Passive situation awareness: a case study

Future conflicts will feature dense threats to low flying aircraft in the vicinity of the battle area. Lethal defenses will include anti-aircraft artillery (AAA) and surface-to-air missiles (SAMs). Survival of the aircraft in these environments is of the highest priority, and often depends on knowing the locations of such threats so that evasive maneuvers can be taken. This paper describes a unique, totally passive technique based on terrain reflections for precisely locating hostile radar emitters. Unlike conventional techniques for emitter location and passive ranging, this method does not require direct line of sight to the emitter; all that is required is that there exist terrain areas that are mutually visible to both the emitter and the receiver. On board a tactical aircraft, it would allow the pilot to locate threats before being exposed, thus allowing him to alter his route to utilize terrain masking. This will result in higher survivability and more stealthy ingress and egress through defended areas. The results of brassboard tests operating on signals from a non-cooperative emitter indicate that the system has the potential to locate non-cooperative emitters within less than 500 meters CEP, independent of range and azimuth, Below line of sight, and within 10-20 seconds of initial signal detection.