Research On Imaging Algorithm For The Missile-borne Synthetic Aperture Radar

Missile-borne synthetic aperture radar (SAR) is an active microwave remote sensor. It can work all-time, all-weather, gain high resolution images of the target from which we can get the characters of the targets by imaging detection, such as shape and size. In addition, by using the imaging results, we can also correct the accumulative errors of INS and then control the missile to hit the target by matching operation. So, missile-borne SAR imaging has played an important role in modern war and becoming an important developing direction. But in the need of attacking the target, the movement of missile-borne SAR discussed here is larger different with the conventional airborne and spaceborne SAR. The velocity in the vertical direction and some special flights for evading antiaircraft area make the missile's movement flexibility. So, for the missile-borne SAR system, the classical imaging algorithms available for airborne and spaceborne SAR are disabled. Our focus is on the issues such as side-looking SAR, squint SAR and forward-looking SAR when the missile moves in high-speed, non-level straight line. The main contributions can be summarized as follows.1.According to the SAR 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 1st, 2nd and third order expansion coefficients are gained. From the expansion coefficients and the given movement parameter of missile-borne SAR, the range history in a synthetic aperture length is analyzed in details. The accurate two dimensional frequency spectrum expression is derived by the method of series reversion, followed a new imaging algorithm for missile-borne SAR diving acceleration movement is proposed. Finally, simulation results are presented to demonstrate the accuracy and validity of the proposed algorithms.2.The large range migration produced by missile diving acceleration flight make SAR image difficult. The echo model of the missile-borne SAR is established by using the high order range model based on the characters of the missile movements. Considering the large scene, the change of the slant range is analyzed in details. Then, the two-dimensional point target spectrum is derived by the method of series reversion. A large scene imaging algorithm used for diving acceleration flight is presented. Finally, simulation results are presented to demonstrate the accuracy and validity of the proposed algorithms. The resolution of range and azimuth are identical with the theoretical values.3.According to the characters of the large squint angle which cause degradations for the imaging results, an efficient chirp scaling algorithm was proposed. Firstly, the range cell migration was corrected in the time domain based on the characters of the SAR system and received echo, followed by the CS operation. Then the range compression and secondary range compression (SRC) were finished in the two-dimensional frequency domain. When azimuth compressed, the third order phase is compensated. So, the quality of the focus improved. Several simulation and processing results are presented to demonstrate the validity of the proposed algorithms. Finally, several problems for squint SAR imaging were analyzed and explained.4.For the SAR imaging theory, an angle between the direction of the SAR platform and the beam is necessary. In the end stage of missile flight, the directions of the missile and the beam are the same which make the SAR imaging disabled. Based on the bistatic theory, a bistatic forward-looking SAR model is proposed. The transmitter and receiver are mounted on the plane and missile, respectively. The ambiguity function is derived from the geometry of bistatic missile-borne forward-looking SAR system. The results of the simulations demonstrate the efficacy of the proposed ambiguity function. Further, by analyzing the ambiguity function, it is clear to demonstrate the influence of the bistatic missile-borne configuration on the system resolution, which provides principle for the selection of system parameters.5.An innovative missile-borne forward-looking SAR system is proposed based on the problem that side-looking SAR can not image in flight direction with high azimuth resolution. According to the spatial geometry and the model of echo signal, the signal spectrum derived by the method of series reversion and the phase compensation factors and realized steps of algorithm are given. Several simulation and processing results are presented to demonstrate the validity of the proposed algorithms.