CN112505710B - Multi-beam synthetic aperture sonar three-dimensional imaging algorithm - Google Patents
Multi-beam synthetic aperture sonar three-dimensional imaging algorithm Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
- G01S15/88—Sonar systems specially adapted for specific applications
- G01S15/89—Sonar systems specially adapted for specific applications for mapping or imaging
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
- G01S7/539—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
Abstract
The application provides a multi-beam synthetic aperture sonar three-dimensional imaging algorithm, which is characterized in that a large aperture array is virtually synthesized by the motion of a carrier in the course of flight according to the multi-beam synthetic aperture sonar transducer array structure, and the arrival azimuth of an echo is resolved by utilizing a multi-receiving array element structure in the horizontal direction. And through the movement of the carrier, the detection areas are irradiated at positions of different tracks, and the received echo signals are coherently compensated and accumulated, so that the three-dimensional sonar image output under the track-slope distance-angle coordinate system can be obtained. And transforming the sonar image into a three-dimensional sonar image under a horizontal-track-depth coordinate system which is easier to observe by using coordinate transformation. The method breaks through the mechanism limitations of multi-beam sounding sonar and conventional synthetic aperture sonar, obtains constant imaging resolution in the track direction, obtains higher imaging resolution in the horizontal direction and depth direction planes, and realizes the fine sonar imaging in the three-dimensional space.
Description
Technical Field
The application relates to a multi-beam synthetic aperture sonar three-dimensional imaging algorithm, and belongs to the field of sonar signal processing.
Background
For the development and utilization of ocean resources, the primary premise is to carry out fine detection on the topography and the special target of interest of the underwater area, and the method has the technical requirements of large detection area, high fine requirement and strong anti-interference capability. Compared with a conventional optical imaging system, the imaging sonar system can perform remote detection under the working conditions of low underwater visibility and turbid water quality, and has a wider application range. Imaging sonar technology has been developed for decades, and has undergone the development processes of two-dimensional image to three-dimensional image, single-point detection to multi-point detection, single carrier to multi-carrier flexible application and single-frequency signal to multi-frequency signal, so that engineering application is gradually developed from theoretical research, and a serialized and differentiated commercial product system is formed. At present, imaging sonar commonly found in the market mainly comprises three types of side-scan sonar, synthetic aperture sonar and multi-beam sounding sonar.
The side-scan sonar has the technical advantages of simple array structure and lower cost, and is widely applied to the field of large-area sea-sweeping measurement. The basic working principle is that transducer matrixes with certain directivity are arranged on two sides of a sonar carrier such as a towed fish, echo signals returned from the water bottom are radiated and received, each time an echo energy line which is arranged according to time is formed by detection, and a two-dimensional sonar image under a track direction-distance direction plane is formed by splicing along with the navigation of the carrier. The defect of the method is mainly that a detection gap exists right below the imaging device, two-dimensional sonar imaging can be only carried out, and the imaging resolution of the track direction is reduced along with the increase of the acting distance. The synthetic aperture sonar array structure is similar to a side-scan sonar, echo signals of a plurality of sampling positions in the track are subjected to coherent processing, and finally imaging resolution independent of the acting distance and the detection frequency is obtained in the track. The method has the advantages that a small-aperture matrix can be used, a large-aperture matrix is virtually synthesized through movement of the carrier, and the track resolution of the image is improved. The system is complex, the cost is extremely high, the problem of detecting gaps under the system still exists, and the gap is usually required to be supplemented by carrying the multi-beam sonar. The multi-beam sounding sonar adopts tens or even hundreds of receiving array elements which are arranged along the horizontal direction, a two-dimensional sonar image under an oblique distance-angle coordinate system is obtained by each detection, and a three-dimensional full-coverage sonar image of a detection area is formed by navigation and splicing of a carrier. The method has the advantages of high depth estimation accuracy, no detection gap under the device, and full coverage measurement. The disadvantage is that its track and horizontal imaging resolution is limited by the footprint of the beam, decreasing with increasing detection distance.
The multi-beam synthetic aperture sonar is a novel imaging sonar combining the technical characteristics of multi-beam sounding sonar and side scanning synthetic aperture sonar, and can obtain finer imaging results in a three-dimensional space. The basic working principle of the multi-beam synthetic aperture sonar is similar to that of a conventional multi-beam sounding sonar, and the high-precision wave arrival azimuth estimation is realized by the arrangement of a multi-array element linear array in the horizontal direction; and the space sampling interval of the track is increased in a mode of large-space array in the track direction. The core idea of the multi-beam synthetic aperture sonar imaging algorithm provided by the application is to utilize the transducer array manifold of a two-dimensional area array to virtually synthesize a large aperture array in the course of navigation through a carrier, so as to obtain the constant image resolution of the course. And carrying out beam forming processing on a horizontal plane and a depth plane by utilizing a horizontal multi-array element linear array to obtain a sonar image under an oblique direction-angle direction coordinate system, and finally obtaining a three-dimensional sonar image under a track direction-horizontal direction-depth direction coordinate system after coordinate transformation.
Disclosure of Invention
The application aims to provide a multi-beam synthetic aperture sonar three-dimensional imaging algorithm according to a two-dimensional multi-beam synthetic aperture sonar array structure and combining a track-oriented aperture synthesis theory and a horizontal beam forming method, so that the three-dimensional target detection refinement degree of the sonar is improved.
The purpose of the application is realized in the following way: the method comprises the following steps:
step one: performing orthogonal transformation on the received array signals, converting the real signals into analytic signals, and performing band-pass filtering on the received signals according to the transmitting signal parameters;
step two: performing matched filtering processing on the analytic signal by using the transmitted reference signal to realize the pulse compression process of the linear frequency modulation signal, and correcting the signal delay brought by convolution processing;
step three: performing track-oriented aperture synthesis processing on the pulse compressed signals, virtually synthesizing receiving subarrays at the same horizontal position into a large-aperture receiving array along the track direction, and obtaining a plurality of two-dimensional sonar complex images under a track-oriented-oblique coordinate system, wherein the array is equivalent to a horizontal uniform linear array with a virtual large aperture in the track direction;
step four: performing horizontal beam forming treatment on the obtained virtual large-aperture horizontal linear array, and performing coherent accumulation on two-dimensional complex images at different track positions to obtain a three-dimensional sonar image under a track-pitch-angle coordinate system;
step five: and carrying out coordinate transformation on the three-dimensional sonar image under the track direction-oblique direction-angle direction coordinate system to obtain a horizontal direction-track direction-depth direction sonar image which is easier to observe directly.
The application also includes such structural features:
1. the first step is as follows: the time domain signal received by the transducer array element is a real signal, and the phase shift processing can be performed only by converting the real signal into an analysis signal, so that the analysis form of the signal is obtained by adopting orthogonal transformation in a sonar system, and the following formula is shown:
wherein s is i (n) is the echo signal received by the i array element at the nth sampling time, f 0 For signal frequency, f s Is the sampling rate.
2. In the second step, the sonar system adopts a linear frequency modulation signal as a detection signal, and a complex signal form of a transmitting signal s (t) is as follows:
wherein: t is the pulse width of a transmitting signal, and k is the modulation slope;
the matched filter outputs are:
wherein: s is(s) R (t) is a received signal, and h (t) is a reference signal.
3. The third step is as follows: assume that the number of track directional array elements of the two-dimensional area array is N y The number of the array elements of the horizontal line is N x The imaging result requires the formation of M beams in the horizontal direction; n to be in the same horizontal position y The array elements are processed by a synthetic aperture algorithm to obtain 1 array element with a virtual aperture; n for all horizontal positions x After traversing each array element, N with virtual large aperture in the course direction can be obtained x A cell linear array, and N is obtained at the same time x A synthetic aperture sonar image is displayed; imaging results in the track-to-range (y-r) coordinate system are shown below:
wherein: b is signal bandwidth, c is acoustic velocity in water, Γ is virtual synthetic aperture scale, lambda is signal wavelength, v is carrier movement velocity, and n is array element sequence number.
4. Carrying out wave beam forming processing on the obtained two-dimensional sonar complex image, and distinguishing the arrival azimuth of the echo at the position of the same track; on the basis of the complex image obtained in the step three, carrying out beam forming algorithm processing on the obtained uniform linear array with the virtual aperture on the flight path, so as to realize sonar imaging in a three-dimensional space:
wherein θ is the beamScan angle, theta T For the direction of the target, d x Is the horizontal array element spacing.
5. For the obtained three-dimensional sonar image I MBSAS (y, r, θ) coordinate transformation to convert the track-pitch-angle-oriented image into a more easily observable horizontal-track-depth-oriented image I MBSAS (x,y,z)；
x＝rsinθ
y＝rcosθ。
Compared with the prior art, the application has the beneficial effects that:
1. the full-coverage three-dimensional measurement can be realized within a certain angle range, a vertical bottom detection blind area does not exist, and no extra equipment is needed for gap compensation. The sonar carrier is flexible, and the navigation measurement can be realized through a shipborne or AUV carrying mode.
2. The image resolution is the same as that of the side-scanning synthetic aperture sonar in the track direction, and the carrier is used for navigating the virtual synthetic large aperture array to obtain constant imaging resolution independent of the detection distance and the signal frequency.
3. The target echo direction can be resolved by the beam forming technology in the horizontal direction, the distance and direction resolution are higher, and a three-dimensional sonar image with higher resolution can be finally obtained after coordinate transformation.
Drawings
FIG. 1 is a basic schematic diagram of multibeam synthetic aperture sonar imaging;
FIG. 2 is a schematic diagram of a multibeam synthetic aperture sonar imaging procedure;
FIG. 3 is a schematic diagram of track-to-aperture synthesis;
FIG. 4 is a schematic diagram of horizontal beamforming;
fig. 5 (a) - (f) are multi-beam sounding sonar versus multi-beam synthetic aperture sonar simulation imaging results, wherein fig. 5 (a) is a multi-beam sounding sonar imaging result, fig. 5 (b) is a multi-beam synthetic aperture sonar imaging result, fig. 5 (c) is a multi-beam sounding sonar imaging top surface slice, fig. 5 (d) is a multi-beam synthetic aperture sonar imaging top surface slice, fig. 5 (e) is a multi-beam sounding sonar imaging bottom surface slice, and fig. 5 (f) is a multi-beam synthetic aperture sonar imaging bottom surface slice;
fig. 6 (a) shows the detection result of the multi-beam sounding sonar imaging target, and fig. 6 (b) shows the detection result of the multi-beam synthetic aperture sonar imaging target.
Detailed Description
The application is described in further detail below with reference to the drawings and the detailed description.
The application relates to a multi-beam synthetic aperture sonar three-dimensional imaging algorithm, which comprises the following steps:
step (1): and performing orthogonal transformation on the received array signals, converting the real signals into analytic signals, and performing band-pass filtering on the received signals according to the transmitting signal parameters.
Step (2): and carrying out matched filtering processing on the analysis signal by using the transmitted reference signal, so as to realize the pulse compression process of the linear frequency modulation signal and correct the signal delay caused by convolution processing.
Step (3): and carrying out track-oriented aperture synthesis processing on the pulse compressed signals, virtually synthesizing receiving subarrays at the same horizontal position into a large-aperture receiving array along the track direction, and obtaining a plurality of two-dimensional sonar complex images under a track-oriented-oblique coordinate system, wherein the array is equivalent to a horizontal uniform linear array with a virtual large aperture in the track direction.
Step (4): and performing horizontal beam forming processing on the obtained virtual large-aperture horizontal linear array, and performing coherent accumulation on two-dimensional complex images at different track positions to obtain a three-dimensional sonar image under a track-pitch-angle coordinate system.
Step (5): and carrying out coordinate transformation on the three-dimensional sonar image under the track direction-oblique direction-angle direction coordinate system to obtain a horizontal direction-track direction-depth direction sonar image which is easier to observe directly.
In the step (1), the time domain signal received by the transducer array element is a real signal, and the phase shift processing can be performed only by converting the real signal into an analysis signal, and the conversion method mainly comprises methods such as Hilbert conversion, orthogonal conversion and the like. The hilbert transform is implemented by means of signal integration, and the operand thereof is not suitable for use in a real-time system, so that an analysis form of signals is obtained by orthogonal transform in a sonar system, as shown in the following formula.
Wherein s is i (n) is the echo signal received by the i array element at the nth sampling time, f 0 For signal frequency, f s Is the sampling rate.
In the step (2), the analysis signal is subjected to matched filtering processing by utilizing the transmitted reference signal, so that the pulse compression process of the linear frequency modulation signal is realized, and the signal time delay caused by convolution processing is corrected. Pulse compression of a chirp signal, also known as a matched filter, is a linear filter that is based on the maximum signal-to-noise ratio of the output signal. The method for realizing the time domain pulse compression is to carry out convolution operation on the received signal and the time reversal reference signal, is suitable for shorter reference signals and can be realized in real time in a sampling interval. The sonar system adopts a linear frequency modulation signal as a detection signal, and the following complex signal form is adopted if the emission signal is s (t)
Wherein T is the signal time, T is the pulse width of the linear frequency modulation signal, and k is the frequency modulation slope of the signal. Delay tau after reflecting sound wave by target 0 And returning to the receiving matrix, the signal delayed by the time is expressed as:
the reference signal, i.e., the system unit impulse response signal h (t), can be expressed as:
the output of the matched filter is shown as follows, the output form of the matched filter is represented as a sine function, the position of the matched filter correlation peak is the arrival time of the echo, and the signal can be applied to a multi-beam synthetic aperture sonar imaging algorithm. The matched filter output is shown as follows:
wherein s is R (t) is a received signal, and h (t) is a reference signal.
The matched filtering process introduces filter delay, and the output signal needs to be translated according to the half filter length, so that the starting position of the pulse compressed signal is ensured to be the same as the original signal.
In the step (3), track synthetic aperture processing is carried out on subarray echo signals at the same horizontal position, and a two-dimensional sonar image under a plurality of track-oblique coordinate systems is obtained.
The flow of the multi-beam synthetic aperture imaging algorithm is shown in figure 2, and the number of the track array elements of the two-dimensional area array is assumed to be N y The number of the array elements of the horizontal line is N x The imaging result requires the formation of M beams in the horizontal direction. Firstly, performing track-oriented synthetic aperture algorithm processing, and secondly, performing horizontal beam forming algorithm processing. This treatment scheme will first be N in the same horizontal position y And (3) performing synthetic aperture algorithm processing on each array element to obtain 1 array element with a virtual aperture. N for all horizontal positions x After traversing each array element, N with virtual large aperture in the course direction can be obtained x A cell linear array, and N is obtained at the same time x And (5) synthesizing an aperture sonar image. The linear array is subjected to beam forming processing, namely N is provided x M wave beams of the element are formed, and a needed multi-wave beam synthetic aperture sonar image with M wave beams is obtained.
The present application is described taking a two-dimensional area array structure of 4×32 reception cells as an example. First, 4 array elements arranged along the track direction in the same horizontal sequence number are divided into the same subarrays, and the total plan is divided into 32 subarrays distributed along the horizontal direction. And respectively compensating phase shifts of 4 primitives in the same subarray, integrating in a synthetic aperture range to obtain an SAS image, and accumulating complex images in the subarray to obtain a two-dimensional SAS image under a track direction-inclined distance direction coordinate system. The array structure at this time is similar to a conventional multi-beam sounding sonar, and beam forming processing can be performed in the horizontal direction. And traversing all scanning moments, and presetting beam angles and track direction positions to obtain three-dimensional sonar image output under a track-slant range-angle coordinate system. The sonar image can be converted into a three-dimensional sonar image under a horizontal-track-depth coordinate system which is easier to observe through coordinate transformation, and the reflection characteristic of the target on the sound wave can be reflected after the inverse weighting treatment such as propagation loss compensation and gain removal.
The principle of track-to-aperture synthesis is shown in fig. 3, by moving the small-aperture sonar array upwards in the track, irradiating the detection area for multiple times, and performing coherent accumulation processing on the received echo signals according to the moving position to obtain an equivalent virtual large-aperture receiving array, thereby obtaining higher track-to-resolution. The synthetic aperture imaging algorithm needs to calculate the distances from each scanning pixel point to the transmitting position and the receiving position when in different sampling positions, so as to calculate the propagation time of sound waves and compensate the phase difference of different positions. For the multi-beam synthetic aperture imaging algorithm, the track-to-synthetic aperture algorithm processing can be expressed as an integral form of phase-shifted track-to-signal, wherein Γ is a synthetic aperture period, and the motion of the carrier can be calculated according to the applied virtual aperture length and the carrier motion speed, namely, the motion of the carrier starts from- Γ/2 moment to Γ/2 moment.
Setting the scanning pixel point to be positioned under the track direction-distance direction coordinate system, wherein the coordinate position is (y) T ,r T ) The real aperture size of the transmitting transducer is D, and the virtual aperture length is calculated according to the footprint of the transmitting beamThen a combination can be obtainedPore diameter period Γ=l/v=r T λ/(Dv), the analytical solution for deriving the synthetic aperture imaging algorithm is expressed as:
wherein t is 0 The linear translation of the time center of the synthetic aperture is the moment when the matrix is positioned at the center of the virtual aperture.
Imaging results under the track direction-distance direction (y-r) coordinate system are shown as follows, the formula can be generalized to any horizontal position array element, and only the slant distance r from the target to the receiving array element needs to be recalculated T And (3) obtaining the product. In order to break through the limitations of the synthetic aperture sonar imaging technology, on the basis of a conventional synthetic aperture imaging algorithm, a horizontal uniform linear array structure similar to multi-beam sounding sonar is utilized to perform beam forming processing on a two-dimensional sonar complex image, and arrival directions of echoes at the same track position are distinguished, so that a high-resolution sonar image in a three-dimensional space is realized. On the basis of the obtained complex image, carrying out beam forming algorithm processing on the obtained uniform linear array with the virtual aperture in the track upwards, and deducing an imaging result to any array element on the linear array:
wherein B is signal bandwidth, c is acoustic velocity in water, Γ is virtual synthetic aperture scale, lambda is signal wavelength, v is carrier movement velocity, n is array element sequence number, r T (n) is the slant range from the target in the horizontal-to-depth coordinate system to the n-th receiving array element.
In the step (4), the obtained two-dimensional sonar complex image is subjected to beam forming processing, and the arrival direction of the echo at the position of the same track is distinguished. In the obtained complex imageOn the basis, carrying out beam forming algorithm processing on the obtained uniform linear array with the virtual aperture on the track to realize sonar imaging in a three-dimensional space, wherein θ is a beam scanning angle, and θ T For the direction of the target, d x Is the horizontal array element spacing.
And performing horizontal beam forming processing on the obtained virtual large-aperture horizontal linear array, and performing coherent accumulation on two-dimensional complex images at different track positions to obtain a three-dimensional sonar image under a track-pitch-angle coordinate system. The horizontal beam forming principle is shown in fig. 4, the received acoustic wave signals are coherently accumulated, the interested preset angle direction signals are enhanced, the rest direction signals are restrained, and the time-direction diagram output in the required coverage range is finally obtained through traversing the preset beam angles. Firstly, deducing an imaging result of a designed two-dimensional area array horizontal multi-array element system, wherein the far-field plane wave is assumed to be a special condition of spherical wave expansion. According to the beam forming theory, when the beam scanning angle of the N-element receiving array is theta, the time delay difference of the N-number array element relative to the reference array element can be expressed as
Thus, when the target is at (r T ,θ T ) When in position, the sound path difference of the n-number array element relative to the reference array element is expressed as
r T -r T (n)＝nd x sinθ T
Pair I SAS After the time delay accumulation of the complex images of (y, r, n), the three-dimensional synthetic aperture sonar imaging result under the y-r-theta coordinate system can be obtained:
the complex image result processed by the multi-beam synthetic aperture imaging algorithm comprises an amplitude part and a phase part, and the complex image result is obtained by recombining the above formulas:
I MBSAS (y,r,θ)＝I SAS (y,r)·I MBES (θ)
from the deduction of the formula, the imaging result of the multibeam synthetic aperture sonar is obtained by I SAS (y, r) and I MBES (θ) multiplication, I SAS (y, r) is the imaging result of the track direction, I MBES (θ) is similar to the conventional phase shift beamforming result. Thus, the multi-beam synthetic aperture imaging result in the three-dimensional space can be obtained.
In the step (5), the obtained three-dimensional sonar image I MBSAS (y, r, θ) coordinate transformation to convert the track-pitch-angle-oriented image into a more easily observable horizontal-track-depth-oriented image I MBSAS (x,y,z)。
And carrying out coordinate transformation on the three-dimensional sonar image under the track direction-oblique direction-angle direction coordinate system to obtain a horizontal direction-track direction-depth direction sonar image which is easier to observe directly.
x＝rsinθ
y＝rcosθ
The application is illustrated in more detail below with reference to the examples of fig. 5 and 6:
in order to verify the effectiveness of the multi-beam synthetic aperture three-dimensional imaging algorithm, the simulation imaging analysis is carried out on the three-dimensional target by comparing the method with the multi-beam sounding sonar imaging algorithm to improve the image fineness. Setting simulation conditions: the geometric center position coordinates (2, 5, -15) of the solid target are 2.0mX0.8mX2.0m, the dividing distance of the bright spots on the surface is 20cm,10cm and 20cm, and the dividing distance of the bright spots on the water bottom is 40cm and 20cm on the assumption that the cube bottom is on the water bottom plane. And respectively carrying out data processing by utilizing a multi-beam sounding sonar imaging algorithm and a multi-beam synthetic aperture sonar imaging algorithm, carrying out slice display on the obtained three-dimensional sonar image, wherein the track slice spacing is 10cm, the horizontal slice spacing is 5cm, and respectively carrying out slice at the depth direction-15 m and-16 m, so that the imaging effect of observing the upper surface of the target and the water bottom position is shown in figure 5.
As shown in fig. 5 (a), the imaging result of the multi-beam sounding sonar is that the sonar image is seriously defocused, and because the bright point target not only comprises a cube surface target but also comprises a reflection echo from the water bottom, the track echo is seriously aliased, and the three-dimensional target and the water bottom area cannot be clearly reflected. The imaging result after processing by the multi-beam synthetic aperture algorithm is clearly visible, as shown in fig. 5 (b). The imaging result can not only clearly reflect the position and the size of the stereoscopic target, but also clearly see the 'sound spot' separated from the surface and the 'sound spot' of the underwater target. The sonar track echo is not aliased, and the arrival direction of the horizontal echo can be effectively resolved, so that the three-dimensional space imaging result is obviously superior to that of the multi-beam sounding sonar. Fig. 5 (c) and fig. 5 (d) show the top surface cutting results of the image respectively, and it can be observed that the multi-beam synthetic aperture sonar imaging result is clearly visible at the bright spot position on the upper surface compared with the conventional multi-beam sounding sonar result, and the echo energy is more concentrated, which indicates that the algorithm effectively focuses the sonar echo. As shown in fig. 5 (e) and 5 (f), the imaging result of the lower surface and the underwater portion is partially close-up slice, since the lower surface cannot be irradiated with the acoustic wave, the echo energy thereof is weak, and an shadow area and a dark area portion are formed around the object. The imaging algorithm of the multi-beam synthetic aperture sonar effectively improves the defects of the multi-beam sounding sonar, can clearly reflect the changes of the energy intensity of the bottom surface, the water bottom area and the forming shadow area of a target, and also verifies the technical superiority of the three-dimensional imaging algorithm of the multi-beam synthetic aperture sonar.
In order to evaluate the three-dimensional refined detection capability of the multi-beam synthetic aperture imaging algorithm on the target form, a cube target with a side length of 30cm is selected for a pool detection test, data processing is respectively carried out by using different imaging algorithms, and image output and target detection results thereof are compared. The cube target was placed 13m from the transducer position and the transducer array walked 5 positions up the track at 25cm sampling intervals, forming a virtual aperture of 1.0m in size. The center frequency of the transmitted signal is 135kHz, the bandwidth of the linear frequency modulation signal is 10kHz, the pulse length of the signal is 10ms, the theoretical resolution of the track direction is 8cm, and the imaging result pairs of different algorithms are shown in figure 6.
In practical engineering application, the three-dimensional fully-covered sonar image has the technical advantages of fine imaging effect, water body target display and target strength reflection, but also has the technical characteristics of huge data volume, too dense point cloud distribution and adverse direct observation, so that the three-dimensional imaging result is usually required to be thinned through a target detection method, and the effective echo points are restrained through an energy detection threshold, so that the three-dimensional form of the detection target is more intuitively reflected. Therefore, the three-dimensional sonar image is further thinned by adopting a target detection algorithm aiming at the imaging result, and the form and the position of the cube target are observed in a three-dimensional space. The three-dimensional detection result of the target imaged by the multi-beam sounding sonar and the top view thereof are shown in fig. 6 (a), the detection result of the target is an ellipsoid, and effective detection points are also displayed inside the ellipsoid. The detection result between the pixels is continuous and smooth, and is due to the fact that the imaging resolution of the multi-beam sounding sonar is low, and image energy leaks to irrelevant pixels, so that energy detection artifacts appear in a detection area. The detection result of the multi-beam sounding sonar imaging algorithm shows that the positions of not only six surfaces of the multi-beam sounding sonar imaging algorithm detect uniform and continuous echo bright spots, but also the positions inside the cube contain bright spot targets, and the analysis of a manager can know that the bright spot results are all image 'artifacts' caused by lower imaging resolution and target echo leakage to irrelevant pixel points. From this, it can be concluded that the multi-beam sounding sonar imaging algorithm can obtain the approximate position of the target, but the detection refinement degree is insufficient, and the detection of the scale, shape and detail parts of the target is easy to be distorted.
The target detection result of multi-beam synthetic aperture sonar imaging and the top view thereof are shown in fig. 6 (b), the target basically presents a cube shape, the track scale is the same as the preset target parameter, and the horizontal scale expansion accords with the theoretical calculation result. The surface pixel points of the detection result are characterized by roughness and discontinuity, the effective bright points on the upper surface and the front surface are arranged fully, the number of the effective bright points on the side surface and the rear surface is less and discontinuous, the effective bright points on the lower surface are relatively sparse, and the bright points in the target are arranged very sparsely. Because the space sampling position of the track direction is asymmetric, the irradiation times of the rear surface are less than those of the front surface, the lower surface and the side surfaces are less irradiated by sound waves, the effective bright spots are arranged relatively sparsely, and the sound waves penetrating and returning by high-frequency sound waves in the cube are less, so that the arrangement of the internal bright spots is very sparse. The detection result of the multi-beam synthetic aperture sonar imaging algorithm is that the target surface is rough and discontinuous, the theoretical analysis and the approaching reality situation are more met, no obvious detection 'false image' appears, and the multi-beam synthetic aperture sonar three-dimensional imaging algorithm provided by the application has obvious technical advantages for the refined detection of the underwater target.
In summary, the application discloses a multi-beam synthetic aperture sonar three-dimensional imaging algorithm. According to the multi-beam synthetic aperture sonar transducer array structure, the large aperture array is virtually synthesized through the motion of the carrier in the course direction, and the arrival azimuth of the echo is resolved by utilizing the multi-receiving array element structure in the horizontal direction. And through the movement of the carrier, the detection areas are irradiated at positions of different tracks, and the received echo signals are coherently compensated and accumulated, so that the three-dimensional sonar image output under the track-slope distance-angle coordinate system can be obtained. By means of coordinate transformation, the sonar image can be converted into a three-dimensional sonar image under a horizontal-track-depth coordinate system which is easier to observe. The algorithm processing flow mainly comprises four steps of orthogonal change, pulse compression, track-oriented aperture synthesis and horizontal beam forming. The method breaks through the mechanism limitations of multi-beam sounding sonar and conventional synthetic aperture sonar, obtains constant imaging resolution in the track direction, obtains higher imaging resolution in the horizontal direction and depth direction planes, and finally realizes the fine sonar imaging in the three-dimensional space.
Claims (2)
1. A multi-beam synthetic aperture sonar three-dimensional imaging algorithm is characterized in that: the method comprises the following steps:
step one: performing orthogonal transformation on the received array signals, converting the real signals into analytic signals, and performing band-pass filtering on the received signals according to the transmitting signal parameters;
the time domain signal received by the transducer array element is a real signal, and the phase shift processing can be performed only by converting the real signal into an analysis signal, so that the analysis form of the signal is obtained by adopting orthogonal transformation in a sonar system, and the following formula is shown:
wherein s is i (n) is the echo signal received by the i array element at the nth sampling time, f 0 For signal frequency, f s Is the sampling rate;
step two: performing matched filtering processing on the analysis signal by using the transmitted reference signal to realize the pulse compression process of the linear frequency modulation signal and correct the signal delay caused by convolution processing;
the sonar system adopts a linear frequency modulation signal as a detection signal, and a complex signal form with a transmitting signal s (t) is as follows:
wherein: t is the pulse width of a transmitting signal, and k is the modulation slope;
the matched filter outputs are:
wherein: s is(s) R (t) is a received signal, h (t) is a reference signal;
step three: performing track-oriented aperture synthesis processing on the pulse compressed signals, virtually synthesizing receiving subarrays at the same horizontal position into a large-aperture receiving array along the track direction, and obtaining a plurality of two-dimensional sonar complex images under a track-oriented-oblique coordinate system, wherein the array is equivalent to a horizontal uniform linear array with a virtual large aperture in the track direction;
assume that the number of track directional array elements of the two-dimensional area array is N y The number of the array elements of the horizontal line is N x The imaging result requires the formation of M beams in the horizontal direction; n to be in the same horizontal position y The array elements are processed by a synthetic aperture algorithm to obtain 1 array element with a virtual aperture; n for all horizontal positions x After traversing each array element, N with virtual large aperture in the course direction can be obtained x A cell linear array, and N is obtained at the same time x A synthetic aperture sonar image is displayed; imaging results in the track-to-range (y-r) coordinate system are shown below:
wherein: b is signal bandwidth, c is acoustic velocity in water, Γ is virtual synthetic aperture scale, lambda is signal wavelength, v is carrier movement velocity, n is array element sequence number;
step four: performing horizontal beam forming treatment on the obtained virtual large-aperture horizontal linear array, and performing coherent accumulation on two-dimensional complex images at different track positions to obtain a three-dimensional sonar image under a track-pitch-angle coordinate system;
carrying out wave beam forming processing on the obtained two-dimensional sonar complex image, and distinguishing the arrival azimuth of the echo at the position of the same track; on the basis of the complex image obtained in the step three, carrying out beam forming algorithm processing on the obtained uniform linear array with the virtual aperture on the flight path, so as to realize sonar imaging in a three-dimensional space:
wherein the method comprises the steps ofθ is the beam scan angle, θ T For the direction of the target, d x Is the distance between horizontal array elements;
step five: and carrying out coordinate transformation on the three-dimensional sonar image under the track direction-oblique direction-angle direction coordinate system to obtain a horizontal direction-track direction-depth direction sonar image which is easier to observe directly.
2. A multibeam synthetic aperture sonar three-dimensional imaging algorithm according to claim 1, wherein: the fifth step is specifically as follows: for the obtained three-dimensional sonar image I MBSAS (y, r, θ) coordinate transformation to convert the track-pitch-angle-oriented image into a more easily observable horizontal-track-depth-oriented image I MBSAS (x,y,z)；
x＝rsinθ
y＝rcosθ。
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