Passive direction-finding and distance-measuring method for large-aperture array near-field target

Abstract

The invention discloses a large-aperture array near-field target passive direction-finding and distance-measuring method, and belongs to the technical field of underwater acoustic detection and array signal processing. According to the method, the problems of low azimuth estimation precision and large calculation amount of an existing near-field target passive positioning method are solved. The invention provides a far-field and near-field target distinguishing method by utilizing the property that spectrum peak broadening occurs when near-field target spatial spectrum estimation is performed under a far-field model. According to the method, the distance measurement step length is set, the mode of first coarse focusing scanning and then fine focusing scanning is adopted, the near-field target azimuth and distance are obtained at the position with the narrowest spectral peak width of the spatial spectrum, and the azimuth estimation precision is remarkably improved. Moreover, the method directly achieves the far and near field discrimination of the target on the basis of the remote one-dimensional azimuth spectrum, can achieve the far and near field discrimination of the target without two-dimensional space scanning, and remarkably reduces the calculation amount of the far and near field discrimination of the target. The method can be applied to passive direction finding and distance measuring of the near-field target.

Description

Technical Field

[0001] The present invention belongs to the technical field of underwater acoustic detection and array signal processing, and particularly relates to a method for passive direction finding and ranging of near-field targets by a large-aperture array. Background Technique

[0002] To obtain better spatial processing performance, modern sonar systems are developing towards large-scale and large-aperture sonar arrays. Due to the increase in the array aperture relative to the target distance, the gain and resolution of far-field targets will be significantly improved. However, with the increase in the array aperture, the near-field spatial coverage range of the array will also increase significantly. When compensating for the azimuth of near-field targets, it no longer conforms to the far-field plane wave compensation assumption. Directly using the far-field compensation model for azimuth estimation will cause a serious decline in gain and direction-finding performance, and a near-field compensation model needs to be adopted. Therefore, the problem of distinguishing between far-field and near-field targets detected by a large-aperture array is very important.

[0003] However, there is relatively little research on the discrimination of far-field and near-field targets at present. The following main literatures have carried out research on the passive positioning of near-field targets:

[0004] Literature 1: Ji Qing, Cheng Jinfang, Xiao Dawei. Research on the localization algorithm of near-field small targets based on vector array [J]. Ship Science and Technology, 2018, 40(07): 105-109.

[0005] Literature 2: Mei Jidan, Shi Wenpei, Ma Chao, etc. Near-field deconvolution focusing beamforming sonogram measurement [J]. Acta Acustica, 2020, 45(01): 15-28.

[0006] Literature 3: Jia Xiaoyun, Wang Xiuqing, Hu Ye. Source localization of pipeline defects based on MVDR near-field focusing beamforming [J]. Journal of Tianjin University of Science & Technology, 2021, 36(06): 37-43.

[0007] Literature 4: Shang Zhigang, Qu Xinghao, Qiao Gang, etc. Beam deconvolution localization of far-field and near-field mixed sources [J]. Acta Acustica, 2023, 48(03): 447-458.

[0008] Literature 1, Literature 2 and Literature 3 mainly carried out research on near-field source localization methods, and realized passive localization of near-field targets through two-dimensional spatial scanning. However, they all rely on near-field focusing scanning, and the computational complexity of the localization method is relatively large.

[0009] Reference 4 proposes a high-resolution azimuth estimation processing method in the case of a mixed near-field and far-field source. Its main purpose is to solve the problem of high-resolution processing of multiple targets in the case of a mixed near-field and far-field multi-target situation, which improves the azimuth estimation accuracy to a certain extent, but cannot achieve the discrimination of the near-field and far-field of the target. Therefore, the spatial spectrum estimation is still affected by the mixed near-field and far-field source. Therefore, the accuracy of azimuth estimation for near-field targets is still relatively limited.

[0010] In summary, the existing passive localization methods for near-field targets have problems that the spatial spectrum estimation is affected by the mixed near-field and far-field source and the calculation amount of the localization method is large. Therefore, it is an urgent problem to be solved to propose a new passive localization method for targets. Summary of the Invention

[0011] The purpose of the present invention is to solve the problems of low azimuth estimation accuracy and large calculation amount of the existing passive localization methods for near-field targets, and a passive direction finding and ranging method for near-field targets with a large-aperture array is proposed.

[0012] The technical solution adopted by the present invention to solve the above technical problems is: a passive direction finding and ranging method for near-field targets with a large-aperture array, and the method specifically includes the following steps:

[0013] Step 1: Obtain an array far-field beamforming beamwidth calculation model according to the natural directivity of a uniform linear array;

[0014] Step 2: Initialize the time t = 0;

[0015] Step 3: Perform far-field beamforming on the array received signal at time t to obtain a far-field azimuth spectrum, and then obtain the rough measurement angles of each target to be measured and the peak widths of the azimuth spectra of each target to be measured according to the far-field azimuth spectrum;

[0016] Step 4: Initialize the number of targets to be measured n = 1;

[0017] Step 5: Substitute the rough measurement angle of the nth target to be measured into the array far-field beamforming beamwidth calculation model, and compare the calculated beamwidth with the peak width of the azimuth spectrum of the nth target to be measured to perform near-far-field judgment on the nth target to be measured;

[0018] Step 6: If the nth target to be measured is in the near field, continue to execute Step 7;

[0019] If the nth target to be measured is in the far field, directly use the rough measurement angle as the azimuth estimation result of the nth target to be measured; then let n = n + 1, and return to execute Step 5;

[0020] Step 7: Perform near-field focusing scanning on the nth target to be measured to obtain the peak widths of the azimuth spectra corresponding to each scanning distance, that is, obtain the main lobe widths corresponding to each scanning distance of the nth target to be measured;

[0021] Determine the initial azimuth value and the initial distance value of the nth target to be measured according to the main lobe width corresponding to each scanning distance;

[0022] Step Eight: Determine the scanning range of the near-field focusing fine scan according to the initial azimuth value and the initial distance value of the nth target to be measured, and perform a near-field focusing fine scan within the determined scanning range to obtain the final azimuth and distance of the nth target to be measured;

[0023] Step Nine: Determine whether n reaches the maximum number of near-field targets;

[0024] If it reaches, execute Step Ten;

[0025] If it does not reach, let n = n + 1, and return to execute Step Five;

[0026] Step Ten: Let t = t + 1, and return to execute Step Three.

[0027] Furthermore, the uniform linear array is specifically: with the due east direction as the positive x-axis direction and the due north direction as the positive y-axis direction, each array element is evenly arranged at equal intervals on the positive half-axis of the x-axis, and the distance between two adjacent array elements is d.

[0028] Furthermore, the specific process of Step One is as follows:

[0029] Define the target incident azimuth as the angle in the north-by-east direction, and denote the incident azimuth of the kth target as θ k , k = 1, 2, …, K, where K is the number of incident targets. For the incident azimuth θ k of the kth target, the natural directivity G(θ) of the uniform linear array is:

[0030]

[0031] where M is the number of array elements, and |·| represents taking the absolute value;

[0032] According to |G(θ)| 2 = 1 / 2, the calculation model of the array far-field beamforming beam width corresponding to the incident azimuth of the kth target is obtained as:

[0033]

[0034] where θ DT is the array far-field beamforming beam width corresponding to the incident azimuth of the kth target, and λ is the signal wavelength.

[0035] Furthermore, the array received signal at time t is:

[0036]

[0037] Among them, \(x(t)\) represents the array received signal at time \(t\), \(s(t)\) represents the target signal received by the array at time \(t\), and \(n(t)\) represents the noise signal received by the array at time \(t\). \(a(\theta k ) represents the array steering vector corresponding to the angle \(\theta k .

[0038] Furthermore, the specific process of the third step is as follows:

[0039] Step 3-1: In the range of \(\theta\in[-90^{\circ},90^{\circ}]\), perform far-field beamforming on the array received signal \(x(t)\) to obtain the far-field azimuth spectrum \(p(\theta)\):

[0040] \(p(\theta)=a(\theta) H R a(\theta)

[0041] where \(R\) represents the covariance matrix, \(R = E[x(t)x H (t)]\), \(E[\cdot]\) represents taking the expectation, \(x H (t)\) is the conjugate transpose of \(x(t)\), \(a(\theta)\) is the array steering vector corresponding to the scanning angle \(\theta\) on the azimuth scanning grid, and \(a(\theta) H is the conjugate transpose of \(a(\theta)\);

[0042]

[0043] where \(e\) is the base of the natural logarithm, \(j\) is the imaginary unit, and the superscript \(T\) represents the transpose;

[0044] Step 3-2: Find the maximum peak position of each local spectrum peak in the far-field azimuth spectrum \(p(\theta)\), and take the angle corresponding to the maximum peak position of each local spectrum peak as the rough measurement angle of each target to be measured. Denote the rough measurement angle of the \(k\)-th target to be measured as \(\theta k '\);

[0045] Taking the maximum peak position of any one local spectrum peak as an example, the calculation method of the azimuth spectrum peak width of the target to be measured is as follows:

[0046] Denote the azimuth corresponding to a 3 dB drop on the left side of the maximum peak position of this local spectrum peak as \(\theta L , and denote the azimuth corresponding to a 3 dB drop on the right side of the maximum peak position of this local spectrum peak as \(\theta R . Then the azimuth spectrum peak width \(\Delta\theta\) of the target corresponding to the maximum peak position of this local spectrum peak is:

[0047] \(\Delta\theta = |\theta R -\theta L |

[0048] Similarly, obtain the azimuth spectrum peak width of the target corresponding to the maximum peak position of each local spectrum peak respectively.

[0049] Further, the specific process of step five is as follows:

[0050] For any target to be measured, substitute the rough measurement angle of the target to be measured into the far-field beamforming beamwidth calculation model of the array to obtain the beamwidth θ corresponding to the target to be measured DT , and then compare the θ DT corresponding to the target to be measured with the peak width Δθ of the azimuth spectrum corresponding to the target to be measured:

[0051] If Δθ ≥ ε·θ DT , where ε is the decision threshold, then the target to be measured is located in the near field;

[0052] Otherwise, Δθ < ε·θ DT , and the target to be measured is located in the far field.

[0053] Further, the specific process of step seven is as follows:

[0054] Step seven one: Perform distance grid division within the search range θ ∈ [-90°, 90°], where D represents the array aperture, D = (M - 1)d;

[0055] Then, perform near-field focusing scanning on the nth target to be measured at each distance respectively, obtain the azimuth spectrum corresponding to each scanning distance respectively, and then obtain the main lobe width corresponding to each scanning distance according to the azimuth spectrum;

[0056] Step seven two: Sort the main lobe widths corresponding to each scanning distance in ascending order of the main lobe width:

[0057] If the smallest main lobe width corresponds to only one scanning distance, then take the scanning distance corresponding to the smallest main lobe width as the initial distance value of the target to be measured, and obtain the peak of the azimuth spectrum obtained when the initial distance value is used as the scanning distance, and take the angle corresponding to the obtained peak as the initial azimuth value of the target to be measured;

[0058] If the smallest main lobe width corresponds to multiple scanning distances, then calculate the azimuth spectrum power drop degree under each scanning distance corresponding to the smallest main lobe width respectively, take the scanning distance with the largest azimuth spectrum power drop degree as the initial distance value of the target to be measured, and take the angle corresponding to the peak of the azimuth spectrum obtained when the initial distance value is used as the scanning distance as the initial azimuth value of the target to be measured;

[0059] The calculation method of the azimuth spectrum power drop degree is as follows:

[0060] Taking the azimuth spectrum at any scanning distance as an example, record the peak power of the azimuth spectrum as P max, to the left of the grid point where the peak power is located, the power corresponding to the grid point with an interval of 1° from the grid point where the peak power is located is denoted as P -1° , to the right of the grid point where the peak power is located, the power corresponding to the grid point with an interval of 1° from the grid point where the peak power is located is denoted as P 1° , then the degree of decrease ΔP of the azimuth spectrum power is:

[0061] ΔP = (|P max - P -1° | + |P max - P 1° ) / 2.

[0062] Furthermore, the azimuth spectrum in Step 71 is:

[0063] Taking the i-th scanning distance as an example, the azimuth spectrum p(θ, r i ) corresponding to the i-th scanning distance r i is:

[0064] p(θ, r i ) = a(θ, r i ) H Ra(θ, r i )

[0065] where a(θ, r i ) represents the array steering vector at the i-th scanning distance, and the steering vector a(θ i ) at the i'-th scanning azimuth θ i′ of the array steering vector a(θ i′ , r i ) is:

[0066]

[0067] where, when M is odd, N = (M - 1) / 2, when M is even, N = M / 2, a(θ i′ , r i ) ∈ C 2N×1 , γ i′ and φ i′ are respectively expressed as:

[0068]

[0069] Even further, the specific process of Step 8 is:

[0070] Find the scanning distance in Step 7 that is closest to and less than the distance initial value, and then find the scanning distance in Step 7 that is closest to and greater than the distance initial value. The found scanning distances are respectively used as the lower and upper limits of the distance for the near-field focusing fine scan;

[0071] Substitute the initial azimuth value into the beam width calculation model to obtain the beam width calculation result. Then, use the difference between the initial azimuth value and the beam width calculation result as the lower limit of the angle for fine scanning in near-field focusing, and use the sum of the initial azimuth value and the beam width calculation result as the upper limit of the angle for fine scanning in near-field focusing;

[0072] Perform fine scanning in near-field focusing within the scanning range to obtain the near-field focusing azimuth spectrum corresponding to each scanning distance respectively, and then obtain the main lobe width corresponding to each scanning distance according to the near-field focusing azimuth spectrum;

[0073] If the minimum main lobe width in each near-field focusing azimuth spectrum corresponds to only one scanning distance, then use the scanning distance corresponding to the minimum main lobe width as the final distance value of the target to be measured, and obtain the spectral peak of the near-field focusing azimuth spectrum obtained when the final distance value is used as the scanning distance, and use the angle corresponding to the obtained spectral peak as the final azimuth value of the target to be measured;

[0074] If the minimum main lobe width in each near-field focusing azimuth spectrum corresponds to multiple scanning distances, then calculate the power decline degree of the near-field focusing azimuth spectrum at each scanning distance corresponding to the minimum main lobe width respectively, and use the scanning distance with the largest power decline degree of the near-field focusing azimuth spectrum as the final distance value of the target to be measured, and use the azimuth corresponding to the spectral peak of the near-field focusing azimuth spectrum with the largest power decline degree as the final azimuth value of the target to be measured.

[0075] The beneficial effects of the present invention are as follows:

[0076] Aiming at the problem that it is difficult to distinguish the influence of far and near source targets on spatial spectrum estimation during passive detection of large-aperture arrays in the prior art, and using the property that spectral peak broadening occurs during spatial spectrum estimation of near-field targets under the far-field model, the present invention proposes a method for distinguishing far and near-field targets. By setting the ranging step size, the present invention adopts a method of first rough focusing scanning and then fine focusing scanning, and obtains the azimuth and distance of the near-field target at the place where the spatial spectrum spectral peak width is the narrowest; through the ranging method of the present invention, multiple targets can be detected one by one to obtain the ranging and direction-finding results of multiple near-field targets and the direction-finding results of far-field targets, and significantly improves the accuracy of azimuth estimation.

[0077] Moreover, the method of the present invention directly realizes the discrimination of the far and near fields of the target based on the remote one-dimensional azimuth spectrum, and can realize the discrimination of the far and near fields of the target without two-dimensional spatial scanning, significantly reducing the calculation amount of the discrimination of the far and near fields of the target. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 is a flowchart of a method for passive direction-finding and ranging of near-field targets by a large-aperture array of the present invention;

[0079] Figure 2 is a schematic diagram of the natural directivity of a conventional beamforming array;

[0080] Figure 3 This is a comparison chart of the azimuth spectrum results obtained by focusing and scanning at different distances for a 2km near-field target;

[0081] Figure 4 It is the far-field azimuth course measurement result of a near-field target moving in a straight line of 2km-4km;

[0082] Figure 5 It is the rough measurement result of the azimuth and distance of the 2km near-field target;

[0083] Figure 6 It is the precise measurement result of the azimuth and distance of the 2km near-field target;

[0084] Figure 7 is the ranging result of the moving target;

[0085] Figure 8 It is the direction finding result of moving target. DETAILED DESCRIPTION

[0086] Specific implementation method 1: Combination Figure 1 This embodiment describes a large aperture array near-field target passive direction finding and ranging method, the method specifically comprising the following steps:

[0087] Step 1: Obtain a calculation model for array far-field beamforming beam width according to the natural directivity of the uniform linear array;

[0088] Step 2: Initialization time t=0;

[0089] Step 3: Perform far-field beamforming on the array receiving signal at time t to obtain a far-field azimuth spectrum, and then obtain a rough measurement angle of each target to be measured and a peak width of the azimuth spectrum of each target to be measured according to the far-field azimuth spectrum;

[0090] Step 4: Initialize the number of targets to be measured n=1;

[0091] Step 5: Bring the coarse measurement angle of the nth target to be measured into the array far-field beam forming beam width calculation model, and compare the calculated beam width with the azimuth spectrum peak width of the nth target to be measured to make a near-field and far-field judgment on the nth target to be measured;

[0092] Step 6: If the nth target to be measured is in the near field, proceed to step 7;

[0093] If the nth target to be measured is in the far field, the rough angle is directly used as the azimuth estimation result of the nth target to be measured, and the distance cannot be measured; then set n=n+1, and return to step 5;

[0094] Step 7: Perform near-field focusing scanning on the nth target to be measured, obtain the azimuth spectrum peak width corresponding to each scanning distance, that is, obtain the main lobe width corresponding to each scanning distance of the nth target to be measured;

[0095] Determine the initial azimuth value and the initial distance value of the nth target to be measured according to the main lobe widths corresponding to each scanning distance;

[0096] Step 8: Determine the scanning range of the near-field focusing fine scanning according to the initial azimuth value and the initial distance value of the nth target to be measured, and perform near-field focusing fine scanning within the determined scanning range to obtain the final azimuth and distance of the nth target to be measured;

[0097] Step 9: Determine whether n reaches the maximum number of near-field targets;

[0098] If it reaches, execute Step 10;

[0099] If it does not reach, let n = n + 1, and return to execute Step 5;

[0100] Step 10: Let t = t + 1, and return to execute Step 3.

[0101] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that the uniform linear array is specifically: taking the due east direction as the positive x-axis direction and the due north direction as the positive y-axis direction, each array element is evenly arranged at equal intervals on the positive half-axis of the x-axis, and the distance between two adjacent array elements is d.

[0102] Other steps and parameters are the same as those in Specific Embodiment 1.

[0103] Specific Embodiment 3: The difference between this embodiment and Specific Embodiment 1 or 2 is that the specific process of Step 1 is:

[0104] Define the target incident azimuth as the included angle in the north-east direction, and denote the incident azimuth of the kth target as θ k , k = 1, 2,..., K, where K is the number of incident targets. For the incident azimuth θ k of the kth target, the natural directivity G(θ) of the uniform linear array is:

[0105]

[0106] where M is the number of array elements, and |·| represents taking the absolute value;

[0107] According to |G(θ)| 2 = 1 / 2, that is, let |G(θ)| 2 = 1 / 2, and the θ derived from the above formula is the beam width. Then, the calculation model of the far-field beamforming beam width of the array corresponding to the incident azimuth of the kth target is obtained as:

[0108]

[0109] where, θ DT is the beamwidth of the array far-field beamforming corresponding to the k-th target incident azimuth, λ is the signal wavelength, λ = c / f, c is the underwater sound speed, usually taken as 1500 m / s, and f is the signal frequency.

[0110] Other steps and parameters are the same as those in the first or second specific implementation manner.

[0111] Specific implementation manner four: The difference between this implementation manner and one of the first to third specific implementation manners is that the array received signal at time t is:

[0112]

[0113] where, x(t) represents the array received signal at time t, s(t) represents the target signal received by the array at time t, n(t) represents the noise signal received by the array at time t, and a(θ k ) represents the array steering vector corresponding to the angle θ k .

[0114] Other steps and parameters are the same as those in one of the first to third specific implementation manners.

[0115] Specific implementation manner five: The difference between this implementation manner and one of the first to fourth specific implementation manners is that the specific process of step three is as follows:

[0116] Step three one: In the range of θ ∈ [-90°, 90°], perform far-field beamforming on the array received signal x(t) to obtain the far-field azimuth spectrum p(θ):

[0117] p(θ) = a(θ) H Ra(θ)

[0118] where, R represents the covariance matrix, R = E[x(t)x H (t)], E[·] represents taking the expectation, x H (t) is the conjugate transpose of x(t), a(θ) is the array steering vector corresponding to the scanning angle θ on the azimuth scanning grid, and a(θ) H is the conjugate transpose of a(θ);

[0119]

[0120] where, e is the base of the natural logarithm, j is the imaginary unit, and the superscript T represents the transpose;

[0121] Step 32: Find the maximum peak positions of each local spectral peak in the far-field azimuth spectrum p(θ), and use the angles corresponding to the maximum peak positions of each local spectral peak as the rough measurement angles of each target to be measured. Denote the rough measurement angle of the k-th target to be measured as θ k ′;

[0122] Taking the maximum peak position of any one local spectral peak as an example, the calculation method for the spectral peak width of the target azimuth spectrum is as follows:

[0123] Denote the azimuth corresponding to a 3 dB drop on the left side of the maximum peak position of this local spectral peak as θ L , and denote the azimuth corresponding to a 3 dB drop on the right side of the maximum peak position of this local spectral peak as θ R . Then, the spectral peak width Δθ of the azimuth spectrum corresponding to the maximum peak position of this local spectral peak for the target to be measured is:

[0124] Δθ = |θ R - θ L |

[0125] Similarly, obtain the spectral peak widths of the azimuth spectra corresponding to the maximum peak positions of each local spectral peak for each target to be measured respectively.

[0126] Other steps and parameters are the same as those in any one of the specific embodiments 1 to 4.

[0127] Figure 2 is a schematic diagram of the natural directivity of a far-field conventional beamforming array with an array element number of 10 and a half-wavelength array arrangement; according to Figure 2 the results, measure the spectral peak width of the target to be measured of interest, find the maximum peak position of the local spectral peak, and record the angle θ′ k = 0°. Use this angle as the rough measurement angle of the target azimuth, and use the azimuths corresponding to a 3 dB drop on the left and right sides of the maximum value as the left and right half-power beamwidth angles respectively. The difference Δθ = |θ R - θ L |. Δθ is the main lobe width, and θ L and θ R are the left and right half-power beamwidth angles respectively.

[0128] It should be noted that: when the target is near the end-fire, use the method of spatial spectrum periodic extension to perform periodic extension of the spatial spectrum in the range of -90° to 90° to the two end-fire directions respectively, and then select the left and right 3 dB half-power point angles of the spectral peak according to the spatially spectrally extended spatial spectrum.

[0129] Specific Embodiment 6: The difference between this embodiment and any one of the specific embodiments 1 to 5 is that the specific process of the said Step 5 is as follows:

[0130] For any target to be measured, substitute the roughly measured angle of the target to be measured into the far-field beamforming beamwidth calculation model of the array to obtain the beamwidth θ corresponding to the target to be measured. DT Then, compare the θ DT corresponding to the target to be measured with the peak width Δθ of the azimuth spectrum corresponding to the target to be measured:

[0131] If Δθ ≥ ε·θ DT , where ε is the decision threshold, then the target to be measured is located in the near field;

[0132] Otherwise, Δθ < ε·θ DT , and the target to be measured is located in the far field.

[0133] Other steps and parameters are the same as those in any one of the specific embodiments one to five.

[0134] Similarly, the method of this embodiment can be used to perform near and far field judgments on each target to be measured respectively. ε is usually greater than 1.5 to prevent the influence of beam broadening caused by signal-to-noise ratio.

[0135] Specific embodiment seven: The difference between this embodiment and any one of the specific embodiments one to six is that the specific process of step seven is as follows:

[0136] Step seven one: Perform distance grid division (with an interval of 1 km) within the search range θ ∈ [-90°, 90°], where D represents the array aperture, and D = (M - 1)d;

[0137] The upper limit of the distance search range set by the present invention is one-fourth of the Rayleigh distance (D 2 / λ), and the lower limit of the distance is Then perform near-field focusing scanning;

[0138] Then perform near-field focusing scanning on the nth target to be measured at each distance respectively, obtain the azimuth spectrum corresponding to each scanning distance respectively, and then obtain the main lobe width corresponding to each scanning distance according to the azimuth spectrum;

[0139] Step seven two: Sort the main lobe widths corresponding to each scanning distance in ascending order of the main lobe width:

[0140] If the smallest main lobe width corresponds to only one scanning distance, then use the scanning distance corresponding to the smallest main lobe width as the initial distance value of the target to be measured, and obtain the peak of the azimuth spectrum obtained when using the initial distance value as the scanning distance, and use the angle corresponding to the obtained peak as the initial azimuth value of the target to be measured;

[0141] The ranging error will cause multiple peaks to appear in the azimuth spectrum estimation result near the target azimuth. Therefore, the main lobe widths at different distances may be equal. If the smallest main lobe width corresponds to multiple scanning distances, calculate the degree of azimuth spectrum power drop at each scanning distance corresponding to the smallest main lobe width respectively. Take the scanning distance with the largest degree of azimuth spectrum power drop as the initial value of the distance of the target to be measured, and take the angle corresponding to the azimuth spectrum peak obtained when the initial value of the distance is used as the scanning distance as the initial value of the azimuth of the target to be measured;

[0142] The calculation method of the degree of azimuth spectrum power drop is as follows:

[0143] Taking the azimuth spectrum at any one scanning distance as an example, denote the peak power of this azimuth spectrum as P max , on the left side of the grid point where the peak power is located, denote the power corresponding to the grid point with an interval of 1° from the grid point where the peak power is located as P -1° , on the right side of the grid point where the peak power is located, denote the power corresponding to the grid point with an interval of 1° from the grid point where the peak power is located as P 1° , then the degree of power drop ΔP of this azimuth spectrum is:

[0144] ΔP = (|P max - P -1° | + |P max - P 1° |) / 2.

[0145] Other steps and parameters are the same as those in any one of the specific embodiments one to six.

[0146] The higher the degree of power drop, the higher the angular resolution of the azimuth spectrum result, that is, the smaller the ranging error. Add a small amount to the main lobe width corresponding to the azimuth spectrum with a lower degree of power drop. The size of the small amount is 1 / 10 of the grid quantization accuracy. Take the azimuth and distance corresponding to the narrowest main lobe width as the initial values for fine scanning.

[0147] Specific embodiment eight: The difference between this embodiment and any one of the specific embodiments one to seven is that the azimuth spectrum in step seven one is:

[0148] Taking the i-th scanning distance as an example, the azimuth spectrum p(θ, r i ) corresponding to the i-th scanning distance r i is:

[0149] p i = p(θ, r i )

[0150] p(θ, r i ) = a(θ, r i ) H Ra(θ, r i )

[0151] Among them, a(θ, r i ) represents the array steering vector at the i-th scanning distance. The i'-th scanning azimuth θ i of the array steering vector a(θ, r i′ The steering vector a(θ i′ , r i ) is as follows:

[0152]

[0153] Among them, when M is odd, N = (M - 1) / 2; when M is even, N = M / 2. a(θ i′ , r i ) ∈ C 2N×1 , γ i′ and φ i′ are respectively expressed as:

[0154]

[0155] Other steps and parameters are the same as those in any one of the first to seventh specific embodiments.

[0156] Specific Embodiment Nine: The difference between this embodiment and any one of the first to eighth specific embodiments is that the specific process of Step 8 is as follows:

[0157] Find the scanning distance closest to and less than the distance initial value in Step 7, and then find the scanning distance closest to and greater than the distance initial value in Step 7. The found scanning distances are respectively used as the lower and upper limits of the distance for the near-field focusing fine scan;

[0158] That is, among the respective scanning distances corresponding to the azimuth initial value, find the grid point closest to the grid point where the distance initial value is located, and use the scanning distance corresponding to the found grid point as the distance interval for the near-field focusing fine scan;

[0159] Substitute the azimuth initial value into the beam width calculation model to obtain the beam width calculation result. Then, use the difference between the azimuth initial value and the beam width calculation result as the lower limit of the angle for the near-field focusing fine scan, and use the sum of the azimuth initial value and the beam width calculation result as the upper limit of the angle for the near-field focusing fine scan;

[0160] Perform a near-field focusing fine scan within the scan range formed by the lower distance limit, upper distance limit, lower angle limit, and upper angle limit, respectively obtain the near-field focusing azimuth spectrum corresponding to each scanning distance, and then respectively obtain the main lobe width corresponding to each scanning distance according to the near-field focusing azimuth spectrum;

[0161] If the minimum main lobe width in each near-field focusing azimuth spectrum corresponds to only one scanning distance, the scanning distance corresponding to the minimum main lobe width is used as the final distance value of the target to be measured, and the peak of the near-field focusing azimuth spectrum obtained when the final distance value is used as the scanning distance is obtained, and the angle corresponding to the obtained peak is used as the final azimuth value of the target to be measured;

[0162] If the minimum main lobe width in each near-field focusing azimuth spectrum corresponds to multiple scanning distances, the degree of power reduction of the near-field focusing azimuth spectrum at each scanning distance corresponding to the minimum main lobe width is calculated respectively, and the scanning distance with the largest power reduction of the near-field focusing azimuth spectrum is taken as the final distance value of the target to be measured, and the azimuth corresponding to the azimuth spectrum peak of the near-field focusing azimuth spectrum with the largest power reduction is taken as the final azimuth value of the target to be measured.

[0163] The other steps and parameters are the same as those in Specific Embodiments 1 to 8.

[0164] The near-field focused fine scanning interval is grid-divided according to the actual accuracy requirements of direction and distance measurement. Generally, the grid quantization accuracy is less than 1 / 5 of the measurement accuracy requirement.

[0165] Experimental Section

[0166] The array used in the experiment is a 512-element linear array with an evenly spaced half-wavelength distribution. The element spacing is 1.2m, and the speed of sound in water is 1500m / s. The azimuth spectrum results obtained by focusing and scanning at different distances to a 2km near-field target are compared. Figure 3 The accuracy of the near-field target distance estimation affects the azimuth estimation result. When the distance estimation error is large, the spatial spectrum will have multiple peaks, and the target azimuth cannot be determined. Figure 3 It can be seen that the smaller the distance estimation error is, the narrower the width of the spatial spectrum peak is, which illustrates the feasibility of the method of the present invention.

[0167] Assume that at time t = 0, there is a target in space incident on the linear array, the near-field moving target has an incident azimuth of -40° and a distance of 2000m, a signal-to-noise ratio of 10dB, and moves away from the receiving array in the north-east direction. The spatial spectrum of the target is estimated at each time. Figure 4 Taking t=0 as an example for analysis, when the far-field model is used for spatial spectrum estimation, the target in the -40° direction has a spectrum divergence. According to the far-field and near-field target judgment formula, it can be seen that the target is located in the near field.

[0168] The azimuth and distance of the target in the near field are roughly measured. A rough distance grid (1 km interval) is drawn within the distance search range, and the azimuth spectrum is obtained by near-field focus compensation. Figure 5 For the convenience of analysis, Figure 5Only the azimuth spectrograms at different distances within 1 - 10 km are intercepted. For comparison Figure 5 with the main lobe widths at different distances in

[0169] Take the azimuth and distance results corresponding to the narrowest main lobe as the initial values for near - field focusing scanning. Figure 5 Use the azimuth and distance dimensions within the box in Figure 6 as the scanning range for near - field focusing fine scanning as shown in Figure 6 It can be seen that the near - field target is approximately located at the position of (-40°, 2 km). Perform the above - mentioned processing on the azimuth curves at each moment in Figure 4 to obtain the azimuth spectrogram history of the near - field target and obtain the azimuth and distance information.

[0170] At the same time, record the azimuth and distance information of the target at the current moment, and obtain the tracking distance history curve and azimuth history curve as shown in Figure 7 and Figure 8 respectively. The fine scanning finely divides the distance grid at intervals of 5 m. It can be seen from Figure 7 and Figure 8 that the method of the present invention can track the target position, and the size of the ranging error is affected by the division accuracy of the distance scanning grid.

[0171] The above - mentioned numerical examples of the present invention are only to illustrate the calculation model and calculation process of the present invention in detail, rather than to limit the implementation manner of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made on the basis of the above description. It is impossible to list all the implementation manners here. Any obvious changes or modifications derived from the technical solutions of the present invention still fall within the protection scope of the present invention.

Claims

1. A large aperture array near-field target passive direction finding and ranging method, characterized in that: The method specifically comprises the following steps: Step 1: Obtain a calculation model for array far-field beamforming beam width according to the natural directivity of the uniform linear array; Step 2: Initialization time t=0; Step 3: Perform far-field beamforming on the array receiving signal at time t to obtain a far-field azimuth spectrum, and then obtain a rough measurement angle of each target to be measured and a peak width of the azimuth spectrum of each target to be measured according to the far-field azimuth spectrum; Step 4: Initialize the number of targets to be measured n=1; Step 5: Bring the coarse measurement angle of the nth target to be measured into the array far-field beam forming beam width calculation model, and compare the calculated beam width with the azimuth spectrum peak width of the nth target to be measured to make a near-field and far-field judgment on the nth target to be measured; Step 6: If the nth target to be measured is in the near field, proceed to step 7; If the nth target to be measured is located in the far field, the rough measured angle is directly used as the azimuth estimation result of the nth target to be measured; then n=n+1, and the process returns to step 5; Step 7: Perform near-field focusing scanning on the nth target to be measured to obtain the azimuth spectrum peak width corresponding to each scanning distance, that is, obtain the main lobe width corresponding to each scanning distance of the nth target to be measured; Determine the initial azimuth value and the initial distance value of the nth target to be measured according to the main lobe width corresponding to each scanning distance; Step 8: determining a scanning interval of near-field focusing fine scanning according to the initial value of the azimuth and the initial value of the distance of the nth target to be measured, performing near-field focusing fine scanning within the determined scanning interval, and obtaining the final azimuth and distance of the nth target to be measured; Step nine, determining whether n reaches the maximum number of near-field targets; If reached, execute step 10; If not reached, set n=n+1 and return to step 5; Step 10: Set t=t+1 and return to step 3.

2. A large aperture array near-field target passive direction finding and ranging method according to claim 1, characterized in that: The uniform linear array is specifically: with due east as the positive direction of the x-axis and due north as the positive direction of the y-axis, each array element is evenly arranged at equal intervals on the positive semi-axis of the x-axis, and the distance between two adjacent array elements is d.

3. A large aperture array near-field target passive direction finding and ranging method according to claim 2, characterized in that: The specific process of step one is: The target incident direction is defined as the angle in the north-east direction, and the incident direction of the kth target is recorded as θ k , k = 1, 2, ..., K, K is the number of incident targets, and the incident azimuth θ of the kth target k , the natural directivity G(θ) of a uniform linear array is: Where M is the number of array elements, |·| represents the absolute value; According to |G(θ)| 2 =1 / 2, then the array far-field beamforming beam width calculation model corresponding to the k-th target incident azimuth is obtained as: Among them, θ DT is the array far-field beamforming beam width corresponding to the k-th target incident azimuth, and λ is the signal wavelength.

4. A large aperture array near-field target passive direction finding and ranging method according to claim 3, characterized in that: The array receiving signal at time t is: Among them, x(t) represents the array received signal at time t, s(t) represents the target signal received by the array at time t, n(t) represents the noise signal received by the array at time t, and a(θ k ) represents the angle θ k The corresponding array steering vector.

5. A large aperture array near-field target passive direction finding and ranging method according to claim 4, characterized in that: The specific process of step three is: Step 31: Within θ∈[-90°,90°], perform far-field beamforming on the array receiving signal x(t) to obtain the far-field azimuth spectrum p(θ): p(θ)=a(θ) H Ra(θ) Where R represents the covariance matrix, R = E[x(t)x H (t)], E[·] means to find the expectation, x H (t) is the conjugate transpose of x(t), a(θ) is the array steering vector corresponding to the scanning angle θ on the azimuth scanning grid, and a(θ) H is the conjugate transpose of a(θ); Where e is the base of the natural logarithm, j is the imaginary unit, and the superscript T represents the transpose; Step 32: Find the maximum peak position of each local spectral peak in the far-field azimuth spectrum p(θ), take the angle corresponding to the maximum peak position of each local spectral peak as the rough measurement angle of each target to be measured, and record the rough measurement angle of the kth target to be measured as θ k ′; Taking the maximum peak position of any local spectrum peak as an example, the calculation method of the peak width of the target azimuth spectrum to be measured is: The position corresponding to the maximum peak position of the local spectrum peak dropping 3dB to the left is recorded as θ L The direction corresponding to the maximum peak position of the local spectrum peak dropping 3dB to the right is recorded as θ R , then the azimuth spectrum peak width Δθ of the target to be measured corresponding to the maximum peak position of the local spectrum peak is: Δθ=|θ R -θ L | Similarly, the maximum peak position of each local spectrum peak is obtained, corresponding to the azimuth spectrum peak width of the target to be measured.

6. A large aperture array near-field target passive direction finding and ranging method according to claim 5, characterized in that: The specific process of step five is as follows: For any target to be measured, the coarse measurement angle of the target to be measured is brought into the array far-field beamforming beam width calculation model to obtain the beam width θ corresponding to the target to be measured DT , and then the θ corresponding to the target to be measured DT Compare with the azimuth spectrum peak width Δθ corresponding to the target to be measured: If Δθ≥ε·θ DT , ε is the decision threshold, then the target to be measured is located in the near field; Otherwise, Δθ<ε·θ DT , the target to be measured is located in the far field.

7. A large aperture array near-field target passive direction finding and ranging method according to claim 6, characterized in that: The specific process of step seven is as follows: Step 71: In the search range Distance grid division is performed within θ∈[-90°,90°], where D represents the array aperture, D=(M-1)d; Then, a near-field focusing scan is performed on the nth target to be measured at each distance, and an azimuth spectrum corresponding to each scanning distance is obtained, and then a main lobe width corresponding to each scanning distance is obtained according to the azimuth spectrum; Step 72: Sort the main lobe widths corresponding to each scanning distance in order from small to large: If the minimum main lobe width corresponds to only one scanning distance, the scanning distance corresponding to the minimum main lobe width is used as the initial distance value of the target to be measured, and the peak of the azimuth spectrum obtained when the initial distance value is used as the scanning distance is obtained, and the angle corresponding to the obtained peak is used as the initial azimuth value of the target to be measured; If the minimum main lobe width corresponds to multiple scanning distances, the degree of decrease in azimuth spectrum power at each scanning distance corresponding to the minimum main lobe width is calculated respectively, and the scanning distance with the largest degree of decrease in azimuth spectrum power is taken as the initial distance value of the target to be measured, and the angle corresponding to the azimuth spectrum peak obtained when the initial distance value is taken as the scanning distance is taken as the initial azimuth value of the target to be measured; The calculation method of the azimuth spectrum power reduction degree is: Taking the azimuth spectrum at any scanning distance as an example, the peak power of the azimuth spectrum is recorded as P max , on the left side of the grid point where the peak power is located, the power corresponding to the grid point with an interval of 1° from the grid point where the peak power is located is recorded as On the right side of the grid point where the peak power is located, the power corresponding to the grid point with an interval of 1° from the grid point where the peak power is located is recorded as Then the decrease degree ΔP of the azimuth spectrum power is:

8. The large aperture array near-field target passive direction finding and ranging method according to claim 7, characterized in that: The orientation spectrum in step 71 is: Taking the i-th scanning distance as an example, the i-th scanning distance r i The corresponding azimuth spectrum p(θ,r i )for: p(θ,r i )=a(θ,r i ) H Ra(θ,r i ) Among them, a(θ,r i ) represents the array steering vector at the i-th scanning distance, and the array steering vector a(θ, r i )’s i′th scanning position θ i′ The steering vector a(θ i′ ,r i )for: When M is an odd number, N = (M-1) / 2, and when M is an even number, N = M / 2. i′ ,r i )∈C 2N×1 , γ i′ and φ i′ Respectively expressed as:

9. A large aperture array near-field target passive direction finding and ranging method according to claim 8, characterized in that: The specific process of step eight is as follows: Find the scanning distance in step 7 that is closest to the initial distance value and smaller than the initial distance value, and then find the scanning distance in step 7 that is closest to the initial distance value and larger than the initial distance value, and use the found scanning distances as the lower limit and upper limit of the distance of the near-field focusing fine scanning respectively; Substitute the initial azimuth value into the beam width calculation model to obtain the beam width calculation result, then use the difference between the initial azimuth value and the beam width calculation result as the lower limit of the angle of near-field focusing fine scanning, and use the sum of the initial azimuth value and the beam width calculation result as the upper limit of the angle of near-field focusing fine scanning; Perform near-field focusing fine scanning within the scanning interval to obtain the near-field focusing azimuth spectrum corresponding to each scanning distance, and then obtain the main lobe width corresponding to each scanning distance according to the near-field focusing azimuth spectrum; If the minimum main lobe width in each near-field focusing azimuth spectrum corresponds to only one scanning distance, the scanning distance corresponding to the minimum main lobe width is used as the final distance value of the target to be measured, and the peak of the near-field focusing azimuth spectrum obtained when the final distance value is used as the scanning distance is obtained, and the angle corresponding to the obtained peak is used as the final azimuth value of the target to be measured; If the minimum main lobe width in each near-field focusing azimuth spectrum corresponds to multiple scanning distances, the degree of power reduction of the near-field focusing azimuth spectrum at each scanning distance corresponding to the minimum main lobe width is calculated respectively, and the scanning distance with the largest power reduction of the near-field focusing azimuth spectrum is taken as the final distance value of the target to be measured, and the azimuth corresponding to the azimuth spectrum peak of the near-field focusing azimuth spectrum with the largest power reduction is taken as the final azimuth value of the target to be measured.

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