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

Abstract

The invention discloses a time domain azimuth wide bunching imaging method based on sub-pulse diversity, and the method comprises the steps: constructing a multi-frequency sub-pulse wide bunching signal model, enabling different sub-pulse beams of a radar to point to different imaging regions, and obtaining a total echo signal received by the radar; according to the characteristics of a radar system, the pulse repetition frequency and synthetic aperture time of the radar are designed; and based on the pulse repetition frequency and synthetic aperture time of the radar, performing wide-range bunching imaging processing on the total echo signal received by the radar to obtain a complete image with large azimuth imaging breadth. According to the invention, the single pulse of the radar is divided into a plurality of sub-pulses, and the different sub-pulses point to different azimuth positions, so that the radar has the azimuth wide bunching capability; the application of a sub-pulse diversity technology in an imaging system is effectively realized, and the problems of high operation complexity and distance resolution loss of a scene edge part of a method for applying a frequency scanning radar to an azimuth array are solved.

Description

Technical Field

[0001] The invention belongs to the technical field of synthetic aperture radar imaging, and in particular relates to a time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity. Background Art

[0002] Synthetic aperture radar has the advantage of all-day and all-weather observation, and plays an important role in target detection, traffic control, geological disaster warning and many other aspects. High-resolution imaging can obtain more detailed information about the target, and has always been an important research direction. Initially, strip imaging was limited by the synthetic aperture time, and the theoretical azimuth resolution could only reach half of the antenna azimuth aperture. The spotlight imaging system controls the direction of the radar beam and focuses the beam on a certain area on the ground, effectively improving the azimuth resolution of the observation area. However, the azimuth imaging width is limited to the main lobe beam width at this time. In order to improve the azimuth imaging width, the sliding spotlight imaging mode was proposed. In this mode, the radar beam rotates around a virtual center, which increases the target illumination time and expands the azimuth imaging width to a certain extent. However, this method sacrifices the azimuth resolution. How to ensure high azimuth resolution of the target while achieving a large azimuth imaging width is a problem worth studying.

[0003] Frequency scanning radar is a new type of radar proposed in recent years. It realizes the intra-pulse beam scanning function by introducing a fixed time delay between array elements, thus having a wide beam coverage characteristic. Based on the wide beam coverage characteristic, a high-resolution wide-width imaging system in elevation is proposed. This method uses the mapping relationship between elevation angle and range frequency to construct different range frequency bandpass filters in the receiving process, thereby achieving separation of targets at different distances, and then using the traditional strip imaging method to obtain the range wide-width imaging results. The method of applying frequency scanning radar to the azimuth array realizes wide beam coverage in azimuth, and by setting specific array weights, the wide beam is directed to the same area, thereby realizing wide azimuth width beam imaging.

[0004] Sub-pulse diversity technology improves the signal processing performance of the radar system by dividing a single pulse into multiple sub-pulses and modulating the sub-pulses into different signals (frequency, phase, pulse width). Multi-frequency sub-pulse technology is one of them, which obtains the sub-pulse dimensional freedom by modulating the sub-pulses into signals separated from each other in the range-frequency domain. The multi-frequency sub-pulse wide-band imaging method in elevation achieves a coverage width that exceeds the original elevation beam width by pointing sub-pulses in different frequency ranges to different areas in the elevation direction. Then, the range fuzzy separation is achieved through frequency bandpass to realize the range wide-band imaging.

[0005] The method of applying frequency scanning radar to azimuth array requires constructing bandpass filters to separate different distance data, which has high computational complexity. In addition, since the effective bandwidth of the scene target in the range direction is modulated by the beam pattern, the edge points of the scene will have some range resolution loss, which reduces the imaging resolution. Although sub-pulse diversity technology has been studied in the wide-band synthetic aperture radar ground moving target detection technology, it has been less studied in the imaging system and is now mainly focused on the application of strip-type range imaging. Summary of the invention

[0006] In order to solve the above problems existing in the prior art, the present invention provides a time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity. The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0007] The present invention provides a time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity, comprising:

[0008] S1: Construct a multi-frequency sub-pulse wide-width beamforming signal model, point different sub-pulse beams of the radar to different imaging areas, and obtain the total echo signal received by the radar;

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

[0010] 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.

[0011] In one embodiment of the present invention, the S1 includes:

[0012] S1.1: Obtain the 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 operating in a spotlight imaging mode;

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

[0014] S1.3: directing different sub-pulse beams to different imaging areas where beamforming is desired, respectively, to obtain a total echo signal of each imaging area;

[0015] S1.4: Obtain a total echo signal received by the radar according to the total echo signal of each imaging area.

[0016] In one embodiment of the present invention, the S1.2 includes:

[0017] Modulate different sub-pulses to different distance frequency regions to obtain the frequency offset Δf between the nth sub-pulse and the first sub-pulse n :

[0018]

[0019] Where Δf represents the frequency increment between two adjacent sub-pulses;

[0020] According to the frequency shift Δf n Get the transmitted signal of the nth sub-pulse of the radar

[0021]

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

[0023] In one embodiment of the present invention, the S1.3 includes:

[0024] Assuming that the nth sub-pulse points to the nth imaging area, the echo signal of the pth target in the nth imaging area is obtained.

[0025]

[0026] Among them, σ np is the complex scattering coefficient of the pth target, For fast time, t k is the azimuth slow time, w a (t k ) is the target azimuth window function, c is the speed of light, Rnp(tk) is the instantaneous slant range history between the p-th target in the n-th imaging area and the radar;

[0027] According to the echo signal of the pth target in the nth imaging area Obtain the total echo signals of all targets in the nth imaging area.

[0028] In one embodiment of the present invention, the pulse repetition frequency of the radar satisfies:

[0029]

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

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

[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 carrier frequency of the radar, 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 the time, p = 1, 2, 3…P, P represents the total number of targets in the nth scene.

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

[0036] S3.1: construct an echo down-conversion function, and conjugate-multiply the echo down-conversion function with 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 range Fourier transform on the separated echo data of each imaging region to obtain echo data of each imaging region after range Fourier transform;

[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: construct a corresponding azimuth deramp function with the center of each imaging area as a reference to process the echo data, and obtain the corresponding BP imaging result of each imaging area;

[0042] S3.7: According to the corresponding BP imaging result of each imaging area, a beam image of each imaging area is obtained, thereby obtaining a complete image with a large azimuth imaging width.

[0043] In one embodiment of the present invention, the S3.2 includes:

[0044] Construct a bandpass filter H near the range frequency domain fliter (f r ):

[0045]

[0046] Among them, f r Indicates the distance frequency of the echo signal, B sp represents the bandwidth of the radar;

[0047] Down-convert the signal After conversion to the range frequency domain and the bandpass filter H fliter (f r ) and then converted to the range-time domain to obtain the echo data after separation of the nth imaging area.

[0048]

[0049] Where 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, represents fast time, c is the speed of light, R np (t k ) is the instantaneous slant range history between the pth target in the nth imaging area and the radar, f c represents the carrier frequency of the radar, j represents the complex unit, Δf n represents the frequency offset between the nth sub-pulse and the first sub-pulse, Δτ n represents the transmission delay between the nth sub-pulse and the first sub-pulse, The baseband sub-pulse signal representing the nth sub-pulse of the radar;

[0050] Different bandpass filters are constructed respectively and the down-converted signals are Processing is performed to separate the echo data of different imaging areas.

[0051] In one embodiment of the present invention, the S3.4 includes:

[0052] Construct the correction function H in the range-frequency domain match (f r ):

[0053]

[0054] Convert the distance to the echo data Y after Fourier transformation n (f r ,t k ) and the correction function H match (f r ) and then converted into the range-time domain to obtain the echo data of the nth imaging area in the range-time domain:

[0055]

[0056] In one embodiment of the present invention, the S3.6 includes:

[0057] The corresponding azimuth deramp function is constructed with the center of the nth imaging area as the reference. The echo data of the nth imaging area in the range time domain is processed using the azimuth deramp back-projection algorithm. The echo data after deramp is obtained as

[0058] Get the corresponding BP imaging result of the nth imaging area:

[0059]

[0060] Among them, T a represents the synthetic aperture time, Express fast time The echo data after deramp.

[0061] Compared with the prior art, the present invention has the following beneficial effects:

[0062] 1. The present invention proposes a time-domain azimuth wide-beam imaging method based on sub-pulse diversity. By dividing a single pulse of a traditional radar into multiple sub-pulses and pointing different sub-pulses to different azimuth positions, the ability of azimuth wide-beam coverage is achieved, thereby enabling the radar to have azimuth wide-beam focusing capability; the application of sub-pulse diversity technology in the imaging system is effectively realized, and the problems of high computational complexity and loss of range resolution at the edge of the scene are solved when the frequency scanning radar is applied to the azimuth array.

[0063] 2. The present invention separates different sub-pulses in the range frequency domain, and then designs different bandpass filters at the receiving end to realize different sub-scene echoes, which is equivalent to azimuth Doppler deambiguation. Compared with the traditional phased array radar, under the same array and system pulse repetition frequency, the azimuth imaging width is expanded by several times of the sub-pulses. After processing with the range frequency bandpass filter, the present invention separates the echoes of different sub-scenes well, splices them according to the imaging grid, and obtains the final azimuth wide-width imaging result.

[0064] 3. The method of the present invention achieves the expansion of the azimuth imaging width by several times of the number of sub-pulses without losing the azimuth imaging resolution through sub-pulse diversity design. It only needs to perform bandpass filtering several times of the sub-pulses in the range frequency domain to separate the unambiguous echoes in different azimuths, and the algorithm complexity is low. In addition, the process of sub-pulse separation can be equivalent to Doppler deambiguation, so when realizing the traditional phased array imaging width, after applying the invented method, the required system pulse repetition frequency is reduced by several times of the sub-pulses compared with the traditional phased array radar.

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

[0066] Figure 1 It is a flow chart of a time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity provided by an embodiment of the present invention;

[0067] Figure 2 It is a geometrical schematic diagram of a multi-frequency sub-pulse wide-amplitude beamforming signal model provided by an embodiment of the present invention;

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

[0069] Figure 4 is a schematic diagram of imaging results after imaging processing of images of each imaging area provided by an embodiment of the present invention;

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

[0071] Figure 6 It is a schematic diagram of azimuth deramp processing results of a conventional phased array provided by an embodiment of the present invention;

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

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

[0074] Fig. 9 It is a schematic diagram of the imaging result of the scene target processed by the traditional phased array;

[0075] Fig.10 It is a schematic diagram of the imaging result of a scene target processed by the time-domain azimuth wide-band spotlight imaging method provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0076] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined purpose of the invention, a time-domain azimuth wide-band spotlight imaging method based on sub-pulse diversity proposed by the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0077] The above and other technical contents, features and effects of the present invention are clearly presented in the following detailed description of the specific implementation modes in conjunction with the accompanying drawings. Through the description of the specific implementation modes, the technical means and effects adopted by the present invention to achieve the predetermined purpose can be more deeply and specifically understood. However, the attached drawings are only for reference and explanation purposes and are not used to limit the technical solutions of the present invention.

[0078] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants are intended to cover non-exclusive inclusion, so that an article or device including a series of elements includes not only those elements, but also other elements that are not explicitly listed. In the absence of more restrictions, the elements defined by the statement "including one..." do not exclude the existence of other identical elements in the article or device including the elements.

[0079] Embodiment 1

[0080] See also Figure 1 , Figure 1 1 is a flow chart of a method for time-domain azimuth wide-band spotlight imaging based on sub-pulse diversity provided by an embodiment of the present invention. The method for time-domain azimuth wide-band spotlight imaging includes:

[0081] S1: Construct a multi-frequency sub-pulse wide-bandwidth beamforming signal model, point different sub-pulse beams of the radar to different imaging areas, and obtain the total echo signal received by the radar.

[0082] See also Figure 2 , Figure 2Schematic diagram of a multi-frequency sub-pulse wide-amplitude spotlight signal model provided by an embodiment of the present invention. Consider that the phased array radar works in the spotlight imaging mode, and the platform where the radar is located moves along the x-axis at an azimuth velocity v.

[0083] Step S1 of this embodiment specifically includes:

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

[0085] Assuming that the pulse repetition frequency of the radar is PRF, and each pulse emitted by the radar contains N sub-pulses, the baseband sub-pulse signal of the nth sub-pulse of the radar is It can be expressed as:

[0086]

[0087] in, represents fast time, rect(x) represents the envelope function, Δτ n =(n-1)T Δ represents the transmission delay between the nth sub-pulse and the first sub-pulse, n = 1, 2, ..., N; T Δ , T sp and μ represent the sub-pulse time interval, sub-pulse duration and sub-pulse modulation frequency of the radar respectively.

[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, different sub-pulses are modulated to different range frequency regions to obtain a frequency offset Δf between the nth sub-pulse and the first sub-pulse. n , which can be expressed as:

[0090]

[0091] Wherein, Δf represents the frequency increment between two adjacent sub-pulses.

[0092] Furthermore, according to the frequency shift Δf n Get the transmitted signal of the nth sub-pulse of the radar:

[0093]

[0094] Among them, f c represents the carrier frequency of the radar, j represents a complex unit, and n=1,2,3…N.

[0095] S1.3: Direct different sub-pulse beams to different imaging areas where beam-forming imaging is desired, respectively, to obtain a total echo signal in each imaging area.

[0096] Since different sub-pulses are independent of each other in time, the function of directing different sub-pulses to different areas can be achieved by controlling the phases between the phased array elements. This feature is similar to the wide beam coverage of multiple-transmit multiple-receive radar (MIMO). To this end, this embodiment points different sub-pulse beams to different imaging areas where beamforming is desired, and performs corresponding imaging and splicing on the echo data of each imaging area, so as to obtain an azimuth imaging range that is N times larger than that of the traditional beamforming system (a compromise between the range bandwidth and the azimuth imaging width).

[0097] like Figure 2 As shown, different sub-pulse beams are directed to different imaging areas where beam focusing imaging is desired. Assuming that the nth sub-pulse is directed to the nth imaging area, the coordinates of the center o of the nth imaging area are (x n ,y n ,0), at the azimuth 0, the oblique angle of the imaging area center o relative to the radar is θ nc0 , the closest slope distance is R Bnc , and there is a coordinate (x np ,y np ,0). At azimuth 0, the oblique angle of the target relative to the radar is θ np0 , the closest slope distance is R Bnp , and the distance between the radar and the target is Rnp0 = R B np / cos(θnp0), and xnp=R B np·tan(θnp0). The imaging plane coordinates of the target can be expressed as (x np ,R Bnp ), and then the echo signal of the pth target in the nth imaging area can be obtained

[0098]

[0099] Among them, σ np is the complex scattering coefficient of the pth target, t k is the azimuth slow time, w a (t k ) is the target azimuth window function. In the spotlight imaging mode, the target azimuth window function can be considered as a constant and its influence can be ignored in the subsequent derivation. c is the speed of light. np (t k ) is the instantaneous slant range history between the pth target in the nth imaging area and the radar, expressed as:

[0100]

[0101] Among them, x n ' 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, according to the echo signal of the pth target in the nth imaging area The total echo signal of all targets in the nth imaging area can be obtained as:

[0103]

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

[0105] Then, the total echo signal received by the radar can be obtained according to the total echo signal of each imaging area, which is expressed as:

[0106]

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

[0108] S2: Design the radar's pulse repetition frequency and synthetic aperture time based on the characteristics of the radar system.

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

[0110]

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

[0112]

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

[0114] Then, the relevant Doppler parameters of the pth target in the nth imaging area can be obtained as:

[0115]

[0116] Among them, f dc_np represents the Doppler frequency, f dr_np represents the Doppler modulation frequency, Indicates wavelength.

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

[0118]

[0119] Among them, θ 3dB is the 3dB beamwidth of the radar in azimuth.

[0120] After the azimuth deramp processing is used, the corresponding target will have a certain residual modulation bandwidth in the edge area because the azimuth deramp processing cannot be completely matched. However, this residual modulation bandwidth is generally small and can be ignored. Therefore, the Doppler bandwidth B of the nth imaging area at this time is subn It can be approximated as:

[0121] B subn =Δf dcn

[0122] Considering the single wide beam system of 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] in, represents the angle of the middle sub-pulse, is the wavelength of the intermediate sub-pulse.

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

[0128] BPA =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 at this time:

[0131]

[0132] It can be seen from the above formula that when a wide beam is used to illuminate the same large azimuth width under the 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, there are differences in the Doppler support areas corresponding to the echo signals of different sub-scenes, 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 for 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 approximately 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 that there is no ambiguity in each sub-scene, and 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). At this time, 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 in 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 smaller 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 the moment, p = 1, 2, 3...P, P represents the total number of targets in 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 of the beamforming system, the center resolution is used to represent the resolution of the entire sub-scene at this time), and the second constraint indicates that the azimuth resolution of each sub-scene must meet the system requirements. Radar system design is a complex process. The present invention mainly exemplifies several key indicators and design processes of the sub-pulse system.

[0138] S3: Based on the pulse repetition frequency and synthetic aperture time of the radar, the total echo signal received by the radar is processed by wide-band spotlight imaging 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, perform conjugate multiplication of the echo down-conversion function and the total echo signal received by the radar, and obtain a down-conversion signal.

[0141] Specifically, when the radar receives the echo signal, the echoes of different sub-pulses are mixed together in the time domain, and the imaging geometric relationship of different imaging areas is obviously different, so the aliased echoes of different imaging areas will cause image aliasing, and thus high-resolution target imaging cannot be achieved. In order to separate the echo signals of different sub-pulses, different range frequency regions are assigned to different sub-pulses when the sub-pulses are emitted. To this end, the corresponding echo down-conversion function is constructed:

[0142]

[0143] Then, the echo down-conversion function The total echo signal received by the 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 bandpass filter, and use the bandpass filter to separate the down-converted signal to obtain separated echo data of each imaging area.

[0147] This embodiment constructs a bandpass filter near the range frequency domain:

[0148]

[0149] Among them, f r Indicates the distance frequency of the echo signal, B sp Indicates the bandwidth of the radar.

[0150] Then, the down-converted signal After conversion to the range frequency domain, the bandpass filter H fliter (f r ) and then converted to the range-time domain to obtain the echo data of the separated nth imaging area. It is expressed as:

[0151]

[0152] Similarly, different bandpass filters can be constructed and the aliased signals (ie) can be processed respectively, so as to achieve separation of echo data of different imaging areas.

[0153] S3.3: Perform range-transformation on the separated echo data of each imaging region to obtain the echo data of each imaging region after range-transformation.

[0154] Since there is a coupling relationship between the sub-pulses and different imaging areas (sub-scenes), and there is a fixed time delay between different sub-pulses. This time delay will cause the images of different imaging areas to have range deviations. For this reason, the echo data is further separated and range registration is performed. Using the principle of stationary phase, the separated echo data is subjected to range Fourier transformation to obtain the echo data of the nth imaging area after the range Fourier transformation:

[0155]

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

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

[0158] From the above formula, we can see that at this time f r With Δτ n There is a certain coupling, which leads to envelope errors in the echo data of different imaging areas. For this reason, this step constructs a correction function in the range frequency domain, and its expression is:

[0159]

[0160] Convert the distance to the echo data Y after Fourier transformation n (f r ,t k ) and the correction function H match (f r ) and then converted into the range-time domain to obtain the echo data of the nth imaging area in the range-time domain:

[0161]

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

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

[0164]

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

[0166]

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

[0168] S3.6: Using the center of each imaging area as a reference, construct a corresponding azimuth deramp function to process the echo data, and obtain the corresponding BP imaging result of each imaging area.

[0169] Due to the existence of motion errors and other factors in the actual imaging process, this embodiment selects the classic azimuth deramp back projection algorithm (BP) for imaging processing. The center of each imaging area is used as a reference to construct the corresponding azimuth deramp function to process the echo data, and the echo data after deramp is obtained as follows: Then, the corresponding BP imaging result of the nth imaging area is obtained, which is expressed as follows:

[0170]

[0171] Among them, T a represents the synthetic aperture time, Express fast time The echo data after deramp.

[0172] S3.7: According to the corresponding BP imaging result of each imaging area, a beam image of each imaging area is obtained, thereby obtaining a complete image with a large azimuth imaging width.

[0173] Since the calibration of each sub-scene of the spotlight 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 with a large azimuth imaging width can be obtained.

[0174] Furthermore, in order to verify the effectiveness of the sub-pulse diversity-based time-domain azimuth wide-band spotlight imaging method proposed in the present invention, the following is a comparative experiment of point targets and extended targets under the sub-pulse system and the traditional phased array system. Some 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. Furthermore, this experiment considers the case of three sub-pulses, and each sub-pulse is distributed in three adjacent areas in azimuth, corresponding to an effective imaging range of about 3.6km. The azimuth imaging positions of the sub-scenes are set to (-1838.3, -611.2), (-611.2, 611.2), and (611.2, 1838.3)m, respectively. And 9 point targets are arranged with the center of each sub-scene as the center, and the adjacent point targets in the same scene are spaced 410m in azimuth and 166m in range. After deRamp processing, the maximum Doppler bandwidth of a single sub-scene is about 528Hz. For this reason, according to the PRF design constraints, the corresponding system PRF is set to 550Hz. The Doppler bandwidth of the corresponding scene covering three sub-scenes with a single beam is calculated to be 1525.4Hz. At this time, when the PRF is selected to be greater than this value, aliasing-free imaging can be achieved, and the sub-pulse imaging results are obtained as shown in the figure. Figure 3 As shown, Figure 3 (a) shows the range-frequency domain distribution of the transmitted sub-pulse signal. Figure 3 It can be seen in (a) that at this time, different sub-pulses are located in different distance frequency domains; Figure 3 (b) and Figure 3 (c) shows the signals of the first area after the aliased echo signal and the sub-pulse are separated. The horizontal axis represents the azimuth upward time (radar platform movement time), and the vertical axis represents the range gate.

[0176] Table 1 System simulation parameters

[0177]

[0178]

[0179] After the three regional echoes are separated, the images of each region are processed separately. The imaging results are as follows: Figure 4 As shown, Figure 4 (a)(c)(e) respectively represent the range-azimuth information of 9 target points in three areas. The horizontal axis represents the azimuth gate, and the vertical axis represents the range gate. Figure 4 (b)(c)(d) represent the imaging results of radar in azimuth sampling and range sampling respectively. The horizontal axis represents azimuth sampling and the vertical axis represents range sampling. Figure 4 (a)(b) are the results of the first imaging area. Figure 4 (c)(d) are the results of the second imaging area. Figure 4 (e)(f) are the results of the third imaging area. Figure 4 It can be seen that although the PRF of the radar system is low at this time, the sub-pulse separation is equivalent to the Doppler deambiguation processing, and good imaging is achieved in all three areas at this time, which verifies the azimuth-free wide-band imaging capability of the sub-pulse system. The imaging quality of the point targets in the three scenes in the figure is evaluated, and the corresponding imaging indicators meet the system requirements. Then, using the geometric information of the sub-scenes, the sub-scene imaging is registered and spliced ​​to obtain the final azimuth wide-band spotlight imaging results, as shown in Figure 1. Figure 5 shown.

[0180] In order to further compare the processing results of the method proposed in the present invention with those of the traditional phased array method, the imaging results of the traditional phased array with the same azimuth imaging width (the same azimuth beam coverage cannot be achieved using the same array, and the number of array elements is reduced in this case to expand the beam width) are given below. Figure 6 is the result of deramp processing in azimuth, where the horizontal axis represents the azimuth frequency. It can be seen from the figure that since the system PRF is low at this time, Doppler aliasing occurs in echo signals in different regions. Figure 7 The corresponding wide-band imaging results are given, where the red circle is the target imaging result and the black dotted line is the aliasing energy. Since the radar cannot separate the echoes from different regions at this time, the Doppler aliasing phenomenon still exists during the imaging process, which reduces the corresponding imaging quality.

[0181] This experiment further gives the simulation results of the extended target (scene target), in which only the 200*200m scene target is arranged in the central area of ​​each sub-scene. The corresponding theoretical images of different areas are as follows: Figure 8 shown.

[0182] Fig. 9 This is the result of traditional phased array processing. At this time, due to the low system PRF, the echo data in the three regions are aliased, and good imaging cannot be achieved. Fig.10This is the imaging result processed by the sub-pulse wide-width imaging method. Although there is azimuth Doppler ambiguity in the three regions of the original echo due to the low PRF, after the range frequency bandpass filtering, the echoes of different sub-scenes are well separated. Then, each sub-scene is BP imaged separately and spliced ​​according to the imaging grid to obtain the final azimuth wide-width imaging result.

[0183] The present invention proposes a time-domain azimuth wide-band focusing imaging method based on sub-pulse diversity. By dividing a single pulse of a traditional radar into multiple sub-pulses and pointing different sub-pulses to different azimuth positions, the ability of wide beam coverage in azimuth is achieved, thereby enabling the radar to have wide-band focusing capability in azimuth; the application of sub-pulse diversity technology in the imaging system is effectively realized, and the problem of high computational complexity and loss of range resolution at the edge of the scene in the method of applying frequency scanning radar to the azimuth array is solved. The present invention separates different sub-pulses from each other in the range frequency domain, and then designs different bandpass filters at the receiving end, so as to realize echoes of different sub-scenes, and equivalently performs azimuth Doppler deambiguation. Compared with traditional phased array radars, under the same array surface and system pulse repetition frequency, the azimuth imaging width is expanded by several times of the number of sub-pulses. After processing with the range frequency bandpass filter, the present invention separates the echoes of different sub-scenes well, splices them according to the imaging grid, and obtains the final azimuth wide-band imaging result.

[0184] The traditional phased array beamforming system is limited by the azimuth beam width of the radar, and can only achieve the azimuth imaging width of the main lobe single beam width; the sliding beamforming system can expand part of the azimuth imaging width, but loses the azimuth imaging resolution; the frequency scanning array radar achieves wide coverage in azimuth by introducing time delay between array elements, and then achieves wide-width beamforming imaging in azimuth, but this method requires the design of corresponding bandpass filters for different range gates to achieve the separation of data in different ranges, and the algorithm operation complexity is very high. And because the target range frequency is modulated by the beam pattern, the beam coverage range that can be achieved by the frequency scanning radar is smaller than the beam coverage range of the present invention. The method of the present invention achieves the expansion of the azimuth imaging width by several times of the sub-pulses without losing the azimuth imaging resolution through sub-pulse diversity design. It only needs to perform bandpass filtering for the number of sub-pulses in the range frequency domain to separate the unambiguous echoes in different azimuths, and the algorithm complexity is low. Moreover, the process of sub-pulse separation can be equivalent to Doppler deambiguation. Therefore, when realizing the conventional phased array imaging width, after applying the method of the present invention, the required system pulse repetition frequency is reduced by several times the number of sub-pulses compared with the conventional phased array radar.

[0185] In the several embodiments provided by the present invention, it should be understood that the apparatus and method disclosed by the present invention can be implemented in other ways. For example, the apparatus embodiments described above are only illustrative, for example, the division of the modules is only a logical function division, and there may be other division methods in actual implementation, 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 functional module in each embodiment of the present invention may be integrated into one processing module, or each module may exist physically separately, or two or more modules may be integrated into one module. The above-mentioned integrated module may be implemented in the form of hardware or in the form of hardware plus software functional modules.

[0187] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

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-width beamforming signal model, point 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 according to 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.

2. The method for time-domain azimuth wide-band spotlight imaging based on sub-pulse diversity according to claim 1, characterized in that: The S1 includes: S1.1: Obtain the 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 operating in a spotlight imaging mode; S1.2: Modulate different sub-pulses to different range frequency regions to obtain the transmission signal of each sub-pulse of the radar S1.3: directing different sub-pulse beams to different imaging areas where beamforming is desired, respectively, to obtain a total echo signal of each imaging area; S1.4: Obtain a total echo signal received by the radar according to the total echo signal of each imaging area.

3. The method for time-domain azimuth wide-band spotlight imaging based on sub-pulse diversity according to claim 2, characterized in that: The S1.2 includes: Modulate different sub-pulses to different distance frequency regions to obtain the frequency offset Δf between the nth sub-pulse and the first sub-pulse n : Where Δf represents the frequency increment between two adjacent sub-pulses; According to the frequency shift Δf n Get the transmitted signal of the nth sub-pulse of the radar Among them, f c represents the carrier frequency of the radar, j represents a complex unit, Δτ n =(n-1)T Δ represents the transmission delay between the nth sub-pulse and the first sub-pulse, T Δ Indicates the time interval between sub-pulses.

4. The method for time-domain azimuth wide-band spotlight imaging based on sub-pulse diversity according to claim 3, characterized in that: The S1.3 includes: Assuming that the nth sub-pulse points to the nth imaging area, the echo signal of the pth target in the nth imaging area is obtained. Among them, σ np is the complex scattering coefficient of the pth target, For fast time, t k is the azimuth slow time, w a (t k ) is the target azimuth window function, c is the speed of light, Rnp(tk) is the instantaneous slant range history between the p-th target in the n-th imaging area and the radar; According to the echo signal of the pth target in the nth imaging area Obtain the total echo signals of all targets in the nth imaging area.

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

6. The method for time-domain azimuth wide-band spotlight imaging based on sub-pulse diversity according to claim 1, characterized in that: The synthetic aperture time T of the radar a The constraints that must be met are: 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 represents the frequency offset between the nth sub-pulse and the first sub-pulse, f c represents the carrier frequency of the radar, 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 the time, p = 1, 2, 3…P, P represents the total number of targets in the nth scene.

7. The method for time-domain azimuth wide-band spotlight imaging based on sub-pulse diversity according to claim 1, characterized in that: The S3 includes: S3.1: construct an echo down-conversion function, and conjugate-multiply the echo down-conversion function with 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 range Fourier transform on the separated echo data of each imaging region to obtain echo data of each imaging region after range Fourier transform; 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: constructing a corresponding azimuth deramp function with the center of each imaging area as a reference to process the echo data, and obtaining a corresponding BP imaging result of each imaging area; S3.7: According to the corresponding BP imaging result of each imaging area, a beam image of each imaging area is obtained, thereby obtaining a complete image with a large azimuth imaging width.

8. The method for time-domain azimuth wide-band spotlight imaging based on sub-pulse diversity according to claim 7, characterized in that: The S3.2 includes: Construct a bandpass filter H near the range frequency domain fliter (f r ): Among them, f r Indicates the distance frequency of the echo signal, B sp represents the bandwidth of the radar; Down-convert the signal After conversion to the range frequency domain and the bandpass filter H fliter (f r ) and then converted to the range-time domain to obtain the echo data after separation of the nth imaging area. Where 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, represents fast time, c is the speed of light, R np (t k ) is the instantaneous slant range history between the pth target in the nth imaging area and the radar, f c represents the carrier frequency of the radar, j represents the complex unit, Δf n represents the frequency offset between the nth sub-pulse and the first sub-pulse, Δτ n represents the transmission delay between the nth sub-pulse and the first sub-pulse, The baseband sub-pulse signal representing the nth sub-pulse of the radar; Different bandpass filters are constructed respectively and the down-converted signals are Processing is performed to separate the echo data of different imaging areas.

9. The method for time-domain azimuth wide-band spotlight imaging based on sub-pulse diversity according to claim 8, characterized in that: The S3.4 includes: Construct the correction function H in the range-frequency domain match (f r ): Convert the distance to the echo data Y after Fourier transformation n (f r ,t k ) and the correction function H match (f r ) and then converted into the range-time domain to obtain the echo data of the nth imaging area in the range-time domain:

10. The method for time-domain azimuth wide-band spotlight imaging based on sub-pulse diversity according to claim 9, characterized in that: The S3.6 includes: The corresponding azimuth deramp function is constructed with the center of the nth imaging area as the reference. The echo data of the nth imaging area in the range time domain is processed using the azimuth deramp back-projection algorithm. The echo data after deramp is obtained as Get the corresponding BP imaging result of the nth imaging area: Among them, T a represents the synthetic aperture time, Express fast time The echo data after deramp.

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