Time-varying modulation radar polarization characteristic converter

Abstract

The invention provides a time-varying modulation radar polarization feature converter. The time-varying modulation radar polarization feature converter comprises the following steps: step 1, establishing an active linear polarization conversion surface ALPCM equivalent polarization scattering matrix model; 2, establishing a multi-channel equivalent modulation signal model based on a PolSAR (Polarized Synthetic Aperture Radar) imaging system; step 3, establishing a PolSAR image modulation model; and step 4, generating a PolSAR image modulation result. According to the invention, the limitation of a passive structure is broken through, and the characteristics of incident electromagnetic waves can be flexibly and dynamically controlled. According to the method, electromagnetic regulation and control parameters are combined with PRI of a time-sharing polarization measurement system, a multichannel equivalent signal modulation model based on five types is established, and a theoretical basis is provided for the PolSAR image electromagnetic regulation and control technology. According to the method, combined regulation and control of spatial position distribution and polarization characteristics of the target scattering center in the PolSAR image are realized, and the method has great significance in improving the radar target characteristic regulation and control capability in a complex electromagnetic environment.

Description

[Technical field]

[0001] The present invention discloses a time-varying modulated radar polarization characteristic converter, which can effectively act on polarimetric synthetic aperture radar (PolSAR) to transform the polarization characteristics of the resulting image. Specifically, the converter relates to the field of active polarization conversion metasurfaces (APCMs). Specifically, the APCM, controlled by a time-varying signal, alters the energy relationship between polarimetric radar channels, thereby jointly regulating the spatial distribution of target scattering centers and their polarization characteristics in the image. [Background technology]

[0002] Synthetic Aperture Radar (SAR) is an active microwave imaging sensor that has demonstrated important application value in environmental monitoring, geological exploration, disaster assessment and other fields. In addition to high-resolution imaging and target recognition algorithms, active control of target electromagnetic scattering characteristics has become another key research direction in the information perception dimension of radar systems. Traditional feature modulation technology mainly controls the amplitude, phase and other characteristics of the target scattered wave through means such as geometric shape optimization and loading frequency selective surfaces, thereby achieving functions such as radar cross-section (RCS) control and scattering center offset. However, existing methods mostly focus on controlling features such as spatial position or intensity, and lack systematic research on the control of polarization features. Polarization features contain information about the physical properties of the target and can reflect properties such as the target material, surface roughness and structural symmetry. Its coordinated modulation with spatial features can significantly enhance the multi-dimensional adaptability of electromagnetic control. In particular, in polarimetric synthetic aperture radar (PolSAR) imaging systems that integrate multiple polarimetric transmit and receive channels, the manipulation of single-channel features is easily compromised by polarization decomposition algorithms and multi-channel fusion recognition techniques. Therefore, developing new technologies that can manipulate target polarization scattering characteristics is crucial for improving radar target feature manipulation capabilities in complex electromagnetic environments.

[0003] Passive electromagnetic control technologies, exemplified by metamaterials and metasurfaces, achieve control over the polarization state of electromagnetic waves through subwavelength structural design. However, the electromagnetic response of passive devices is rigid due to their physical structure, making dynamic, real-time control impossible. While some studies have attempted to achieve limited reconfigurability through mechanical structures, the equipment involved is complex, and the control speed struggles to adapt to the radar's pulse repetition interval (PRI), limiting their applicability in remote sensing scenarios.

[0004] The Active Polarization Conversion Metasurface (APCM) breaks through the limitations of passive structures by integrating semiconductor devices (such as PIN diodes and varactor diodes) or Micro-Electro-Mechanical Systems (MEMS). By controlling signals to change the on / off state of surface units, flexible dynamic control of the characteristics of incident electromagnetic waves can be achieved. Current technologies have realized functions such as linear-cross linear polarization conversion and linear-circular polarization conversion based on electrically controlled signals. However, existing research mainly focuses on the optimization of the basic properties of materials, and the potential of feature modulation in the PolSAR imaging system has not been explored. In view of this problem, the present invention proposes a feature regulation theory and implementation method for the PolSAR imaging mechanism.

Summary of the Invention

[0005] In view of the problem that the existing electromagnetic regulation technology lacks adaptability to the PolSAR imaging mechanism, the present invention proposes a radar polarization feature converter for time-varying modulation for PolSAR. It realizes the energy conversion between full-polarization radar channels through an Active linear polarization conversion metasurface (ALPCM). After being affected by this radar polarization feature converter, the spatial position distribution and polarization features of targets in the PolSAR image will be jointly modulated. To achieve the above PolSAR image feature modulation process, the following steps are adopted in this method:

[0006] Step 1: Establish an equivalent polarization scattering matrix model of ALPCM

[0007] The radar polarization feature converter proposed by the present invention is based on ALPCM and consists of three parts: an active impedance layer loaded with PIN diodes, a dielectric layer, and a metal bottom plate. In different on / off states of the PIN diodes, ALPCM can achieve two functions of co-polarization conversion and cross-polarization conversion for linearly polarized incident electromagnetic waves. The so-called time-varying modulation is to apply a periodically changing excitation signal to ALPCM, so that it switches regularly between the two functional states. By establishing an equivalent polarization scattering matrix model in the two functional states, the variation relationship of the amplitude coefficients of electromagnetic waves in each channel under the modulation of ALPCM controlled by time-varying signals can be obtained.

[0008] Step 2: Establish a multi-channel equivalent modulation signal model based on the PolSAR imaging system

[0009] According to the ALPCM equivalent polarization scattering matrix model, the equivalent amplitude coefficients of each channel can be obtained in the two states of diode on and off. By applying a periodically varying excitation signal to the ALPCM, the equivalent modulation signals of the four channels corresponding to full polarization will also show a periodic variation law. For the PolSAR with a time-sharing full polarization measurement system, the modulation dimensions can be divided into range modulation and azimuth modulation. The former is modulated within the pulse of the radar signal, and the latter is modulated between the pulses of the radar signal. Since the polarization mode of the transmitted electromagnetic wave switches with the PRI as the period, the target polarization characteristics are mainly affected by the azimuth modulation parameters. In the present invention, through the relationship between the switching period T of the excitation signal s and the azimuth parameter PRI of the PolSAR imaging azimuth, a multi-channel equivalent modulation signal model suitable for the time-sharing full polarization measurement system PolSAR is established.

[0010] Step 3: Establish a PolSAR image modulation model

[0011] According to the multi-channel equivalent modulation signal model, the influence of the modulation on the radar echo can be obtained. By analyzing the effect of the modulation on the pulse compression of the chirp signal, the corresponding PolSAR echo modulation signal model can be further generalized.

[0012] Step 4: Generate the PolSAR image modulation result

[0013] According to the PolSAR echo modulation signal model, it can be known that the periodic ALPCM modulation will jointly control the spatial position distribution and polarization characteristics of the targets in the PolSAR image. The parameters of the modulation signal mainly affect the spatial position distribution of the targets, while the conversion rate of the ALPCM mainly affects the polarization characteristics of the targets. By performing Pauli decomposition and Cameron decomposition on the modulated image under different parameters respectively, it can be found that the modulation can effectively control the polarization characteristics of the radar targets, thereby 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 radar polarization feature converter with time-varying modulation for PolSAR is proposed, breaking through the limitation of the passive structure and enabling flexible dynamic control of the characteristics of the incident electromagnetic wave.

[0016] Second, aiming at the problem that the existing electromagnetic regulation technologies lack adaptability to the PolSAR imaging mechanism, the present invention combines the electromagnetic regulation parameters with the PRI of the time-sharing polarization measurement system to establish five basic multi-channel equivalent signal modulation models, providing a theoretical basis for the electromagnetic regulation technology of PolSAR images.

[0017] Thirdly, by modulating the parameters of the signal and the conversion rate of ALPCM, the energy distribution relationship between channels can be changed, realizing the joint regulation of the spatial position distribution and polarization characteristics of target scattering centers in PolSAR images. This invention is of great significance for enhancing the radar target feature regulation ability in complex electromagnetic environments.

Description of the Drawings

[0018] Figure 1 It is a flowchart of the PolSAR image feature modulation method.

[0019] Figure 2 It is the signal timing relationship of the time-division full-polarization measurement system.

[0020] Figure 3 It is the waveform of the type 1 external excitation change.

[0021] Figures 4(a) - 4(d) It is the equivalent modulation signal of each channel of type 1.

[0022] Figure 5 It is the waveform of the type 2 external excitation change.

[0023] Figures 6(a) - 6(d) It is the equivalent modulation signal of each channel of type 2.

[0024] Figure 7 It is the waveform of the type 3 external excitation change.

[0025] Figures 8(a) - 8(d) It is the equivalent modulation signal of each channel of type 3.

[0026] Figure 9 It is the waveform of the type 4 external excitation change.

[0027] Figures 10(a) - 10(d) It is the equivalent modulation signal of each channel of type 4.

[0028] Figure 11 It is the waveform of the type 5 external excitation change.

[0029] Figures 12(a) - 12(d) It is the equivalent modulation signal of each channel of type 5.

[0030] Figures 13(a) and 13(b) are the generation results of the type 3 harmonic components.

[0031] Figures 14(a) and 14(b) are the generation results of the type 4 harmonic components.

Detailed Embodiments

[0032] 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.

[0033] The applicable object of the present invention is a PolSAR system with a time-sharing full-polarization measurement system. The transmitted signal uses a relatively common linear frequency modulation pulse signal, with a center frequency of 3 GHz, a bandwidth of 150 MHz, a pulse width of 2 μs, a pulse repetition interval of 2.86 ms, an airborne platform height of 1 km, a platform speed of 200 m / s, a side-looking imaging scenario, and the Range Doppler (RD) algorithm is used for imaging. The ALPCM with a flat structure is located in the center of the imaging scenario.

[0034] The present invention relates to the following specific steps, and the specific process is as Figure 1 shown.

[0035] Step 1: Establish an equivalent polarization scattering matrix model of ALPCM

[0036] The basic structure of the radar polarization feature converter proposed by the present invention is ALPCM, which is composed of an active impedance layer loaded with PIN diodes, a dielectric layer, and a metal bottom plate. By changing the control voltage, the on-off state of the PIN diodes on the active impedance layer can be changed, so that it can switch between the two functions of co-polarization conversion and cross-polarization conversion of incident electromagnetic waves.

[0037] When the ALPCM is in the co-planar polarization conversion state, the electromagnetic wave incident on the ALPCM with x polarization will be totally reflected in the form of x polarization, and the electromagnetic wave incident with y polarization will be totally reflected in the form of y polarization. For a full-polarization radar with two orthogonal transmit and receive channels, writing the reflection coefficients corresponding to each channel in matrix form, the physical meaning of this matrix is the same as that of the polarization scattering matrix, both reflecting the scattering ability of the target under the corresponding polarization configuration. Therefore, the reflection coefficient matrix in the co-planar polarization conversion state can be expressed as:

[0038]

[0039] Among them, S xx represents the coefficient obtained by transmitting with x polarization and receiving with x polarization, S xy represents the coefficient obtained by transmitting with y polarization and receiving with x polarization, S yx represents the coefficient obtained by transmitting with x polarization and receiving with y polarization, S yy represents the coefficient obtained by transmitting with y polarization and receiving with y polarization. These four coefficients are also the corresponding elements of the scattering matrix. It can be seen that in this state, the modulus values of the coefficients in the co-polarization channels reach the maximum, and the modulus values of the coefficients in the cross-polarization channels reach the minimum. Therefore, corresponding to the co-polarization conversion function, the scattering matrix in this state is denoted as S co-c .

[0040] When the ALPCM is in the cross-polarization conversion state, ideally, the electromagnetic wave incident on the ALPCM in the x polarization will be totally reflected in the y polarization form, and the electromagnetic wave incident in the y polarization will be totally reflected in the x polarization form. Therefore, the reflection coefficient matrix in the cross-polarization conversion state can be expressed as:

[0041]

[0042] It can be seen that in this state, the modulus value of the co-polarization channel coefficient reaches the minimum, and the modulus value of the cross-polarization channel coefficient reaches the maximum. Therefore, corresponding to the cross-polarization conversion function, the scattering matrix in this state is denoted as S cr-c .

[0043] When the actual linear polarization conversion surface performs energy conversion between channels, it is very difficult for the electromagnetic wave incident in one polarization state to be completely converted into the electromagnetic wave reflected in another polarization state. Assuming that the actual polarization conversion rate is γ (0 ≤ γ ≤ 1), the reflection coefficient matrix is expressed as:

[0044]

[0045] To sum up, the co-cross polarization conversion is equivalent to the conversion of the reflection coefficients of each channel between S co-c and S cr-c This is the two equivalent polarization scattering matrix models of the ALPCM.

[0046] Step 2: Establish a multi-channel equivalent modulation signal model based on the PolSAR imaging system

[0047] According to the ALPCM equivalent polarization scattering matrix model shown in formula (1) and formula (3), it can be obtained that the equivalent amplitude coefficients of each channel in the two states of diode on and off are converted between S co-c and S cr-c When a periodically varying excitation signal is applied to the ALPCM, the equivalent modulation signals of the four channels will also show a periodic variation law. Since the polarization mode of the electromagnetic wave emitted by the PolSAR with the time-division full-polarization measurement system switches with the PRI as the period, the target polarization characteristics are mainly affected by the azimuth modulation parameters. Therefore, based on only modulating the azimuth direction of the radar echo, the equivalent modulation signal model under the relationship between different modulation parameters and the radar azimuth parameter PRI is established in the present invention.

[0048] The measurement timing relationship between the transmitted and received signals of the time-division full-polarization measurement system is as Figure 2As shown in the figure, this timing relationship assumes that the radar transmits H-polarized signals within odd PRIs and simultaneously receives the target's H- and V-polarized echo components; within even PRIs, it transmits V-polarized signals and simultaneously receives the target's H- and V-polarized echo components. For ALPCM, it is assumed that when the applied excitation is at a high level, the corresponding function is co-polarization conversion, and when the applied excitation is at a low level, the corresponding function is cross-polarization conversion. For the convenience of discussion, assume that the period T of the modulation signal a is an integer multiple of the PRI. Therefore, the equivalent modulation signals of each channel mainly depend on two factors. The first is the relationship between the period of the modulation signal and the PRI, and the second is the level of the initial PRI excitation signal.

[0049] Based on the above factors, the present invention establishes multi-channel equivalent modulation signal models in five typical cases, and the remaining cases can be regarded as linear superpositions of these five cases. In the analysis of the five typical cases, p HH (t), p VH (t), p VV (t) and p HV (t) are used to represent the equivalent modulation signal models of the HH, VH, VV, and HV channels respectively. And considering that the subsequent processing is independent of the time delay, the influence in this aspect is ignored in the expressions.

[0050] (1) Type 1: Modulation period 2×PRI, high level in odd PRI

[0051] The applied excitation levels corresponding to this modulation parameter are as Figure 3 shown, and the equivalent modulation waveforms of each channel are as Figures 4(a) - 4(d) shown. Among them, Fig. 4(a) is the HH channel, Fig. 4(b) is the VH channel, Fig. 4(c) is the VV channel, and Fig. 4(d) is the HV channel. In this modulation type, the equivalent modulation waveforms of the four channels are all constants, as shown in Equation (4). Compared with the modulation with any other parameter, this type of modulation makes the energy of the HH channel the highest and the energy of the VH channel the lowest.

[0052]

[0053] (2) Type 2: Modulation period 2×PRI, high level in even PRI

[0054] The applied excitation levels corresponding to this modulation parameter are as Figure 5 shown, and the equivalent modulation waveforms of each channel are as Figures 6(a) - 6(d) shown. Among them, Fig. 6(a) is the HH channel, Fig. 6(b) is the VH channel, Fig. 6(c) is the VV channel, and Fig. 6(d) is the HV channel. In this modulation type, the equivalent modulation waveforms of the four channels are all constants, as shown in Equation (5). Compared with the modulation with any other parameter, this type of modulation makes the energy of the VV channel the highest and the energy of the HV channel the lowest.

[0055]

[0056] (3) Type 3: Modulation period 4×PRI (equivalent for odd / even PRI and high-level)

[0057] The externally applied excitation level corresponding to this type of modulation parameter is as Figure 7 shown, and the equivalent modulation waveforms of each channel are as Figures 8(a) - 8(d) shown. Among them, Fig. 8(a) is the HH channel, Fig. 8(b) is the VH channel, Fig. 8(c) is the VV channel, and Fig. 8(d) is the HV channel. Under this type of modulation, the equivalent waveforms of the HH and VV channels are the same, and the equivalent waveforms of the HV and VH channels are the same, as shown in Equation (6).

[0058]

[0059] (4) Type 4: Modulation period 6×PRI, odd PRI high level

[0060] The externally applied excitation level corresponding to this type of modulation parameter is as Figure 9 shown, and the equivalent modulation waveforms of each channel are as Figures 10(a) - 10(d) shown. Among them, Fig. 10(a) is the HH channel, Fig. 10(b) is the VH channel, Fig. 10(c) is the VV channel, and Fig. 10(d) is the HV channel. Under this type of modulation, the equivalent waveforms of each channel are different from each other, as shown in Equation (7).

[0061]

[0062] (5) Type 5: Modulation period 6×PRI, even PRI high level

[0063] The externally applied excitation level corresponding to this type of modulation parameter is as Figure 11 shown, and the equivalent modulation waveforms of each channel are as Figures 12(a) - 12(d) shown. Among them, Fig. 12(a) is the HH channel, Fig. 12(b) is the VH channel, Fig. 12(c) is the VV channel, and Fig. 12(d) is the HV channel. Under this type of modulation, the equivalent waveforms of each channel are different from each other, as shown in Equation (8).

[0064]

[0065] Step 3: Establish the PolSAR image modulation model

[0066] For the convenience of analysis, the signal models of the five basic types represented by formulas (4)-(8) are divided into two categories according to the co-polarization channel and the cross-polarization channel, and expressed as more general expressions. Then, the time-domain response of the modulation signal can be expressed as:

[0067]

[0068] Among them, the subscript co represents the co-polarization channel, the subscript cr represents the cross-polarization channel, t represents the time variable, rect(·) is 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 . It can be seen that the equivalent modulation signals of each channel are all linear superpositions of rectangular pulses. Therefore, the modulation characteristics are analyzed below using the co-polarization channel modulation waveform p co (t) as an example, and the same conclusion can be obtained for the cross-polarization channel.

[0069] Performing a Fourier transform on the co-polarization channel periodic coding modulation waveform, its spectral expression is:

[0070]

[0071] Among them, the amplitude coefficient 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-polarization channel periodic coding modulation waveform is composed of a series of harmonic frequency components, and the amplitude envelope follows the sinc function distribution.

[0072] The commonly used radar transmit waveform in the PolSAR system is a chirp signal, and its expression is:

[0073]

[0074] Among them, represents the fast time variable of the SAR system, T p is the pulse width, f c is the signal carrier frequency, K r = B / T p is the range chirp rate, and B is the signal bandwidth.

[0075] After being incident on the ALPCM and reflected, the received echo signal of the radar signal is:

[0076]

[0077] Among them, τ = 2R t / c, where c represents the propagation speed of electromagnetic waves in free space, and R t is the distance between ALPCM and the radar.

[0078] Performing range-direction matched filtering on the echo baseband signal, the pulse compression output result of the modulated echo signal can be obtained as:

[0079]

[0080] where, n = ±1, ±2, …, ±N is the order of discrete peaks along the range direction, σ is the point scattering intensity, which is a complex constant. It can be seen that the pulse compression output result of the modulated echo signal is a weighted superposition of multiple sinc discrete peaks.

[0081] This output result shows that the output position of the nth-order harmonic peak is:

[0082]

[0083] The interval between peaks of each order is:

[0084]

[0085] Therefore, in the one-dimensional range profile of the radar, the interval between peaks of each order is:

[0086]

[0087] For the zero-order peak, its amplitude is:

[0088] I0 = γα + 1 - γ (18)

[0089] The amplitude of the nth-order peak is:

[0090]

[0091] It can be seen from the formula that the positions and intervals of the peaks of the waveform after modulation are mainly affected by the frequency modulation slope K r of the radar transmitted signal and the modulation frequency f s . Specifically, it is inversely proportional to the frequency modulation slope K r and directly proportional to the modulation frequency f s ; the amplitudes of the peaks of each order are mainly affected by the duty cycle α and the conversion rate γ of the modulation waveform. Specifically, it is directly proportional to the duty cycle α of the modulation waveform and directly proportional to the conversion rate γ (when n ≥ 1).

[0092] Further extended to the PolSAR system with a time-sharing full-polarization measurement system, it can be obtained that the original imaging results of each channel are

[0093]

[0094] where \(t\) mH and \(t\) mV are slow-time variables consisting of the times when corresponding polarized electromagnetic waves are emitted, \(S\) HH and \(S\) HV and \(S\) VH and \(S\) VV represent the polarization scattering coefficients of the HH, HV, VH, and VV channels respectively, \(R\) B is the closest distance between the target and the PolSAR platform's trajectory, \(T\) L represents the synthetic aperture time, \(K\) a = \(2v\) 2 \(f\) c / \(cR\) B is the azimuth Doppler modulation frequency, 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:

[0095]

[0096] where \(I_{pq}(\cdot)\) represents the PolSAR image with \(q\)-polarized emission and \(p\)-polarized reception, \(t\) mq is the slow-time variable consisting of the times when \(q\)-polarized electromagnetic waves are emitted, \(S\) pqi represents the scattering coefficient corresponding to the \(i\)-th scatterer with \(q\)-polarized emission and \(p\)-polarized reception, and \(R\) Bi represents the closest distance between the \(i\)-th point target and the PolSAR platform's trajectory.

[0097] The co-polarized channel PolSAR imaging result after modulation and two-dimensional pulse compression in the range and azimuth directions is:

[0098]

[0099] where the amplitude coefficient \(B\) n = \(\gamma\alpha\) r \(\text{sinc}(n\alpha\) r ), \(\alpha\) r = \(T\) wr / \(T\) r is the range modulation duty cycle, the amplitude coefficient \(B\) m = \(\alpha\) a \(\text{sinc}(m\alpha\) a ), \(\alpha\) a = \(T\) wa / \(T\) ra is the azimuth modulation duty cycle, \(t\) m is the slow-time variable composed of \(t\) mH and \(t\) mV , and \(f\) 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.

[0100] It can be seen that the PolSAR imaging result under periodic ALPCM modulation shows a two-dimensional reticular harmonic array in the range and azimuth directions. Moreover, 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, 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.

[0101] Step 4: Generate the modulation result of the PolSAR image

[0102] According to the PolSAR image modulation model shown in formula (22), 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 result shows a two-dimensional reticular harmonic array in the range and azimuth directions. 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 harmonics can be changed by changing the modulation period, thereby realizing the regulation of the spatial position distribution of targets in the PolSAR image.

[0103] In addition, according to the expressions of the amplitude coefficients B n 、B m in formula (22), the amplitude of each channel is mainly determined by the conversion rate and the modulation duty cycle. And the change of the amplitude of each channel will cause the change of the target polarization characteristics. Therefore, periodic ALPCM modulation will jointly regulate the spatial position distribution and polarization characteristics of targets in the PolSAR image.

[0104] The five types of modulation signal models established in step two represent five typical situations. After modulating the five types of modulation signals, images are generated, and the target peak values of each channel are extracted as the elements of the polarization scattering matrix for Pauli decomposition and Cameron decomposition to extract polarization features. Among them, Pauli decomposition decomposes the polarization scattering matrix S into a weighted sum of the complex forms of four Pauli matrices. The four matrices represent the odd-order scattering mechanism of a flat surface, the dihedral angle scattering mechanism with a direction angle of 0°, the dihedral angle scattering mechanism with a direction angle of 45°, and the asymmetric scattering mechanism. Cameron decomposition divides the target into 11 categories: non-reciprocal component, asymmetric scatterer, symmetric scatterer, left-handed helix, right-handed helix, trihedral reflector, dihedral reflector, dipole scatterer, cylinder scatterer, narrow dihedral reflector, and quarter-wave device. The two decomposition methods are widely used in polarization feature decomposition and can obtain the basic scattering mechanism of the target through the polarization scattering matrix.

[0105] In type 1, the modulation waveforms of each channel are constants, so there is no harmonic effect. The equivalent polarization scattering matrix of the target is When the conversion rates are 0.9, 0.5, and 0.1 respectively, the Pauli decomposition is mainly dominated by the odd-order scattering component of the flat surface, and the Cameron classification results are dipole, cylinder, and trihedral respectively.

[0106] In type 2, the modulation waveforms of each channel are constants, and there is also no harmonic effect. The equivalent polarization scattering matrix of the target is The polarization feature decomposition result is similar to that of type 1.

[0107] In type 3, harmonic components are generated in the azimuth direction of each channel, as shown in Fig. 13(a) and Fig. 13(b). For the Pauli decomposition result, the proportion of the odd-order scattering component of the flat surface increases as the conversion rate decreases, and the proportions of the dihedral angle scattering component with a direction angle of 0°, the dihedral angle scattering component with a direction angle of 45°, and the asymmetric component decrease as the conversion rate decreases. The Cameron classification result is similar to that of type 1.

[0108] In type 4, harmonic components are generated in the azimuth direction of each channel, as shown in Fig. 14(a) and Fig. 14(b). The polarization feature decomposition result is similar to that of type 3. The same result also applies to type 5.

[0109] In summary, the radar polarization feature converter based on ALPCM can jointly regulate the spatial position distribution and polarization features of the targets on the PolSAR image, which proves the effectiveness of this method.

Claims

1. A time-varying modulation radar polarization feature transformer, characterized in that, It includes the following steps: Step 1: Establish an equivalent polarization scattering matrix model of the active linear polarization conversion surface (ALPCM). The radar polarization feature converter is implemented based on the ALPCM. Under different on-off states of the PIN diodes, the ALPCM realizes two functions of co-polarization conversion and cross-polarization conversion for linearly polarized incident electromagnetic waves. Time-varying modulation is to apply a periodically changing excitation signal to the ALPCM, so that it switches regularly between the two functional states. By establishing an equivalent polarization scattering matrix model in the two functional states, the variation relationship of the amplitude coefficients of electromagnetic waves in each channel under the modulation of the ALPCM controlled by the time-varying signal is obtained. Step 2: Establish a multi-channel equivalent modulation signal model based on the polarization synthetic aperture radar (PolSAR) imaging system. According to the ALPCM equivalent polarization scattering matrix model, the equivalent amplitude coefficients of each channel in the two states of diode on and off are obtained; when a periodically varying excitation signal is applied to the ALPCM, the equivalent modulation signals of the four channels corresponding to full polarization will also show a periodic variation law; for the PolSAR with a time-sharing full polarization measurement system, the modulation dimensions are divided into range modulation and azimuth modulation. The former is modulated within the pulse of the radar signal, and the latter is modulated between the pulses of the radar signal; through the switching period T of the excitation signal s The relationship with the azimuth parameter PRI of PolSAR imaging is used to establish a multi-channel equivalent modulation signal model suitable for five types of time-sharing full polarization measurement system PolSAR Step 3: Establish a PolSAR image modulation model. According to the multi-channel equivalent modulation signal model, the influence of the modulation on the radar echo is obtained. By analyzing the effect of the modulation on the pulse compression of the chirp signal, the corresponding PolSAR echo modulation signal model is obtained. Step 4: Generate the PolSAR image modulation result. According to the PolSAR echo modulation signal model, the periodic ALPCM modulation will jointly regulate the spatial position distribution and polarization characteristics of the targets in the PolSAR image. The parameters of the modulation signal affect the spatial position distribution of the targets, while the conversion rate of the ALPCM affects the polarization characteristics of the targets. Perform Pauli decomposition and Cameron decomposition on the modulated image under different parameters respectively.

2. A time-varying modulation radar polarization feature transformer according to claim 1, characterized in that: In Step 1, the reflection coefficient matrix in the co-polarization conversion state is expressed as: Among them, S xx represents the coefficient obtained by transmitting in x polarization and receiving in x polarization, and S xy represents the coefficient obtained by transmitting in y polarization and receiving in x polarization, and S yx represents the coefficient obtained by transmitting in x polarization and receiving in y polarization, and S yy represents the coefficient obtained by transmitting in y polarization and receiving in y polarization. These four coefficients are also the corresponding elements of the scattering matrix. At this time, the modulus value of the co-polarization channel coefficient reaches the maximum, and the modulus value of the cross-polarization channel coefficient reaches the minimum. Therefore, corresponding to the co-polarization conversion function, the scattering matrix in this state is denoted as S co-c ; The reflection coefficient matrix in the cross-polarization conversion state is expressed as: At this time, the modulus value of the co-polarization channel coefficient reaches the minimum, and the modulus value of the cross-polarization channel coefficient reaches the maximum. Therefore, corresponding to the cross-polarization conversion function, the scattering matrix in this state is denoted as S cr-c ; Assume that the actual polarization conversion rate is γ (0 ≤ γ ≤ 1), then the reflection coefficient matrix is expressed as: The co-cross polarization conversion is equivalent to the conversion of the reflection coefficients of each channel between S co-c and S cr-c This is the two equivalent polarization scattering matrix models of ALPCM.

3. A time-varying modulation radar polarization feature transformer according to claim 1, characterized in that: In step 2, assume that the radar transmits H-polarized signals within odd PRIs and simultaneously receives the target's H- and V-polarized echo components; within even PRIs, it transmits V-polarized signals and simultaneously receives the target's H- and V-polarized echo components; for ALPCM, 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; assume that the period T of the modulation signal a is an integer multiple of the PRI; therefore, the equivalent modulation signals of each channel depend on two factors. The first is the relationship between the period of the modulation signal and the PRI, and the second is the level of the initial PRI excitation signal.

4. A time-varying modulation radar polarization feature converter according to claim 1 or 3, characterized in that: In step two, in the analysis of five cases, p HH (t), p VH (t), p VV (t) and p HV (t) represent the equivalent modulation signal models of the four channels HH, VH, VV, and HV. Considering that the subsequent processing is independent of delay, this is therefore ignored; (1) Type 1: Modulation period 2×PRI, high level for odd PRI; This type of modulation makes the energy of the HH channel the highest and the energy of the VH channel the lowest; (2) Type 2: Modulation period 2×PRI, high level for even PRI; This type of modulation makes the energy of the VV channel the highest and the energy of the HV channel the lowest; (3) Type 3: Modulation period 4×PRI, equivalent high levels for odd and even PRIs; The equivalent waveforms of the HH and VV channels are the same, and the equivalent waveforms of the HV and VH channels are the same, as shown in Equation (6); (4) Type 4: Modulation period 6×PRI, high level for odd PRI; The equivalent waveforms of each channel are different from each other, as shown in Equation (7); (5) Type 5: Modulation period 6×PRI, high level for even PRI; The equivalent waveforms of each channel are different from each other, as shown in Equation (8); 5. A time-varying modulation radar polarization feature transformer according to claim 1, characterized in that: In Step 3, the time-domain response of the modulation signal is expressed as: Among them, the subscript co represents the co-polarization channel, the subscript cr represents the cross-polarization channel, t represents the time variable, rect(·) is a rectangular pulse signal, and T w is the modulation symbol width at a 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.

6. A time-varying modulation radar polarization characteristic converter according to claim 5, characterized in that: In Step 3, perform Fourier transform on the co-polarization channel periodic coding modulation waveform, and its spectral expression is: Among them, the amplitude coefficient 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; The radar transmission waveform of the PolSAR system is a chirp signal, and its expression is: Among them, represents the fast-time variable of the SAR system, T p is the pulse width, f c is the signal carrier frequency, K r = B / T p is the range chirp rate, and B is the signal bandwidth; After being incident on the ALPCM and reflected, the echo signal received by the radar is: where τ = 2R t / c, where c represents the propagation speed of electromagnetic waves in free space, and R t is the distance between the ALPCM and the radar.

7. A time-varying modulation radar polarization feature transformer according to claim 6, characterized in that: In Step 3, perform range-direction matching filtering on the echo baseband signal, and the pulse compression output result of the modulated echo signal is: Among them, n = ±1, ±2, ···, ±N is the order of discrete peaks along the range direction, σ is the point scattering intensity, which is a complex constant; the pulse compression output result of the modulated echo signal is the weighted superposition of multiple sinc discrete peaks.

8. A time-varying modulation radar polarization feature converter according to claim 7, characterized in that: In Step 3, the output position of the peak of the nth harmonic is: The interval between the peaks of each order is: Therefore, the interval between peaks of each order in the radar one-dimensional range profile is as follows: For the zero-order peak, its amplitude is: I0 = γα + 1 - γ (18) The amplitude of the nth-order peak is: The positions and intervals of the peaks of each order of the modulated waveform are affected by the frequency modulation slope K of the radar transmitted signal r and the modulation frequency f s Specifically, it is inversely proportional to the frequency modulation slope K r and directly proportional to the modulation frequency f s The amplitudes of the peaks of each order are affected by the duty cycle α and the conversion rate γ of the modulation waveform. Specifically, it is directly proportional to the duty cycle α of the modulation waveform and directly proportional to the conversion rate γ when n ≥ 1 9. A time-varying modulation radar polarization feature transformer according to claim 8, wherein: In step three, for the PolSAR system with a time-sharing full-polarization measurement system, the original imaging results of each channel are: where t mH and t mV are slow time variables consisting of the moments when corresponding polarized electromagnetic waves are emitted, S HH , S HV , S VH , S VV represent the polarization scattering coefficients of the HH, HV, VH, and VV channels respectively, R B is the closest distance between the target and the trajectory of the PolSAR platform, T L represents the synthetic aperture time, K a = 2v 2 f c / cR B is the azimuth Doppler modulation frequency, 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-polarization transmission and p-polarization reception, and t mq represents the slow-time variable composed of the moments when the q-polarized electromagnetic wave is transmitted, and S pqi represents the scattering coefficient corresponding to the i-th scattering point when transmitting with q-polarization and receiving with p-polarization, and R Bi represents the closest distance between the i-th point target and the trajectory of the PolSAR platform; The co-polarization channel PolSAR imaging results after modulation and two-dimensional pulse compression in the range and azimuth directions are: 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.

10. A time-varying modulation radar polarization feature transformer according to claim 1, characterized in that: In Step 4, under periodic ALPCM modulation, the internal energy of each channel of the PolSAR system is redistributed. The energy that was concentrated at the zero-order peak position before modulation is shifted at integer multiples of the modulation frequency. The imaging result shows a two-dimensional reticular harmonic array in the range and azimuth directions, and the interval between each peak is proportional to the modulation frequencies f r and f a , that is, inversely proportional to the modulation periods T r and T a . Therefore, by changing the modulation period, the position distribution of the harmonics can be changed, thereby achieving the regulation of the spatial position distribution of the targets in the PolSAR image.

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