13,657 research outputs found
On the evolution of snow roughness during snow fall
The deposition and attachment mechanism of settling snow crystals during snowfall dictates the very initial structure of ice within a natural snowpack. In this letter we apply ballistic deposition as a simple model to study the structural evolution of the growing surface of a snowpack during its formation. The roughness of the snow surface is predicted from the behaviour of the time dependent height correlation function. The predictions are verified by simple measurements of the growing snow surface based on digital photography during snowfall. The measurements are in agreement with the theoretical predictions within the limitations of the model which are discussed. The application of ballistic deposition type growth models illuminates structural aspects of snow from the perspective of formation which has been ignored so far. Implications of this type of growth on the aerodynamic roughness length, density, and the density correlation function of new snow are discusse
Dynamic trust models for ubiquitous computing environments
A significant characteristic of ubiquitous computing is the need for interactions of highly mobile entities to be secure: secure both for the entity and the environment in which the entity operates. Moreover, ubiquitous computing is also characterised by partial views over the state of the global environment, implying that we cannot guarantee that an environment can always verify the properties of the mobile entity that it has just received. Secure in this context encompasses both the need for cryptographic security and the need for trust, on the part of both parties, that the interaction is functioning as expected. In this paper we make a broad assumption that trust and cryptographic security can be considered as orthogonal concerns (i.e. an entity might encrypt a deliberately incorrect answer to a legitimate request). We assume the existence of reliable encryption techniques and focus on the characteristics of a model that supports the management of the trust relationships between two entities during an interaction in a ubiquitous environment
Object Matching in Distributed Video Surveillance Systems by LDA-Based Appearance Descriptors
Establishing correspondences among object instances is still challenging in multi-camera surveillance systems, especially when the cameras’ fields of view are non-overlapping. Spatiotemporal constraints can help in solving the correspondence problem but still leave a wide margin of uncertainty. One way to reduce this uncertainty is to use appearance information about the moving objects in the site. In this paper we present the preliminary results of a new method that can capture salient appearance characteristics at each camera node in the network. A Latent Dirichlet Allocation (LDA) model is created and maintained at each node in the camera network. Each object is encoded in terms of the LDA bag-of-words model for appearance. The encoded appearance is then used to establish probable matching across cameras. Preliminary experiments are conducted on a dataset of 20 individuals and comparison against Madden’s I-MCHR is reported
Maser Source Finding Methods in HOPS
The {\bf H}{\bf O} Southern Galactic {\bf P}lane {\bf S}urvey (HOPS) has
observed 100 square degrees of the Galactic plane, using the Mopra radio
telescope to search for emission from multiple spectral lines in the 12\,mm
band (19.5\,--\,27.5\,GHz). Perhaps the most important of these spectral lines
is the 22.2\,GHz water maser transition. We describe the methods used to
identify water maser candidates and subsequent confirmation of the sources. Our
methods involve a simple determination of likely candidates by searching peak
emission maps, utilising the intrinsic nature of water maser emission -
spatially unresolved and spectrally narrow-lined. We estimate completeness
limits and compare our method with results from the {\sc Duchamp} source
finder. We find that the two methods perform similarly. We conclude that the
similarity in performance is due to the intrinsic limitation of the noise
characteristics of the data. The advantages of our method are that it is
slightly more efficient in eliminating spurious detections and is simple to
implement. The disadvantage is that it is a manual method of finding sources
and so is not practical on datasets much larger than HOPS, or for datasets with
extended emission that needs to be characterised. We outline a two-stage method
for the most efficient means of finding masers, using {\sc Duchamp}.Comment: 8 pages, 1 table, 4 figures. Accepted for publication in PASA special
issue on Source Finding & Visualisatio
Security Attributes Based Digital Rights Management
Most real-life systems delegate responsibilities to different authorities. We apply this model to a digital rights management system, to achieve flexible security. In our model a hierarchy of authorities issues certificates that are linked by cryptographic means. This linkage establishes a chain of control, identity-attribute-rights, and allows flexible rights control over content. Typical security objectives, such as identification, authentication, authorization and access control can be realised. Content keys are personalised to detect illegal super distribution. We describe a working prototype, which we develop using standard techniques, such as standard certificates, XML and Java. We present experimental results to evaluate the scalability of the system. A formal analysis demonstrates that our design is able to detect a form of illegal super distribution
Initial states and infrared physics in locally de Sitter spacetime
The long wavelength physics in a de Sitter region depends on the initial
quantum state. While such long wavelength physics is under control for massive
fields near the Hartle-Hawking vacuum state, such initial states make unnatural
assumptions about initial data outside the region of causal contact of a local
observer. We argue that a reasonable approximation to a maximum entropy state,
one that makes minimal assumptions outside an observer's horizon volume, is one
where a cutoff is placed on a surface bounded by timelike geodesics, just
outside the horizon. For sufficiently early times, such a cutoff induces
secular logarithmic divergences with the expansion of the region. For massive
fields, these effects sum to finite corrections at sufficiently late times. The
difference between the cutoff correlators and Hartle-Hawking correlators
provides a measure of the theoretical uncertainty due to lack of knowledge of
the initial state in causally disconnected regions. These differences are
negligible for primordial inflation, but can become significant during epochs
with very long-lived de Sitter regions, such as we may be entering now.Comment: 19 pages, 4 figures, references adde
Hydrodynamic induced deformation and orientation of a microscopic elastic filament
We describe simulations of a microscopic elastic filament immersed in a fluid
and subject to a uniform external force. Our method accounts for the
hydrodynamic coupling between the flow generated by the filament and the
friction force it experiences. While models that neglect this coupling predict
a drift in a straight configuration, our findings are very different. Notably,
a force with a component perpendicular to the filament axis induces bending and
perpendicular alignment. Moreover, with increasing force we observe four shape
regimes, ranging from slight distortion to a state of tumbling motion that
lacks a steady state. We also identify the appearance of marginally stable
structures. Both the instability of these shapes and the observed alignment can
be explained by the combined action of induced bending and non-local
hydrodynamic interactions. Most of these effects should be experimentally
relevant for stiff micro-filaments, such as microtubules.Comment: three figures. To appear in Phys Rev Let
Hydrogen slush density reference system
A hydrogen slush density reference system was designed for calibration of field-type instruments and/or transfer standards. The device is based on the buoyancy principle of Archimedes. The solids are weighed in a low-mass container so arranged that solids and container are buoyed by triple-point liquid hydrogen during the weighing process. Several types of hydrogen slush density transducers were developed and tested for possible use as transfer standards. The most successful transducers found were those which depend on change in dielectric constant, after which the Clausius-Mossotti function is used to relate dielectric constant and density
The Performance of Alfalfa Synthetics in the First and Advanced Generations
During alfalfa breeding investigations conducted at the Nebraska Agricultural Experiment Station, numerous superior clones were selected and tested as clones, and in polycross progeny tests. Information was needed on the performance of synthetic varieties in the first and advanced generations, on the optimum number of clones to include in a synthetic variety, and on parent-progeny relationships. Clones with high general combining ability for forage yield as measured by polycross progeny tests, and in certain instances specific combining ability based on single-cross tests, were intercrossed in various ways to produce synthetic varieties. A group of synthetics varying in number of parents from 2 to 6 clones, having in some instances certain clones as common parents, was tested initially in the first generation of synthesis (referred to as Syn-1 from here on), later in the Syn-1 versus the Syn-2, and in some instances in the Syn-1, Syn-2, and Syn-3, and ultimately in the Syn-1,-2,-3, and -4 generations. The purposes of this bulletin are to report (1) comparative results obtained in yield trials involving the Syn-1,-2,-3, and -4 generations of 5 two-clone and 14 multiple-clone synthetics at Lincoln, Nebraska, and Ithaca, New York, and (2) parent-progeny relationships
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