703 research outputs found

    A theory of the imaging mechanism of underwater bottom topography by real and synthetic aperture radar

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    A simple theoretical model of the imaging mechanism of underwater bottom topography in tidal channels by real and by synthetic aperture radar (SAR) is presented. The imaging is attributed to surface effects induced by current variations over bottom topography. The current modulates the short-scale surface roughness, which in turn gives rise to changes in radar reflectivity. The bottom topography- current interaction is described by the continuity equation, and the current-short surface wave interac- tion is described by weak hydrodynamic interaction theory in the relaxation time approximation. This theory contains only one free parameter, which is the relaxation time. It is shown that in the case of tidal flow over large-scale bottom topographic features, e.g., over sandbanks, the radar cross-section modulation is proportional to the product of the relaxation time and the gradient of the surface current velocity, which is proportional to the slope of the water depth divided by the square of the depth. To first order, tiffs modulation is independent of wind direction. In the case of SAR imaging, in addition to the above mentioned hydrodynamic modulation, phase modulation or velocity bunching also contributes to the imaging. However, in general, the phase modulation is small in comparison to the hydrodynamic modu- lation. The theory is confronted with experimental data which show that to first order our theory is capable of explaining basic features of the radar imaging mechanism of underwater bottom topography in tidal channels. I n order to explain the large observed modulation of radar reflectivity we are compelled to assume a large relaxation time, which for Seasat SAR Bragg waves (wavelength 34 cm) is of the order of 30-40 s, corresponding to 60-80 wave periods

    Now Is the Time: Experts vs. the Uninitiated as Future Nominees to the U.S. Court of Appeals for Veterans Claims

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    Two-thirds of judges appointed to the Court of Appeals for Veterans Claims ( CAVC or Court ) could and should be drawn from among lawyers experienced in the U.S. Department of Veterans Affairs ( VA ) benefits claims adjudication system. It is a specialty court, and like other such courts, its judges would benefit from specialized experience. All stakeholders in the claims system and the Court\u27s work, and most importantly, veterans, would benefit from a Court that has appointees steeped in VA law and adjudication

    Now Is the Time: Experts vs. the Uninitiated as Future Nominees to the U.S. Court of Appeals for Veterans Claims

    Full text link
    Two-thirds of judges appointed to the Court of Appeals for Veterans Claims ( CAVC or Court ) could and should be drawn from among lawyers experienced in the U.S. Department of Veterans Affairs ( VA ) benefits claims adjudication system. It is a specialty court, and like other such courts, its judges would benefit from specialized experience. All stakeholders in the claims system and the Court\u27s work, and most importantly, veterans, would benefit from a Court that has appointees steeped in VA law and adjudication

    Experimental performance of liquid hydrogen and liquid fluorine in regeneratively cooled rocket engines

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    Performance of liquid hydrogen-fluorine propellant combination in regeneratively cooled rocket engin

    On the Spectrum of Field Quadratures for a Finite Number of Photons

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    The spectrum and eigenstates of any field quadrature operator restricted to a finite number NN of photons are studied, in terms of the Hermite polynomials. By (naturally) defining \textit{approximate} eigenstates, which represent highly localized wavefunctions with up to NN photons, one can arrive at an appropriate notion of limit for the spectrum of the quadrature as NN goes to infinity, in the sense that the limit coincides with the spectrum of the infinite-dimensional quadrature operator. In particular, this notion allows the spectra of truncated phase operators to tend to the complete unit circle, as one would expect. A regular structure for the zeros of the Christoffel-Darboux kernel is also shown.Comment: 16 pages, 11 figure

    A dilemma in representing observables in quantum mechanics

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    There are self-adjoint operators which determine both spectral and semispectral measures. These measures have very different commutativity and covariance properties. This fact poses a serious question on the physical meaning of such a self-adjoint operator and its associated operator measures.Comment: 10 page

    Aberrant computational mechanisms of social learning and decision-making in schizophrenia and borderline personality disorder

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    Psychiatric disorders are ubiquitously characterized by debilitating social impairments. These difficulties are thought to emerge from aberrant social inference. In order to elucidate the underlying computational mechanisms, patients diagnosed with major depressive disorder (N = 29), schizophrenia (N = 31), and borderline personality disorder (N = 31) as well as healthy controls (N = 34) performed a probabilistic reward learning task in which participants could learn from social and nonsocial information. Patients with schizophrenia and borderline personality disorder performed more poorly on the task than healthy controls and patients with major depressive disorder. Broken down by domain, borderline personality disorder patients performed better in the social compared to the non-social domain. In contrast, controls and MDD patients showed the opposite pattern and SCZ patients showed no difference between domains. In effect, borderline personality disorder patients gave up a possible overall performance advantage by concentrating their learning in the social at the expense of the non-social domain. We used computational modeling to assess learning and decision-making parameters estimated for each participant from their behavior. This enabled additional insights into the underlying learning and decision-making mechanisms. Patients with borderline personality disorder showed slower learning from social and non-social information and an exaggerated sensitivity to changes in environmental volatility, both in the non-social and the social domain, but more so in the latter. Regarding decision-making the modeling revealed that compared to controls and major depression patients, patients with borderline personality disorder and schizophrenia showed a stronger reliance on social relative to non-social information when making choices. Depressed patients did not differ significantly from controls in this respect. Overall, our results are consistent with the notion of a general interpersonal hypersensitivity in borderline personality disorder and schizophrenia based on a shared computational mechanism characterized by an over-reliance on beliefs about others in making decisions and by an exaggerated need to make sense of others during learning specifically in borderline personality disorder

    Analysis of the low-energy electron-recoil spectrum of the CDMS experiment

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    We report on the analysis of the low-energy electron-recoil spectrum from the CDMS II experiment using data with an exposure of 443.2 kg-days. The analysis provides details on the observed counting rate and possible background sources in the energy range of 2 - 8.5 keV. We find no significant excess in the counting rate above background, and compare this observation to the recent DAMA results. In the framework of a conversion of a dark matter particle into electromagnetic energy, our 90% confidence level upper limit of 0.246 events/kg/day at 3.15 keV is lower than the total rate above background observed by DAMA by 8.9σ\sigma. In absence of any specific particle physics model to provide the scaling in cross section between NaI and Ge, we assume a Z^2 scaling. With this assumption the observed rate in DAMA differs from the upper limit in CDMS by 6.8σ\sigma. Under the conservative assumption that the modulation amplitude is 6% of the total rate we obtain upper limits on the modulation amplitude a factor of ~2 less than observed by DAMA, constraining some possible interpretations of this modulation.Comment: 4 pages, 3 figure
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