Three decades of active microwave imaging achievements, difficulties and future challenges

This paper provides an overview of the microwave imaging techniques, which have been developed during the last three decades for ISM applications. This overview is not limited to the state of the art in terms of software and equipment issues. Taking profit of a significant time stand off, the real impact of these techniques with respect to their targeted applications is critically analyzed. Beyond undisputable achievements, specific persisting difficulties are identified as well as future challenges.