A Deception and Jamming Method for ISAR Shape Features Based on Template Product and Time Delay

Abstract

This invention provides an ISAR shape feature deception jamming method based on template product and time delay, comprising the following steps: Step 1: generating a false target template; Step 2: template product frequency shift compensation; Step 3: data interval sampling and template signal generation; Step 4: template cancellation of time-shifted signals. This method can accurately locate the position coordinates of ISAR image scattering points, precisely altering target characteristics and causing the radar to misidentify the target as another target, taking into account the target's motion and the relative relationship between the scattered echoes from each strong scattering point and the jamming signal.

Description

Technical Field

[0001] This invention relates to signal and information processing technology, specifically to an ISAR shape feature deception jamming method based on template product and time delay. Background Technology

[0002] Inverse Synthetic Aperture Radar (ISAR) can achieve high-resolution two-dimensional imaging of aerial targets around the clock and in all weather conditions. The radar transmits linear frequency modulated (LFM) signals, and target feature extraction based on ISAR images is widely used in air defense, missile defense, and other fields. How to jam ISAR images has become a hot topic of widespread interest in radar countermeasures in recent years. Currently, ISAR jamming is categorized into suppression jamming and deception jamming based on its effectiveness. Since the advent of Digital Radio Frequency Memory (DRFM) technology, time-frequency modulation techniques based on signals have become possible. By time-frequency modulating the intercepted radar echo and forwarding it to the radar receiver, the effect of generating dense ISAR false targets or striped target coverage can be achieved. However, both dense false targets and striped target coverage, which resemble real targets, are easily identified as being jammed. In such cases, the radar can filter out the jamming signal by adjusting the transmitted radar signal parameters or imaging parameters, thus failing to achieve the deception effect. Therefore, figuring out how to prevent radar from detecting interference and misidentifying enemy targets has more practical application value.

[0003] Current technologies rely on direct imaging of jamming signals emitted through template multiplication, without considering target motion or the relative relationship between the scattered echoes from strong scattering points and the jamming signal. Therefore, they cannot accurately pinpoint the location coordinates of scattering points in ISAR images. Furthermore, feature extraction based on ISAR imaging only needs to consider the target's inherent characteristics, not target location or window size. Thus, time-frequency modulation methods can be used to precisely alter target features, causing the radar to misidentify the target as something else. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and propose an ISAR shape feature deception and interference method based on template product and time delay, comprising the following steps:

[0005] Step 1: Generating false target templates. The jammer intercepts the LFM signal and extracts the information of scattering points that need to be compensated and the information of scattering points that need to be eliminated by comparing and ORing the true target template and the false target.

[0006] Step 2: Template product frequency shift compensation. Perform Fourier transform on the obtained pseudo-target template and compensate for the corresponding frequency shift parameters.

[0007] Step 3: Data interval sampling and template signal generation. Interference template data is generated by sampling the false target template at intervals. The generated interference template data is multiplied with the LFM signal intercepted by the jammer to generate an interference signal, which is then forwarded to the radar receiver.

[0008] Step 4: Template cancellation of time-shifted signals. Based on the scattering point signals captured by the jammer, the time-shifted amount of the corresponding scattering point to be eliminated is added, and then the corresponding phase compensation is performed to obtain the corresponding jamming signal. The radar receiver receives the jamming signal and superimposes it with the target echo signal and performs imaging. A false target identical to the template is generated at the corresponding position of the ISAR image to achieve signal cancellation.

[0009] Furthermore, step 1 specifically includes:

[0010] Let the true target template A be an ISAR image obtained in advance based on the scattering characteristics of the true target where the jammer is located, and the false target template B be a false target image obtained by the radar side imagery desired by the jammer. The specific steps are as follows:

[0011] (1) Perform 0 / 1 binarization on A and B respectively, that is, if there is a value at the corresponding pixel position of the template, it is 1, and otherwise it is 0, and they are denoted as A and B respectively;

[0012] (2) Multiply template A and template B, which gives A∩B;

[0013] (3) Perform the following operations on templates A and B:

[0014]

[0015] The additional strong scattering points obtained from the two templates are as follows: This indicates the number of additional scattering points in A compared to B. This indicates that B has more scattering points than A;

[0016] Where, assuming the template pixel is p r ×p a There exists a scattering point on the template with a scattering coefficient of 1, denoted as m(r,a)=1. Assume that the range resolution and azimuth resolution of the ISAR image corresponding to the template are Δr and Δα, respectively, and therefore the range length is L. r =p r ×Δr, azimuth length is L a =p a If the zero point of the coordinate system is set to the center of the template, then the coordinates of the scattering point become (rΔr-L). r / 2,aΔa-La / 2), where r is the row variable of the m matrix and a is the column variable of the m matrix, corresponding to the range and azimuth directions of the ISAR image, respectively.

[0017] Furthermore, step 2 specifically includes:

[0018] First, the radar transmits LFM signals s(t) r ,t a ), represented as

[0019]

[0020] Among them, t r t indicates fast time. a T represents slow time. p Indicates the pulse width. j represents an imaginary number, fc Indicates the carrier frequency, k r This represents the frequency modulation slope. The signal is intercepted by the jammer, frequency-modulated, and then forwarded. The jamming signal at this time is represented as...

[0021]

[0022] Among them, Ψ r and Ψ a Here, λ represents the frequency shift parameters in the range and azimuth directions, respectively; c is the ideal transmission velocity of electromagnetic waves in space; λ is the radar transmission wavelength; and ω represents the equivalent rotational angular velocity of the target on the turntable model. Direct imaging of this signal reveals a frequency shift in both the range and azimuth directions compared to the intercepted LFM signal.

[0023]

[0024] Among them, f r _ d For the range image frequency shift, f a _ d Due to the azimuth frequency shift, the interfering signal is processed by pulse compression to obtain the ISAR image of the interfering signal, and the peak positions along the range and azimuth directions are respectively...

[0025]

[0026] Among them, R r _ d R represents the distance to the image position. a _ d Given the azimuth position, the frequency shift parameters Ψ in the range and azimuth directions are obtained from the formula. r and Ψ a Considering the radar echo delay, in order to generate false target points at the desired location in the ISAR image of the jamming signal emitted by the jammer, the frequency shift parameter Ψ is derived.r and Ψ a The correspondence between the scattering points of the fake target and the scattering points of the fake target template;

[0027] Next, the frequency shift parameter Ψ is derived. r and Ψ a ;

[0028] The jamming signal received by the radar from the jammer is equivalent to receiving the reflected frequency-shifted signal from the scattering point corresponding to the jammer's location, denoted as:

[0029]

[0030] Among them, R c R is the distance from the scattering point to the radar. k Let k be the path of the reflected echo from the scattering point k at the location of the jammer. After performing de-chirp processing on the jamming signal and ignoring the echo envelope slant term and the residual video phase (RVP) term, we obtain the following:

[0031]

[0032] Among them, s ref * (t r ,t a ) is the conjugate of the interference reference signal. Assume the center point of the target, i.e., the reference point Ref, is the origin of the turntable model, where R... Δ =R c -R ref R represents the distance between the jammer and the target reference center. ref Define the distance from the selected reference point on the flight target to the radar. Substituting this into the equation, we get...

[0033]

[0034] Then consider the time t in equation (8) Δ Performing a Fast Fourier Transform (FFT) can yield a one-dimensional high-resolution range vector, i.e.

[0035]

[0036] Among them, the second item The RVP term and the third term are R. Δ ≠0 o'clock After compensating and removing the skewed term of the echo envelope, we obtain the one-dimensional high-resolution range vector (HRRP) of multiple echoes, i.e.

[0037]

[0038] Where M represents the number of pulse echoes, and the position coordinates of the jammer on the target in the turntable model are assumed to be (x...). k ,y k ), x k Represents the azimuth coordinate, y k Let the range coordinate be the coordinate system. The echo distance from the location of the jammer to the echo distance from the reference point has the following relationship:

[0039] R Δ =x k ωt a +y k (11)

[0040] Where ω represents the rotational angular velocity of the target on the equivalent turntable model, after substituting the equation into the equation and rearranging, the corresponding t is extracted. a From the phase term, we can obtain the expression for the interference signal:

[0041]

[0042] The slow time t of the equation a By performing an FFT, a two-dimensional ISAR image can be obtained, i.e.

[0043]

[0044] Among them, f a For full time, t a In the frequency domain representation, the phase term can be ignored as a constant. Therefore, it can be seen that the positions of the false target scattering points formed by the received interference signal in the range and azimuth directions are respectively...

[0045]

[0046] Where, ΔR r Let ΔR be the location of the false target scattering point formed by the received interference signal in the range direction. a Let t be the location of the false target scattering point formed by the received interference signal in the azimuth direction, in one echo. a It can be considered a constant, therefore the scattering point position R in the range direction is... Δ ≈y k To ensure that the location of the scattering point in the ISAR image of the interfering signal is the same as the location of the false scattering target point in the template, thus guaranteeing the elimination of the scattering point at the corresponding location in the echo of the real target, the following equation applies.

[0047]

[0048] Where r is the range direction of the ISAR image corresponding to the template, Δr is the range resolution of the ISAR image corresponding to the template, and L r L represents the range length of the ISAR image corresponding to the template, a represents the azimuth length of the ISAR image corresponding to the template, Δa represents the azimuth resolution of the ISAR image corresponding to the template, and L represents the range length of the ISAR image corresponding to the template. a Given the azimuth length of the ISAR image corresponding to the template, the frequency shift parameter of the jammer on the intercepted LFM signal should be:

[0049]

[0050] This shows that the jammer shifts the frequency of the intercepted LFM signal by Ψ along both the range and azimuth directions. r and Ψ a The interference signal received by the radar receiver is processed by pulse compression to generate an ISAR image, and the coordinates of the false target scattering point are (rΔr-L). r / 2,aΔa-L a / 2), which is the location of the false target on the template.

[0051] Furthermore, step 3 specifically includes:

[0052] The template m(r,a) used to generate the interference signal is preprocessed, and then the adjusted frequency shift template m'(t) is used. r ,t a Multiplying the jamming signal by the intercepted LFM signal, the frequency shift template for multiplying the jamming signal by the LFM signal when the jammer transmits the jamming signal is derived from the formulas:

[0053]

[0054] To obtain the frequency-shifting template, the false target template image m(r,a) is processed in the following three steps:

[0055] Step (1) performs a two-dimensional FFT on the template m(r,a) of the false target, and obtains:

[0056]

[0057] Where 1≤f r ≤N r 1≤f a ≤N a N r N is the number of points from the upward FFT. a The number of points in the azimuth-up FFT is given, and r' and a' represent the position coordinates of the specific spurious scattering point on the template, respectively.

[0058] Step (2) multiplies the equation by a correction matrix H( xr ,f aThis involves performing phase compensation on the frequency domain expression of the false target template image, where the correction matrix H(f) is used. r ,f a ) represents

[0059]

[0060] Thus, the frequency domain expression of the interference signal template is obtained, i.e.

[0061]

[0062] Comparing the formulas, we can see that if an equation exists...

[0063]

[0064] Then the interference signal template m'(t) is obtained. r ,t a This expression is considered as the time-domain form of the interference signal template;

[0065] Step (3) In order to obtain the equation relationship of the formula, for m h (f r ,f a Perform equal-interval sampling, assuming F s The sampling rate of the jammer's analog-to-digital converter (A / D) is represented by , and PRT represents the pulse repetition time. Therefore, there is a correspondence between the fast and slow times and the range and azimuth directions.

[0066]

[0067] Let η be the intervals along the template distance direction and azimuth direction. r and η a The points sampled on the template are represented as f. r 'and f a ', then there exists an equation

[0068]

[0069] Substituting the expressions into the equation, we derive:

[0070]

[0071] Due to η r and η a The value is usually not an integer. It is decimated by linear interpolation. After sampling, a frequency shift template can be obtained. The template is multiplied with the intercepted LFM signal and then forwarded to generate an interference signal that induces false targets in the radar ISAR image.

[0072] Furthermore, step 4 specifically involves:

[0073] Signal cancellation is achieved using a template time-shift method. A time-shift amount corresponding to the scattering point to be eliminated is added to the scattering point signal captured by the jammer, along with a phase π. The radar receiver receives an interference signal with the same amplitude but opposite phase as the echo signal from the real scattering point, thus achieving signal cancellation. Here, i represents the point to be eliminated in the false target template, and its coordinates in the false target template are (x...). i ,y i ), where the jammer's position k is the origin, and the jammer's position coordinates on the target are (x, k). k ,y k If the actual scattering point i on the target is reflected back to the radar, then the echo path R is... i It can be represented as

[0074]

[0075] Where (X) ref ,Y ref The initial coordinates of the true target reference point ref in the radar ISAR imaging coordinate system are represented by the echo path R reflected back to the radar from the scattering point k where the jammer is located. k Represented as

[0076]

[0077] The time difference between the reflected echoes from positions k and i is expressed as follows:

[0078]

[0079] Therefore, once the transmitter acquires the radar's LFM signal, it directly forwards the signal without considering hardware delay and uses the false target template. After adjusting the coordinate parameters of each scattering point i in the signal and applying a corresponding time delay, and then adding a π phase before transmission, the cancellation signal transmitted by the jammer is expressed as:

[0080]

[0081] Where, Δt ki Let the time difference between the reflected echoes from positions k and i be the time difference, then the signal received at the radar receiver is:

[0082]

[0083] The signal is canceled because it has the same amplitude as the real echo signal returned from scattering point i, but opposite in phase.

[0084] The beneficial effects of this invention are as follows: by using the ISAR shape feature deception jamming method based on template product and time delay, the motion of the target and the relative relationship between the scattered echoes of each strong scattering point and the jamming signal of the jammer can be taken into account, so as to accurately locate the position coordinates of the scattering points of the ISAR image, precisely change the target characteristics, and make the radar identify the target as other targets. Attached Figure Description

[0085] Figure 1 This is a flowchart illustrating the interference signal generation process of the present invention.

[0086] Figure 2 This is a diagram of a signal cancellation model;

[0087] Figure 3 A four-point model diagram of true / false targets; Figure 3 (a) represents four points of the real target, with coordinates (-18.5, 6.5), (-18.5, -18.5), (6.5, 6.5), and (6.5, -18.5). Figure 3 (b) is a false target imitated by four points. Figure 3 (c) represents two points eliminated from template A after XOR processing of the interfering template. Figure 3 (d) represents the compensated scattering point in template A;

[0088] Figure 4 ISAR image of the actual target echo / interference signal; Figure 4 (a) is the echo ISAR image of the real target. Figure 4 (b) is an ISAR image of the interference signal generated based on the false target template. Figure 4 (c) represents the ISAR image of the scattering point interference signal that needs to be eliminated, obtained based on template elimination. Figure 4 (d) represents the ISAR image of the interference signal that needs to be added with scattering points. Figure 4 (e) represents the radar echo after the scattering points have been eliminated. Figure 4 (f) represents the ISAR image of the false target B;

[0089] Figure 5 ISAR image for simulating aircraft scattering point echo / interference signal; Figure 5 (a) represents the 29-point true target aircraft ISAR image. Figure 5 (b) indicates a 54-point false target aircraft template. Figure 5 (c) represents the scattering interference signals that need to be eliminated based on the 54-point dummy target aircraft template. Figure 5 (d) indicates the interference signal that needs to be added with scattering points based on the 54-point fake target aircraft template. Figure 5 (e) represents the ISAR image of the 54-point false target aircraft. Detailed Implementation

[0090] The present invention will be further described below with reference to the accompanying drawings and examples.

[0091] A method for deceiving and interfering with ISAR shape features based on template product and time delay includes the following steps:

[0092] Step 1: Generating false target templates. The jammer intercepts the LFM signal and extracts the information of scattering points that need to be compensated and the information of scattering points that need to be eliminated by comparing and ORing the true target template and the false target.

[0093] Step 2: Template product frequency shift compensation. Perform Fourier transform on the obtained pseudo-target template and compensate for the corresponding frequency shift parameters.

[0094] Step 3: Data interval sampling and template signal generation. Interference template data is generated by sampling the false target template at intervals. The generated interference template data is multiplied with the LFM signal intercepted by the jammer to generate an interference signal, which is then forwarded to the radar receiver.

[0095] Step 4: Template cancellation of time-shifted signals. Based on the scattering point signals captured by the jammer, the time-shifted amount of the corresponding scattering point to be eliminated is added, and then the corresponding phase compensation is performed to obtain the corresponding jamming signal. The radar receiver receives the jamming signal and superimposes it with the target echo signal and performs imaging. A false target identical to the template is generated at the corresponding position of the ISAR image to achieve signal cancellation.

[0096] Step 1 specifically involves:

[0097] To generate a false target by superimposing the jamming signal and the original target signal, a cancellation / product jamming method is used to generate the jamming signal. Therefore, in the template generation stage, the desired false target image (false target B) and the echo signal of the original target (target A) need to be XORed accordingly. The real target A is an ISAR image obtained in advance based on the scattering characteristics of the real target where the jammer is located. False target B is the false target image obtained by the jammer from the radar image it desires. Performing AND / OR operations on A and B yields a template of additional scattering points in B compared to A. These points are multiplied by the intercepted LFM signal, i.e., forward phase processing. For the additional scattering points in A compared to B, reverse phase processing is performed, ensuring that the signals received by the radar receiver from these points have the same amplitude and opposite phase (π-direction) to the original target echo signal, thus canceling out the corresponding points in the echo signal. To cancel out the corresponding scattering point echo signal using signal cancellation, the jammer needs to rapidly modulate the intercepted radar transmission signal to ensure precise alignment between the scattering points and the echo signal. The specific operation is described below:

[0098] Let the true target template A be an ISAR image obtained in advance based on the scattering characteristics of the true target where the jammer is located, and the false target template B be a false target image obtained by the radar side imagery desired by the jammer. The specific steps are as follows:

[0099] (1) Perform 0 / 1 binarization on A and B respectively, that is, if there is a value at the corresponding pixel position of the template, it is 1, and otherwise it is 0, and they are denoted as A and B respectively;

[0100] (2) Multiply template A and template B, which gives A∩B;

[0101] (3) Perform the following operations on templates A and B:

[0102]

[0103] The additional strong scattering points obtained from the two templates are as follows: This indicates the number of additional scattering points in A compared to B. This indicates that B has more scattering points than A;

[0104] Where, assuming the template pixel is p r ×p a There exists a scattering point on the template with a scattering coefficient of 1, denoted as m(r,a)=1. Assume that the range resolution and azimuth resolution of the ISAR image corresponding to the template are Δr and Δα, respectively, and therefore the range length is L. r =p r ×Δr, azimuth length is L a =p a If the zero point of the coordinate system is set to the center of the template, then the coordinates of the scattering point become (rΔr-L). r / 2,aΔa-L a / 2), where, r Let 'a' represent the row variables of matrix m, and 'a' represent the column variables of matrix m, corresponding to the range and azimuth directions of the ISAR image, respectively.

[0105] The interference signal received by the radar receiver can be equivalent to the signal reflected from the scattering point corresponding to the target location where the jammer is located. Since a false target needs to be created on the ISAR image, this interference signal should be a frequency-shifted signal. The specific frequency-shifting parameters are derived based on the principle that the position of the false target scattering point on the ISAR image is equal to the corresponding position of the false target on the template. The theoretical derivation process of the transmitted interference signal is as follows: Step 2 specifically involves:

[0106] First, the radar transmits LFM signals s(t) r ,t a ), represented as

[0107]

[0108] Among them, t r t indicates fast time. a T represents slow time. p The pulse width is represented by j, which represents the imaginary number, and f is the pulse width. c Indicates the carrier frequency, k r This represents the frequency modulation slope. The signal is intercepted by the jammer, frequency-modulated, and then forwarded. The jamming signal at this time is represented as...

[0109]

[0110] Among them, Ψ r and Ψ a Here, λ represents the frequency shift parameters in the range and azimuth directions, respectively; c is the ideal transmission velocity of electromagnetic waves in space; λ is the radar transmission wavelength; and ω represents the equivalent rotational angular velocity of the target on the turntable model. Direct imaging of this signal reveals a frequency shift in both the range and azimuth directions compared to the intercepted LFM signal.

[0111]

[0112] Among them, f r_d For the range image frequency shift, f a_d Due to the azimuth frequency shift, the interfering signal is processed by pulse compression to obtain the ISAR image of the interfering signal, and the peak positions along the range and azimuth directions are respectively...

[0113]

[0114] Among them, R r_d R represents the distance to the image position. a_d Given the azimuth position, the frequency shift parameters Ψ in the range and azimuth directions are obtained from the formula. r and Ψ a Considering the radar echo delay, in order to generate false target points at the desired location in the ISAR image of the jamming signal emitted by the jammer, the frequency shift parameter Ψ is derived. r and Ψ a The correspondence between the scattering points of the fake target and the scattering points of the fake target template;

[0115] Next, the frequency shift parameter Ψ is derived. r and Ψ a ;

[0116] The jamming signal received by the radar from the jammer is equivalent to receiving the reflected frequency-shifted signal from the scattering point corresponding to the jammer's location, denoted as:

[0117]

[0118] Among them, R c R is the distance from the scattering point to the radar. kLet k be the path of the reflected echo from the scattering point k at the location of the jammer. After performing de-chirp processing on the jamming signal and ignoring the echo envelope slant term and the residual video phase (RVP) term, we obtain the following:

[0119]

[0120] Among them, s ref * (t r ,t a ) is the conjugate of the interference reference signal. Assume the center point of the target, i.e., the reference point Ref, is the origin of the turntable model, where R... Δ =R c -R ref R represents the distance between the jammer and the target reference center. ref Define the distance from the selected reference point on the flight target to the radar. Substituting this into the equation, we get...

[0121]

[0122] Then consider the time t in equation (8) Δ Performing a Fast Fourier Transform (FFT) can yield a one-dimensional high-resolution range vector, i.e.

[0123]

[0124] Among them, the second item The RVP term and the third term are R. Δ ≠0 o'clock After compensating and removing the skewed term of the echo envelope, we obtain the one-dimensional high-resolution range vector (HRRP) of multiple echoes, i.e.

[0125]

[0126] Where M represents the number of pulse echoes, and the position coordinates of the jammer on the target in the turntable model are assumed to be (x...). k ,y k ), x k Represents the azimuth coordinate, y k Let the range coordinate be the coordinate system. The echo distance from the location of the jammer to the echo distance from the reference point has the following relationship:

[0127] R Δ =x k ωt a +y k (11)

[0128] Where ω represents the rotational angular velocity of the target on the equivalent turntable model, after substituting the equation into the equation and rearranging, the corresponding t is extracted. a From the phase term, we can obtain the expression for the interference signal:

[0129]

[0130] The slow time t of the equation a By performing an FFT, a two-dimensional ISAR image can be obtained, i.e.

[0131]

[0132] Among them, f a For full time, t a In the frequency domain representation, the phase term can be ignored as a constant. Therefore, it can be seen that the positions of the false target scattering points formed by the received interference signal in the range and azimuth directions are respectively...

[0133]

[0134] Where, ΔR r Let ΔR be the location of the false target scattering point formed by the received interference signal in the range direction. a Let t be the location of the false target scattering point formed by the received interference signal in the azimuth direction, in one echo. a It can be considered a constant, therefore the scattering point position R in the range direction is... Δ ≈y k To ensure that the location of the scattering point in the ISAR image of the interfering signal is the same as the location of the false scattering target point in the template, thus guaranteeing the elimination of the scattering point at the corresponding location in the echo of the real target, the following equation applies.

[0135]

[0136] Where r is the range direction of the ISAR image corresponding to the template, Δr is the range resolution of the ISAR image corresponding to the template, and L r L represents the range length of the ISAR image corresponding to the template, a represents the azimuth length of the ISAR image corresponding to the template, Δa represents the azimuth resolution of the ISAR image corresponding to the template, and L represents the range length of the ISAR image corresponding to the template. a Given the azimuth length of the ISAR image corresponding to the template, the frequency shift parameter of the jammer on the intercepted LFM signal should be:

[0137]

[0138] This shows that the jammer shifts the frequency of the intercepted LFM signal by Ψ along both the range and azimuth directions. r and Ψ aThe interference signal received by the radar receiver is processed by pulse compression to generate an ISAR image, and the coordinates of the false target scattering point are (rΔr-L). r / 2,aΔa-L a / 2), which is the location of the false target on the template.

[0139] To generate interference signals that can produce strong scattering points at corresponding locations on the ISAR image, the template m(r,a) used to generate the interference signals first needs to be preprocessed, and then the adjusted frequency-shifting template m'(t) needs to be preprocessed. r ,t a Step 3 involves multiplying the intercepted LFM signal by the signal itself.

[0140] The template m(r,a) used to generate the interference signal is preprocessed, and then the adjusted frequency shift template m'(t) is used. r ,t a Multiplying the jamming signal by the intercepted LFM signal, the frequency shift template for multiplying the jamming signal by the LFM signal when the jammer transmits the jamming signal is derived from the formulas:

[0141]

[0142] To obtain the frequency-shifting template, the fake target template image m(r,a) needs to be processed in three steps: first, FFT; second, phase compensation; and third, equal-interval sampling. The specific steps are as follows:

[0143] Step (1) performs a two-dimensional FFT on the template m(r,a) of the false target, and obtains:

[0144]

[0145] Where 1≤f r ≤N r 1≤f a ≤N a N r N is the number of points from the upward FFT. a The number of points in the azimuth-up FFT is given, and r' and a' represent the position coordinates of the specific spurious scattering point on the template, respectively.

[0146] Step (2) multiplies the equation by a correction matrix H(f) r ,f a This involves performing phase compensation on the frequency domain expression of the false target template image, where the correction matrix H(f) is used. r ,f a ) represents

[0147]

[0148] Thus, the frequency domain expression of the interference signal template is obtained, i.e.

[0149]

[0150] Comparing the formulas, we can see that if an equation exists...

[0151]

[0152] Then the interference signal template m'(t) is obtained. r ,t a This expression is considered as the time-domain form of the interference signal template;

[0153] Step (3) In order to obtain the equation relationship of the formula, for m h (f r ,f a Perform equal-interval sampling, assuming F s The sampling rate of the jammer's analog-to-digital converter (A / D) is represented by , and PRT represents the pulse repetition time. Therefore, there is a correspondence between the fast and slow times and the range and azimuth directions.

[0154]

[0155] Let η be the intervals along the template distance direction and azimuth direction. r and η a The points sampled on the template are represented as f. r 'and f a ', then there exists an equation

[0156]

[0157] Substituting the expressions into the equation, we derive:

[0158]

[0159] Due to η r and η a The value is usually not an integer. It is decimated by linear interpolation. After sampling, a frequency shift template can be obtained. The template is multiplied with the intercepted LFM signal and then forwarded to generate an interference signal that induces false targets in the radar ISAR image.

[0160] Because the reflected echoes from the actual target scattering points contain many phase terms, and the interference signals emitted by the template frequency shifting method cannot compensate for the actual echo phase, the scattering points on the ISAR image cannot be eliminated. Therefore, a template time shifting method is used here to achieve signal cancellation. The signal cancellation model is as follows: Figure 2 As shown; step 4 specifically involves:

[0161] Signal cancellation is achieved using a template time-shift method. A time-shift amount corresponding to the scattering point to be eliminated is added to the scattering point signal captured by the jammer, along with a phase π. The radar receiver receives an interference signal with the same amplitude but opposite phase as the echo signal from the real scattering point, thus achieving signal cancellation. Here, i represents the point to be eliminated in the false target template, and its coordinates in the false target template are (x...). i ,y i ), where the jammer's position k is the origin, and the jammer's position coordinates on the target are (x, k). k ,y k If the actual scattering point i on the target is reflected back to the radar, then the echo path R is... i It can be represented as

[0162]

[0163] Where (X) ref ,Y ref The initial coordinates of the true target reference point ref in the radar ISAR imaging coordinate system are represented by the echo path R reflected back to the radar from the scattering point k where the jammer is located. k Represented as

[0164]

[0165] The time difference between the reflected echoes from positions k and i is expressed as follows:

[0166]

[0167] Therefore, once the transmitter acquires the radar's LFM signal, it directly forwards the signal without considering hardware delay and uses the false target template. After adjusting the coordinate parameters of each scattering point i in the signal and applying a corresponding time delay, and then adding a π phase before transmission, the cancellation signal transmitted by the jammer is expressed as:

[0168]

[0169] Where, Δt ki Let the time difference between the reflected echoes from positions k and i be the time difference, then the signal received at the radar receiver is:

[0170]

[0171] The signal is canceled because it has the same amplitude as the real echo signal returned from scattering point i, but opposite in phase.

[0172] The simulation experiment analysis is as follows:

[0173] The radar parameters used are shown in Table 1. To more clearly simulate and reproduce the template product / elimination principle, a four / six-point model simulating real and false targets was set up, as shown in Table 1. Figure 1 As shown, firstly, based on the ISAR image of the model, scattering points are added / cancelled, and then the cancellation experiment is verified by a 29-point / 54-point aircraft model that mimics the ideal shape of the aircraft.

[0174] Table 1 Radar Simulation Parameters

[0175] parameter numerical values parameter numerical values <![CDATA[f0(GHz)]]> 10 PRF(Hz) 200 B(MHz) 300 ω(rad) 0.02 <![CDATA[T p (us)]]> 1 α (rad) 0

[0176] Figure 3 (a) is a four-point model for true / false targets. Figure 3 (a) Draw four points representing the real target with coordinates (-18.5, 6.5), (-18.5, -18.5), (6.5, 6.5), and (6.5, -18.5). The jammer is located on the real target with coordinates (3, 4). Figure 3 (b) represents the false targets simulated by four points. After XOR processing of the interference template, the two points that need to be eliminated from template A are obtained, such as... Figure 3 (c), and the scattering points that need to be compensated from template A, such as Figure 3 (d) The scattering coefficients are assumed to be the ideal value of 1.

[0177] Figure 4 The image is an ISAR image composed of the superposition of the echo of the real target and the interference signal. The template product / cancellation algorithm is used to process the four real and false targets. Based on the false target template, the additional scattering points of the false target compared to the real target are added to compensate for the additional scattering points of the real target compared to the false target, while the additional scattering points of the real target compared to the false target are eliminated to obtain an ISAR image that is the same as the false target template. The difficulty of the compensation work lies in aligning the signal coordinates of the scattering points. Figure 4 (a) is the echo ISAR image of the real target. Figure 4 (c) shows the ISAR image of the scattering interference signal that needs to be eliminated, obtained based on template elimination, in comparison. Figure 4 (a) and Figure 4 (c) It can be found that the scattering point that needs to be eliminated has the same coordinates and amplitude as the scattering point generated by the interference signal. By adding the two signals, the radar echo after the scattering point is eliminated can be obtained, such as... Figure 4 As shown in (e). Figure 4 (b) To facilitate subsequent comparisons of added scattering point locations, an interfering ISAR image is generated based on the fake target template; however, no fake target ISAR image is actually generated in the process. Generating the interfering signal using the template multiplication method compensates for the additional scattering points in the fake target template compared to the real target in the real target ISAR image. Figure 4 (d) ISAR image of the interference signal to be added with scattering points, for comparison. Figure 4 (b) and Figure 4 (d) It can be found that the coordinates and amplitudes of the scattering points that need to be added are the same as those of the scattering points generated by the interference signal. After compensation and elimination of the interference signal at the scattering points, the radar can finally receive the signal superimposed on the echo of the real target and the interference signal, thus obtaining the ISAR image of the false target B, such as... Figure 4 As shown in (f).

[0178] Figure 5 An ISAR image composed of superimposed echoes and interference signals from a real aircraft model; Figure 5 (a) shows a 29-point ISAR image of a real target aircraft, based on a 54-point template of a false target aircraft. Figure 5 (b) The scattering interference signal that needs to be eliminated ( Figure 5 (c) and the interference signal that needs to have scattering points added ( Figure 5 (d) After compensation and elimination of the scattering interference signal, the radar can finally receive the signal superimposed on the true target echo and the interference signal, obtaining an ISAR image of the 54-point false target aircraft, such as... Figure 5 As shown in (e).

[0179] This invention requires two prerequisites: first, the template must be known, and the location of the scattering point must be accurately confirmed; second, this model requires knowledge of the electromagnetic scattering model of the true target to ensure that the ISAR images of the true target under different radar illumination directions are known. As an active jammer, it is possible to obtain the model of the true target (the cover target). However, storing and processing the electromagnetic scattering model requires considerable storage space and a fast processing speed.

[0180] The beneficial effects of this invention are as follows: by using the ISAR shape feature deception jamming method based on template product and time delay, the motion of the target and the relative relationship between the scattered echoes of each strong scattering point and the jamming signal of the jammer can be taken into account, so as to accurately locate the position coordinates of the scattering points of the ISAR image, precisely change the target characteristics, and make the radar identify the target as other targets.

Claims

1. A method for deceiving and interfering with ISAR shape features based on template product and time delay, comprising the following steps: Step 1: Generating false target templates. The jammer intercepts the LFM signal and extracts the information of scattering points that need to be compensated and the information of scattering points that need to be eliminated by comparing and ORing the true target template and the false target. Step 2: Template product frequency shift compensation. Perform Fourier transform on the obtained pseudo-target template and compensate for the corresponding frequency shift parameters. Step 3: Data interval sampling and template signal generation. Interference template data is generated by sampling the false target template at intervals. The generated interference template data is multiplied with the LFM signal intercepted by the jammer to generate an interference signal, which is then forwarded to the radar receiver. Step 4: Template cancellation of time-shifted signals. Based on the scattering point signals captured by the jammer, the time-shifted amount of the corresponding scattering point to be eliminated is added, and then the corresponding phase compensation is performed to obtain the corresponding jamming signal. The radar receiver receives the jamming signal and superimposes it with the target echo signal and performs imaging. A false target identical to the template is generated at the corresponding position of the ISAR image to achieve signal cancellation.

2. The ISAR shape feature deception and interference method based on template product and time delay as described in claim 1, characterized in that: Step 1 specifically involves: Let the true target template A be an ISAR image obtained in advance based on the scattering characteristics of the true target where the jammer is located, and the false target template B be a false target image obtained by the radar side imagery desired by the jammer. The specific steps are as follows: (1) Perform 0 / 1 binarization on A and B respectively, that is, if there is a value at the corresponding pixel position of the template, it is 1, and otherwise it is 0, and they are denoted as A and B respectively; (2) Multiply template A and template B, which gives A∩B; (3) Perform the following operations on templates A and B: The additional strong scattering points obtained from the two templates are as follows: This indicates that A has more scattering points than B. This indicates that B has more scattering points than A; Where, assuming the template pixel is p r ×p a There exists a scattering point on the template with a scattering coefficient of 1, denoted as m(r,a)=1. Assume that the range resolution and azimuth resolution of the ISAR image corresponding to the template are Δr and Δa, respectively, and therefore the range length is L. r =p r ×Δr, azimuth length is L a =p a If the zero point of the coordinate system is set to the center of the template, then the coordinates of the scattering point become (rΔr-L). r / 2,aΔa-L a / 2), where r is the row variable of the m matrix and a is the column variable of the m matrix, corresponding to the range and azimuth directions of the ISAR image, respectively; Furthermore, step 2 specifically includes: First, the radar transmits LFM signals s(t) r ,t a ), represented as Among them, t r t indicates fast time. a T represents slow time. p The pulse width is represented by j, which represents the imaginary number, and f is the pulse width. c Indicates the carrier frequency, k r This represents the frequency modulation slope. The signal is intercepted by the jammer, frequency-modulated, and then forwarded. The jamming signal at this time is represented as... Among them, Ψ r and Ψ a Here, λ and ω represent the frequency shift parameters in the range and azimuth directions, respectively; c is the ideal transmission velocity of electromagnetic waves in space; λ is the radar transmission wavelength; and ω represents the equivalent rotational angular velocity of the target on the turntable model. Direct imaging of this signal reveals a frequency shift in both the range and azimuth directions compared to the intercepted LFM signal. Among them, f r_d For the range-direction frequency shift, f a_d Due to the azimuth frequency shift, the interfering signal is processed by pulse compression to obtain the ISAR image of the interfering signal, and the peak positions along the range and azimuth directions are respectively... Among them, R r_d R represents the distance to the location. a_d Given the azimuth position, the frequency shift parameters Ψ in the range and azimuth directions are obtained from formula (3). r and Ψ a Considering the radar echo delay, in order to generate false target points at the desired location in the ISAR image of the jamming signal emitted by the jammer, the frequency shift parameter Ψ is derived. r and Ψ a The correspondence between the scattering points of the fake target and the scattering points of the fake target template; Next, the frequency shift parameter Ψ is derived. r and Ψ a ; The jamming signal received by the radar from the jammer is equivalent to receiving the reflected frequency-shifted signal from the scattering point corresponding to the jammer's location, denoted as: Among them, R c R is the distance from the scattering point to the radar. k Let k be the path of the reflected echo from the scattering point k at the location of the jammer. After performing difference frequency processing on the jamming signal and ignoring the echo envelope skew term and the remaining video phase term, we obtain the following: Among them, s ref * ( t r ,t a) To ensure the conjugate of the interference reference signal, it is assumed that the center point of the target, i.e., the reference point Ref, is the origin of the turntable model, where R Δ =R c -R ref R represents the distance between the jammer and the target reference center. ref Define the distance from the selected reference point on the flight target to the radar. Substituting into equation (7), we can obtain Then consider the time t in equation (8) Δ Performing an FFT yields a one-dimensional high-resolution range vector, i.e. Among them, f r For time t Δ The frequency domain representation, the second term The RVP term and the third term are R. Δ ≠0 o'clock After compensating and removing the skewed term of the echo envelope, we obtain the one-dimensional high-resolution range direction of multiple echoes, i.e. Where M represents the number of pulse echoes, and the position coordinates of the jammer on the target in the turntable model are assumed to be (x...). k ,y k ), x k Represents the azimuth coordinate, y k Let the range coordinate be the coordinate system. The echo distance from the location of the jammer to the echo distance from the reference point has the following relationship: R Δ =x k ωt a +y k (11) Where ω represents the rotational angular velocity of the target on the equivalent turntable model, after substituting equation (11) into equation (10) and rearranging to extract the corresponding t a From the phase term, we can obtain the expression for the interference signal: For the slow time t of equation (12) a By performing an FFT, a two-dimensional ISAR image can be obtained, i.e. Among them, f a For slow time t a In the frequency domain representation, the bit term can be ignored as a constant. Therefore, it can be seen that the positions of the false target scattering points formed by the received interference signal in the range and azimuth directions are respectively... Where, ΔR r Let ΔR be the location of the false target scattering point formed by the received interference signal in the range direction. a Let t be the location of the false target scattering point formed by the received interference signal in the azimuth direction, in one echo. a It can be considered a constant, therefore the scattering point position R in the range direction is... Δ ≈y k To ensure that the location of the scattering point in the ISAR image of the interfering signal is the same as the location of the false scattering target point in the template, thus guaranteeing the elimination of the scattering point at the corresponding location in the echo of the real target, the following equation applies. Where r is the range direction of the ISAR image corresponding to the template, Δr is the range resolution of the ISAR image corresponding to the template, and L r L represents the range length of the ISAR image corresponding to the template, a represents the azimuth length of the ISAR image corresponding to the template, Δa represents the azimuth resolution of the ISAR image corresponding to the template, and L represents the range length of the ISAR image corresponding to the template. a Given the azimuth length of the ISAR image corresponding to the template, the frequency shift parameter of the jammer on the intercepted LFM signal should be: This shows that the jammer shifts the frequency of the intercepted LFM signal by Ψ along both the range and azimuth directions. r and Ψ a The interference signal received by the radar receiver is processed by pulse compression to generate an ISAR image, and the coordinates of the false target scattering point are (rΔr-L). r / 2,aΔa-L a / 2), which is the location of the false target on the template.

3. The ISAR shape feature deception and interference method based on template product and time delay as described in claim 1, characterized in that: Step 3 specifically involves: The template m(r,a) used to generate the interference signal is preprocessed, and then the adjusted frequency shift template m'(t) is used. r ,t a Multiplying the jamming signal by the intercepted LFM signal, the frequency shift template for multiplying the jamming signal by the LFM signal when the jammer transmits the jamming signal is derived from equations (3) and (16): To obtain the frequency-shifting template, the false target template image m(r,a) is processed in the following three steps: Step 3.1 Perform a two-dimensional FFT on the template m(r,a) of the dummy target to obtain: Where 1≤f r ≤N r 1≤f a ≤N a N r N is the number of points from the upward FFT. a The number of points in the azimuth-up FFT is given, and r' and a' represent the position coordinates of the specific spurious scattering point on the template, respectively. Step 3.2 Multiply equation (18) by a correction matrix H(f) r ,f a This involves performing phase compensation on the frequency domain expression of the false target template image, where the correction matrix H(f) is used. r ,f a ) represents Thus, the frequency domain expression of the interference signal template is obtained, i.e. Comparing formula (20) and formula (17), we can see that if an equation exists... Then the interference signal template m'(t) is obtained. r ,t a This expression is considered as the time-domain form of the interference signal template; Step 3.3 In order to obtain the equation relationship of formula (21), for m h (f r ,f a Perform equal-interval sampling, assuming F s The sampling rate of the jammer's A / D converter is represented by , and PRT represents the pulse repetition time. Therefore, there is a correspondence between the fast and slow times and the range and azimuth directions. Let η be the intervals along the template distance direction and azimuth direction. r and η a The points sampled on the template are represented as f. r 'and f a ', then there exists an equation Substituting equations (23) and (22) into equation (21), we derive the following: Due to η r and η a The value is usually not an integer. It is decimated by linear interpolation. After sampling, a frequency shift template can be obtained. The template is multiplied with the intercepted LFM signal and then forwarded to generate an interference signal that induces false targets in the radar ISAR image.

4. The ISAR shape feature deception and interference method based on template product and time delay as described in claim 1, characterized in that: Step 4 specifically involves: Signal cancellation is achieved using a template time-shift method. A time-shift amount corresponding to the scattering point to be eliminated is added to the scattering point signal captured by the jammer, along with a phase π. The radar receiver receives an interference signal with the same amplitude but opposite phase as the echo signal from the real scattering point, thus achieving signal cancellation. Here, i represents the point to be eliminated in the false target template, and its coordinates in the false target template are (x...). i ,y i ), where the jammer's position k is the origin, and the jammer's position coordinates on the target are (x, k). k ,y k If the actual scattering point i on the target is reflected back to the radar, then the echo path R is... i It can be represented as Where (X) ref ,Y ref The initial coordinates of the true target reference point ref in the radar ISAR imaging coordinate system are represented by the echo path R reflected back to the radar from the scattering point k where the jammer is located. k Represented as The time difference between the reflected echoes from positions k and i is expressed as follows: Therefore, once the transmitter acquires the radar's LFM signal, it directly forwards the signal without considering hardware delay and uses the false target template. After adjusting the coordinate parameters of each scattering point i in the signal and applying a corresponding time delay, and then adding a π phase before transmission, the cancellation signal transmitted by the jammer is expressed as: Where, Δt ki Let the time difference between the reflected echoes from positions k and i be the time difference, then the signal received at the radar receiver is: The signal is canceled because it has the same amplitude as the real echo signal returned from scattering point i, but opposite in phase.

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