Scattering-based Wideband Echo Signal Simulation Generation Method

Abstract

The present invention provides a method for simulating and generating wideband echo signals based on scattering, comprising the following steps: Step 1: Establish a spatial geometric model of a detection radar and a very low altitude target; Step 2: Design a radar antenna model carried on the radar platform according to the established spatial geometric model; Step 3: Analyze the coordinate transformation of the target model when the radar detects a very low altitude target according to the established spatial geometric model; Step 4: Divide the radar antenna illumination area into multiple scattering units by using the spatial decomposition method, and decompose the transmitted pulse signal into narrow pulses by using the time decomposition method, so as to obtain the echo response of each scattering unit on these narrow pulses; Step 5: Model the echo signal based on the above analysis.

Description

Technical Field

[0001] The present invention belongs to the technical field of radar, and particularly relates to a method for simulating and generating broadband echo signals based on scattering. Background Art

[0002] When a radar seeker detects a target in a down-looking mode, it will be affected by environmental scattering and the coupled scattering between the target and the environment. The echo of the radar seeker includes the target echo generated by target scattering, the environmental clutter generated by environmental scattering, and the multipath echo generated by the coupled scattering between the target and the environment. When the signal emitted by the radar seeker is a single-carrier frequency pulse signal, the clutter interference is strong, and the mirror target signal generated by multipath interference is mixed with the target signal and cannot be separated. The clutter interference and mirror interference will affect the detection of the target by the radar seeker, making the angular error larger and pulling the beam direction of the radar seeker off. In order to reduce the influence of clutter interference and mirror interference, the radar seeker can adopt a broadband waveform, that is, the transmitted signal adopts a broadband linear frequency modulation signal. At this time, the range resolution of the radar seeker becomes larger, and the clutter interference will be reduced, which is beneficial to improving the target detection probability; the larger range resolution also enables the target and mirror interference that are difficult to distinguish under narrowband to be distinguished, which is beneficial to reducing the angular error and making the beam direction of the radar seeker always stable around the target signal. The broadband design of the radar seeker can effectively improve the target detection and tracking effects. After broadbanding, the target signal, multipath signal, and clutter signal have very different signal characteristics from those under narrowband, and they will be affected by various parameters of the target environment.

[0003] The acquisition of broadband signal echoes is the premise for studying the characteristics of broadband echoes. The applicability of traditional echo generation methods is relatively limited. The method for simulating and generating broadband echo signals proposed by the present invention can model the target signal, multipath signal, and clutter signal respectively, and generate the total broadband echo signal by the method of superimposing echo sequences. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for simulating and generating broadband echo signals based on scattering in view of the defects and deficiencies of the prior art.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A method for simulating and generating broadband echo signals based on scattering includes the following steps:

[0007] Step 1: Establish a spatial geometric model of a detection radar and a very low altitude target;

[0008] 101: Design a radar antenna model carried on a radar platform;

[0009] 102: Analyze the coordinate transformation of the target model when the radar detects a very low altitude target;

[0010] Step 2: Based on the established spatial geometric model of the detection radar and the ultra-low altitude target, the spatial decomposition method is used to divide the radar antenna illumination area into multiple scattering units, and the time decomposition method is used to decompose the transmitted pulse signal to obtain narrow pulses, so as to obtain the echo response of each scattering unit on these narrow pulses;

[0011] Step 3: Based on the decomposition results in Step 2, model the echo signal.

[0012] Preferably, in Step 1, in the spatial geometric model, the radar is on a moving platform, and the direction of the platform velocity vector is v r , and the direction of the antenna main axis is The direction of the target velocity vector is v t , and the direction of the mirror target velocity vector is v' t , and the vector direction from the platform center to the target center is The vector direction from the platform center to the mirror target center is ΔR represents the size of the range ring determined by the radar minimum range resolution, represents the vector direction from the platform center to a certain environmental scattering unit;

[0013] The antenna on the radar platform is a phased array antenna composed of several antenna elements. The direction of the antenna main axis is changed by mechanical or electronic control means to align it with the target direction. According to the angle by which the vector direction at the target center deviates from the antenna main axis, the amplitude of the incident wave at this position can be determined; according to the antenna main axis direction and the main beam width of the antenna pattern, the effective illumination area of the platform antenna on the environmental plane can be determined. The main lobe clutter is generated in the main lobe illumination area, the sidelobe clutter is generated in the sidelobe, and the height line clutter generated directly below the platform will cause Doppler frequencies according to the relative motion between the platform and the target, the mirror target, and the environmental unit, and they are respectively:

[0014]

[0015] In the formula, λ represents the radiation frequency of the incident electromagnetic wave.

[0016] Preferably, in Step 2, the antenna model coordinate system is established as follows: Define the main beam direction of the antenna as the z-axis, the x-axis is parallel to the horizontal direction and points to the right of the z-axis, and the y-axis is determined accordingly. Let the z-axis always be parallel to the environmental plane. Along the azimuth direction, let the y-axis be along the pitch direction, so as to form a four-quadrant antenna model, which can be regarded as composed of four array antennas arranged in a square. In the four-quadrant antenna model, Pa, Pb, Pc, and Pd are the equivalent phase centers of each quadrant, and their mutual spacing is D; f Σ represents the sum beam direction pattern, f Δ1 represents the pitch difference beam direction pattern, fΔ2 Indicates the azimuth difference beam pattern, and its expression is as follows:

[0017]

[0018] Among them, f qb (θ E , θ A ) represents the Chebyshev distribution array pattern, and λ represents the radiation frequency of the incident electromagnetic wave.

[0019] Preferably, in step 3, the position and attitude of the target model relative to the transmitting and receiving antennas will both change. Especially when the radar detects ultra-low altitude targets, the axis of the target will deflect with the change of the motion parameters, and it is necessary to transform the target model according to the axis. The transformation formula can be written as:

[0020]

[0021] Among them, θ α represents the elevation angle of the target axis with respect to the horizontal plane, represents the angle between the horizontal projection direction of the antenna axis and x, θ β represents the angle of the target rotating along the axis, (a x , a y , a z ) represents the coordinate point on the target.

[0022] Preferably, perform spatio-temporal decomposition on the antenna illumination area:

[0023] When the radar antenna detects ultra-low altitude targets in the down-looking mode, the antenna illumination area is very large. It is not possible to simply regard the target environment as an equivalent scattering center. A spatial division method based on range resolution is used to divide it in space:

[0024] First, it is divided into several equidistant rings according to the different distances from the antenna. The distance ring interval is ΔR, and the azimuth direction is divided into strips with an interval of Δθ according to the Doppler frequency. Finally, several ΔR×Δθ units are obtained. Among them, the distance ring interval ΔR is determined by the range resolution of the pulsed LFM signal, and thus:

[0025]

[0026] Among them, f prf represents the pulse repetition frequency, N p represents the number of pulses within the coherent processing time, v r represents the magnitude of the radar velocity, represents the sine of the angle between the radar velocity vector and the incident wave direction, and λ represents the electromagnetic wave wavelength.

[0027] Preferably, during the process of radar detecting a target, both the target environment and the radar are in motion. Then, the echo signal of the radar is a dynamic echo. In the pulse regime, the pulse signal transmitted by the radar antenna is a wide-time pulse linear frequency modulation signal. After pulse compression, the equivalent pulse width is τ′ = 1 / B, that is, a single pulse contains multiple sub-pulse resolution units. It is necessary to subdivide the pulse. The time decomposition method is used to decompose the broadband pulse signal into multiple narrow pulse signals. The transmitted signal can be rewritten as:

[0028]

[0029] where τ′ is the sub-pulse width and N b is the number of narrow pulses obtained by decomposing the wide pulse.

[0030] Preferably, the echo signal received by the radar antenna at ultra-low altitude is the superposition of the target echo, the mirror echo, and the environmental clutter. Then, the echo signal is modeled, that is, the target echo, the mirror echo, and the environmental clutter are modeled separately.

[0031] Preferably, the process of modeling the target echo:

[0032] (1) Transfer function of the target scattering unit

[0033] The target surface elements on which the target scattering calculation depends are of the same order of magnitude as the wavelength, and the surface element size is usually smaller than the scattering unit size. It is necessary to group the target surface elements by scattering unit. The transfer function of the m-th scattering unit under the excitation of the time-decomposed narrowband signal can be expressed as

[0034]

[0035] where N f represents the number of target surface elements within the scattering unit, and the phase φ i represents the phase generated by the path difference of the target surface element relative to the phase zero point, that is, the transmitting antenna. R iS and R iT represent the distances of the target surface element relative to the transmitting antenna and the receiving antenna respectively, and c represents the speed of light;

[0036] (2) Echo response expression of the scattering unit

[0037] It can be seen from the form of the transfer function of the target that the time delay in the transfer function is equal to the total time delay of the transmitted signal from the transmitting antenna to the propagation and finally back to the receiving antenna. Combining the Doppler frequency generated by the relative motion of the radar and the target, the transfer function of each scattering unit is convolved with the transmitted signal to obtain the echo response of each scattering unit;

[0038] The echo response of the n-th target scattering unit is

[0039]

[0040] In the formula, P t represents the transmission power, G t and G r respectively represent the gains of the transmitting antenna and the receiving antenna, R tn represents the distance between the scattering unit and the transmitting antenna, R rn represents the distance between the scattering unit and the receiving antenna, R iS represents the distance between the i-th scattering unit and the transmitting antenna, R iT represents the distance between the i-th scattering unit and the receiving antenna, s inc (t) is a chirp signal, f dn represents the Doppler frequency corresponding to the n-th scattering unit, is the convolution symbol, λ represents the wavelength of the electromagnetic wave, and j represents the imaginary unit.

[0041] Preferably, the mirror echo modeling process:

[0042] The coupling scattering between the target and the environment is the basis for the generation of mirror echoes. The coupling scattering can be regarded as the mirror image of the target. The higher-order coupling scattering corresponds to the higher-order mirror image. Then, the coupling scattering can be regarded as the scattering generated with the target mirror as the equivalent scattering source. The echo generated based on the coupling scattering is the mirror echo;

[0043] (1) Coupling scattering transfer function

[0044] The m-th scattering unit contains multiple target surface elements and environmental surface elements. There are induced currents generated by the coupling of ray irradiation on these surface elements. The transfer function of these multipath scatterings under the excitation of narrowband signals is

[0045]

[0046] In the formula, N f represents the number of target and environmental surface elements irradiated by rays in the scattering unit, N k represents the total number of rays that reach the surface element after several bounces, represents the RCS calculated under the irradiation of the i-th ray on the k-th surface element, and the phase φ ki represents the phase generated by the path difference at the first bounce point of each ray on the surface element relative to the phase zero point, R ki represents the total path length of the ray from the transmitting antenna, through bounces, and finally reaching the receiving antenna. It can be seen from the formula of the multipath scattering transfer function that each scattering unit may contain both target units and clutter units. It can also be seen that the incident rays can finally leave the target environment only after multiple bounces. In practice, the maximum bounce number truncation method is used to reduce the weak coupling effect caused by high-order coupling and improve the operation efficiency;

[0047] (2) Calculate the Doppler frequency of the multipath scattering units

[0048] To calculate the echo response of each scattering unit, the Doppler frequency generated due to the relative motion between the radar and the target should be considered. For the Doppler frequency of the multipath echo, the motion between the bounce points of each ray should be considered. Thus, the Doppler frequency generated by each ray after n bounces should be expressed as

[0049]

[0050] In the formula, n represents the maximum number of bounces of each ray; represents the direction of the main axis of the antenna, and λ represents the wavelength of the electromagnetic wave;

[0051] By convolving the transfer function of each scattering unit with the transmitted signal, the echo response of each scattering unit can be obtained. The echo response of the nth multipath scattering unit is

[0052]

[0053] In the formula, P t represents the transmitted power, G t and G r respectively represent the gains of the transmitting antenna and the receiving antenna, s inc (t) is a chirp signal, R ki represents the distance between the scattering unit and the transmitting antenna, R iS represents the distance from the transmitting antenna to the first bounce point of the ith ray, R iT represents the distance from the last bounce point of the ith ray to the receiving antenna, f dki represents the ray, the Doppler frequency corresponding to the kth scattering unit, λ represents the wavelength of the electromagnetic wave, and j represents the imaginary unit, represents the transfer function of the multipath scattering unit.

[0054] Preferably, the environmental clutter modeling process:

[0055] (1) Clutter scattering unit transfer function

[0056] Using space-time decomposition, the transfer function excited by the discrete narrowband signal under the wide-time pulse signal decomposition can be obtained for a grid of a certain size of ΔR i ×Δθ i according to the environmental parameters at the grid position:

[0057]

[0058] In the formula, γ i represents the scattering coefficient of the environmental scattering unit, Ri denotes the distance between the scattering unit and the transmitting antenna, R iS and R iT respectively denote the distances of the scattering unit relative to the transmitting antenna and the receiving antenna, and δ denotes the impulse function;

[0059] (2) Echo response expression of clutter scattering unit

[0060] Convolving the transmitted signal with the transfer function of the clutter scattering unit can obtain the echo response of the clutter scattering unit. Therefore, the echo response of the nth clutter scattering unit is:

[0061]

[0062] In the formula, R nS denotes the distance between the nth clutter unit and the transmitting antenna, R nT denotes the distance between the nth clutter unit and the receiving antenna, f dn denotes the Doppler frequency corresponding to the nth clutter scattering unit, λ denotes the electromagnetic wave wavelength, j denotes the imaginary unit, denotes the transfer function of the clutter scattering unit.

[0063] Advantages of the present invention: The present invention determines the size of the minimum resolution unit by using the specific form and waveform of the radar detection signal, and then divides the scattering units in the target environment model spatially; the target environment model is secondarily dissected inside the scattering unit and electromagnetic calculations are performed to obtain the scattering intensity of the scattering unit. Based on this, the transfer functions of each scattering unit under the radar detection signal are calculated, and convolved with the transmitted signal to obtain the echo sequences of each scattering unit; finally, the echo sequences are linearly superimposed to obtain the total echo sequence, and the target signal, multipath signal, and clutter signal can be modeled separately. With the help of the broadband echo signal obtained by simulation, the target characteristics, multipath characteristics, and clutter characteristics under broadbanding can be analyzed, which can provide a reference for suppressing clutter interference and reducing multipath interference. Description of the drawings

[0064] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0065] Figure 1 is the geometric relationship diagram of the radar looking down at ultra-low altitude;

[0066] Figure 2 is the radiation pattern of the uniform array antenna;

[0067] Figure 3 is the radiation pattern of the Chebyshev array antenna;

[0068] Figure 4 is the "four-quadrant" antenna model;

[0069] Figure 5 is the schematic diagram of the coordinate transformation of the target model;

[0070] Figure 6 is the schematic diagram of the coordinate transformation of the missile model; (a) before transformation (b) after transformation;

[0071] Figure 7 is the schematic diagram of space division;

[0072] Figure 8 is the schematic diagram of time division;

[0073] Figure 9 is the schematic diagram of the generation of the multipath scattering Doppler frequency;

[0074] Figure 10 is the diagram of the relationship between the target environment model and the radar position;

[0075] Figure 11 is the range-Doppler diagram of each component of the echo;

[0076] Figure 12 is the schematic diagram of the sea surface echo acquisition experiment;

[0077] Figure 13 is the comparison of the range-Doppler diagrams of the simulated and experimental sea surface echoes;

[0078] Figure 14 is the comparison of the simulation and experimental results of the sea clutter and the signal-to-clutter ratio;

[0079] Figure 15 is the comparison of the simulation and experimental results of the sea surface signal-to-interference ratio. Specific implementation manner

[0080] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0081] The present invention provides a method for simulating and generating broadband echo signals based on scattering, including the following steps:

[0082] Step 1: Establish a spatial geometric model of a detection radar and a very low altitude target;

[0083] 101: Design a radar antenna model carried on a radar platform;

[0084] 102: Analyze the coordinate transformation of the target model when the radar detects ultra-low altitude targets;

[0085] Step 2: Based on the established spatial geometric model of the detection radar and the ultra-low altitude target, use the spatial decomposition method to divide the radar antenna illumination area into multiple scattering units, and use the time decomposition method to decompose the transmitted pulse signal to obtain narrow pulses, so as to obtain the echo response of each scattering unit on these narrow pulses;

[0086] Step 3: Based on the decomposition results of Step 2, model the echo signal.

[0087] In Step 1, in the spatial geometric model, the radar is on a moving platform, and the direction of the platform velocity vector is v r , and the direction of the antenna main axis is The direction of the target velocity vector is v t , and the direction of the mirror target velocity vector is v' t , is the vector direction from the platform center to the target center, is the vector direction from the platform center to the mirror target center. ΔR represents the size of the range ring determined by the minimum range resolution of the radar, represents the vector direction from the platform center to a certain environmental scattering unit.

[0088] The antenna on the radar platform is usually a phased array antenna composed of several antenna elements. The direction of the antenna main axis is changed by mechanical or electronic control means to align it with the target direction. The amplitude of the incident wave at that position can be determined according to the angle by which the vector direction at the target center deviates from the antenna main axis; the effective illumination area of the platform antenna on the environmental plane can be determined according to the antenna main axis direction and the main beam width of the antenna pattern. The main lobe clutter is generated in the main lobe illumination area, the sidelobe clutter is generated in the sidelobe, and the height line clutter is generated directly below the platform. As Figure 1 shown, the relative motion between the platform and the target, mirror target, and environmental unit will cause Doppler frequencies, which are respectively:

[0089]

[0090] In the formula, λ represents the radiation frequency of the incident electromagnetic wave. Usually, the target environment is composed of many scattering centers, and the above formula can also be used for calculation, but the velocity vectors of each scattering center are required.

[0091] Array antenna

[0092] When a radar detects ultra-low altitude targets, it receives all echo signals within the beam illumination range, and these signals are weighted by the antenna pattern. Therefore, radar guidance antenna modeling is very important for radar echo modeling. The antenna in the radar should have high gain, high directivity, and beam scanning ability. Using an array antenna can achieve both high gain and beam scanning. The area of the effective radiation aperture determines the antenna gain, so an antenna with a large aperture should be selected, and the aperture is restricted by the size of the radar cross-section. Beam scanning can be achieved by mechanical scanning or electrical scanning. The advantages of electrical scanning compared to mechanical scanning are: good real-time performance; no need for a rotating device, which reduces the design difficulty, improves the stability of the equipment, and also provides more space for the design of the antenna aperture. The array antenna can obtain a radiation pattern with low side lobes through array optimization technology, which can reduce interference from side lobes.

[0093] Let d x be the element spacing in the x direction, and d y be the element spacing in the y direction. is a spatial direction vector, and θ and are respectively the elevation angle and azimuth angle in the coordinate system. Additionally, the angle between E and the xoz plane is θ A , and its angle with the yoz is θ and According to the array synthesis theory, the radiation pattern expression in this direction can be obtained as f(θ

[0094] , θ E , θ A ) = f 0 (θ E , θ A )·f x ·f y

[0095]

[0096] In the formula, f 0 (θ E , θ A ) represents the element radiation pattern, usually with a relatively wide beam width. f x and f y are the array factors in the x and y directions respectively. N x and N y are the number of elements in the x and y directions. and represent the normalized amplitudes of each element along the x and y directions respectively. And and is the initial phase of each array element. When and are both 0, the maximum radiation direction of the antenna is along the z-axis direction. By changing the phase of the array elements, beam scanning can be achieved. The following gives the parameters of a uniform array antenna. Assume the element pattern f 0 (θ E , θ A ) is is the cosine of the angle between the maximum radiation direction, i.e., the z-axis direction, and the observation direction . The initial phases of the array elements are all 0, and the normalized amplitudes are all 1, that is, the amplitudes and phases of each element are the same. d x and d y are both 0.5λ. Figure 5 .4 shows the antenna pattern in the plane where θ E = 0 degrees. It can be seen that the sidelobe level is -13 dB.

[0097] In actual use, requirements are often put forward for the sidelobe level of the antenna to make it able to be reduced to the set value. To meet the requirement of reducing the sidelobe design value, array optimization methods can be used, such as the Chebyshev array optimization method. This optimization method can optimize the amplitudes of the array elements according to the preset sidelobe level, and thus a Chebyshev array antenna with low sidelobes is obtained. The specific parameters are: the initial phases of the array elements are all 0, d x and d y are both 0.5λ, and the sidelobe level is -20 dB. The following gives the antenna pattern in the plane where θE = 0 degrees obtained by optimization. From Figure 3 it can be seen that: for the antenna pattern obtained by optimization, the sidelobe level meets the requirements.

[0098] "Four-quadrant" antenna model

[0099] The antenna on the radar will move with the movement of the radar, and the pointing of the antenna will also change accordingly, which is very unfavorable for the radar to detect and track targets. In order to enable the radar antenna to always point to the target direction that needs to be pointed at and continuously track the target, it is necessary to isolate the adjustment of the beam from the radar movement to achieve decoupling. This brings convenience to the modeling of the antenna, without considering the change of the radar attitude and only needing to always point the beam of the planar antenna at the target. Based on such considerations, the way to establish the antenna model coordinate system is: define the main beam direction of the antenna as the z-axis, the x-axis is parallel to the horizontal direction and points to the right of the z-axis, and the y-axis is then determined, as Figure 4 shown. Since there is no need to consider the influence of the radar attitude change on the antenna coordinate system, the x-axis can be made to be always parallel to the environmental plane, along the azimuth direction, and the y-axis is along the elevation direction.

[0100] In order to achieve angle tracking, a radar antenna needs to construct sum and difference beams. For this purpose, the antenna model can adopt a "four-quadrant" model, which can be regarded as composed of four array antennas arranged in a square. Figure 4 Among them, Pa, Pb, Pc, and Pd are the equivalent phase centers of each quadrant, and their mutual spacing is D; f Σ represents the sum beam pattern, and f Δ1 represents the elevation difference beam pattern, and f Δ2 represents the azimuth difference beam pattern, and its expression is as follows:

[0101]

[0102] Among them, f qb (θ E , θ A ) represents the Chebyshev distribution array pattern. In order to intuitively understand the sum and difference beams, an example is given here, where the spacing D of the "four-quadrant" antenna is 0.5λ.

[0103] In step 3, the position and attitude of the target model relative to the illuminating and receiving antennas will change. Especially when the radar detects ultra-low altitude targets, the axis of the target will deflect with the change of motion parameters. This requires transforming the target model along the axis.

[0104] Figure 5 is a schematic diagram of target transformation. In the figure, the local coordinate system x' of the target is along the axis of the target, y' is parallel to the horizontal plane and points to the left of the axis, and z' forms a right-handed screw relationship with their two directions. θ α represents the elevation angle of the target axis with respect to the horizontal plane, represents the angle between the horizontal projection direction of the antenna axis and x. When the vector direction of the target axis changes from (1, 0, 0) to and the model rotates by an angle θ β , the coordinate point (a x , a y , a z ) on the target model will change to (a’ x , a’ y , a’ z ). In order to achieve such a coordinate transformation, the following three transformations are given, namely roll transformation, pitch transformation, and azimuth transformation. The total transformation formula can be written as

[0105]

[0106] Suppose the axis direction of a missile model coincides with the positive direction of the x-axis, and it is transformed. The specific parameters are: roll angle θ β =-40°, θ α =20°, The transformed model is as follows Figure 6 shown in the figure. This figure shows that the target coordinate transformation can timely adjust the attitude change of the target model according to the transformation of the target axis, which brings convenience to the electromagnetic calculation of the target scattering. It can also timely adjust the pointing of the target axis according to the trajectory and motion vector of the target, thus establishing a connection between the original target model and the motion parameter model. In practice, the target scattering is completed in the target coordinate system, that is, the local coordinate system, while the radar motion model is established in the global coordinate system. The relative motion of the target relative to the radar will inevitably cause the direction of the antenna incident on the target to change continuously. The coordinate transformation can establish the necessary mutual position relationship between the antenna and the target for the calculation of the target electromagnetic scattering.

[0107] Perform space-time decomposition on the antenna illumination area

[0108] When the radar antenna detects ultra-low altitude targets in the down-looking mode, the antenna illumination area is very large. The target environment model cannot be simply regarded as an equivalent scattering center. Instead, it should be divided in space so that the antenna gain, Doppler frequency shift, radar pitch angle, clutter reflectivity, etc. within each scattering unit are constants, thus ensuring that the signals of each scattering unit have no coherence. Considering that the radar detection signal waveform uses LFM modulation within the pulse, its minimum range resolution is much smaller than that of narrowband or unmodulated pure pulse signals. The azimuth resolution is related to both the pulse repetition frequency and the relevant processing time. In theory, the smaller the scattering unit is divided, the higher the calculation accuracy, but the calculation amount will become unbearable. Here, a space division method based on range resolution is adopted. As shown in Figure 7 the figure, the target environment model in the figure is first divided into several equidistant rings according to the different distances from the antenna. The distance ring interval is ΔR, and the azimuth direction is divided into strips with an interval of Δθ according to the Doppler frequency, obtaining several scattering units of ΔR×Δθ. Among them, the distance ring interval ΔR is determined by the range resolution of the pulse linear frequency modulation signal, and it can be obtained that

[0109]

[0110] In the formula, f prf represents the pulse repetition frequency, N p represents the number of pulses within the coherent processing interval (CPI), v r represents the magnitude of the radar velocity, represents the sine of the angle between the radar velocity vector and the incident wave direction. In actual use, the selection of the minimum value of Δθ can be appropriately relaxed.

[0111] Time decomposition of broadband pulse signals

[0112] During the process of radar detecting a target, both the target environment and the radar are in motion, which determines that the radar echo is a dynamic echo. In the pulse system, the radar transmitting and receiving systems need to perform real-time echo signal processing on multiple pulses received within a certain coherent processing interval (CPI). Since the transmitted pulse signal is a wide-time pulse linear frequency modulation (LFM) signal, the equivalent pulse width after pulse compression is τ′ = 1 / B, that is, a single pulse contains multiple sub-pulse resolution units. When modeling the signal, it cannot be modeled with pulses as units like the conventional radar signal model. Instead, the pulse needs to be further subdivided. Using the time decomposition method, the wide-time pulse signal is decomposed into multiple narrow pulse signals, and the transmitted signal can be rewritten as

[0113]

[0114] where τ′ is the sub-pulse width, and N b is the number of narrow pulses obtained by decomposing the wide pulse.

[0115] Based on the narrow pulses obtained by time decomposition, the echo response of each scattering unit on these narrow pulses is obtained. During the narrow pulse time, the quasi-static method can be used, that is, it is assumed that the relative positions of the scattering center units of each target environment decomposed in space remain unchanged, so as to obtain the echo response of each scattering unit on the narrow time signal. Finally, the response echoes of each scattering unit excited by the wide-time pulse signal are superimposed to obtain the total echo. The whole process is as Figure 8 shown.

[0116] The echo signal received by the radar antenna at ultra-low altitude is the superposition of the target echo, the mirror echo, and the environmental clutter. The following details the modeling processes of the target echo, the mirror echo, and the environmental clutter.

[0117] Target Echo Modeling

[0118] The antenna model was introduced earlier. The transmitted signal uses an LFM signal with in-pulse modulation and contains N pulses within the coherent processing time. Thus, the form of the transmitted signal can be written as

[0119]

[0120] where (nT r −τ / 2) ≤ t − nT r ≤ (nT r +τ / 2), n represents the pulse sequence number of the signal, satisfying 0 ≤ n ≤ N, and T r represents the pulse period.

[0121] (1) Target Scattering Unit Transfer Function

[0122] Targets can be divided into point targets and extended targets according to their actual sizes and the size of the radar resolution unit. When the radial size of the target is smaller than the minimum range resolution of the radar, the target can be regarded as a point target, and the equivalent scattering center can be taken as the geometric center of the target. The RCS is the vector sum of the complex square root RCS of all surface elements. When the radial size of the target is larger than the minimum range resolution, the target is divided into multiple scattering units in the radial direction. Each scattering unit can be regarded as an equivalent scattering center, and its RCS can be obtained by the vector sum of the complex square root RCS of the target surface elements within the scattering unit.

[0123] The size of the scattering unit in the range dimension is determined according to the range resolution of the radar, and the target surface elements on which the target scattering calculation depends are on the order of the wavelength. The surface element size is usually much smaller than the scattering unit size, so it is necessary to group the target surface elements by scattering units. Then, the transfer function of the m-th scattering unit under the excitation of the time-resolved narrowband signal can be expressed as

[0124]

[0125] In the formula, N f represents the number of target surface elements within the scattering unit, and the phase φ i represents the phase generated by the path difference of the target surface element relative to the phase zero point, that is, the transmitting antenna. R iS and R iT respectively represent the distances of the target surface element relative to the transmitting antenna and the receiving antenna, and c represents the speed of light. It can be seen that compared with the traditional single scattering center model, the transfer function of the target scattering center unit obtained based on scattering calculation utilizes the positional relationship between the target and the antenna, takes into account the influence of radar signal parameters, different target postures, etc., and the calculated transfer function of the target scattering unit is closer to the transfer function generated by target scattering at a certain moment.

[0126] (2) Echo response expression of the scattering unit

[0127] From the form of the transfer function of the target, it can be seen that they all utilize the efficient calculation method of target environment scattering, and the time delay in the transfer function is exactly equal to the total time delay of the transmitted signal from the transmitting antenna to propagation and finally back to the receiving antenna. Combining with the Doppler frequency generated by the relative motion between the radar and the target, convolving the transfer function of each scattering unit with the transmitted signal can obtain the echo response of each scattering unit.

[0128] The echo response of the n-th target scattering unit is

[0129]

[0130] In the formula, P t represents the transmitted power, G t and G rrespectively represent the gains of the transmitting antenna and the receiving antenna, R tn represents the distance between the scattering unit and the transmitting antenna, R rn represents the distance between the scattering unit and the receiving antenna, R iS represents the distance between the i-th scattering unit and the transmitting antenna, R iT represents the distance between the i-th scattering unit and the receiving antenna, s inc (t) is a chirp signal, f dn represents the Doppler frequency corresponding to the n-th scattering unit, is the convolution symbol, λ represents the wavelength of the electromagnetic wave, and j represents the imaginary unit.

[0131] Modeling of mirror echoes

[0132] The coupling scattering between the target and the environment is the basis for the generation of mirror echoes. The coupling scattering can also be regarded as the mirror image of the target, and the high-order coupling scattering corresponds to the high-order mirror image. In this way, the coupling scattering can be regarded as the scattering generated by the equivalent scattering source of the target mirror image, and the echo generated based on the coupling scattering is the mirror echo.

[0133] (1) Coupling scattering transfer function

[0134] The coupling scattering is essentially the coupling scattering between the target and the environment. The SBR method can calculate the coupling scattering RCS generated under the irradiation of each ray. The m-th scattering unit contains multiple target facets and environmental facets. There are induced currents generated by the coupling of the rays on these facets. The transfer function of these multipath scatterings under the excitation of narrowband signals is

[0135]

[0136] In the formula, N f represents the number of target and environmental facets irradiated by the rays in the scattering unit, N k represents the total number of rays that reach the facet after several bounces, represents the RCS calculated under the irradiation of the i-th ray on the k-th facet, and the phase φ ki represents the phase generated by the path difference at the first bounce point of each ray on the facet relative to the phase zero point, R ki represents the total path length of the ray from the transmitting antenna, through bounces, and finally reaching the receiving antenna. It can be seen from the formula of the multipath scattering transfer function that each scattering unit may contain both target facets and clutter units. It can also be seen that the incident ray can finally leave the target environment only after multiple bounces. In practice, the maximum bounce number truncation method is used to reduce the weak coupling effect caused by high-order coupling and improve the operation efficiency.

[0137] (2) Calculate the Doppler frequency of the multipath scattering unit

[0138] To calculate the echo response of each scattering unit, the Doppler frequency generated due to the relative motion between the radar and the target should be considered, as Figure 9 shown. Finally, by convolving the transfer function of each scattering unit with the transmitted signal, the echo response of each scattering unit can be obtained. The Doppler frequencies of the target scattering unit and the clutter scattering unit have been given by

[0139] f dt = 2(v r - v t )·k t / λ

[0140]

[0141] Given this, the Doppler frequency of the multipath echo should consider the motion between the bounce points of each ray. Thus, the Doppler frequency generated by each ray after n bounces should be expressed as

[0142]

[0143] where n represents the maximum number of bounces of each ray; represents the direction of the main axis of the antenna, and λ represents the wavelength of the electromagnetic wave.

[0144] The echo response of the nth multipath scattering unit is

[0145]

[0146]

[0147] where P t represents the transmitted power, G t and G r represent the gains of the transmitting antenna and the receiving antenna respectively, s inc (t) is a chirp signal, R ki represents the distance between the scattering unit and the transmitting antenna, R iS represents the distance of the ith ray from the transmitting antenna to the first bounce point, R iT represents the distance of the ith ray from the last bounce point to the receiving antenna, f dki represents the ray, the Doppler frequency corresponding to the kth scattering unit, λ represents the wavelength of the electromagnetic wave, j represents the imaginary unit, represents the transfer function of the multipath scattering unit.

[0148] Environmental Clutter Modeling

[0149] (1) Transfer Function of Clutter Scattering Unit

[0150] Using space-time decomposition, a grid of size ΔR i ×Δθ i can be obtained. According to the environmental parameters at the position of this grid, the transfer function excited by the discrete narrowband signal under the wide-time pulse signal decomposition is obtained:

[0151]

[0152] In the formula, γ i represents the scattering coefficient of this environmental scattering unit, R i represents the distance between this scattering unit and the transmitting antenna, R iS and R iT respectively represent the distances of this scattering unit relative to the transmitting antenna and the receiving antenna. Inside the clutter scattering unit, the environmental clutter scattering coefficient at the position actually irradiated by the antenna can be calculated according to the geographical environment information, without the need for secondary dissection or grouping within the clutter unit, greatly reducing the calculation amount. And this is also the advantage of the dual-scale combination method.

[0153] In addition, the distance resolution of the clutter scattering unit is low under narrowband, and the number of equivalent distance rings is less than that in the case of wideband. In this way, the calculation time and calculation efficiency are very high. However, when it is necessary to establish the echo signal of wideband clutter, the calculation amount will be very large. This can be achieved by removing the regions with small scattering contributions according to the radar antenna pattern to speed up the operation and realize the rapid generation of clutter signals.

[0154] (2) Echo response expression of clutter scattering unit

[0155] Convolving the transmitted signal with the transfer function of the clutter scattering unit can obtain the echo response of the clutter scattering unit. Therefore, the echo response of the nth clutter scattering unit is:

[0156]

[0157] In the formula, R nS represents the distance between the nth clutter unit and the transmitting antenna, R nT represents the distance between the nth clutter unit and the receiving antenna, f dn represents the Doppler frequency corresponding to the nth clutter scattering unit.

[0158] Radar echo model verification

[0159] Radar echo simulation verification

[0160] (1) Verification of target environmental parameters

[0161] ● Verification of target RCS calculation model

[0162] Set the calculation conditions as follows:

[0163] Motion parameters: The radar position vector is S p (5000 m, 0 m, 1500 m), and the velocity vector is V s (-500 m / s, 0, 0), and the target position vector is T p (0, 0, 40 m), and the velocity vector V t is (100 m / s, 0, 0), the target radar elevation angle is θ, and the relationship between the target environment model and the radar position is as Figure 10 shown.

[0164] The radar parameters are as follows: active system, that is, both the transmitting and receiving antennas are inside the radar, and the parameters of the two antennas are the same. The sidelobe level is -20 dB, the operating frequency is in the Ku band, the number of accumulated pulses is 256, the frequency modulation bandwidth is 5 MHz, the antenna beam points to the target center, and both the incident and receiving are VV polarization.

[0165] Target environment composite model: The target model is a conductor sphere with a diameter of 30 cm, the environmental surface is the sea surface, the wind speed is 2 m / s, the wind direction is 0 degrees relative to the x-axis direction, and the relative dielectric constant of the sea surface is ε r = 42 - j40.

[0166] Since the frequency modulation bandwidth B is 5 MHz, the minimum range resolution is 30 m, and the radial size of the target is 0.3 m, the target can be approximately represented by a point target model, and the RCS of the conductor sphere can be directly calculated, that is, σ = πr 2 = 0.0707 m 2 . Next, a comparison is made between the point target model and the scattering-based model proposed in this paper.

[0167] From Figure 11 the results, it can be seen that: in the range-Doppler diagram under the point target model, the calculated target energy value is 85.718, while in the diagram under the scattering-based model, the calculated target energy value is 85.719. The target calculated values under the two models are the same, indicating that the target RCS value calculated based on scattering is consistent with the theoretical value. The echo simulation model based on scattering is reasonable.

[0168] (2) Calculation and display verification of each component of the echo

[0169] Set the calculation conditions as follows:

[0170] Motion parameters: The radar position vector is S p (8000 m, 0 m, 1500 m), and the velocity vector is V s (-400 m / s, 0, 0), and the target position vector is T p (0, 0, 50), and the velocity vector V t is (200 m / s, 0, 0), and the target radar elevation angle is θ.

[0171] The radar parameters are as follows: active system, sidelobe level of -20 dB, operating frequency in the Ku band, number of integrated pulses of 256, frequency modulation bandwidth of 10 MHz, main beam of the antenna pointing to the target center, and both incident and received polarizations are VV polarization.

[0172] Target environment composite model: The target model is a conductor sphere with a diameter of 30 cm, the environmental surface is the sea surface, the wind speed is 2 m / s, the wind direction is 0 degrees relative to the x-axis direction, and the relative dielectric constant of the sea surface is ε r = 42 – j40.

[0173] Figure 11 The range-Doppler diagrams of each component of the echo and the total echo are given. The echo model mentioned in the present invention is established based on the target environment composite scattering echo model, and the target echo, multipath echo, and environmental clutter are modeled separately. This indicates that the echo model of the present invention can reflect each component of the echo and can superimpose each part as needed. This can facilitate the analysis of the effects of target parameters, environmental parameters, and radar parameters on each part in actual use.

[0174] Radar echo experimental verification

[0175] Sea-borne radar echo experiment

[0176] The airborne radar and acquisition equipment fly horizontally at an altitude of 1500 m, and the model is as Figure 12 shown. The same aircraft is regarded as the target and is made to fly horizontally at an altitude of 100 m. The flight speeds of the two aircraft are kept the same, approximately 65 m / s.

[0177] During the flight of the carrier aircraft, the antenna beam is always pointed at the target aircraft, and the echo acquisition equipment installed on it is used to continuously acquire the echo from the target. During the experiment, the relative dielectric constant ε of the sea water r = 47 - j37. The sea surface wind speed is about 5 m / s, and the wind direction is 45 degrees relative to the flight direction of the carrier aircraft. The airborne radar operates in the Ku band, and both incident and received polarizations are VV polarization. The carrier aircraft and the target aircraft conduct test flights and data acquisition when they are 15 km apart horizontally.

[0178] After performing operations such as mixing, pulse compression, rearrangement, FFT, etc. on the collected test data, range-Doppler diagrams are obtained, and signal detection methods are used to acquire the data of the target, clutter, and multipath signals to obtain the final test results. The simulation experiment is also modeled and calculated under the above calculation conditions and the simulation calculation results are obtained according to the same signal processing process.

[0179] First, the comparison of the simulation test results of the range-Doppler diagram at a certain moment with a frequency modulation bandwidth of 80 MHz is given, asFigure 13 As shown in the figure, it can be seen that the appearance of the clutter pattern is a curve, which is caused by the narrow beam illumination of the antenna. The illuminated area of the sea surface is actually an approximate long strip, and the entire clutter distribution conforms to the situation when the radar looks down. That is, the main lobe clutter occupies the stronger part, and the side lobe clutter illuminates other areas. As the distance gets closer, the Doppler frequency corresponding to the side lobe clutter area on the sea surface decreases, showing a "V" shape, and there is also relatively strong clutter at the position of the altitude line. In the right test result, there is a bright straight line band. That is to ensure that the beam of the carrier aircraft always points to the target aircraft during the test, and a strong signal transmitter is installed on the target aircraft to ensure that the carrier aircraft can always point to the strong signal generator, so as to ensure the stable illumination of the beam direction on the target. Due to the difference between the relative speed of the airborne radar and the target and the relative speed of the airborne radar and the sea surface, the target signal and the clutter signal can be separated in the Doppler dimension.

[0180] In order to compare the characteristics of the clutter signal with the change of bandwidth, signal acquisitions were carried out under three FM bandwidths in the test. Figure 14 The comparison of the maximum clutter power (C(dB)) between simulation and test under three FM bandwidths is given, and the comparison of the ratio of the maximum target signal to the clutter power (SCR(dB), signal-to-clutter ratio) is also given. The clutter power in the test is taken as the maximum value in the clutter range-Doppler map, and the target power is also taken as the maximum value of the target. From Figure 15 it can be seen that when the grazing angle increases, the clutter backscattering coefficient increases, which also makes the clutter backscattering cross-section increase, thus causing the enhancement of the clutter power.

[0181] When the FM bandwidth increases, the minimum range resolution of the radar improves, and the backscattering cross-section of the clutter cell will decrease, which is equivalent to reducing the correlation of the clutter in the range. In this way, the clutter power will decrease. The theoretically decreased value of the clutter power ΔC is:

[0182]

[0183] In the formula, B 1 and B 2 represent the FM bandwidth. When B 1 = 20MHz, B 2 = 80MHz, ΔC = -6dB. There is a difference between the measured result in the figure and this theoretical value, which is mainly due to the continuous change of the sea surface wind speed over time, and the change of the wind speed causes the change of the sea surface backscattering.

[0184] The signal-to-multipath interference ratio (SIR(dB), signal-to-interference ratio) was also compared in the test. From Figure 15From the comparison results, it can be seen that the sea surface Brewster angle obtained by simulation is approximately around 6.5 degrees, while the measured result is around 8 degrees, with a difference of less than 2 degrees. The calculation error mainly comes from the estimation of the relative permittivity of the sea surface and the setting of the radar depression angle. Specifically: the actual salt content and temperature of seawater change dynamically over time, which will cause changes in the relative permittivity, and then lead to the offset of the Brewster angle position; during the simulation process, the actual depression angle of the radar needs to be set, however, there are relatively large errors in the acquisition of longitude, latitude and altitude information itself, which will also cause the offset of the Brewster angle in the final result. During actual testing, the target and interference signals are mixed together and cannot be separated, which increases the difficulty of obtaining the signal-to-interference ratio (SIR). In the simulation, the target echo and multipath echo are modeled and processed separately, and the difference between the two results in the difference in the magnitude of the signal-to-interference ratio comparison curve.

[0185] As described above, it is only used to illustrate the technical solution of the present invention and not to limit it. Any other modifications or equivalent replacements made by those of ordinary skill in the art to the technical solution of the present invention should be covered within the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for simulating and generating wideband echo signals based on scattering, characterized in that, it includes the following steps: Step 1: Establish a spatial geometric model of a detection radar and a very low altitude target; 101: Design a radar antenna model carried on a radar platform; 102: Analyze the coordinate transformation of the target model when the radar detects a very low altitude target; Step 2: Based on the established spatial geometric model of the detection radar and the very low altitude target, use the spatial decomposition method to divide the radar antenna illumination area into multiple scattering units, and use the time decomposition method to decompose the transmitted pulse signal into narrow pulses, so as to obtain the echo response of each scattering unit on these narrow pulses; Step 3: Based on the decomposition results of Step 2, model the echo signal; In step 1, in the spatial geometric model, the radar is on a moving platform, and the direction of the platform velocity vector is v r , and the direction of the main axis of the antenna is The direction of the target velocity vector is v t , and the direction of the mirror target velocity vector is v' t , and the vector direction from the platform center to the target center is The vector direction from the platform center to the mirror target center is ΔR represents the size of the range ring determined by the minimum range resolution of the radar, represents the vector direction from the platform center to a certain environmental scattering unit; The antenna on the radar platform is a phased array antenna composed of several antenna elements. The main axis direction of the antenna is changed by mechanical or electronic control means to align it with the target direction. According to the main axis direction of the antenna and the main beam width of the antenna pattern, the effective illumination area of the platform antenna on the environmental plane is determined. The main lobe clutter is generated in the main lobe illumination area, and the side lobe clutter is generated in the side lobe. The height line clutter generated directly below the platform will cause Doppler frequency shift according to the relative motion between the platform and the target, the mirror target, and the environmental unit. They are respectively: In the formula, λ represents the radiation frequency of the incident electromagnetic wave; In step 101, the antenna model coordinate system is established as follows: Define the main beam direction of the antenna as the z-axis, the x-axis is parallel to the horizontal direction and points to the right of the z-axis, and the y-axis is then determined. Keep the z-axis always parallel to the environmental surface. Along the azimuth direction, let the y-axis be along the elevation direction, thus forming a four-quadrant antenna model, which is regarded as composed of four array antennas arranged in a square. In the four-quadrant antenna model, Pa, Pb, Pc, and Pd are the equivalent phase centers of each quadrant, and their mutual spacing is D; f Σ represents the sum beam pattern, f Δ1 represents the elevation difference beam pattern, f Δ2 represents the azimuth difference beam pattern, and its expression is as follows: where f qb (θ E , θ A ) represents the Chebyshev distribution array pattern, and λ represents the radiation frequency of the incident electromagnetic wave.

2. The method for simulating and generating wideband echo signals based on scattering according to claim 1, characterized in that, In step 102, the position and attitude of the target model relative to the transmitting and receiving antennas will both change. Especially when the radar detects a very low altitude target, the axis of the target will deflect with the change of the motion parameters. It is necessary to transform the target model according to the axis, and its transformation formula can be written as: Among them, θ α represents the elevation angle of the target axis with respect to the horizontal plane, represents the angle between the horizontal projection direction of the antenna axis and x, θ β represents the angle of rotation of the target along the axis, (a x , a y , a z ) represents the coordinate point on the target.

3. The method for simulating and generating wideband echo signals based on scattering according to claim 1, characterized in that, Perform space-time decomposition on the antenna illumination area: It is divided into several equidistant rings according to the different distances from the antenna. The distance ring interval is ΔR, and the azimuth direction is divided into strips with an interval of Δθ according to the Doppler frequency. Finally, several ΔR×Δθ units are obtained. Among them, the distance ring interval ΔR is determined by the range resolution of the pulse LFM signal, and we get: where, f prf represents the pulse repetition frequency, N p represents the number of pulses within the coherent processing time, v r represents the magnitude of the radar velocity, represents the sine of the angle between the radar velocity vector and the direction of the incident wave, and λ represents the electromagnetic wave wavelength.

4. The method for simulating and generating wideband echo signals based on scattering according to claim 3, characterized in that, During the process of the radar detecting the target, the pulse signal transmitted by the radar antenna is a wide-time pulse linear frequency modulation signal. The equivalent pulse width after pulse compression is τ′ = 1 / B, that is, a single pulse contains multiple sub-pulse resolution units. It is necessary to subdivide the pulse. The time decomposition method is used to decompose the wideband pulse signal into multiple narrow pulse signals, and the transmitted signal is written as: where τ′ is the sub-pulse width, and N b is the number of narrow pulses obtained by decomposing the wide pulse.

5. The method for simulating and generating wideband echo signals based on scattering according to claim 1, characterized in that, The echo signal received by the radar antenna at very low altitude is the superposition of the target echo, the mirror echo, and the environmental clutter. Then, model the echo signal, that is, model the target echo, the mirror echo, and the environmental clutter respectively.

6. The method for simulating and generating wideband echo signals based on scattering according to claim 5, characterized in that, The target echo modeling process: (1) Transfer function of target scattering unit The target surface elements are grouped by scattering units. The transfer function of the m-th scattering unit under the excitation of a time-decomposed narrowband signal can be expressed as Where N f represents the number of target surface elements in the scattering unit, and the phase φ i represents the phase generated by the path difference of the target surface element relative to the phase zero point, i.e., the transmitting antenna. R iS and R iT respectively represent the distances of the target surface element relative to the transmitting antenna and the receiving antenna, and c represents the speed of light; (2) Echo response expression of scattering unit Convolving the transfer functions of each scattering unit with the transmitted signal can obtain the echo responses of each scattering unit; The echo response of the n-th target scattering unit is where P t represents the transmit power, G t and G r represent the gains of the transmit antenna and the receive antenna respectively, R tn represents the distance between the scattering unit and the transmit antenna, R rn represents the distance between the scattering unit and the receive antenna, R iS represents the distance between the i-th scattering unit and the transmit antenna, R iT represents the distance between the i-th scattering unit and the receive antenna, s inc (t) is a chirp signal, f dn represents the Doppler frequency corresponding to the n-th scattering unit, is the convolution symbol, λ represents the electromagnetic wave wavelength, and j represents the imaginary unit.

7. The method for simulating and generating a broadband echo signal based on scattering according to claim 5, wherein, The mirror echo modeling process: (1) Coupled scattering transfer function The m-th scattering unit contains multiple target surface elements and environmental surface elements. There are induced currents generated by coupling due to ray irradiation on these surface elements. The transfer function of these multipath scatterings under the excitation of a narrowband signal is Where, N f represents the number of targets and environmental surface elements irradiated by rays in the scattering unit, N k represents the total number of rays reaching the surface element after several bounces, represents the RCS calculated under the irradiation of the i-th ray on the k-th surface element, and the phase φ ki represents the phase generated by the path difference at the first bounce point of each ray on the surface element relative to the phase zero point, R ki represents the total path length of the ray from the transmitting antenna, through bounces and finally reaching the receiving antenna; (2) Calculating the Doppler frequency of the multipath scattering unit The Doppler frequency of the multipath echo should consider the movement between the bounce points of the rays. The Doppler frequency generated after n bounces of each ray should be expressed as Where n represents the maximum number of bounces per ray; represents the direction of the antenna main axis, and λ represents the electromagnetic wave wavelength; Convolving the transfer functions of each scattering unit with the transmitted signal can obtain the echo responses of each scattering unit. The echo response of the n-th multipath scattering unit is where P t represents the transmit power, G t and G r represent the gains of the transmit antenna and the receive antenna respectively, s inc (t) is a chirp signal, R ki represents the distance between the scattering unit and the transmit antenna, R iS represents the distance of the i-th ray from the transmit antenna to the first bounce point, R iT represents the distance of the i-th ray from the last bounce point to the receive antenna, f dki represents the Doppler frequency corresponding to the k-th scattering unit of the ray, λ represents the wavelength of the electromagnetic wave, j represents the imaginary unit, represents the transfer function of the multipath scattering unit.

8. The method for simulating and generating a broadband echo signal based on scattering according to claim 5, wherein, The environmental clutter modeling process: (1) Transfer function of clutter scattering unit A grid of a certain size of ΔR i ×Δθ i can be obtained by space-time decomposition. According to the environmental parameters at the grid position, the transfer function excited by the discrete narrowband signal under the wide-time pulse signal decomposition is obtained: where γ i represents the scattering coefficient of the environmental scattering unit, R i represents the distance between the scattering unit and the transmitting antenna, R iS and R iT respectively represent the distances of the scattering unit relative to the transmitting antenna and the receiving antenna, and δ represents the impulse function; (2) Echo response expression of clutter scattering unit Convolving the transmitted signal with the transfer function of the clutter scattering unit can obtain the echo response of the clutter scattering unit; Therefore, the echo response of the n-th clutter scattering unit is: wherein, R nS represents the distance between the nth clutter cell and the transmitting antenna, and R nT represents the distance between the nth clutter cell and the receiving antenna, f dn represents the Doppler frequency corresponding to the nth clutter scattering cell, λ represents the electromagnetic wave wavelength, and j represents the imaginary unit, represents the transfer function of the clutter scattering cell.

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