Multireceiver Synthetic Aperture Sonar Research Articles

A wide-beam NCS algorithm for multi-receiver SAS based on azimuth spectrum superposition

The existing multi-receiver synthetic aperture sonar (SAS) imaging algorithms are suitable for narrow-beam width, which will lead to a decrease in imaging quality under wide-beam condition and are not in line with the development needs of SAS. We propose a non-linear chirp scaling algorithm (NCSA) for wide beam multi-receiver SAS. Firstly, the point target reference spectrum (PTRS) of each receiver is obtained by the Lagrange inversion theorem (LIT), and then the under-sampled signal in the azimuth frequency domain is obtained through azimuth spectrum extension; Then, considering the cubic term of range frequency in the PTRS and the linear variation of equivalent frequency modulation slope with range, each receiver is imaged using the NCSA, and coherent superposition is performed in the azimuth frequency domain to eliminate spectrum aliasing caused by azimuth spectrum extension; Finally, the azimuth inverse transform is performed on the superimposed signal to obtain the focusing imaging. Computer simulation experiments and field data verify that this method is superior to the existing SAS imaging algorithm, improving the quality of wide-beam imaging, avoiding the interpolation operation of the traditional range-Doppler algorithm, and saving computation cost.

An omega‐k algorithm for multireceiver synthetic aperture sonar

AbstractIn this Letter, the authors present a novel imaging algorithm for multireceiver synthetic aperture sonar (SAS). The Loffeld's bistatic formula (LBF) including quasi‐monostatic (QM) and multireceiver deformation (MD) phases is first reformulated into range‐variant and range‐invariant phases. After compensating the range‐invariant phase, the subsequent steps are the extended Stolt mapping for each bistatic SAS. Based on simulations, the presented method can also provide high‐resolution result which is similar to that of back projection (BP) algorithm.

Synthetic aperture imagery for high-resolution imaging sonar

Synthetic aperture sonar (SAS) can provide high-resolution underwater images. Traditional fast imaging algorithms designed for multi-receiver synthetic aperture sonar (MSAS) are complex because the point target reference spectrum (PTRS) deduction and imaging algorithm development are complicated. This paper proposes an imaging algorithm for the MSAS system to solve this issue. The proposed method first approximates the two-round slant range based on the phase center approximation method. The PTRS, including the quasi-monostatic and bistatic deformation terms, can be easily deduced. After compensating for the bistatic deformation term based on the interpolation and complex multiplication with the preprocessing step, the MSAS imagery can be simplified to the focus of the traditional monostatic SAS. Therefore, the conventional imaging algorithms designed for traditional monostatic SAS can be used directly. The proposed method providing high-resolution imaging results is more efficient than the traditional methods.

INVESTIGATION ON THE CONSTRUCTION OF THE RECEIVED SIGNAL MODEL FOR MULTI-RECEIVER SYNTHETIC APERTURE SONAR IN VIETNAM’S SEA

In this article,the received signal models are constructed for multi-receiver synthetic aperture sonar (SAS) using CW pulses and LFM pulses when considering the change of acoustic velocity in seawater and the Doppler effect. With a more complete description of the physical processes than the traditional models and the mathematical representation of signals, the proposed model can provide data sets for the research and thedevelopment of SAS imaging algorithms to save time and cost in actual data collection. The simulation results illustrate the characteristics of the signals generated from the proposed model with real data of acoustic velocity in Vietnam’s sea.

Investigation on beamforming solution for multi-receiver synthetic aperture sonar using CW pulse with sound velocity profiles in Vietnam’s sea

This paper proposes a novel beamforming solution for multi-receiver synthetic aperture sonar (SAS), which uses gated continuous-wave (CW) pulses, based on the average value of equivalent sound velocity (ESV) when considering the change of the sound velocity with depth. The main beam can be steered to desirable positions with the proposed solution. Besides, the computation time for the phase distribution from the proposed solution reduces compared with the conventional solution by dividing the propagation trajectory into straight lines, where the sound velocity is unchanged. The effectiveness of the proposed solution is validated and evaluated by the results of the beam pattern and the time for determining the phase distribution with the sound velocity profiles (SVPs) in Vietnam’s sea.

Back Projection Algorithm for Multi-Receiver Synthetic Aperture Sonar Based on Two Interpolators

The back projection (BP) algorithm is characterized by its high performance for multi-receiver synthetic aperture sonar (SAS). For this reason, it is usually used to evaluate the imaging performance of Fourier-domain methods. However, this algorithm suffers from a large computation load, and the imaging efficiency is seriously lowered. In order to improve the imaging performance, this paper proposes focusing the multi-receiver SAS data using the BP algorithm based on two interpolators, including the linear interpolation and nearest-neighbor interpolation. The former interpolation is used to decrease the interpolation error based on adjacent sampled data; the latter estimates the data at the desired moment by assigning the data value of the nearest sample as estimated data. Then, the imaging performance of the presented method is discussed in detail based on simulations and real-data processing. With the presented method, the imaging performance can be improved without a loss of efficiency compared to nearest-neighbor interpolation without an upsampling operation. In comparison with the traditional BP algorithm, the presented method can be used to improve the imaging efficiency without any loss of performance.

Efficient imaging method for multireceiver SAS

With the Loffeld's bistatic formula, the point target reference spectrum (PTRS) of multireceiver synthetic aperture sonar (SAS) can be decomposed into the quasi monostatic term and bistatic deformation (BD) term. The former term is analogous to the monostatic SAS PTRS while the latter one describes the bistatic phase introduced by the bistatic displacement between the transmitter and receiver. The multireceiver SAS echo signal can be transformed into monostatic SAS equivalent data after correcting the BD term. Based on the range sub-block processing method, this data transformation is conducted by using the preprocessing step. Then, fast imaging processors based on monostatic SAS system can be directly exploited to process the monostatic SAS equivalent data. In this paper, the authors mainly focus on the application of non-linear chirp scaling algorithm. At last, the simulations and real data are used to validate the presented method. The processing results show that the imaging performance of presented method is nearly close to that of traditional method. Besides, the imaging efficiency is highly improved compared to the phase centre approximation (PCA) method.

Multireceiver Synthetic Aperture Sonar Chirp Scaling Algorithm Considering Intrapulse Doppler Shift

In synthetic aperture imaging, the interpulse Doppler shift is commonly used, while the intrapulse Doppler shift is ignored. However, for the linear frequency modulation signal, the intrapulse Doppler shift can cause an additional range cell migration (ARCM) and a phase error term. Additionally, the ARCM may exceed a range resolution cell in certain cases of multireceiver synthetic aperture sonar (SAS), which will result in image defocus. Therefore, the intrapulse Doppler shift in multireceiver SAS should not be ignored in such cases. In this study, the condition of the negligible intrapulse Doppler shift is presented, that is, the moving distance of the sonar platform within a pulse should be much less than an azimuth resolution cell. Based on the multireceiver accurate time-delay model, a multireceiver SAS intrapulse Doppler shift signal model is developed by introducing the intrapulse Doppler frequency into the echo signal. After simplifying the accurate range history under the narrow-beam assumption, the range-Doppler spectrum is derived. Finally, a multireceiver SAS chirp scaling algorithm considering the intrapulse Doppler shift (IDS-CSA) is derived. The proposed algorithm can correct the ARCM and compensate the phase error (although not required), and then obtain a well-focused synthetic aperture image. The effectiveness of the proposed model and algorithm is verified by simulations and field data. The results show that the proposed algorithm provides a better image quality than the conventional multireceiver CSA when the condition of the negligible intrapulse Doppler shift is not met.

An Improved Imaging Algorithm for Multi-Receiver SAS System with Wide-Bandwidth Signal

When the multi-receiver synthetic aperture sonar (SAS) works with a wide-bandwidth signal, the performance of the range-Doppler (R-D) algorithm is seriously affected by two approximation errors, i.e., point target reference spectrum (PTRS) error and residual quadratic coupling error. The former is generated by approximating the PTRS with the second-order term in terms of the instantaneous frequency. The latter is caused by neglecting the cross-track variance of secondary range compression (SRC). In order to improve the imaging performance in the case of wide-bandwidth signals, an improved R-D algorithm is proposed in this paper. With our method, the multi-receiver SAS data is first preprocessed based on the phase center approximation (PCA) method, and the monostatic equivalent data are obtained. Then several sub-blocks are generated in the cross-track dimension. Within each sub-block, the PTRS error and residual quadratic coupling error based on the center range of each sub-block are compensated. After this operation, all sub-blocks are coerced into a new signal, which is free of both approximation errors. Consequently, this new data is used as the input of the traditional R-D algorithm. The processing results of simulated data and real data show that the traditional R-D algorithm is just suitable for an SAS system with a narrow-bandwidth signal. The imaging performance would be seriously distorted when it is applied to an SAS system with a wide-bandwidth signal. Based on the presented method, the SAS data in both cases can be well processed. The imaging performance of the presented method is nearly identical to that of the back-projection (BP) algorithm.

A New Imaging Geometry Model for Multi-receiver Synthetic Aperture Sonar Considering Variation of The Speed of Sound in Seawater

This paper proposes a new imaging geometry model for multi-receiver synthetic aperture sonar (SAS). The model considers the change of the speed of sound in seawater, the effect of platform movement on the acoustic velocity vector (AVV), and the Doppler effect. Based on the proposed model, a solution to determine the phase distribution was generated to improve the SAS image quality. The simulation results demonstrate the merits of proposed model compared to the traditional models that consider the speed of sound in seawater as a fixed value, ignore the change of AVV during transmission, and suppress the Doppler effect.

Multireceiver SAS Imagery Based on Monostatic Conversion

To use monostatic based imaging algorithms for multireceiver synthetic aperture sonar, the monostatic conversion is often carried out based on phase centre approximation, which is widely exploited by multireceiver SAS systems. This article presents a novel aspect for dealing with the multireceiver SAS imagery, which still depends on the idea of monostatic conversion. The approach in this article is based on Loffeld's bistatic formula that consists of two important terms, i.e., quasi monostatic and bistatic deformation terms. Our basic idea is to preprocess the bistatic deformation term and then incorporate the quasi monostatic term into an analogous monostatic spectrum. With this new spectrum, traditional imaging algorithms designed for monostatic synthetic aperture sonar can be easily exploited. In this article, we show that Loffeld's bistatic formula can be reduced to the same formula as spectrum based on phase centre approximation when certain conditions are met. Based on our error analysis, the maximum error magnitude of PCA method is about 1 rad, which would noticeably affect the SAS imagery. Fortunately, the error magnitude of presented method can be always kept within ${{\rm{\pi }} \mathord{/ {\vphantom {{\rm{\pi }} 4}} \kern-\nulldelimiterspace} 4}$ . It means that Loffeld's bistatic formula provides a more accurate approximation of the spectrum compared to that based on phase centre approximation. After that, this article develops a new imaging scheme and presents imaging results. Based on quantitative comparisons, the presented method well focuses multireceiver SAS data, and it provides better image compared to phase centre approximation method.

High‐resolution imaging for the multireceiver SAS

The area coverage rate is dramatically improved by using the multireceiver technique. However, this configuration leads to the complexity of the synthetic aperture image formation. In order to solve this issue, the target based on the phase centre approximation (PCA) is to be reconstructed. The data related to the multireceiver SAS system is coerced into the data like traditional monostatic synthetic aperture sonar (SAS) system. Then, the data can be processed by traditional imaging methods. In this paper, the chirp scaling algorithm is exploited as an example. Moreover, the influence of some important steps with the monostatic conversion, i.e. pulse compression in the range dimension and the micro range cell migration correction (RCMC), is further discussed in detail. The simulations show that the presented method can focus the target well.

BP algorithm for the multireceiver SAS

The imaging performance and efficiency are two important issues for the multireceiver synthetic aperture sonar (SAS). Back projection (BP) algorithm is characterised by the high performance and low efficiency. In this study, the authors first recall the standard BP algorithm based on the interpolation for the multireceiver SAS. Then, two improved BP algorithms based on the range Fourier transformation (FT) are presented. Considering the fact that the time delay in the time domain can be carried out by the phase shifting in the frequency domain, an FT shifting based BP algorithm avoiding the interpolation error is presented. Although this method produces the focusing result with high performance, it is very time consuming. In order to improve the imaging efficiency without loss of imaging performance, the authors propose an oversampling-based BP algorithm, which is based on the fact that the zero-padding in the frequency domain is equivalent to the interpolation in the time domain. After that, the computation complexity of three BP algorithms is analysed in detail. Finally, simulations and real data are exploited to validate the presented methods.

An Imaging Algorithm for Multireceiver Synthetic Aperture Sonar

For the multireceiver synthetic aperture sonar (SAS), the point target reference spectrum (PTRS) in the two-dimensional (2D) frequency domain and azimuth modulation in the range Doppler domain were first deduced based on a numerical evaluation method and accurate time delay. Then, the difference between the PTRS and azimuth modulation generated the coupling term in the 2D frequency domain. Compared with traditional methods, the PTRS, azimuth modulation and coupling term was better at avoiding approximations. Based on three functions, an imaging algorithm is presented in this paper. Considering the fact that the coupling term is characterized by range variance, the range-dependent sub-block processing method was exploited to perform the decoupling. Simulation results showed that the presented method improved the imaging performance across the whole swath in comparison with existing multireceiver SAS processor. Furthermore, real data was used to validate the presented method.

Imaging Algorithm for Multireceiver Synthetic Aperture Sonar

This paper presents a range migration algorithm for the multireceiver synthetic aperture sonar (SAS) based on Loffeld’s bistatic formula, which consists of the quasi monostatic term and the bistatic deformation (BD) term. It is expected that the multireceiver data can be easily focused by the monostatic SAS imaging algorithm after compensating the BD term. In this paper, the BD term is first decomposed into the range variant phase and the range invariant phase. At this point, the BD term is compensated via two steps. One step is the compensation of the range variant phase through the range-dependent sub-block processing in the 2-D frequency domain. The other one is the arrangement of the multireceiver data in a sequential order in the 2-D time domain. After these operations, we obtain the monostatic SAS equivalent data, which is viewed as the input of the traditional range migration algorithm. The processing results of the simulated data and the real data show that the proposed method can work well for the focusing of the multireceiver SAS data. More importantly, the presented method dramatically improves the imaging efficiency without loss of performance since the complex multiplication and the data arrangement are only exploited by the presented method.

Focusing Multireceiver SAS Data Based on the Fourth-Order Legendre Expansion

Using Legendre polynomials and the derivation method of the implicit function, this paper proposes a range-Doppler algorithm for multireceiver synthetic aperture sonar. Based on Legendre polynomials, the two-way slant range is first expanded into a power series with respect to the slow time. Then, the derivation method of the implicit function is exploited to deduce the point of stationary phase and point target reference spectrum (PTRS). Based on this PTRS, the paper presents an imaging algorithm, which uses the range-dependent sub-block processing method to cancel the space-variant coupling between the range and azimuth dimensions. Simulation results and real data processing are presented to validate the proposed method.

A Nonlinear Chirp Scaling Algorithm for Processing Multi-Receiver SAS Data with Small Squint Angle

Squint multi-receiver synthetic aperture sonar (SAS) data is more challenging to process than data about its common side-looking counterpart because the geometry is complicated. While traditional multi-receiver algorithms mainly handle general side-looking cases, they do not work well for squint cases. Up to now, no multi-receiver algorithm for handling squint multi- receiver SAS data has been reported. This paper focuses on an imaging algorithm for the squint multi-receiver SAS. Firstly, the “non-stop-go-stop” signal model is established for squint multi- receiver SAS. Then a squint multi-receiver nonlinear chirp scaling algorithm (SMANLCSA) is proposed that is suited to process multi-receiver SAS data with small squint angle and SMANLCSA involves three key steps. The first step is to turn squint multi-receiver data into squint single-receiver data. The second step is a Doppler centroid frequency shift. The third step is range cell migration correction using nonlinear chirp scaling. Simulations have shown that SMANLCSA can handle squint multi-receiver data more accurately and effectively than the previous algorithm..

An Indirect Range-Doppler Algorithm for Multireceiver Synthetic Aperture Sonar Based on Lagrange Inversion Theorem

This paper presents a novel imaging algorithm which has the capacity of processing multireceiver synthetic aperture sonar (SAS) data based on Vu’s spectrum. There are two important issues when developing the fast Fourier-based imaging algorithms. One is the point target reference spectrum (PTRS) with analytical expression in the 2-D frequency domain; the other is the space variance of the PTRS. Vu has solved the first issue based on the Lagrange inversion theorem. However, the second issue has not been solved yet. Our approach in this paper is to apply Vu’s spectral result in an indirect range-Doppler (R-D) imaging algorithm. For each transmitter/receiver pair, the close-form azimuth modulation is derived first, and then, range/azimuth coupling can be obtained via the difference between the PTRS and the azimuth modulation. Unlike traditional R-D algorithm, which utilizes the interpolation operation to eliminate the range cell migration (RCM), the RCM correction (RCMC) in this paper is carried out through two steps, i.e., bulk RCMC and differential RCMC. Bulk RCMC accounts for the RCMC in one reference range. Differential RCMC accomplished with a range-dependent subblock postprocessing method is used to compensate for the space variant deviation phase error. Via the proposed method, the echoed signal corresponding to each transmitter/receiver pair is processed first. Following which, a high-resolution SAS complex image is obtained by coherent superposition of all the coarse subimages. Numerical simulations and real data processing results show that the proposed method has the similar focusing capabilities with the accurate back projection algorithm across the whole swath. Although the presented method has the disadvantage of latency, it is very easy to make a complete phase center approximation correction, allowing for the tandem nature of the collected data. Besides, the proposed algorithm can handle general case with explicit point of stationary phase (PSP) no matter how complicated the PSP is, and it is also suitable for the imagery of tandem synthetic aperture radar.

Multireceiver Correction for the Chirp Scaling Algorithm in Synthetic Aperture Sonar

The phase center approximation (PCA) and the motion of the receiver array during reception are two main conditions that must be solved to use fast-Fourier-based imaging techniques in multireceiver synthetic aperture sonar (SAS). The former issue affects focusing results at close range while the latter one influences imaging performances in far distance. The processing result is that the uncompensated phase errors, introduced by those two issues, would degrade SAS imaging quality seriously. Through the analysis of outgoing and incoming acoustic paths, this paper derives an improved PCA method, considering the influence under those two approximations, describes the residual effects of narrow-beam approximations for PCA and “stop-and-hop”(S&H) errors in 2-D space domain, and presents imagery processed with chirp scaling (CS) algorithm. Based on the simulation and lake-trial results, the new processing method provides image quality better than conventional algorithms.