Evaluation of a new autofocus algorithm within the framework of Fast Factorized Back-Projection

Rapport, 2011

The following report deals with a new autofocus algorithm within the framework of time domain SAR processing (Fast Factorized Back-Projection). The strategy, developed at SAAB EDS, relies on varying flight track parameters until a sharp image is obtained. Focus measures are provided by a predetermined object function. After a short introduction and some preliminaries, the algorithm is described in detail, results for two different data sets (collected by the CARABAS II system) are then presented, analyzed and discussed. The analysis emphasizes some promising attributes. Two of the autofocused images are for example sharper than corresponding references. It is also shown that two of the variation parameters are separable, this reduces the required number of geometry hypotheses substantially. However, some issues are also raised. A point-like target is for example easily distorted unless the data is upsampled prior to the processing. For the moment, it is not clear if this problem is related to the back-projection or the autofocus algorithm itself. The time consumption is another distinct downside. The report concludes that a lot of work remains to be done before the algorithm can be considered as functional. A sensitivity study with respect to the variation parameters, as well as a search for alternative object functions should be prioritized. A more conventional autofocus approach is also proposed, to keep options open for the future.