Terahertz video synthetic aperture radar system based on vortex electromagnetic waves and imaging method

Abstract

The invention relates to a terahertz video synthetic aperture radar system based on vortex electromagnetic waves and an imaging method. The system comprises a uniform concentric circle array antenna, a beam control network, an intermediate frequency filter amplifier and an upper computer, and the upper computer is connected with the beam control network to control the uniform concentric circle array antenna to emit equal-amplitude phase-modulated electromagnetic waves to synthesize vortex electromagnetic waves to irradiate a target, receive echoes, carry out intermediate-frequency filtering and amplification, return the echoes to the upper computer, and carry out signal processing through an improved imaging algorithm. Based on phase modal information characteristics of vortex electromagnetic waves, a vortex wave radar imaging model of a multi-transmitting and single-receiving array is established in combination with synthetic aperture radar slant-range imaging geometry; by combining the design of the time-varying orbital angular momentum, the corresponding processing method is improved, and the azimuth modulation rate is increased to bring additional frame rate; according to the imaging system, vortex electromagnetic waves are combined with a traditional video synthetic aperture radar technology, and the frame rate can be improved under the condition that the resolution ratio is kept.

Description

TECHNICAL FIELD

[0001] The present application relates to a radar signal processing system and method, in particular to a terahertz video synthetic aperture radar system and imaging method based on vortex electromagnetic waves. BACKGROUND

[0002] The all-weather high-frame-rate imaging technology of video synthetic aperture radar (ViSAR) has attracted widespread attention in recent years. Traditional ViSAR realizes two-dimensional imaging based on the distance-doppler principle of plane waves: in the azimuth direction, a virtual aperture is formed by the radar platform movement to obtain high-resolution information, and in the range direction, the large time-bandwidth product characteristics of linear frequency modulation signals are used to realize the resolution capability. However, the high-resolution imaging in the azimuth direction requires a long synthetic aperture time, resulting in an inherent contradiction between short observation time and high azimuth resolution. Although the carrier frequency can be increased to increase the azimuth frequency to shorten the synthetic aperture time, this method still has technical bottlenecks in the trade-off between frame rate and resolution.

[0003] In recent years, vortex electromagnetic waves carrying orbital angular momentum have unique physical characteristics that provide new degrees of freedom for information modulation and have been widely studied. With its free information modulation capability and unique information acquisition characteristics, it is widely used in wireless communication, quantum state manipulation, and rotating object detection, and has achieved important research results. Vortex electromagnetic waves couple the mode and target azimuth angle information into the azimuth angle term of the return signal. It is expected to realize two-dimensional imaging with higher frame rate under the same resolution as traditional ViSAR.

[0004] Many current vortex-based radar researches mainly focus on staring and improving resolution, showing its advantages over traditional plane wave imaging methods and potential. The research of vortex electromagnetic waves in the field of ViSAR is still in its infancy, and there are many problems to be solved in imaging principles, imaging models and methods. In view of this, the present application proposes a vortex ViSAR system based on time-varying mode design, which can realize higher frame rate under the same resolution as traditional ViSAR. SUMMARY

[0005] The purpose of the present application is to provide a terahertz video synthetic aperture radar system and imaging method based on vortex electromagnetic waves, which integrates the ViSAR system and imaging method of vortex electromagnetic wave technology to solve the problem of difficult to balance frame rate and resolution in the prior art, and realize high-frame-rate and high-resolution radar imaging.

[0006] Technical scheme: The vortex electromagnetic wave based terahertz video synthetic aperture radar system, namely the vortex electromagnetic wave based terahertz ViSAR system, comprises a uniform concentric circle multi-transmitting single-receiving antenna array, a phase shifter, a digital analog converter, a power divider, a power amplifier, a coupler, a signal source, a beam control system, an upper computer, an internal calibration module, an analog-digital converter, an intermediate frequency filter amplifier and a low-noise amplifier.

[0007] The vortex electromagnetic wave terahertz ViSAR system is based on an imaging model established based on a vortex electromagnetic wave imaging model, and the mode of the transmitted signal is designed;

[0008] The mode is introduced to change with the slow time l η The signal transmission mode is |ξ|·η, wherein ξ is the time-varying rate of the orbital angular momentum mode.

[0009] The vortex wave time-varying mode introduces a frame rate, and the Taylor expansion approximation condition needs to be met to avoid the influence of high-order terms.

[0010] The time-varying order of the mode is controlled in different frames in a frame division manner, so as to shorten the synthetic aperture length and realize high frame rate.

[0011] The signal source generates a linear frequency modulation signal with large time-width and bandwidth product.

[0012] The coupler is electrically connected with the signal source and is used for extracting the signal from the signal source as a reference to correct the phase and amplitude.

[0013] The power amplifier is electrically connected with the coupler and is used for amplifying the low-power radio frequency signal output from the signal source.

[0014] The power divider is electrically connected with the power amplifier and divides the input radio frequency signal in proportion to multiple channels to ensure that each antenna obtains the same power.

[0015] The beam control module is electrically connected with the digital analog conversion module and the upper computer, and dynamically adjusts the parameters of the current transmitting signal phase shifter according to the instruction of the upper computer.

[0016] The digital analog conversion module is electrically connected with the beam control module and the power divider, and converts the digital signal into an analog signal.

[0017] The phase shifter is electrically connected with the digital analog module and applies a corresponding phase difference to the signal transmitted by each antenna.

[0018] The even concentric circle multi-transmit single-receive antenna array is electrically connected with a phase shifter and a low-noise amplifier, transmits electromagnetic waves through a multi-antenna array mode, and receives vortex back waves using a single antenna, synthesizes vortex electromagnetic waves carrying different orbital angular momentum modes, and places the array antenna at an angle of 45° with the Z axis, so that the radar works in a forward-looking mode and forms a strip-shaped observation area.

[0019] The low-noise amplifier is electrically connected with the antenna array, amplifies weak signals at the front end of the receiving link, and reduces noise.

[0020] The intermediate frequency filter amplifier is electrically connected with the low-noise amplifier, filters and amplifies the received signals, and improves the signal-to-noise ratio.

[0021] The analog-to-digital converter is electrically connected and converts the received analog signals into digital signals for subsequent processing.

[0022] The internal calibration module is electrically connected with the analog-to-digital conversion module, the coupler and the upper computer, and compensates for errors in system parameters.

[0023] The upper computer is electrically connected with the internal calibration module, and performs imaging processing on the signals after the compensation error and the improved algorithm.

[0024] Further, the signal processing method in the upper computer module: compensate the Bessel term and the Taylor expansion linear term in the slow time domain; due to the introduction of an additional frequency modulation rate by the vortex wave time-varying design, the traditional matched filter is invalid, and the improved range migration correction is used, and the correction result is filtered and processed by a new filter.

[0025] Further, the Bessel function eliminates the amplitude modulation of the azimuth information by the center point method.

[0026] Further, the imaging method of the vortex electromagnetic wave-based terahertz video synthetic aperture radar imaging system comprises the following steps:

[0027] (1) Echo data: the echo data is obtained through formula (1) to obtain vortex electromagnetic echoes with additional azimuth items and Bessel function items;

[0028]

[0029] t is the fast time, η represents the slow time, λ is the center wavelength of the transmitted signal, c represents the speed of light, k is the number of beams, represents the array radius corresponding to the number of transmitted different orbital angular momentum modes, σ is the backscattering coefficient, ω r [·] and ω a [·] are the distance envelope and azimuth envelope, respectively, represents the first Bessel function, l ηdenotes the mode varying with slow time, denotes the instantaneous azimuth angle, θ denotes the instantaneous elevation angle, η T denotes the slow time of target position, R denotes the instantaneous slant range of target, j denotes the imaginary unit.

[0030] (2) Range matched filter: the same range matched filter as the traditional matched filter is used for range direction matched filtering;

[0031] (3) Linear term elimination: there is a first-order linear term after Taylor expansion of the azimuth angle term introduced by the vortex electromagnetic wave, which is eliminated by the method of conjugate phase factor correction;

[0032] (4) Bessel function elimination and normalization: the azimuth direction center point approximation method is used to eliminate the Bessel amplitude modulation term introduced by the vortex electromagnetic wave, and the distance direction normalization is performed to avoid the residual modulation;

[0033] (5) Slow time domain Fourier transform: the signal is transformed to the slow time frequency domain by Fourier transform;

[0034] (6) Range migration correction: due to the influence of the newly introduced azimuth direction modulation frequency, the range migration correction is correspondingly changed to avoid errors;

[0035] (7) Azimuth direction matched filter: after introducing the new modulation frequency, the traditional matched filter is invalid, and the new azimuth direction matched filter uses the traditional modulation frequency corresponding to each range unit plus the additional modulation frequency brought by the introduction of vortex to eliminate the phase;

[0036] (8) Slow time domain inverse Fourier transform: after matched filtering, the inverse Fourier transform two-dimensional time domain result is performed;

[0037] (9) ViSAR two-dimensional imaging: the time domain result is imaged.

[0038] Further, the ViSAR two-dimensional imaging frame rate is expressed as formula (11),

[0039]

[0040] A transmission signal mode, according to the Taylor expansion approximation condition needed to be met in order to ignore the influence of high-order terms in the time-varying mode, the transmission signal traverses all modes in different frame transmission modes, and the synthetic aperture length is shortened; the transmission signal is a linear frequency modulation signal, and different orbital angular momentum mode number functions varying with slow time are designed in different frames: l η = |ξ|·η, wherein ξ is the time-varying rate of the orbital angular momentum mode, and η represents the slow time.

[0041] The improved algorithm after imaging processing includes:

[0042] First, the distance compression consistent with the traditional ViSAR is adopted; next, the linear term of the Taylor expansion of the azimuth angle term introduced by the vortex electromagnetic wave is removed by the conjugate cancellation method to remove the influence of the linear deviation; then, the Bessel function term introduced by the vortex electromagnetic wave is weakened by the center point approximation method to weaken the amplitude modulation of the azimuth information; finally, the azimuth dimension uses the mode orthogonal characteristic to construct a new azimuth frequency modulation, and designs the corresponding improved matching filter and distance migration correction unit.

[0043] The beneficial effects obtained by adopting the present application are as follows: the present application combines the vortex electromagnetic wave with the traditional ViSAR imaging technology, and through the design of the vortex electromagnetic wave orbital angular momentum mode number transmission and the re-design and optimization of the system, the frame rate is effectively improved and two-dimensional imaging is obtained under the condition of the same resolution as the traditional ViSAR, which can provide reference and reference for the development of the new system and high frame rate imaging technology of the vortex electromagnetic wave-based ViSAR imaging. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 System composition block diagram of the present application;

[0045] Figure 2 System model geometric relationship diagram;

[0046] Figure 3 System frame time-varying design diagram;

[0047] Figure 4 Improved RD algorithm flow chart. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical scheme and advantages of the present application more clear, the technical scheme of the present application will be further described below.

[0049] The present application will be described in detail below in combination with specific embodiments.

[0050] The present application designs a ViSAR system and imaging method based on vortex electromagnetic wave, Figure 1 The system diagram shows the ViSAR system block diagram designed in the present application.

[0051] The establishment of the imaging geometric scene is as shown in the Figure 2 The radar antenna is located in the XOY β plane of the Cartesian coordinate system, and flies at a speed v at a height H from the ground. The X-axis direction is the motion direction of the airborne platform, the Z β axis is the normal direction of the concentric circle array, and the Y β axis is determined by the right-hand rule. Another coordinate system O-XYZ in the figure is consistent with the coordinate axes in the traditional ViSAR imaging model. In the O-XYZ coordinate system, Pr = (x, 0, 0) and P T =(x T y T , z T Let R represent the radar's position and the coordinates of an arbitrary ideal target point in the observation scene, respectively. The instantaneous slant range between the radar and the target is R. η The shortest slant distance is R T η represents the slow-time variable (the time variable related to radar motion), η T Represents the target's location coordinates x T The corresponding slow time interval, with the subscript T indicating the target's marked position; pitch angle θ η O-XY β Z β Target and Z in coordinate system β Angle along the positive axis; azimuth It is the instantaneous slant distance R η In XOY β The angle between the projection on the plane and the positive X-axis.

[0052] During signal transmission, the signal source generates a raw radio frequency signal and inputs it to the coupler; after receiving the signal, the coupler transmits it to the power amplifier and synchronizes it with the internal calibration module; the power amplifier amplifies the power and sends it to the power divider to ensure that the power on each antenna is the same; the host computer sends mode signals to the beam control network and controls the phase difference transmission signal s corresponding to each antenna. t By combining the characteristics of vortex electromagnetic waves with the transmission and reception history of traditional ViSAR signals, the echo s is derived. r . See formula (1) for details

[0053]

[0054] It can be observed that the echo of a vortex electromagnetic wave introduces an additional azimuth term on top of the traditional plane wave. and Bessel function terms t is the fast time, λ is the center wavelength of the transmitted signal, and k is the number of beams. The array radius represents the number of different orbital angular momentum modes emitted, σ is the backscattering coefficient, and ω is the array radius. r [·] and ω a [·] represents the range envelope and the azimuth envelope, respectively. θ represents the instantaneous azimuth angle, and θ represents the instantaneous pitch angle. Figure 2 θ T This represents the angle corresponding to the shortest slant distance to the target in the O-XYZ range.

[0055] According to the imaging model and echo signal, it is observed that the additional term introduced by the vortex electromagnetic wave designed with time-varying mode modulates the phase of the echo, and the Bessel function modulates the amplitude of the echo.

[0056]

[0057] y β represents the projection of the distance between the target point and the beam center in the Y β axis in the O-XY β coordinate system. β axis in the O-XY η coordinate system. At this time, the time-varying design of the vortex wave mode l is introduced as l T = | ξ | ( η - η T ), and the first-order term is expanded to the second-order term. To avoid the modulation effect of the high-order term, the approximate condition needs to be met in the Taylor expansion:

[0058]

[0059] wherein x T = v η s , and x = v η. To solve this problem, the present application introduces the frame division idea. The specific method is to divide the complete imaging time into multiple smaller synthetic aperture time periods, and then combine the imaging results after independent imaging of each period. The scene width of each frame is equivalent to the length of a synthetic aperture. As shown in Fig. Figure 3 , in the imaging process of the first frame data, the aircraft needs to fly twice the length of the synthetic aperture 2L a . At this time, the vortex wave mode linearly changes from the -L mode to the +L mode with the slow time. In the imaging process of the second frame, the vortex mode starts from the 0 mode, first linearly changes to the +L mode, and then gradually decreases from the -L mode to the 0 mode. Similarly, the mode design of each frame meets the requirement of linear change with the slow time in a single frame. After completing the imaging of each frame, the data is adjusted to form a complete regional imaging result.

[0060] After the center red single antenna receives the signal and performs low-noise and intermediate frequency filtering processing, the improved range Doppler algorithm of the present application is used for further imaging processing, and the algorithm flow chart is shown in Fig. Figure 4 . First, the range matching filter is used. The expression of the echo after range compression is obtained as shown in equation (1);

[0061]

[0062] wherein the constant coefficient K0 is the first-order frequency after Taylor expansion, K a is the traditional azimuth frequency introduced by the synthetic aperture, and K l is the new frequency introduced by the time-varying mode design. They can be expressed as:

[0063]

[0064] At this time, the azimuth frequency is K a raised to a new azimuth frequency K A = K a + K l This design significantly shortens the synthetic aperture time while maintaining the same resolution, thereby improving the frame rate of the ViSAR.

[0065] The Bessel function and linear term in equation (4) will affect the imaging results, and the modulation effect needs to be eliminated to improve the imaging quality. This part is introduced below. Compensation of both in the slow time domain, respectively, in the slow time time domain multiplied by the conjugate phase compensation factor h bessel and h linear , represents the instantaneous slant range of the target, η i represents the i-th sampling point of the slow time variable η.

[0066] h linear = exp{-jπK0(η-η T )) (6)

[0067] After compensating the Bessel function, there is a certain degree of elimination error, and the distance normalization method is used for compensation and elimination.

[0068]

[0069] The azimuth is Fourier transformed, and then the range migration correction is corrected using the azimuth joint frequency K A = K l + K a . In the embodiment, the range migration correction needs to be dynamically adjusted according to the change of the projection distance, so (8) is used to correct the normalized echo.

[0070]

[0071] In the formula, R t is the distance unit that changes with the fast time. In the correction process, the current range migration is recalculated according to the projection distance of each distance unit, thereby realizing more accurate correction. f η represents the frequency domain variable corresponding to the slow time variable η.

[0072] The pulse compression technology is used to realize the acquisition of target azimuth high-resolution information, specifically: the new azimuth matching filter uses K l corresponding to the vortex electromagnetic wave introduced in each distance unit.The traditional frequency modulation rate K a The new azimuth direction frequency modulation rate K A The matched filter is performed in the frequency domain using formula (9).

[0073]

[0074] After the azimuth direction compression is completed, inverse Fourier transform is performed to convert the data to the time domain.

[0075]

[0076] σ B Constant, represents the envelope after the range direction pulse compression, p a (η-η T ) represents the envelope after the azimuth direction pulse compression. Thus, the information acquisition in the range direction and the azimuth direction is completed.

[0077] According to the frame rate expression (11) of the ViSAR based on the vortex electromagnetic wave provided in the application, compared with the traditional ViSAR frame rate The first term corresponds to the traditional frame rate, and the second term represents the frame rate improvement brought by the vortex wave.

[0078]

[0079] It is worth noting that this improvement is irrelevant to the radar carrier frequency. In the case that other parameters are fixed, under the Taylor expansion approximation condition, the projection distance y β will increase with the decrease of the carrier frequency, thereby causing the frame rate improvement to decrease. With the increase of the time-varying rate ξ, the proportion of the azimuth angle term introduced by the vortex wave gradually increases, further affecting the frame rate improvement.

[0080] In summary, the application provides a terahertz video synthetic aperture radar system and an imaging method based on the vortex electromagnetic wave. At present, the improvement of the frame rate mainly focuses on the method of improving the radar carrier frequency, and the application attempts to introduce a new beam system to improve this situation; the influence of the Taylor expansion approximation condition on the system is considered, and the system is improved accordingly; the approximation of the Bessel function is more effective, and for the additional frequency modulation rate introduced by the vortex electromagnetic wave, the azimuth direction matched filter is improved according to the characteristics that the change of different distance units is intensified; the system has the ability to obtain a higher imaging frame rate under the same imaging conditions.

[0081] The above merely describes the preferred embodiments of the present application and does not limit the present application in any way. Any person skilled in the art can make any form of equivalent replacement or modification to the technical solutions and technical contents disclosed by the present application without departing from the scope of the technical solutions of the present application, and such changes still belong to the protection scope of the present application.

Claims

1. A terahertz video synthetic aperture radar imaging system based on vortex electromagnetic waves, characterized in that, include: Signal source 7 is used to generate a linear frequency modulated signal with a large time-width-bandwidth product; Coupler 6, which is electrically connected to signal source 7, is used to extract a signal from the signal source as a reference to correct the phase and amplitude; Power amplifier 5, which is electrically connected to coupler 6, is used to amplify the low-power radio frequency signal output from signal source 7; The power divider 4, which is electrically connected to the power amplifier 5, distributes the input radio frequency signal proportionally to the multiple channels to ensure that each antenna receives the same power. The beam control module 8 is electrically connected to the digital-to-analog converter module 3 and the host computer 9, and dynamically adjusts the parameters of the current transmit signal phase shifter 2 according to the instructions of the host computer 9. The digital-to-analog conversion module 3, which is electrically connected to the beam control module 8 and the power divider 4, converts digital signals into analog signals; Phase shifter 2, which is electrically connected to digital-analog module 3, applies a corresponding phase difference to the signal transmitted by each antenna; A uniform concentric circle multi-transmitter single-receiver antenna array 1 is electrically connected to a phase shifter 2 and a low noise amplifier 13. It synthesizes vortex electromagnetic waves through a multi-antenna array and uses a single antenna to receive the vortex echo. The uniform concentric circle multi-transmit single-receive antenna array 1: uses a concentric circle array to effectively limit the beam angle after the vortex electromagnetic wave is generated, and avoids the beam angle from widening as the mode l increases; The low noise amplifier 13 is electrically connected to a uniform concentric circle multi-transmit single-receive antenna array 1, which amplifies weak signals at the front end of the receiving link and reduces noise. The intermediate frequency filter amplifier 12 is electrically connected to the low noise amplifier 13 to perform intermediate frequency filtering and amplification on the received signal, thereby improving the signal-to-noise ratio. The analog-to-digital converter 11, which is electrically connected to 12, converts the received analog signal into a digital signal for easier subsequent processing. The internal calibration module 10 is electrically connected to the analog-to-digital conversion module 11, the coupler 6, and the host computer 9 to correct and compensate for errors in the system parameters. The host computer 9 is electrically connected to the internal calibration module 10, which performs imaging processing on the signal after error compensation using an improved algorithm.

2. The terahertz video synthetic aperture radar imaging system based on vortex electromagnetic waves according to claim 1, characterized in that, The signal processing method in the host computer (9) module is as follows: the Bessel term and the Taylor expansion linear term are compensated in the slow time domain; since the vortex wave time-varying design introduces an additional frequency modulation, the traditional matched filter fails, and the improved distance migration correction is used to filter the correction result through a new filter.

3. The terahertz video synthetic aperture radar imaging system based on vortex electromagnetic waves according to claim 2, characterized in that, Bessel functions eliminate amplitude modulation of positional information by using the center point method.

4. An imaging method based on the terahertz video synthetic aperture radar imaging system based on vortex electromagnetic waves as described in any one of claims 1-3, characterized in that, Includes the following steps: (1) Echo data: The echo data is obtained by formula (1) to obtain the vortex electromagnetic echo with additional azimuth term and Bessel function term; t represents fast time, η represents slow time, λ represents the center wavelength of the transmitted signal, c represents the speed of light, and k represents the number of beams. The array radius represents the number of different orbital angular momentum modes emitted, j is the backscattering coefficient, and ω is the array radius. r [·] and ω a [·] represents the range envelope and the azimuth envelope, respectively. Let l represent the first kind of Bessel function. η This represents a mode that changes with slow time. η represents the instantaneous azimuth angle, θ represents the instantaneous elevation angle, and η represents the instantaneous pitch angle. T The slow time represents the target's position, R represents the target's instantaneous slant range, and j represents the imaginary unit. (2) Range matched filter: The same range matched filter as the traditional matched filter is used for range-direction matched filtering; (3) Linear term elimination: After Taylor expansion of the azimuth term after the introduction of vortex electromagnetic wave, there is a first-order linear term, which is eliminated by the method of conjugate phase factor correction. (4) Bessel function elimination and normalization: The Bessel amplitude modulation term introduced by the vortex electromagnetic wave is eliminated by the azimuth center point approximation method, and normalization is performed along the range direction to avoid the existence of residual modulation. (5) Slow-time domain Fourier transform: The signal is transformed to the slow-time frequency domain through Fourier transform; (6) Range migration correction: Due to the influence of the newly introduced azimuth tuning frequency, the range migration correction is modified accordingly to avoid errors; (7) Azimuth Matched Filter: After introducing a new frequency modulation rate, the traditional matched filter becomes ineffective. The new azimuth matched filter uses the traditional frequency modulation rate corresponding to each range unit plus the additional frequency modulation rate brought about by the introduction of vortex to perform phase elimination. (8) Slow time domain inverse Fourier transform: The two-dimensional time domain result of inverse Fourier transform after matched filtering; (9) ViSAR two-dimensional imaging: Imaging the time domain results.

5. The imaging method of the terahertz video synthetic aperture radar imaging system based on vortex electromagnetic waves according to claim 4, characterized in that, The frame rate of ViSAR two-dimensional imaging is expressed as in equation (11).

6. A transmission signal mode based on the terahertz video synthetic aperture radar imaging system based on vortex electromagnetic waves as described in any one of claims 1-3, characterized in that: To ignore the influence of higher-order terms in time-varying modes, the Taylor expansion approximation condition must be satisfied. In different frame transmission modes, the transmitted signal traverses all modes, shortening the synthetic aperture length. The transmitted signal is a linear frequency modulated signal, and different functional relationships between the number of orbital angular momentum modes and slow time are designed for different frames: l η =|ξ|·η, where ξ is the time-varying rate of the orbital angular momentum mode and η represents the slow time.

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