Fine-Time Resolution Passive RF Source Imaging

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Radio Frequency (RF) sensing and communicating is now ubiquitous. Moreover, with RF emissions being generated using generic semiconductor technologies and with the available spectrum becoming ever more crowded, there is an inexorable convergence of civilian and military systems. Thus all systems are now characterised by frequency, beam-pattern and pulse-pattern agility; because the technology allows it. A radar system will exploit agility to avoid detection and to avoid interference or ambiguity while interleaving multiple service functions whereas communication systems again exploit agility in the search for bandwidth matching varying data rate needs. Thus the problem of gaining situation awareness, by passively monitoring RF transmissions, is becoming ever more fraught with difficulty. Radar and communications systems are no longer characterised by steady repetitive signal structures and as a result it is difficult to collect enough energy from individual sources to detect them, let alone to characterise or locate them. Thus there is a need to exploit not just the direct line-of-sight signals but also to multi-path reflections of the primary source. Of particular interest is the analysis of finer details in the signals relating to a specific source in order to reconstruct more detailed understanding of the source itself, via what might be described as a source image. Reflected signals will tend to have lower power than the direct interception and it is likely that acceptable performance will require the use of multiple-sensors able to share information and synthetic aperture techniques to gain resolution.