A method and system for positioning an underwater target at a known depth

Abstract

The present application provides a method and system for positioning an underwater target with a known depth. The method includes: using a cross array as a receiving array; two hydrophones in the horizontal direction form the first group of hydrophones, and two hydrophones in the vertical direction form the second group of hydrophones; receiving the pulse signal of the target sound source, and estimating the time difference of arrival by the method of quadratic cross-correlation; estimating the elevation angle of the target sound source according to the time difference of arrival of the second group of hydrophones; estimating the azimuth angle of the target sound source according to the elevation angle of the target sound source and the time difference of arrival of the first group of hydrophones; estimating the distance of the target sound source according to the depth and elevation angle of the target sound source. The advantages of the present application are as follows: in the case where only the depth of the target position is known and there is no communication between the sound source and the receiving array, the present application can accurately and quickly direction-find and position the target sound source. The present application can be widely applied to underwater passive sonar systems with known target depths and can be extended to weak signal detection systems.

Description

Technical Field

[0001] This application belongs to the field of underwater acoustic positioning, direction finding and passive sonar system design, and particularly relates to a positioning method and system for underwater targets with known depths. Background Art

[0002] Sonar positioning utilizes the characteristics of sound waves propagating in water to determine the position of a target object by measuring the time difference and propagation direction between the emission and reception of sound waves. According to the working principle, sonar systems are mainly divided into two categories: active sonar and passive sonar. The active sonar system actively emits sound wave signals and determines the position of the target by receiving the reflected echo signals. However, due to the integration of transmission and reception, the active sonar is more likely to be detected by the target ship and evaded during operation.

[0003] Compared with active sonar, passive sonar has obvious advantages in terms of concealment, flexibility and low power consumption. Since it does not need to actively emit signals, it can achieve "one-way" transparency in the underwater battlefield, and is particularly suitable for ships that cannot make sounds to expose themselves but need to detect target activities. Moreover, passive sonar can be deployed on the hull, underwater cables or buoys, etc., and is relatively convenient for installation and use. It is widely used in fields such as reconnaissance and early warning, surveying and mapping, fish activity monitoring, and marine environment monitoring. However, due to the lack of distance information of the active echo, passive sonar technology usually can only obtain the approximate azimuth of the target, and it is difficult to achieve precise positioning.

[0004] With the rapid improvement of the requirements for marine resource development and marine security defense, as well as the development of intelligent technology and adaptive systems, passive sonar target recognition is a prerequisite for ships to discover the enemy first, conduct covert attacks and win victories in naval battles, and is also a key technology for realizing the intelligence of other underwater offensive and defensive systems. In the 1970s, the successful development of passive ranging sonar was a milestone in the development of sonar technology. Currently, the commonly used passive positioning methods include triple ranging, target motion analysis (TMA) and matched field processing (MFP) methods. However, the triple passive sonar ranging method is limited to near-field positioning due to its positioning principle, and for distant sound sources, the positioning accuracy drops significantly. The target motion analysis method is suitable for solving long-range positioning problems. The commonly used one is pure bearing TMA, which requires high bearing measurement accuracy and accurately known own ship's track. In practice, the maneuvering behavior of the own ship may not be convenient or even not allowed sometimes because it may expose itself. The solution is to add new observables such as frequency, arrival time and time delay, etc., but its estimation quality improves with the increase of observation data, so a large amount of computation is required to improve the estimation accuracy. The ranging performance of matched field processing is extremely sensitive to sea depth, sound speed profile, seabed acoustic parameters and hydrophone depth, has a strong dependence on environmental parameters and generally requires the deployment of a relatively large-scale vertical array. Summary of the Invention

[0005] The purpose of this application is to overcome the defect of low positioning accuracy of the target sound source in the prior art.

[0006] To achieve the above object, this application proposes a method for positioning an underwater target with a known depth, including:

[0007] Using a cross array as the receiving array; the receiving array is placed perpendicular to the geodetic coordinate system; two hydrophones in the horizontal direction form the first group of hydrophones, and two hydrophones in the vertical direction form the second group of hydrophones;

[0008] Receiving the pulse signal of the target sound source, and estimating the time difference of arrival by the cross-correlation method;

[0009] Estimating the elevation angle of the target sound source according to the time difference of arrival of the second group of hydrophones;

[0010] Estimating the azimuth angle of the target sound source according to the elevation angle of the target sound source and the time difference of arrival of the first group of hydrophones;

[0011] Estimating the distance of the target sound source according to the depth and elevation angle of the target sound source.

[0012] As an improvement of the above method, the estimation of the time difference of arrival by the quadratic cross-correlation method includes:

[0013] Setting the window width = 2T according to the prior information, where T is the maximum width of the sound source pulse; performing a fast Fourier transform on the received signal window by window, detecting whether a pulse signal arrives through a set frequency domain peak threshold, and confirming the peak frequency of the pulse signal;

[0014] Performing a short-time Fourier transform on the time window when the pulse signal arrives to obtain the frequency range; combining the prior information to confirm the pulse width of the transmitted pulse Bandwidth B and center frequency f c ;

[0015] Adding a Doppler frequency shift to the transmitted pulse signal to obtain s(t), and performing a fast Fourier transform on it to generate a frequency domain replica signal S rep , S rep = DFT(s(t), N), where N is the number of points of the fast Fourier transform, N = fs×2×T, and fs is the sampling frequency;

[0016] Setting the concerned frequency range to the maximum possible value of the Doppler frequency shift difference between the upper and lower limits of the frequency band of the transmitted pulse signal;

[0017] Multiplying the received signal within the concerned frequency range with to obtain Z i，n , and obtaining the frequency domain result of the matched filtering of each channel of the pulse appearance frame; Indicates that the pulse signal appears in the nth frame, and the frequency-domain data received by the ith channel of the array;

[0018] Multiply the frequency-domain results of matched filtering for the first channel and the third channel to obtain: Y y = Z 3，n * Z′ 1，n ; ·′ represents matrix transpose;

[0019] Multiply the frequency-domain results of matched filtering for the second channel and the fourth channel to obtain: Y z = Z 4，n * Z′ 2，n ;

[0020] Perform a Chirp-Z transform (CZT) on the received signal within the frequency range of interest to obtain:

[0021] P y = CZT(Y y , M, ω, a)

[0022] P z = CZT(Y z , M, ω, a)

[0023] where M is the number of points of the CZT; a is the complex starting point, a = exp(-j×2πΔτ begin / N), [Δτ begin ,Δτ end is the range where the time difference of arrival may be located; ω = exp(j2π(Δτ end -Δτ begin ) / (NM));

[0024] Perform peak search on the CZT transform result. The peak of P y corresponds to the time difference Δt y when the signal arrives at the first group of hydrophones; The peak of P z corresponds to the time difference Δt z when the signal arrives at the second group of hydrophones.

[0025] As an improvement of the above method, estimating the elevation angle of the target sound source based on the time difference of arrival includes:

[0026]

[0027] where, represents the estimated value of the elevation angle of the target sound source; c represents the speed of sound; Δt z represents the time difference when the signal arrives at the second group of hydrophones; d represents the spacing between two array elements in the horizontal or vertical direction.

[0028] As an improvement of the above method, estimating the azimuth angle of the target sound source based on the pitch angle and time difference of arrival of the target sound source includes:

[0029]

[0030] wherein, represents the estimated value of the azimuth angle of the target sound source; represents the estimated value of the pitch angle of the target sound source; c represents the speed of sound; Δt y represents the time difference between the signals arriving at the first group of hydrophones; d represents the distance between two array elements in the horizontal or vertical direction.

[0031] As an improvement of the above method, estimating the distance of the target sound source based on the depth and pitch angle of the target sound source includes:

[0032]

[0033] wherein, R represents the distance of the target sound source; p z represents the depth of the target sound source; represents the estimated value of the pitch angle of the target sound source.

[0034] The present application also provides a positioning system for underwater targets with known depth, which is implemented based on the above method. The system includes:

[0035] A receiving array in the form of a cross array; the receiving array is placed perpendicular to the geodetic coordinate system;

[0036] A time difference of arrival estimation module, configured to process the pulse signals received by the receiving array and estimate the time difference of arrival through the cross-correlation method;

[0037] A pitch angle estimation module, configured to estimate the pitch angle of the target sound source according to the time difference of arrival;

[0038] An azimuth angle estimation module, configured to estimate the azimuth angle of the target sound source according to the pitch angle and time difference of arrival of the target sound source;

[0039] A target sound source distance estimation module, configured to estimate the distance of the target sound source according to the depth and pitch angle of the target sound source.

[0040] Compared with the prior art, the advantages of the present application are:

[0041] The present invention innovatively proposes a positioning method for underwater targets with known depth, using a cross array as the receiving array and a moving target sound source near the water surface as the transmitting sound source. In the case where only the depth of the target position is known and communication between the sound source and the receiving array is not possible, the present application can accurately and quickly determine the direction and position of the target sound source. The present application can be widely applied to underwater passive sonar systems with known target depth and can be extended to weak signal detection systems. Description of the Drawings

[0042] Figure 1 Shown is a target positioning scenario diagram;

[0043] Figure 2 Shown are the time-domain and time-frequency domain schematic diagrams of the pulse signal arrival window;

[0044] Figure 3 Shown is the schematic diagram of the received signals of 4 channels;

[0045] Figure 4 Shown is the time-frequency diagram of the data received by element 1;

[0046] Figure 5 Shown is the output diagram after the cross-correlation of channel 1 and channel 3 through matched filtering;

[0047] Figure 6 Shown is the flow chart of the method for positioning a target with a known depth underwater. Detailed Embodiment

[0048] The technical solution of the present application will be described in detail below with reference to the drawings.

[0049] During the passive sonar target recognition process, the cross array, as a typical small-aperture array, is more suitable for the current situation where the application requirements of small sonar platforms are increasing. Aiming at the problem that when using passive sonar technology for positioning, due to the lack of synchronization between the platform and the target, the time difference between the transmitted signal and the received signal cannot be determined. The present application provides a method for detecting a target with a known depth underwater. By estimating the time delay between the elements of the cross-shaped array to obtain the azimuth and combining the target depth to determine the target distance and position, the direction finding and positioning of a moving target can be realized. This working mode has little influence on the sound source system, has high concealment, strong flexibility, greatly reduces the amount of calculation and improves the positioning accuracy, and has strong operability and good application prospects.

[0050] Aiming at the positioning and tracking requirements of underwater moving targets, the present application uses a cross array as the receiving array and uses the pulse signal of the target for passive positioning. This mode uses the target depth as prior information, does not require synchronization between the target and the receiving platform, and both can be in motion, which is particularly suitable for the positioning and tracking of near-surface targets.

[0051] Suppose the target sound source is placed under a surface ship or other platform, and a pulse signal is transmitted. The receiving array is installed on an underwater mobile object such as a UUV, or can also be installed on a fixed platform such as a moored buoy. It should be noted in particular that the pulse signal transmitted by the target sound source is not completely known a priori information for the receiving platform. The receiving platform may know several possible forms of the pulse, such as LFM signal / CW signal, as well as possible pulse widths, frequencies, bandwidths, periods, etc. of the pulse. However, the specific signal actually used, the accurate radial velocity between the target and the platform is unknown, and the starting time of the transmitted pulse cannot be obtained. Therefore, the method of time-of-arrival estimation cannot be used to obtain the distance between the platform and the target. Moreover, the signals received by the platform and the transmitted pulses will inevitably have Doppler frequency shifts as well as pulse compression and broadening. Therefore, the method proposed in this application mainly estimates the azimuth of the target based on the time difference of arrival between array elements, and then combines the target depth information for positioning.

[0052] 1. Array design;

[0053] Since the cross array has the advantages of simple structure, convenient installation, and the ability to perform two-dimensional azimuth estimation simultaneously, we design the receiving array as a cross array, with a total of four receiving hydrophones. According to simulation analysis, this method is particularly suitable for small-aperture designs, and the vertical and horizontal apertures are set to be greater than 0.5 times the wavelength. Taking a center frequency of 5 kHz as an example, the distance between the 4 array elements and the array center is d / 2, which can be set to not less than 0.1 m.

[0054] 2. System scenario construction;

[0055] The array is placed perpendicular to the geodetic coordinate system. Let the center of the receiving array be the origin, and the normal direction of the array be the x-axis direction. The distance between the sound source and the array center is R. The position of the target sound source is [p x , p y , p z . T . A depth sensor is installed on the platform to obtain the depth of the platform itself, plus the known depth of the target, so that p z is known. Assuming that the working depth of the platform is 205 m and the depth of the sound source is 5 m, then p z should be 200.

[0056] As Figure 1 shown, a three-dimensional coordinate system is established. The sound source is a red five-pointed star, and the array elements of the receiving array are 4 red dots. The direction of arrival of the sound source pulse: the elevation angle (the angle between the incoming wave and the xoy plane) θ, the azimuth angle (the angle between the projection of the incoming wave on the xoy plane and the x-axis) A similar three-dimensional relationship can also be established for the target. The difference lies in the elevation angle. Taking upward as positive and downward as negative, then the elevation angle θ of the sound source-target pulse incoming wave > 0.

[0057] 3. Geometric relationship analysis;

[0058] 1) The relationship between coordinates, distance, and angle is as shown in the following equation:

[0059]

[0060] 2) The relationship between Time of Arrival (TOA) and distance;

[0061] The emission time of the sound source pulse signal is t0, and the arrival times of the sound source pulse at the 1st to 4th array elements are t j , where j = 1, 2, 3, 4, that is, the arrival time of the sound source pulse at the 2nd array element is t2.

[0062] Ignoring the change in the speed of sound and assuming the speed of sound is constant at c, we can approximately obtain:

[0063]

[0064] Unfortunately, since t0 cannot be known, distance estimation cannot be performed through the Time of Arrival (TOA).

[0065] 3) Sound source angle estimation;

[0066] The receiving array is divided into two groups. The first group consists of the first and third hydrophones, and the second group consists of the second and fourth hydrophones. For the direct wave of the sound source, the distances from the sound source to array elements 1 - 4 are R j . Taking the second group as an example, under the assumption of a far - field plane wave, the angles between the direct waves of the sound source reaching array element 2, array element 4, and the array center and the z - axis are approximately equal, that is, γ z2 ≈γ z4 ≈γ z , that is, the polar angle, which is complementary to the elevation angle. Based on Figure 1 the trigonometric relationship shown, we get:

[0067]

[0068] The acoustic path difference reaching the second - group hydrophones is R4 - R2, and the corresponding time difference TDOA is Δt z , Δt z = t4 - t2. Similarly, the time difference reaching the first - group hydrophones is Δt y , Δt y = t3 - t1, and the acoustic path difference is R3 - R1, so Therefore, the elevation angle can be estimated from TDOA, and then the azimuth angle can be estimated by combining the estimated value of the elevation angle and the estimated value of TDOA. The estimation method of TDOA is described in detail in "6. Time Difference of Arrival (TDOA) Estimation".

[0069]

[0070] 4) Sound source distance estimation;

[0071] According to p z the distance between the sound source and the platform can be obtained

[0072] Furthermore, the three-dimensional coordinates of the sound source relative to the center of the platform array can be obtained, and t0 can be obtained from Equation (2).

[0073] 4. Pulse reconnaissance;

[0074] 1) Windowing processing;

[0075] Set the window width = 2T according to the prior information, where T is the maximum width of the sound source pulse. Perform FFT on the received signal window by window, and detect whether there is a pulse signal arriving through the peak threshold in the frequency domain, and confirm its peak frequency.

[0076] 2) Estimate pulse parameters;

[0077] Process the time window when the pulse signal arrives. As Figure 2 shown, perform short-time Fourier transform on this window, detect the bright diagonal line in Figure 2 to obtain the frequency range. Combine the prior information to confirm the pulse width bandwidth B and center frequency f c .

[0078] 5. Matched filtering;

[0079] 1) Generate a replica signal;

[0080] Add the Doppler frequency shift to the transmitted pulse signal to obtain s(t), perform fast Fourier transform FFT to generate the frequency-domain replica signal S rep . S rep = DFT(s(t), N), where N is the number of points of FFT, N = fs×2×T, and fs is the sampling frequency. Assume that the pulse appears in the nth frame, and the frequency-domain data received by the array is X n , and the first channel is X 1,n ,

[0081] 2) Set the frequency band of interest;

[0082] The frequency range of the received pulse obtained from the pulse parameter estimation is further expanded to obtain the frequency band range of interest: f1 - f2. It should differ from the upper and lower limits of the frequency band of the transmitted pulse by f dmax , that is, the maximum possible value of the Doppler frequency shift.

[0083] 3) Frequency-domain multiplication;

[0084] Multiply the received signal within the frequency range of interest by Multiply to obtain Z 1,n , and obtain the frequency-domain result of the first-channel matched filtering of this frame. The other channels are processed accordingly.

[0085] 6. Time Difference of Arrival (TDOA) Estimation;

[0086] The time difference of arrival can be directly obtained by subtracting the arrival times of each channel. However, this processing will cause quadratic accumulation of errors due to the need to separately estimate the arrival times for solution. We use the cross-correlation method to solve it, avoiding the quadratic accumulation of errors.

[0087] 1) Cross-correlation of the first group of hydrophone matched filtering;

[0088] Multiply the frequency-domain results of the matched filtering of the first channel and the third channel to obtain: Y y = Z 3,n * Z′ 1,n . Where, ·′ represents matrix transpose.

[0089] 2) Cross-correlation of the second group of hydrophone matched filtering;

[0090] Multiply the frequency-domain results of the matched filtering of the second channel and the fourth channel to obtain: Y z = Z 4,n * Z′ 2,n .

[0091] 3) Perform chirp-z (CZT) transform on the cross-correlation function to search for the time-delay difference spectral peak;

[0092] We perform chirp-z transform, only perform local processing within the concerned range, and can further refine within this range, thus greatly reducing the computational complexity of searching for the time-delay difference spectral peak.

[0093] Set CZT parameters:

[0094] [Δτ begin , Δτ end is the range where the time difference of arrival may be located, M is the number of points of the CZT transform, and [Δτ begin , Δτ end is divided into M - 1 intervals. Q is the refinement multiple, which can be set to 4. M is the smallest power of 2 greater than or equal to the number of points in the time-delay difference range after refining by Q times and rounding up.

[0095] The sampling on the z-plane is z = aω -m , where m = 0,..., M - 1, and a = exp(-j×2πΔτ begin / N), a is the complex starting point, and ω = exp(j2π(Δτ end - Δτ begin ) / (NM)).

[0096] Perform the CZT transform:

[0097] P y = CZT(Y y , M, ω, a)

[0098] P z = CZT(Y z , M, ω, a)

[0099] Peak search to obtain TDOA:

[0100] P y At the peak of, the time difference Δt when the signal arrives at the first group of hydrophones y . Similarly, P z The peak of corresponds to the time difference Δt when the signal arrives at the second group of hydrophones z . Combining the geometric relationships in the third part, parameters such as the distance between the target and the platform, the target pitch angle, and the three-dimensional coordinates can be estimated.

[0101] Simulation experiment: Next, we use the cross-correlation spectrum method to estimate TDOA to obtain parameter estimates such as the distance and azimuth of the target and the sound source, so as to illustrate the feasibility of this scheme. Set the element spacing between array elements 1 and 3, and 2 and 4 of the cross array to 0.2 m. The center position of the array is the origin. The position of the target sound source is [600; 400; 200], the distance from the array center is 748.33 m, emit an LFM signal of 5 kHz, the bandwidth is 1000 Hz, and the pulse width is 300 ms. The speed of the target sound source is 8.9 m / s, and the angle between the moving direction and the positive x-axis is 180°. The speed of the platform is 12 m / s, and the sailing direction and the positive x-axis angle is 0°. Set the SNR of the sound source signal to 10 dB (actually far exceeding), and the SNR of the target signal to 10 dB.

[0102] The distance from the sound source to the platform is 748.33 m. According to the matched filter cross-correlation calculation, 746.57 m is obtained, and the error is 0.2%R. According to the cross-correlation spectrum method for TDOA estimation, with the center position of the platform as the origin, the relevant parameters of the target sound source are shown in Table 1:

[0103] Table 1 Relevant parameters of the target sound source

[0104] Actual orientation Estimated orientation Actual coordinates Estimated coordinates (33.69°，15.50°) (33.69°，15.53°) [600；400；200] [598.55；398.88；200]

[0105] As Figure 6 shown, based on the above method, this application proposes a positioning method for underwater targets with known depth, including:

[0106] Adopt a cross array as the receiving array; the receiving array is placed perpendicular to the geodetic coordinate system; two hydrophones in the horizontal direction form the first group of hydrophones, and two hydrophones in the vertical direction form the second group of hydrophones;

[0107] Receive the pulse signal of the target sound source, and estimate the time difference of arrival (TDOA) by the cross-correlation method;

[0108] Estimate the elevation angle of the target sound source according to the TDOA of the second group of hydrophones;

[0109] Estimate the azimuth angle of the target sound source according to the elevation angle of the target sound source and the TDOA of the first group of hydrophones;

[0110] Estimate the distance of the target sound source according to the depth and elevation angle of the target sound source.

[0111] This application also provides a positioning system for underwater targets with known depth, which is implemented based on the above method. The system includes:

[0112] A receiving array in the form of a cross array; the receiving array is placed perpendicular to the geodetic coordinate system;

[0113] An estimated TDOA module, which is used to process the pulse signal received by the receiving array and estimate the TDOA by the cross-correlation method;

[0114] An estimated elevation angle module, which is used to estimate the elevation angle of the target sound source according to the TDOA;

[0115] An estimated azimuth angle module, which is used to estimate the azimuth angle of the target sound source according to the elevation angle of the target sound source and the TDOA;

[0116] An estimated target sound source distance module, which is used to estimate the distance of the target sound source according to the depth and elevation angle of the target sound source.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and not to limit them. Although the present application has been described in detail with reference to the embodiments, those of ordinary skill in the art should understand that any modification or equivalent replacement of the technical solutions of the present application does not depart from the spirit and scope of the technical solutions of the present application, and they should all be covered by the scope of the claims of the present application.

Claims

1. A method for locating an underwater target at a known depth, comprising: Using a cross array as the receiving array; Placing the receiving array perpendicular to the geodetic coordinate system; Two hydrophones in the horizontal direction form the first group of hydrophones, and two hydrophones in the vertical direction form the second group of hydrophones; Receiving the pulse signal of the target sound source, and estimating the time difference of arrival (TDOA) by the quadratic cross-correlation method; Estimating the elevation angle of the target sound source according to the TDOA of the second group of hydrophones; Estimating the azimuth angle of the target sound source according to the elevation angle of the target sound source and the TDOA of the first group of hydrophones; Estimating the distance of the target sound source according to the depth and elevation angle of the target sound source; The estimating the TDOA by the quadratic cross-correlation method includes: Setting the window width = 2T according to the prior information, where T is the maximum width of the sound source pulse; performing a fast Fourier transform on the received signal window by window, detecting whether a pulse signal arrives through a set frequency domain peak threshold, and confirming the peak frequency of the pulse signal; Perform short-time Fourier transform on the time window when the pulse signal arrives to obtain the frequency range; confirm the pulse width of the transmitted pulse in combination with prior information Bandwidth B and center frequency f c ; The transmitted pulse signal is added with a Doppler frequency shift to obtain s(t), and a fast Fourier transform is performed on it to generate a frequency-domain replica signal S rep , S rep = DFT(s(t), N), where N is the number of points of the fast Fourier transform, N = fs × 2 × T, and fs is the sampling frequency; Setting the concerned frequency range to the maximum possible value of the Doppler frequency shift difference between the upper and lower limits of the frequency band of the transmitted pulse signal; The received signal within the frequency range of interest is multiplied by to obtain Z i,n , and the frequency-domain result of the matched filtering for each channel of the pulse-occurrence frame is obtained; represents the frequency-domain data received by the i-th channel of the array when the pulse signal appears in the n-th frame; Multiply the frequency-domain results of the matched filtering of the first channel and the third channel to obtain: Y y = Z 3,n * Z' 1,n ; ·′ represents matrix transpose; Multiply the frequency-domain results of the matched filtering of the second channel and the fourth channel to obtain: Y z = Z 4,n * Z' 2,n ; Performing a Chirp-Z transform (CZT) on the received signal within the concerned frequency range to obtain: P y = CZT(Y y ,M,ω,a) P z = CZT(Y z , M, ω, a) where M is the number of points of the CZT transform; a is the complex starting point, a = exp(-j×2πΔτ begin / N), [Δτ begin , Δτ end is the range where the time difference of arrival may be located; ω = exp(j2π(Δτ end -Δτ begin ) / (NM)); Perform peak search on the CZT transform result, and the peak of P y corresponds to the time difference Δt when the signal arrives at the first group of hydrophones y ; the peak of P z corresponds to the time difference Δt when the signal arrives at the second group of hydrophones z .

2. The positioning method for an underwater target at a known depth according to claim 1, characterized in that, Estimating the elevation angle of the target sound source according to the TDOA, including: Among them, represents the estimated value of the elevation angle of the target sound source; c represents the speed of sound; Δt z represents the time difference of the signal arriving at the second group of hydrophones; d represents the distance between two array elements in the horizontal or vertical direction.

3. The positioning method for an underwater target at a known depth according to claim 1, characterized in that, The estimating the azimuth angle of the target sound source according to the elevation angle of the target sound source and the TDOA includes: Among them, represents the estimated value of the azimuth angle of the target sound source; represents the estimated value of the elevation angle of the target sound source; c represents the speed of sound; Δt y represents the time difference for the signal to reach the first group of hydrophones; d represents the spacing between two array elements in the horizontal or vertical direction.

4. The method for positioning a target at a known depth underwater according to claim 1, wherein The estimating the distance of the target sound source according to the depth and elevation angle of the target sound source includes: Among them, R represents the distance of the target sound source; p z represents the depth of the target sound source; represents the estimated value of the pitch angle of the target sound source.

5. A positioning system for underwater targets at a known depth, implemented based on the method according to any one of claims 1-4, characterized in that, The system includes: A receiving array in the form of a cross array; the receiving array is placed perpendicular to the geodetic coordinate system; A TDOA estimating module for processing the pulse signal received by the receiving array and estimating the TDOA by the cross-correlation method; An elevation angle estimating module for estimating the elevation angle of the target sound source according to the TDOA; An azimuth angle estimating module for estimating the azimuth angle of the target sound source according to the elevation angle of the target sound source and the TDOA; and A target sound source distance estimating module for estimating the distance of the target sound source according to the depth and elevation angle of the target sound source.

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