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Editor Work2015 Vol. 4, No. 3
Previous Issue
2015, 4(3): 241-253.
Abstract
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The concept of Geosynchronous Circular SAR (Geo-CSAR) is introduced in this paper. With the design of the geosynchronous orbit parameters, a near-circular satellite sub-track could be formed to enable the staring imaging mode, which supports the advanced applications for wide-field and 3-D information acquisition under long-term consistent observation. This paper also analyzes Geo-CSAR's imaging formation capabilities, and concludes its attractive advantages over low-earth orbit spaceborne SAR in terms of instantaneous coverage, consistent observing area, 3-D positioning accuracy and etc.. Encouraging expectations for Geo-CSAR thus could be positively predicted in military investigation and disaster monitoring management applications.
The concept of Geosynchronous Circular SAR (Geo-CSAR) is introduced in this paper. With the design of the geosynchronous orbit parameters, a near-circular satellite sub-track could be formed to enable the staring imaging mode, which supports the advanced applications for wide-field and 3-D information acquisition under long-term consistent observation. This paper also analyzes Geo-CSAR's imaging formation capabilities, and concludes its attractive advantages over low-earth orbit spaceborne SAR in terms of instantaneous coverage, consistent observing area, 3-D positioning accuracy and etc.. Encouraging expectations for Geo-CSAR thus could be positively predicted in military investigation and disaster monitoring management applications.
2015, 4(3): 254-264.
Multiaspect Synthetic Aperture Radar (SAR) can generate high resolution images and target scattering signatures in different azimuth angles from the coherent integration of all subaperture images. However, mixed anisotropic scatters limit the application of traditional imaging theory. Anisotropic scattering may introduce errors in polarimetric parameters by decreasing the reliability of terrain classification and detection of variability. Thus a method is proposed for estimating and removing anisotropic scattering in multiaspect polarimetric SAR images. The proposed algorithm is based on the maximum likelihood and likelihood-ratio tests for the two-class case, while considering the speckle effect, the mechanism of removing the anisotropic scattering, and the monotonicity of the Constant False Alarm Rate (CFAR) detection function. We compare the polarimetric entropy before and after removing the anisotropic subapertures, and then validate the algorithm's potential in retrieving the target signature using a P-band quad-pol airborne SAR with circular trajectory.
Multiaspect Synthetic Aperture Radar (SAR) can generate high resolution images and target scattering signatures in different azimuth angles from the coherent integration of all subaperture images. However, mixed anisotropic scatters limit the application of traditional imaging theory. Anisotropic scattering may introduce errors in polarimetric parameters by decreasing the reliability of terrain classification and detection of variability. Thus a method is proposed for estimating and removing anisotropic scattering in multiaspect polarimetric SAR images. The proposed algorithm is based on the maximum likelihood and likelihood-ratio tests for the two-class case, while considering the speckle effect, the mechanism of removing the anisotropic scattering, and the monotonicity of the Constant False Alarm Rate (CFAR) detection function. We compare the polarimetric entropy before and after removing the anisotropic subapertures, and then validate the algorithm's potential in retrieving the target signature using a P-band quad-pol airborne SAR with circular trajectory.
2015, 4(3): 265-275.
Conventional ground moving target imaging algorithms mainly focus on the range cell migration correction and the motion parameter estimation of the moving target. However, in real Synthetic Aperture Radar (SAR) data processing, non-ideal motion error compensation is also a critical process, which focuses and has serious impacts on the imaging quality of moving targets. Non-ideal motion error can not be compensated by either the stationary SAR motion error compensation algorithms or the autofocus techniques. In this paper, two sorts of non-ideal motion errors that affect the Doppler centroid of the moving target is analyzed, and a novel non-ideal motion error compensation algorithm is proposed based on the Inertial Navigation System (INS) data and the range walk trajectory. Simulated and real data processing results are provided to demonstrate the effectiveness of the proposed algorithm.
Conventional ground moving target imaging algorithms mainly focus on the range cell migration correction and the motion parameter estimation of the moving target. However, in real Synthetic Aperture Radar (SAR) data processing, non-ideal motion error compensation is also a critical process, which focuses and has serious impacts on the imaging quality of moving targets. Non-ideal motion error can not be compensated by either the stationary SAR motion error compensation algorithms or the autofocus techniques. In this paper, two sorts of non-ideal motion errors that affect the Doppler centroid of the moving target is analyzed, and a novel non-ideal motion error compensation algorithm is proposed based on the Inertial Navigation System (INS) data and the range walk trajectory. Simulated and real data processing results are provided to demonstrate the effectiveness of the proposed algorithm.
2015, 4(3): 276-286.
Passive corner reflectors and active transponders are often used as man-made reference targets in Synthetic Aperture Radar (SAR) radiometric calibration, With the emergence of new radar systems and the increasing demand for greater accuracy, wide-band and wide-beam radars challenge the hypothesis that the Radar Cross Section (RCS) of reference targets is constant. In this study, the FEKO electromagnetic simulation software is used to obtain the change curve of the target RCS as a function of frequency and aspect angle while incorporating high-resolution point-target SAR simulation, and quantitatively analyzing the effect of the modulation effect on SAR images. The simulation results suggest that the abovementioned factors affect the SAR calibration by more than 0.2 dB within a fractional bandwidth greater than 10% or azimuth beam width of more than 20, which must be corrected in the data processing.
Passive corner reflectors and active transponders are often used as man-made reference targets in Synthetic Aperture Radar (SAR) radiometric calibration, With the emergence of new radar systems and the increasing demand for greater accuracy, wide-band and wide-beam radars challenge the hypothesis that the Radar Cross Section (RCS) of reference targets is constant. In this study, the FEKO electromagnetic simulation software is used to obtain the change curve of the target RCS as a function of frequency and aspect angle while incorporating high-resolution point-target SAR simulation, and quantitatively analyzing the effect of the modulation effect on SAR images. The simulation results suggest that the abovementioned factors affect the SAR calibration by more than 0.2 dB within a fractional bandwidth greater than 10% or azimuth beam width of more than 20, which must be corrected in the data processing.
2015, 4(3): 287-294.
This paper proposes a novel rotating interferometer-based algorithm for parameter estimation of nearfield source. The algorithm exploits the rotation of a single long baseline interferometer and the integration operation of the phase to unwrap the phase ambiguity. Thus the contradiction between the maximal unambiguous angle and the direction finding accuracy in the single baseline interferometer can be efficiently eliminated in the near-field source scenario. The algorithm can obtain the closed form solutions of the elevation and azimuth angles as well as range estimations for near-field source with two receiving sensors. It avoids the construction of high-order cumulant matrices and multi-dimension search. Simultaneously, it alleviates the requirment for multi-baseline channel consistency. Compared with the conventional double long baselines interferometer method, the proposed algorithm achieves higher parameter estimation accuracy and better performance in ambiguity resolution. Moreover, the proposed algorithm enjoys a simple structure, which is easy to be implemented in engineering application. Simulation results demonstrate the effectiveness and validity of the proposed algorithm.
This paper proposes a novel rotating interferometer-based algorithm for parameter estimation of nearfield source. The algorithm exploits the rotation of a single long baseline interferometer and the integration operation of the phase to unwrap the phase ambiguity. Thus the contradiction between the maximal unambiguous angle and the direction finding accuracy in the single baseline interferometer can be efficiently eliminated in the near-field source scenario. The algorithm can obtain the closed form solutions of the elevation and azimuth angles as well as range estimations for near-field source with two receiving sensors. It avoids the construction of high-order cumulant matrices and multi-dimension search. Simultaneously, it alleviates the requirment for multi-baseline channel consistency. Compared with the conventional double long baselines interferometer method, the proposed algorithm achieves higher parameter estimation accuracy and better performance in ambiguity resolution. Moreover, the proposed algorithm enjoys a simple structure, which is easy to be implemented in engineering application. Simulation results demonstrate the effectiveness and validity of the proposed algorithm.
2015, 4(3): 295-300.
Compared with conventional FIR and IIR broadband beamformers, the Laguerre broadband beamformer based on the Frost structure can achieve perfect performance. However, it needs the optimal solution of the solo pole, and it suffers high computational complexity and slow convergence speed. To address these problems, a novel Laguerre wideband beamforming algorithm based on GSC is proposed. A novel solution of the poles based on the least square method with low computational complexity is presented on the basis of the GSC structure of a Laguerre wideband beamformer. Thereafter, the broadband beamformer is achieved using the normalized LMS method. The simulations illustrate that without the optimal process of adjusting the solo pole in the Laguerre broadband beamformer based on the Frost structure, the proposed method can ensure high output SINR, reduce the computational complexity, and improve the convergence speed.
Compared with conventional FIR and IIR broadband beamformers, the Laguerre broadband beamformer based on the Frost structure can achieve perfect performance. However, it needs the optimal solution of the solo pole, and it suffers high computational complexity and slow convergence speed. To address these problems, a novel Laguerre wideband beamforming algorithm based on GSC is proposed. A novel solution of the poles based on the least square method with low computational complexity is presented on the basis of the GSC structure of a Laguerre wideband beamformer. Thereafter, the broadband beamformer is achieved using the normalized LMS method. The simulations illustrate that without the optimal process of adjusting the solo pole in the Laguerre broadband beamformer based on the Frost structure, the proposed method can ensure high output SINR, reduce the computational complexity, and improve the convergence speed.
2015, 4(3): 301-308.
The classical Track Segment Association (TSA) algorithm suffers from low accuracy and is impractical to use in concentrated targets, branching, and cross-tracking environment. Thus, a new statistical binary track segment association algorithm is proposed. The new algorithm is more appropriate as it increases the sample size for the 2 distribution threshold detection. Simulation results show that in air cross tracking and for ballistic targets, the global correct association rate and the average correct association rate of the proposed algorithm are remarkably improved, which proves the good performance of the proposed algorithm.
The classical Track Segment Association (TSA) algorithm suffers from low accuracy and is impractical to use in concentrated targets, branching, and cross-tracking environment. Thus, a new statistical binary track segment association algorithm is proposed. The new algorithm is more appropriate as it increases the sample size for the 2 distribution threshold detection. Simulation results show that in air cross tracking and for ballistic targets, the global correct association rate and the average correct association rate of the proposed algorithm are remarkably improved, which proves the good performance of the proposed algorithm.
2015, 4(3): 309-316.
The traditional MUltiple SIgnal Classification (MUSIC) algorithm requires significant computational effort and can not be employed for the Direction Of Arrival (DOA) estimation of targets in a low-altitude multipath environment. As such, a novel MUSIC approach is proposed on the basis of the algorithm of Adaptive Step Glowworm Swarm Optimization (ASGSO). The virtual spatial smoothing of the matrix formed by each snapshot is used to realize the decorrelation of the multipath signal and the establishment of a fullorder correlation matrix. ASGSO optimizes the function and estimates the elevation of the target. The simulation results suggest that the proposed method can overcome the low altitude multipath effect and estimate the DOA of target readily and precisely without radar effective aperture loss.
The traditional MUltiple SIgnal Classification (MUSIC) algorithm requires significant computational effort and can not be employed for the Direction Of Arrival (DOA) estimation of targets in a low-altitude multipath environment. As such, a novel MUSIC approach is proposed on the basis of the algorithm of Adaptive Step Glowworm Swarm Optimization (ASGSO). The virtual spatial smoothing of the matrix formed by each snapshot is used to realize the decorrelation of the multipath signal and the establishment of a fullorder correlation matrix. ASGSO optimizes the function and estimates the elevation of the target. The simulation results suggest that the proposed method can overcome the low altitude multipath effect and estimate the DOA of target readily and precisely without radar effective aperture loss.
2015, 4(3): 317-325.
The Equivalent Plane Model (EPM) and Phase Correction Algorithm (PCA) that are based on Transmission Line Theory (TLT) are proposed to satisfy the resource and efficiency requirements of ElectroMagnetic (EM) scattering analysis of large and complex multilayered dielectric targets. The proposed method accurately predicts the EM scattering characteristics of reference targets. To simplify the analysis, the multilayered dielectric slab structure is considered planar. On the basis of the TLT, the reflection and transmission coefficients of the plane are determined by using network analysis methods typically adopted in circuit analysis. Moreover, the reflection and transmission phases are corrected by considering the thickness of the multilayered dielectric slab and the direction of incidence and observation. Simulation results verify the applicability of the proposed method.
The Equivalent Plane Model (EPM) and Phase Correction Algorithm (PCA) that are based on Transmission Line Theory (TLT) are proposed to satisfy the resource and efficiency requirements of ElectroMagnetic (EM) scattering analysis of large and complex multilayered dielectric targets. The proposed method accurately predicts the EM scattering characteristics of reference targets. To simplify the analysis, the multilayered dielectric slab structure is considered planar. On the basis of the TLT, the reflection and transmission coefficients of the plane are determined by using network analysis methods typically adopted in circuit analysis. Moreover, the reflection and transmission phases are corrected by considering the thickness of the multilayered dielectric slab and the direction of incidence and observation. Simulation results verify the applicability of the proposed method.
2015, 4(3): 326-333.
The internal wave travels beneath the sea surface and modulate the roughness of the sea surface through the wave-current interaction. This makes some dark and bright bands can be observed in the Synthetic Aperture Radar (SAR) images. In this paper, we first establish the profile of the internal wave based on the KdV equations; then, the action balance equation and the wave-current interaction source function are used to modify the sea spectrum; finally, the two-scale theory based facet model is combined with the modified sea spectrum to calculate the scattering characteristics of the sea. We have simulated the scattering coefficient distribution of the sea with an internal wave traveling through. The influence on the scattering coefficients and the Doppler spectra under different internal wave parameters and sea state parameters are analyzed.
The internal wave travels beneath the sea surface and modulate the roughness of the sea surface through the wave-current interaction. This makes some dark and bright bands can be observed in the Synthetic Aperture Radar (SAR) images. In this paper, we first establish the profile of the internal wave based on the KdV equations; then, the action balance equation and the wave-current interaction source function are used to modify the sea spectrum; finally, the two-scale theory based facet model is combined with the modified sea spectrum to calculate the scattering characteristics of the sea. We have simulated the scattering coefficient distribution of the sea with an internal wave traveling through. The influence on the scattering coefficients and the Doppler spectra under different internal wave parameters and sea state parameters are analyzed.
2015, 4(3): 334-342.
In case of high-resolution, low grazing angle, high sea state, and horizontal transmitting, horizontal receiving polarization, the radar returns are strengthened, resulting in sea spikes. The sea spikes have the characteristics of high amplitudes, nonstationary, and non-Gaussian, which have a strong impact on the radar detection of weak marine moving targets. This study proposes a method for sea clutter suppression. Firstly, based on the sea spikes identification and selection method, the amplitude, temporal correlation, Doppler spectrum, and fractional power spectrum properties of sea spikes are analyzed. Secondly, the data to be detected are chosen by selecting the background clutter with minimum mean power, which can also eliminate the sea spikes. Correspondingly, sea clutter is suppressed with improved Signal-to-Clutter Ratio (SCR). Finally, the results of experiment with real radar data verify the effectiveness of the proposed method.
In case of high-resolution, low grazing angle, high sea state, and horizontal transmitting, horizontal receiving polarization, the radar returns are strengthened, resulting in sea spikes. The sea spikes have the characteristics of high amplitudes, nonstationary, and non-Gaussian, which have a strong impact on the radar detection of weak marine moving targets. This study proposes a method for sea clutter suppression. Firstly, based on the sea spikes identification and selection method, the amplitude, temporal correlation, Doppler spectrum, and fractional power spectrum properties of sea spikes are analyzed. Secondly, the data to be detected are chosen by selecting the background clutter with minimum mean power, which can also eliminate the sea spikes. Correspondingly, sea clutter is suppressed with improved Signal-to-Clutter Ratio (SCR). Finally, the results of experiment with real radar data verify the effectiveness of the proposed method.
2015, 4(3): 343-350.
In this paper, the wideband backscattering fields of two-Dimensional (2-D) linear and nonlinear sea surfaces are numerically simulated employing the Weighted Curvature Approximation (WCA) method. A large number of Monte Carlo trials are performed to investigate the statistical characteristics of the rang-resolved sea clutter, especially for the sea spike phenomenon. Simulation results demonstrate that the long tail of the sea clutter intensity Probability Density Function (PDF) tends to be more evident with finer radar resolution, higher wind speed, and when the radar sight changes from the crosswind direction to the upwind direction. Meanwhile, it is found that the nonlinear sea surfaces are more likely to have sea spikes. In addition, the Pareto distribution is demonstrated to describe the statistics of the sea clutter intensities better than the Kdistribution and Weibull distribution at low grazing angles.
In this paper, the wideband backscattering fields of two-Dimensional (2-D) linear and nonlinear sea surfaces are numerically simulated employing the Weighted Curvature Approximation (WCA) method. A large number of Monte Carlo trials are performed to investigate the statistical characteristics of the rang-resolved sea clutter, especially for the sea spike phenomenon. Simulation results demonstrate that the long tail of the sea clutter intensity Probability Density Function (PDF) tends to be more evident with finer radar resolution, higher wind speed, and when the radar sight changes from the crosswind direction to the upwind direction. Meanwhile, it is found that the nonlinear sea surfaces are more likely to have sea spikes. In addition, the Pareto distribution is demonstrated to describe the statistics of the sea clutter intensities better than the Kdistribution and Weibull distribution at low grazing angles.
2015, 4(3): 351-360.
Aiming at meeting the requirement of the amass high-quality SAR images needed by template-based ground target recognition engineering practice, a novel efficient SAR signal level simulation method is proposed. The electromagnetic scattering interaction mechanisms including coherent clutter of ground, multiple reflection and edge diffraction of vehicle, coupling-scattering between vehicle and ground are accurately estimated by utilizing an efficient ray-tracing technique. High quality SAR images are finally created through the SAR imaging procedure. Simulation results show that, the new method is feasible and effective.
Aiming at meeting the requirement of the amass high-quality SAR images needed by template-based ground target recognition engineering practice, a novel efficient SAR signal level simulation method is proposed. The electromagnetic scattering interaction mechanisms including coherent clutter of ground, multiple reflection and edge diffraction of vehicle, coupling-scattering between vehicle and ground are accurately estimated by utilizing an efficient ray-tracing technique. High quality SAR images are finally created through the SAR imaging procedure. Simulation results show that, the new method is feasible and effective.
2015, 4(3): 361-366.
Discontinuous Galerkin Time Domain (DGTD) method appears to be very promising which combines the advantages of unstructured mesh in Finite Element Time Domain (FETD) and explicit scheme in Finite Difference Time Domain (FDTD). This paper first describes principle of DGTD base on vector basis function. Secondly, Specific method for incident plane wave is given for scattering problem. At last, the monostatic Radar Cross Section (RCS) of PEC sphere, medium sphere and the PEC bullet are computed by DGTD method. The numerical results illustrate the feasibility and correctness of the presented scheme. The study of this paper is a foundation for analyzing the RCS of complex target.
Discontinuous Galerkin Time Domain (DGTD) method appears to be very promising which combines the advantages of unstructured mesh in Finite Element Time Domain (FETD) and explicit scheme in Finite Difference Time Domain (FDTD). This paper first describes principle of DGTD base on vector basis function. Secondly, Specific method for incident plane wave is given for scattering problem. At last, the monostatic Radar Cross Section (RCS) of PEC sphere, medium sphere and the PEC bullet are computed by DGTD method. The numerical results illustrate the feasibility and correctness of the presented scheme. The study of this paper is a foundation for analyzing the RCS of complex target.
2015, 4(3): 367-373.
To deal with the problem of false alarm in the ship detection, a method base on proportion of spiral scattering in the peak zone is proposed. By comparing the proportion of spiral scattering in the peak zone, which is available from Krogager decomposition, the ships and interfering targets are identified and analyzed. The effectiveness of this method is justified with C-band full-polarization data from RADARSAT-2. The result show that this method can discriminate ships from interfering targets such as island, water-break, nautical platforms and bridges, thus reducing the false alarm rate of ship targets detection in SAR images.
To deal with the problem of false alarm in the ship detection, a method base on proportion of spiral scattering in the peak zone is proposed. By comparing the proportion of spiral scattering in the peak zone, which is available from Krogager decomposition, the ships and interfering targets are identified and analyzed. The effectiveness of this method is justified with C-band full-polarization data from RADARSAT-2. The result show that this method can discriminate ships from interfering targets such as island, water-break, nautical platforms and bridges, thus reducing the false alarm rate of ship targets detection in SAR images.
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