Squint Mode Synthetic Aperture Radar Research Articles

Novel acceleration compensation method for highly squint mode SAR with curve trajectory

Due to the influence of air currents and the control of the platform's own power system, acceleration is often present in the motion platform equipped with synthetic aperture radar (SAR) system, which causes the actual trajectory of the platform to be curved. When the acceleration is large or the azimuth accumulation time is long, a large envelope error or phase error will be caused. This problem will be more obvious in highly squint mode SAR. On the other hand, the acceleration may also cause the situation where the absolute value of the Doppler frequency rate is extremely small, resulting in the azimuthal time-bandwidth product that is too small to satisfy the requirements of the stationary phase principle. To solve the above problems, a novel acceleration compensation method based on inertial navigation information is proposed in this article. By integrating the acceleration and velocity information provided by the inertial navigation system, the position information of maneuvering platform is obtained. Then the actual trajectory of the platform is constructed. On this basis, the envelope and phase errors caused by the acceleration are estimated. The results of real data processing verify the correctness of the analysis and the effectiveness of the proposed method.

A Novel Weighted Doppler Centroid Estimation Approach Based on Electromagnetic Scattering Model for Multichannel in Azimuth HRWS SAR System

Similar to the conventional squint mode synthetic aperture radar (SAR) imaging processing, the estimation of Doppler centroid is one key problem for the low-squint-mode (within the range of [−5°, 5°]) multichannel in an azimuth high-resolution and wide-swath (HRWS) SAR system. In this paper, based on the electromagnetic scattering model, a novel weighted Doppler centroid estimation approach is proposed for the multichannel in the azimuth HRWS (MC-HRWS) SAR system. First, Maxwell’s equations are employed to derive the echo model for the multichannel SAR system. Then, an improved backscattering model is presented based on the small perturbation method and the available echo model, which is adopted to produce the weights for Doppler centroid estimation. More importantly, in order to improve the precision of Doppler centroid estimation, a weighted ambiguity-free Doppler centroid estimation approach is proposed, where the range-variant characteristic of Doppler centroid is employed to resolve the ambiguity number of Doppler centroid, and the echoes from different range bins with different signal-to-noise ratios are considered. In addition, the Cramer–Rao low bound of the estimated Doppler centroid is also discussed in this paper. The effectiveness of the proposed Doppler centroid estimation approach is verified via simulated and real measured low-squint-mode MC-HRWS SAR data.

A Novel Focus Approach for Squint Mode Multi-Channel in Azimuth High-Resolution and Wide-Swath SAR Imaging Processing

An imaging processing for the squint mode multi-channel in azimuth high-resolution and wide-swath (MC-HRWS) synthetic aperture radar (SAR) system is presented in this paper. First, the linear range cell migration correction (RCMC) is employed to correct the skew Doppler spectrum for each channel echo. Then, a robust Doppler spectrum reconstruction method is proposed, which is based on the robust Capon beamforming and multi-Doppler-direction restriction technique. After the reconstruction, a two-step focus approach is utilized to focus the image, where the first step is utilized to deal with the azimuth-invariance components and the second addresses the azimuth-variance component introduced by the linear RCMC. Since the Doppler centroid estimation is crucial to squint mode SAR imaging, this paper also presents a robust entropy-based Doppler centroid estimation algorithm for an MC-HRWS SAR system, where the correlation function approach is employed to estimate the baseband Doppler centroid. More importantly, an entropy-based Doppler ambiguity number resolving approach is proposed and the bisection algorithm is utilized to accelerate the searching process. Some simulation experiments are conducted to verify our proposal. Besides, the results of some real MC-HRWS SAR data also show that the proposed method works well.

Estimation of Aircraft Altitude Based on Squint Mode SAR Data

The aircraft altitude relative to the illuminated region is a significant parameter for the geometric correction in squint mode synthetic aperture radar (SAR) imaging. Error in the parameter causes geometric distortion, which degrades the image quality. To circumvent this problem, a relative altitude estimator (RAE) is proposed based on the relationship between the relative aircraft altitude and the range variation of the Doppler centroid. The high performance of the RAE is investigated compared to the Cramer-Rao lower bound of the relative aircraft altitude. Finally, the effectiveness of the RAE in practical applications is verified by real data processing.

Strategy of Doppler centroid estimation in synthetic aperture radar

Doppler centroid is an important parameter in synthetic aperture radar (SAR) image processing. Owing to measurement uncertainties, many clutter-lock algorithms are proposed to estimate the Doppler centroid. One of the factors that affect the estimation is ‘chirp coupling’ [linear FM signal is used as transmitting signal in SAR, and this makes echoes of targets from different range bins overlap with each other at the same range bin, which could be called ‘chirp coupling (CHCO)’]. In broadside or lowly squint mode SAR, CHCO is often ignored. However, with large squint angle, its influence on Doppler centroid estimation will increase and even affect the final image. The azimuth-frequency shift of echoes caused by CHCO is analysed and a method proposed to calculate the threshold angle which is used to judge its influence on Doppler centroid estimation. With the help of the threshold angle, a novel strategy of Doppler centroid estimation is proposed. After theoretic analyses, simulations and experimental results are provided to demonstrate the proposed strategy of Doppler centroid estimation.

A new two-dimensional squint mode SAR processor

This paper extends the processing capability of the wide angle synthetic aperture radar (WASAR) algorithm (1992) to synthetic aperture radar (SAR) data acquired with the radar beam directed with an offset angle (squint angle) from the zero Doppler direction. The key point of this work is the analytical evaluation of the system transfer function for the squinted geometry via the stationary phase method. The range dependence of this new transfer function is compensated in two steps: a phase multiplication and a modified version of the basic fractional Fourier transform (FT). The performance of the extended algorithm is demonstrated by experiments with real and simulated data.

A chirp scaling approach for processing squint mode SAR data

Image formation from squint mode synthetic aperture radar (SAR) is limited by image degradations caused by neglecting the range-variant filtering required by secondary range compression (SRC). Introduced here is a nonlinear FM chirp scaling, an extension of the chirp scaling algorithm, as an efficient and accurate approach to range variant SRC. Two methods of implementing the approach are described. The nonlinear FM filtering method is more accurate but adds a filtering step to the chirp scaling algorithm, although the extra computation is less than that of a time domain residual compression filter. The nonlinear FM pulse method consists of changing the phase modulation of the transmitted pulse, thus avoiding an increase in computation. Simulations show both methods significantly improve resolution width and sidelobe level, compared with existing SAR processors for squint angles above 10 deg for L-band and 20 deg for C-band.