CN114966682B - ISAR range profile compensation method for plasma sheath cladding space high-speed target - Google Patents

Abstract

The present invention discloses an ISAR range image compensation method for a high-speed target in space covered by a plasma sheath, which mainly solves the problem that the prior art cannot compensate for the different intra-pulse coupling velocities caused by the plasma sheath, resulting in serious broadening and offset of the ISAR range dimension. The implementation scheme is: 1) establishing an echo signal model of a high-speed target in space covered by a plasma sheath after dechirp; 2) estimating the coupling velocity caused by the plasma sheath based on the fractional Fourier transform method to construct a phase compensation factor for compensating the echo signal in 1); 3) performing range dimension FFT on the compensated echo signal to obtain the compensated ISAR range image. The present invention solves the range dimension broadening and offset caused by different coupling velocities of the plasma sheath, effectively weakens the range dimension defocusing phenomenon, and lays the foundation for obtaining a well-focused ISAR image later, and can be used for accurate imaging and recognition of high-speed targets in space covered by a plasma sheath.

Description

ISAR range profile compensation method for high-speed targets in space covered by plasma sheath

The invention belongs to the field of radar imaging technology, and in particular relates to an ISAR range image compensation method for a space high-speed target, which can be used for accurate imaging and recognition of space high-speed targets covered by a plasma sheath.

Background technique

Due to the high speed of near-space hypersonic aircraft, its surface is covered with a plasma sheath, which produces a series of complex electromagnetic effects on electromagnetic waves, such as amplitude attenuation and phase distortion. Because the plasma sheath has fluid characteristics, it is affected by the high-speed flow, and the plasma sheath and the aircraft are relatively displaced to form a velocity field. Due to the distribution characteristics of the plasma velocity field, the coupling speed introduced to the echo at different positions of the target is different, which will cause the expansion and offset of the distance dimension during ISAR imaging, making it impossible for the ISAR image to form a good focus, affecting subsequent target recognition and detection.

In existing research, the idea of parameterized sparse representation is used for ISAR imaging of high-speed moving targets to achieve accurate reconstruction of ISAR images of high-speed moving targets. Luo Wenmao et al. proposed a range image method to compensate for the movement of echo pulses. This method estimates the target pulse motion parameters through frequency modulation Fourier transform, and compensates for the offset in the distance dimension caused by high speed by constructing a phase compensation factor. However, this method has not yet solved the problem of plasma sheath coupling, resulting in waveform thickening and defocusing in the distance dimension. It is impossible to obtain the ISAR range image under the plasma sheath, which is not conducive to subsequent ISAR imaging and cannot accurately identify the target.

Summary of the invention

The purpose of the present invention is to address the deficiencies of the above-mentioned prior art and to propose an ISAR range image compensation method for a high-speed target in space covered by a plasma sheath. The method is used to estimate the velocity of the target scattering point due to sheath coupling, and to construct a compensation factor for compensation, so as to solve the problem of expansion and offset in the distance dimension caused by the intra-pulse coupling effect of the plasma sheath on different positions of the target. Finally, a good ISAR range image can be obtained, which is convenient for the subsequent acquisition of a well-focused ISAR image, thereby facilitating the detection and identification of high-speed targets in space.

To achieve the above object, the technical solution of the present invention comprises the following steps:

S1) Establish the echo signal model of the high-speed target in space after dechirp under the plasma sheath:

S11) Input the modulation frequency μ, pulse width T _p , carrier frequency f _c , and fast time of the linear frequency modulation pulse signal of the inverse synthetic aperture radar Slow time _tm and full time t, according to these parameters to determine the transmission signal of the inverse synthetic aperture radar

S12) Input the pulse width T ₀ and reference distance R ₀ of the signal at the reference target, and construct a reference signal based on the radar transmission signal of S11)

S13) Input the complex reflection coefficient Γ _k and amplitude A ₀ at each scattering point of the space high-speed target covered by the plasma sheath, and use the parameters of S11) to determine the echo signals of Q scattering points of the space high-speed target covered by the plasma sheath

S14) performs frequency difference processing on the reference signal in S12) and the echo signal in S13) to obtain the echo signal of the high-speed target in space after dechirp under the plasma sheath

S2) Estimation of coupling velocity based on fractional Fourier transform (FRFT)

S21) for the echo signal Perform FRFT transformation and calculate the result after FRFT transformation Perform peak search to obtain the coordinates of the peak point Estimate the starting frequency of the linear frequency modulation signal based on the coordinates Modulation frequency

S22) according to the estimated modulation rate The coupling velocity of the kth scattering point is obtained by smoothing calculation for M pulses:

Where n = 1, 2, 3...M is the number of pulse echoes, k = 1, 2, 3...Q is the number of scattering points, and c is the speed of light;

S3) Using the coupling velocity estimated in S2) Construct the phase compensation factor H _k (t):

S4) Use the phase compensation factor H _k (t) to perform phase compensation on the echo signal after dechirp, and then Make fast time The compensated ISAR range image is obtained by FFT transformation.

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

First, the present invention constructs a phase compensation factor in the distance dimension, which can not only weaken the target's deviation in the distance dimension caused by high speed, but also effectively weaken the intra-pulse movement caused by the different coupling speeds of the plasma sheath to each scattering point of the target, thereby obtaining a better ISAR range image;

Secondly, since the present invention uses fractional Fourier transform FRFT to calculate the coupling velocity introduced by the plasma sheath, compared with the existing Wigner-Vera distribution WVD method, it can avoid the interference of cross terms, make parameter estimation more accurate, improve the accuracy of the phase compensation factor, and thus enhance the defocus suppression effect in the distance dimension.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an implementation flow chart of the present invention;

Figure 2 is a single-cycle ISAR one-dimensional range image of an uncompensated plasma sheath-enveloped high-speed target;

FIG3 is a single-period ISAR one-dimensional range image of a high-speed target covered by a plasma sheath obtained by the present invention;

Figure 4 is a multi-period ISAR range image of an uncompensated plasma sheath-encapsulated high-speed target;

FIG. 5 is a multi-period ISAR range image obtained by using the plasma sheath obtained by the present invention to cover a high-speed target after compensation.

Detailed ways

In order to make the purpose and technical solution of the present invention clearer, the embodiments and effects of the present invention are further described in detail below with reference to the accompanying drawings.

The current distance dimension compensation method for inverse synthetic aperture radar ignores the problem of intra-pulse mismatch caused by the coupling velocity caused by the sheath changing with time in the pulse at high speed, which leads to more serious broadening and offset of the ISAR distance dimension. The present invention studies and discusses the ISAR distance dimension defocus compensation method for plasma sheath-enclosed high-speed targets. Aiming at the ISAR distance dimension defocus phenomenon caused by the coupling velocity of the plasma sheath, based on the echo signal of the high-speed target in space after dechirp under the plasma sheath, the echo signal is subjected to fractional Fourier transform and parameter estimation, so as to obtain the coupling velocity caused by the sheath at the scattering point, and the phase compensation factor is constructed by using the coupling velocity to compensate for the echo signal of the high-speed target in space after dechirp under the plasma sheath, which can effectively weaken the distance dimension defocus caused by the coupling of different scattering points of the high-speed target covered by the plasma sheath with different intra-pulse velocities.

Referring to Figure 1, the implementation steps of this example are as follows:

Step 1: Establish the echo signal model of the high-speed target in space after dechirp under the plasma sheath.

The echo signal after dechirp of the high-speed space target covered by the plasma sheath is an echo signal that takes into account the modulation effect of the plasma sheath. It is based on the delay of the transmission signal to construct a reference signal, and the echo data of the high-speed space target coupled with the plasma sheath is processed by dechirping the echo data, which is implemented as follows:

1.1) Input the modulation frequency μ, pulse width T _p , carrier frequency f _c , and fast time of the linear frequency modulation pulse signal of the inverse synthetic aperture radar Slow time _tm and full time t, according to these parameters to determine the transmission signal of the inverse synthetic aperture radar

Where rect() is a rectangle function;

1.2) Input the pulse width T ₀ and reference distance R ₀ of the signal at the reference target, based on the radar's transmitted signal Constructing a reference signal

Where τ ₀ =R ₀ /c is the reference delay, c is the speed of light;

1.3) Input the complex reflection coefficient Γ _k and amplitude A ₀ at each scattering point of the space high-speed target covered by the plasma sheath, and use the parameters of 1.1) to determine the echo signals of Q scattering points of the space high-speed target covered by the plasma sheath

Where k = 1, 2, 3... Q is the number of scattering points, is the time delay of the echo signal of the kth scattering point, _Rk is the distance between the kth scattering point and the radar, and _{vk is} the coupling velocity of the kth scattering point caused by the sheath;

1.4) The reference signal and echo signal Perform difference frequency processing to obtain the echo signal of the high-speed target in space after dechirp under the plasma sheath

in for conjugate signal.

Step 2: Estimation of coupling velocity based on fractional Fourier transform (FRFT)

Fractional Fourier Transform (FRFT) is a method of expressing a signal in a fractional Fourier domain formed by rotating the coordinate axis counterclockwise around the origin at any angle in the time-frequency plane. It can be regarded as a representation of the signal rotating counterclockwise at any angle on the time axis to the u-axis, which is called the fractional Fourier domain. The specific implementation of this step is as follows:

2.1) Echo signal After FRFT transformation, the transformed result is obtained

Where Γ _FRFT represents FRFT transformation, (θ, u) is the coordinate plane after FRFT, A is the amplitude constant introduced in the calculation process;

2.2) Results after FRFT transformation Perform peak search to obtain the coordinates of the peak point

Among them, argmax() is a function that finds the maximum independent variable;

2.3) Using the peak point coordinate parameters, estimate the starting frequency of the linear frequency modulation signal Modulation frequency

in, are the peak point coordinates respectively;

2.4) Based on the estimated modulation rate The coupling velocity of the kth scattering point is obtained by smoothing calculation for M pulses:

Among them, n = 1, 2, 3...M is the number of pulse echoes, and k = 1, 2, 3...Q is the number of scattering points.

Step 3, using the estimated coupling velocity Construct the phase compensation factor H _k (t).

Using the estimated coupling velocity Based on echo signal The phase term in is used to construct the phase compensation factor H _k (t) based on the form of the linear frequency modulation signal:

Step 4: Use the phase compensation factor H _k (t) to obtain the compensated ISAR range image.

The echo signal is compensated by the phase compensation factor H _k (t). Compensation can eliminate the intra-pulse velocity v _k and fast time The coupling of the generated linear and quadratic terms is implemented as follows:

4.1) The echo signal Multiplying with the phase compensation factor H _k (t) yields the compensated dechirp signal

4.2) Yes Make fast time The FFT transform of tm is used to obtain the compensated ISAR range image s(f, _tm ):

The effect of the present invention can be further illustrated by the following simulation.

1. Simulation conditions:

Given the radar parameter information, the radar carrier frequency is 10GHz, the bandwidth is 1000MHz, the pulse width is 100us, the pulse repetition frequency is 1000Hz, the number of multi-cycles is 128, and the flight altitude is 50Km.

Given the target motion parameters, the target motion speed is 15 Mach. Input the target motion parameter information, select 5 scattering points, the motion speeds are 5130m/s, 4975m/s, 3020m/s, 2985m/s, 4824m/s respectively. The signal-to-noise ratio SNR = 0dB.

Simulation software: Matlab.

2. Simulation content:

Simulation 1 is to perform single-cycle FFT of the distance dimension on the dechirp echo signals of the selected 5 scattering points under the above simulation conditions using the existing technology, and obtain the single-cycle ISAR one-dimensional range image of the space high-speed target covered by the plasma sheath, as shown in Figure 2. In Figure 2, the horizontal axis is the range direction and the vertical axis is the amplitude.

As can be seen from Figure 2, due to the coupling effect of the sheath in the distance dimension, each scattering point shows obvious broadening and offset. Under low signal-to-noise ratio, the signal is submerged and the position information of the scattering point cannot be obtained.

Simulation 2, under the above simulation conditions, the dechirp echo signals of the selected 5 scattering points are first compensated by the compensation method of the present invention, and then the FFT of the range dimension is performed to obtain the compensated single-cycle ISAR one-dimensional range image, and the result is shown in Figure 3. In Figure 3, the horizontal axis is the range direction and the vertical axis is the amplitude.

As can be seen from FIG3 , under low signal-to-noise ratio, five obvious peaks still appear, and the offset and broadening of the one-dimensional range image are effectively suppressed, so that the peak is focused.

Simulation 3, under the above simulation conditions, the echo signals after dechirp of the selected 5 scattering points are subjected to multi-cycle FFT of the distance dimension using the existing technology, and the ISAR range image of the space high-speed target covered by the plasma sheath at 128 cycles is obtained, as shown in Figure 4. In Figure 4, the horizontal axis is the slow time and the vertical axis is the distance dimension.

As can be seen from FIG4 , multiple curves appear in the distance direction, and the distance image has a serious defocusing phenomenon, which is caused by the coupling of different intra-pulse velocities of the sheath.

Simulation 4, under the above simulation conditions, the range image simulation of the dechirp echo signals of the selected 5 scattering points under multiple cycles is performed. The dechirp echo signals are first compensated by the compensation method of the present invention, and then the FFT of the range dimension is performed to obtain the compensated 128-cycle ISAR range image, as shown in Figure 5. In Figure 5, the horizontal axis is the slow time and the vertical axis is the range dimension.

It can be seen from Figure 5 that all straight lines are perpendicular to the range axis, and the straight line energy gain at the actual position of the target is significantly increased, and the defocusing in the range dimension is effectively suppressed.

In summary, the present invention makes up for the deficiency of the existing inverse synthetic aperture radar range dimension compensation method that cannot compensate for the waveform thickening caused by the plasma sheath coupling intra-pulse velocity, which causes the range dimension defocusing. The FRFT method is used to estimate the different intra-pulse coupling velocities of the target scattering point caused by the sheath, and a compensation factor is constructed to compensate for it. The distance offset and broadening phenomenon caused by the plasma sheath coupling velocity can be effectively weakened, the ISAR range dimension defocusing suppression effect is improved, and the foundation for the subsequent realization of reliable ISAR images is laid, which is beneficial to the detection and identification of high-speed space targets under the plasma sheath.

Claims (9)

1. A method for compensating an ISAR range image of a high-speed target in space covered by a plasma sheath, characterized in that it comprises the following: S1) Establish the echo signal model of the high-speed target in space after dechirp under the plasma sheath: S11) Input the modulation frequency μ, pulse width T _p , carrier frequency f _c , and fast time of the linear frequency modulation pulse signal of the inverse synthetic aperture radar Slow time _tm and full time t, according to these parameters to determine the transmission signal of the inverse synthetic aperture radar S12) Input the pulse width T ₀ and reference distance R ₀ of the signal at the reference target, and construct a reference signal based on the radar transmission signal of S11) S13) Input the complex reflection coefficient Γ _k and amplitude A ₀ at each scattering point of the space high-speed target covered by the plasma sheath, and use the parameters of S11) to determine the echo signals of Q scattering points of the space high-speed target covered by the plasma sheath S14) performs frequency difference processing on the reference signal in S12) and the echo signal in S13) to obtain the echo signal of the high-speed target in space after dechirp under the plasma sheath S2) Estimation of coupling velocity based on fractional Fourier transform (FRFT) S21) for the echo signal Perform FRFT transformation and calculate the result after FRFT transformation Perform peak search to obtain the coordinates of the peak point Estimate the starting frequency of the linear frequency modulation signal based on the coordinates Modulation frequency S22) according to the estimated modulation rate The coupling velocity of the kth scattering point is obtained by smoothing calculation for M pulses: Where n = 1, 2, 3...M is the number of pulse echoes, k = 1, 2, 3...Q is the number of scattering points, and c is the speed of light; S3) Using the coupling velocity estimated in S2) Construct the phase compensation factor H _k (t): S4) Use the phase compensation factor H _k (t) to perform phase compensation on the echo signal after dechirp, and then Make fast time The compensated ISAR range image is obtained by FFT transformation. 2. The method according to claim 1, characterized in that the inverse synthetic aperture radar transmission signal determined in S11) It is expressed as follows: Where rect() is a rectangle function. 3. The method according to claim 1, characterized in that the reference signal is determined in S12) It is expressed as follows: Wherein τ ₀ =R ₀ /c is the reference delay, and c is the speed of light. 4. The method according to claim 1, characterized in that the echo signal is determined in S13) It is expressed as follows: Where k = 1, 2, 3... Q is the number of scattering points, is the echo signal delay of the kth scattering point, _Rk is the distance between the kth scattering point and the radar, and _vk is the coupling velocity of the kth scattering point caused by the sheath. 5. The method according to claim 1, characterized in that the echo signal of the high-speed target in space after dechirp is obtained in said S14) It is expressed as follows: in for conjugate signal. 6. The method according to claim 1, characterized in that the echo signal in S21) The result after FRFT transformation is expressed as follows: Where Γ _FRFT represents FRFT transformation, (θ, u) is the coordinate plane after FRFT, A is the amplitude constant introduced in the calculation process. 7. The method according to claim 1, characterized in that in said S21), a peak search is performed on the result after FRFT transformation to obtain the coordinates of the peak point It is expressed as follows: Among them, argmax() is a function that finds the maximum independent variable. 8. The method according to claim 1, characterized in that the starting frequency of the linear frequency modulation signal is estimated in S21) Modulation frequency The formula is as follows: in are the peak point coordinates. 9. The method according to claim 1, characterized in that the dechirp signal after compensation in S4) Make fast time The FFT transformation of is used to obtain the compensated ISAR range image, which is implemented as follows: S41) The echo signal Multiplying with the phase compensation factor H _k (t) yields the compensated dechirp signal S42) Yes Make fast time The FFT transform of tm is used to obtain the compensated ISAR range image s(f, _tm ):