Research On 3D Imaging Techniques Of Space Target Via Spaceborne Interferometric ISAR

As a new kind of imaging radar, Inverse Synthetic Aperture Radar (ISAR) can generate images of moving targets such as aircrafts, missiles, ships, satellites, and therefore plays important roles in both military area including strategic defense, tactics arms, and civil applications such as intending air and space traffic control. In order to image space targets such as spaceship, space station, artifact satellite and space debris more accurately and effectively, this paper proposes that ISAR imaging equipments be mounted on satellite formation platforms to establish interferometric 3D ISAR space target imaging system. On the one hand, such configuration avoids the influence of atmospheric window effectively; on the other hand, as another dimensional information added to the target compared with traditional 2D image, the solid geometry structure can be discripted more clearly, so we can achieve the same target recognition ratio from only one 3D image instead of series of 2D images. Howerver, spaceborne ISAR/interferometric ISAR imaging copes with new problems that tranditional ones never fall across, this paper ananlyses and investigates in depth these new problems, then proposes corresponding solutions. Main research contents are listed as below:(1) Principles of ISAR and interferometric ISAR imaging are elaborated. Turntable imaging and motion compasation techniques of ISAR imaging are enphasize, which lay groundwork for the following researches on spaceborne ISAR imaging under the condition of radar moving platform. Key techniques of traditional interferometric ISAR imaging are summarized, which lay groundwork for comparison and analysis with key techniques under the satellites formation interferometric ISAR imaging in the following researches.(2) Aiming at the new problems resulted from motion of both observation satellite and space target, imaging characters of spaceborne ISAR and applied conditions of RD algorithm are analyzed. Firstly, combined with the orbital motion description of space object by six essential elements and motion compensation theories of ISAR imaging, the influence of orbital motion of observation satellite on ISAR imaging is analyzed; then from signal echoes, the effects of dithering amplitude, frequency and original phase on ISAR imaging are investigated, and results of quantitative analysis are given; and then, two kinds of imaging modes, orbital motion primary and rotational motion domination, are investigated separately according to motion characters of space target; finally, the applied condition of RD algorithm is pointed out. This part affords the fundamentals for further spaceborne interferometric ISAR investigation. (3) In order to carry out 3D imaging of space target, spaceborne interferometric ISAR imaging system based on satellite formation is established and relative algorithms and problems are investigated. Firstly, as spaceborne ISAR formation no longer satisfies the requirement of orthogonal configured antennae geometry for tranditional interferometric ISAR, this paper brings forward interferometric ISAR algorithm with non-orthogonal configured antennae. According to the new imaging algorithm, formation round-flying effect on 2D imaging and interferometric imaging are investigated quantitatively, and through analyzing correlation of signal echoes and the principal value interval of height, the limitation of vertical and horizontal baseline are assured. Based on this, multi-attitude ISAR imaging algorithm is put forword, which makes use of variation of antanna view angle induced by long baseline to get multi-attitude images. This algorithm can obtain more shape and structure information compared with tranditional ISAR. More over, interferometric ISAR 3D imaging algorithm for target with unknown orbital motion target is proposed, in order to resolve the problem that cross-range can't be estimated from ISAR image when equivalent rotational angle is unknown. Through decomposing, predigesting and compensating phase of the echoes of three channels induced by target motion, cross-range and height information are directly estimated from phase of the echoes.(4) Multi-baseline phase unwrapping method for five satellites formation is put forward to enhance height accuracy in the whole round-flying period. Here, six height accuracy elements are deduced firstly from interferometric ISAR algorithm under non-orthogonal configured antennae, and the relationship between height imaging accuracy and phase unwrapping is determined by practical influence analysis of the six elements. And then, two phase unwrapping methods using scattering points correlation are researched: minimum spanning tree algorithm and colony optimization algorithm, while the results show that these methods will make error when numerous height discontinuous points are present. Thus, ultilizing ISAR satellite self-preponderant, this paper investigates multi-baseline unwrapping algorithm based on satellites formation which makes full use of horizontal and vertical variation during the period of satellite formation round-flying. The proposed method reduces phase error and meliorates the horizontal to vertical ratio, so height presicion can be enhanced in the whole flying period. Computer simulaton and results comparation demonstrate that this method is effective.