# Passive Electromagnetic Reconnaissance **Entity class:** Concept **Research spine:** [[research/Chronology of Signals, Silos, and Sovereign Software - Research|Chronology of Signals, Silos, and Sovereign Software]] ## Definition Passive electromagnetic reconnaissance locates and characterizes emitters by receiving their signals without transmitting a revealing signal of its own. ## Chronological Proximity It matured during the 1942–1945 electronic war alongside ferret aircraft, H2S exploitation, and specialized jammers. ## Relational Proximity [[wiki/Ferret Aircraft|Ferret aircraft]] performed the collection; the results drove [[wiki/Radar Countermeasures|countermeasure]] design and the [[wiki/Recursive Electronic Warfare|reconnaissance–jamming loop]]. ## Collection Routes [[collections/The Civilian Intelligence Marketplace|The Civilian Intelligence Marketplace]] · [[collections/Terrorism, Counterterrorism, and the Intelligence Environment|Terrorism, Counterterrorism, and the Intelligence Environment]] · [[wiki/Signals, Silos, and Sovereign Software|Signals, Silos, and Sovereign Software]] ## Sources / Provenance - [[research/Chronology of Signals, Silos, and Sovereign Software - Research|Chronology of Signals, Silos, and Sovereign Software - Research]].