Research On Aerostat Borne Inverse Synthetic Aperture Radar Imaging Techinique For Aircraft

Inverse synthetic aperture radar(ISAR) is a high-resolution two-dimensional imaging radar which can generate the image of some moving targets, such as aircrafts, satellites, missiles and ships. It is capable of working in all-weather, all-day and long-distance condition and it has the important application value in strategic defense, battle reconnaissance, fire evaluation and so on. Therefore, ISAR is a critical component in radar early warning. Nowadays, many countries including China are formulating the integrated ground-ocean-air-space early warning system. In this system, implementing the aerostat to the radar surveillance has become an frontier research topic as the advantages of long working time, high cost-effectiveness and excellent performance of stealth and anti-stealth. The aerostat borne ISAR imaging for the aircraft is deeply researched in his paper. The major subjects include: the influence of the unstationary motion of the aerostat on the ISAR imaging, the high-resolution imaging and motion compensation used in aerosate ISAR imaging for aircrafts, the approach of reducing the micro-motion interference in propeller-driven aircraft imaging, the imaging method for multiple aircrafts in a single radar beam and the cross-range scaling method for the aircraft image generated by aerostat borne ISAR.At the beginning of the paper, the fundamental theroies and approaches of ISAR imaging for aircraft under the stationary platform is studied. In this part, the motion model and the echo model of the surveillant aircraft is built. Moreover, the principle of two-dimensional ISAR imaging and the Range-Doppler(RD) algorithm is presented. The classical motion compensation approaches, including the maximum cross- correlation criterion and the module-1 criterion of range alignment and the phase gradient autofocus, are also expressed. Then, as the aerostat is not stationary actually, the three-dimentional motion model of aerostat is built. On the basis of kinetic equations and kinematical equations, this paper analysed the motion characteristics of aerostat in the circumstance of airflow disturbance. By the simulation, the influence of the unstationary motion of aerostat on ISAR imaging for aircraft is shown in detail.As the unstationary motion of aerostat may induce complicated Doppler components, the results generated by traditional imaging and compensation methods are unsatisfactory. For solving this problem, the high-resolution imaging algorithm which can be employed in the unstationary aerostat borne ISAR for the aircraft is proposed. The basic idea is that the echoes received during the airflow disturbance can be removed firstly by processing raw data into time sparse form and then the high-resolution ISAR image of target can be generated by compressed sensing(CS) technique. As the phase errors induced by translational motion of the aircraft is still existing after removing the affected echoes, a novel autofocus approach on the basis of two optimisation methods: the gradient projection method and the conjugate gradient method, called GP-CG autofocus, is also proposed in this paper. The phase errors can be estimated and the focused ISAR image of aircraft can be obtained simultaneously by dual iterative computation of GP-CG autofocus on the conditon of some echoes missing. This approach is verified by simulated data and real data processing so it can be employed in the engineering.The major goal of aerostat borne ISAR imaging for the aircraft is generating high quality image of main body. However, some aircrafts are equipped with external propellers whose motion is different from that of the main body. The rigid body assumption of the whole target is destroyed by the fast rotation of propeller and the micro-Doppler effect is caused. The micro-Doppler effect generates sidebands about the return Doppler frequency of the main body. These sidebands may cover some parts of the body in the ISAR image and make the image defocus, which interferes with the future target recognition. It is considered as an micro-motion interference and should be reduced. Most of the current reducing approaches spend much time. In this paper, we analyze the signal characteristic of aircraft propeller in detail in the aerostat borne ISAR by taking the blade as a unit and discover the flickering feature of the propeller signals in the high-range resolution(HRR) image. Based on this feature and the compressed sensing(CS) technique, we propose a novel approach which can fast reduce the micro-motion interference of propeller and generate the image of the aircraft body with acceptable quality simultaneously. An available method for detecting the range bins containing the propeller by the use of waveform entropy in the HRR image is also presented. This reducing approach meets the requirement of the real-time application as the calculated amount is small. The validity is demonstrated by processing the simulated data and real data and the results are satisfactory.As the aerostat borne radar has far surveillant distance, the radar beam covers wide scope and many aircrafts fly in formation sometimes, Multiple aircrafts may be observed in a single radar beam. Since the movements of these aircrafts may not be same, using the imaging algorithm for single target will blur the image. There are two types of imaging methods for multiple aircrafts: direct imaging and separated imaging. This paper proposes a range instantaneous Doppler(RID) imaging method based on chirplet-WVD to generate a high resolution ISAR image of multiple aircrafts directly when the aerostat is unstationary. This method can also suppress many kinds of clutter. Besides, a method of imaging multiple aircrafts separately based on K-means clustering algorithm is also proposed. It is suitable for the conditon that the original ISAR image of these aircrafts are obviously are overlapped in some parts.The ultimate purpose of generating the high resolution aircraft image by aerostat borne ISAR is realizing the aircraft recognition and classification. The size is a key feature for aircraft recognition whereas it cannot be obtained directly from the ISAR image. Since the range resolution is known in the ISAR imaging but the cross-range resolution is generally unknown because the motion is noncooperative during the ISAR imaging period. The cross-range scaling approach is needed to estimate the cross-range resolution. In this paper, a cross-range scaling approach for ISAR image of aircraft based on adaptive chirplet transformation is investigated. It extracts the chirp rates of the chirplet atoms matching with the signals of multiple range bins and uses least square linear fitting to reduce the scaling error. For dealing with the condition that the rotation angle of the aircraft target is large, the modified approach is also proposed. This cross-range scaling approach can estimate the cross-range resolution exactly and it is also effective in processing the aircraft ISAR image interfered by the micro-Doppler effect.