Method and system for positioning underwater target with known depth

Abstract

The invention provides a method and a system for positioning an underwater target with known depth. The method comprises the following steps: adopting a cross array as a receiving array; the two hydrophones in the horizontal direction form a first group of hydrophones, and the two hydrophones in the vertical direction form a second group of hydrophones; receiving a pulse signal of a target sound source, and estimating a time difference of arrival through a secondary cross-correlation method; estimating the pitch 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 pitch angle of the target sound source and the time difference of arrival of the first group of hydrophones; and estimating the target sound source distance according to the target sound source depth and the pitch angle. The method has the advantages that under the condition that only the depth of the target position is known and the sound source and the receiving array cannot communicate, the target sound source can be accurately and rapidly subjected to direction finding and positioning. The method can be widely applied to an underwater passive sonar system with a known target depth, and can be expanded to a weak signal detection system.

Description

Technical Field

[0001] The present application belongs to the field of underwater acoustic positioning, direction finding and passive sonar system design, and specifically relates to a method and system for positioning underwater targets of known depth. Background Art

[0002] Sonar positioning uses the characteristics of sound waves propagating in water to determine the location of the target object by measuring the time difference between the emission and reception of sound waves and the propagation direction. According to the working principle, sonar systems are mainly divided into two categories: active sonar and passive sonar. The active sonar system actively transmits sound wave signals and determines the location of the target by receiving the reflected echo signals. However, since active sonar is a transmitter-receiver system, it is easier for the target ship to detect it and avoid it when it is working.

[0003] Compared with active sonar, passive sonar has obvious advantages in concealment, flexibility and low power consumption. Because it does not need to actively send out signals, it can achieve "one-way" transparency in the underwater battlefield. It is particularly suitable for ships that cannot make sounds to expose themselves but need to detect target activities. In addition, passive sonar can be deployed on the hull, underwater cables or buoys, etc., and is relatively easy to install and use. It is often widely used in reconnaissance and early warning, surveying and mapping, fish activity monitoring, marine environment monitoring and other fields. However, due to the lack of distance information of active echoes, passive sonar technology can usually only obtain the approximate direction of the target, and accurate positioning is very difficult.

[0004] With the rapid increase in the demand for marine resource development and marine security defense, as well as the development of intelligent technology and adaptive systems, passive sonar target identification is a prerequisite for ships to detect the enemy first, attack covertly, and defeat the enemy in naval battles. It 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. At present, the most widely used passive positioning methods are ternary ranging, target motion analysis (TMA) and matched field processing (MFP) methods. However, the ternary passive sonar ranging method is limited to its positioning principle and is only applicable to near-field positioning. For distant sound sources, the positioning accuracy is greatly reduced. The target motion analysis method is suitable for solving long-range positioning problems. The commonly used method is pure bearing TMA, which requires high bearing measurement accuracy and accurately known own ship track. In practice, the maneuvering behavior of the own ship is not necessarily convenient and sometimes even not allowed, because it may expose itself. The solution is to add new observations such as frequency, arrival time and delay, but its estimation quality improves with the increase of observation data. Therefore, a large amount of calculation is required to improve the estimation accuracy. The ranging performance of matched field processing is extremely sensitive to sea depth, sound velocity profile, seabed acoustic parameters and hydrophone depth. It has a strong dependence on environmental parameters and generally requires the deployment of a larger vertical array. Summary of the invention

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

[0006] In order to achieve the above object, the present application proposes a method for positioning an underwater target of known depth, comprising:

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

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

[0009] Estimate the elevation angle of the target sound source based on the time difference of arrival of the second group of hydrophones;

[0010] The azimuth of the target sound source is estimated according to the pitch angle of the target sound source and the time difference of arrival of the first group of hydrophones;

[0011] Estimate the distance of the target sound source based on the depth and elevation angle of the target sound source.

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

[0013] According to the prior information, the window width is set to 2T, where T is the maximum width of the sound source pulse; the received signal is subjected to fast Fourier transform window by window, and the peak frequency of the pulse signal is confirmed by detecting whether a pulse signal has arrived through the set frequency domain peak threshold;

[0014] Perform short-time Fourier transform on the time window of the pulse signal arrival to obtain the frequency range; combine prior information to confirm the pulse width of the transmitted pulse Bandwidth B and center frequency f c ;

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

[0016] The frequency range of interest is set to the maximum possible value of the Doppler frequency shift between the upper and lower limits of the frequency band of the transmitted pulse signal;

[0017] The received signal in the frequency range of interest and Multiply to get Z i，n , obtain the frequency domain result of matched filtering of each channel of the pulse occurrence frame; Indicates that the pulse signal appears in the nth frame, and the frequency domain data received by the i-th channel of the array;

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

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

[0020] Performing CZT transformation on the received signal in the frequency range of interest yields:

[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 CZT transformation; a is the starting point of the complex, a = exp(-j×2πΔτ begin / N), [Δτ begin ,Δτ end ] is the possible range of arrival time difference; ω=exp(j2π(Δτ end -Δτ begin ) / (NM));

[0024] Perform peak search on the CZT transformation result, P y The peak value of the corresponding signal reaches the first group of hydrophones. y ;P z The peak value of the corresponding signal reaches the second group of hydrophones. z .

[0025] As an improvement of the above method, the estimating the pitch angle of the target sound source according to the arrival time difference includes:

[0026]

[0027] in, represents the estimated pitch angle of the target sound source; c represents the speed of sound; Δt z represents the time difference of the signal reaching 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 of the target sound source according to the pitch angle and the time difference of arrival of the target sound source includes:

[0029]

[0030] in, Indicates the estimated azimuth of the target sound source; represents the estimated pitch angle of the target sound source; c represents the speed of sound; Δt y represents the time difference of the signal reaching the first group of hydrophones; d represents the spacing between two array elements in the horizontal or vertical direction.

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

[0032]

[0033] Where R represents the distance of the target sound source; p z Indicates the target sound source depth; Indicates the estimated pitch angle of the target sound source.

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

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

[0036] The time difference of arrival estimation module is used to process the pulse signal received by the receiving array and estimate the time difference of arrival by using the cross-correlation method;

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

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

[0039] The target sound source distance estimation module is used to estimate the target sound source distance according to the target sound source depth and pitch angle.

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

[0041] The present invention innovatively proposes a method for locating underwater targets of known depth, using a cross array as a receiving array and a moving target sound source near the water surface as a transmitting sound source. When 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 find the direction and locate the target sound source. The present application can be widely used in underwater passive sonar systems with known target depths, and can be expanded to weak signal detection systems. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0043] Figure 2 Shown is a schematic diagram of the time domain and time-frequency domain of the pulse signal arrival window;

[0044] Figure 3 Shown is a schematic diagram of receiving signals of 4 channels;

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

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

[0047] Figure 6 Shown is a flow chart of a method for locating underwater targets at known depths. DETAILED DESCRIPTION

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

[0049] In the process of passive sonar target recognition, the cross array, as a typical small-aperture array, is more suitable for the current situation where the demand for small sonar platforms is constantly increasing. When using passive sonar technology for positioning, the time difference between the transmitted signal and the received signal cannot be determined due to the lack of synchronization between the platform and the target. The present application provides a method for detecting underwater targets of known depths. The direction is estimated by the time delay between the elements of the cross-shaped array and the target distance and position are determined by combining the target depth, so that the direction finding and positioning of the moving target can be achieved. This working mode has little impact 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] In response to the positioning and tracking needs of underwater moving targets, this application uses a cross array as the receiving array and uses the target's pulse signal 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 positioning and tracking near the water surface.

[0051] Assume that the target sound source is placed under a surface ship or other platform and emits a pulse signal. The receiving array is installed on an underwater mobile such as a UUV, or it can be installed on a fixed platform such as a buoy. It should be noted that the pulse signal emitted by the target sound source is not completely known a priori information to the receiving platform. The receiving platform may know several possible forms of the pulse, such as LFM signal / CW signal, as well as the possible pulse width, frequency, bandwidth, period, etc. of the pulse. However, the specific signal actually used, the accurate radial velocity between the target and the platform are unknown, and the starting time of the emitted pulse cannot be known. Therefore, the arrival time estimation method cannot be used to obtain the distance between the platform and the target. In addition, the signal received by the platform and the pulse emitted must have Doppler frequency shift, pulse compression and broadening. Therefore, the method proposed in this application mainly obtains the direction of the target based on the arrival time difference estimation between array elements, and then locates the target in combination with the target depth information.

[0052] 1. Formation design;

[0053] Since the cross array has the advantages of simple structure, easy installation and simultaneous two-dimensional azimuth estimation, we designed the receiving array as a cross array with a total of four receiving hydrophones. According to simulation analysis, this method is particularly suitable for the design of small apertures, and the vertical and horizontal apertures are set to be greater than 0.5 times the wavelength. Taking the 5kHz center frequency as an example, the distance between the four array elements and the array center is d / 2, which can be set to be no less than 0.1m.

[0054] 2. System scenario construction;

[0055] The array is placed perpendicular to the earth 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 center of the array is R. The position of the target sound source is [p x ,p y ,p z ] T The depth sensor installed on the platform obtains the depth of the platform itself, plus the depth of the known target, so as to obtain the depth of the platform itself. z Assuming the platform working depth is 205m and the sound source depth is 5m, then p z Should be 200.

[0056] like Figure 1 As 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 four red dots. The direction of arrival of the sound source pulse: the pitch 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 with the target. The difference lies in the pitch angle, with upward being positive and downward being negative. The pitch angle θ of the sound source target pulse wave is > 0.

[0057] 3. Geometric relationship analysis;

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

[0059]

[0060] 2) The relationship between time of arrival (TOA) and distance;

[0061] The time when the sound source pulse signal is sent is t0, and the time when the sound source pulse reaches array elements 1 to 4 is t j , j = 1, 2, 3, 4, that is, the time when the sound source pulse arrives at array element No. 2 is t2.

[0062] Ignoring the change of the speed of sound and assuming that the speed of sound is constant at c, we can approximate:

[0063]

[0064] Unfortunately, since t0 is unknown, distance estimation cannot be performed using 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 from the sound source, the distances from the sound source to array elements 1 to 4 are R j Taking the second group as an example, under the far-field plane wave assumption, the direct wave from the sound source to element 2, element 4, and the center of the array has an approximately equal angle with the z-axis, that is, γ z2 ≈γ z4 ≈γ z , that is, the polar angle, is complementary to the pitch angle. Figure 1 The triangular relationship shown in , we get:

[0067]

[0068] The path difference to the second group of hydrophones is R4-R2, and the corresponding time difference TDOA is Δt z , Δt z =t4-t2. Similarly, the time difference of reaching the first group of hydrophones is Δt y , Δt y =t3-t1, the path difference is R3-R1, so Therefore, the pitch angle can be estimated by TDOA, and then the azimuth angle can be estimated by combining the estimated value of the pitch angle and the estimated value of TDOA. For details on the TDOA estimation method, see "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] The three-dimensional coordinates of the sound source relative to the center of the platform array can be further obtained, and t0 can be obtained from equation (2).

[0073] 4. Pulse reconnaissance;

[0074] 1) Window processing;

[0075] According to the prior information, the window width is set to 2T, where T is the maximum width of the sound source pulse. The received signal is subjected to FFT window by window, and the frequency domain peak threshold is used to detect whether a pulse signal has arrived and confirm its peak frequency.

[0076] 2) Estimation of pulse parameters;

[0077] Process the time window of the pulse signal arrival. Figure 2 As shown, the window is short-time Fourier transformed. Figure 2 Detect the bright diagonal lines in the image to obtain the frequency range. Confirm the pulse width of the transmitted pulse based on prior information Bandwidth B, center frequency f c .

[0078] 5. Matched filtering;

[0079] 1) Generate a replica signal;

[0080] The transmitted pulse signal is added with Doppler frequency shift to obtain s(t), and then a fast Fourier transform FFT is performed to generate a frequency domain replica signal S rep . S rep =DFT(s(t),N), where N is the number of FFT points, 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 , 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 range that needs attention: f1-f2. The difference between the upper and lower limits of the frequency band of the transmitted pulse should be f dmax , which is the maximum possible value of the Doppler frequency shift.

[0083] 3) Frequency domain multiplication;

[0084] The received signal in the frequency range of interest and Multiply to get Z 1,n , get the frequency domain result of the matched filtering of the first channel of the frame. Other channels are processed accordingly.

[0085] 6. Time difference of arrival (TDOA) estimation;

[0086] The arrival time difference can be obtained by directly subtracting the arrival time of each channel, but this process will lead to secondary accumulation of errors because the arrival time needs to be estimated separately. We use the cross-correlation method to solve it and avoid the secondary accumulation of errors.

[0087] 1) Matched filter cross-correlation of the first group of hydrophones;

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

[0089] 2) matched filter cross-correlation of the second group of hydrophones;

[0090] The frequency domain results of the second channel and the fourth channel matched filter are multiplied to obtain: Y z =Z 4,n *Z′ 2,n .

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

[0092] Through chirp-z transform, we only perform local processing within the range of interest, and can further refine within this range, thereby greatly reducing the computational complexity of the delay difference spectrum peak search.

[0093] Set CZT parameters:

[0094] [Δτ begin , Δτ end ] is the possible range of arrival time difference, M is the number of points of CZT transformation, and [Δτ begin , Δτ end ] is divided into M-1 intervals. Q is the refinement multiple, which can be set to 4. M is equivalent to the number of points in the range of the delay difference refined by Q times, rounded up to the nearest power of 2.

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

[0096] Perform CZT transformation:

[0097] P y =CZT(Y y , M, ω, a)

[0098] P z =CZT(Y z , M, ω, a)

[0099] Peak search to get TDOA:

[0100] P y The time difference Δt corresponding to the peak value of the signal reaching the first group of hydrophones y Similarly, P z The peak value of the corresponding signal reaches the second group of hydrophones. z Combined with the geometric relationship in the third part, parameters such as the distance between the target and the platform, the target pitch angle and three-dimensional coordinates can be estimated.

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

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

[0103] Table 1 Target sound source related parameters

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

[0105] like Figure 6 As shown, based on the above method, the present application proposes a method for positioning an underwater target of known depth, including:

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

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

[0108] Estimate the elevation angle of the target sound source based on the time difference of arrival of the second group of hydrophones;

[0109] The azimuth of the target sound source is estimated according to the pitch angle of the target sound source and the time difference of arrival of the first group of hydrophones;

[0110] Estimate the distance of the target sound source based on the depth and elevation angle of the target sound source.

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

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

[0113] The time difference of arrival estimation module is used to process the pulse signal received by the receiving array and estimate the time difference of arrival by using the cross-correlation method;

[0114] A pitch angle estimation module is used to estimate the pitch angle of the target sound source according to the time difference of arrival;

[0115] An azimuth estimation module is used to estimate the azimuth of the target sound source according to the pitch angle and the time difference of arrival of the target sound source;

[0116] The target sound source distance estimation module is used to estimate the target sound source distance according to the target sound source depth and pitch angle.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application and are not intended to limit it. Although the present application is described in detail with reference to the embodiments, a person skilled in the art should understand that any modification or equivalent replacement of the technical solution of the present application does not depart from the spirit and scope of the technical solution of the present application and should be included in the scope of the claims of the present application.

Claims

1. A method for locating an underwater target at a known depth, comprising: A cross array is used as the receiving array; The receiving array is placed perpendicular to the earth coordinate system; The two hydrophones in the horizontal direction form the first group of hydrophones, and the two hydrophones in the vertical direction form the second group of hydrophones; Receive the pulse signal of the target sound source and estimate the arrival time difference by the quadratic cross-correlation method; Estimate the elevation angle of the target sound source based on the time difference of arrival of the second group of hydrophones; The azimuth of the target sound source is estimated according to the pitch angle of the target sound source and the time difference of arrival of the first group of hydrophones; Estimate the distance of the target sound source based on the depth and elevation angle of the target sound source.

2. The method for locating an underwater target of known depth according to claim 1, characterized in that: The method of estimating the time difference of arrival by using the quadratic cross-correlation method comprises: According to the prior information, the window width is set to 2T, where T is the maximum width of the sound source pulse; the received signal is subjected to fast Fourier transform window by window, and the peak frequency of the pulse signal is confirmed by detecting whether a pulse signal has arrived through the set frequency domain peak threshold; Perform short-time Fourier transform on the time window of the pulse signal arrival to obtain the frequency range; combine prior information to confirm the pulse width of the transmitted pulse Bandwidth B and center frequency f c ; The transmitted pulse signal is added with Doppler frequency shift to obtain s(t), and then a fast Fourier transform is performed on it to generate a frequency domain replica signal S rep , S rep =DFT(s(t),N), N is the number of fast Fourier transform points, N=fs×2×T, fs is the sampling frequency; The frequency range of interest is set to the maximum possible value of the Doppler frequency shift between the upper and lower limits of the frequency band of the transmitted pulse signal; The received signal in the frequency range of interest i=1,2,3,4, and Multiply to get Z i,n , obtain the frequency domain result of matched filtering of each channel of the pulse occurrence frame; Indicates that the pulse signal appears in the nth frame, and the frequency domain data received by the i-th channel of the array; Multiply the frequency domain results of the matched filtering of the first channel and the third channel to get: Y y =Z 3,n *Z′ 1,n ; ·′ represents matrix transposition; Multiply the frequency domain results of the second channel and the fourth channel matched filter to get: Y z =Z 4,n *Z′ 2,n ; Performing CZT transformation on the received signal in the frequency range of interest yields: P y =CZT(Y y ,M,ω,a) P z =CZT(Y z ,M,ω,a) Where M is the number of points of CZT transformation; a is the starting point of the complex, a = exp(-j×2πΔτ begin / N), [Δτ begin ,Δτ end ] is the possible range of arrival time difference; ω=exp(j2π(Δτ end -Δτ begin ) / (NM)); Perform peak search on the CZT transformation result, P y The peak value of the corresponding signal reaches the first group of hydrophones. y ;P z The peak value of the corresponding signal reaches the second group of hydrophones. z .

3. The method for locating an underwater target of known depth according to claim 1, characterized in that: The method of estimating the pitch angle of the target sound source according to the time difference of arrival includes: in, represents the estimated pitch angle of the target sound source; c represents the speed of sound; Δt z represents the time difference of the signal reaching the second group of hydrophones; d represents the spacing between two array elements in the horizontal or vertical direction.

4. The method for locating an underwater target of known depth according to claim 1, characterized in that: The estimating the azimuth of the target sound source according to the pitch angle and the time difference of arrival of the target sound source comprises: in, Indicates the estimated azimuth of the target sound source; represents the estimated pitch angle of the target sound source; c represents the speed of sound; Δt y represents the time difference of the signal reaching the first group of hydrophones; d represents the spacing between two array elements in the horizontal or vertical direction.

5. The method for locating an underwater target of known depth according to claim 1, characterized in that: The estimating the distance of the target sound source according to the target sound source depth and the pitch angle comprises: Where R represents the distance of the target sound source; p z Indicates the target sound source depth; Indicates the estimated pitch angle of the target sound source.

6. A positioning system for underwater targets of known depth, implemented based on the method described in any one of claims 1 to 5, characterized in that: The system comprises: A receiving array in the form of a cross array; the receiving array is placed perpendicular to the geodetic coordinate system; The time difference of arrival estimation module is used to process the pulse signal received by the receiving array and estimate the time difference of arrival by using the cross-correlation method; A pitch angle estimation module is used to estimate the pitch angle of the target sound source according to the time difference of arrival; an azimuth estimation module, used to estimate the azimuth of the target sound source according to the pitch angle and the time difference of arrival of the target sound source; and The target sound source distance estimation module is used to estimate the target sound source distance according to the target sound source depth and pitch angle.

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