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In this paper we confine our attention to the analysis of amplitude and phase data acquired by monitoring VLF/LF radio signals emitted by four European transmitters during a seven-year period (2008-2014). All the data were recorded at a Belgrade site (44.85$^{0}$ N, 20.38$^{0}$ E) by the Stanford University ELF/VLF receiver AWESOME. Propagation of VLF/LF radio signal takes place in the Earth-ionosphere waveguide and strongly depends on ionization level of the D-region, which means that it is mainly controlled by solar conditions. Some results of amplitude and phase variations on GQD/22.10 kHz, DHO/23.40 kHz, ICV/20.27 kHz and NSC/45.90 kHz radio signals measurements at short distances ($D < 2$ Mm) over Central Europe and their interpretation are summarized in this paper. Attention is restricted to regular diurnal, seasonal and solar variations including sunrise and sunset effects on propagation characteristics of four VLF/LF radio signals. We study VLF/LF propagation over short path as a superposition of different number of discrete modes which depends on the variations of the path parameters. Although the solar X-ray flare effects on propagation of VLF/LF radio signals are well recognized on all paths, similarities and differences between them are defined under existing conditions over the paths. Statistical results show that the size of amplitude and phase perturbations on VLF/LF radio signal is in correlation with the intensity of X-ray flux. We present the calculations of electron density enhancements in the D-region caused by different classes of solar X-ray flares during the period of ascending phase and maximum of the solar cycle 24.

Obstacle detection plays an important role in unmanned surface vehicles (USV). The USVs operate in highly diverse environments in which an obstacle may be a floating piece of wood, a scuba diver, a pier, or a part of a shoreline, which presents a significant challenge to continuous detection from images taken onboard. This paper addresses the problem of online detection by constrained unsupervised segmentation. To this end, a new graphical model is proposed that affords a fast and continuous obstacle image-map estimation from a single video stream captured onboard a USV. The model accounts for the semantic structure of marine environment as observed from USV by imposing weak structural constraints. A Markov random field framework is adopted and a highly efficient algorithm for simultaneous optimization of model parameters and segmentation mask estimation is derived. Our approach does not require computationally intensive extraction of texture features and comfortably runs in real-time. The algorithm is tested on a new, challenging, dataset for segmentation and obstacle detection in marine environments, which is the largest annotated dataset of its kind. Results on this dataset show that our model outperforms the related approaches, while requiring a fraction of computational effort., Comment: This is an extended version of the ACCV2014 paper [Kristan et al., 2014] submitted to a journal. [Kristan et al., 2014] M. Kristan, J. Pers, V. Sulic, S. Kovacic, A graphical model for rapid obstacle image-map estimation from unmanned surface vehicles, in Proc. Asian Conf. Computer Vision, 2014

Electron density and temperature changes in the D-region of the ionosphere are sensitively manifested as changes in the amplitude and phase of subionospheric Very Low Frequency (VLF) signals propagating beneath the perturbed region. Disturbances (either in electron density or temperature) in the D region cause significant scattering of VLF waves propagating in the earth-ionosphere waveguide, leading to measurable changes in the amplitude and phase of the VLF waves. We analyze Lightning-induced electron precipitation (LEP) events during period 2008 - 2009 at Belgrade station on subionospheric VLF signals from four transmitters (DHO/23.4 kHz, Germany; GQD/22.1 kHz, UK; NAA/24.0 kHz USA and ICV/20.9 kHz Italy)., Comment: 5 pages, 2 figs

Very Low Frequency, VLF and Low Frequency, LF signal perturbations were examined to study ionospheric disturbances induced by solar X-ray flares. The aim was to understand processes in propagation VLF/LF radio signals over short paths, and to estimate specific characteristics of each short path. The receiver at Belgrade station continuously monitor the amplitude and phase of coherent and subionospherically propagating LF signal operated in Sicily, NSC at 45.90, kHz and VLF signal operated in Isola di Tavolara ICV at 20.27 kHz, with great circle distances of 953 km and 976 km, respectively. Geographical locations of transmitters and receiver site result that these short paths have many similarity. The main difference is in transmitter frequencies. In period from 2008 to February 2014 were selected around 200 events for further examination. In all selected examples amplitude and phase on VLF and LF signals were perturbed by occurrence of solar X-ray flares. This six years period covers minimum and maximum of solar activity. Also, simultaneous measurement of amplitude and phase on the VLF/LF signals during occurrence of solar flare were employed for constructing electron density profile versus altitude and also in a function of time over middle Europe., Comment: published in Serb.Astron.J

In this paper, we present a model for determination of a weakly time dependent effective recombination coefficient for the perturbed terrestrial ionospheric D-region plasma. We study consequences of a class M1.0 X-ray solar flare, recorded by GOES-15 satellite on February 18, 2011 between 14:00 UT and 14:15 UT, by analyzing the amplitude and phase real time variations of very low frequency (VLF) radio waves emitted by transmitter DHO (located in Germany) at frequency 23.4 kHz and recorded by the AWESOME receiver in Belgrade (Serbia). Our analysis is limited to ionospheric perturbations localized at altitudes around 70 km where the dominant electron gain and electron loss processes are the photo-ionization and recombination respectively.

In this paper, we study the influence of solar flares on electron concentration in the terrestrial ionospheric D-region by analyzing the amplitude and phase time variations of very low frequency (VLF) radio waves emitted by DHO transmitter (Germany) and recorded by the AWESOME receiver in Belgrade (Serbia) in real time. The rise of photo-ionization rate in the ionospheric D-region is a typical consequence of solar flare activity as recorded by GOES-15 satellite for the event on March 24, 2011 between 12:01 UT and 12:11 UT. At altitudes around 70 km, the photo-ionization and recombination are the dominant electron gain and electron loss processes, respectively. We analyze the relative contribution of each of these two processes in the resulting electron concentration variation in perturbed ionosphere.

One of the methods of detection and analysis of solar flares is observing the time variations of certain solar spectral lines. During solar flares, a raise of electron concentration occurs in Earth's ionosphere which results in amplitude and phase variations of the recorded very low frequency (VLF) waves. We compared the data obtained by the analysis of recorded VLF signals and line spectra for different solar flares. In this paper we treated the DHO VLF signal transmitted from Germany at the frequency of 23.4 kHz recorded by the AWESOME system in Belgrade (Serbia) during solar flares in the period between 10:40 UT and 13:00 UT on 2011 April 22.

We use ab initio methods and neutron inelastic scattering (NIS) to study the structure, energetics, and dynamics of pure and Ti-doped sodium alanate (NaAlH_4), focusing on the possibility of substitutional Ti doping. The NIS spectrum is found to exhibit surprisingly strong and sharp two-phonon features. The calculations reveal that substitutional Ti doping is energetically possible. Ti prefers to substitute for Na and is a powerful hydrogen attractor that facilitates multiple Al--H bond breaking. Our results hint at new ways of improving the hydrogen dynamics and storage capacity of the alanates., Comment: 5 pages, with 4 postscript figures embedded. Uses REVTEX4 and graphicx macros