Abstract: Though accurate motion compensation for SAR satelliteimaging is not an acute problem at present, it willprobably become one in the near future with increase inthe ground resolution of available sensors. Thisproblem is already nowadays extremely pertinent forairborne SAR imaging, especially with slow and/orunmanned aircrafts. Image registration is often thefirst and highly critical step in many operational usesof SAR imaging. We illustrate some of thoseapplications with highly distorted airborne SAR images.Those applications are of two main classes, one beingthe registration of two similar SAR images without anexternal reference, and the second being theregistration of a SAR image with another type ofgeographical data such as an optical image or map. Thelocal similarity of the images in the first class ofproblems leads us to adapt a technique from opticalflow motion evaluation. We used a local correlationtechnique with a low dimensional parametrization of themotion to be estimated. The second class of problems isdealt by first extracting linear (sometimes pointwiseor areal) features and then matching them by means of amulti hypothesis filtering. We shall describe here fourapplications of image registration: (1) The first oneis the registration of single-look images obtainedwithin the same overflight in order to compensate fortrajectory errors not detected by the aircraftnavigation system. This registration not only allowsthe multi-look incoherent summation, but also therefocusing of the single-look images to increaseazimuth resolution. This process is the type of'auto-focusing' known as frame-drift. (2) Multi-lookregistration also provides an accurate evaluation ofground speed of the carrier aircraft that can beexploited for internal navigation unit hybridization,even in the absence of cartographic knowledge of thearea flown over. This application is called'odometering.' (3) Multi-look registration isfurthermore extremely critical for moving vehiclesdetection and their proper motion estimation usingmulti-look imaging on wide beam antennas. This is oneversion of the problem known as 'moving targetindication.' (4) Registration of a SAR image with a mapor an optical image of a given area is also used foraccurate localization of objects detected ingeographical coordinates, for their designation toother systems. It may also be used for accuratelylocalizing the carrier air-vehicle, especially whenunmanned. This is the 'navigation resetting problem.'!9
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