On the detection of underwater bottom topography by imaging radars

A theoretical model which explains basic properties of radar imaging of underwater bottom topography in tidal channels is presented. The surface roughness modulation is described by weak hydrodynamic interaction theory in the relaxation time approximation. In contrast to previous theories on short wave modulation by long ocean waves, a different approximation is used to describe short wave modulation by tidal flow over underwater bottom topography. The modulation depth is proportional to the relaxation time of the Bragg waves. The large modulation of radar reflectivity observed in SEASAT-SAR imagery of sand banks in the Southern Bight of the North Sea are explained by assuming that the relaxation time of 34 cm Bragg waves is of the order of 30-40 seconds

Numerical study on signatures of atmospheric convective cells in radar images of the ocean

Current and wind variations at the ocean surface can give rise to a modulation of the sea surface roughness and thus become visible in radar images. The discrimination between radar signatures of oceanic and atmospheric phenomena can be quite difficult, since signatures of different origin can have very similar shapes and magnitudes and are often superimposed upon each other. In this work we employ a numerical radar imaging model for an investigation of typical properties of radar signatures of atmospheric convective cells and of theoretical differences between such atmospherically induced radar signatures and those of oceanic phenomena. We show that main characteristics of observed multifrequency/multipolarization radar signatures of atmospheric convective cells over the Gulf Stream are reproduced quite well by the proposed model. This encourages us to vary wind and radar parameters systematically in order to get a general overview of the dependency of atmospherically induced radar signatures on these parameters. Finally, we compare typical characteristics of radar signatures of atmospheric and oceanic phenomena, and we present simulated radar images of a scenario of superimposed atmospheric convective cells and oceanic internal waves. We show that the proposed model supports the experimental finding that radar signatures of oceanic phenomena are stronger at horizontal (HH) than at vertical (VV) polarization, while atmospherically induced radar signatures are better visible at VV polarization

A perturbation approach to coherent propagation of energetic charged particles in random magnetic fields

The Fokker-Planck equation describing the propagation of charged particles in magnetic fields that consist of a dominant constant guiding field and superposed random fluctuations is solved by applying the perturbation method of dividing the total particle density into an averaged isotropic and a small anisotropic component. A particle transport equation is derived which describes the 'coherent' propagation of a particle pulse whose center moves at half the constant total particle velocity in either the positive or negative direction. The range of validity of the coherent solution is examined, and the general formulas for coherent propagation are applied to the slab, isotropic, and Alfven-wave models of magnetic-field fluctuations. Ranges of magnetic-fluctuation spectral indices are identified over which diffusive and coherent particle-transport modes can exist in the three models considered

The smallest sets of points not determined by their X-rays

Let $F$ be an $n$-point set in $\mathbb{K}^d$ with $\mathbb{K}\in\{\mathbb{R},\mathbb{Z}\}$ and $d\geq 2$. A (discrete) X-ray of $F$ in direction $s$ gives the number of points of $F$ on each line parallel to $s$. We define $\psi_{\mathbb{K}^d}(m)$ as the minimum number $n$ for which there exist $m$ directions $s_1,...,s_m$ (pairwise linearly independent and spanning $\mathbb{R}^d$) such that two $n$-point sets in $\mathbb{K}^d$ exist that have the same X-rays in these directions. The bound $\psi_{\mathbb{Z}^d}(m)\leq 2^{m-1}$ has been observed many times in the literature. In this note we show $\psi_{\mathbb{K}^d}(m)=O(m^{d+1+\varepsilon})$ for $\varepsilon>0$. For the cases $\mathbb{K}^d=\mathbb{Z}^d$ and $\mathbb{K}^d=\mathbb{R}^d$, $d>2$, this represents the first upper bound on $\psi_{\mathbb{K}^d}(m)$ that is polynomial in $m$. As a corollary we derive bounds on the sizes of solutions to both the classical and two-dimensional Prouhet-Tarry-Escott problem. Additionally, we establish lower bounds on $\psi_{\mathbb{K}^d}$ that enable us to prove a strengthened version of R\'enyi's theorem for points in $\mathbb{Z}^2$

The epidemiology of kuru: monitoring the epidemic from its peak to its end

Kuru is a fatal transmissible spongiform encephalopathy restricted to the Fore people and their neighbours in a remote region of the Eastern Highlands of Papua New Guinea. When first investigated in 1957 it was found to be present in epidemic proportions, with approximately 1000 deaths in the first 5 years, 1957–1961. The changing epidemiological patterns and other significant findings such as the transmissibility of kuru are described in their historical progression. Monitoring the progress of the epidemic has been carried out by epidemiological surveillance in the field for 50 years. From its peak, the number of deaths from kuru declined to 2 in the last 5 years, indicating that the epidemic is approaching its end. The mode of transmission of the prion agent of kuru was the local mortuary practice of transumption. The prohibition of this practice in the 1950s led to the decline in the epidemic, which has been prolonged into the present century by incubation periods that may exceed 50 years. Currently, the epidemiological surveillance is being maintained and further studies on human genetics and the past mortuary practices are being conducted in the kuru-affected region and in communities beyond it

Measurements at 13.9 GHz of the radar backscattering cross section of the North Sea covered with an artificial surface film

The reduction of the Ku‐band (13.9 GHz) normalized radar cross section (NRCS) by an artificial monomolecular surface film (oleyl alcohol) on the sea surface was measured in the North Sea during the 1975 Joint North Sea Wave Project, JONSWAP 75 experiment. The aim of the surface film experiment was to simulate natural surface films which often occur on the ocean surface and are produced by plankton or fish. NRCS measurements were obtained from an aircraft at incidence angles of 41° and 47° at vertical and horizontal polarizations. For winds between 3.5 and 4.4 m/sec the maximum measured reduction was 7.3 ± 3.5 dB relative to the mean. In‐situ measurements showed that the oleyl alcohol film reduced the surface tension from 74 to 43 dyne/cm. Similar reductions in surface tension have also been measured on the ocean due to natural surface films of biological origin. It is noted that variations of the NRCS due to natural surface film effects may significantly limit the techniques used currently to infer surface wind vector over biologically active ocean regions

3D particle tracking velocimetry using dynamic discrete tomography

Particle tracking velocimetry in 3D is becoming an increasingly important imaging tool in the study of fluid dynamics, combustion as well as plasmas. We introduce a dynamic discrete tomography algorithm for reconstructing particle trajectories from projections. The algorithm is efficient for data from two projection directions and exact in the sense that it finds a solution consistent with the experimental data. Non-uniqueness of solutions can be detected and solutions can be tracked individually

Qualitätssicherung bei der Langzeitlagerung ökologisch erzeugter Möhren

Von 25 Möhrenschlägen auf Marschböden im Bereich der schleswig-holsteinischen Westküste wurden in der zweiten Septemberhälfte 2003 Pflanzenproben gezogen und deren Besatz mit Lagerkrankheiten qualitativ bestimmt. Von den Ende Oktober maschinell geernteten Möhren wurde je Fläche eine Probe gezogen und in einem maschinengekühlten Lager bei 0 – 0,5 °C und ca. 95 % rel. Luftfeuchte etwa 4 Monate lang gelagert. Anfang März 2004 wurden die krankheitsbedingten Lagerverluste und die sie verursachenden Lagerkrankheiten bestimmt. Obwohl zwischen dem Krankheitsbesatz vor und nach der Auslagerung nur wenige Übereinstimmungen bestanden, ließ sich unter Verwendung weiterer Daten zu Fruchtfolge, Stickstoffniveau im Boden und dem Anteil beschädigter Möhren im Lagergut ein Index zur Abschätzung der voraussichtlichen krankheitsbedingten Lagerverluste ermitteln, der mit r = 0,45 * mit den tatsächlichen Verlusten korreliert war. Die zusätzliche Berücksichtigung der sortenbedingten Lagereignung ergab r = 0,66 ***. Dennoch sind der Index und der darauf basierende Schätzrahmen nur eingeschränkt praxistauglich, da ohne Berücksichtigung der sortenbedingten Lagereignung in nur 16 von 25 Fällen (= 64 %) ein enger Zusammenhang zwischen Index und Lagerverlusten gefunden wurde. Bei Anpassung des Index an die Lagereignung der Sorten waren es 18 von 25 Fällen (= 72 %). In der Praxis erscheint eine auf gut lagerfähige Sorten beschränkte Anwendung des Schätzrahmens sinnvoll, wobei der Index hier in 16 von 23 Fällen (= 70 %) einen engen Zusammenhang mit den Lagerverlusten aufwies und mit r = 0,58 ** eine gut signifikante Korrelation zwischen Index und Lagerverlusten bestand. In allen Fällen, in denen kein Zusammenhang zwischen Index und Lagerverlusten bestand, traten trotz sehr niedriger Indices hohe bis sehr hohe Lagerverluste auf