PolSAR image modulation method based on polarization conversion surface

Abstract

The invention provides a PolSAR image modulation method based on a polarization conversion surface. The method comprises the following steps: step 1, establishing a polarization conversion surface modulation signal model; step 2, establishing a PolSAR echo modulation model; 3, generating a PolSAR image target scattering center position transformation modulation result; and 4, extracting the target polarization characteristics of the PolSAR image. According to the invention, the application range of the reconfigurable metasurface in the field of radar image modulation is expanded. According to the PolSAR image modulation method based on the polarization conversion surface provided by the invention, modulation effects can be generated on images of four channels of complete polarization, and the generated scattering center position transformation effect has the advantage of flexibility and controllability. According to the invention, the peak amplitude relation of the target scattering center of each channel can be flexibly regulated and controlled, so that the polarization conversion surface of a fixed physical structure can show a plurality of scattering mechanisms, and the modulation of PolSAR image target polarization characteristics is innovatively realized.

Description

Technical Field

[0001] A method for modulating Polarimetric Synthetic Aperture Radar (PolSAR) images based on a polarization conversion surface according to the present invention belongs to the field of radar imaging effects. Specifically, it relates to the field of modulating polarimetric synthetic aperture radar images. Further, it actively changes its own scattering characteristics through a polarization conversion surface, so that the position distribution and polarization characteristics of targets on polarimetric radar images change, and this effect can be controlled by parameters.

Background Art

[0002] Electromagnetic metasurface is an artificial array structure composed of patch or slot elements. It can make electromagnetic waves undergo total reflection or total transmission at specific frequencies, thereby realizing flexible control of the characteristics of electromagnetic waves. Currently, it has been widely used in fields such as electromagnetic shielding, antenna design, and target protection. However, in practical applications, the metasurface structure is fixed and has a single function, making it difficult to meet the application requirements in complex and changeable environments. To solve this problem, reconfigurable metasurfaces have emerged. It integrates elements that can be dynamically regulated, such as PIN diodes or varactor diodes, into the metasurface, so that the on-off of the diode can be controlled by changing the applied excitation, making the impedance characteristics of the material have adaptable variability. This working mechanism greatly improves the application value of traditional electromagnetic metasurfaces and shows great potential in radar, communication, etc.

[0003] Applying excitation signals under different time encodings to the reconfigurable metasurface can control the characteristics of electromagnetic waves. This is the Time-modulated metasurfaces (TMMs). TMMs can have various effects on incident electromagnetic waves, such as absorption / reflection switching, transmission / reflection switching, etc. This effect is reflected in the modulation of the echo amplitude, phase, etc. For Synthetic Aperture Radar (SAR), the influence of TMMs on the echo will cause special transformation effects in the imaging result. For example, under the control of a periodic encoding signal, TMMs can generate multiple false harmonic peaks in the SAR image and can control the positions of the peaks. Under the control of a non-periodic encoding signal, TMMs will cause a defocusing effect in the SAR image. However, existing research has ignored the influence of polarization, and the relevant conclusions only apply to single-polarization SAR. For the PolSAR system integrated with multi-polarization transceiver antennas, TMMs can only produce modulation effects in the co-polarization channels and cannot affect the cross-polarization channels.

[0004] The Active Polarization Conversion Metasurface (APCM) is a new type of reconfigurable metasurface that can effectively control the polarization state of incident electromagnetic waves. Different from single-polarization TMMs, the APCM controlled by time-varying signals can modulate multiple polarization channels simultaneously. Therefore, this feature can be applied to polarization radar imaging modulation technology to effectively expand the application scope of reconfigurable metasurfaces in the field of radar image modulation.

Summary of the Invention

[0005] Aiming at the deficiencies of the existing radar imaging modulation method based on TMMs, the present invention proposes a PolSAR image modulation method based on a polarization conversion surface. The core idea is to use an Active linear polarization conversion metasurface (ALPCM) to perform energy conversion on the echo signals of each channel of a polarization radar. After the echo signals of each channel are processed by imaging, the position features and polarization features of the targets on the PolSAR image will change significantly. To achieve the above radar image modulation process, the present method is implemented by the following steps:

[0006] Step 1: Establish a modulation signal model for the polarization conversion surface

[0007] The basis of the method proposed in this application is the ALPCM that can achieve co-cross linear polarization conversion of incident electromagnetic waves. It mainly consists of an active impedance layer, a dielectric layer, and a metal bottom plate. The basic structure for the ALPCM to achieve polarization conversion is the active impedance layer. By changing the control voltage, the on-off state of the switching elements (usually PIN diodes) on the active impedance layer can be changed, thereby changing the scattering characteristics of the ALPCM. Applying a periodic high-low level control signal to the ALPCM enables the linearly polarized incident electromagnetic wave to switch between co-polarization conversion and cross-polarization conversion. For a polarization radar system with two orthogonal transmit and receive channels, the modulation signals of each channel are periodic pulse trains. If the high level corresponds to co-polarization conversion and the low level corresponds to cross-polarization conversion, then the modulation period T s corresponds to the switching period of the control signal, and the pulse width T w of the co-polarization conversion state corresponds to the high-level pulse width. In addition, this signal model is closely related to the energy conversion rate γ of the ALPCM itself. The amplitude coefficient of the modulation signal in the co-polarization channel switches periodically between 1 and 1 - γ. Correspondingly, the amplitude coefficient of the modulation signal in the cross-polarization channel switches periodically between 0 and γ. This modulation signal is flexible and variable and acts on the dual-channel incident signals of the polarization radar system.

[0008] Step 2: Establish the PolSAR echo modulation model

[0009] According to the equivalent modulation signal model of ALPCM, the echo signal model received by the dual-channel of the polarimetric radar system after modulation can be obtained. In the PolSAR image feature modulation, it can be divided into range modulation and azimuth modulation. The former is modulated within the pulse of the radar signal, corresponding to the fast time axis, and the latter is modulated between the pulses of the radar signal, corresponding to the slow time axis. By designing the corresponding periodic coding modulation waveforms on the two time axes respectively and then multiplying them, a two-dimensional joint periodic coding modulation waveform can be established, thereby obtaining the modulated PolSAR echo signal model.

[0010] Step 3: Generate the modulation result of the target scattering center position transformation in the PolSAR image

[0011] According to the PolSAR echo modulation model, the ALPCM under the control of the periodic coding waveform can redistribute the energy in each channel of the PolSAR system. After imaging processing, multiple harmonic components will be generated in the image outside the original target position, producing a position transformation effect of the scattering center. By changing the parameters of the range and azimuth modulation signals, the target position in the PolSAR image can be controlled to achieve effective modulation of the PolSAR image.

[0012] Step 4: Extract the target polarization features of the PolSAR image

[0013] According to the PolSAR echo modulation model, in addition to the position transformation effect of the scattering center, the periodic ALPCM modulation can also control the amplitude relationship of the harmonic components between channels, thereby controlling the polarization features of the scattering centers in the PolSAR image and producing a polarization feature modulation effect. By performing polarization feature extraction on multiple scattering centers in the modulated image based on the Cameron feature decomposition algorithm, it can be found that the target polarization features after modulation change significantly compared with the original target polarization features, thus proving the effectiveness of the method proposed in the present invention.

[0014] The beneficial effects of the present invention are as follows:

[0015] First, an innovative PolSAR image modulation method based on the polarization conversion surface is proposed, expanding the application scope of the reconfigurable metasurface in the field of radar image modulation.

[0016] Second, aiming at the limitations of the existing radar image feature modulation methods based on TMMs, considering the influence of the polarization factor, the proposed PolSAR image modulation method based on the polarization conversion surface can produce modulation effects on the images of all four full-polarization channels, and the generated position transformation effect of the scattering center has the advantages of flexible controllability.

[0017] Thirdly, by changing the parameters of the range and azimuth modulation signals and the conversion rate of the polarization conversion surface, the peak amplitude relationship of the target scattering centers in each channel can be flexibly regulated, so that the polarization conversion surface with a fixed physical structure can exhibit multiple scattering mechanisms, and innovatively realize the modulation of the polarization characteristics of the targets in PolSAR images.

Description of the Drawings

[0018] Figure 1 It is a flow chart of the PolSAR image modulation method.

[0019] Figure 2 It is a periodic externally applied excitation control signal.

[0020] Figure 3(a) - Figure 3(d) It is the waveform of the equivalent modulation signals of each channel for periodic linear polarization conversion.

[0021] Figure 4(a) - Figure 4(d) It is the PolSAR imaging result of the flat structure ALPCM without modulation.

[0022] Figure 5(a) - Figure 5(d) It is the PolSAR imaging results under different range modulation periods.

[0023] Figure 6(a) - Figure 6(d) It is the PolSAR imaging results under different azimuth modulation periods.

[0024] Figure 7(a) - Figure 7(d) It is the PolSAR imaging results under different duty cycles of the modulation waveforms.

[0025] Figure 8 It is the Cameron polarization decomposition circle.

[0026] Figure 9 It is the annotation of the target positions of the extracted polarization characteristics after modulation.

Detailed Embodiments

[0027] To better understand the method of the present invention, the technical solutions of the present invention will be further described below in conjunction with the drawings and embodiments.

[0028] The applicable object of the present invention is a PolSAR system adopting a time-division polarization measurement system using linear frequency modulation signals. Taking an airborne PolSAR system as an example, the radar uses an orthogonal dual-polarization channel to alternately transmit and simultaneously receive pulses during pulse intervals. Therefore, the radar can measure two columns of the target polarization scattering matrix within two Pulse Repetition Intervals (PRIs), thereby obtaining a complete polarization scattering matrix. Its transmitted signal is a linear frequency modulation pulse signal with a center frequency of 3 GHz, a bandwidth of 150 MHz, a pulse width of 2 μs, an azimuth beam width of 0.03 rad, a platform speed of 200 m / s, a pulse repetition interval of 2.86 ms, and uses the Range Doppler (RD) algorithm for imaging. The ALPCM with a flat structure is located in the center of the image scene.

[0029] The present invention relates to the following specific steps, and the specific process is as Figure 1 shown.

[0030] Step 1: Establish an ALPCM modulation signal model

[0031] By changing the control voltage, the on-off state of the diodes in the ALPCM can be changed, so that the linearly polarized incident electromagnetic wave can be switched between the co-polarization conversion and cross-polarization conversion cases. By applying different coded control signals, the time lengths of the reflection coefficients in the two states can be controlled. The method proposed in the present invention is to periodically control the ALPCM to regularly switch between two functional states.

[0032] It is assumed that when the external excitation is at a high level, the corresponding function is co-polarization conversion, and when the external excitation is at a low level, the corresponding function is cross-polarization conversion. The control signal of the periodic external excitation is as Figure 2 shown, and the corresponding modulation signal models for each channel are shown in Figures 3(a) - 3(d). Figure 3(a) is the xx channel, Figure 3(b) is the yx channel, Figure 3(c) is the yy channel, and Figure 3(d) is the xy channel. When the ALPCM switches to the co-polarization conversion state, the co-polarization channels (xx and yy) are in a high-scattering state, and the amplitude coefficients are normalized to "1". Since the cross-polarization channels (yx and xy) do not receive the converted energy, the amplitude coefficients are "0". When switching to the cross-polarization conversion state, part of the energy in the co-polarization channels (xx and yy) will be converted to the cross-polarization channels. When the conversion rate is γ, the amplitude coefficients are "1 - γ", and the amplitude coefficients of the corresponding cross-polarization channels (yx and xy) are "γ".

[0033] The time-domain responses of the equivalent modulation signals for each channel can be expressed as:

[0034]

[0035] Among them, the subscript co represents the co-polarized channel, the subscript cr represents the cross-polarized channel, t represents the time variable, represents a rectangular pulse signal, T w is the modulation symbol width at high level, δ(·) is the impulse function, T s is the coding period, n is the order, is the convolution operation, and the duty cycle α of the modulation waveform = T w / T s . It can be seen that the equivalent modulation signals of each channel are all linear superpositions of rectangular pulses. Therefore, the modulation waveform p co (t) of the co-polarized channel is taken as an example to analyze the modulation characteristics, and the modulation characteristics of the cross-polarized channel are the same as it.

[0036] Perform Fourier transform on the co-polarized channel periodic coding modulation waveform, and its spectral expression is:

[0037]

[0038] Among them, A0, A n are amplitude coefficients, and A0 = γT w f s +1 - γ, A n = γT w f s sinc(nT w f s ), f s = 1 / T s is the modulation frequency. It can be seen that the spectrum of the co-polarized channel periodic coding modulation waveform is composed of a series of harmonic frequency components, and the amplitude envelope follows the sinc function distribution.

[0039] Step 2: Establish the PolSAR image modulation model

[0040] For the PolSAR system with the time-sharing polarization measurement system, assume that the radar transmits H-polarized signals within odd PRIs and simultaneously receives the H and V polarization echo components of the target. Then, the S HH , S VH elements in the polarization scattering matrix are measured correspondingly; within even PRIs, the radar transmits V-polarized signals and simultaneously receives the H and V polarization echo components of the target. Then, the S HV , S VV elements are measured correspondingly. Assume that the airborne PolSAR imaging adopts the forward-looking strip working mode and uses the RD imaging algorithm for imaging processing. The imaging results of each channel can be obtained as follows:

[0041]

[0042] Among them, Denote the fast-time variable of the SAR system, \(t\). mH and \(t\). mV r is the slow-time variable composed of the moments when corresponding polarized electromagnetic waves are transmitted, \(K\). p p is the range chirp rate, \(B\) is the signal bandwidth, \(T\). p is the pulse width, \(R\). B is the closest distance between the target and the trajectory of the PolSAR platform, \(T\). L is the synthetic aperture time, \(K\). a 2 c is the azimuth Doppler modulation rate, \(f\). c / cR B is the signal carrier frequency, \(f\). c is the flight speed of the airborne platform, \(v\), and \(G\) represents the two-dimensional matched filtering gain. According to the scatterer model, a complex target can be approximated as the superposition of \(N\) (where \(N\) is a positive integer) scatterers on the target. Therefore, the two-dimensional image of the target can be expressed as:

[0043]

[0044] where \(I_{pq}(\cdot)\) represents the PolSAR image with \(q\)-polarization transmission and \(p\)-polarization reception, \(t\). mq represents the slow-time variable composed of the moments when \(q\)-polarized electromagnetic waves are transmitted, \(S\). pqi represents the scattering coefficient corresponding to the \(i\)-th scatterer when \(q\)-polarization is transmitted and \(p\)-polarization is received, \(R\). Bi represents the closest distance between the \(i\)-th point target and the trajectory of the PolSAR platform.

[0045] In PolSAR image feature modulation, it can be divided into range modulation and azimuth modulation. The former is modulated within the pulse of the radar signal, corresponding to the fast-time axis, and the latter is modulated between pulses of the radar signal, corresponding to the slow-time axis.

[0046] Taking the co-polarization channel as an example, in two-dimensional periodic coding modulation, the range coding modulation waveform can be expressed as:

[0047]

[0048] where \(T\). wr is the modulation symbol width at the high level in the range direction, \(T\). r is the range coding period.

[0049] The azimuth coding modulation waveform can be expressed as:

[0050]

[0051] where \(T\). wa is the modulation symbol width at the high level in the azimuth direction, \(T\).a is the azimuth encoding period.

[0052] The co-polarized channel PolSAR imaging result after two-dimensional pulse compression in the range and azimuth directions after modulation is:

[0053]

[0054] Among them, the amplitude coefficient B n = γα r sinc(nα r ), α r = T wr / T r is the range modulation duty cycle, and the amplitude coefficient B m = α a sinc(mα a ), α a = T wa / T ra is the azimuth modulation duty cycle. t m is the slow time variable composed of t mH , t mV , f r = 1 / T r is the range modulation frequency, f a = 1 / T a is the azimuth modulation frequency, T L is the synthetic aperture time, and n and m are both orders.

[0055] Formula (8) is the PolSAR image modulation model. It can be seen that the PolSAR imaging result under periodic ALPCM modulation shows a two-dimensional mesh-distributed harmonic array in the range and azimuth directions, and the peak interval is proportional to the modulation frequency of each dimension modulation waveform, that is, inversely proportional to the modulation period, and the amplitude of the peak is related to the modulation waveform duty cycle and the conversion rate of ALPCM. Therefore, the distribution of harmonics can be controlled by changing the modulation parameters, and the amplitude relationship of harmonic components between channels can also be controlled, thereby controlling the polarization characteristics.

[0056] Step 3: Generate the modulation result of the target scattering center position transformation of the PolSAR image

[0057] Assume that the transmitted signal is a linear frequency modulation pulse signal with a center frequency of 3 GHz, a bandwidth of 150 MHz, a pulse width of 2 μs, an azimuth beam width of 0.03 rad, a platform speed of 200 m / s, and a pulse repetition interval of 2.86 ms. Simulate the time-sharing polarization measurement system PolSAR, set the target imaging area to 200 m × 200 m, and the APCM with a flat structure is located at the origin (0, 0). The ideal polarization conversion matrix of the flat structure is Therefore, its imaging results are only shown in the co-polarization channel. Its original imaging results without modulation are as follows Figure 4(a) - Figure 4(d) shown

[0058] According to the modulation model shown in formula (8), under periodic ALPCM modulation, the internal energy of each channel of the PolSAR system is redistributed. The energy concentrated at the zero-order peak position before modulation is shifted at integer multiples of the modulation frequency. The imaging results are manifested as a two-dimensional mesh-distributed harmonic array in the range direction and azimuth direction, and the interval between each peak is proportional to the modulation frequencies f r 、f a in each dimension, that is, inversely proportional to the modulation periods T r 、T a . Therefore, the position distribution of the harmonics can be changed by changing the modulation period. In addition, according to the expressions of the amplitude coefficients B n and B m , when the range modulation duty cycle α r = 0.5 and n is an even number, the amplitude coefficient B n = 0. When the azimuth modulation duty cycle α a = 0.5 and m is an even number, the amplitude coefficient B m = 0. Therefore, the presence or absence of even harmonics can be controlled by changing the modulation duty cycle.

[0059] Set the range and azimuth modulation duty cycles α r = α a = 0.5, the conversion rate γ = 0.8. Use the imaging results of the HH and HV channels to represent the effects of the co-polarization channel and cross-polarization channel, and change the range modulation period T r and the azimuth modulation period T a . The imaging results under different range modulation periods are as follows Figure 5(a) - Figure 5(d) shown Figure 5(a) - Figure 5(b) represents the imaging results under the conditions of T r = 0.4 us, T a = 4×PRI, Figure 5(c) - Figure 5(d) represents the imaging results under the conditions of T r = 0.2 us, T a = 4×PRI.

[0060] The imaging results under different azimuth modulation periods are as follows Figure 6(a) - Figure 6(d) shown Figure 6(a) - Figure 6(b) represents the imaging results under the conditions of T r = 0.4 us, T a = 4×PRI, Figure 6(c) - Figure 6(d) represents the imaging results under the conditions of T r = 0.4 us, T a= Imaging results under the condition of 8×PRI. It can be seen that under the two-dimensional periodic ALPCM modulation model, the position transformation effect of the scattering center as theoretically deduced occurs in the PolSAR image. First, the APCM of the flat structure at the zero point is displayed in all four channels in the imaging result after modulation, reflecting an energy distribution different from the nature of the flat structure. Second, in addition to the original target peak points located at the zero point, a harmonic peak array showing a two-dimensional network distribution appears in the image, and the interval of the distribution is inversely proportional to the modulation periods in the corresponding range direction and azimuth direction.

[0061] Set the range modulation period T r = 0.4 us, and the azimuth modulation period T a = 8×PRI, conversion rate γ = 0.8, and change the range modulation duty cycle α r and the azimuth modulation duty cycle α a , and the imaging results are as Figure 7(a) - Figure 7(d) shown. Figure 7(a) - Figure 7(b) represents the imaging result under the condition of α r = α a = 0.5, Figure 7(c) - Figure 7(d) represents the imaging result under the condition of α r = α a = 0.25. It can be seen that under the two-dimensional periodic ALPCM modulation model, the interval of the harmonic peak array distribution generated in the PolSAR image has nothing to do with the duty cycle. The duty cycle mainly affects the amplitude characteristics of the peaks. When the duty cycle is 0.5, the even-order peaks disappear, which is consistent with the theory.

[0062] In summary, by controlling the modulation period and duty cycle of the modulation waveform, the distance of the peak position in the PolSAR image and the presence or absence of even-order peaks can be controlled, realizing the position transformation effect of the scattering center.

[0063] Step 4: Extract the target polarization characteristics of the PolSAR image

[0064] According to formula (8), the amplitude coefficient is related to the conversion rate γ of the ALPCM. Therefore, the periodic ALPCM modulation can also control the amplitude of the harmonic components between channels. Since the polarization characteristics are mainly determined by the amplitude relationships of each channel, changing the conversion rate of the ALPCM can control the polarization characteristics of the scattering center in the PolSAR image, generating a polarization characteristic modulation effect.

[0065] On the basis of generating the PolSAR modulation image, use the regional peak detection method to locate the peaks of each channel, and extract the polarization characteristics based on the Cameron feature decomposition algorithm. The basic idea of the Cameron decomposition is to divide the target into 11 types of basic scatterers, among which 8 typical scattering mechanisms are such as Figure 8As shown, complex targets can all be decomposed into combinations of these basic scatterers. To simplify the analysis process and highlight the influence of the energy conversion rate, the present invention analyzes the target polarization characteristics of PolSAR images modulated by different conversion rate ALPCMs under only azimuth modulation.

[0066] Set the azimuth modulation duty cycle to 0.5 and the modulation period to 4×PRI. After modulation, the energy is only distributed on the first harmonic component, and the three target positions generated are as Figure 9 shown. The middle is the zero-order peak, which is the placement position of the flat ALPCM. The left and right sides are the -1st order peak and the +1st order peak respectively, which are the generated harmonic components. Set the ALPCM energy conversion rates γ to 0.9, 0.5, and 0.1 respectively. The polarization scattering matrices of the zero-order peak target and the ±1st order peaks under the three conversion rates can be obtained. Perform Cameron-based eigen-decomposition on the polarization scattering matrices. The decomposition results of the zero-order peak show several scattering mechanisms such as dipoles, cylinders, and trihedrals. The ±1st order peaks basically show the dipole scattering mechanism. Therefore, the ALPCM controlled by the time-varying signal can effectively modulate the characteristics of the PolSAR image, which is consistent with the theory.

[0067] In summary, the polarization conversion surface controlled by the time-varying signal can effectively modulate the position characteristics and polarization characteristics of the targets on the PolSAR image, proving the effectiveness of the present method.

Claims

1. A PolSAR image modulation method based on a polarization conversion surface, characterized in that It includes the following steps: Step 1: Establish a polarization conversion surface modulation signal model The structure of the active linear polarization conversion surface (ALPCM) for realizing the polarization conversion function is an active impedance layer. By changing the control voltage, that is, changing the on-off state of the switching elements on the active impedance layer, the scattering characteristics of the ALPCM can be changed; when a periodic high-low level control signal is applied to the ALPCM, the ALPCM can make the linearly polarized incident electromagnetic wave switch between the co-polarization conversion and cross-polarization conversion cases; Step 2: Establish a polarization synthetic aperture radar (PolSAR) echo modulation model According to the equivalent modulation signal model of the ALPCM, the echo signal model received by the two channels of the polarized radar system after modulation is obtained; in the PolSAR image feature modulation, it is divided into range modulation and azimuth modulation. The former is modulated within the pulse of the radar signal, corresponding to the fast time axis, and the latter is modulated between pulses of the radar signal, corresponding to the slow time axis; corresponding periodic coding modulation waveforms are designed on the two time axes and then multiplied to establish a two-dimensional joint periodic coding modulation waveform, so as to obtain the modulated PolSAR echo signal model, which is the PolSAR echo modulation model; Step 3: Generate the modulation result of the position transformation of the target scattering center in the PolSAR image According to the PolSAR echo modulation model, the ALPCM under the control of the periodic coding waveform redistributes the energy in each channel of the PolSAR system. After imaging processing, multiple harmonic components will be generated in the image outside the original target position, producing a position transformation effect of the scattering center; by changing the parameters of the range and azimuth modulation signals, the position of the PolSAR image target is controlled to achieve effective modulation of the PolSAR image; Step 4: Extract the polarization characteristics of the target in the PolSAR image According to the PolSAR echo modulation model, in addition to the position transformation effect of the scattering center, the periodic ALPCM modulation also controls the amplitude relationship of the harmonic components between channels, thereby controlling the polarization characteristics of the scattering center in the PolSAR image, producing a polarization characteristic modulation effect; the polarization characteristics of multiple scattering centers in the modulated image are extracted based on the Cameron eigen decomposition algorithm.

2. A PolSAR image modulation method based on a polarization conversion surface according to claim 1, characterized in that: In Step 1, for a polarimetric radar system with two orthogonal transmit and receive channels, the modulation signals of each channel appear as periodic pulse trains; if the high level corresponds to co-polarization conversion and the low level corresponds to cross-polarization conversion, then the modulation period T of this modulation signal s corresponds to the switching period of the control signal, and the pulse width T of the co-polarization conversion state w corresponds to the high-level pulse width.

3. A PolSAR image modulation method based on a polarization conversion surface according to claim 2, characterized in that: In Step 1, the signal model is closely related to the energy conversion rate γ of the ALPCM itself. The amplitude coefficient of the co-polarization channel modulation signal switches periodically between 1 and 1 - γ. Correspondingly, the amplitude coefficient of the cross-polarization channel modulation signal switches periodically between 0 and γ; the modulation signal acts on the two-channel incident signal of the polarized radar system.

4. A PolSAR image modulation method based on a polarization conversion surface according to claim 1 or 2 or 3, characterized in that: In Step 1, when the external excitation is at a high level, the corresponding function is co-polarization conversion, and when the external excitation is at a low level, the corresponding function is cross-polarization conversion; when the ALPCM switches to the co-polarization conversion state, the co-polarization channel is in a high-scattering state, and the amplitude coefficient is normalized to "1", while the cross-polarization channel has an amplitude coefficient of "0" because it does not receive the converted energy; when switching to the cross-polarization conversion state, part of the energy in the co-polarization channel will be converted to the cross-polarization channel. When the conversion rate is γ, its amplitude coefficient is "1 - γ", and the corresponding amplitude coefficient of the cross-polarization channel is "γ".

5. A PolSAR image modulation method based on a polarization conversion surface according to claim 1 or 2 or 3 or 4, characterized in that: In Step 1, the time-domain response of the equivalent modulation signal of each channel is expressed as: Among them, the subscript co represents the co-polarization channel, the subscript cr represents the cross-polarization channel, and t represents the time variable. represents a rectangular pulse signal, and T w is the modulation symbol width at the high level, δ(·) is the impulse function, and T s is the coding period, n is the order. is the convolution operation, and the duty cycle α of the modulation waveform is α = T w / T s ; The equivalent modulation signals of each channel are all linear superpositions of rectangular pulses. Therefore, the modulation waveform p co (t) of the co-polarization channel is taken as an example to analyze the modulation characteristics, and the modulation characteristics of the cross-polarization channel are the same as it.

6. A PolSAR image modulation method based on a polarization conversion surface according to claim 5, characterized in that: In Step 1, the Fourier transform is performed on the co-polarization channel periodic coding modulation waveform, and its spectral expression is: Among them, A0, A n are amplitude coefficients, and A0 = γT w f s + 1 - γ, A n = γT w f s sinc(nT w f s ), f s = 1 / T s is the modulation frequency; the spectrum of the co-polarization channel periodic coding modulation waveform is composed of a series of harmonic frequency components, and the amplitude envelope follows the sinc function distribution.

7. A PolSAR image modulation method based on a polarization conversion surface according to claim 1, characterized in that: In step 2, for the PolSAR system with a time-sharing polarization measurement system, assume that the radar transmits H-polarized signals within odd PRIs and simultaneously receives the H- and V-polarized echo components of the target. Then, the S HH and S VH elements in the polarization scattering matrix are measured accordingly. Within even PRIs, the radar transmits V-polarized signals and simultaneously receives the H- and V-polarized echo components of the target. Then, the S HV and S VV elements are measured accordingly. Assume that the airborne PolSAR imaging adopts a side-looking strip working mode and uses the RD imaging algorithm for imaging processing.

8. A PolSAR image modulation method based on a polarization conversion surface according to claim 7, characterized in that: In Step 2, the imaging results of each channel are: Among them, represents the fast-time variable of the SAR system, t mH , t mV and t r are the slow-time variables composed of the moments when the corresponding polarized electromagnetic waves are transmitted, K r = B / T p is the range chirp rate, B is the signal bandwidth, T p is the pulse width, R B is the closest distance between the target and the trajectory of the PolSAR platform, T L is the synthetic aperture time, K a = 2v 2 f c / cR B is the azimuth Doppler modulation rate, f c is the signal carrier frequency, v is the flight speed of the airborne platform, and G represents the two-dimensional matched filtering gain; according to the scatterer model, a complex target is approximated as the superposition of N scatterers on the target. Therefore, the two-dimensional image of the target is expressed as: Among them, Ipq(·) represents the PolSAR image with q-polarized transmission and p-polarized reception, and t mq represents the slow-time variable composed of the moments when q-polarized electromagnetic waves are transmitted, and S pqi represents the scattering coefficient corresponding to the i-th scattering point when q-polarized transmission and p-polarized reception occur, and R Bi represents the closest distance between the i-th point target and the PolSAR platform trajectory.

9. A PolSAR image modulation method based on a polarization conversion surface according to claim 8, characterized in that: In Step 2, in the two-dimensional periodic coding modulation, the range-direction coding modulation waveform is expressed as: Among them, T wr is the modulation symbol width when the range direction is at a high level, and T r is the coding period in the range direction; The azimuth-direction coding modulation waveform is expressed as: Among them, T wa is the modulation symbol width at the azimuth high level, and T a is the azimuth coding period.

10. A PolSAR image modulation method based on a polarization conversion surface according to claim 9, characterized in that: In Step 2, the co-polarization channel PolSAR imaging result after modulation and two-dimensional pulse compression in the range direction and azimuth direction is: Among them, the amplitude coefficient B n =γα r sinc(nα r ), α r =T wr / T r is the duty cycle of range modulation, and the amplitude coefficient B m =α a sinc(mα a ), α a =T wa / T ra is the duty cycle of azimuth modulation, t m is the slow time variable composed of t mH , t mV , f r =1 / T r is the range modulation frequency, and f a =1 / T a is the azimuth modulation frequency, T L is the synthetic aperture time, and n and m are both orders; Formula (8) is the PolSAR image modulation model. The PolSAR imaging result under periodic ALPCM modulation shows a two-dimensional networked harmonic array in the range direction and azimuth direction, and the peak interval is proportional to the modulation frequency of each dimension modulation waveform, that is, inversely proportional to the modulation period, and the amplitude of the peak is related to the duty cycle of the modulation waveform and the conversion rate of ALPCM; therefore, by changing the modulation parameters, the distribution of harmonics can be controlled, and the amplitude relationship of harmonic components between channels can be controlled, thereby controlling the polarization characteristics.

Images