Jianzhi Lin

COHERENT PHASE COMPENSATION METHOD BASED ON DIRECT IF SAMPLING IN WIDEBAND RADAR

In order to eliminate the negative in∞uence of the rotational phase component (RPC) of target prominent scattering centres on the performance of Doppler centroid tracking (DCT) method, a coherent phase compensation method is proposed. The coherence of echo pulses sampled directly in intermediate frequency (IF) is flrstly analyzed and proved. Based on the coherence property, the proposed approach improves the translational phase component (TPC) estimation accuracy of DCT. Compared to the modifled Doppler centroid tracking (MDCT) algorithm, the proposed method achieves better phase compensation performance with simpler operations. Both the theoretical analysis and experimental results based on the real ISAR data prove the efiectiveness and e-ciency of the presented strategy.

Wideband Direction of Arrival Estimation in the Presence of Unknown Mutual Coupling

This paper investigates a subarray based algorithm for direction of arrival (DOA) estimation of wideband uniform linear array (ULA), under the presence of frequency-dependent mutual coupling effects. Based on the Toeplitz structure of mutual coupling matrices, the whole array is divided into the middle subarray and the auxiliary subarray. Then two-sided correlation transformation is applied to the correlation matrix of the middle subarray instead of the whole array. In this way, the mutual coupling effects can be eliminated. Finally, the multiple signal classification (MUSIC) method is utilized to derive the DOAs. For the condition when the blind angles exist, we refine DOA estimation by using a simple approach based on the frequency-dependent mutual coupling matrixes (MCMs). The proposed method can achieve high estimation accuracy without any calibration sources. It has a low computational complexity because iterative processing is not required. Simulation results validate the effectiveness and feasibility of the proposed algorithm.

Maximum likelihood-based range alignment for polarimetric inverse synthetic aperture radar

ABSTRACT The range alignment technique is extended and applied to full polarization processing technique using the fully polarized data for a target. By the maximum likelihood-based span, the fused high-resolution range profile (HRRP) is achieved. Then the range alignment is carried out on the fused HRRPs and the offsets are used to align the original HRRPs of each polarimetric channel. Applying this technique, the signal-to-noise ratio (SNR) of the HRRP is increased and the performance of the range alignment is improved in comparison with that of the conventional techniques using singly polarized data, and therefore the inverse synthetic aperture radar images of the target can be clearly obtained even in low SNR condition. Experiments from measured data under field conditions are presented to validate the proposed technique.

IF digitization receiver of wideband digital array radar test-bed

In this paper, an X-band, 8-element wideband digital array radar (DAR) test-bed is presented, which makes use of a novel digital backend coupled with highly-integrated, multi-channel intermediate frequency (IF) digital receiver. Radar returns are received by the broadband antenna and then down-converted to the IF of 0.6GHz-3.0GHz. Four band-pass filters are applied in the front-end to divide the IF returns into four frequency bands with the instantaneous bandwidth of 500MHz. Every four array elements utilize a digital receiver, which is focused in this paper. The digital receivers are designed in a compact and flexible manner to meet the demands of DAR system. Each receiver consists of a fourchannel ADC, a high-performance FPGA, four DDR3 chips and two optical transceivers. With the sampling rate of up to 1.2GHz each channel, the ADC is capable of directly sampling the IF returns of four array elements at 10bits. In addition to serving as FIFO and controller, the onboard FPGA is also utilized for the implementation of various real-time algorithms such as DDC and channel calibration. Data is converted to bit stream and transferred through two low overhead, high data rate and multi-channel optical transceivers. Key technologies such as channel calibration and wideband DOA are studied with the measured data which is obtained in the experiments to illustrate the functionality of the system.

Precise Radial Velocity Estimation for Inverse Synthetic Aperture Radar

This paper describes a convenient technique of precise radial velocity estimation for inverse synthetic aperture radar (ISAR). In order to keep both the range profile and phase history of the echoes coherent, direct sampling with high sampling rate using high performance analog-to-digital converter and matched-filter correlation processing in pulse compression are used for the ISAR system. Due to the coherence property of the echoes, the translational motion compensation parameters for ISAR imaging are just the radial motion parameters of the target. Thus, the coarse velocity estimation is obtained by range alignment and fine velocity estimation is achieved by phase adjustment. The fine velocity estimation is ambiguous and the coarse velocity estimation is used for ambiguity resolution. The advantage of this technique is the high precision with range error values at sub wavelength levels, and it achieves velocity information and translational motion compensation at the same time. Both simulated and experimental validations are presented to verify the effectiveness of the proposed method.

SEGMENT NONCOHERENT INTEGRATION BASED INVERSE SYNTHETIC APERTURE RADAR IMAGING UNDER LOW SIGNAL-TO-NOISE RATIO

In this paper, a novel scheme for inverse synthetic aperture radar (ISAR) imaging under low signal-to-noise ratio (SNR) condition is proposed. The method is a preprocess of the high-resolution range profiles and relies on the oversampling in the azimuth direction. It divides the entire coherent processing interval into segments according to the down sampling factor. In each segment, original low SNR echoes are noncoherently integrated to obtain a new high SNR echo. With the new high SNR echoes, conventional methods for ISAR imaging can perform much better and obtain a better focused ISAR image. The presented algorithm has the advantage of effectiveness under low SNR condition and computational efficiency. Experimental results based on both the simulated and real radar data of an airplane verify the superiority of the proposed strategy.

Polarimetric Calibration Based on Lexicographic-Basis Decomposition

A novel polarimetric calibration scheme on the basis of lexicographic matrix decomposition is proposed, and a new polarimetric active radar calibrator (PARC) with two independently rotatable antennas is designed to obtain the lexicographic matrices. Thus, the proposed method is realizable and can operate with the lexicographic target vectors instead of the polarization scattering matrix (PSM). The base elements of the lexicographic target vectors are the vectors corresponding to the new PARC with the antennas at different orientations. Moreover, the elements of the vector are just the elements of the PSM. Hence, the error coefficients corresponding to a polarimetric measurement system are directly achieved, and the polarimetric calibration is simple and accurate. Experimental results confirm the superiority of the presented strategy. A 35-dB improvement in the system cross-polarization isolation is obtained.

Channel calibration for digital array radar in the presence of amplitude-phase and mutual coupling errors

Amplitude-phase errors and mutual coupling errors among multi-channels in digital array radar (DAR) will seriously deteriorate the performance of signal processing such as digital beam-forming (DBF) and high resolution direction finding. In this paper, a combined algorithm for error calibration in DAR has been demonstrated. The algorithm firstly estimates the amplitude-phase errors of each channel using interior calibration sources with the help of the calibration network. Then the signals from far field are received and the amplitude-phase errors are compensated. According to the subspace theories, the relationship between the principle eigenvectors and distorted steering vectors is expressed, and the cost function containing the mutual coupling matrix (MCM) and incident directions is established. Making use of the properties of MCM of uniform linear array, Gauss-Newton method is implied to iteratively compute the MCM and the direction of arrival (DOA). Simulation results have shown the effectiveness and performance of proposed algorithm. Based on an 8-elements DAR test-bed, experiments are carried out in anechoic chamber. The results illustrate that the algorithm is feasible in actual systems.

Correction of amplitude-phase distortion for polarimetric active radar calibrator

Abstract. The polarimetric active radar calibrator (PARC) is extensively used as an external test target for system distortion compensation and polarimetric calibration for the high-resolution polarimetric radar. However, the signal undergoes distortion in the PARC, affecting the effectiveness of the compensation and the calibration. The system distortion compensation resulting from the distortion of the amplitude and phase in the PARC was analyzed based on the “method of paired echoes.” Then the correction method was proposed, which separated the ideal signals from the distorted signals. Experiments were carried on real radar data, and the experimental results were in good agreement with the theoretical analysis. After the correction, the PARC can be better used as an external test target for the system distortion compensation.