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Spotlight Synthetic Aperture Radar Signal Processing Algorithms

The imagery of highly squinted synthetic aperture radar mounted on maneuvering platforms with nonlinear trajectory is a challenging task due to the existence of acceleration and the cross-range-dependent range migration and Doppler parameters. In order to accommodate these issues, a frequency-domain imaging algorithm based on tandem two-step nonlinear chirp scaling (TNCS) with small aperture is proposed. For the cross-range-dependent range cell migration (RCM) caused by the linear range walk correction and acceleration, the first-step NCS is introduced to suppress this dependence and realize the unified RCM correction. Based on the differences between full-aperture and small-aperture data in the cross-range processing, the second-step NCS is introduced in frequency domain to equalize the cross-range-dependent Doppler parameters, for cross-range processing is more sensitive to the cross-range dependence than range processing. Furthermore, a novel geometric correction method based on inverse projection is utilized to eliminate the negative effects caused by the imaging processing. Simulation results and real data processing are presented to validate the proposed approach.

A Frequency-Domain Imaging Algorithm for Highly Squinted SAR Mounted on Maneuvering Platforms With Nonlinear Trajectory

The challenge in the imaging of high-squint-mode synthetic aperture radar (SAR) mounted on maneuvering platforms is the azimuth dependence of both the range migration and the azimuth focusing parameters (the azimuth frequency-modulation rate and higher order coefficients), which are caused by range walk correction and acceleration In order to accommodate the dependence, a modified subaperture imaging algorithm is proposed Based on the fact that the azimuth times corresponding to the same Doppler frequency are different for targets in the same range gate, after making blocks in the azimuth frequency domain, the azimuth-dependent range cell migration correction is performed in the azimuth time domain for each block Considering that the regions of support in the azimuth frequency domain are different for targets in the same range gate, the equalization of the azimuth focusing parameters is achieved by a new azimuth nonlinear chirp scaling method in the azimuth frequency domain In order to verify the effectiveness of the proposed algorithm, the simulation of a point target array is presented Furthermore, the real SAR data with a squint angle of 70° are processed, and high-quality images with a resolution of 1 m are provided

Subaperture Approach Based on Azimuth-Dependent Range Cell Migration Correction and Azimuth Focusing Parameter Equalization for Maneuvering High-Squint-Mode SAR

A method of motion status estimation of airborne synthetic aperture radar (SAR) platform in short subapertures via parametric sparse representation is proposed for high-resolution SAR image autofocusing. The SAR echo is formulated as a jointly sparse signal through a parametric dictionary matrix, which converts the problem of SAR motion status estimation into a problem of dynamic representation of jointly sparse signals. A full synthetic aperture is decomposed into several subapertures to estimate the dynamic motion parameters of a platform, and SAR motion compensation is achieved by refining the estimation of the equivalent platform motion parameters, i.e., the azimuth velocity and the radial acceleration of the radar platform, at each subaperture in an iterative fashion. Experimental results based on both simulated and real data demonstrate that: 1) the proposed algorithm outperforms the map-drift algorithm and the phase gradient autofocus algorithm in terms of the imaging quality and 2) compared to the iterative minimum-entropy autofocus, the proposed algorithm produces the comparative imaging quality with less computational complexity in complex motion environment.

Motion Compensation for Airborne SAR via Parametric Sparse Representation

Since synthetic aperture technology was employed in radar signal processing, a lot of imaging algorithms have been developed for highly squinted synthetic aperture radar (SAR). However, the high-resolution imaging for highly squinted SAR data is still a difficult issue due to the large range migration and strong space-variant characteristic. To accommodate for this problem, a new fractional chirp scaling algorithm (FrCSA) for highly squinted missile-borne SAR is proposed from high resolution point of view in this paper. The performance of the FrCSA is compared to the classical CSA based on fast Fourier transform (FFT). Simulation and real data results indicate that the FrCSA offers better focusing capabilities, greater peak sidelobe ratio (PSLR), and integrated sidelobe ratio (ISLR) by appropriately choosing the rotation angles for the range and azimuth fractional Fourier transform (FrFT).

A New Chirp Scaling Algorithm for Highly Squinted Missile-Borne SAR Based on FrFT

Synthetic aperture radar (SAR) imaging is usually performed along a straight path. In practice, variations from the ideal caused by maneuvers with acceleration are inevitable. The main problem is the complex signal processing, particularly in the highly squinted spotlight mode. In this paper, acceleration model is analyzed in more details. Impacts are obtained how the acceleration influences the Doppler parameters and space-variant terms, including the SAR resolution and azimuth bandwidth. According to the special signal properties of the acceleration case, an approximation of the range history is made to avoid using the method of series reversion (MSR) to obtain an accurate spectrum. Based on the high-accurate spectrum, a spotlight algorithm with a modified two-step approach is given. Simulation results show the effectiveness of the modified algorithm.

Acceleration Model Analyses and Imaging Algorithm for Highly Squinted Airborne Spotlight-Mode SAR with Maneuvers

This letter proposes an omega-K algorithm for missile-borne synthetic aperture radar (SAR) diving with constant acceleration. The essence of the novel method is to derive a reference function multiplication from the equivalent range history. The true range history is divided into two parts: range independent and range dependent. After compensating the range-independent part with considering all the terms, we can regard the range-dependent part as an equivalent range history with a form similar to that of conventional SAR. Then, an analytical 2-D spectrum can be used for the omega-K algorithm. The proposed algorithm can handle general missile configurations with wide swaths and high squint angles. Simulation results are presented to validate the proposed method.

An Omega-K Algorithm for Highly Squinted Missile-Borne SAR With Constant Acceleration

In the traditional model of monostatic synthetic aperture radar (SAR), the flying platform should have a uniform linear trajectory. In practice, the flight path of the SAR platform, which is highly nonlinear or curvy caused by 3-D velocity and acceleration, cannot be used in the traditional model. In this paper, a geometrical model of the monostatic SAR with general configurations is graphically illustrated by vector notation. Moreover, the gradient method is utilized to point out the effects of the motion parameters, namely, velocity vector and acceleration vector, on total bandwidth and image resolution. Based on the accurate model, a general frequency-domain algorithm, with the incorporations of pre- and postprocessing, is proposed to focus the raw data. Applicability is studied through theoretical analysis and numerical experiments. All these general analyses can be applied to different realizations of monostatic SAR mode, including spotlight SAR, sliding spotlight SAR, and Terrain Observation by Progressive Scans (TOPS) SAR.

Processing of Monostatic SAR Data With General Configurations

For different applications with diverse requirements, many synthetic aperture radar (SAR) modes have been developed in the literature, such as, spotlight SAR, terrain observation by progressive scans (TOPS) SAR, and sliding spotlight SAR. For highly squinted beam steering SAR (BS-SAR; spotlight SAR, sliding spotlight SAR, and TOPS SAR), the main challenge is to develop an efficient full-aperture focusing approach since the mostly traditional algorithms fail to process the highly squinted BS-SAR data without subaperture technique. In this study, a unified full-aperture focusing algorithm is presented for BS-SAR modes in the case of highly squint angle. The key of the algorithm lies in the linear multiplications applied in the range-wavenumber and azimuth-space domain, which can remove the azimuth spectral folding effect and the geometrical distortion caused by spectrum skewing. Moreover, there is no any approximation in the focusing procedure; thus, the presented algorithm can perform well. Since the subaperture is avoided, this improved focusing approach is highly efficient. Simulation results are presented to validate the proposed method.

Improved focusing approach for highly squinted beam steering SAR

The invention discloses a multi-sending-multi-receiving interference synthetic aperture radar time frequency two-dimension signal waveform designing method which mainly solves the problem that in the prior art, waveform orthogonality is poor, and sending signal pulse repetition frequency is high. The implementation steps of the method are that 1. carrier frequency variable quantity delta f i j needed by waveform design is computed; 2. according to the carrier frequency variable quantity delta f i j, the carrier frequency f i j of a sending signal of a sending-receiving combination Ai-Aj is computed; 3. the carrier frequency f i j is subjected to non-redundancy processing through the frequency shift consistency of the sending-receiving combination Ai-Aj and Aj-Ai; 4. according to the non-redundancy carrier frequency number, the sending pulse of an i array element Ai is subjected to sub-pulse number dividing, carrier frequency is distributed to each sub pulse; and 5. the sending sub pulses of array elements are subjected to modulation respectively through the carrier frequency f i j, and a radar waveform signal si(t) is obtained. According to the method, pulse repetition frequency is effectively lowered, waveform orthogonality is improved, larger surveying and mapping band width is obtained, and the method can be used for multi-sending-multi-receiving interference synthetic aperture radar imaging.

Ping Guo

Papers

Acceleration Model Analyses and Imaging Algorithm for Highly Squinted Airborne Spotlight-Mode SAR with Maneuvers

An Omega-K Algorithm for Highly Squinted Missile-Borne SAR With Constant Acceleration

Processing of Monostatic SAR Data With General Configurations

Improved focusing approach for highly squinted beam steering SAR

Multi-sending-multi-receiving interference synthetic aperture radar time frequency two-dimension signal waveform designing method