Ground slow moving target detection and imaging method based on time-varying vortex SAR (Synthetic Aperture Radar)

Abstract

The invention relates to the technical field of target detection and imaging, in particular to a ground slow moving target detection and imaging method based on a time-varying vortex SAR (synthetic aperture radar), which comprises the steps of target detection and target imaging. Target detection: firstly, carrying out target detection on a ground slow moving target by utilizing a vortex SAR (Synthetic Aperture Radar) in a side-looking banded graph scene to obtain target imaging of a pitch angle theta (delta eta) and an azimuth angle # imgabs0 #, and firstly, estimating motion parameters of the ground slow moving target by utilizing the azimuth angle and the pitch angle in an echo signal; then, an RDA algorithm is improved to achieve ground slow moving target focusing imaging with azimuth resolution; according to the invention, the problems of defocusing of a target imaging result and deviation of a target imaging position during detection and tracking of a target in the prior art are solved.

Description

Technical Field

[0001] The invention relates to the technical field of target detection and imaging, and in particular to a ground slow-moving target detection and imaging method based on time-varying eddy SAR. Background Art

[0002] Radar imaging technology is an important means of obtaining target information under all-day, all-weather and long-distance conditions. It can effectively reflect the target's scattering ability of electromagnetic waves in various non-ideal environments, and plays a pivotal role in space surveillance, earth observation, land surveying and mapping, battlefield reconnaissance, precision guidance and other fields. Traditional real aperture radar relies on time delay to measure the target distance and detects the target azimuth position according to the antenna beam pointing, but it cannot obtain more detailed and higher resolution target information and cannot meet the actual application needs. The "range-Doppler" principle imaging radar represented by synthetic aperture radar (SAR) was first proposed in the 1950s. After nearly 60 years of development, it has a fairly mature theoretical basis and system technology. By emitting a large time-bandwidth product signal to obtain high resolution in the range direction, a large virtual antenna aperture is formed through synthetic aperture technology to achieve high spatial resolution in the azimuth direction that is independent of the target distance. Compared with the relatively easy high resolution in the range direction, the acquisition of higher resolution information in the target azimuth direction depends on the accumulation of longer observation time, which requires new breakthroughs in imaging principles and technologies.

[0003] Nowadays, SAR has been widely used for detecting and tracking ground moving targets. However, studies have found that when the target has a velocity component in the direction parallel to the radar carrier track (hereinafter referred to as azimuth velocity), it will cause the target imaging result to be defocused. When the target has a velocity component in the direction perpendicular to the radar carrier flight track (hereinafter referred to as range velocity), it will cause the target imaging position to shift, resulting in serious distortion of the imaging result. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a ground slow-motion target detection and imaging method based on time-varying vortex SAR, so as to solve the problem that when the prior art detects and tracks the target, the target imaging result is defocused and the target imaging position is offset. The invention discloses a ground slow-moving target detection and imaging method based on time-varying eddy SAR, which is characterized by: mainly including target detection and target imaging; Step 1: Target detection: First, eddy SAR is used to detect slow-moving targets on the ground in the side-view strip map scene to obtain the pitch angle θ (Δη) and azimuth angle Step 2, target imaging, first use the azimuth and pitch angles in the echo signal to estimate the motion parameters of the slow-moving ground target, and then improve the RDA algorithm to achieve focused imaging of the slow-moving ground target with azimuth resolution.

[0005] The specific steps of target detection are: in the Cartesian coordinate system O-XY β Z β The medium-sized aircraft radar moves linearly along the X-axis at a constant speed v at a height of H above the ground to form an azimuth direction; the receiving array element of the radar uniform concentric circle array antenna is located at point O of the coordinate system, the transmitting array element is the outer ring, and the number of transmitting array elements is N; Z β A reference Cartesian coordinate system O-XYZ is established perpendicular to the UCCA plane and pointing to the imaging area. The Y axis is horizontally perpendicular to the X axis to form the distance direction, and the Z axis is perpendicular to O-XY and points to the height direction. There is a rotation angle β between the two coordinate systems.

[0006] As a further qualification for target detection, the initial position of the ground slow-moving target and the radar in the system O-XYZ is P R =(0,0,0), x T =vη T , η T is the target coordinate x T The corresponding azimuth time; the target moves at a constant speed Moving on the ground, the position of the target and the radar after the movement is P R '=(x,0,0), The shortest slant range between the aircraft radar track and the target is θ 0 and are the initial elevation angle and azimuth angle between the target and the aircraft radar respectively; θ(Δη), and R(Δη) are the movement time Δη=η-η T The instantaneous pitch angle, azimuth angle and slant range after the measurement.

[0007] The specific steps of target imaging are: Step 1: Extract the pitch angle θ (Δη) and azimuth angle Step 2, parameter estimation, based on the extracted pitch angle θ (Δη) and azimuth angle Get the azimuth speed and distance speed of the slow-moving target on the ground; Step 3, Bessel term compensation; Step 4, azimuth compensation; Step 5, improve the RDA algorithm, and use the RDA algorithm to compress the data obtained from the angle compensation.

[0008] The beneficial effects of the present invention are:

[0009] The present invention firstly uses vortex SAR to detect slow-moving targets on the ground in a side-view strip diagram scene, estimates the motion parameters of the ground moving targets through the Bessel terms and spatial phase terms of the vortex echo signal, improves the traditional range Doppler algorithm (RDA), compensates the Bessel terms and phase terms of the echo signal, designs a new azimuth matched filter using the estimated target Doppler parameters to compress the echo signal to obtain a two-dimensional imaging result, verifies the effectiveness of the algorithm through theoretical derivation and simulation, and proves that the algorithm has better azimuth resolution and improves the quality parameters of SAR images compared with the moving target imaging results of the plane wave SAR system. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 Schematic diagram of the geometric scene of eddy SAR imaging.

[0011] Figure 2 This is the algorithm flow chart.

[0012] Figure 3 This is the cross-sectional view of the RDA algorithm target imaging.

[0013] Figure 4 3D image of the RDA algorithm target.

[0014] Figure 5 This is the azimuth section diagram of the vortex SAR.

[0015] Figure 6 This is the range section diagram of eddy SAR.

[0016] Figure 7 It is the azimuth section diagram of planar SAR and eddy SAR.

[0017] Figure 8 The compression results of velocities in different azimuthal directions are shown in Figure 2.

[0018] Fig. 9 The results of speed compression for different distances.

[0019] Fig.10 It is a two-dimensional cross-sectional diagram of the SNR=10dB target.

[0020] Fig.11 This is a three-dimensional image of the target with SNR=10dB.

[0021] Fig.12 It is a two-dimensional cross-sectional diagram of the SNR=15dB target.

[0022] Fig.13 This is a three-dimensional image of the target with SNR=15dB. DETAILED DESCRIPTION

[0023] In order to clearly understand the technical solution of the present application, a ground slow-moving target detection and imaging method based on time-varying eddy SAR provided by the present application will be described in detail below in combination with specific embodiments and drawings.

[0024] The terms used in the following embodiments are only for the purpose of describing specific embodiments, and are not intended to be used as limitations to the present application. As used in the specification and claims of the present application, the singular expressions "one", "a kind of", "above", "the" and "this" are intended to also include expressions such as "one or more", unless there is a clear indication to the contrary in the context. It should also be understood that in the following embodiments of the present application, "at least one", "one or more" refer to one, two or more than two.

[0025] References to "one embodiment" or "some embodiments" etc. described in this specification mean that a particular feature, structure or characteristic described in conjunction with the embodiment is included in one or more embodiments of the present application. Thus, the phrases "one embodiment", "in some embodiments", "in some other embodiments", "in some other embodiments", etc. appearing in different places in this specification do not necessarily all refer to the same embodiment, but mean "one or more but not all embodiments", unless otherwise specifically emphasized in other ways. The terms "including", "comprising", "having" and their variations all mean "including but not limited to", unless otherwise specifically emphasized in other ways.

[0026] Example 1

[0027] This embodiment provides a ground slow-moving target detection and imaging method based on time-varying eddy SAR, which includes two steps: target detection and target imaging.

[0028] SAR moving target parameter estimation mainly includes two aspects: azimuth velocity and range velocity. The traditional azimuth velocity estimation method is mainly based on the single-channel method to estimate the reflectivity displacement method (RDM) of the target's complete velocity and acceleration vector using SAR raw echo data, but the algorithm is highly complex and requires a large amount of data to be processed, which is not conducive to engineering implementation; the Wigner-Ville Distribution (WVD) method can complete the estimation of target motion parameters in the time domain, but the algorithm is susceptible to cross-term interference, that is, interference caused by factors such as target movement and background changes, and is not suitable for occasions with high precision requirements; the fractional Fourier transform (FrFT) is used to estimate the target azimuth motion parameters. This method can effectively avoid the influence of cross-terms, but it cannot synchronously estimate the range velocity; the self-focusing two-dimensional velocity search algorithm can estimate the azimuth velocity and range velocity at the same time, but this method requires multiple iterations of the phase history, and the azimuth velocity needs to be traversed and searched during the iteration, which has high time complexity. The main algorithm for estimating range velocity is based on the multi-channel along-track interferometry (ATI) algorithm. ATI estimates the range velocity based on the relationship between the interference phase and the range velocity, effectively solving the velocity ambiguity problem. However, the ATI algorithm cannot estimate the azimuth velocity and requires multiple radars to detect the target at the same time, which is costly. When imaging the moving target after estimating the motion parameters, the single-channel SAR system that emits plane waves has a low spatial degree of freedom, a high time complexity for detecting moving targets, and the imaging results are greatly affected by the side lobes of the electromagnetic waves. Although the multi-channel SAR system that emits plane waves improves the spatial degree of freedom, it increases the cost of the radar system and the complexity of information processing.

[0029] The present invention firstly detects slow-moving targets on the ground using vortex SAR in a side-view strip diagram scene, estimates the motion parameters of the ground moving targets by using the Bessel terms and spatial phase terms of the vortex echo signal, improves the traditional range Doppler algorithm (RDA), compensates for the Bessel terms and phase terms of the echo signal, and designs a new azimuth matched filter using the estimated target Doppler parameters to compress the echo signal to obtain a two-dimensional imaging result. The effectiveness of the algorithm is verified through theoretical derivation and simulation, and compared with the moving target imaging results of the plane wave SAR system, it is proved that the algorithm has better azimuth resolution and improves the quality parameters of the SAR image.

[0030] Step 1: Target detection. The specific steps are as follows:

[0031] like Figure 1 As shown, in the Cartesian coordinate system O-XY β Z βThe medium-sized aircraft radar moves linearly along the X-axis (hereinafter referred to as the azimuth) at a constant speed v at a height of H above the ground. The receiving array element of the radar uniform concentric circular array antenna (UCCA) is located at point O of the coordinate system, and the transmitting array element is the outer ring. The number of transmitting array elements is N; Z β Perpendicular to the UCCA plane pointing to the imaging area. Establish a reference Cartesian coordinate system O-XYZ, with the Y axis horizontally perpendicular to the X axis (hereinafter referred to as the range direction), and the Z axis perpendicular to O-XY pointing to the height direction. There is a rotation angle β between the two coordinate systems. The initial position of the target P and the radar in the system O-XYZ is P R =(0,0,0), Among them, x T =vη T , η T is the target coordinate x T The corresponding azimuth time. The target moves at a constant speed Moving on the ground, after a period of movement, the position of the target and the radar is P R '=(x,0,0), The shortest slant range between the aircraft radar track and the target is θ 0 and are the initial elevation angle and azimuth angle between the target and the aircraft radar, θ(Δη), and R(Δη) are the movement time Δη=η-η T The instantaneous pitch angle, azimuth angle and slant range after the measurement.

[0032] The model uses a linear frequency modulation signal (LFM) to generate vortex electromagnetic waves. According to the UCCA array antenna, the nth array element transmits the signal under the staring condition:

[0033]

[0034] Therefore, when the radar is in the multi-transmit-single-receive working mode, the received target echo signal is:

[0035]

[0036] Where σ is the backscatter coefficient of the target under the viewing angle of the UCCA antenna, l η =|ξ|·η, ξ is the time-to-azimuth ratio (TVR) of the OAM mode, η and t are the time in azimuth and time in range, a lη is the radius of UCCA, T is the pulse width of the transmitted signal, f c is the center carrier frequency, is the LFM wave number, λ is the wavelength, K r is the LFM signal modulation frequency. l is the OAM mode number of the vortex electromagnetic wave, J l[] is the order of the first kind Bessel function, t 0 =2R / c is the target echo delay.

[0037] In the electromagnetic vortex SAR imaging model, the movement of the radar in azimuth will introduce azimuth time variables into the azimuth factor term, which is given by Figure 1 The geometric relationship between the target and the radar shows that when the target is stationary, the instantaneous slant range is:

[0038]

[0039] At this time, the instantaneous elevation angle and azimuth angle expressions introduced by OAM are as follows:

[0040]

[0041]

[0042] The target has speed The moving target has displacement components in both azimuth and range. At this time, the instantaneous slant range, instantaneous pitch angle and azimuth angle of the moving target and the carrier radar are:

[0043]

[0044]

[0045]

[0046] Substituting equations (6), (7) and (8) into equation (2), the normalized echo is:

[0047]

[0048] Among them, ω r [] and ω a [] are the range envelope and azimuth envelope respectively. The following algorithm processing is s(t,η; l η ) is used to process the signal. η ) where the known quantities are the ground scattering coefficient σ, the radius of the concentric uniform circular array Instantaneous slant distance R(Δη), linear modulation frequency K r , speed of light c, wavelength λ. The quantities to be extracted are the instantaneous pitch angle and the instantaneous azimuth angle θ(Δη),

[0049] Step 2, target imaging, the specific steps are as follows:

[0050] In this step, the azimuth and elevation angles in the echo signal are first used to estimate the motion parameters of the moving target, and then the RDA algorithm is improved to achieve focused imaging of slow-moving ground targets with azimuth resolution.

[0051] A. Extracting the pitch angle and azimuth angle

[0052] The carrier aircraft carries a vortex radar and a conventional radar, which transmit OAM radar signals and conventional plane wave signals one after the other in a short time interval. Because the transmission interval is short, the radar can be considered stationary. By filtering the echo data of vortex SAR imaging using the received echo data of conventional plane wave imaging, the Bessel amplitude term and azimuth phase term can be obtained. That is:

[0053]

[0054] Since the Bessel amplitude term changes with the number of OAM modes, when kasinθ1 and kasinθ≥l, the approximate expression of the Bessel term is:

[0055]

[0056] Since the side-looking radar platform is relatively far away from the target, the change of the pitch angle is small in a short time. Therefore, the change of the pitch angle in a short time can be represented by taking the average of the pitch angle change value. Then the expression of the Bessel term is:

[0057]

[0058] The phase error of the Bessel approximation term increases with the increase of the mode, so its phase error needs to be compensated. After the Bessel term is compensated for the phase error Δφ, the extraction formula of the instantaneous pitch angle is as follows:

[0059]

[0060] After obtaining the pitch angle information and obtaining the Bessel term, the spatial phase term can be obtained by separating it. The azimuth angle can be obtained by mathematical transformation.

[0061] B. Parameter Estimation

[0062] According to the azimuth extracted above By transforming equations (7) and (8) with the pitch angle θ(Δη), the azimuth velocity and range velocity of the moving target can be obtained.

[0063]

[0064]

[0065] x T =vηT Substituting into formula (6), the instantaneous slope distance is:

[0066]

[0067] Applying Fresnel approximation to equation (16) yields:

[0068]

[0069] At this time, the phase error caused by the motion of the radar and the target is:

[0070]

[0071] Among them, f 0 is the center frequency of the echo signal, and c is the speed of light. According to formula (18), the Doppler frequency f of the aircraft radar is obtained t The Doppler frequency f of the moving target η The expression is as follows:

[0072]

[0073]

[0074] After obtaining the motion parameters of the moving target, the echo signal of the target can be compressed. Due to the existence of Bessel amplitude term and spatial phase term, its processing process is different from that of the plane wave. The specific analysis is as follows.

[0075] C. Bessel term compensation

[0076] Since the number of OAM modes varies linearly with slow time, the order of the Bessel term varies with slow time and needs to be compensated in the azimuth echo. Assume that the radar linear motion time is η i , the instantaneous slant range between the radar and the target is R i At this time, the azimuth difference between the radar and the target on the x-axis is Δθ=|vη i -x T -v x η i |, since Δθ=|vη i -x T -v x η i |Then The instantaneous pitch angle at this time can be obtained as:

[0077]

[0078] Substituting equation (21) into the Bessel expression, the compensation function of the Bessel term is obtained as:

[0079]

[0080] For the echo signal s(t,η;l η ) The result after Bessel compensation is:

[0081]

[0082] D. Azimuth compensation

[0083] In the RDA algorithm, the azimuth resolution of the target depends on the quadratic term of the azimuth term, so it is necessary to analyze the azimuth term introduced by OAM. Perform Taylor expansion, ignoring terms of third order and above.

[0084]

[0085] The linear term (first term) in equation (24) introduces an additional offset term that needs to be compensated. The second term is a quadratic term with linear frequency modulation characteristics that can broaden the Doppler bandwidth of the echo signal, so that the azimuth resolution increases with the increase of the Doppler bandwidth. A (t,η;l η ) is used to perform linear compensation to obtain the echo signal as follows:

[0086]

[0087] in,

[0088] E. Improved RDA algorithm

[0089] According to the above analysis and compensation, the RDA algorithm is used to compress equation (25). Since the range compression reference function of the signal is constructed using radar system parameters, the range compression processing of moving targets is consistent with the compression of stationary targets in traditional planar SAR. The range compression function is designed as follows:

[0090]

[0091] For the echo signal s AA (t,η;l η ) is compressed in the distance direction and the result is:

[0092]

[0093] According to equations (16) and (18), the instantaneous slant distance is η = η T The Taylor formula at is expanded, ignoring the third order and above, and we have:

[0094]

[0095] Among them, fdc is the Doppler center frequency between the carrier radar and the moving target, f dr is the Doppler modulation frequency. It is divided into range migration and range bending. Since the carrier radar transmission beam in this system is a narrow beam, the range bending in the range migration (RCM) can be ignored, and only the range migration is considered. The range migration R is obtained from formula (28): walk The expression of is,

[0096]

[0097] in, is the estimated value of the Doppler center frequency of the radar system. The number of moving units of the azimuth signal distance is:

[0100] ΔN=R walk / δ r =-λf dc η / 2δ r (30);

[0101] Among them, δ r Represents the resolution unit in the range direction. After the interpolation operation is performed to perform precise range migration correction (RCMC), the echo signal is compressed in azimuth. From equation (17), it can be obtained that the azimuth term introduced by OAM in this radar system, the movement of the target and the movement of the radar platform all cause the change of the Doppler modulation frequency. Therefore, when performing azimuth compression, the Doppler modulation frequencies of the above three items must be included in the azimuth matching filter. According to equations (19), (20) and (24), the Doppler modulation frequencies of the three items are obtained as follows:

[0102]

[0103]

[0104]

[0105] f tc 、f ηc and f ηc are respectively the carrier radar Doppler modulation frequency, the target motion Doppler modulation frequency and the OAM modulation frequency. The azimuth compression function is:

[0106]

[0107] For the echo signal s B (t,η;l η ) is used for azimuthal compression to obtain the two-dimensional compression result:

[0108] s 2D =σ AA |K r|Tsinc(|K r |T t)|K x |T s sinc(|K x |T s η)(35);

[0109] Among them, K x =f tc +f ηc +f lc The range resolution is determined by the radar signal bandwidth B r =K r T is determined and expressed as:

[0110]

[0111] K r Because the LFM ratio is determined by the radar system itself, the range resolution does not change compared with traditional plane wave SAR.

[0112] The resolution of plane wave SAR is determined by the antenna aperture length, while in vortex SAR, due to the introduction of additional azimuth terms by OAM, the resolution is expressed as:

[0113]

[0114] K x Compared with the Doppler modulation frequency of traditional plane wave SAR, the OAM modulation frequency term is added, so the vortex radar has better azimuth resolution.

[0115] in, Figure 2 Flowchart of the improved RDA algorithm processing of moving targets.

[0116] The experimental evidence is as follows: Table 1 shows the main parameters of the numerical simulation. In the imaging scene, the position of the target in the O-XYZ coordinate system is set as It has velocity components in both azimuth and range directions. Table 1 Simulation parameters

[0117] First, the azimuth velocity and range velocity of the target are estimated using equations (14) and (15). The estimation results and errors are shown in Table 2. Table 2 Moving target parameter estimation and error

[0118] Using the estimated parameters of the moving target combined with the theory in Section 3, the echo signal of the moving target is processed, and the imaging result is as follows: Figure 3-6 As shown in Figure 2, after processing the eddy SAR signal, the azimuth defocus and imaging position offset of the target are compensated and corrected.

[0119] When plane wave SAR (TVR = 0) and vortex SAR use RDA algorithm to perform compression imaging on the same moving target, the following can be obtained: Figure 7 The azimuth section diagram shown shows the comparison with planar SAR when the time-varying ratio of OAM is different. From the azimuth section diagram, it can be seen that as the TVR increases, the azimuth sidelobes of the vortex SAR gradually decrease, the main lobe becomes narrower, and the azimuth imaging effect is improved.

[0120] The significant improvement of this method is also reflected in the azimuth performance indicators, namely impulse response width (IRW), peak sidelobe ratio (PSLR) and integrated sidelobe ratio (ISLR). IRW refers to the 3dB main lobe width of the impulse response, PSLR and ISLR refer to the ratio of the height of the maximum sidelobe to the main lobe and the ratio of the total energy of all sidelobes to the main lobe energy, respectively. The comparison of azimuth performance parameters of eddy SAR imaging and planar SAR imaging is shown in Table 3, where the enhancement ratio θ = (ρ' x -ρ x ) / ρ' x (ρ' x is the azimuth resolution of the planar SAR, ρ x is the azimuth resolution of eddy SAR). The azimuth performance parameters are enhanced to different degrees according to different TVRs. The values ​​of IRW, PSLR and ISLR of plane wave (TVR=0) SAR imaging are 0.5471m, -8.7417dB and -6.4604dB respectively. When TVR=70 and the modal range is (-47,47), the values ​​of IRW, PSLR and ISLR of eddy SAR imaging are 0.2094m, -12.9675dB and -10.7831dB respectively. The enhancement ratio of azimuth resolution is 61.73%, indicating that the azimuth imaging results of the algorithm in the low and medium modes of OAM are effectively improved. When the time ratio reaches more than 100, the image compression results begin to deteriorate and the azimuth performance parameters decrease. Table 3 Azimuth performance indicators

[0121] To verify whether the algorithm is applicable to high-speed moving targets, we conducted experiments on the azimuth speed and distance speed of the moving targets. Figure 8 The algorithm is able to detect the azimuth velocity range, and when the target azimuth velocity is higher than 84 m / s, the compression effect of the target weakens, the side lobes begin to increase, and false target points appear. Fig. 9 The algorithm was verified to have a detectable range of speed in the range direction. When the target speed is higher than 20m / s, the side lobes increase significantly, the compression effect deteriorates, and false target points appear. According to this experiment, the algorithm can detect the best azimuth speed of the target should not be higher than 84m / s, and the best range speed should be lower than 20m / s. The experiments were conducted under ideal conditions. However, in actual detection, the detection of the target will be interfered by noise, so we need to consider the imaging results of the algorithm under different signal-to-noise ratio (SNR) conditions. The parameters of the point target are the same as those in the ideal case. Fig.10 and Fig.11 is the imaging result of the algorithm under SNR=10dB, Fig.12 and Fig.13 The imaging result under SNR=15dB shows that the algorithm can effectively reconstruct the target without causing the target position offset and imaging defocus problem. It proves that the algorithm has good anti-noise performance within a certain signal-to-noise ratio range.

Claims

1. A method for detecting and imaging ground slow-moving targets based on time-varying eddy SAR, characterized in that: Mainly includes target detection and target imaging; S1, target detection, first use eddy SAR to detect the slow-moving ground target in the side-view strip map scene, and obtain the pitch angle θ (Δη) and azimuth angle S2, target imaging, firstly uses the azimuth and pitch angles in the echo signal to estimate the motion parameters of the slow-moving ground target, and then improves the RDA algorithm to achieve focused imaging of the slow-moving ground target with azimuth resolution.

2. The method for detecting and imaging ground slow-moving targets based on time-varying eddy SAR according to claim 1, characterized in that: The specific steps of target detection are: in the Cartesian coordinate system O-XY β Z β The medium-sized aircraft radar moves linearly along the X-axis at a constant speed v at a height of H above the ground to form an azimuth direction; the receiving array element of the radar uniform concentric circle array antenna is located at point O of the coordinate system, the transmitting array element is the outer ring, and the number of transmitting array elements is N; Z β A reference Cartesian coordinate system O-XYZ is established perpendicular to the UCCA plane and pointing to the imaging area. The Y axis is horizontally perpendicular to the X axis to form the distance direction, and the Z axis is perpendicular to O-XY and points to the height direction. There is a rotation angle β between the two coordinate systems.

3. The method for detecting and imaging ground slow-moving targets based on time-varying eddy SAR according to claim 2, characterized in that: The initial position of the ground slow-moving target and the radar in the system O-XYZ is P R =(0,0,0), x T =vη T , η T is the target coordinate x T The corresponding azimuth time; the target moves at a constant speed Moving on the ground, the position of the target and the radar after the movement is P R '=(x,0,0), The shortest slant range between the aircraft radar track and the target is θ0 and are the initial elevation angle and azimuth angle between the target and the aircraft radar respectively; θ(Δη), and R(Δη) are the movement time Δη=η-η T The instantaneous pitch angle, azimuth angle and slant range after the measurement.

4. The method for detecting and imaging ground slow-moving targets based on time-varying eddy SAR according to claim 1, characterized in that: The specific steps of target imaging are: S1, extract the pitch angle θ (Δη) and azimuth angle S2, parameter estimation, based on the extracted pitch angle θ (Δη) and azimuth angle Get the azimuth speed and distance speed of the slow-moving target on the ground; S3, Bessel term compensation; S4, azimuth compensation; S5, improve the RDA algorithm, and use the RDA algorithm to compress the data obtained from the azimuth compensation.

Images