Direct wave signal suppression method of continuous wave active sonar detection system

Abstract

The invention belongs to the technical field of underwater acoustic signal processing, and provides a direct wave signal suppression method of a continuous wave active sonar detection system, which comprises the following steps of: acquiring a received signal of a hydrophone and calculating an original time-frequency spectrum of sound intensity of the received signal; performing first filtering processing on the original time-frequency spectrum, wherein the first filtering processing is used for filtering a continuous direct wave principal component item in the original time-frequency spectrum; second filtering processing is carried out on the result of the first filtering processing, the second filtering processing is used for filtering continuous direct wave-target echo interference terms in the result of the first filtering processing, and logarithmic transformation is carried out on the result of the first filtering processing before the second filtering processing is carried out. According to the method provided by the invention, the high-intensity direct wave signal component contained in the original underwater acoustic signal received by the bistatic continuous wave active sonar system can be effectively inhibited.

Description

Technical Field

[0001] The present application belongs to the technical field of underwater acoustic signal processing, and specifically provides a method for suppressing direct wave signals of a continuous wave active sonar detection system. Background Art

[0002] A bistatic active sonar detection system uses a split-transmitter / receiver architecture, with the active sonar transmitting and the hydrophone receiving the underwater acoustic signals. Traditional active sonars mostly emit pulsed signals. Due to the intervals between pulses, it's relatively easy to separate the direct-arrival signal from the echo signal in the time domain within the acoustic signal received by the hydrophone. In recent years, continuous-wave active sonar detection systems have become increasingly popular in underwater detection due to their high time gain and extended detection range. Currently, continuous-wave active sonars can transmit various forms of long-pulse-width continuous-wave signals, such as linear frequency modulation (LFM) and hyperbolic frequency modulation (HFM). These long pulse widths cause the direct-arrival signal (the signal transmitted directly from the active sonar to the hydrophone) to be significantly stronger than the target echo signal, thereby masking weak echoes and creating large detection blind spots.

[0003] Currently, many schemes have been proposed to suppress the direct wave component in the received signal of continuous wave active sonar detection system. However, these schemes have significant limitations: First, matched filter sidelobe suppression technology reduces the matched filter sidelobes through window function method or signal design (such as nonlinear frequency modulation), but its performance degrades or even fails when the signal is distorted or Doppler shifted in shallow water dispersive channel environment; Second, spatial domain filtering method forms a null in the azimuth of the direct wave based on beamforming technology (such as blocking matrix and zero point constraint) to suppress interference, but it will also suppress the echo signal in that azimuth, resulting in azimuth blind spots; Third, adaptive interference cancellation technology requires a pure direct wave reference signal, but in actual environment, the reference signal is easily mixed with echo or other interference, resulting in performance degradation, its computational complexity is high, and it is sensitive to channel time variation; Finally, acoustic shielding technology cancels the direct wave by adjusting the transmitted signal, but it requires precise known channel information, which is difficult to achieve in actual marine environment.

[0004] It can be seen that for the signal and information processing of the dual-base continuous wave active sonar detection system, there is an urgent need for a method with simple structural setting, low computational complexity, strong robustness to shallow sea dispersive channels, and applicability to various FM signal forms. It can effectively suppress the direct wave while completely retaining the echo signal, so as to improve the recognizability of the target echo in the time-frequency structure diagram in a low signal-to-noise ratio environment. Summary of the Invention

[0005] The purpose of this application is to provide a method for suppressing direct wave signals of a continuous wave active sonar detection system, the method comprising the following steps: Obtaining a received signal from a hydrophone and calculating the original time-frequency spectrum of its sound intensity, wherein the received signal includes a continuous direct wave signal from an active sonar and a target echo signal; Performing a first filtering process on the original time-frequency spectrum, wherein the first filtering process is used to filter the continuous direct wave principal component items in the original time-frequency spectrum; The result of the first filtering process is subjected to a second filtering process, and the second filtering process is used to filter the continuous direct wave-target echo interference term in the result of the first filtering process, wherein before the second filtering process is performed, the result of the first filtering process is logarithmically transformed.

[0006] Furthermore, the target echo signal is a signal received by the hydrophone after the continuous signal emitted by the active sonar is reflected and scattered by the moving target; the number of the active sonar and the hydrophone is 1, and the positions of the active sonar and the hydrophone remain unchanged during the detection process.

[0007] Furthermore, both filtering processes include a two-dimensional Fourier transform operation, a binary mask operation, and a two-dimensional inverse Fourier transform operation.

[0008] Furthermore, the two-dimensional Fourier transform operation and the two-dimensional inverse Fourier transform operation are performed by equation (1) and equation (2), respectively: (1), (2), in, 、 are time variables and frequency variables respectively, Two-dimensional spectrum - The expression of the domain, 、 are the independent variables of the two-dimensional Fourier transform, is the imaginary unit, for Transform to - The expression of the domain; And, the binary mask operation is performed based on formula (3): (3), in, is a binary mask matrix, is the one-to-one multiplication of matrix elements, For The result of the binary mask operation, and The dimensions of Stay consistent.

[0009] Furthermore, the binary mask matrix As shown in formula (4): (4).

[0010] Further, and are all two-dimensional matrices, where and The dimensions remain consistent, and middle Dimensions and middle The dimensions are equal odd numbers.

[0011] Preferably, the energy of the principal component of the target echo signal is - The distribution of domains satisfies the energy dispersion condition shown below: , in, The sound intensity time spectrum of the main component of the target echo signal is transformed into - The expression of the domain, is the summation function, is the preset dispersion threshold.

[0012] Furthermore, in the original time-frequency spectrum, the total energy of the continuous direct wave principal component terms is much greater than the total energy of the continuous direct wave-target echo interference terms, and the total energy of the continuous direct wave-target echo interference terms is much greater than the total energy of the target echo signal principal component terms.

[0013] The embodiment of the present application provides a method for suppressing direct wave signals of a continuous wave active sonar detection system, which fully utilizes the different distribution characteristics of the target echo signal of a moving target and the direct wave signal of a stationary active sonar in the two-dimensional Fourier transform domain, and uses two filtering processes to filter out the continuous direct wave main component term and the continuous direct wave-target echo interference term in turn, so that the interference structure characteristics reflected by the processed sound intensity time-frequency spectrum are dominated by the main component term of the target echo signal, and the intensity is distributed within a relatively reasonable upper and lower limit. The method adopted in the present application has a strong interference resistance capability in shallow sea dispersive channels from the perspective of sound field interference and the characteristic difference of different signal interference fringes, and is not restricted by the direction of the target echo and the direct wave. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a schematic diagram of the principle of the continuous wave active sonar detection system; Figure 2 is the sound intensity time-frequency spectrum of the target echo signal under ideal conditions; Figure 3 is the sound intensity time-frequency spectrum of the direct wave signal; Figure 4 is the sound intensity time-frequency spectrum of the original received signal of the hydrophone; Figure 5 is the two-dimensional Fourier transform result of the time-frequency spectrum of the target echo signal under ideal conditions; Figure 6 is the two-dimensional Fourier transform result of the sound intensity time-frequency spectrum of the direct wave signal; Figure 7 Flowchart of a method for suppressing direct wave signals in a continuous wave active sonar detection system according to an embodiment of the present application; Figure 8 is the two-dimensional Fourier transform result of the sound intensity time-frequency spectrum of the original received signal; Figure 9 A schematic diagram of the result after the first filtering process is performed on the original time-frequency spectrum provided in an embodiment of the present application; Figure 10 The figure is a schematic diagram of the results after the original time-frequency spectrum is filtered twice according to an embodiment of the present application. DETAILED DESCRIPTION

[0015] Hereinafter, the present application will be further described based on preferred embodiments with reference to the accompanying drawings.

[0016] In the description of the embodiments of the present application, it should be noted that if the terms "upper", "lower", "inner", "outer" and the like indicate an orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or are the orientation or positional relationship in which the products of the embodiments of the present application are usually placed when in use, they are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application. In addition, in the description of the present application, in order to distinguish different units, words such as first and second are used in this specification, but these are not limited by the order of manufacture, nor can they be understood as indicating or implying relative importance. Their names may be different in the detailed description and claims of the present application.

[0017] The vocabulary in this specification is used to illustrate the embodiments of the present application, but is not intended to limit the present application. It should also be noted that, unless otherwise clearly specified and limited, the terms "disposed", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, a direct connection, an indirect connection through an intermediate medium, or a communication between the two components. For those skilled in the art, the specific meanings of the above terms in this application can be specifically understood.

[0018] In order to clearly explain the technical solution of the present application, the working principle of the continuous wave active sonar detection system and the implementation mechanism of the technical solution adopted in the present application are first explained.

[0019] Figure 1 This is a schematic diagram of the principle of the continuous wave active sonar detection system. In the figure, the X direction and the Y direction represent two orthogonal directions on the sea level, such as Figure 1 As shown in the figure, in this detection system, the sound source is an active sonar that can continuously emit underwater acoustic signals in the 300-500Hz frequency band. The active sonar is located at the horizontal plane (0km, 5km), and the hydrophone is located at the horizontal plane (0km, 0km). At the same time, there is a moving target in the sea area where the detection system is located, and its position and movement direction are Figure 1 Indicated by red line.

[0020] like Figure 1 As shown in the figure, the underwater acoustic signal emitted by the active sonar first undergoes one-way acoustic propagation to the target's real-time position. It is then scattered by the target. The scattered sound wave undergoes another one-way acoustic propagation from the target to the hydrophone, forming the target echo signal. In addition, the acoustic signal emitted by the sound source can directly reach the hydrophone, forming a direct wave signal. The direct wave signal and the target echo signal together constitute the original underwater acoustic signal received by the bistatic continuous wave sonar detection system.

[0021] When the target moves relative to the active sonar and hydrophone in the XOY plane, the target echo signal will show obvious time-frequency interference characteristics. Figure 1 Taking the embodiment shown in the figure as an example, when the target takes the horizontal plane (2km, -8km) as the initial position and the horizontal plane (20km, 10km) as the final position, and moves at a constant speed (5m / s, 5m / s) along the red line, the target echo signal can be short-time Fourier transform to obtain the following: Figure 2 The time spectrum of the target echo signal intensity is shown ,in 、 are time and frequency variables, respectively.

[0022] from Figure 2 Two groups of interference fringes with different degrees of curvature can be observed. These two groups of interference fringes are caused by the movement of the target relative to the sound source and the target relative to the hydrophone. Figure 2 The time-frequency spectrum of the target echo signal is shown It is a processing result under ideal conditions without direct wave interference. However, when continuous wave active sonar continuously transmits underwater acoustic signals, the original underwater acoustic signals received by the hydrophone also include direct wave signals from the sound source to the hydrophone. Since there is no relative motion between the sound source and the hydrophone, the sound intensity time-frequency spectrum obtained by short-time Fourier transform of the direct wave signal is like Figure 3 As shown, there is only one kind of interference fringe without bending.

[0023] Moreover, since the target echo signal is the underwater acoustic signal emitted by the active sonar and then reflected by the moving target and received by the hydrophone, the intensity of the direct wave is much higher than the target echo intensity, such as Figure 4 As shown in the figure, the original time-frequency spectrum of the sound intensity obtained by performing short-time Fourier transform on the original underwater acoustic signal received by the hydrophone is , which is almost completely dominated by the direct wave signal component. Obviously, if the direct wave signal cannot be effectively suppressed, it is impossible to extract effective information of the target from the received signal.

[0024] However, as analyzed in the background technology section, the various direct wave signal component suppression schemes currently used for continuous wave active sonar are either sensitive to the environment, or have azimuth blind spots, or are computationally complex and difficult to adapt to the actual ocean environment.

[0025] In order to achieve simple, efficient and universal suppression of direct wave signal components, the applicant first analyzed the characteristic distribution of the echo signal and direct wave signal of the moving target in the time-frequency space to obtain the characteristic differences and separation difficulty of the two in different computational domains, and then determined the optimal solution for suppressing the direct wave component.

[0026] Specifically, Figure 5 For Figure 2 The time-frequency spectrum of the target echo signal The result of performing a two-dimensional Fourier transform (2D FFT) is Figure 6 For Figure 3 The time-frequency spectrum of the direct wave signal The result of two-dimensional Fourier transform. Figure 5 and Figure 6 It can be seen that when the two-dimensional Fourier transform is performed on the sound intensity time spectrum, Domain transformation to After the domain is formed, due to the movement of the target relative to the sound source and the hydrophone, and the relative position between the sound source and the hydrophone remains unchanged, the target echo and the direct wave are in the same The distribution characteristics of the domain are obviously different, among which the energy of the direct wave component is Focused in space At the same time, since there is an order of magnitude difference in intensity between the two (in some specific embodiments, the intensity of the direct wave component is much greater than that of the target echo component, for example, the intensity difference between the two exceeds three orders of magnitude), and the image features of the direct wave component are extremely simple after two-dimensional Fourier transform, it can be easily filtered out by using a simple and convenient mask operation.

[0027] Based on the above analysis, this application provides a direct wave signal suppression method for a continuous wave active sonar detection system that is different from various existing direct wave suppression technologies. Figure 7 , the method comprises the following steps: Step S1: Obtain the received signal of the hydrophone and calculate the original time-frequency spectrum of its sound intensity.

[0028] According to the above analysis, Figure 1 The continuous wave active sonar detection system shown, the receiving signal of the hydrophone Contains the continuous direct wave signal of active sonar and target echo signal In some specific embodiments, a shorter time window can be used to slide the received signal of the hydrophone and perform a short-time Fourier transform to obtain the spectrum at each time window, and then the square of the modulus is taken to obtain the original time-frequency spectrum of the received signal intensity. , for example, in a specific embodiment, for processing the received signal The duration is not less than 3600 seconds, and a time window of 3.6 seconds can be used for Perform short-time Fourier transform to generate the original time-frequency spectrum The dimension is , i.e. 201 frequency points and 1001 time points, with a lower frequency limit of 300Hz and an upper frequency limit of 500Hz. The intensity diagram can be found in Figure 4 .

[0029] Step S2: the original time spectrum Perform the first filtration process.

[0030] In an embodiment of the present application, the first filtering process includes the following steps: First, yes Perform a two-dimensional Fourier transform operation to obtain - Domain expression ; Then, Perform binary mask operation to obtain the masked result ; Finally, perform a two-dimensional inverse Fourier transform operation to obtain the processing result of the first filtering .

[0031] In general, for any - Two-dimensional spectrum of the domain , its two-dimensional Fourier transform is shown in formula (1): (1), in, 、 are the independent variables of the two-dimensional Fourier transform, is the imaginary unit, for Transform to - The expression for the domain.

[0032] Accordingly, from - Domain transformation back - The two-dimensional inverse Fourier transform operation of the domain is shown in formula (2): (2).

[0033] It should be known that in the field of signal processing technology, the above 、 Both are in the form of two-dimensional matrices.

[0034] Obviously, for the original time spectrum , substituting it into formula (1), we can get the result of the two-dimensional Fourier transform operation as shown in the following formula: .

[0035] Figure 8 Shows the Figure 4 The original time-frequency spectrum shown The two-dimensional Fourier transform is obtained ,pass Figure 8 It can be seen that since the intensity of the continuous direct wave part is much greater than the target echo part in the original time-frequency spectrum, after the two-dimensional Fourier transform exist The intensity of the bright line at that position is much higher than that at other positions, so that the characteristics of the target echo signal part are compressed within the intensity range.

[0036] In this application - The filtering of the continuous direct wave components in the domain is performed using a binary mask operation. The general expression of the binary mask operation is shown in formula (3): (3), (3) In the formula is a binary mask matrix, For the general and The matrix elements of are multiplied one by one, For The result of a binary mask operation, where and The dimensions of Stay consistent.

[0037] Accordingly, for Obtained by two-dimensional Fourier transform , substituting it into formula (3), we can implement the binary mask operation as shown below: , In order to distinguish it from the subsequent steps, the result of this binary mask operation is recorded as .

[0038] The form determines the - The signal components filtered by the domain are transformed by two-dimensional Fourier transform. Transform to - After the domain, the distribution of the continuous direct wave components becomes very easy to characterize, so the form of the binary mask matrix will also become very simple. Preferably, It can be constructed using the form of formula (4): (4), That is, only = 0, and the matrix elements on the axis where the matrix is ​​set to zero, and the other positions remain unchanged, so that the matrix can be completed. - The domain is used to filter out the continuous direct wave components.

[0039] Since the binary mask operation is The matrix elements on the axis are set to zero. To facilitate the implementation of this operation, in some preferred embodiments, the two two-dimensional spectra involved in equations (1) and (2) can be set to and The dimensions remain consistent, and variable and Medium variables The dimensions are the same odd number, for example, 1001 points.

[0040] In completing the The binary mask operation is performed and the Then, substitute it into formula (2) to get the result of the first filtering process. Without loss of generality, the result of the first filtering process is recorded as .

[0041] Figure 9 Shows the Figure 8 shown The first filtering process , we can see that through the first binary mask operation, The continuous direct wave components that remain unchanged over time are filtered out, and the interference structure characteristics of the darker target echo signal are revealed.

[0042] However, Figure 9 The result cannot be used to extract target echo features based on signal strength. This is because after the first binary mask operation, - There are still some brighter areas scattered in various parts of the domain, and the intensity of this part is much higher than the intensity of the location where the interference structure of the target echo signal is located. This causes the intensity of the location where the interference structure of the target echo signal is located to be still compressed to a low level. Moreover, since those continuous direct wave components that remain unchanged over time are filtered out after the first binary mask operation, the intensity of the filtered area also becomes extremely low. Therefore, the two cannot be distinguished based on the difference in signal strength.

[0043] The following is an analysis of the generation mechanism of the brighter part. As we can see from the previous article, the signal received by the hydrophone is Continuous direct wave component and target echo component Composition, that is , then the sound intensity of the received signal It can be expressed as formula (5): (5), Among them, the superscript represents the conjugate transpose operation, This is the real part operation.

[0044] By observing formula (5), we can find that It consists of three parts, among which: represents the sound intensity when only continuous direct waves exist in the received signal. In this application, this part is called the continuous direct wave principal component term; similarly, It represents the sound intensity when only the target echo signal exists in the received signal. In this application, this part is called the principal component of the target echo signal.

[0045] In addition to the two ideal principal components mentioned above, due to the interference between the continuous direct wave and the target echo, there are also This part is the result of the interference between the continuous direct wave and the target echo. In this application, it is called the continuous direct wave-target echo interference term.

[0046] During the first filtering process, the principal component of the continuous direct wave can be transformed into All concentrated in - Domain axis, so the first binary mask operation can be Eliminate the direct wave principal component .

[0047] However, the first filtering process does not The reason for filtering is that this part is the result of interference between the continuous direct wave with unchanged time-frequency characteristics and the target echo with time-frequency characteristics showing time-varying characteristics. Therefore, directly performing two-dimensional Fourier transform cannot make this part - The domain is concentrated in The simple structural features of the shaft, therefore, cannot be filtered out by the first filtering process.

[0048] At the same time, considering , for example in some specific embodiments, Exceed Three orders of magnitude, therefore, much higher than At the same time, Also much higher than .

[0049] Through the above analysis, we can draw the conclusion that after the first filtering process, the principal component of the continuous direct wave will be After filtering, The bright spots in the image are caused by the interference of the continuous direct wave and the target echo, i.e. caused by.

[0050] In this application, this part is much larger than the principal component of the target echo signal The component filtering is carried out in step S3, specifically, due to , that is, the sound intensity time spectrum at this time is the inner product of the continuous direct wave sound pressure and the target echo sound pressure, so it is necessary to first Perform logarithmic transformation, Perform logarithmic operations on each element in , and after logarithmic operations, as shown in formula (6), we can The interference term becomes an additive term: (6), (6) In the formula, is the cosine of the phase angle between the continuous direct wave signal and the target echo signal, is a constant.

[0051] From formula (6), we can see that after logarithmic operation, the continuous direct wave component and the target echo component that were originally coupled together in the time spectrum of the sound intensity are separated from each other. Logarithmic operation does not affect its characteristics. - In the form of domain, after logarithmic processing, in step S3, the same steps as the first filtering process are adopted to transform Transform to - domain, and then use (4) Perform a binary mask operation to remove the continuous direct wave component, and finally perform a two-dimensional inverse Fourier transform operation on the binary masked result to obtain the result of the second filtering process. : .

[0052] Figure 10 Shows the Figure 9 in After the second filtration ,pass Figure 10 It can be seen that through two filtering processes, the continuous direct wave principal component and interference term contained in the sound intensity time-frequency spectrum of the hydrophone received signal can be filtered out in turn, so that the interference structure characteristics reflected by the filtered time-frequency spectrum are dominated by the principal component term of the target echo signal. Since the intensity of the interference structure of the target echo signal is higher than that of other positions at this time, it is convenient to extract its intensity-based features.

[0053] Due to the binary mask-based filtering operation adopted in this application, the time-frequency spectrum is directly - In order to avoid the loss of effective information of the target echo signal, in some preferred embodiments, the echo signal characteristics of the moving target can also be limited. Specifically, the energy of the main component of the target echo signal is - The distribution of the domain must satisfy the energy dispersion condition shown in formula (7): (7), in, The sound intensity time spectrum of the main component of the target echo signal is transformed into - The expression of the domain, is the summation function, is the preset dispersion threshold, which can be set to 0.1 or smaller. By using formula (7), the interference characteristics of the target echo signal are avoided from being concentrated on axis, resulting in excessive target echo information being filtered out as continuous direct wave components in the binary mask process.

[0054] The above is a detailed introduction to the specific implementation methods of the present application. For those skilled in the art, several improvements and modifications can be made to the present application without departing from the principles of the present application. These improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. A method for suppressing direct wave signals of a continuous wave active sonar detection system, characterized in that: The following steps are involved: Obtaining a received signal from a hydrophone and calculating the original time-frequency spectrum of its sound intensity, wherein the received signal includes a continuous direct wave signal from an active sonar and a target echo signal; Performing a first filtering process on the original time-frequency spectrum, wherein the first filtering process is used to filter the continuous direct wave principal component items in the original time-frequency spectrum; The result of the first filtering process is subjected to a second filtering process, and the second filtering process is used to filter the continuous direct wave-target echo interference term in the result of the first filtering process, wherein before the second filtering process is performed, the result of the first filtering process is logarithmically transformed.

2. The direct wave signal suppression method of the continuous wave active sonar detection system according to claim 1, characterized in that: The target echo signal is a signal received by the hydrophone after the continuous signal emitted by the active sonar is reflected and scattered by the moving target; The number of the active sonar and the number of the hydrophone are both 1, and the positions of the active sonar and the hydrophone remain unchanged during the detection process.

3. The direct wave signal suppression method of the continuous wave active sonar detection system according to claim 1, characterized in that: Both filtering processes include a two-dimensional Fourier transform operation, a binary mask operation, and a two-dimensional inverse Fourier transform operation.

4. The direct wave signal suppression method of the continuous wave active sonar detection system according to claim 3, characterized in that: The two-dimensional Fourier transform operation and the two-dimensional inverse Fourier transform operation are performed by equation (1) and equation (2), respectively: （1）， （2）， in, 、 are time variables and frequency variables respectively, Two-dimensional spectrum - The expression of the domain, 、 are the independent variables of the two-dimensional Fourier transform, is the imaginary unit, for Transform to - The expression of the domain; And, the binary mask operation is performed based on formula (3): （3）， in, is a binary mask matrix, is the one-to-one multiplication of matrix elements, For The result of the binary mask operation, and The dimensions of Stay consistent.

5. The direct wave signal suppression method of the continuous wave active sonar detection system according to claim 4, characterized in that: The binary mask matrix As shown in formula (4): （4）。 6. The direct wave signal suppression method of the continuous wave active sonar detection system according to claim 5, characterized in that: and are all two-dimensional matrices, where and The dimensions remain consistent, and middle Dimensions and middle The dimensions are equal odd numbers.

7. The direct wave signal suppression method of the continuous wave active sonar detection system according to claim 5, characterized in that: The energy of the principal component of the target echo signal is - The distribution of the domains satisfies the energy dispersion condition shown below: ， in, The sound intensity time spectrum of the main component of the target echo signal is transformed into - The expression of the domain, is the summation function, is the preset dispersion threshold.

8. The direct wave signal suppression method of the continuous wave active sonar detection system according to claim 1, characterized in that: In the original time-frequency spectrum, the total energy of the continuous direct wave principal component items is much greater than the total energy of the continuous direct wave-target echo interference items, and the total energy of the continuous direct wave-target echo interference items is much greater than the total energy of the target echo signal principal component items.

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