A deep-sea submarine planar array target positioning method based on multipath coherent focusing

Abstract

The present invention belongs to the fields of underwater acoustic array signal processing, underwater acoustic detection, underwater acoustic positioning, and relates to a deep-sea seabed planar array target positioning method based on multi-path coherent focusing. The method utilizes the physical property that sound sources in a direct sound zone reach the array mainly through direct paths and single-reflection paths from the sea surface. By establishing three-dimensional grid points of possible sound source positions at different angles, distances, and depths, the propagation time and amplitude corresponding to the direct path D and the single-reflection path S from each grid point to each array element under a given sound velocity profile are calculated. A new steering vector is constructed to perform beamforming to achieve coherent accumulation of multi-path sound energy. The grid point position corresponding to the peak of the ambiguity function is finally obtained, which is the position of the desired sound source. The present invention only requires measuring the sound velocity profile and is suitable for estimating the azimuth, distance, and depth of broadband and narrowband targets in the direct sound zone, with low computational complexity.

Description

Technical Field

[0001] The present invention belongs to the fields of underwater acoustic array signal processing, underwater acoustic detection, underwater acoustic positioning, and the like, and relates to a deep-sea seabed planar array target positioning method based on multi-path coherent focusing. Background Art

[0002] Deep-coastal arrays, deployed on the seafloor, offer high security and concealment, can be deployed in large numbers, and offer superior detection performance, making them a key means of passive underwater target detection. Target depth information is a key criterion for classifying surface and underwater targets. However, deep-coastal arrays currently face numerous difficulties in locating targets in the direct sound zone. Matching fields are a classic approach to address this problem, but they require a comprehensive understanding of the ocean environment. Using an underwater acoustic model, the amplitude and phase of the receiving array's sound field are calculated to form a copy field vector, which is then matched with the acoustic data received by the array to localize underwater targets. This method requires the calculation of the entire sound pressure field, which is computationally expensive in the case of a three-dimensional spatial grid of multi-frequency, broadband signals. In practical applications, even if the sound pressure copy field is calculated and stored in advance, this method is highly sensitive to environmental parameter mismatches, resulting in large positioning errors or even failure.

[0003] Deep-sea sound fields usually have obvious and distinguishable multi-path arrival structures. For example, in the deep-sea direct sound zone, the signals received by the hydrophone come from the direct path, the first reflection path on the sea surface, the first reflection path from the seabed to the sea surface, etc. Existing methods make full use of multi-path information to conduct positioning research on targets. For example, the Chinese patent "A method and device for underwater sound source detection and positioning based on deep-sea multi-path focusing" (patent number ZL 202411119049.9) discloses a method and device for underwater sound source detection and positioning based on deep-sea multi-path focusing. This method uses the spatial power spectrum estimation method to calculate the beam output sound field of the sound pressure signal radiated by the target sound source at different frequency points and different azimuth angles within a set frequency range; for any frequency point, the beam output sound field of the frequency point at two different azimuth angles is phase-compensated and added according to different assumed arrival delays to obtain the focusing result; the multi-path sound field focusing results of multiple frequency points are added to obtain a broadband multi-path three-dimensional sound field focusing result; the two angles and arrival delays corresponding to the maximum value of the broadband focusing result are compared with the template values under different assumed target distances and depths to determine the estimated values of the target sound source distance and depth. This method realizes the coherent superposition detection of deep-sea multipath signals, improving the detection and positioning capabilities of weak underwater sound sources in multipath environments. The method requires that the pitch angles of the direct sound line reaching the array and the sound line reflected once from the sea surface can be distinguished. However, the equivalent vertical aperture of the seabed planar array is limited, and the pitch angles of the two paths reaching the array are usually very close, making them difficult to distinguish.

[0004] The paper "Near-field Localization of Aerial Sound Sources by Deep-sea Horizontal Arrays" (Mao Junjie, Zhang Bo, et al., Acta Acoustica Sinica, Issue 1, 2025) utilizes near-field focused beamforming using sound rays from different paths—direct sound incident from air into the deep sea, sound rays reflected once from the seabed and once from the surface, and sound rays reflected twice from the seabed and twice from the surface. To address the problem of false peaks in the spatial spectrum, the spatial spectra corresponding to the multiple paths are weighted and summed, enabling the submarine horizontal array to locate aerial sound sources in the near-field Fresnel zone. However, this weighted summation of the spatial spectra is an incoherent summation of the multipath signal energy, which cannot improve the output signal-to-noise ratio and thus the weak target detection capability. Summary of the Invention

[0005] To address the existing problems of large computational complexity and large copy field storage requirements for passive target positioning using deep-sea planar arrays, the present invention proposes a deep-sea planar array target positioning method based on multi-path coherent focusing. This method utilizes the physical property that sound sources in the direct sound zone reach the array primarily through direct paths and single-reflection paths from the sea surface. By establishing three-dimensional grid points of possible sound source positions at different angles, distances, and depths, the propagation time and amplitude corresponding to the direct path D and single-reflection path S from each grid point to each array element under a given sound velocity profile are calculated. A new steering vector is constructed for beamforming to achieve coherent accumulation of multi-path sound energy. The grid point position corresponding to the peak of the ambiguity function is ultimately the location of the desired sound source. The present invention only requires measuring the sound velocity profile and is suitable for estimating the azimuth, distance, and depth of broadband and narrowband targets in the direct sound zone, with low computational complexity. At the same time, the present invention only requires storing the multi-path propagation time corresponding to the sound source center frequency in the search space. and amplitude As the parameter template required for the steering vector, the storage space is greatly reduced, which greatly saves hardware costs.

[0006] The specific steps of this method are as follows:

[0007] Step 1: Establish a three-dimensional rectangular coordinate system and obtain the sound pressure signal of the seabed plane array;

[0008] Step 2: Construct a three-dimensional search space grid of all possible locations of potential targets. Use the BELLHOP ray model to calculate the propagation time and amplitude corresponding to the direct path D and the single reflection path S from each grid point to each array element.

[0009] Step 3: At each frequency point, construct the corresponding multipath coherent focusing steering vector based on the searched sound source depth, horizontal distance, and horizontal azimuth, and perform normalization processing;

[0010] Step 4: Beamforming is performed for a given frequency point to obtain an ambiguity function to achieve coherent accumulation of multipath acoustic energy. The value of each point in the ambiguity function represents the multipath focused energy of the sound source at the corresponding grid point propagating along two paths to the array. The maximum value of the ambiguity function corresponds to the depth, horizontal distance, and horizontal azimuth of the desired sound source.

[0011] The form of beamforming is not limited to conventional beamforming, and may also include MVDR (Minimal Variance Distortion Response) method, etc. If the signal is wideband, wideband beam energy synthesis may be further performed to obtain a wideband ambiguity function.

[0012] The step 1 is specifically as follows:

[0013] Take any position on the sea surface as the coordinate origin O, establish a rectangular coordinate system with the east and north directions as the x-axis and y-axis respectively, and assume that the plane array has M array elements, and the coordinates of the mth array element are , where x m is the x-axis coordinate of the m-th array element, y m is the y-axis coordinate of the mth array element, H is the known sea depth. The depth of the sound source is z s , the horizontal distance and horizontal azimuth to the coordinate origin are r s and θ s , where the horizontal azimuth angle θ s It is defined as the angle between the sound source projected onto the xoy plane and the x-axis, and its value ranges from 0 degrees to 360 degrees.

[0014] Assume that the spectrum of the target sound source radiation signal received by the mth array element at frequency f is ,in , 、 The sound pressure signals received by the M array elements are expressed in vector form as an M×1-dimensional column vector:

[0015] (1)

[0016] The superscript T indicates the transpose of a vector.

[0017] The step 2 is specifically as follows:

[0018] Assume that the depth, horizontal distance and horizontal azimuth of the sound source in the search space are z, r and θ respectively, then its coordinates in the rectangular coordinate system can be written as When the sound source is located in the direct sound zone of the array, only the direct path D and the single reflection path S on the sea surface are considered. When the sound velocity profile and sea depth are known (the sound velocity profile and sea depth can be measured at the sea trial site using equipment such as a speed meter and a depth sounder), the sound source position and the receiver position (here, the mth array element) are substituted into the ray model BELLHOP to calculate the propagation time from the sound source through the direct path D and the single reflection path S on the sea surface to the mth array element. and amplitude ,in They correspond to the direct path D and the sea surface single reflection path S respectively.

[0019] When the sound line bending is not considered, that is, when linear propagation is assumed, the propagation distance of the sound pressure signal emitted by the actual sound source through the lth path to the mth array element is:

[0020] (2)

[0021] Among them .

[0022] At this time, the propagation time and amplitude of the sound pressure signal emitted by the actual sound source through the lth path to the mth array element can be approximated by the propagation law of spherical waves, that is:

[0023]

[0024] Where c is the speed of sound in seawater. Abbreviated as , Abbreviated as ;

[0025] Since the sound speed in the real ocean environment has a certain distribution along the depth, as the distance between the sound source and the receiver increases, formulas (2)-(4) no longer hold. In this case, the ray model BELLHOP is used to calculate the propagation time of the sound source in the search space through the direct path D and the sea surface single reflection path S to the mth array element. and amplitude More accurate, where the calculation uses the signal center frequency Once the calculation is complete, the propagation time and amplitude Store as a template and call it directly later. Matching field technology needs to calculate the sound pressure copy field for each frequency point. Under deep-sea conditions, the amount of calculation is very large and difficult to use for real-time calculation. The present invention uses the advantage that the multipath propagation time and multipath amplitude calculated by the ray model are insensitive to the signal frequency. For direct calls to broadband signals, the amount of calculation is greatly reduced. At the same time, the matching field needs to store the broadband complex sound pressure field received by multiple primitive arrays as matching templates, while the present invention only stores the multipath propagation time corresponding to the center frequency of the sound source in the search space. and amplitude , the storage space is greatly reduced, which greatly saves hardware costs.

[0026] The step 3 is specifically as follows:

[0027] Given the acoustic signal frequency f, the multipath coherent focusing steering vector is constructed based on the signal amplitude and propagation time of the direct acoustic path D and the first reflection acoustic path S on the sea surface under the actual sound source depth z, horizontal distance r and horizontal azimuth θ. : (5)

[0028] Steering vector Perform normalization:

[0029] (6)

[0030] represents the normalized steering vector;

[0031] The step 4 comprises:

[0032] The sound pressure signal received by the plane array Perform conventional beamforming processing to obtain a beam domain signal pointing to the target :

[0033] (7)

[0034] When the sound pressure signal is a narrowband signal, the energy of the output narrowband signal is The greatest advantage of the above processing is that it achieves coherent synthesis of multipath signals similar to matched field processing, thereby improving the signal-to-noise ratio.

[0035] The beamforming method in formula (7) is not limited to conventional beamforming, but may also include the MVDR (Minimal Variance Distortion Response) method, namely:

[0036] (8)

[0037] in is the cross-spectral density matrix of the observed data, Indicates the inversion of the matrix.

[0038] When the sound pressure signal is a broadband signal, broadband energy synthesis is further performed to obtain the broadband ambiguity function:

[0039] (9)

[0040] The value of each point in the narrowband or wideband ambiguity function represents the multipath focused energy of the sound source at the corresponding grid point propagating along two paths to the array. The maximum value of the ambiguity function corresponds to the depth, horizontal range, and horizontal azimuth of the desired sound source.

[0041] Traverse the search for the maximum value of the ambiguity function, and its corresponding position is the estimated value of the horizontal azimuth angle of the desired sound source , horizontal distance estimate , depth estimation :

[0042] .

[0043] Compared with the prior art, the advantages of the present invention are:

[0044] When a deep-sea plane array uses beamforming to estimate target parameters under a far-field plane wave model, it is affected by multipath interference such as the direct path D from the shallow source and the single-reflection sound line path S from the sea surface, making it difficult to estimate the target's pitch angle, distance, and depth. The present invention proposes a deep-sea seabed plane array target positioning method based on multipath coherent focusing. It uses a deep-sea multipath structure to improve the steering vector in the beamforming process: when calculating the propagation time and amplitude of the target reaching each element of the plane array through the D and S paths, the three factors of sound source depth, horizontal distance, and horizontal azimuth are considered. Therefore, the improved steering vector contains more target parameter information. After multipath beam focusing, not only can the depth, horizontal distance, and horizontal azimuth of the sound source be estimated simultaneously by solving the position corresponding to the maximum value of the ambiguity function. In addition, this method further realizes the energy coherent accumulation of deep-sea multipath path signals, enhances the maximum value of the ambiguity function, and is beneficial to the detection of weak targets in a multipath environment. In practical applications, the construction of the steering vector only requires the calculation of the multipath propagation time and amplitude at a single frequency point, which requires little computation and greatly reduces the template storage space, thus greatly saving hardware costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is the coordinate system used in the embodiment: Figure 1 (a) is a top view, in which the solid circle is a circular array. Figure 1 (b) is the sound velocity profile;

[0046] Figure 2This is a flow chart of a target positioning method using a deep-sea seabed horizontal array for multi-path coherent focusing;

[0047] Figure 3 is the three-dimensional broadband ambiguity function corresponding to the sound source: Figure 3 (a) is the three-dimensional broadband ambiguity function corresponding to sound source 1, Figure 3 (b) is the three-dimensional broadband ambiguity function corresponding to sound source 2;

[0048] Figure 4 is a slice of the broadband ambiguity function of sound source 1: Figure 4 (a) is the horizontal range-angle slice of the broadband ambiguity function of sound source 1, Figure 4 (b) is the depth-horizontal distance slice of the broadband ambiguity function of sound source 1;

[0049] Figure 5 is a slice of the broadband ambiguity function of sound source 2: Figure 5 (a) is the horizontal range-angle slice of the broadband ambiguity function of sound source 2, Figure 5 (b) is the depth-horizontal distance slice of the broadband ambiguity function of sound source 2;

[0050] Figure 6 The results of using this method to perform coherent synthesis of multiple paths for sound source 2 are compared with the results of using a traditional single path beamforming method: Figure 6 (a) is the result of the ambiguity function of this method in the distance-depth slice, Figure 6 (b) is the result of the ambiguity function on the distance-depth slice considering only the direct path, Figure 6 (c) is the result of the ambiguity function on the range-depth slice considering only the single reflection path on the sea surface. DETAILED DESCRIPTION

[0051] The present invention proposes a target positioning method suitable for multi-path energy coherent focusing of a deep-sea planar array. First, a deep-sea planar array with horizontal and vertical apertures is used to collect broadband sound pressure signals radiated by a target sound source. Then, all possible grid points of potential targets are searched, and the propagation time and amplitude corresponding to the direct path D and the single reflection path S from the sound source grid point to each array element at different depths, target horizontal distances, and target horizontal azimuths are calculated. The corresponding steering vector is constructed and normalized. Finally, beamforming is performed on each frequency point, and broadband energy synthesis is performed to obtain a broadband ambiguity function. The desired target is positioned according to the position corresponding to the maximum value of the ambiguity function.

[0052] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0053] Step 1: Establish a three-dimensional rectangular space coordinate system and obtain the sound pressure signal of the seabed plane array:

[0054] In the embodiment, without loss of generality, a uniform circular array of deep seabed is considered. Figure 1 As shown in (a), the receiving array has a total of 128 elements, which are deployed on the seabed with an element spacing of d=5m. The coordinate origin is projected at the center of the circular array, and element 1 is located on the x-axis. The array locates two sound sources at different positions, where the coordinates of sound source 1 and sound source 2 are (500m, 500m) and (1000m, 1000m), respectively, and the depths are both 80m. Therefore, the horizontal distances of the two sound sources to the origin are 707m and 1414m, respectively, and the horizontal azimuths relative to the x-axis are both 45°. Both sound sources radiate broadband continuous signals with a frequency range of 50Hz~160Hz. The selected ocean environment is an incomplete deep-sea sound channel environment with a sea depth of 1245m. The sound velocity profile is as follows Figure 1 (b) The speed of sound on the seabed is 1600 m / s and the density is 1.6 g / cm 3 , the absorption attenuation is 0.6dB / λ.

[0055] Step 2: Construct a three-dimensional search space grid of all possible locations of potential targets. Use the BELLHOP ray model to calculate the propagation time and amplitude corresponding to the direct path D and the single reflection path S from each grid point to each array element:

[0056] In the embodiment, the sound source distance search range is 0.01km-8km, the distance search interval is 0.01km; the sound source search depth range is 1m-150m, the depth search interval is 1m; the sound source angle range is 0~360°, and the angle interval is 1°. The calculation frequency is the sound source center frequency of 105Hz. According to the ocean environment parameters and array position corresponding to step 1, the position of each grid point in the search space is calculated. , combined with the ray acoustic field calculation program BELLHOP, the propagation time and amplitude of the potential target reaching the mth array element through the D and S paths are calculated as follows: and ,in These correspond to the D and S paths, respectively. Once calculated, they can be stored as propagation time and amplitude templates. This template is suitable for broadband signals and can be directly applied to different frequencies, significantly reducing the computational effort.

[0057] Step 3: At each frequency point, construct the corresponding multipath coherent focusing steering vector based on the searched sound source depth, horizontal distance, and horizontal azimuth, and perform normalization processing;

[0058] In the embodiment, the calculated value obtained in step 2 is and , select the frequency f of the signal of interest, substitute it into formula (5) and formula (6), and get the normalized steering vector .

[0059] Step 4: Beamforming is performed at a given frequency point to obtain a narrowband ambiguity function. This step achieves coherent accumulation of multipath acoustic energy. Beamforming is not limited to conventional beamforming and can also include methods such as MVDR (Minimal Variance Distortion Response). For broadband signals, broadband beam energy synthesis can be further performed to obtain a broadband ambiguity function. The value of each point in the narrowband or broadband ambiguity function represents the multipath focused energy of the sound source at the corresponding grid point propagating along two paths to the array. The maximum value of the ambiguity function corresponds to the depth, horizontal distance, and horizontal azimuth of the desired sound source.

[0060] In the embodiment, the sound source 1 and the sound source 2 are located respectively, and the implementation process is as follows: Figure 2 shown.

[0061] Figure 3 (a) is the three-dimensional broadband ambiguity function corresponding to sound source 1, Figure 3 (b) is the three-dimensional broadband ambiguity function corresponding to sound source 2, broadband ambiguity function The depth, target horizontal distance and target horizontal azimuth corresponding to the maximum value are the depth, horizontal distance and horizontal azimuth of the desired sound source. Figure 4 (b) shows the horizontal distance-angle slice and depth-horizontal distance slice of sound source 1 respectively. The maximum values of the ambiguity functions of the two sub-images can respectively obtain the estimated results of the target's distance, depth and horizontal azimuth angle as 700m, 80m and 45°. Figure 5 (a) and Figure 5 (b) shows the horizontal range-angle slice and depth-horizontal range slice of sound source 2. The maximum values of the ambiguity functions in the two sub-graphs can be used to estimate the target's range, depth, and horizontal azimuth angle to be 1410m, 80m, and 45°, respectively. The estimated values for sound sources 1 and 2 are in good agreement with the actual values, proving that this method can achieve three-dimensional positioning of underwater targets. To further illustrate the advantages of this method, we compare the results of the ambiguity function of this method in the range-depth slice ( Figure 6 (a)) and considering only direct paths ( Figure 6 (b)) or the sea surface primary reflection path ( Figure 6 (c)) is compared with the results of the ambiguity function on the distance-depth slice. It can be seen that when only a single path is considered, both the depth and distance estimation are ambiguous, while this method can accurately estimate the distance of the sound source 2. In addition, Figure 6 The maximum value of this method in (a) is -35.5dB, Figure 6 (b) and Figure 6(c) The maximum values obtained using only a single path are -37.6 and -37.9 dB, respectively. Therefore, the maximum beam output energy of this method is more than 2 dB higher than that using only a single path, demonstrating the advantage of coherent energy focusing of multiple paths.

Claims

1. A deep-sea bottom planar array target positioning method based on multipath coherent focusing, characterized in that: The specific steps of this method are as follows: Step 1: Establish a three-dimensional rectangular coordinate system and obtain the sound pressure signal of the seabed plane array; Step 2: Construct a three-dimensional search space grid of all possible locations of potential targets. Use the BELLHOP ray model to calculate the propagation time and amplitude corresponding to the direct path D and the single reflection path S from each grid point to each array element. Step 3: At each frequency point, construct the corresponding multipath coherent focusing steering vector based on the searched sound source depth, horizontal distance, and horizontal azimuth, and perform normalization processing; Step 4: Beamforming is performed for a given frequency point to obtain an ambiguity function to achieve coherent accumulation of multipath acoustic energy. The value of each point in the ambiguity function represents the multipath focused energy of the sound source at the corresponding grid point propagating along two paths to the array. The maximum value of the ambiguity function corresponds to the depth, horizontal distance, and horizontal azimuth of the desired sound source.

2. The deep-sea bottom planar array target positioning method based on multipath coherent focusing according to claim 1 is characterized in that: The step 1 is specifically as follows: Take any position on the sea surface as the coordinate origin O, establish a rectangular coordinate system with the east and north directions as the x-axis and y-axis respectively, and assume that the plane array has M array elements, and the coordinates of the mth array element are , where x m is the x-axis coordinate of the m-th array element, y m is the y-axis coordinate of the mth array element, H is the known sea depth; the sound source depth is z s , the horizontal distance and horizontal azimuth to the coordinate origin are r s and θ s , where the horizontal azimuth angle θ s It is defined as the angle between the sound source projected onto the xoy plane and the x-axis, ranging from 0 degrees to 360 degrees; Assume that the spectrum of the target sound source radiation signal received by the mth array element at frequency f is ,in , 、 are the lowest frequency and the highest frequency of the sound pressure signal respectively; the sound pressure signal received by the M array elements is expressed in vector form as an M×1-dimensional column vector: (1) The superscript T indicates the transpose of a vector.

3. The deep-sea bottom planar array target positioning method based on multipath coherent focusing according to claim 1 is characterized in that: The step 2 is specifically as follows: Assume that the depth, horizontal distance and horizontal azimuth of the sound source in the search space are z, r and θ respectively, then its coordinates in the rectangular coordinate system can be written as When the sound source is located in the direct sound zone of the array, only the direct path D and the sea surface first reflection path S are considered; when the sound velocity profile and sea depth are known, the sound source position and the receiver position are substituted into the ray model BELLHOP to calculate the propagation time of the sound source through the direct path D and the sea surface first reflection path S to reach the mth array element and amplitude ,in They correspond to the direct path D and the sea surface single reflection path S respectively; When the sound line bending is not considered, that is, when linear propagation is assumed, the propagation distance of the sound pressure signal emitted by the actual sound source through the lth path to the mth array element is: (2) in ; At this time, the propagation time and amplitude of the sound pressure signal emitted by the actual sound source through the lth path to the mth array element are approximated by the spherical wave propagation law, where c is the speed of sound in seawater: 。 4. The deep-sea bottom planar array target positioning method based on multipath coherent focusing according to claim 1 is characterized in that: The step 3 is specifically as follows: Given the acoustic signal frequency f, the multipath coherent focusing steering vector is constructed based on the signal amplitude and propagation time of the direct acoustic path D and the first reflection acoustic path S on the sea surface under the actual sound source depth z, horizontal distance r and horizontal azimuth θ. : (5) Steering vector Perform normalization: (6) represents the normalized steering vector.

5. The deep-sea bottom planar array target positioning method based on multipath coherent focusing according to claim 1 is characterized in that: The step 4 comprises: The sound pressure signal received by the plane array Perform conventional beamforming processing to obtain a beam domain signal pointing to the target : (7) When the sound pressure signal is a narrowband signal, the energy of the output narrowband signal is As a narrowband ambiguity function; when the sound pressure signal is a broadband signal, further broadband energy synthesis is performed to obtain the broadband ambiguity function: (8) The value of each point in the narrowband or broadband ambiguity function represents the multipath focused energy of the sound source at the corresponding grid point propagating along two paths to the array; the maximum value of the ambiguity function corresponds to the depth, horizontal range and horizontal azimuth of the desired sound source; Traverse the search for the maximum value of the ambiguity function, and its corresponding position is the estimated value of the horizontal azimuth angle of the desired sound source , horizontal distance estimate , depth estimation : 。 6. The deep seabed planar array target positioning method based on multipath coherent focusing according to claim 5 is characterized in that The beamforming form in formula (7) also includes the MVDR method, that is, (9) in is the cross-spectral density matrix of the observed data, Indicates the inversion of the matrix.

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