A synthetic aperture sonar system and method with adjustable down-angle

Abstract

This invention relates to the field of synthetic aperture sonar technology, and discloses a synthetic aperture sonar system and method with adjustable downward viewing angle. The invention establishes the relative spatial relationship between the bottom and the surface of the water, and based on an iterative optimization algorithm for ray tracing, it senses and predicts the propagation path and angle of the echo signal in advance, providing accurate target incident angle correction, evaluating the optimal downward viewing angle of the sonar system, and simultaneously using an angle-adjustable device to adjust the downward viewing angle of the sonar system's transducer array, thereby achieving suppression of surface reverberation by the synthetic aperture sonar system. This system significantly improves the responsiveness to environmental changes, ensures the imaging quality of the synthetic aperture sonar system, and enhances stable operation in complex underwater environments, avoiding false alarms caused by environmental changes, thus achieving high-precision target detection and localization.

Description

Technical Field

[0001] This invention relates to the field of synthetic aperture sonar technology, specifically to a synthetic aperture sonar transducer array under-view optimization system, and more particularly to a synthetic aperture sonar system and method with adjustable under-view. Background Technology

[0002] Synthetic Aperture Sonar (SAS) is a high-resolution sonar imaging technology that uses multiple receiver array elements and phase compensation techniques to image underwater targets. The core advantage of the SAS system lies in its high-resolution imaging capability, which can accurately depict the shape and features of underwater objects.

[0003] However, in the complex marine exploration environment, especially when operating in shallow water, multipath effect is one of the core factors leading to the degradation of imaging quality. Its essence is the superposition interference of direct waves and reflected waves generated during the transmission and reception of sound waves. The typical characteristics of shallow water areas are shallow water depth and strong reflective interfaces such as the seabed and sea surface. In addition to direct waves (which propagate directly to the target and are reflected back to the receiving array), the sound waves emitted by synthetic aperture sonar also generate multiple reflection path waves, mainly including sea surface-seabed secondary reflection waves (sound waves are first reflected from the sea surface to the seabed and then reflected to the target, or reflected from the seabed to the sea surface and then to the target), sea surface / seabed single reflection waves (sound waves are directly reflected from the sea surface or seabed and then reach the target), and multiple reflection waves (sound waves travel back and forth between the sea surface and the seabed multiple times before reaching the target). These sound waves with different paths are received by the sonar at the same time as the direct waves. Due to the different propagation path lengths, their propagation delay, phase, and amplitude are different, and they eventually coherently superimpose at the receiving end, forming multipath interference. The specific degradation of SAS images by multipath effect is manifested in many aspects.

[0004] First, there are target ghost images and artifacts. Multipath reflected waves can be misjudged by sonar as echoes from a certain "false target", forming a ghost image in the imaging results that is offset from the position of the real target. For example, a real target on the seabed will appear as a "mirror image" artifact in the image due to the interference of sea surface reflected waves. Moreover, the position and shape of the artifact will change with the movement of the sonar platform, and in severe cases, it will cover the outline of the real target.

[0005] Secondly, the image contrast and signal-to-noise ratio decrease. Multipath interference waves are coherent noise that will be superimposed on the echo signal of the real target. This will cause the signal energy of the target area to be diluted by noise, reduce the grayscale difference between the real target and the background, and make the overall image blurry, making it difficult to distinguish between weak targets and the seabed background.

[0006] Third, resolution degradation. The high resolution of synthetic aperture sonar depends on the phase coherence and Doppler information of the echo signal. The phase distortion of multipath waves will destroy the coherent accumulation process of the signal, resulting in a decrease in azimuth and range resolution. The originally clear target edges become blurred and fine structures are lost.

[0007] Fourth, the target positioning error increases. SAS target positioning is based on the propagation delay and phase information of the echo to calculate the distance and azimuth. The time delay offset of the multipath wave will cause the target distance calculated by the sonar to deviate. At the same time, the change of the incident angle of the reflected wave will introduce the azimuth angle error, which ultimately leads to a significant reduction in the positioning accuracy of the real target.

[0008] Fifth, image stripes and periodic fringes. When the sonar platform moves along the route, the length of the multipath path changes periodically with the platform's position, and the phase of the interference wave also fluctuates periodically. This periodic change will form bright and dark stripes or bands distributed along the azimuth in the imaging results, further destroying the uniformity of the image. Compared with deep water areas, the multipath effect in shallow water environments is more significant. This is because the distance between the sea surface and the seabed is shorter in shallow water, and sound waves are more likely to be reflected multiple times. The energy attenuation of reflected waves is smaller, and the interference intensity is higher. At the same time, the sound velocity profile in shallow water areas is more complex (affected by temperature, salinity, and ocean currents). The curvature of sound rays will further change the multipath propagation path and increase the randomness of interference. Moreover, the seabed in shallow water is mostly composed of strong scattering media such as mud, sand, and reefs, which will enhance the energy of seabed reflected waves and exacerbate multipath interference.

[0009] Therefore, it is necessary to design a synthetic aperture sonar system and method with adjustable downward viewing angle to alleviate and solve the problem of multipath interference in shallow water sonar. Summary of the Invention

[0010] This invention overcomes the shortcomings of the prior art and provides a synthetic aperture sonar system and method with adjustable downward viewing angle.

[0011] To achieve the above objectives, the technical solution adopted by this invention is: a method for controlling synthetic aperture sonar with adjustable downward viewing angle, comprising:

[0012] Step S1: Deploy a receiving array on the measurement platform to establish the incident angle of the sonar platform and the relative spatial relationship between the bottom and the surface of the water;

[0013] Step S2: Monitor the echo signal of the receiving array in real time to determine the nominal incident angle deviation of the sound wave reaching the receiving array;

[0014] Step S3: Based on the nominal incident angle deviation of the receiving array and combined with the sound tracking model, calculate the target under-view correction amount of the echo signal receiving array and generate a deflection command.

[0015] Step S4: The controller performs downward viewing angle adjustment according to the deflection command and simultaneously starts virtual phase center reconstruction. It calculates the spatial coordinate displacement of the array elements caused by the deflection in real time and dynamically corrects the coherence compensation parameters in the synthetic aperture imaging process.

[0016] In a preferred embodiment of the present invention, step S2 includes: calculating the phase difference between adjacent array elements, and combining the preset array element spacing and signal wavelength, using the interferometric angle measurement formula to calculate the actual physical incident angle of the echo signal reaching the depth of the receiving array.

[0017] The deviation of the nominal angle of incidence is obtained by performing real-time difference calculation between the actual physical angle of incidence and the nominal angle of incidence.

[0018] In a preferred embodiment of the present invention, in step S2, determining the nominal incident angle deviation includes:

[0019] The echo signal of the receiving array is monitored in real time. By calculating the phase difference between adjacent array elements and combining the preset array element spacing and signal wavelength, the actual physical incident angle of the echo signal reaching the depth of the receiving array is calculated using the interferometric angle measurement formula.

[0020] The deviation of the nominal angle of incidence is obtained by performing real-time difference calculation between the actual physical angle of incidence and the nominal angle of incidence.

[0021] In a preferred embodiment of the present invention, in step S3, the target downward viewing angle correction amount of the echo signal receiving array is calculated using a ray tracking algorithm, including:

[0022] Obtain the sound velocity profile data of the current water area, divide the water layer into N horizontally equal-thickness thin layers, and obtain the sound velocity gradient of each thin layer. ,in, This represents the sound velocity of the k-th thin layer. Indicates the depth of this thin layer;

[0023] Using the measured echo angle of arrival and the local sound velocity at its depth from the receiving array, the ray constant of the sound beam in this acoustic environment is calculated according to Snell's law.

[0024] Using the ray constant as a path constraint and ray acoustics theory, the cumulative horizontal displacement and curvature change of the sound wave as it propagates upward from the bottom of the water to the depth of the receiving array are calculated. By iteratively integrating the refraction path of each thin layer, the true continuous geometric trajectory of the sound beam in the non-uniform medium is restored.

[0025] Based on the restored geometric trajectory, the instantaneous vector direction of the sound beam when it reaches the depth of the receiving array is calculated and denoted as the predicted incident angle. The predicted incident angle is then subjected to vector difference operation with the preset ideal viewing angle of the receiving array under the assumption of straight-line propagation, and the nominal incident angle deviation is output.

[0026] In a preferred embodiment of the present invention, in step S3, generating a deflection command includes:

[0027] Based on the aforementioned ray tracing model, combined with the current water depth and sound speed profile, the theoretical optimal incident vector when the sound beam propagates from the target area to the depth of the receiving array is calculated, and the direction of this vector is recorded as the predicted incident angle.

[0028] The predicted incident angle is compared with the current physical axis of the receiving array, and the angle between the two is calculated as the target downward viewing angle correction amount.

[0029] The controller generates deflection commands for the receiving array based on the target's downward viewing angle correction.

[0030] In a preferred embodiment of the present invention, in step S4, the virtual phase center reconstruction includes:

[0031] Establish a local coordinate system with the rotation axis of the receiving array as the origin. Based on the angle correction in the deflection command, use the rotation matrix to calculate the three-dimensional spatial displacement vector of each physical element in the array relative to its original position. ;

[0032] Based on formula Calculate the two-way acoustic path phase error caused by deflection for each array element, where Where λ is the wavelength of the sound wave, and n is the unit vector in the line-of-sight direction of the sound wave;

[0033] In the beamforming stage of synthetic aperture imaging, the phase error is superimposed on the echo data of the corresponding array element in the form of a conjugate phase, thus canceling the nonlinear drift of the phase center caused by physical deflection at the algorithm level.

[0034] In a preferred embodiment of the present invention, the controller performs the following functions when adjusting the viewing angle:

[0035] Adjust the physical elevation angle of the receiving array according to the deflection command to change the physical coverage area of ​​the sonar beam;

[0036] By adjusting the receiving weighted delay time of each element in the receiving array, the electronic beam can be deflected rapidly at the microsecond level to compensate for the transient lag error of the mechanical actuator during operation.

[0037] In a preferred embodiment of the present invention, the coherence compensation parameters are dynamically corrected, including: real-time monitoring of the phase center overlap change caused by the deflection of the receiving array;

[0038] When the deflection angle causes a change in the equivalent phase center spacing between adjacent pulses, the azimuth resampling grid spacing in the synthetic aperture processing algorithm is dynamically adjusted, and the lever arm parameter in the motion compensation module is updated.

[0039] In a preferred embodiment of the present invention, the time gap within the pulse repetition period of the sonar system is used as a time delay window; the time delay window is used to compensate for the system response time required for calculating deflection commands and downward angle modulation.

[0040] A synthetic aperture sonar control system with adjustable downward viewing angle, comprising:

[0041] Measurement platform;

[0042] The main imaging emission array is mounted on the measurement platform;

[0043] A receiving array, mounted on the measurement platform and coaxially arranged with the main imaging transmitting array, is used to capture wavefront distortion caused by changes in the acoustic environment in advance at deep water levels.

[0044] A controller, connected to the receiving array, is configured to:

[0045] Real-time monitoring of the echo signal from the receiving array is used to determine the nominal incident angle deviation of the sound wave reaching the receiving array.

[0046] Based on the nominal incident angle deviation and combined with the ray tracking model, the target viewing angle correction amount for the echo signal reaching the receiving array is calculated, and a deflection command is generated; and

[0047] The lower viewing angle of the receiving array is adjusted by the lower viewing angle adjustment mechanism according to the deflection command, and the virtual phase center reconstruction is started simultaneously. The spatial coordinate displacement of the array elements caused by the deflection is calculated in real time, and the coherence compensation parameters in the synthetic aperture imaging process are dynamically corrected.

[0048] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0049] This invention provides a synthetic aperture sonar control method and system with adjustable downward viewing angle. By establishing the relative spatial relationship between the bottom and the surface of the water, and utilizing the time difference of sound wave propagation in the vertical water layer to capture wavefront distortion caused by the environment, it can acquire propagation information from different angles before the echo signal reaches the receiving array. This allows for advance perception and prediction of the propagation path and angle of the echo signal, thereby providing the receiving array with accurate target incident angle correction. This significantly improves the system's responsiveness to environmental changes, ensures the imaging quality of the synthetic aperture sonar system, enhances the stable operation of the synthetic aperture sonar system in complex underwater environments, avoids false alarms caused by environmental changes, and thus achieves high-precision target detection and positioning.

[0050] This invention provides a synthetic aperture sonar control method and system with adjustable downward viewing angle. By reconstructing the virtual phase center, the phase and distance compensation can be updated in real time when the angle of the receiving array is adjusted, thereby ensuring the accurate alignment of the echo signal, avoiding phase error and image distortion caused by array deflection, and improving the imaging quality of the synthetic aperture sonar system. Especially in complex waters and changing environmental conditions, it can still provide clear and stable images and realize real-time dynamic adjustment.

[0051] This invention is based on the detection feature of real-time comparison between the known emission angle and the measured reception angle of the array. By extracting the nominal incident angle deviation and combining it with the ray tracking model for cross-depth path prediction, it eliminates the beam pointing inaccuracy caused by sound speed gradient refraction, and further improves the detection stability and terrain mapping accuracy of sonar in the harsh environment of variable sound speed tiers. Attached Figure Description

[0052] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0053] Figure 1 This is a flowchart of a preferred embodiment of the present invention for controlling a synthetic aperture sonar with an adjustable downward viewing angle;

[0054] Figure 2 This is a timing diagram of a synthetic aperture sonar system with adjustable downward viewing angle according to a preferred embodiment of the present invention. Detailed Implementation

[0055] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0056] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0057] In the description of this application, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the scope of protection of this application. Furthermore, the terms "", "", etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, features defined with "", "", etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0058] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art will understand the specific meaning of the above terms in this application based on the specific circumstances.

[0059] Application Overview:

[0060] In existing technologies, the downward angle of view of sonar systems is usually set to a fixed value at the factory or before deployment. However, the sound velocity profile in actual operating sea areas is non-uniformly distributed due to the influence of temperature, salinity, and pressure, causing the sound rays to refract and bend. This bending effect causes the actual incident angle of the echo signal to deviate from the center of the array's physical main lobe, resulting in a loss of receiving gain and even target loss. Although some advanced systems have introduced electronic beam deflection, the adjustment process often has serious lag due to the lack of real-time measurement and perception of distortions in the deep-water environment. Furthermore, frequent movements of the physical array can disrupt the phase center consistency required for synthetic aperture imaging.

[0061] This invention constructs a depth diversity dual-receiver array architecture, utilizes a deep-water array to capture wavefront distortion in advance and combines it with a ray tracking model for feedforward prediction, and employs virtual phase center reconstruction technology to achieve dynamic and precise alignment of the lower viewpoint while ensuring high resolution of coherent imaging.

[0062] For ease of implementation, the key objects involved in this embodiment are first uniformly defined:

[0063] A measurement platform refers to an underwater vehicle or towed vehicle that carries a main imaging transmitting array, receiving array, controller, attitude and positioning measurement device and downward viewing angle adjustment mechanism. It can be an autonomous underwater vehicle, a towed body, an ROV or other underwater measurement platform.

[0064] The measurement platform has available instantaneous position, heading, and attitude information during operation. The attitude information includes at least pitch angle, roll angle, and heading angle.

[0065] A receiving array is a main receiving array used for synthetic aperture imaging. It is coaxially arranged with the main imaging transmitting array or installed with a fixed geometric relationship to receive the main echo data used for imaging processing.

[0066] In this invention, the receiving array has adjustable viewing angle, preferably with the receiving array as a whole physically pitched around a preset rotation axis; when a segmented structure is used, it can also be physically pitched around a preset rotation axis by array sub-modules that can rotate independently as a whole. To avoid ambiguity, the change in receiving array angle in this specification refers to the change in the physical pitch angle of the whole or the whole sub-module, and not to pure electronic beam deflection achieved only through delay weighting.

[0067] Simultaneously, the receiving array can be used as an auxiliary receiving array for early detection of wavefront distortion in deep water. By receiving the echo and sampling its spatial phase distribution, the actual physical angle of incidence and the nominal angle of incidence deviation can be estimated, thus providing a feedforward observation starting point for ray tracking and deflection command generation.

[0068] The acoustic environment refers to the set of environmental factors that cause distortion of the sound wave propagation wavefront, including at least the effects of changes in sound velocity profile, turbulence disturbance, and bubbles on sound wave propagation. Changes in sound velocity profile are related to changes in water temperature, salinity, and pressure with depth, which cause continuous refraction of sound rays, forming a nonlinear, curved propagation path.

[0069] The nominal angle of incidence refers to the direction angle of echo propagation, determined by the known emission geometry of the transmitting array and the platform attitude, under the assumption of straight-line propagation in an ideal homogeneous medium.

[0070] The actual physical incident angle refers to the actual propagation direction angle of the echo when it reaches the receiving array after being subjected to the action of the real acoustic environment.

[0071] The nominal angle of incidence deviation is the difference or vector difference between the actual physical angle of incidence and the nominal angle of incidence, which characterizes the degree of deviation caused by the nonlinear bending of the sound ray from the angle of incidence.

[0072] The downward angle refers to the pitch angle of the main response direction of the receiving array relative to the platform's reference coordinate system, used to describe the angle at which the array is facing downwards for observation.

[0073] The target-angle correction refers to the angle correction that needs to be applied to make the main response direction of the receiving array consistent with the predicted true echo incident direction.

[0074] The virtual phase center refers to the equivalent observation reference point introduced in the synthetic aperture imaging processing chain to maintain phase reference consistency between pulses.

[0075] Virtual phase center reconstruction refers to the process of using geometric modeling and phase compensation to map the echo data back to the un-deflected phase center position at the algorithm level when the spatial coordinates of the array elements change due to physical pitch adjustment of the receiving array. This is done to counteract the nonlinear drift of the phase center caused by physical deflection and restore the azimuth coherence accumulation conditions of the synthetic aperture.

[0076] The beamforming front stage refers to the processing stage located before azimuth synthesis or focusing processing, and which can apply phase compensation on an element-by-element or channel-by-channel basis. It may include processing steps such as pulse compression, matched filtering, channel equalization, primary motion compensation, and range resampling.

[0077] This invention addresses synthetic aperture sonar imaging missions in complex marine exploration environments. Synthetic aperture sonar acquires multi-pulse echoes by moving along a measurement platform along its trajectory, performing phase compensation and coherent superposition to achieve high azimuth resolution. However, in actual underwater acoustic propagation, the water medium exhibits significant inhomogeneity. Temperature, salinity, and pressure vary with depth, forming a sound velocity profile that causes the sound wave propagation path to undergo continuous refraction and nonlinear bending according to ray acoustics. The direct consequence is a significant deviation between the actual propagation angle of the echo signal reaching the receiving array and the ideal angle calculated based on a straight-line propagation model. This leads to a misalignment between the main lobe direction of the receiving array and the direction of echo energy propagation, resulting in decreased array gain, reduced target echo signal-to-noise ratio, and, in severe cases, imaging instability, target loss, or increased positioning errors.

[0078] If existing technologies rely solely on fixed array geometry and linear propagation assumptions, it is difficult to maintain beam alignment in scenarios where the sound velocity profile changes rapidly. If only electronic beam deflection is used, although the receiving direction can be changed quickly, the lack of advance observation of deep-water wavefront distortion and cross-depth propagation prediction can easily lead to hysteresis compensation problems where it is too late to adjust when the deviation is observed.

[0079] If a mechanical method is used to physically adjust the array to achieve large-angle alignment, it will introduce changes in the spatial coordinates of the array elements and phase center drift, thereby disrupting the phase reference consistency of the synthetic aperture and causing azimuth defocus.

[0080] Together, these constitute the core problem that this invention attempts to solve: to achieve real-time alignment of the lower viewing angle under complex sound velocity profile conditions, and to maintain phase consistency of the coherent accumulation of the synthetic aperture under array physical angle adjustment conditions.

[0081] To address this, the present invention provides a synthetic aperture sonar system with adjustable lower viewing angle. By establishing the relative spatial relationship between the bottom and the surface of the water and utilizing the time difference of the echo propagation in the vertical water layer to form a propagation delay window, the system can capture wavefront distortion and estimate the nominal incident angle deviation in advance before the echo reaches the receiving array. Then, by combining the ray tracking model, the deviation observed in the deep water level is mapped to the depth of the receiving array, outputting the target lower viewing angle correction amount and generating deflection commands, thereby realizing dynamic alignment of the lower viewing angle of the receiving array.

[0082] While completing energy alignment, this invention further introduces virtual phase center reconstruction to calculate and conjugate compensate for the two-way acoustic path phase error caused by the displacement of array elements due to the overall physical angle adjustment of the receiving array, and dynamically updates the coherence compensation parameters, thereby offsetting the geometric changes and phase errors caused by the angle adjustment and avoiding defocusing of synthetic aperture imaging.

[0083] Example 1: As Figure 1 and Figure 2 As shown, a synthetic aperture sonar system with adjustable downward viewing angle includes the following steps:

[0084] Step S1: Deploy a receiving array on the measurement platform to establish the incident angle of the sonar platform and the relative spatial relationship between the bottom and the surface of the water;

[0085] Step S2: Monitor the echo signal of the receiving array in real time to determine the nominal incident angle deviation of the sound wave reaching the receiving array;

[0086] Step S3: Based on the nominal incident angle deviation of the receiving array, use the ray tracking algorithm to calculate the target downward viewing angle correction of the echo signal receiving array and generate a deflection command.

[0087] Step S4: The controller performs downward viewing angle adjustment according to the deflection command and simultaneously starts virtual phase center reconstruction. It calculates the spatial coordinate displacement of the array elements caused by the deflection in real time and dynamically corrects the coherence compensation parameters in the synthetic aperture imaging process.

[0088] In the specific implementation of step S1, at least a main imaging transmitting array, a receiving array, a controller, an attitude and positioning measuring device, and a downward viewing angle adjustment mechanism are installed on the measurement platform.

[0089] The receiving array and the main imaging transmitting array are preferably coaxially arranged to ensure that the transmitting and main receiving geometric references are consistent, thereby reducing system deviations caused by installation errors.

[0090] To ensure the system has sufficient time for calculations and mechanical actions, this embodiment utilizes the time intervals within the pulse repetition cycle of the sonar system as a time delay window. The controller is configured to adjust the physical viewing angle using the time delay window after receiving the current pulse echo and completing the angle calculation, before the next pulse is emitted or the imaging accumulation window opens.

[0091] To reduce error propagation, calibration can be performed using a combination of static calibration in a water tank and dynamic calibration during sea trials. Static calibration is used to determine geometric dimensions, while dynamic calibration is used to correct the offset between the attitude sensor and the array pointer.

[0092] The wavefront distortion caused by changes in the acoustic environment in step S1 includes at least the wavefront phase fluctuations caused by factors such as changes in sound speed, turbulence, and bubbles.

[0093] In the specific implementation of step S2, the controller monitors the echo signal of the receiving array in real time and calculates the nominal incident angle deviation. To ensure that the angle estimation is reproducible, the controller performs synchronous sampling and time alignment on the echo of each element of the receiving array. The time alignment can be completed based on the system common clock or synchronous trigger signal.

[0094] To improve the signal-to-noise ratio and suppress out-of-band noise, the controller performs bandpass filtering and matched filtering or pulse compression processing on the array element signals, and performs time gating according to the target range window, selecting the complex envelope data within the range gate where the target echo is located as the angle estimation object.

[0095] During angle estimation, the controller calculates the phase difference between adjacent array elements. Let the distance between adjacent array elements be d, the wavelength of the operating signal be λ, and the phase difference between the received signals of adjacent array elements be Δφ. Then, the actual physical angle of incidence of the echo signal reaching the depth of the receiving array can be calculated using interferometric angle measurement relationships.

[0096] To improve noise immunity, the controller can perform weighted least-squares fitting on the phase differences of multiple adjacent array element pairs. The fitting result characterizes the spatial phase gradient of the wavefront across the array aperture, thereby obtaining an estimated incident angle. Since seabed scattering, multipath propagation, and reverberation may cause phase disturbances, this embodiment preferably combines the statistical stability of the phase differences of multiple sampling points within the range gate to screen reliable samples, or performs robust estimation of the phase difference sequence to reduce the impact of outliers.

[0097] After estimating the actual physical incident angle, the controller calculates the nominal incident angle, which is determined by the array geometry and platform attitude under the assumption of rectilinear propagation. It is at least related to the transmission direction of the main imaging transmitting array, the instantaneous pitch and roll of the platform, and the preset ideal viewing angle of the receiving array.

[0098] The controller performs real-time differential calculations between the measured actual physical angle of incidence and the nominal angle of incidence to obtain the nominal angle of incidence deviation. .

[0099] The nominal angle of incidence deviation is the amount of deflection of a sound ray relative to its straight-line propagation as it reaches the receiving array. It can be used as the initial observation for subsequent sound ray tracking. To ensure the continuity and stability of the nominal angle of incidence deviation when used for feedforward control, the controller can perform time smoothing on the nominal angle of incidence deviation. The smoothing method can be moving average, exponential smoothing, or Kalman filtering. The length of the smoothing window should be constrained by the propagation delay window t to avoid reducing the feedforward angle adjustment efficiency due to excessive lag introduced by smoothing.

[0100] In the specific implementation of step S3, considering that the change in sound speed is related to underwater temperature, salinity, and pressure, the formation of a sound speed profile causes sound rays to refract and undergo nonlinear bending. Based on the nominal incident angle deviation and the sound ray tracking algorithm, the controller establishes a sonar echo quality model, calculates the further refraction and deflection during the propagation of the sound beam to the depth of the receiving array, obtains the predicted incident angle when the sound beam reaches the receiving array, and calculates the target under-view angle correction to generate a deflection command.

[0101] The reason why this step must introduce a ray tracking model is that the receiving arrays are at different depths, and the sound rays will continue to be deflected by the sound speed gradient between the two arrays. If the nominal incident angle deviation is directly used as the correction amount for the receiving array, systematic errors will occur under conditions of strong nonlinearity in the sound speed profile or the presence of a layer jump, causing the receiving array to still be unable to align with the true echo direction.

[0102] To construct the sonar echo quality model, the controller divides the water layer between the receiving array and the seabed into N horizontal thin layers of equal thickness along the depth direction, with each thin layer having a thickness of z, satisfying N=H / z.

[0103] Divide the water layer into N horizontal, equally thick layers, and obtain the sound velocity gradient of each layer as follows: ,in, This represents the sound velocity of the k-th thin layer. Indicates the depth of this thin layer;

[0104] The value of z should balance computational accuracy and real-time performance, allowing the sound velocity within the thin layer to vary approximately linearly. The controller determines the local sound velocity Ck and sound velocity gradient gk for each thin layer, where k is the thin layer index. Ck can be calculated from temperature, salinity, and depth data or obtained through sound velocity profile interpolation, while gk can be obtained from the sound velocity difference between adjacent thin layers. To maintain model updates during rapid environmental changes, the controller can update the Ck sequence at a preset period, or trigger a more frequent update when a rapid change in the nominal incident angle deviation is detected.

[0105] Ray tracing employs ray acoustics principles and uses the ray constant as a path constraint. The controller calculates the ray constant based on Snell's law, utilizing the measured echo angle of arrival and local sound velocity at the depth of the receiving array. .

[0106] With K as a constraint, the controller performs iterative integration on each thin layer to calculate the changes in the propagation direction of the sound beam within the thin layer, the cumulative horizontal displacement and curvature changes, and accumulates them to obtain the true continuous geometric trajectory of the sound beam as it deflects upward from the bottom of the water to the depth of the receiving array.

[0107] Subsequently, based on the reconstructed geometric trajectory, the controller calculates the instantaneous vector direction of the sound beam when it reaches the depth of the receiving array, denoted as the predicted incident angle. To ensure that this predicted incident angle can be directly used to control the angle adjustment of the receiving array, the controller also needs to obtain the current direction of the receiving array's physical axis. The current direction of the receiving array's physical axis can be determined jointly by the array installation calibration angle, the platform attitude, and the angle feedback from the downward viewing angle adjustment mechanism. When the receiving array is in an adjustable angle state, it is preferable to use an angle encoder to obtain the actual pitch angle of the array and convert it to the platform coordinate system.

[0108] The controller compares the predicted angle of incidence with the current physical axis of the receiving array and calculates the angle between the two. , This refers to the target's downward viewing angle correction. The controller generates deflection commands based on the target's downward viewing angle correction.

[0109] The deflection command includes a dynamic compensation component, which is used for fine-tuning during the mechanical response or for constraining angular velocity and angular acceleration to avoid mechanical shock.

[0110] The generation of deflection commands must meet the constraints of the actuator's maximum angular velocity, maximum angular acceleration, mechanical travel limit, and platform attitude stability. The controller can apply slope limits or use S-shaped acceleration / deceleration curves to the target downward view correction, making pitch adjustment smooth and continuous, and avoiding additional noise or signal phase stability caused by array vibration.

[0111] This embodiment achieves feedforward prediction and downward viewing angle alignment for the incident angle deviation caused by the nonlinear bending of the acoustic ray, significantly reducing the risk of received energy loss caused by the huge deviation between the actual incident angle and the ideal angle of the straight line model, so that the main response direction of the receiving array is aligned with the true echo incident direction before the echo arrives, thereby improving the imaging signal-to-noise ratio and target detection stability.

[0112] In the specific implementation of step S4, the controller performs downward viewing angle adjustment according to the deflection command. Preferably, the controller first drives the mechanical servo mechanism to adjust the physical pitch angle of the receiving array according to the deflection command, thereby changing the physical coverage area of ​​the sonar beam. During the mechanical servo operation, due to the transient lag in the servo response, the controller can further adjust the receiving weighted delay time of each element of the receiving array to achieve microsecond-level rapid deflection of the electronic beam, thereby compensating for the transient lag error of the mechanical actuator during the operation.

[0113] At the same time, the controller synchronously initiates virtual phase center reconstruction, calculates the spatial coordinate displacement of the array elements due to deflection in real time, and dynamically corrects the coherence compensation parameters in the synthetic aperture imaging process to avoid the physical angle adjustment of the receiving array from destroying the coherent accumulation conditions of the synthetic aperture.

[0114] It should be noted that, in order to ensure that the wavefront distortion captured in advance by the receiving array can be stably converted into a nominal incident angle deviation that can be used for control, and further stably mapped into the target downward viewing angle correction amount of the receiving array.

[0115] First, the controller should limit the processing of the receiver array echo in each pulse cycle to within the propagation delay window t, and preferably complete at least one effective target down-view correction update before the echo arrives at the receiver array.

[0116] Secondly, if changes in platform speed or operating altitude alter the echo propagation delay window, the controller should synchronously update the estimate of t and adjust the processing cycle to ensure that feedforward control always has a time margin.

[0117] Third, the angle estimation of the receiving array should preferably be completed within the same range gate to ensure that the phase difference calculation corresponds to the same target scattered echo. The echo phases of different range gates and different paths should not be mixed, otherwise it will cause the angle estimation deviation.

[0118] To reduce the impact of multipath and reverberation on angle estimation, this embodiment preferably sets multiple candidate range gates for the range-to-echo signal and uses consistency constraints to select a reliable range gate. Specifically:

[0119] The controller can acquire the amplitude peak or energy concentration region in the range direction after pulse compression and set a main range gate around this region; at the same time, it sets several bypass range gates to evaluate the multipath energy ratio.

[0120] When the phase difference sequence within the main range gate shows a stable trend across multiple consecutive sampling points and is significantly different from the phase difference distribution of the bypass range gate, the main range gate is determined to be a reliable echo gate and used for angle estimation. If the phase difference of the main range gate fluctuates drastically and is highly similar to the phase difference of the bypass range gate, significant multipath interference is determined to exist, and the controller can reduce the weight of the nominal incident angle deviation in this period or trigger stronger smoothing and robust estimation.

[0121] By using the aforementioned distance gate consistency screening, the nominal incident angle deviation can remain usable even in scenarios with complex seabed topography and significant multipath propagation.

[0122] In the estimation of the nominal incident angle deviation, in order to avoid ambiguity in the angle estimation caused by the limited array aperture, this embodiment preferably adopts a multi-pair phase difference fitting method rather than using only a single adjacent array element pair.

[0123] For example, suppose the receiving array has M elements in a certain dimension, and the coordinates of the elements are... m is the array element number. This represents the position coordinates of the array element along the array axis. The controller obtains the complex envelope signal for each array element within the main distance gate. Its phase is .

[0124] The controller constructs an objective function that minimizes the linear trend of the wavefront phase with respect to the array element coordinates. ,in Here, k represents the wavefront spatial phase slope, and k represents the weight. This is the phase intercept.

[0125] The incident angle is estimated by solving for k and combining it with the wavelength λ and the array element coordinate scale. The advantage of this method is that it can improve robustness by utilizing multi-element information in the presence of noise, and can adjust the array elements to handle abnormal noise in some areas. Reduce its impact. Weight The weight of array elements with larger echo amplitudes is related to the amplitude of the echoes, so as to reduce the contamination of phase slope estimation by low signal-to-noise array elements.

[0126] Example 2: In synthetic aperture sonar imaging systems, high-resolution imaging in the azimuth direction is highly dependent on the phase consistency of the echo signal during the virtual aperture synthesis process.

[0127] In this embodiment, in order to compensate for the incident angle deviation caused by the nonlinear bending of the sound velocity profile, the receiving array must perform a physical-level pitch deflection according to the deflection command.

[0128] However, the physical deflection action is a nonlinear motion centered on the array's rotation center, causing significant geometric displacements in three-dimensional space for each receiving element other than the center. This displacement deviation can be converted into a large phase jump through the two-way acoustic path effect. If the physical displacement caused by the active adjustment of the viewing angle is not reconstructed and compensated, the imaging algorithm will be unable to distinguish whether the phase change is caused by the seabed target or by the array adjustment action. This results in coherent cancellation during azimuth matched filtering, directly leading to severe defocusing, blurring, and geometric distortion in the sonar image.

[0129] Therefore, reconstructing the deflected real array position through a virtual phase center and virtually pushing it back to the ideal motion trajectory is a key prerequisite for achieving dynamically adjustable viewing angles without sacrificing imaging quality.

[0130] Therefore, this type of problem needs to be solved by reconstructing the phase center.

[0131] Under the condition that the receiving array is physically pitched around a preset rotation axis as a whole, a geometric model of the array attitude change is established, the two-way acoustic path phase error caused by the three-dimensional displacement of the array elements is calculated, and conjugate phase compensation is applied to each array element channel in the pre-beamforming stage so that the compensated echo data is equivalently mapped back to the ideal phase center trajectory when the angle is not adjusted on the phase reference. At the same time, the azimuth resampling grid spacing and motion compensation lever arm parameters are dynamically updated to ensure that the synthetic aperture coherent accumulation condition still holds during the dynamic angle adjustment process, thereby avoiding imaging defocus and geometric distortion.

[0132] In this embodiment, the angle adjustment of the receiving array is clearly defined as the overall physical pitch adjustment. The receiving array, as a whole, or as a subarray module that can be rotated independently, changes pitch angle around a preset rotation axis.

[0133] The pre-set rotation axis is fixed to the array mounting frame, and its center position and axial direction are determined and recorded during the installation and calibration phase. The physical pitch angle change is given by the deflection command output by the controller. The controller can also obtain the actual pitch angle feedback value from the angle encoder for closed-loop updates in the compensation calculation. For simplicity, θ represents the actual pitch angle change of the receiving array at the current moment in the following geometric modeling.

[0134] Virtual phase center reconstruction first establishes a local coordinate system. This local coordinate system uses a right-handed Cartesian coordinate system with the rotation axis of the receiving array as its origin. To ensure consistency with platform motion calculations, the x-axis can be defined as the platform's forward direction, the y-axis as the platform's lateral rightward direction, and the z-axis as the vertical downward direction. The relationship between the local coordinate system and the platform coordinate system is obtained through installation calibration and saved as a fixed transformation in the controller. With this setting, the positions of array elements before and after array angle adjustment can be expressed within a unified coordinate framework.

[0135] In this local coordinate system, the initial position coordinates of each element in the receiving array are measured and stored during the system calibration phase.

[0136] Wherein, the initial position coordinates of the i-th array element are , where i is the element number and T represents transpose.

[0137] The overall physical pitch angle of the array can be adjusted by the rotation matrix. describe. Used to map the positions of array elements before angle adjustment to the positions of array elements after angle adjustment.

[0138] If the axis of rotation coincides with one of the coordinate axes of the local coordinate system, then The standard rotation matrix of this axis can be used; when the rotation axis does not coincide with the coordinate axis, It can be constructed using an axis-angle configuration.

[0139] This embodiment uses a unit vector u with a preset rotation axis and an angle θ to construct... In this way. Let's assume... If we define a unit vector as the axis of rotation, then the rotation matrix can be constructed using Rodriguez's formula. In engineering implementation, the controller can store u along with the initial element coordinates and calculate it each time θ is updated. .

[0140] The controller gets Then, calculate the new position coordinates of each array element after angle adjustment. Its satisfaction Array element three-dimensional displacement vector . This represents the spatial displacement of the array elements due to physical angle adjustment, and is the direct input for subsequent calculations of sound path change and phase error.

[0141] Before calculating the phase error, it is necessary to define a unit vector n for the acoustic wave line of sight. n is used to describe the unit direction vector from the array to the target scattering center or imaging grid point, and it must be normalized to a unit length.

[0142] The value of n is determined by the geometry of the imaging grid: the controller constructs a difference vector based on the current position of the platform, the position of the array reference point and the spatial coordinates of the target grid point and normalizes it to obtain n.

[0143] The projection of the array element displacement along the line of sight is: Where "·" represents the dot product operation. Since sonar imaging involves two-way propagation, the change in path length for the round trip is approximately... .

[0144] This two-way path length variation will lead to a phase change. Its operating wavelength is λ, which is determined by the equivalent sound velocity C and the center frequency f, satisfying λ = C / f. Therefore, the two-way path length phase error of the i-th element is... The following relationship must be satisfied: ,in Let be the phase error caused by physical angle adjustment of the i-th element, λ be the wavelength of the sound wave, and n be the unit vector in the line-of-sight direction of the sound wave. Let i be the three-dimensional displacement vector of the i-th element. This represents the sensitivity of the phase to path changes in two-way propagation. Since λ is usually small, when f is high, even a small displacement can cause a significant phase change, therefore, it is necessary to calculate and compensate for this in real time. .

[0145] After obtaining the phase error, the virtual phase center reconstruction applies conjugate phase compensation before beamforming. The recovered echo data at a certain distance sampling point through the i-th element channel is... Where t is the time or distance sampling index, then the compensated echo data satisfy: .

[0146] in, For the compensated echo data, The imaginary unit, Represents an exponential function. This indicates that a conjugate phase opposite to the error is applied to compensate for the phase deviation caused by physical angle adjustment. This is used to counteract the nonlinear drift of the phase center caused by physical deflection, ensuring the coherence of synthetic aperture imaging.

[0147] To achieve dynamic correction of coherent compensation parameters, this embodiment defines the equivalent phase center of the receiving array for each pulse period. ,in, For beamforming weights or effective weights of array elements, The reference point for motion compensation and azimuth sampling of the synthetic aperture sonar; its constraints are: It should be consistent with the actual imaging processing, otherwise It will shift.

[0148] Phase center overlap is used to measure the relative consistency of the equivalent phase centers between adjacent pulses. It can be obtained by comparing the phase center spacing between adjacent pulses with the system's desired azimuth sampling spacing.

[0149] By reconstructing the virtual phase center, this embodiment achieves real-time cancellation of phase center drift under the condition of overall physical angle adjustment of the receiving array, and maintains the consistency of azimuth sampling and motion compensation by dynamically updating the coherent compensation parameters.

[0150] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for controlling synthetic aperture sonar with adjustable downward viewing angle, characterized in that, Includes the following steps: Step S1: Deploy a receiving array on the measurement platform to establish the incident angle of the sonar platform and the relative spatial relationship between the bottom and the surface of the water; Step S2: Monitor the echo signal of the receiving array in real time to determine the nominal incident angle deviation of the sound wave reaching the receiving array; Step S3: Based on the nominal incident angle deviation of the receiving array and combined with the sound tracking model, calculate the target downward viewing angle correction amount of the echo signal receiving array and generate a deflection command; wherein, calculating the target downward viewing angle correction amount specifically includes: S31. Obtain the sound velocity profile data of the current water area, divide the water layer into N horizontally equal-thickness thin layers, and obtain the sound velocity gradient of each thin layer as follows: ,in, This represents the sound velocity of the k-th thin layer. The depth of this thin layer is represented; the echo arrival angle and the local sound velocity at its depth are measured by the receiving array; the cumulative horizontal displacement and curvature change of the sound wave as it deflects upward from the bottom of the water to the depth of the receiving array are calculated to reconstruct the true continuous geometric trajectory of the sound beam in the non-uniform medium; based on the reconstructed geometric trajectory, the instantaneous vector direction of the sound beam when it reaches the depth of the receiving array is calculated and denoted as the predicted incident angle. S32. Compare the predicted incident angle with the direction of the current physical axis of the receiving array, and calculate the angle between the two as the target downward viewing angle correction amount; S33. Generate a deflection command for adjusting the viewing angle of the receiving array based on the target viewing angle correction amount; Step S4: Execute the lower viewing angle adjustment according to the deflection command, and simultaneously start the virtual phase center reconstruction. Calculate the spatial coordinate displacement of the array elements caused by the deflection in real time, and dynamically correct the coherence compensation parameters in the synthetic aperture imaging process.

2. The method for controlling synthetic aperture sonar with adjustable downward viewing angle according to claim 1, characterized in that: In step S2, the following steps are included: by calculating the phase difference between adjacent array elements, and combining the preset array element spacing and signal wavelength, the actual physical incident angle of the echo signal reaching the depth of the receiving array is calculated. The deviation of the nominal angle of incidence is obtained by comparing the actual physical angle of incidence obtained by measurement with the nominal angle of incidence.

3. The method for controlling synthetic aperture sonar with adjustable downward viewing angle according to claim 1, characterized in that: In step S4, the virtual phase center reconstruction includes: Establish a local coordinate system with the rotation axis of the receiving array as the origin, and calculate the three-dimensional spatial displacement vector of each physical element in the array relative to its original position using the rotation matrix based on the angle correction amount in the deflection command. Calculate the two-way acoustic path phase error of each array element caused by deflection based on three-dimensional spatial displacement vector calculation; In the beamforming stage of synthetic aperture imaging, the phase error is superimposed on the echo data of the corresponding array element.

4. The method for controlling synthetic aperture sonar with adjustable downward viewing angle according to claim 1, characterized in that: The downward perspective adjustment includes: Adjust the physical elevation angle of the receiving array according to the deflection command; The electronic beam deflection is achieved by adjusting the weighted delay time of each element in the receiving array.

5. The method for controlling synthetic aperture sonar with adjustable downward viewing angle according to claim 1, characterized in that: The coherent compensation parameters are dynamically corrected, including: real-time monitoring of changes in phase center overlap caused by receiver array deflection; When the deflection angle causes a change in the equivalent phase center spacing between adjacent pulses, the azimuth resampling grid spacing of the synthetic aperture is dynamically adjusted, and the lever arm parameters in the motion compensation module are updated.

6. The method for controlling synthetic aperture sonar with adjustable downward viewing angle according to claim 1, characterized in that: The time gap within the pulse repetition period of the sonar system is used as a time delay window; the time delay window is used to compensate for the system response time required to resolve deflection commands and down-angle modulation.

7. A synthetic aperture sonar control system with adjustable downward viewing angle, characterized in that, include: Measurement platform; The main imaging emission array is mounted on the measurement platform; A receiving array, mounted on the measurement platform and coaxially arranged with the main imaging transmitting array, is used to capture wavefront distortion caused by changes in the acoustic environment in advance at deep water levels. A controller, connected to the receiving array, is configured to: Real-time monitoring of the echo signal from the receiving array is used to determine the nominal incident angle deviation of the sound wave reaching the receiving array. Based on the nominal incident angle deviation and combined with the ray tracking model, the target downward viewing angle correction amount for the echo signal reaching the receiving array is calculated, and a deflection command is generated; wherein, calculating the target downward viewing angle correction amount specifically includes: Obtain the sound velocity profile data of the current water area, divide the water layer into N horizontally equal-thickness thin layers, and obtain the sound velocity gradient of each thin layer. ,in, This represents the sound velocity of the k-th thin layer. The depth of this thin layer is represented; the echo arrival angle and the local sound velocity at its depth are measured by the receiving array; the cumulative horizontal displacement and curvature change of the sound wave as it deflects upward from the bottom of the water to the depth of the receiving array are calculated to reconstruct the true continuous geometric trajectory of the sound beam in the non-uniform medium; based on the reconstructed geometric trajectory, the instantaneous vector direction of the sound beam when it reaches the depth of the receiving array is calculated and denoted as the predicted incident angle. The predicted incident angle is compared with the direction of the current physical axis of the receiving array, and the angle between the two is calculated as the target downward viewing angle correction amount; Generate a deflection command for adjusting the downward viewing angle of the receiving array based on the target downward viewing angle correction amount; and The lower viewing angle of the receiving array is adjusted by the lower viewing angle adjustment mechanism according to the deflection command, and the virtual phase center reconstruction is started simultaneously. The spatial coordinate displacement of the array elements caused by the deflection is calculated in real time, and the coherence compensation parameters in the synthetic aperture imaging process are dynamically corrected.

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