25,769 research outputs found
Performance of a 12-GHz fiber-optic system for beam-waveguide antenna stability testing
A 12-GHz fiber-optic system is a critical part of a test configuration that was proposed for measuring the fractional frequency stability of the Deep Space Station (DSS)-13 beam-waveguide (BWG) antenna. This fiber-optic system is used to carry Ku-band (12-GHz) signals from a reference antenna to the DSS-13 BWG pedestal room. Tests performed only on the fiber-optic system portion of the overall test configuration showed that the 12-GHz fiber-optic system (installed at DSS-13) has a frequency stability of about 1.1 x 10(exp -16) for sampling time tau = 1000 sec for a nighttime run. This preliminary result establishes the lowest noise floor that can probably be achieved for the test configuration that will be used to measure the frequency stability of the DSS-13 BWG antenna
The Lorentzian distance formula in noncommutative geometry
For almost twenty years, a search for a Lorentzian version of the well-known
Connes' distance formula has been undertaken. Several authors have contributed
to this search, providing important milestones, and the time has now come to
put those elements together in order to get a valid and functional formula.
This paper presents a historical review of the construction and the proof of a
Lorentzian distance formula suitable for noncommutative geometry.Comment: 16 pages, final form, few references adde
Can the effects of anthropogenic pressures and environmental variability on nekton fauna be detected in fishery data? Insights from the monitoring of the artisanal fishery within the Venice lagoon
Nekton communities in transitional ecosystems are naturally adapted to stressful conditions associated with high environmental variability. Human activities in these systems are likely to determine additional stress with a possible effect on fish fauna, hence on fisheries. In order to test the relative importance of natural and anthropogenic factors in determining changes in nekton community, catches (incl. bycatch) from artisanal fisheries (fyke nets) were monitored seasonally in different areas of the Venice lagoon (Italy) between 2001 and 2013. Changes in nekton community composition and in the biomass of target and non-target species/groups were analysed, and the results were related to temporal factors, environmental characteristics and to the variability in anthropogenic pressures. Statistical tests were carried out using a model-based analysis of both univariate and multivariate data. Results highlighted that temporal factors and environmental conditions (i.e. the main chemico-physical descriptors) are more relevant than anthropogenic pressures in explaining spatial and temporal changes in the lagoon nekton assemblage, but that several characteristics of the assemblage, in particular the biomass of some particular categories and of the whole assemblage, are sensitive to human impacts. A particularly negligible effect seemed to be associated with fishing effort, thus suggesting that the monitoring of the local artisanal fishery is suitable also to provide useful information on the evaluation of the status of nekton assemblage
Synchronization universality classes and stability of smooth, coupled map lattices
We study two problems related to spatially extended systems: the dynamical
stability and the universality classes of the replica synchronization
transition. We use a simple model of one dimensional coupled map lattices and
show that chaotic behavior implies that the synchronization transition belongs
to the multiplicative noise universality class, while stable chaos implies that
the synchronization transition belongs to the directed percolation universality
class.Comment: 6 pages, 7 figure
Revenue-Recycling and the Efficiency and Spatial Distributional Impacts of Development Taxes
Recent studies that compare the efficiency and distributional impacts of alternative instruments to curb sprawl typically ignore what to do with the revenues from anti-sprawl policies, such as development taxes. This paper extends first-best analysis of development taxes aimed at preserving land at the urban fringe to account for interactions with other distortions within the urban system. By incorporating urban decline at the city core, which in turn, generates negative neighborhood spillover effects and extra pressure for development at the urban fringe, we provide a more complete framework to evaluate the efficiency and distributional impacts of development taxes. We consider three potential alternative schemes to recycle the revenues: lump sum recycling, earmarked revenues to purchase conservation easements that permanently save open space and earmarked revenues to subsidize a revitalization program at the city core. In this setting, when revenues from the development tax are earmarked to fund a conservation easement there is an additional welfare gain (relative to the lump sum case) because the threat of future conversation of open space is fully eliminated. Similarly, when revenues are earmarked to fund a revitalization program at the city core, there are additional sources of welfare that make this policy preferred relative to the lump-sum recycling scheme. Finally, we also explore the spatial distributional impacts of these three alternative recycling schemes.Urban Sprawl, Revenue-Recycling, Regional Coordination, Spatial Modeling, Environmental Economics and Policy, Land Economics/Use, Resource /Energy Economics and Policy, Q31, R14, R38,
Welfare Effects of Anti-Sprawl Policies in the Presence of Urban Decline
This paper extends first-best analysis of anti-sprawl policies, such as development taxes, and examines the welfare effects of development taxes in the presence of urban decline at the city core. We find that anti-sprawl policies generate several important feedbacks within the urban system, generating additional welfare gains and affecting the level of urban decline and suburban sprawl. Further, the optimal development tax exceeds the (first-best) Pigouvian level, irrespective of whether or not revenues are returned lump-sum to all landowners or earmarked for urban decline mitigation.urban sprawl, development taxes, second-best policies, spatial modeling, Land Economics/Use,
The electrical conductivities of the DSS-13 beam-waveguide antenna shroud material and other antenna reflector surface materials
A significant amount of noise temperature can potentially be generated by currently unknown dissipative losses in the beam waveguide (BWG) shroud. The amount of noise temperature contribution from this source is currently being studied. In conjunction with this study, electrical conductivity measurements were made on samples of the DSS-13 BWG shroud material at 8.420 GHz. The effective conductivities of unpainted and painted samples of the BWG shroud were measured to be 0.01 x 10(exp 7) and 0.0036 x 10(exp 7) mhos/m, respectively. This value may be compared with 5.66 x 10(exp 7) mhos/m for high conductivity copper
Dynamics of a magnetic dimer with exchange, dipolar and Dzyalozhinski-Moriya interaction
We investigate the dynamics of a magnetic system consisting of two magnetic
moments coupled by either exchange, dipole-dipole, or Dzyalozhinski-Moriya
interaction. We compare the switching mechanisms and switching rates as induced
by the three couplings. For each coupling and each configuration of the two
anisotropy axes, we describe the switching modes and, using the kinetic theory
of Langer, we provide (semi-)analytical expressions for the switching rate. We
then compare the three interactions with regard to their efficiency in the
reversal of the net magnetic moment of the dimer. We also investigate how the
energy barriers vary with the coupling. For the dipole-dipole interaction we
find that the energy barrier may either increase or decrease with the coupling
depending on whether the latter is weak or strong. Finally, upon comparing the
various switching rates, we find that the dipole-dipole coupling leads to the
slowest magnetic dimer, as far as the switching of its net magnetic moment is
concerned.Comment: 20 pages, 18 Figures, 2 table
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