A dual-transmit multiple-receive multi-beam synthetic aperture sonar array and imaging method thereof

Abstract

The present invention belongs to the field of measurement technology, and relates to a dual-transmit, multi-receive, multi-beam synthetic aperture sonar array and an imaging method thereof. The array includes two mutually parallel transmitting array elements arranged along the track direction, and the two transmitting array elements are symmetrically placed along the center point of the array; the same transmitting signal source is used to synchronously excite the transmitted sound signal; a number of horizontally distributed parallel receiving arrays are evenly arranged along the track direction, and the horizontal spacing of the receiving array elements is half a wavelength, and the spacing in the track direction is much larger than half a wavelength; the transmitting array elements and the receiving arrays are perpendicular to each other. The array of the present invention adopts a large-spacing array arrangement in the track direction, which effectively reduces the scale of the system and improves the sonar detection efficiency. Its imaging algorithm only scans and images the area within a small angle range directly below the array point by point each time it detects, and synthesizes the detection area image by irradiating the target at multiple positions, avoiding the "grating lobe" problem introduced by the large-spacing array, thereby improving the detection efficiency of the sonar system.

Description

Technical Field

[0001] The invention belongs to the field of measurement technology, and in particular relates to a dual-transmit and multi-receive multi-beam synthetic aperture sonar array and an imaging method thereof. Background Art

[0002] Multi-beam synthetic aperture sonar combines the technical advantages of multi-beam bathymetric sonar and side-scan synthetic aperture sonar. It can overcome the shortcoming that the imaging resolution of multi-beam bathymetric sonar decreases with the increase of detection distance, and breaks through the problem of vertical bottom detection gap of side-scan synthetic aperture sonar from the detection mechanism. It is a new high-resolution detection technology in three-dimensional space. However, the current multi-beam synthetic aperture sonar mostly adopts the traditional multi-beam bathymetric sonar transmitting array and receiving array vertically set base array. The single-element transmitting array design causes the equivalent phase center of the base array to shift, which is not conducive to the motion posture correction of the base array. The single-line array receiving array design also limits the sampling interval of the multi-beam synthetic aperture sonar in the track direction, reducing the detection efficiency of the system. Summary of the invention

[0003] In order to solve the problem of low detection efficiency caused by the half-wavelength array limitation of the sampling interval of the multi-beam synthetic aperture sonar track, this application proposes a dual-transmit multi-receive multi-beam synthetic aperture sonar array structure, and also proposes a three-dimensional point-by-point imaging algorithm for the array structure. The array form of dual-transmit array elements and multiple receiving linear arrays is adopted to make the array have a track axis symmetrical structure, which is conducive to the six-degree-of-freedom motion attitude compensation of the receiving array element position.

[0004] The present invention provides a dual-transmit multi-receive multi-beam synthetic aperture sonar array, comprising: two transmitting array elements arranged in parallel along the track direction, the two transmitting array elements are symmetrically placed along the center point of the array; the same transmitting signal source is used to synchronously excite the transmitting acoustic signal;

[0005] Several parallel receiving linear arrays are evenly arranged along the track, with the horizontal spacing of the receiving array elements being half a wavelength, and the spacing along the track being much larger than half a wavelength;

[0006] The transmitting array element and the receiving array are perpendicular to each other.

[0007] Based on the base array, the imaging method provided by the present invention includes:

[0008] Step (1): using the array motion data recorded by the attitude sensor and the position sensor to perform six-degree-of-freedom motion attitude compensation on the transmitting array element and the receiving array element position of the multi-beam synthetic aperture sonar;

[0009] Step (2): At multiple track sampling positions, the distance between the scanning pixel point and the equivalent transmission center and the position of each receiving array element is calculated point by point, and the time delay between each receiving array element and the scanning pixel point is calculated according to the on-site sound speed;

[0010] Step (3): Perform phase shift compensation calculation on the echo signal received by the receiving array element, and then perform multi-beam synthetic aperture algorithm imaging processing on the echo signal within the same synthetic aperture interval to obtain a multi-beam synthetic aperture imaging output in three-dimensional space.

[0011] Preferably, in step (1), the real coordinate position of each receiving array element at any time and the real coordinate position of the equivalent transmitting array element The six-degree-of-freedom motion posture compensation is obtained by matrix operation of the translation error matrix, attitude error matrix and initial position:

[0012] ;

[0013] ;

[0014] in, represents the translation error matrix formed by the translation motion of the carrier at any time; , , They represent the attitude error matrices of rotation around each axis caused by the swing of the carrier, For rolling, For pitch, For the bow to be tilted; represents the initial coordinate position of all receiving array elements, Represents the initial coordinate position of the equivalent emission center.

[0015] Preferably, in step (2), the distance between the scanning pixel point and the equivalent transmission center and each receiving array element position is:

[0016] ;

[0017] ;

[0018] in, For the The distance between the receiving array element and the scanning pixel point; is the distance between the equivalent transmitting array element and the scanning pixel point; , , Respectively represent The receiving array element , , The current position on the axis; , , They represent the equivalent emission centers at , , Current position on the axis; ( , , ) is the coordinate of the scanned pixel point;

[0019] Depend on and Calculate the The receiving array elements are in the navigation position The time delay between the initial position and ,in The speed of sound at the scene is:

[0020] .

[0021] Preferably, the multi-beam synthetic aperture imaging output in three-dimensional space is:

[0022] ;

[0023] in, is the signal center frequency, is the symbol of the imaginary part; Indicates The position of the receiving array element in the current synthetic aperture interval Department, The echo signal received at every moment; is the total number of navigation positions within the current synthetic aperture interval; is the total number of receiving array elements.

[0024] The present application proposes a dual-transmit, multi-receive, multi-beam synthetic aperture sonar array and its imaging method. A large-spacing array is used in the track upward direction, and a narrower receiving beam can be obtained by using fewer array elements, which effectively reduces the system scale and improves the sonar detection efficiency. A dual-transmit, multi-receive transducer array is designed so that the array has a track axis-symmetrical structure, which is conducive to the six-degree-of-freedom motion attitude compensation of the received signal. The three-dimensional point-by-point imaging method proposed in the present application relies on the designed two-dimensional array structure. Each detection only scans and images the area within a small angle range directly below the array point by point. By irradiating the target at multiple positions, the image of the detection area is synthesized, avoiding the "grating lobe" problem introduced by the large-spacing array, thereby improving the detection efficiency of the sonar system. By compensating the six-degree-of-freedom motion attitude of the array element position, the imaging "defocusing" problem is effectively overcome, and the imaging accuracy of the sonar system is significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1The schematic diagram of the structure of the dual-transmit multiple-receive multi-beam synthetic aperture sonar array in the embodiment of the present invention; wherein, 1. the first transmitting array element; 2. the second transmitting array element; 3. the receiving linear array; 4. the equivalent transmitting center;

[0026] Figure 2 Comparison of directivity between half-wavelength array and the array structure of the present invention; (a) 32×32 array elements with half-wavelength array; (b) 4×32 array elements with large spacing;

[0027] Figure 3 The following is a comparison of the imaging effects when the motion compensation method is used or not; (a) the imaging result without motion compensation; (b) the imaging result after motion compensation;

[0028] Figure 4 It is the slice effect of single line array imaging;

[0029] Figure 5 It is a half-wavelength array imaging slice effect;

[0030] Figure 6 This is the track-wise large-spacing imaging slice proposed by the present invention. DETAILED DESCRIPTION

[0031] The present invention is described more completely and clearly below in conjunction with the accompanying drawings and specific embodiments. The described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] Embodiment 1 This embodiment provides a dual-transmit multiple-receive multi-beam synthetic aperture sonar array. The structure design of the array is as follows: Figure 1 As shown, two mutually parallel first transmitting array elements 1 and second transmitting array elements 2 are arranged along the array track, the first transmitting array element 1 and the second transmitting array element 2 are symmetrically placed along the center point of the array, and the same transmitting signal source is used to synchronously excite the transmitting acoustic signal. The transmitting array elements and the receiving linear array are in a mutually perpendicular position relationship.

[0033] Each transmitting array element is 16cm long, and the distance between the equivalent transmitting center 4 and the origin of the track coordinate is 9cm. There are horizontally distributed parallel receiving line arrays 3, each receiving line array 3 has The receiving array element has a horizontal length of half a wavelength: The track length is 10cm. To prevent crosstalk between signals, the shortest distance between each two receiving arrays is 1cm. The center distance between each two receiving arrays is .

[0034] Taking the geometric center point of the array as the coordinate origin, the center frequency of the transmitted and received sound waves , effective bandwidth The total number of array elements in the base array designed by the present invention is , the horizontal beam width of the transmitting array element is , the beam width on the track is The horizontal coverage of the receiving array is , the center beam width is , the beam width on the track is The beam width satisfies the relationship , ensuring that the echo signal within the coverage of the transmitting beam can be completely received by the four receiving line arrays.

[0035] The array designed by the present invention has a narrow beam directivity in the track direction and a wide coverage in the horizontal direction for the transmitting array element; each receiving line array has a narrow beam directivity in the track direction and a wide coverage in the horizontal direction. It can significantly break through the sampling interval limitation in the track direction, increase the running speed of the sonar carrier, and thus improve the sonar imaging detection efficiency.

[0036] Example 2 The simulation compares the array directivity obtained by the half-wavelength array in the track direction and the large-spacing array in the track direction proposed in this application. The half-wavelength array can theoretically completely avoid the beam grating lobe problem, but if you want to obtain a narrower beam width, you need to arrange a larger number of receiving array elements. When the number of array elements is the same, the large-spacing array will obtain a larger array aperture, so that its main beam width will be significantly reduced, but when the array element spacing is greater than half a wavelength, the "grating lobe" problem will occur. When the number of array elements is In theory, there will be no grating lobe problem in the entire scanning range, and a good main-sidelobe ratio is maintained. However, limited by the half-wavelength array structure, its real aperture in the track direction is still small, and its beam width in the track direction is only equivalent to that in the horizontal direction. ,like Figure 2 When the large spacing array structure proposed in this application is adopted in the upward direction of the track, the number of array elements is , the "grating lobe" problem inevitably appeared, and multiple "grating lobes" appeared in the upward direction of the track. However, when observed in a small scale range directly below, the beam width in the direction of the track is better than that of the equidistant array, that is, when only 4 array elements are used, a larger array aperture and smaller beam width are still obtained than that of the 32-element equidistant array. , effectively reducing the array size, such as Figure 2 As shown in (b).

[0037] Embodiment 3 Based on the matrix provided in Embodiment 1, the imaging method provided in this embodiment includes the following steps:

[0038] Step (1): Use the array motion data recorded by the attitude sensor and the position sensor to perform six-degree-of-freedom motion attitude compensation on the transmitting element and the receiving element position of the multi-beam synthetic aperture sonar.

[0039] The sonar carrier moves in three-dimensional space, and the six degrees of freedom motion errors generated are recorded as lateral displacement. , Vertical Shift , Heave and roll , Pitch , bow deviation Assume that the sonar carrier moves at a speed Moving along the track, the translation error matrix formed by the translation motion of the carrier at any time is Denoted as:

[0040] .

[0041] The attitude error matrix of the rotation around each axis caused by the swing of the carrier , , Denoted as:

[0042] .

[0043] Assuming that the coordinates of the equivalent center point of the sonar carrier's starting position are marked as the origin of the coordinate system, the first The receiving array element , , The initial coordinate positions on the axis are expressed as , , , the initial coordinate position of the equivalent emission center is recorded as , , ,but represents the initial coordinate position of all receiving array elements, Represents the initial coordinate position of the equivalent emission center.

[0044] The actual coordinate position of each receiving array element at any time and the real coordinate position of the equivalent transmitting array element The six-degree-of-freedom motion posture compensation can be obtained by matrix operation of the translation error matrix, attitude error matrix and initial coordinate position:

[0045] ;

[0046] .

[0047] Step (2): At multiple sampling positions in the track direction, calculate the distance between the scanning pixel point and the equivalent transmission center and the position of each receiving array element point by point, and calculate the time delay between each receiving array element and the scanning pixel point based on the on-site sound speed.

[0048] Assume that the sonar carrier has traveled a total of Positions, that is, in the navigation zone The pore size synthesis is carried out in When the coordinate position of each receiving array element is , the coordinate position of the equivalent transmitting array element is When the pixel is scanned point by point When , we can calculate the The distance between the receiving array element and the scanning pixel is , the distance between the equivalent transmitting array element and the scanning pixel is :

[0049] ;

[0050] ;

[0051] Depend on and Calculate the The receiving array elements are in the navigation position The time delay between the initial position and ,in The speed of sound at the scene is:

[0052] .

[0053] Step (3): Perform phase shift compensation calculation on the echo signal received by the receiving array element, and then perform multi-beam synthetic aperture algorithm imaging processing on the echo signal within the same synthetic aperture interval to obtain a multi-beam synthetic aperture imaging output in three-dimensional space.

[0054] set up is the echo signal received by each receiving array element, then Indicates The position of the receiving array element in the current synthetic aperture interval Department, After scanning and integrating all scanning points within the scanning range point by point, the multi-beam synthetic aperture imaging output in three-dimensional space can be obtained. :

[0055] ;

[0056] in, is the signal center frequency, is the symbol of the imaginary part.

[0057] Example 4 The simulation compares the multi-beam synthetic aperture imaging output before and after six-degree-of-freedom motion attitude compensation. Given the simulation conditions, the carrier sails along the track at a speed of 0.5 m / s, and the error curve is superimposed on the theoretical motion trajectory. The position error of the carrier is set to change according to a sine wave with an amplitude of 1 m, and the attitude error is set to change according to a sine wave with an amplitude of 10°. 25 independent point targets are set on a plane with a depth value of 15 m, and the target spacing is 30 cm. The multi-beam synthetic aperture sonar imaging algorithm is used to perform three-dimensional imaging of the detection area. Figure 3 (a) in the middle is the imaging result without motion posture compensation. It can be found that due to the interference of six-degree-of-freedom motion posture error, the image has become "defocused" and the target position and number have been aliased. Figure 3 (b) is the imaging result after motion posture compensation, the target position and spacing are consistent with the theoretical preset values, and the energy concentration at the target position is good, which proves that the point-by-point imaging method proposed in the present invention can effectively suppress the motion posture mismatch problem.

[0058] Example 5 The imaging effects of different sampling intervals in different track directions under different array arrangements were simulated and compared. Figure 4-6 As shown. Set the simulation conditions: Each horizontally distributed receiving line array has 32 array elements, and the horizontal array element spacing is half a wavelength. The single point target is located 40 m below, that is, its three-dimensional coordinates are (0, 0, 40). Set the length of the transmitting array element The synthetic aperture scanning range is from -20D to 20D in the track direction, that is, the equivalent synthetic virtual aperture array length is 6.4 m. The multi-beam synthetic aperture sonar point-by-point imaging algorithm is used to process the transducer array echo signals of the single-line array structure, the half-wavelength array structure in the track direction, and the large-spacing array structure in the track direction. The sampling intervals in the track direction are designed to be 0.5D, 4.0D, and 10.0D, respectively, and the imaging slice results in the track direction are observed.

[0059] The single-line array track imaging slice results are as follows Figure 4 As shown in the figure, when the sampling interval in the track direction is 0.5D, the imaging result of the single-line array does not show "grating lobes" in the track direction, and its sidelobe level is equivalent to that of conventional synthetic aperture sonar. When the sampling interval in the track direction is 4.0D, the sidelobe level of the imaging result is significantly improved, which has interfered with the imaging result. When the sampling interval in the track direction increases to 10.0D, the sonar image is severely defocused, and multiple "grating lobes" and sidelobe peaks appear in the track direction. This shows that the sampling interval in the track direction of the multi-beam synthetic aperture sonar with a single-line array structure cannot be too high, that is, the speed of the sonar carrier cannot be too high, and the mapping efficiency is limited.

[0060] The results of the half-wavelength array imaging slice are as follows: Figure 5As shown, compared with the single-line array arrangement, The base array of the structure can obviously improve the sampling interval in the track direction. When the sampling interval is increased to 4.0D, no obvious side lobes or "grating lobes" appear. When the sampling interval is increased to 10.0D, the main lobe of the beam is widened, but the side lobe level is still lower than that of the single-line array structure. This shows that the half-wavelength array scheme in the track direction can effectively improve the sampling interval in the track direction of the multi-beam synthetic aperture sonar, that is, when the sonar carrier has a high speed, it can still perform effective imaging, but its array scale is too large, which is not conducive to engineering applications.

[0061] The results of the imaging slices in the large-spacing array are as follows: Figure 6 As shown in the figure, when the sampling interval in the track direction is 0.5D, by comparing the imaging results of the single-line array and the half-wavelength array structure, it can be found that the side lobes in the track direction are effectively suppressed, and the side lobes can still be kept low at the edge of the virtual aperture. When the sampling interval is increased to 4.0D, the side lobes close to the target position increase, but their normalized amplitude is also significantly lower than the single-line array result. When the sampling interval is increased to 10.0D, the main lobe of the beam widens, but the normalized amplitude of the "grating lobe" appearing in the upward direction of the track is also significantly lower than that of the single-line array and the half-wavelength array structure. In addition, although only the sampling interval is used The effective array aperture obtained by using the large-spacing array is still larger than the half-wavelength array, so the beam width obtained by this scheme is narrower. This shows that the track-wise large-spacing array scheme can effectively improve the track-wise sampling interval of the multi-beam synthetic aperture sonar, that is, when the sonar carrier has a high speed, it can still perform effective imaging, and the carrier's navigation speed can reach , that is, exceeding 3 knots in speed, can effectively improve the sonar mapping efficiency and reduce the array size.

Claims

1. A dual-transmit multiple-receive multi-beam synthetic aperture sonar array, characterized in that: include: Two transmitting array elements are arranged parallel to each other along the track direction, and the two transmitting array elements are placed symmetrically along the center point of the base array; the same transmitting signal source is used to synchronously excite the transmitting acoustic signal; Several parallel receiving linear arrays are evenly arranged along the track, with the horizontal spacing of the receiving array elements being half a wavelength, and the spacing along the track being much larger than half a wavelength; The transmitting array element and the receiving linear array are perpendicular to each other; the equivalent transmitting center and the geometric center of the base array coincide.

2. An imaging method based on the dual-transmit multiple-receive multi-beam synthetic aperture sonar array according to claim 1, characterized in that: include: Step (1): using the array motion data recorded by the attitude sensor and the position sensor to perform six-degree-of-freedom motion attitude compensation on the transmitting array element and the receiving array element position of the multi-beam synthetic aperture sonar; The actual position of each receiving element at any time and the actual position of the equivalent transmitting array element , obtained by performing six-DOF motion posture compensation through matrix operation on the translation error matrix, attitude error matrix and initial position: ； ； in, represents the translation error matrix formed by the translation motion of the carrier at any time; , , They represent the attitude error matrices of rotation around each axis caused by the swing of the carrier, For rolling, For pitch, For the bow to be tilted; represents the initial coordinate position of all receiving array elements, represents the initial coordinate position of the equivalent emission center; Step (2): At multiple track sampling positions, the distance between the scanning pixel point and the equivalent transmission center and the position of each receiving array element is calculated point by point, and the time delay between each receiving array element and the scanning pixel point is calculated according to the on-site sound speed; Step (3): Perform phase shift compensation calculation on the echo signal received by the receiving array element, and then perform multi-beam synthetic aperture algorithm imaging processing on the echo signal within the same synthetic aperture interval to obtain a multi-beam synthetic aperture imaging output in three-dimensional space: ； in, is the signal center frequency, is the symbol of the imaginary part; Indicates The position of the receiving array element in the current synthetic aperture interval Department, The echo signal received at all times; is the total number of navigation positions within the current synthetic aperture interval; is the total number of receiving array elements.

3. The imaging method according to claim 2, characterized in that: In step (2), the distance between the scanning pixel point and the equivalent transmission center and the position of each receiving array element is: ； ； in, For the The distance between the receiving array element and the scanning pixel point; is the distance between the equivalent transmitting array element and the scanning pixel point; , , Respectively represent The receiving array element , , The current position on the axis; , , They represent the equivalent emission centers at , , Current position on the axis; ( , , ) is the coordinate of the scanned pixel point; Depend on and Calculate the The receiving array elements are in the navigation position The time delay between the initial position and ,in The speed of sound at the scene is: 。

Images