Autofocus for very high resolution airborne SAR imaging on wide swath

Abstract:
Recent improvement of the experimental SAR facility at ONERA allowed 1.2 GHz (synthetic) bandwidth signal acquisition without deramp on receive, thus allowing imaging swath of about 1 km with range resolution of about 10 cm. Attaining the same resolution along-track requires trajectory knowledge with an accuracy far beyond what is provided by inertial and/or global positioning system (typical need is a millimetre relative accuracy on a time scale of ten seconds). Hence autofocus techniques, resorting to the radar signal itself to update the trajectory estimate, have been introduced. The method, however, required designation by the user of point-like echoes (generally user deployed corner reflector setup) suitable for trajectory triangulation and is not convenient for imaging wide areas. Therefore new autofocus unsupervised algorithms have been designed. They involve automated detection, selection and phase tracking of point-like echoes. They also need to be tolerant to erroneous echoes (most notably reflection on rounded surface that produce "moving" non stationary echoes) that user previously discarded from phase history visual aspect. The dissertation below present the different alternative trajectory update strategy, mainly based on either phase gradient integration or unwrapped phase triangulation. Other autofocus schemes such as frame-drift and contrast based autofocus are also addressed.