resolution by shrinking
an integer aperture (IA), or acceptance region. The incorrect
fixing risk of current IA methods is determined by functional
approximations that are inappropriate for use in safety-of-life
applications. Moreover, generalized IA (GIA) methods incorrectly
assume that the baseline resulting from partial ambiguity resolution
is zero mean. Each of these limitations is addressed by
GIAB, and the claimed improvements are validated by Monte
Carlo simulation. The performance of GIAB is then optimized by
tuning the integer aperture size to maximize the prior probability
of full ambiguity resolution. GIAB is shown to provide higher
availability than EPIC for the same integrity requirements.Aerospace Engineering and Engineering Mechanic
Economic Determinants of Participation in Physical Activity and Sport
This paper examines the economic determinants of participation in physical activity by developing and analyzing a consumer choice model of participation and by testing the predictions of this model using data drawn from the Behavioral Risk Factor Surveillance Survey (BRFSS). Both emphasize that individuals face two distinct decisions: (1) should I participate in sport?; and (2) how much time should I spend participating in sport? The evidence highlights the importance of selectivity. The economic factors that affect these two decisions work in opposite directions; factors that increase the likelihood of participation generally decrease the amount of time spent participating.physical activity, time allocation, health production
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