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Though accurate motion compensation for SAR satellite imaging is not an acute problem at present, it will probably become one in the near future with increase in the ground resolution of available sensors. This problem is already nowadays extremely pertinent for airborne SAR imaging, especially with slow and/or unmanned aircrafts. Image registration is often the first and highly critical step in many operational uses of SAR imaging. We illustrate some of those applications with highly distorted airborne SAR images. Those applications are of two main classes, one being the registration of two similar SAR images without an external reference, and the second being the registration of a SAR image with another type of geographical data such as an optical image or map. The local similarity of the images in the first class of problems leads us to adapt a technique from optical flow motion evaluation. We used a local correlation technique with a low dimensional parametrization of the motion to be estimated. The second class of problems is dealt by first extracting linear (sometimes pointwise or areal) features and then matching them by means of a multi hypothesis filtering. We shall describe here four applications of image registration: (1) The first one is the registration of single-look images obtained within the same overflight in order to compensate for trajectory errors not detected by the aircraft navigation system. This registration not only allows the multi-look incoherent summation, but also the refocusing of the single-look images to increase azimuth resolution. This process is the type of 'auto-focusing' known as frame-drift. (2) Multi-look registration also provides an accurate evaluation of ground speed of the carrier aircraft that can be exploited for internal navigation unit hybridization, even in the absence of cartographic knowledge of the area flown over. This application is called 'odometering.' (3) Multi-look registration is furthermore extremely critical for moving vehicles detection and their proper motion estimation using multi-look imaging on wide beam antennas. This is one version of the problem known as 'moving target indication.' (4) Registration of a SAR image with a map or an optical image of a given area is also used for accurate localization of objects detected in geographical coordinates, for their designation to other systems. It may also be used for accurately localizing the carrier air-vehicle, especially when unmanned. This is the 'navigation resetting problem.'2955
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