Research On ISAR Imaging Of High-speed Space Target Under Plasma Sheath

Inverse Synthetic Aperture Radar(ISAR)is a new radar system capable of high-resolution two-dimensional imaging,which can realize the imaging and recognition of space targets.With the rapid development of high-speed aircraft in adjacent space,radar detection and imaging technology for high-speed space targets have become a research hotspot in recent years.During the flight of a high-speed space target,the aircraft’s surface will rub violently with the air,forming a plasma sheath covering the target surface.Plasma sheath is an electromagnetic medium that will interact with electromagnetic waves.When ISAR imaging the high-speed space target covered by plasma sheath,the radar echo will be affected by the plasma sheath,which will cause energy attenuation and phase error,causing ISAR defocusing.Therefore,according to the spatial distribution of plasma sheath parameters,we calculated the reflection depth of electromagnetic waves in different areas of the plasma sheath,obtained the spatial distribution of plasma sheath reflection characteristics,and studied the mechanism of the influence of the plasma sheath on radar echoes,including the pulse pressure mismatch caused by the movement in the echo pulse,the mechanism of ISAR defocusing on range dimension by the plasma sheath dispersion effect,and the agency of ISAR defocusing on azimuth dimension by the time-varying plasma sheath.ISAR echo model of space high-speed target under plasma sheath was established.An accurate matched filtering method based on estimating intra-pulse motion parameters,a range imaging method based on maximum contrast self-focusing,and an azimuth imaging method based on phase gradient self-focusing is proposed.Finally,ISAR imaging of space high-speed targets under the plasma sheath is realized.The main innovations and research contents of this paper are as follows:1.Aiming at the physical characteristics of plasma sheath,such as velocity field,dispersion effect,and time variation,the ISAR echo model of space high-speed target under the influence of plasma sheath is established.The mechanism of the impact of the plasma sheath on the radar echo is analyzed.The velocity of the plasma sheath flow field introduces a time scale factor into the radar echo.The dispersion effect of the plasma sheath causes phase distortion in the echo pulse,and the time variation of the plasma sheath causes the phase jitter between the echo pulses.These three problems are coupled,causing the ISAR imaging defocusing of the plasma sheath.Based on the influence factors of plasma sheath on radar echo,the ISAR echo model of space high-speed target under plasma sheath is established.2.An accurate matched filtering method for wideband radar echoes covered by plasma sheath is proposed.The plasma sheath’s flow velocity will cause the radar echo’s intra-pulse movement.The study found that the echo of scattering points at different positions on the target surface will have various pulse pressure mismatches when matched filtering.It will not only cause the displacement and energy diffusion of the one-dimensional range profile but also cause the intra-pulse movement to be coupled with the problems caused by the plasma sheath’s dispersion effect and time variability.To achieve accurate pulse compression processing of scattering point echo,firstly,fractional Fourier transform and successive elimination technology are used to estimate the velocity of solid scattering points at different positions of the target to obtain the intra-pulse velocity of strong scattering point echo.According to the velocity estimation results,a set of matching filters are designed for the compensation of each strong scattering point to compensate for the intra-pulse motion of different scattering points accurately.The problem of broadening and migrating of one-dimensional range profile is solved,and the coupling between pulse motion,dispersion effect,and time variation problem are eliminated.By precisely matched filtering,the PSNR of the ISAR image is increased by3.4 d B,and the SSIM is increased by 0.29.3.An ISAR range imaging method based on maximum contrast self-focusing is proposed.The frequency dispersion effect of the plasma sheath will cause nonlinear changes in the phase of the radar echo within the bandwidth.During ISAR imaging,the nonlinearity of the phase will cause scattering points to diffuse in the range dimension,causing ISAR range defocusing.The dispersion phase error model is fitted by the least square method.The quadratic term in the model is the main reason for range diffusion.The method based on maximum contrast autofocusing is used to search and estimate the quadratic coefficients in the phase error model.First,we calculated the initial value of the quadratic term coefficient search and the search step length through the dispersion broadening relationship in the range direction.Then we gradually find the dispersion phase error quadratic term coefficient corresponding to the image’s maximum contrast by circular search compensation value and calculating the image contrast.By constructing the compensation factor of the dispersion phase error,we complete the suppression of ISAR range defocusing caused by the plasma sheath dispersion.After the maximum contrast autofocus method is used for range imaging,the range dimension broadening of the ISAR image is effectively suppressed,the PSNR of the image is increased by 2.3d B,and the SSIM of the image is increased by 0.11.4.An ISAR azimuth imaging method based on phase gradient autofocus is proposed.ISAR azimuthal imaging relies on the inter-pulse coherence of multi-period echoes.While the time variation of the plasma sheath destroys the inter-pulse coherence,the radar echo generates an inter-pulse phase jitter,and the scattering point Doppler expands,causing ISAR azimuthal defocusing.Due to the complex time-varying factors of plasma sheath,it is impossible to obtain an accurate model of inter-pulse phase error.We used the phase gradient autofocus method to obtain the phase error estimation of the complex model.Firstly,the strong scattering point is moved to the Doppler center by cyclic displacement to eliminate the influence of the Doppler phase term.Then the clutter interference is eliminated by adding a window in the center to improve the estimation accuracy.Then,using the linear unbiased minimum variance estimation of phase gradient,the inter-pulse phase error of scattering points is estimated.The phase error compensation factor is obtained to eliminate the ISAR azimuth defocusing caused by the time variation of the plasma sheath.After using the phase gradient autofocus method for azimuth imaging,the problem of ISAR azimuth defocusing is effectively suppressed,the PSNR of the image is improved by 4.2 d B,and the SSIM of the image is improved by 0.11.In summary,this paper profoundly analyzes the influence of plasma sheath velocity field,dispersion effect,and time variation on ISAR echo.We proposed an accurate matched filtering method for radar echo intra-pulse motion for solving the problem of one-dimensional range profile broadening and migration to eliminate the coupling between intra-pulse activity,dispersion effect,and time variation.For the dispersion effect,a range imaging method based on maximum contrast self-focusing is proposed to suppress the range-defocusing problem of ISAR caused by echo phase Intra pulse distortion.An azimuth imaging method based on phase gradient autofocus is proposed to suppress the ISAR azimuth defocusing caused by echo phase jitter.The research results are of great significance to the realization of space high-speed target radar detection,tracking,and recognition.