S. G. Likhansky

The problem of ground-based moving target motion parameters estimation (without ambiguity) with the aid of six synthetic aperture radars (SAR) based on group of six space vehicles is being explored at the paper. Only one SAR of six is working as active one (i.e. both emitting and receiving), other five SAR are passive (i.e. only receiving). Each SAR is Active Phased Antennae Array. The group of space vehicles is being divided into two triplets (groups of three very-close flying spaceships). The whole group is flying along the only trajectory in Greenwich coordinate system (GCS) ? not in Orbital (Sidereal, Inertial) coordinate system (OCS). SAR data processing within the SAR-system is both coherent (Moving Target Detection (MTD) within triplets using along-way interferometry with relatively short base) and non-coherent one (velocity and coordinate ambiguity resolution between different remote triplets using bi-static simultaneous multi-look with «large» base). The imaging mode in each SAR is Stripmap Mode (SMM) having overlapped cadres (i.e. FFT bases for azimuth convolution) and short SAR-look session (10 sec); each SAR-look has very small non-zero squint angle. The complex image synthesis algorithm is Chirp Scaling Algorithm (CSA) «in LFM-approximation». The general problem is being discussed at the paper is «Extended MTD problem» (EMTDP) ? unambiguous estimation of the whole set of MT current motion parameters. The coherent interferometrical MTD in SAR having tree (or four) phase receiving centers in mono-static look provides separation of MT and background pure complex images (or two MT-s and background pure complex images). But it is impossible in mono-static situation to determine four MT current parameters - radial velocity, tangential velocity, along-way coordinate and slant normal range coordinate, because the knowledge of only three values - MT image coordinates pair and the linear shift variation law along the SAR aperture - is not enough for EMTD task decision (in general case). Bi-static non-coherent look between two remote SAR triplets allows to determine the whole set of MT current parameters without ambiguity due to knowledge of «excessively enough» amount of input EMTDP values (i.e. coherent MTD output values) ? different values (non-linearity of global phase variation!) of phase shifts within different triplets ? «active» and «passive», MT image way and range coordinate shifts between different triplets, MT image «absolute» coordinate values in each triplet. Picking up different «fours» from the whole set of coherent MTD output values provides different equation systems (both «in variations» and «absolute» ones). Such of these equation systems which are «non-degenerative» are providing various methods of correct unambiguous estimation of four MT current movement parameters. While obtaining all necessary formulae the MT complex image formation procedure (by CSA) was thoroughly examined with the aid of migration laws accounting the SAR-beam properties and the GCS-trajectory curvature. Specifically, the «reduced velocity» of space vehicle and the «Local coordinate system» of scene have been explored. The analysis of obtained equation systems have been generated three methods (algorithms) of EMTDP decision called as «method 1», «method 2» and «method 3». Each method has the certain range of applicability. The method 1 is the general method applicable always when tangential MT velocity is non-zero (not «very small»). The method 2 is the very-special method applicable only in case when tangential MT velocity is zero or «very small». The method 3 is the universal method of EMTDP decision applicable in the whole velocity range of ground-based targets, but it demands very large overlapping of cadres (more then 64 times, «sliding-window mode») and large amount of calculations (especially on stage of synthesis). This method 3 is intended only for processing of six complex digital holograms (of the whole SAR-look session) in Earth-based high-speed computers (synthesis-MTD-EMTDP). The general method 1 being combined with special method 2 is intended for «mixed» data processing - space-based (synthesis and coherent along-way MTD within triplets) and Earth-based one (EMTDP). The correctness of methods 1, 2 and 3 had been proved by thorough examination of obtained equations and formulae and (partially) by computer simulation.