4 research outputs found
SAR Imagery in Non-Cartesian Geometries
Get PDFABSTRACT The subject of the reported work is the improvement of geometrical models for a SAR scanning in Pushbroom, Spotlight, Scansar or Bistatic imaging modes. These researches have been motivated by the planetary cornerstone mission of ESA's long term programme for European Space Science ("Rendez-vous" with a comet, and Fly-Bys of asteroids). In this specific context, the synthetic aperture radar is destined for an important role, but the rules and standard backgrounds of the cartesian geometry are no longer justified. Several new techniques are proposed to handle with an optimal precision the data relative to celestial bodies with a complex geometry (coherent and non-coherent imagery). On the basis of a mathematical rigour (singleness of solutions, convergence of processes, biunivocity of transformations and generalizations), a lot of scenarios are discussed with key relations established (plane and spherical models, bodies with a symetry of revolution and general bodies, specific sensor(s) trajectories as Fly-Bys or flight into orbit with the possibility of an approaching probe). The four methods developed are the tomographic analogy of radar principles (only known, previously, in the usual case of a straight line flight at constant altitude over a plane surface) and Hilbertian techniques for a direct adaptation to the scanned surface geometry, an automated autofocusing which enhances the contrast resulting from a cartesian reconstruction and the coordinates transformation where the real space is converted into a fictitious space where cartesian algorithms are fully rigorous. Beyond the fact that an interpolation step is often unavoidable, the major conclusion of the research is that all the prospected techniques are complementary and that the choice between the methods has to be made according to geometry, objectives and time requirements (reconstruction on board or not). In particular, coordinates transformation techniques are worthy of commendation in case of plane (wavefront curvature balancing) or spherical models in a monostatic situation. Autofocusing methods (judicious ponderation between the usual reconstruction and a reconstruction of the derivative of the key expression of the mathematical formalism with regard to one of its parameters) has proved its validity in an hilly regions of the East of Belgium with low differences in contrast, while the Hilbertian principles are general methods without any restriction on the paths of the probes, the geometry of the celestial body, the modulation scheme and antennae radiation pattern. On the other hand, the tomographic analogy can be applied in all situations where a correct model of the body relief is available, but there are some approximations in the formalism (no antenna pattern modelling, no balancing of the Range Migration)
