Multiple-receiver Synthetic Aperture Sonar Research Articles

A Novel Imaging Algorithm for Wide-Beam Multiple-Receiver Synthetic Aperture Sonar Systems

In existing imaging algorithms for wide-beam multiple-receiver synthetic aperture sonar (SAS) systems, the double-square-root (DSR) range history of each receiver is generally converted into the sum of a single-square-root (SSR) range history and an error term using displaced phase center aperture (DPCA) approximation. Therefore, before imaging, each receiver’s error term needs to be individually compensated in the azimuth frequency domain, which is computationally expensive. As a result, a novel wide-beam multiple-receiver SAS system algorithm with low complexity and high precision is suggested. First, the translation relationship between the range histories of the reference receiver and other receivers is used to derive an SSR approximation range history that takes into account the azimuth variance of the non-stop-hop-stop time while ignoring differential range curvature (DRC) between the range histories from different receivers. Then, using the principle of stationary phase (POSP), the two-dimensional (2-D) spectrum of the point target is obtained. Finally, the multiple-receiver data are transformed into monostatic SAS-equivalent data for imaging after phase correction, time delay correction, and azimuth reconstruction. The range-Doppler (RD) algorithm is used as an example to explain the specific steps of the proposed approach. Simulation data and ChinSAS data experiments verify that the proposed algorithm achieves an imaging performance that is comparable to that of the existing wide-beam algorithm, but with much higher computational efficiency, making it suitable for real-time imaging.

Azimuth-Invariant Motion Compensation and Imaging Chirp Scaling Algorithm for Multiple-Receiver Synthetic Aperture Sonar

To improve the imaging quality of conventional imaging algorithms without motion compensation (MOCO) and the efficiency of point-by-point MOCO algorithms for multiple-receiver synthetic aperture sonar (SAS) with azimuth-invariant six-degree of freedom (DOF) motion errors, an azimuth-invariant MOCO and imaging chirp scaling (CS) algorithm is presented in this paper. Taylor series approximation is used to process the range history in double square root form, while considering the fourth-order and inner terms with respect to the azimuth time. Using the method of series reversion and Fourier transform (FT) properties, the analytical two-dimensional frequency spectrum of the point target response of each receiver is derived. On this basis, the azimuth-invariant MOCO and imaging CS algorithm is proposed for six-DOF motion error compensation and multiple-receiver SAS imaging. Because it considers the azimuth-invariant six-DOF motion errors, the proposed algorithm has better imaging quality than conventional imaging algorithms without MOCO. Additionally, it has a significantly higher efficiency than point-by-point MOCO algorithms because the CS algorithm is a fast FT-based algorithm and does not require interpolation processing. The imaging simulation and experimental results verified the effectiveness and efficiency of the proposed algorithm.

CZT Algorithm for Multiple-Receiver Synthetic Aperture Sonar

The compensation of slant range motion errors needs to be carried out after range pulse compression in the signal processing of multiple-receiver synthetic aperture sonar (SAS). Whereas range pulse compression cannot be performed as the first step in multiple-receiver SAS chirp scaling (CS) algorithm, as it requires the transmitted signal to be linearly frequency modulated. For this reason, it is difficult to combine CS algorithm with motion compensation. Besides, those imaging algorithms based on interpolation, such as range-Doppler algorithm and wavenumber domain algorithm, have plenty of computation load and large phase errors. Aimed at these problems, a CZT algorithm for multiple-receiver SAS in the non-stop-hop-stop mode is proposed in this paper. The proposed algorithm can correct range cell migration utilizing chirp-z transform, which avoids interpolation. Furthermore, the algorithm is easily combined with motion compensation as it can process pulse-compressed data directly. This paper describes the algorithm in 9 key steps, including the principle of correcting range cell migration utilizing chirp-z transform in discrete domain. In the simulation and sea trial data processing, the proposed algorithm provides image quality equal to the CS algorithm, and the algorithm is slightly more efficient than CS algorithm when combined with motion compensation.

Accelerating range Doppler imaging algorithm for multiple-receiver synthetic aperture sonar on multi-core-based architectures

Synthetic aperture sonar (SAS) is an underwater high-resolution imaging method. But with the increase in resolution and mapping width, the amount of raw data used for imaging increases dramatically. To solve the problem of low imaging efficiency of SAS, an acceleration method of SAS imaging in shared memory environment is proposed. By analyzing the calculation characteristics of each step from the original data received to the synthetic aperture imaging result, the range compression, equivalent conversion from multi-receiver signal to single-receiver signal and azimuth compression are designed in parallel with OpenMP instructions, and the multi-core computing resources are fully utilized to accelerate the imaging process. Simulation experiment verifies the correctness of the parallel imaging algorithm. The experimental result of real data shows that the parallel imaging algorithm has high efficiency and can realize super real-time imaging. The efficiency of the proposed method can be changed with the number of computational kernels. The relationship between the acceleration ratio and the computational kernels is approximately linear, which improves the adaptability of the algorithm. Efficient synthetic aperture sonar imaging algorithm provides conditions for post-processing of image, such as image enhancement, image target detection and recognition.

Range-segment Motion Compensation based on Integrated INS for Wide-swath Synthetic Aperture Sonar

Motion errors have severe effect on synthetic aperture sonar (SAS) imagery if worse than a fraction of a wavelength along the entire aperture. Not only currents, but vehicle instability randomly causes periodic motion errors in the synthetic aperture and grating lobes in the SAS images as well. Displaced phase center antenna referred to as micro-navigation has been developed with multiple-receiver SAS for a long time, but still has challenges against larger motion errors. The integrated inertial navigation system comprising an inertial navigation system (INS) and other aided sensors such as a Doppler Velocity Log (DVL) and a surface ship’s Global Position System (GPS) is an unavoidable and trade-off solution to the problem of motion estimate, which is definitely more robust and reliable with the application of the multi-sensor data fusion. Besides, the motion error is variant along the slant range. For a better resolution of the wide-swath SAS, a range-segment motion compensation method based on the integrated inertial navigation system is presented in the paper. This method relies on the multi-sensor data fusion of the integrated inertial navigation system via Kalman filter and is found to significantly outperform the conventional method.

Fast imaging algorithm for the multiple receiver synthetic aperture sonars

For the multiple receiver synthetic aperture sonar, traditional fast imaging algorithms are limited by the approximation of the point target reference spectrum (PTRS). To solve this problem, a novel imaging algorithm based on a new PTRS is proposed in this study. With the presented method, the matched filtering function in the azimuth dimension must be first derived. Then, the range–azimuth coupling phase is obtained by using the difference between the PTRS and the matched filtering function in the azimuth dimension. To perform the decoupling between the range dimension and the azimuth dimension, the bulk range cell migration correction (RCMC) and the differential RCMC based on the range-dependent sub-block processing method are used. Compared to traditional methods, the new method avoids the complicated series expansion. Furthermore, it can handle the general case with the explicit point of the stationary phase. Processing results of the simulated data and the real data indicate that the high-performance results can be got by using the presented method. Moreover, it further shows that the presented method is very suitable for the wide-swath imagery.

Extended Range Doppler Algorithm for Multiple-Receiver Synthetic Aperture Sonar Based on Exact Analytical Two-Dimensional Spectrum

For the imaging of multiple-receiver synthetic aperture sonar (SAS) under the nonstop-hop-stop case, an exact analytical solution for the two-dimensional (2-D) frequency spectrum is usually difficult to obtain because of the existence of the double-square-root (DSR) term in the range history equation. Several approximate solutions for the 2-D spectrum have been derived to focus the multiple-receiver SAS data. In this paper, the range history geometry for multiple-receiver SAS is newly constructed to meet the demand of the derivation of an analytical 2-D spectrum. According to the geometry-based bistatic formula (GBF) method, a quasi-analytical 2-D spectrum with an unknown variable named half quasi-bistatic angle (HQBA) is reviewed. Based on the method of series reversion (MSR), the fourth order equation with respect to the HQBA is solved and an analytical HQBA is obtained. After substituting the analytical HQBA into the quasi-analytical 2-D spectrum, a completely analytical 2-D spectrum is obtained and the problem of the 2-D spectrum derivation caused by the DSR term is solved successfully. In this paper, an extended range Doppler (RD) algorithm based on the derived 2-D spectrum is proposed for focusing the multiple-receiver SAS data under the non stop-hop-stop case. The results of simulation and real experiment data imaging have validated the effectiveness and accuracy of the proposed algorithm.

Motion-Compensation Improvement for Widebeam, Multiple-Receiver SAS Systems

The effect that uncompensated motion errors have on synthetic aperture sonar (SAS) imagery can be severe. Time-domain beamforming SAS reconstruction is able to compensate arbitrary track errors, but the more efficient frequency-domain reconstruction algorithms, such as the range-Doppler, chirp-scaling, and wave number (aka range migration or Stolt-mapping) algorithms do not allow for simple compensation, especially for widebeam sonars. Data processed via these block algorithms is usually compensated before azimuth compression in a computationally inexpensive preprocessing step. Unfortunately, this compensation assumes a narrowbeam geometry, causing blurring in high-resolution images collected with widebeam sonars. In this paper, we demonstrate a new technique for compensation of large, but known, motion errors in data collected with widebeam geometry sonars. The technique relies on obtaining angle-of-arrival information from the multiple-receiver array configuration typical in high-resolution SAS systems. The new method of compensating for motion errors was found to significantly outperform the previous techniques in a simulation of point-reflector imagery.