Time domain azimuth wide swath imaging method based on sub-pulse diversity

Abstract

The application discloses a time-domain azimuth wide-width beamforming imaging method based on sub-pulse diversity, comprising the following steps: constructing a multi-frequency sub-pulse wide-width beamforming signal model, directing different sub-pulse beams of a radar to different imaging areas respectively, and obtaining total echo signals received by the radar; designing a pulse repetition frequency and a synthetic aperture time of the radar according to characteristics of the radar system; performing wide-width beamforming imaging processing on the total echo signals received by the radar based on the pulse repetition frequency and the synthetic aperture time of the radar, and obtaining a complete image with a large azimuth imaging width. The single pulse of the radar is divided into multiple sub-pulses, and different sub-pulses are directed to different azimuth positions, so that the radar has the azimuth wide-width beamforming capability; the application of the sub-pulse diversity technology in the imaging system is effectively realized, and the problem of high operation complexity of a method for applying a frequency scanning radar in an azimuth array and loss of range resolution of a scene edge part is solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of synthetic aperture radar imaging, and particularly relates to a time-domain azimuth wide-width beamforming imaging method based on sub-pulse diversity. BACKGROUND

[0002] Synthetic aperture radar has the advantages of all-weather and all-time observation, and plays an important role in target detection, traffic control, geological disaster warning and the like. High-resolution imaging can obtain more detailed information of a target, and has been an important research direction. Initially, strip imaging is limited by the synthetic aperture time, and the theoretical azimuth resolution can only reach half of the antenna azimuth aperture. The beamforming imaging system effectively improves the azimuth resolution of the observation area by controlling the radar beam pointing and focusing the beam on a certain area on the ground. However, the azimuth imaging width is limited to the main lobe beam width. In order to improve the azimuth imaging width, a sliding beamforming imaging mode is proposed. In this mode, the radar beam rotates around a virtual center, which increases the target illumination time and also expands the azimuth imaging width to a certain extent. However, this method sacrifices the azimuth resolution. How to ensure high azimuth resolution while achieving large azimuth imaging width is a problem worthy of study.

[0003] Frequency scanning radar is a new radar system proposed in recent years, which realizes the function of intra-pulse beam scanning by introducing a fixed time delay between array elements, and thus has the characteristic of wide beam coverage. Based on the characteristic of wide beam coverage, an elevation high-resolution wide-width imaging system is proposed. This method uses the mapping relationship between the elevation angle and the range frequency to construct different range frequency bandpass filters in the receiving processing, so as to realize the separation of different range targets, and then uses the traditional strip imaging method to obtain the range wide-width imaging result. The method of applying frequency scanning radar to the azimuth array realizes the wide beam coverage in the azimuth direction, and by setting a specific array weight, the wide beam is pointed to the same area, and then the wide azimuth width beamforming imaging is realized.

[0004] Sub-pulse diversity technology divides a single pulse into multiple sub-pulses, and modulates the sub-pulses into different signals (frequency, phase, pulse width), thereby improving the signal processing performance of the radar system. Multi-frequency sub-pulse technology is one of them, which obtains the sub-pulse dimension freedom by modulating the sub-pulses into signals separated from each other in the range frequency domain. The elevation multi-frequency sub-pulse wide-width imaging method points the sub-pulses in different frequency ranges to different areas in the elevation direction, and realizes the coverage width beyond the original elevation beam width. Then, the range ambiguity separation is realized by frequency bandpass, and the range wide-width imaging is realized.

[0005] The method of applying frequency scanning radar in the azimuth array needs to construct a band-pass filter respectively to separate different distance data, and the operation complexity is high. Moreover, due to the modulation of the scene target distance bandwidth by the beam pattern, the scene edge point will have a partial distance resolution loss, so that the imaging resolution is reduced. Although the sub-pulse diversity technology has been studied in the wide-aperture synthetic aperture radar ground moving target detection technology, the research in the imaging system is less, and at present, the application mainly focuses on the strip range imaging. SUMMARY

[0006] In order to solve the above problems existing in the prior art, the application provides a time-domain azimuth wide-aperture beamforming imaging method based on sub-pulse diversity. The technical problem to be solved by the application is solved by the following technical scheme:

[0007] The application provides a time-domain azimuth wide-aperture beamforming imaging method based on sub-pulse diversity, comprising:

[0008] S1: constructing a multi-frequency sub-pulse wide-aperture beamforming signal model, pointing different sub-pulse beams of the radar to different imaging areas respectively, and obtaining total echo signals received by the radar;

[0009] S2: designing a pulse repetition frequency and a synthetic aperture time of the radar according to the characteristics of the radar system;

[0010] S3: performing wide-aperture beamforming imaging processing on the total echo signals received by the radar based on the pulse repetition frequency and the synthetic aperture time of the radar, and obtaining a complete image with a large azimuth imaging width.

[0011] In an embodiment of the application, the S1 comprises:

[0012] S1.1: obtaining a baseband sub-pulse signal of each sub-pulse of the radar Wherein, n=1, 2, 3…N, N represents the number of sub-pulses contained in each pulse, and the radar is a phased array radar working in a beamforming imaging mode;

[0013] S1.2: modulating different sub-pulses to different distance frequency regions to obtain a transmission signal of each sub-pulse of the radar

[0014] S1.3: pointing different sub-pulse beams to different imaging areas expected to be beamformed and imaged respectively, and obtaining total echo signals of each imaging area;

[0015] S1.4: obtaining total echo signals received by the radar according to the total echo signals of each imaging area.

[0016] In an embodiment of the application, the S1.2 comprises:

[0017] The different sub-pulses are modulated to different range frequency regions, and a frequency offset Δf between the nth sub-pulse and the 1st sub-pulse is obtained n :

[0018]

[0019] where Δf represents a frequency increment between two adjacent sub-pulses;

[0020] According to the frequency offset Δf n An emission signal of the nth sub-pulse of the radar is obtained

[0021]

[0022] where f c represents a carrier frequency of the radar, j represents a complex number unit, Δτ n =(n-1)T Δ represents a transmission delay between the nth sub-pulse and the 1st sub-pulse, T Δ represents a time interval of the sub-pulse.

[0023] In an embodiment of the present application, the S1.3 comprises:

[0024] Assuming that the nth sub-pulse is directed to the nth imaging region, an echo signal of a pth target in the nth imaging region is obtained

[0025]

[0026] where σ np is a complex scattering coefficient of the pth target, is a fast time, t k is an azimuth slow time, w a (t k ) is a target azimuth window function, c is a light speed, and Rnp(tk) is an instantaneous slant range history of the pth target in the nth imaging region and the radar;

[0027] According to the echo signal of the pth target in the nth imaging region A total echo signal of all targets in the nth imaging region is obtained.

[0028] In an embodiment of the present application, a pulse repetition frequency of the radar satisfies:

[0029]

[0030] where PRF represents a pulse repetition frequency of the radar, B subn represents a Doppler bandwidth corresponding to the nth imaging region.

[0031] In one embodiment of the present invention, the synthetic aperture time T of the radar a The constraints that must be met include:

[0032]

[0033] max{ρ an}≤ρ a ,

[0034] Among them, ρ an represents the resolution required for effective imaging of the nth imaging area within the same synthetic aperture time, ρ a represents the required resolution of the radar system, k a is the azimuthal broadening factor, c represents the speed of light, Δf n The frequency offset Δf between the nth sub-pulse and the first sub-pulse is n , f c represents the radar carrier frequency, Indicates that the pth target in the nth scene is The radar oblique angle of time, Indicates that the pth target in the nth scene is The radar squint angle at time t, p = 1, 2, 3…P, where P represents the total number of targets in the nth scene.

[0035] In one embodiment of the present invention, the S3 includes:

[0036] S3.1: Construct an echo down-conversion function, and conjugate-multiply the echo down-conversion function by the total echo signal received by the radar to obtain a down-conversion signal;

[0037] S3.2: Constructing a bandpass filter, and using the bandpass filter to separate the down-converted signal to obtain separated echo data of each imaging area;

[0038] S3.3: Performing a range-transform on the separated echo data of each imaging region to obtain range-transformed echo data of each imaging region;

[0039] S3.4: constructing a correction function in the range-frequency domain and obtaining the echo data of each imaging area in the range-time domain after correction;

[0040] S3.5: construct a range pulse compression function to obtain echo data of each imaging area after pulse compression;

[0041] S3.6: processing the echo data by constructing a corresponding azimuth deramp function with the center of each imaging region as a reference to obtain a corresponding BP imaging result of each imaging region;

[0042] S3.7: obtaining a beamforming image of each imaging region according to the corresponding BP imaging result of each imaging region, and further obtaining a complete image of a large azimuth imaging width.

[0043] In an embodiment of the present application, the S3.2 comprises:

[0044] constructing a band-pass filter H fliter (f r ) in the vicinity of the range frequency domain

[0045]

[0046] wherein f r represents the range frequency of the echo signal, B sp represents the bandwidth of the radar;

[0047] converting the down-converted signal to the range frequency domain and multiplying it by the band-pass filter H fliter (f r ), and then converting to the range time domain to obtain the separated echo data of the nth imaging region

[0048]

[0049] wherein p = 1, 2, 3…P, P represents the total number of targets in the nth scene, σ np is the complex scattering coefficient of the pth target, t k is the azimuth slow time, wa(tk) is the target azimuth window function, is the fast time, c is the speed of light, R np (t k ) is the instantaneous slant range history of the pth target in the nth imaging region and the radar, f c represents the carrier frequency of the radar, j represents the complex unit, Δf n represents the frequency offset of the nth sub-pulse and the first sub-pulse, Δτ n represents the transmission delay of the nth sub-pulse and the first sub-pulse, represents the baseband sub-pulse signal of the nth sub-pulse of the radar;

[0050] different band-pass filters are constructed and the down-converted signal is processed to separate the echo data of different imaging regions.

[0051] In an embodiment of the present application, the S3.4 comprises:

[0052] In the distance domain, the correction function H match (f r ) is constructed

[0053]

[0054] The echo data Y n (f r ,t k ) after Fourier transform of the distance direction is multiplied by the correction function H match (f r ), and then converted to the distance time domain to obtain the echo data of the nth imaging area in the distance time domain:

[0055]

[0056] In an embodiment of the present application, the S3.6 comprises:

[0057] The corresponding azimuth deramp function is constructed with the center of the nth imaging area as a reference, the echo data of the nth imaging area in the distance time domain is processed by using the back projection algorithm of the azimuth deramp to obtain the echo data after deramp:

[0058] The corresponding BP imaging result of the nth imaging area is obtained:

[0059]

[0060] Wherein, T a represents the synthetic aperture time, represents the fast time deramp after the echo data.

[0061] Compared with the prior art, the present application has the following advantages:

[0062] 1. The present application proposes a time domain azimuth wide-width beamforming imaging method based on sub-pulse diversity, which divides a single pulse of a traditional radar into multiple sub-pulses, and points different sub-pulses to different azimuth positions, thereby realizing the ability of azimuth wide-beam coverage, and further enabling the radar to have azimuth wide-width beamforming capability; effectively realizing the application of sub-pulse diversity technology in imaging system, and solving the problems of high operation complexity of the method for applying a frequency scanning radar in an azimuth array and loss of distance resolution of the scene edge part.

[0063] 2、The application separates different sub-pulses from each other in the range frequency domain, and then designs different band-pass filters at the receiving end, so that different sub-scene echoes are realized, equivalent to azimuth Doppler demodulation, compared with the traditional phased array radar, under the same array surface and system pulse repetition frequency, the azimuth imaging width is expanded by several times the number of sub-pulses. After processing by the range frequency band-pass filter, the application separates different sub-scene echoes well, splices according to the imaging grid, and obtains the final wide-width azimuth imaging result.

[0064] 3、The method of the application realizes the expansion of the azimuth imaging width by several times the number of sub-pulses based on the sub-pulse diversity design without losing the azimuth imaging resolution. It only needs to perform band-pass filtering several times the number of sub-pulses in the range frequency domain, so that different azimuth non-fuzzy echoes are separated, and the algorithm complexity is low. Moreover, the sub-pulse separation process can be equivalent to Doppler demodulation, so when realizing the imaging width of the traditional phased array, the system pulse repetition frequency required after applying the method of the application is reduced by several times the number of sub-pulses compared with the traditional phased array radar.

[0065] The application will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 is a flow chart of a time-domain azimuth wide-width beamforming imaging method based on sub-pulse diversity provided by an embodiment of the application;

[0067] Figure 2 is a geometric diagram of a multi-frequency sub-pulse wide-width beamforming signal model provided by an embodiment of the application;

[0068] Figure 3 is a schematic diagram of a sub-pulse echo signal provided by an embodiment of the application;

[0069] Figure 4 is a schematic diagram of the imaging result of each imaging area image after imaging processing provided by an embodiment of the application;

[0070] Figure 5 is a schematic diagram of a sub-pulse wide-width beamforming imaging result provided by an embodiment of the application;

[0071] Figure 6 is a schematic diagram of an azimuth deramp processing result of a traditional phased array provided by an embodiment of the application;

[0072] Figure 7 is a schematic diagram of a wide-width imaging result of a traditional phased array provided by an embodiment of the application;

[0073] Figure 8is a theoretical image of a scene target in three different imaging regions provided by an embodiment of the present application;

[0074] Figure 9 is an imaging result diagram of a scene target using a traditional phased array processing;

[0075] Figure 10 is an imaging result diagram of a scene target processed by the time-domain azimuth wide-beam imaging method provided by an embodiment of the present application. DETAILED DESCRIPTION

[0076] In order to further illustrate the technical means and effects taken by the present application to achieve the predetermined purposes, a time-domain azimuth wide-beam imaging method based on sub-pulse diversity according to the present application is described in detail below in combination with the drawings and specific embodiments.

[0077] The foregoing and other technical contents, features and effects of the present application can be clearly presented in the detailed description of the specific embodiments below in combination with the drawings. Through the description of the specific embodiments, the technical means and effects taken by the present application to achieve the predetermined purposes can be understood more deeply and specifically. However, the accompanying drawings are provided for reference and illustration only, and are not intended to limit the technical solutions of the present application.

[0078] It should be noted that, in this document, relational terms such as first and second, and the like, are used solely to distinguish one entity or action from another entity or action, without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element preceded by "comprises... a" does not, without more constraints, foreclose the existence of additional identical elements in the process, method, article, or apparatus that comprises the listed element.

[0079] Embodiment one

[0080] Please refer to Figure 1 , Figure 1 is a flowchart of a time-domain azimuth wide-beam imaging method based on sub-pulse diversity provided by an embodiment of the present application. The time-domain azimuth wide-beam imaging method comprises:

[0081] S1: constructing a multi-frequency sub-pulse wide-beam signal model, pointing different sub-pulse beams of the radar to different imaging regions respectively, and obtaining total echo signals received by the radar.

[0082] Please refer to Figure 2 , Figure 2is a geometric diagram of a multi-frequency sub-pulse wide-beam signal model provided by an embodiment of the present application. Consider that the phased array radar works in a beamforming imaging mode at this time, and the platform where the radar is located moves along the x-axis with a velocity v in the azimuth direction.

[0083] The step S1 of the embodiment specifically comprises:

[0084] S1.1: Obtain the baseband sub-pulse signal of each sub-pulse of the radar.

[0085] Suppose that the pulse repetition frequency of the radar at this time is PRF, and each pulse transmitted by the radar contains N sub-pulses, then the baseband sub-pulse signal of the nth sub-pulse of the radar can be expressed as:

[0086]

[0087] wherein, denotes the fast time, rect(x) denotes the envelope function, Δτ n =(n-1)T Δ denotes the transmission delay of the nth sub-pulse and the 1st sub-pulse, n=1, 2, …, N; T Δ , T sp and μ respectively denote the sub-pulse time interval, the sub-pulse duration and the sub-pulse frequency modulation rate of the radar.

[0088] S1.2: Modulate different sub-pulses to different range frequency regions to obtain the transmission signal of each sub-pulse of the radar.

[0089] Specifically, modulate different sub-pulses to different range frequency regions to obtain the frequency offset Δf n of the nth sub-pulse and the 1st sub-pulse, which can be expressed as:

[0090]

[0091] wherein, Δf denotes the frequency increment of two adjacent sub-pulses.

[0092] Further, according to the frequency offset Δf n , the transmission signal of the nth sub-pulse of the radar is obtained:

[0093]

[0094] wherein, f c denotes the carrier frequency of the radar, j denotes the complex number unit, and n=1, 2, 3, …, N.

[0095] S1.3: Direct different sub-pulse beams to different imaging regions expected to be beamformed and imaged respectively to obtain the total echo signal of each imaging region.

[0096] ​Since different sub-pulses are independent of each other in time, the function of pointing different sub-pulses to different regions can be realized by controlling the phase between the array elements. This characteristic is similar to the wide beam coverage of the multi-transmit multi-receive radar (MIMO). Therefore, different sub-pulse beams are respectively pointed to different imaging regions expected to be bunched and imaged in the embodiment, and the echo data of each imaging region is imaged and spliced accordingly, so that the azimuth imaging range (the compromise between the range bandwidth and the azimuth imaging width) is expanded by N times compared with the traditional bunching system.

[0097] As shown in Figure 2 , different sub-pulse beams are respectively pointed to different imaging regions expected to be bunched and imaged, and it is assumed that the nthsub-pulse is pointed to the nthimaging region, the coordinate of the center o of the nthimaging region is (x n ,y n ,0), at the azimuth 0 time, the slant angle of the center o of the imaging region relative to the radar is θ nc0 , the nearest slant distance is R Bnc , and there is a point target with a coordinate of (x np ,y np ,0) in the imaging region. At the azimuth 0 time, the slant angle of the target relative to the radar is θ np0 , the nearest slant distance is R Bnp , and the distance of the radar relative to the target at this time is R B np / cos(θnp0), and x B np=R np np·tan(θnp0). The imaging plane coordinate of the target can be expressed as (x Bnp , R np ), and the echo signal of the pthtarget in the nthimaging region can be obtained.

[0098]

[0099] Where σ k is the complex scattering coefficient of the pthtarget, t a is the azimuth slow time, w k (t np ) is the target azimuth window function, which can be considered as a constant in the bunching imaging mode and ignored in the subsequent derivation, and c is the speed of light. R k (t n ) is the instantaneous slant range history of the pthtarget in the nthimaging region relative to the radar, which is expressed as:

[0100]

[0101] Where x p= x np - x n is the azimuth distance difference of the target relative to the center of the imaging area (sub-scene).

[0102] Further, the echo signal of the pth target in the nth imaging area is The total echo signal of all targets in the nth imaging area can be obtained as:

[0103]

[0104] where P represents the total number of targets in the nth imaging area, and c represents the speed of light.

[0105] Further, the total echo signal received by the radar can be obtained according to the total echo signal of each imaging area, denoted as:

[0106]

[0107] By dividing the single pulse into multiple sub-pulses and directing them to different imaging areas, the system energy can be effectively distributed in different azimuth imaging directions, thereby realizing wide-beam imaging. However, since the data of different sub-pulses are mixed together during the echo data reception process, relevant wide-beam imaging processing is required.

[0108] S2: According to the characteristics of the radar system, design the pulse repetition frequency and synthetic aperture time of the radar.

[0109] The pulse repetition frequency PRF is an important indicator and has a significant impact on radar system design. Generally, a higher PRF will increase the difficulty of radar system design, reduce the system range-azimuth imaging width, and increase the system data volume under the same accumulation time. Therefore, this embodiment discusses the Doppler characteristics of multi-frequency sub-pulse azimuth wide-beam and the corresponding system PRF requirements. The echo data of the nth imaging area in the range time domain is It can be seen that the azimuth phase of the nth imaging area target at this time is:

[0110]

[0111] At this time, the azimuth phase mainly depends on the target range history and frequency offset. At this time, the second-order Taylor expansion of the instantaneous range history R np (t k ) of the pth target in the nth imaging area and the radar is:

[0112]

[0113] where R np (0) is the range of target p at azimuth 0, and Rn p (0)=vsinθ np0 is the first order Taylor coefficient of the slant range history, is the second order Taylor expansion coefficient.

[0114] Further, the related Doppler parameter of the pth target in the nth imaging area can be obtained as:

[0115]

[0116] where f dc_np represents the Doppler frequency, f dr_np represents the Doppler frequency modulation, represents the wavelength.

[0117] Further, the Doppler center support area size corresponding to the echo data in the nth imaging area can be obtained as:

[0118]

[0119] where θ 3dB is the 3dB beam width of the radar in the azimuth direction.

[0120] After the azimuth deramp processing, the residual modulation bandwidth will exist in the edge area due to the imperfect matching of the azimuth deramp processing. However, the residual modulation bandwidth is generally small and can be ignored. Therefore, the Doppler bandwidth B subn of the nth imaging area at this time can be approximated as:

[0121] B subn = Δf dcn

[0122] Considering the single-wide beam system of the phased array radar, the beam width of the radar needs to be expanded to:

[0123] θ PA ≈ Nθ 3dB

[0124] At this time, the entire scene echo is received with aliasing, and the corresponding Doppler center bandwidth is:

[0125]

[0126] where, represents the angle of the middle sub-pulse, is the wavelength of the middle sub-pulse.

[0127] Similarly, ignoring the influence of the azimuth residual bandwidth, the Doppler bandwidth of the echo data of the single beam covering the complete scene can be approximated as:

[0128] B​PA =ΔB+Δf dc

[0129] Where ΔB represents the residual modulation bandwidth in the edge area when a single beam covers the entire scene.

[0130] Then we have:

[0131]

[0132] From the above formula, it can be seen that when a wide beam is used to illuminate the same large azimuth width in a phased array system, the corresponding signal Doppler bandwidth is about N times that of a single sub-scene. Therefore, the pulse repetition frequency required by the radar system at this time must meet the pulse repetition frequency PRF when a single beam covers the entire scene. PA >B PA . Under the sub-pulse system, the Doppler support areas corresponding to the echo signals of different sub-scenes are different, which will also cause the overall Doppler support area to expand. However, since the sub-pulses can be separated by range-frequency domain bandpass filtering, which is equivalent to azimuth deambiguation processing, the system required by the sub-pulse system only needs to ensure that there is no aliasing in the single sub-pulse scene. It can be seen that under the same azimuth imaging width, the system pulse repetition frequency required by the sub-pulse system is about 1N of that of the traditional phased array system. In other words: under the same PRF, the azimuth width that can be achieved without ambiguity by the sub-pulse system is N times that of the traditional phased array.

[0133] According to the above analysis, in order to ensure that the radar system does not produce Doppler ambiguity, the corresponding PRF needs to satisfy the requirement that there is no ambiguity in each sub-scene. The corresponding PRF should be constrained to be

[0134] Furthermore, in the sub-pulse system, the same radar can achieve coverage of multiple imaging areas (sub-scenes). However, the geometric configurations of different imaging areas relative to the radar are different, which leads to differences in their corresponding azimuth resolutions. In order to ensure that the azimuth resolution of the entire imaging range meets the system design requirements, it is necessary to use the same synthetic aperture time T a The resolution required for effective imaging of each sub-scene is less than the system required resolution ρ a At this time, the corresponding synthetic aperture time T a The following two constraints need to be met:

[0135]

[0136] max{ρ an}≤ρ a

[0137] Among them, ρ an represents the resolution required for effective imaging of the nth imaging area within the same synthetic aperture time, ρa represents the required resolution of the radar system, k a is the azimuth broadening factor, which can be selected as 1. Indicates that the pth target in the nth scene is The radar oblique angle of time, Indicates that the pth target in the nth scene is The radar squint angle at time t, p = 1, 2, 3…P, where P represents the total number of targets within the nth scene. The first constraint characterizes the azimuth resolution of the scene center point of the nth sub-scene at the current synthetic aperture time (due to the small scene size of the spotlight system, the center resolution is used to represent the resolution of the entire sub-scene). The second constraint states that the azimuth resolution of each sub-scene must meet system requirements. Radar system design is a complex process. This paper mainly exemplifies several key indicators and design processes for sub-pulse systems.

[0138] S3: Based on the radar's pulse repetition frequency and synthetic aperture time, wide-band spotlight imaging processing is performed on the total echo signal received by the radar to obtain a complete image with a large azimuth imaging width.

[0139] Step S3 of this embodiment includes:

[0140] S3.1: Construct an echo down-conversion function and perform conjugate multiplication of the echo down-conversion function and the total echo signal received by the radar to obtain a down-conversion signal.

[0141] Specifically, when the radar receives echo signals, the echoes of different sub-pulses are mixed together in the time domain. Since the imaging geometric relationships of different imaging areas are significantly different, the mixed echoes of different imaging areas will cause image aliasing, making it impossible to achieve high-resolution target imaging. To achieve the separation of the echo signals of different sub-pulses, different range frequency regions are assigned to different sub-pulses when the sub-pulses are transmitted. To this end, the corresponding echo down-conversion function is constructed:

[0142]

[0143] Then, the echo down-conversion function Total echo signal received by radar By conjugate multiplication, we can get the down-converted signal:

[0144]

[0145] Here, * represents conjugation. At this time, the corresponding n-th sub-pulse echo signal is down-converted to the baseband frequency.

[0146] S3.2: Construct a band-pass filter, separate the down-converted signal by using the band-pass filter, and obtain the separated echo data of each imaging region.

[0147] This embodiment constructs a band-pass filter near the range frequency domain:

[0148]

[0149] wherein f r represents the range frequency of the echo signal, B sp represents the bandwidth of the radar.

[0150] Subsequently, the down-converted signal is converted to the range frequency domain and multiplied by the band-pass filter H fliter (f r ), and then converted to the range time domain, so that the separated echo data of the nth imaging region is obtained.

[0151]

[0152] Similarly, different band-pass filters can be constructed and used to process the aliasing signal (i.e., ), so that the echo data of different imaging regions can be separated.

[0153] S3.3: Perform a range Fourier transform on the separated echo data of each imaging region, to obtain the range Fourier transformed echo data of each imaging region.

[0154] Due to the coupling relationship between the sub-pulse and different imaging regions (sub-scenarios), and the fixed time delay between different sub-pulses, the images of different imaging regions will have a range deviation. Therefore, the separated echo data is further processed for range registration. According to the principle of stationary phase, the range Fourier transform is performed on the separated echo data, to obtain the range Fourier transformed echo data of the nth imaging region:

[0155]

[0156] wherein μ represents the sub-pulse frequency of the radar.

[0157] S3.4: Construct a correction function in the range frequency domain, and obtain the corrected echo data of each imaging region in the range time domain.

[0158] As can be seen from the above formula, f r and Δτ n are coupled, which causes the envelope error of the echo data of different imaging regions. Therefore, a correction function is constructed in the range frequency domain, and the expression is:

[0159]

[0160] The echo data Y n (f r ,t k ) is multiplied by the correction function H match (f r ), and then converted into the distance time domain to obtain the echo data of the nth imaging area in the distance time domain:

[0161]

[0162] S3.5: Construct a distance pulse compression function to obtain the echo data of each imaging area after pulse compression.

[0163] After the above processing, the echo data of the target from different imaging areas (sub-scenes) is separated and aligned. Then, a corresponding distance pulse compression function is constructed:

[0164]

[0165] Further, the echo data of each imaging area after pulse compression can be obtained as:

[0166]

[0167] where IFFT(·) is an inverse Fourier transform operation.

[0168] S3.6: A corresponding azimuth deramp function is constructed with the center of each imaging area as a reference to process the echo data, and the corresponding BP imaging result of each imaging area is obtained.

[0169] Due to factors such as motion errors in the actual imaging process, the embodiment selects a classic azimuth deramp back projection algorithm (BP) for imaging processing. A corresponding azimuth deramp function is constructed with the center of each imaging area as a reference to process the echo data, and the deramped echo data is obtained as Further, the corresponding BP imaging result of the nth imaging area is obtained, and is represented as:

[0170]

[0171] where T a represents a synthetic aperture time, represents a fast time deramped echo data.

[0172] S3.7: According to the corresponding BP imaging results of each imaging area, a bunching image of each imaging area is obtained, and then a complete image of a large azimuth imaging width is obtained.

[0173] Since the calibration of each sub-scene in the bunching imaging is known, the actual coordinates of each sub-scene are also known. Therefore, the sub-scene images can be spliced according to the coordinates between the sub-scenes, and finally a complete image of a large azimuth imaging width is obtained.

[0174] Further, in order to verify the effectiveness of the sub-pulse diversity based time domain azimuth wide bunching imaging method proposed in the application, the following gives a comparison experiment of point targets and extended targets under the sub-pulse system and the traditional phased array system. Part of the simulation parameters are shown in Table 1.

[0175] Under this condition, the traditional phased array is affected by the beam width, and the effective azimuth imaging width is about 1.2km. Further, this experiment considers three sub-pulses, and each sub-pulse is configured in three azimuthally adjacent regions, and the corresponding effective imaging range is about 3.6km. The azimuth of the sub-scene imaging is set to (-1838.3, -611.2), (-611.2, 611.2), and (611.2, 1838.3)m respectively. And taking the center of each sub-scene as the center, 9 point targets are arranged, and the azimuthally adjacent point targets in the same scene are spaced by 410m, and the distance is spaced by 166m. After deramping processing, the maximum Doppler bandwidth of a single sub-scene is about 528Hz, so according to the PRF design constraint, the system PRF is set to 550Hz. While the single beam covers three sub-scenes, the corresponding scene Doppler bandwidth is calculated as 1525.4Hz, at this time, when the PRF is greater than this value, non-aliasing imaging can be realized, and then the sub-pulse imaging result is as shown in Figure 3 Figure 3 (a) shows the distance frequency domain distribution of the transmitted sub-pulse signal, from Figure 3 (a) can be seen, at this time, different sub-pulses are located in different distance frequency domains; Figure 3 (b) and Figure 3 (c) respectively gives the aliasing echo signal and the signal of the first region after sub-pulse separation, the abscissa represents the time in the azimuth direction (radar platform motion time), and the ordinate represents the distance gate.

[0176] Table 1 System simulation parameters

[0177]

[0178]

[0179] After separating the echoes of the three regions, the image of each region is processed, and the imaging result is as shown in​Figure 4 are shown, where, Figure 4 (a)(c)(e) respectively represent the range-azimuth information of 9 target points in three regions, the horizontal coordinate represents the azimuth gate, and the vertical coordinate represents the range gate, Figure 4 (b)(c)(d) respectively represent the imaging results of the radar in the azimuth sampling-range sampling, the horizontal coordinate represents the azimuth sampling, and the vertical coordinate represents the range sampling; Figure 4 (a)(b) is the result of the first imaging region, Figure 4 (c)(d) is the result of the second imaging region, Figure 4 (e)(f) is the result of the third imaging region. As can be seen from Figure 4 , although the radar system PRF is low at this time, since the sub-pulse separation is equivalent to Doppler demodulation processing, good imaging of the three regions is achieved at this time, which verifies the wide azimuth non-aliasing imaging capability of the sub-pulse system. And the point targets in the three scenes in the figure are imaged for quality evaluation, and the corresponding imaging indicators meet the system requirements. Further, by using the geometric information of the sub-scene, the sub-scene imaging is registered and spliced to obtain the final wide azimuth imaging result, as shown in Figure 5 .

[0180] In order to further compare the processing results of the method proposed in the application and the traditional phased array method, the following gives the imaging results of the traditional phased array with the same azimuth imaging width (the same array cannot realize the same azimuth beam coverage at this time, so the number of array elements is reduced to expand the beam width). Figure 6 is the azimuth deramp processing result, where the horizontal coordinate represents the azimuth frequency. As can be seen from the figure, since the system PRF is low at this time, the Doppler aliasing phenomenon occurs for the echo signals of different regions. Figure 7 The corresponding wide imaging result is given, where the red circle is the target imaging result, and the black dashed line is the aliasing energy. Since the radar cannot realize the echo separation of different regions at this time, the Doppler aliasing phenomenon still exists in the imaging process, which further causes the imaging quality to be reduced.

[0181] The simulation results of the extended target (scene target) are further given in this experiment, at this time, only 200*200m scene targets are arranged in the center region of each sub-scene. The corresponding theoretical images of different regions are shown in Figure 8 .

[0182] Figure 9 is the traditional phased array processing result, at this time, due to the low system PRF, the echo data of the three regions is aliased, and good imaging cannot be realized. Figure 10The imaging result processed by the sub-pulse width imaging method. Although three regions in the original echo are blurred in the azimuth Doppler due to the low PRF, the echoes of different sub-scenes are well separated after the distance frequency band-pass filtering processing. Then, each sub-scene is respectively BP imaged and spliced according to the imaging grid, and the final azimuth wide imaging result is obtained.

[0183] The application proposes a time-domain azimuth wide beamforming imaging method based on sub-pulse diversity. By dividing a single pulse of a traditional radar into multiple sub-pulses and directing different sub-pulses to different azimuth positions, the ability of azimuth wide beam coverage is realized, and the radar has the ability of azimuth wide beamforming. The application of sub-pulse diversity technology in the imaging system is effectively realized, and the problems of high operation complexity and loss of distance resolution at the scene edge in the method of applying a frequency scanning radar to an azimuth array are solved. The application separates different sub-pulses in the distance frequency domain, and then designs different band-pass filters at the receiving end, so that different sub-scene echoes are separated, and the azimuth Doppler is deblurred. Compared with the traditional phased array radar, the azimuth imaging width is expanded by a factor of the number of sub-pulses under the same array and system pulse repetition frequency. After the distance frequency band-pass filter processing, the application separates different sub-scene echoes well, splices according to the imaging grid, and obtains the final azimuth wide imaging result.

[0184] The traditional phased array beamforming imaging system is limited by the azimuth beam width of the radar, and can only realize the azimuth imaging width of the main lobe single beam width. The sliding beamforming imaging system can expand the azimuth imaging width, but the azimuth imaging resolution is lost. The frequency scanning array radar realizes the azimuth wide coverage by introducing the inter-element time delay, and then realizes the azimuth wide beamforming imaging. However, this method needs to design corresponding band-pass filters for different distance gates to separate different distance data, and the algorithm operation complexity is very high. And because the target distance frequency is modulated by the beam pattern, the beam coverage range that can be realized by the frequency scanning radar is smaller than that of the application. The method of the application realizes the expansion of the azimuth imaging width by a factor of the number of sub-pulses without losing the azimuth imaging resolution through sub-pulse diversity design. It only needs to perform band-pass filtering for the number of sub-pulses in the distance frequency domain, and the deblurred echoes of different azimuths can be separated. The algorithm complexity is low. Moreover, the sub-pulse separation process can be equivalent to Doppler deblurring. Therefore, when realizing the imaging width of the traditional phased array radar, the system pulse repetition frequency required by the application is reduced by a factor of the number of sub-pulses compared with the traditional phased array radar.

[0185] In several embodiments provided in the present application, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the above-described device embodiments are merely illustrative, for example, the division of the modules is only a logical function division, and actual implementation can have another division manner, for example, multiple modules or components can be combined or integrated into another system, or some features can be ignored or not executed.

[0186] In addition, each function module in each embodiment of the present application can be integrated into a processing module, or each module can exist physically alone, or two or more modules can be integrated into one module. The integrated module can be realized in the form of hardware or in the form of a hardware plus software function module.

[0187] The above is a further detailed description of the present application in combination with specific preferred embodiments, and the specific implementation of the present application cannot be limited to these descriptions. For ordinary skilled persons in the technical field to which the present application belongs, several simple deductions or substitutions can be made without departing from the concept of the present application, and all of them should be regarded as falling within the protection scope of the present application.

Claims

1. A time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity, characterized in that: include: S1: Construct a multi-frequency sub-pulse wide-bandwidth beamforming signal model, direct different sub-pulse beams of the radar to different imaging areas, and obtain the total echo signal received by the radar; S2: Design the radar’s pulse repetition frequency and synthetic aperture time based on the characteristics of the radar system; S3: Based on the pulse repetition frequency and synthetic aperture time of the radar, wide-band spotlight imaging processing is performed on the total echo signal received by the radar to obtain a complete image with a large azimuth imaging width; Synthetic Aperture Time of the Radar The constraints that must be met include: ， in, Indicates the first n The resolution required for effective imaging of the imaging area, represents the required resolution of the radar system, is the azimuthal broadening factor, , c represents the speed of light, Indicates the The frequency offset between the first sub-pulse and the second sub-pulse is represents the radar carrier frequency, Indicates the n The first p The target is The radar oblique angle of time, Indicates the n The first p The target is The radar oblique angle of time, p =1,2,3… P , P Indicates the n The total number of targets in the scene; The S3 includes: S3.1: Construct an echo down-conversion function, and conjugate-multiply the echo down-conversion function by the total echo signal received by the radar to obtain a down-conversion signal; S3.2: Constructing a bandpass filter, and using the bandpass filter to separate the down-converted signal to obtain separated echo data of each imaging area; S3.3: Performing a range-transform on the separated echo data of each imaging region to obtain range-transformed echo data of each imaging region; S3.4: constructing a correction function in the range-frequency domain and obtaining the echo data of each imaging area in the range-time domain after correction; S3.5: construct a range pulse compression function to obtain echo data of each imaging area after pulse compression; S3.6: Using the center of each imaging region as a reference, construct a corresponding azimuth deramp function to process the echo data and obtain the corresponding BP imaging result for each imaging region; S3.7: Obtain a focused image of each imaging area based on the corresponding BP imaging result of each imaging area, thereby obtaining a complete image with a large azimuth imaging width.

2. The time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity according to claim 1, characterized in that: Said S1 comprises: S1.1: Obtain the baseband sub-pulse signal of each sub-pulse of the radar ,in, n =1,2,3… N , N represents the number of sub-pulses contained in each pulse, and the radar is a phased array radar operating in a spotlight imaging mode; S1.2: Modulate different sub-pulses to different range frequency regions to obtain the transmitted signal of each sub-pulse of the radar ; S1.3: Directing different sub-pulse beams toward different imaging areas where spotlight imaging is desired, and obtaining a total echo signal in each imaging area; S1.4: Obtain a total echo signal received by the radar based on the total echo signal of each imaging area.

3. The time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity according to claim 2, characterized in that: Said S1.2 includes: Modulate different sub-pulses to different distance frequency regions to obtain the The frequency offset of the first sub-pulse from the first sub-pulse : in, Represents the frequency increment between two adjacent sub-pulses; According to the frequency offset Get the radar Sub-pulse emission signal : in, represents the radar carrier frequency, j Indicates plural units, Indicates the n The transmission delay between the first sub-pulse and the second sub-pulse is represents the time interval between sub-pulses, Indicates fast time.

4. The time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity according to claim 3, characterized in that: Said S1.3 includes: Assume that n The sub-pulse points to the n imaging area, and obtain the n The first p The echo signal of a target : in, For the said p The complex scattering coefficient of a target, For quick time, For azimuth slow time, is the target azimuth window function, c is the speed of light, For the n The first p The instantaneous slant range history of a target and the radar; According to n The first p The echo signal of a target Get the n The total echo signal of all targets in the imaging area.

5. The time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity according to claim 1, characterized in that: The pulse repetition frequency of the radar satisfies: in, represents the pulse repetition frequency of the radar, Indicates the n The Doppler bandwidth corresponding to the imaging area.

6. The time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity according to claim 5, characterized in that: Said S3.2 includes: Constructing a bandpass filter near the range-frequency domain : in, Indicates the range frequency of the echo signal, represents the bandwidth of the radar; Downconvert the signal After conversion to the range frequency domain and the bandpass filter Multiply and then convert to the distance time domain to obtain the n The echo data after the imaging area is separated : in, p =1,2,3… P , P Indicates the n The total number of targets in the scene, For the p The complex scattering coefficient of a target, For azimuth slow time, is the target azimuth window function, Indicates fast time, c is the speed of light, For the n The first p The instantaneous slant range history of a target and the radar, represents the radar carrier frequency, j Indicates plural units, Indicates the The frequency offset between the first sub-pulse and the second sub-pulse is Indicates the n The transmission delay between the first sub-pulse and the second sub-pulse is Indicates the radar n Baseband sub-pulse signal of sub-pulses; Different bandpass filters are constructed respectively and the down-converted signals are subjected to Processing is performed to separate the echo data of different imaging areas.

7. The time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity according to claim 6, characterized in that: Said S3.4 includes: Constructing a correction function in the distance-frequency domain : Convert the distance to the echo data after Fourier transformation With the correction function Multiply and then convert to distance time domain to obtain the first n Echo data of the imaging area: 。 8. The time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity according to claim 7, characterized in that: Said S3.6 includes: First n The center of each imaging area is used as the reference to construct the corresponding azimuth deramp function, and the back projection algorithm of azimuth deramp is used to calculate the first n The echo data of each imaging area is processed to obtain the echo data after deramp: ; Get the n The corresponding BP imaging results of the imaging area are: in, represents the synthetic aperture time, Indicates fast time The echo data after deramp.

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