A unified approach to phase and cross-talk calibration of polarimetric data which can be applied to calibrating scattering matrix data or to extraction of the descriptors of distributed targets is described. It relies...
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A unified approach to phase and cross-talk calibration of polarimetric data which can be applied to calibrating scattering matrix data or to extraction of the descriptors of distributed targets is described. It relies on the scene being dominated by targets with uncorrelated like and cross-polarized backscattering coefficients, but provides cross-talk calibration of targets for which this is not true. The algorithm needs unsymmetrized data, but uses only quantities derived from the covariance matrix of large areas. It makes no assumptions about system reciprocity, permits ready interpretation of the terms in the calibration procedure, allows comparison of the relative magnitude of the system-induced mixing of terms in the observed covariance matrix, is noniterative, and produces indicators which allow testing of whether it meets its own underlying assumptions. The linear distortion model is shown to lead to an inconsistent system of equations;this inconsistency can be removed by introducing an extra parameter which has properties expected of system noise. The modulus of the copolarized correlation coefficient, which is important in polarimetric classification and as a phase descriptor, is shown to be invariant under all effects embodied in the linear distortion model. Calibration of the scattering matrix data is based on a minimum least squares principle. This suggests that current methods of symmetrization are not optimal. The same analysis shows that estimates of parameters needed to form an equivalent reciprocal system are also nonoptimal. The method is more general than the well-known van Zyl algorithm for cross-talk removal, and permits an analysis of the conditions under which the van Zyl algorithm will yield valid results. Correction of phase distortion induced by channel imbalance is treated as an optional extra step relying on a known HH-VV phase difference in some region of the image. Results from the algorithm are discussed using scattering matrix data fr
When HF radar systems are used for surveillance or tracking of ships or aircraft the strong signal from the ocean surface is termed clutter since it obscures the required signals. Over 30 years ago it was recognised t...
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When HF radar systems are used for surveillance or tracking of ships or aircraft the strong signal from the ocean surface is termed clutter since it obscures the required signals. Over 30 years ago it was recognised that this sea ‘clutter’had characteristics that could be related to the ocean wave spectrum and the idea of using HF radar for remote sensing was born. This paper describes recent work on the inversion of the Doppler spectrum of HF radar echoes from the ocean surface to provide measurements of the ocean wave directional spectrum.
The self-adaptive near-optimal control of diffusion equations is described. The variable structure feedback design is independent of the initial conditions and the plant parameter values. Rather than identifying the p...
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The self-adaptive near-optimal control of diffusion equations is described. The variable structure feedback design is independent of the initial conditions and the plant parameter values. Rather than identifying the precise parameter values by adaptive correction, the online self-adaptive closed-loop controller organises itself by identifying certain dynamic characteristics associated with sliding motion. Digital simulation results are presented for both linear and nonlinear systems, and the effect of parameter variations and external disturbances are discussed. The system responses are close to the time-optimal paths.
Spiral waves and spatiotemporal chaos are sometimes harmful and should be controlled. In this paper spiral waves and spatiotemporal chaos are successfully eliminated by the pulse with a very specific spatiotemporal co...
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Spiral waves and spatiotemporal chaos are sometimes harmful and should be controlled. In this paper spiral waves and spatiotemporal chaos are successfully eliminated by the pulse with a very specific spatiotemporal configuration. The excited position D of spiral waves or spatiotemporal chaos is first recorded at an arbitrary time (to). When the system at the domain D enters a recovering state, the external pulse is injected into the domain. If the intensity and the working time of the pulse are appropriate, spiral waves and spatiotemporal chaos can finally be eliminated because counter-directional waves can be generated by the pulse. There are two advantages in the method. One is that the tip can be quickly eliminated together with the body of spiral wave, and the other is that the injected pulse may be weak and the duration can be very short so that the original system is nearly not affected, which is important for practical applications.
Following recent successes with semivectorial polarisedẼ field solutions for rib, channel, embedded, and buried waveguides a method is presented for the determination of corresponding polarisedH̃ field modes for simi...
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Following recent successes with semivectorial polarisedẼ field solutions for rib, channel, embedded, and buried waveguides a method is presented for the determination of corresponding polarisedH̃ field modes for similar and other rectangular waveguide structures which may contain any arbitrary distribution of horizontal and vertical dielectric discontinuities. The new finite difference schemes take full account of the discontinuities in the normal gradients of magnetic field components across all internal dielectric interfaces. The polarisedH̃ field results, which are presented for various rib and embedded wave-guide structures, are found to be in excellent agreement with the corresponding polarisedẼ field modes. Application of the zero divergence constraint prevents the occurrence of spurious modes.
The current and the voltage of an X-pinch were measured. The inductance of the X-pinch was assumed to be a constant and estimated by the calculation of the magnetic field based on the well-known Biot–Savart’s Law. T...
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The current and the voltage of an X-pinch were measured. The inductance of the X-pinch was assumed to be a constant and estimated by the calculation of the magnetic field based on the well-known Biot–Savart’s Law. The voltage of the inductance was calculated with L · di/dt and subtracted from the measured voltage of the X-pinch. Then, the resistance of the X-pinch was determined and the following results were obtained. At the start of the current flow the resistance of the exploding wires is several tens of Ohms, one order of magnitude, higher than the metallic resistance of the wires at room temperature, and then it falls quickly to about 1 , which reflects the physical processes occurring in the electrically exploding wires, i.e., a current transition from the highly resistive wire core to the highly conductive plasma. It was shown that the inductive contribution to the voltage of the X-pinch is less than the resistive contribution. For the wires we used, the wires’ material and diameter have no strong influence on the resistance of the X-pinch, which may be explained by the fact that the current flows through the plasma rather than through the metallic wire itself. As a result, the current is almost equally divided between two parallel X-pinches even though the diameter and material of the wires used for these two X-pinches are significantly different.
A powerful tool to investigate hypotheses, verify experimental results and simulate the dynamics of complex systems is mathematical modeling. Different versions and generalizations of the logistic growth model have be...
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Risk-driven behaviour provides a feedback mechanism through which individuals both shape and are collectively affected by an epidemic. We introduce a general and flexible compartmental model to study the effect of het...
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A method is developed for the optimal estimation of the parameters in a fully nonlinear model of flow in a channel. The data assimilated consist of values of the water surface elevation during a given interval. The me...
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A method is developed for the optimal estimation of the parameters in a fully nonlinear model of flow in a channel. The data assimilated consist of values of the water surface elevation during a given interval. The method is based on the adjoint method of optimal control. It is shown that accurate values of the parameters can be estimated, and the estimates are stable with respect to random perturbations of the data provided that data from a sufficient number of locations are available for assimilation.
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