Research On Inverse Synthetic Aperture Radar Imaging Of VHF-band

Imaging radar can get the high resolution of the distance and azimuth of the target through detection,so as to determine the geometric outline and structure of the target.It has the characteristics of all-weather and all-day.It has important meaning and applications in various fields.At present,most of the ISAR research focuses on microwave-band,and great progress has been made in imaging processing algorithm and motion compensation algorithm.However,with the changes of modern war environment,stealth aircraft and other stealth targets have been emerging on the battlefield,posing new challenges to ISAR.In the microwave-band,stealth target RCS is small,so radar is difficult to detect.The VHF-band has "natural" anti-stealth characteristics due to its long wavelength,but the performance of traditional meter-wave radar is not good in all aspects,so it can only treat the target as a "point".In this paper,the resolution of meter-wave radar is improved through the method of super-resolution algorithm,so as to realize the imaging of a stationary flying target at a long distance,establish a preliminary coarse imaging identification and judgment system,improve the early-warning distance and earlywarning ability of radar,and extend the battlefield response time for warfighters.The main work of this paper is as follows:(1)This paper introduces the basic knowledge of ISAR,establishes the equivalent rotary table model of the target in quasi-tangential motion,deduces its azimuthal resolution,and gives the restriction relationship between the size of imaging target and the resolution of ISAR image.For the VHF-band,the electromagnetic characteristics are analyzed to illustrate its advantages in detecting stealth targets.FEKO software was used for electromagnetic simulation of aircraft to observe target RCS at different frequencies and azimuth angles,and ISAR imaging experiments were conducted with ffe file echo data.At the same time,due to the narrow bandwidth of the VHF-band and the detection target is far from the radar,the limitation conditions of range resolution and azimuth resolution are studied,and the observation time required to achieve the desired azimuth resolution is calculated.The possibility of applying modern spectral estimation method to azimuth imaging is analyzed.It is shown that ISAR azimuth processing is equal to frequency estimation,and the processing flow of using super-resolution algorithm in azimuth is introduced.(2)Assuming that the aircraft target is traveling smoothly,it can be put on a rotating platform with uniform rotation after motion compensation algorithm.In this paper,two kinds of super-resolution imaging algorithms based on nonparametric spectral estimation are introduced: CAPON and APES.The algorithm expression is derived and the performance difference of the two algorithm is analyzed theoretically.Through the establishment of simulation signals,observation of experimental results,qualitative analysis of the resolution of the two algorithms and the accuracy of signal amplitude estimation.Then,by establishing scattering point models of two typical aircraft,changing flight speed,SNR,detection distance and other factors,some experiments were carried out.At last,compare the new algorithm with the classical FFT algorithm.(3)Two kinds of super-resolution imaging methods MUSIC and ESPRIT based on parametric spectral estimation are explored.This kind of algorithm belongs to the feature subspace method,which can estimate the parameters by decomposing the signal space.By constructing the echo signal,this kind of technology can be used to measure the frequency,and then restore the image through the relationship between the frequency and the scattering point position.In view of the decrease of estimation accuracy of MUSIC algorithm at low SNR,this paper discusses an improved MUSIC algorithm,which increases the anti-interference ability of the algorithm by modifying the noise space.ESPRIT algorithm requires less computation than MUSIC because it does not need peak search.Firstly introduces the basic principle of the algorithms.Because this algorithm needs to know the number of scattering points to be able to accurate classification of feature space,this paper introduces the scattering points number estimation judgment criterion.Finally,analysis of the algorithm under the conditions of different SNR.At the same time,we know that new method can improve the imaging quality contrast to RD algorithm...