Study Of Imaging And System Design Of Missile-Borne Synthetic Aperture Radr

In modern high-technology war, precision-guided weapons have been widely used and have played an important role in modern air striking force. With the increasingly severe attacking-defense rivalry and more complicated battlefield environment, there is a higher requirement of precision-guided weapons. The long range precision-guided weapons are developed towards high-precision, high-speed, high break-defensive capability, high-intelligence, high destroying capability and low-cost. As the core technology of precision-guided weapon, precision-guided technologies have been used in many strategic and tactical missiles, which greatly improve the efficiency of weapon systems. Because of the increasingly high requirement of striking precison and severe battlefield conditions, the single guidance technology is far from being enough to meet new needs, while combined guidance technology has become an important developing direction. The multiple combined technologies have made their appearance, giving priority to inertial navigation and aided by GPS, infrared ray, terrain matching etc. Yet, they all have their own features and limitations, and hardly satisfy the requrement in severe electromagnetism environment and weather conditions.Synthetic aperture radar (SAR) is an active microwave remote sensor. it can work all-time, all-weather, gain high resolution image of the ground target similar to optical photograph in low-visibility weather conditions. What's more, the working mechanism of SAR has better anti-jamming ability. Thus, it is reasonable to do the following steps in order to greatly improve the striking force of missiles:use missile-borne SAR to obtain the characteristic information of targets or the typical physiognomy near the targets, form real-time image, match it with the reference image already prepared and stored in the missile, calculate the missile's precise position parameters according to the result of matching and related information, make use of this position information to correct the accumulative errors of INS and then control the missile to hit the target. The combination of SAR and INS can be used either in the end-stage of guided weapon operation course for guidiance or in the middle-stage for navigation. As the combination of SAR with other navigation systems used for guided weapon is an important developing direction, it is of great practical meaning to do research on the technology of missile-borne SAR. The present dissertation intends to study the relevant problems in application of SAR to guided missile. The platform movement of missile-borne SAR discussed here is different from that of the conventional airborne and spaceborne SAR. Our focus is on the issues such as SAR system design, imaging, image geometric calibration, image geometric calibration error when the missle moves in high-speed, non-level straight line. The main contribution can be summarized as follow.In chapter 2, the principle of SAR and the special issues faced by missile-borne SAR are showed briefly. The basic principle of general SAR and geometric relation of imaging is firstly introduced. Then SAR's signal model is given and the character of the azimuth signal is illumiated. Followed, the essential of utilizing Doppler information of signal to improve azimuth resolution is explained and a general expression of azimuth resolution is shown which is suited either to strip SAR or to spotlight SAR. Finally the features of platform movement of missile-borne SAR and the special issues faced by it are presented.Chapter 3 investigates the geometric model, signal model and imaging problem of missile-borne SAR. According to platform movement features during imaging, the geometric model is set up and the general expression (or mathematic model) of range between point target and radar in single aperture time is founded. By expanding the range expression, the 1 st,2nd and third order expansion coefficients are gained. From the expansion coeficients and the given movement parameter of missile-borne SAR, the errors of range approximation of 2nd and third degree are calculated in single aperture time and the features of the Doppler parameter variation with time through the entire imaging process are obtained, which indicate that complete-aperture imaging processing is hard to realize. By sub-aperture imaging simulation experiment, the focalized SAR image is attained, which testifies the validity of sub-aperture imaging processing.Because the height of the missile is always varing during the imaging process, serious geometric distortion lies in the SAR images acquired through usual imaging processing, which will greatly affect the subsequent image match. So geometric calibration is necessarily required. Chapter 4 discusses the geometric correction methods of distorted SAR image. The geometric model used for geometric correction is presented. According to the geometric correction model together with sub-aperture R-D(Range-Doppler) and extended CS(Chirp Scaling) imaging algorithm, the geometric correction mathematic models suitable for the mentioned two imaging algorithms respectively are developed. The correctness of the geometric correction methods are demonstrated by simulation experiments.In chapter 5, the error of geometric correction is explored. The mathematic expressions involved with geometric correction error are educed which are suited to sub-aperture R-D and extended CS imaging algorithm respectively. With prepared parameters, the effect of platform height error, level speed error, vertical speed error and slant range error on geometric correction is analyzed and a general conclusion is reached. Simulation experiments of geometric correction error are made respectively for the two imaging algorithms and the validity of error analysis is proved.Chapter 6 discusses the problems related to missile-borne SAR system design. Firstly, the NEσ0(Noise Equivelentσ0, whereσ0 denotes radar reflectivity of the earth's surface) of system for sub-aperture imaging processing is analysed detailedly and the expressions of SNR(Signal-to-Noise Ratio) and NEσ0 are developed. Secondly, the basic composition, major index and system design of missile-borne SAR are introduced. Finally, examples are given to illustrate method of system parameter design according to the major system index.PRF(Pulse Repeat Frequency) is an important parameter in SAR system, and its selection needs considering several factors. In chapter 7, the design of PRF of missle-borne SAR is addressed in detail. Emphasis is placed on the analysis of relations between PRF and signal ambiguity, PRF and the timing sequence restriction of radar signal transmitting and echo signal receiving as well as PRF and antenna size. As an example, a simulation is made to illustrate the selection of PRF when the timing sequence restriction relation is considered. In the end, the design method and procedure of PRF for missile-borne SAR is summarized.Finally, A summary of the present work is made and suggestions of further work are also put forward.