160 research outputs found
Divisibility of the stable Miller-Morita-Mumford classes
We determine the sublattice generated by the Miller-Morita-Mumford classes
in the torsion free quotient of the integral cohomology ring of the
stable mapping class group. We further decide when the mod p reductions
vanish.Comment: 24 pages, 1 figur
Astrometric radial velocities. I. Non-spectroscopic methods for measuring stellar radial velocity
High-accuracy astrometry permits the determination of not only stellar
tangential motion, but also the component along the line-of-sight. Such
non-spectroscopic (i.e. astrometric) radial velocities are independent of
stellar atmospheric dynamics, spectral complexity and variability, as well as
of gravitational redshift. Three methods are analysed: (1) changing annual
parallax, (2) changing proper motion and (3) changing angular extent of a
moving group of stars. All three have significant potential in planned
astrometric projects. Current accuracies are still inadequate for the first
method, while the second is marginally feasible and is here applied to 16
stars. The third method reaches high accuracy (<1 km/s) already with present
data, although for some clusters an accuracy limit is set by uncertainties in
the cluster expansion rate.Comment: 13 pages, 2 figures. Accepted for publication in Astronomy &
Astrophysics (main journal
The homotopy type of the cobordism category
The embedded cobordism category under study in this paper generalizes the
category of conformal surfaces, introduced by G. Segal in order to formalize
the concept of field theories. Our main result identifies the homotopy type of
the classifying space of the embedded d-dimensional cobordism category for all
d. For d=2, our results lead to a new proof of the generalized Mumford
conjecture, somewhat different in spirit from the original one.Comment: 40 pages. v2 has improved notation, added explanations, and minor
mistakes fixed. v3 has minor corrections and improvements. Final submitted
versio
Melting transitions in biomembranes
We investigated melting transitions in biological membranes in their native
state that include their membrane proteins. These membranes originated from
\textit{E. coli}, \textit{B. subtilis}, lung surfactant and nerve tissue from
the spinal cord of several mammals. For some preparations, we studied the
pressure, pH and ionic strength dependence of the transition. For porcine
spine, we compared the transition of the native membrane to that of the
extracted lipids. All preparations displayed melting transitions of 10-20
degrees below physiological or growth temperature, independent of the organism
of origin and the respective cell type. The position of transitions in
\textit{E. coli} membranes depends on the growth temperature. We discuss these
findings in the context of the thermodynamic theory of membrane fluctuations
that leads to largely altered elastic constants, an increase in fluctuation
lifetime and in membrane permeability associated with the transitions. We also
discuss how to distinguish lipid transitions from protein unfolding
transitions. Since the feature of a transition slightly below physiological
temperature is conserved even when growth conditions change, we conclude that
the transitions are likely to be of major biological importance for the
survival and the function of the cell.Comment: 12 pages, 6 Figures, 1 supplement with 1 figur
Software for Probabilistic Risk Reduction
A computer program implements a methodology, denoted probabilistic risk reduction, that is intended to aid in planning the development of complex software and/or hardware systems. This methodology integrates two complementary prior methodologies: (1) that of probabilistic risk assessment and (2) a risk-based planning methodology, implemented in a prior computer program known as Defect Detection and Prevention (DDP), in which multiple requirements and the beneficial effects of risk-mitigation actions are taken into account. The present methodology and the software are able to accommodate both process knowledge (notably of the efficacy of development practices) and product knowledge (notably of the logical structure of a system, the development of which one seeks to plan). Estimates of the costs and benefits of a planned development can be derived. Functional and non-functional aspects of software can be taken into account, and trades made among them. It becomes possible to optimize the planning process in the sense that it becomes possible to select the best suite of process steps and design choices to maximize the expectation of success while remaining within budget
Software for Generating Strip Maps from SAR Data
Jurassicprok is a computer program that generates strip-map digital elevation models and other data products from raw data acquired by an airborne synthetic-aperture radar (SAR) system. This software can process data from a variety of airborne SAR systems but is designed especially for the GeoSAR system, which is a dual-frequency (P- and X-band), single-pass interferometric SAR system for measuring elevation both at the bare ground surface and top of the vegetation canopy. Jurassicprok is a modified version of software developed previously for airborne-interferometric- SAR applications. The modifications were made to accommodate P-band interferometric processing, remove approximations that are not generally valid, and reduce processor-induced mapping errors to the centimeter level. Major additions and other improvements over the prior software include the following: a) A new, highly efficient multi-stage-modified wave-domain processing algorithm for accurately motion compensating ultra-wideband data; b) Adaptive regridding algorithms based on estimated noise and actual measured topography to reduce noise while maintaining spatial resolution; c) Exact expressions for height determination from interferogram data; d) Fully calibrated volumetric correlation data based on rigorous removal of geometric and signal-to-noise decorrelation terms; e) Strip range-Doppler image output in user-specified Doppler coordinates; f) An improved phase-unwrapping and absolute-phase-determination algorithm; g) A more flexible user interface with many additional processing options; h) Increased interferogram filtering options; and i) Ability to use disk space instead of random- access memory for some processing steps
Numerical investigation on the thermo-mechanical behavior of a quadratic cross section pile heat exchanger
Pile heat exchangers are traditional foundation piles with built in heat exchangers. As such, the footing of the building both serves as a structural component and a heating/cooling supply element. The existing geotechnical design standards do not consider the nature of thermo-active foundations and, therefore, there is a need to develop guidelines to design them properly. This paper contributes by studying the thermo-mechanical behavior of the precast piles which are 15-meter long and have a quadratic cross section and a W-shape pipe heat exchanger. This article aims to numerically assess the additional changes in the pile load transfer generated by its heating and cooling. In addressing this objective, a preliminary multi-physical finite element analysis is conducted which serves as a tool for exploring: i) the thermally induced mechanical stresses within the concrete and on the pile-soil axial and shaft resistances; ii) the maximum upward/downward displacements. A one-year time span is considered under operational and extreme thermal boundary conditions. The results show that a typical geothermal utilization of the energy foundation does not generate significant structural implications on the geotechnical capacity of a single energy pile. However, ground thermal loads need to be considered in the design phase to account for potential extreme temperature changes, which could generate thermal stresses that equalize the mechanically generated ones
Radar Interferometer for Topographic Mapping of Glaciers and Ice Sheets
A report discusses Ka-band (35-GHz) radar for mapping the surface topography of glaciers and ice sheets at high spatial resolution and high vertical accuracy, independent of cloud cover, with a swath-width of 70 km. The system is a single- pass, single-platform interferometric synthetic aperture radar (InSAR) with an 8-mm wavelength, which minimizes snow penetration while remaining relatively impervious to atmospheric attenuation. As exhibited by the lower frequency SRTM (Shuttle Radar Topography Mission) AirSAR and GeoSAR systems, an InSAR measures topography using two antennas separated by a baseline in the cross-track direction, to view the same region on the ground. The interferometric combination of data received allows the system to resolve the pathlength difference from the illuminated area to the antennas to a fraction of a wavelength. From the interferometric phase, the height of the target area can be estimated. This means an InSAR system is capable of providing not only the position of each image point in along-track and slant range as with a traditional SAR but also the height of that point through interferometry. Although the evolution of InSAR to a millimeter-wave center frequency maximizes the interferometric accuracy from a given baseline length, the high frequency also creates a fundamental problem of swath coverage versus signal-to-noise ratio. While the length of SAR antennas is typically fixed by mass and stowage or deployment constraints, the width is constrained by the desired illuminated swath width. As the across-track beam width which sets the swath size is proportional to the wavelength, a fixed swath size equates to a smaller antenna as the frequency is increased. This loss of antenna size reduces the two-way antenna gain to the second power, drastically reducing the signal-to-noise ratio of the SAR system. This fundamental constraint of high-frequency SAR systems is addressed by applying digital beam-forming (DBF) techniques to synthesize multiple simultaneous receive beams in elevation while maintaining a broad transmit illumination. Through this technique, a high antenna gain on receive is preserved, thereby reducing the required transmit power and thus enabling high-frequency SARs and high-precision InSAR from a single spacecraft
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