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ABSTRACT: In wideband radar imaging, it is necessary to increase the target observation angle, that is, the imaging time to obtain high azimuth resolution. For wide-angle targets, however, migration through resolution cell will occur and the Doppler information of the scatterers will be time varying, which can lead to image defocusing in both the range and azimuth domain if the range-Doppler (RD) method is applied. Aimed at this problem, this study builds up the imaging model for steadily moving targets with a constant rotation axis. Then it compares the RD algorithm, the Keystone algorithm and the polar format algorithm with theoretical analysis and deduces the maximum observation angles that fit for these methods, respectively. Additionally, this study proposes a new algorithm for imaging of targets with large observation angles based on the complex inverse Radon transform. Instead of applying the Taylor-s series approximation, this method strictly follows the signal model for scatterers with a wide observation angle and utilises curvilinear integral with phase information for image formation. Therefore it is valid in wide-angle scenarios. Finally, imaging results for both the simulated and measured data are given to analyse the performance of the available algorithms and to prove the validity of the proposed algorithm.
IET Radar Sonar ? Navigation 05/2011; · 0.88 Impact Factor
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ABSTRACT: Space/air-borne synthetic aperture radar (SAR) detection and imaging of moving ships at sea are important for ocean reconnaissance and fishery monitoring. A novel algorithm based on sandglass transformation to detect weak ships and form high-quality images under the conditions of low signal-to-clutter-plus-noise ratio is presented here. It requires no prior information about motion parameters of targets. The sandglass transformation can decouple the time and lag time in the instantaneous autocorrelation function of a linear frequency modulated (LFM) signal. Thus the cross-range signals of ship in the approximate form of LFM signals can be integrated coherently via the two-dimensional fast Fourier transformation. The proposed algorithm can not only achieve high-energy accumulation gain, but also suppress the interference of cross terms effectively without loss of resolution. Hence, it is effective to detect weak ships and generate high-resolution images. Numerical and experimental results confirm the effectiveness of the proposed algorithm.
IET Radar Sonar ? Navigation 04/2011; · 0.88 Impact Factor
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ABSTRACT: For the equilibrium immiscible Cu–Mo system characterized by a positive heat of formation (+19 kJ mol <sup>-1</sup>) , thermodynamic calculation showed that by adding an adequate interfacial free energy, the Gibbs free energy of the Cu–Mo multilayered films could be elevated up to a higher level than the convex-shaped free energy curve of the amorphous phase and supersaturated solid solutions. Accordingly, ion beam mixing of the Cu–Mo multilayered samples did result in forming a unique amorphous phase with an alloy composition around 80 at. % Mo and two supersaturated solid solutions of fcc and bcc structures at around 27 at. % and 90 at. % Mo, respectively, matching well with the thermodynamic calculation.
Journal of Applied Physics 11/2010; · 2.17 Impact Factor
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ABSTRACT: A four square loops antenna above a finite ground plane is investigated for circular polarisation and gain enhancement. By choosing proper perimeters of the two pairs of loops, the axial ratio (¿3 dB) bandwidth can reach 16%. It is demonstrated that a new pair of loops added to the original dual-loop can increase the gain to 11.6 dBic. A broadband balun is incorporated for impedance matching. The antenna performance is confirmed by experimental results.
Electronics Letters 05/2010; · 0.96 Impact Factor
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ABSTRACT: A four rhombic loop antenna for circular polarisation is proposed. This antenna has a high gain of about 9.7 dBi and a broad beam of more than 130 half power beam width. Four parasitic loops are added inside the original four loops to increase the axis ratio bandwidth. A broadband balun is incorporated into the antenna for impedance matching. Details of the antenna design and experimental results are presented.
Electronics Letters 12/2009; · 0.96 Impact Factor
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ABSTRACT: Ultraviolet (UV) GaN p-i-n avalanche photodiodes (APDs) on low dislocation density free-standing GaN substrates were grown and fabricated. The GaN APD showed a stable avalanche multiplication gain in a linear mode, using UV illumination. In Geiger-mode operation at room temperature with gated quenching, no after-pulsing effect was observed up to 100 kHz. The single-photon detection efficiency and dark-count probability were measured to be ${sim}$ 1% and ${sim} 3times 10^{- 2}$ at 265 nm, respectively.
IEEE Photonics Technology Letters 11/2009; · 2.19 Impact Factor
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ABSTRACT: Considering the economics and securities for the operation of a power system, a semi-definite programming (SDP) model for the security-constrained unit commitment (SCUC) problem is described here, which is directly solved by the interior-point method for SDP within the polynomial times. The proposed method is promising for the SCUC problems because of its excellent convergence and the ability of handling the non-covex integer variables. No model decomposition and initial relaxation are needed when applying the SDP-based method. When the solution contains minor mismatches in the integer variables, a simple rounding strategy is used to correct the non-integer into integer efficiently. Different test cases from 6 to 118 buses over a 24 h horizon are presented. Extensive numerical simulations have shown that the proposed method is capable of obtaining the optimal UC schedules without any network and bus voltage violations, and minimising the operation cost as well.
IET Generation Transmission & Distribution 03/2009; · 1.20 Impact Factor
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ABSTRACT: We report a 4H-SiC PIN recessed-window avalanche photodiode with a peak responsitivity of 136 mA/W (external quantum efficiency = 60%) at lambda = 262 nm, corresponding to more than a 50% increase in external quantum efficiency compared to nonrecessed structures. The dark current was 90 pA at a photocurrent gain of 1000. Avalanche gains of over 106, k ~ 0.1, and a spatially uniform response were achieved.
IEEE Photonics Technology Letters 10/2008; · 2.19 Impact Factor
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ABSTRACT: This paper addresses the quantification and allocation of peaking costs of hydrothermal power systems under electricity market environment. Mixed-integer linear formations of market scheduling of hydrothermal power systems is described, with the maximum ramping up/down limits and minimum start/off time limits of thermal generators, as well as the generation quantity and hydraulic vibrations of hydropower generators considered. The peaking costs are quantified based on the market scheduling model, with the impacts of transmission flow limit and generating constraints of hydro units and thermal units on peaking costs considered. Cooperative game theory is applied to allocate peaking costs among loads, the superadditivity and convex game characteristics of the peaking costs allocation problem are analyzed; a simplified algorithm for calculating Shapley value is proposed to overcome the combinatorial explosion problems of non-simplified Shapley value or nucleolus. Simulation results show that, operation constraints have severe impacts on the peaking capability of generation units as well as the economical operation of hydro-thermal power systems, and the simplified algorithm for calculating Shapley value is very effective in terms of calculation burden and precision.
Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE; 08/2008
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ABSTRACT: The operation scheduling model in Northwest China Power System (NCPS), which is undergoing industry restructuring, is established. The peaking value of generators is identified. Two peaking cost compensation mechanisms for generators are proposed: the first one is based on the theory of cooperative game, the second one based on engineering concept. Some of the characteristics of the suggested game model are analyzed, and several methods for resolving the combinational explosion problem involved in the game-theoretic computation are described. Two numerical indices for quantifying the peaking capability of generators are proposed, the impacts of generator parameters, load variations, and transmission constraints are investigated. The effectiveness of the proposed methods is showed with the simulation results of a 4-generator 8-bus power system and NCPS. Copyright © 2008 John Wiley & Sons, Ltd.
European Transactions on Electrical Power 08/2008; 19(7):1016 - 1032. · 0.58 Impact Factor
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ABSTRACT: A high resolution EEG study was conducted on healthy human subjects during video game play. Throughout the game playing experiment short time segments of spontaneous activity were recorded. Spectral analysis was performed on these segments for the theta-wave (4-8 Hz) and alpha-wave (8-13 Hz) bands to investigate the modulatory effects of long-lasting game play and the dynamic changes of spectral contribution in range of alpha- and theta-wave. The present results revealed that a) the frontal midline theta-wave activity increased over time relative to the eye open resting condition and b) the parietal alpha-wave activity initially decreased relative to the resting condition, then followed by a slow increase. These experimental results indicate the high resolution EEG provides a useful quantitative analysis tool for studying dynamic brain activity.
Engineering in Medicine and Biology Society, 2007. EMBS 2007. 29th Annual International Conference of the IEEE; 09/2007
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ABSTRACT: Security in mobile ad-hoc networks (MANETs) continues to attract attention after years of research. Recent advances in identity-based cryptography (IBC) sheds light on this problem and has become popular as a solution base. We present a comprehensive picture and capture the state of the art of IBC security applications in MANETs based on a survey of publications on this topic since the emergence of IBC in 2001. In this paper, we also share insights into open research problems and point out interesting future directions in this area.
IEEE Communications Surveys & Tutorials · 6.31 Impact Factor
