In military telecommunications, electronic support or electronic support measures gather intelligence through passive "listening" to electromagnetic radiations of military interest. They are an aspect
RAF Menwith Hill, a large ECHELON site in the United Kingdom, and part of the UK-USA Security Agreement.
French ship Dupuy de Lôme, specialised in SIGINT
Signals intelligence
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Signals intelligence is the act and field of intelligence-gathering by interception of signals, whether communications between people or from electronic signals not directly used in communication. As
Electronic warfare support measures
…of military interest. They are an aspect of electronic warfare involving actions taken under direct control of an operational commander to detect, intercept, identify, locate, record, and/or analyze sources of radiated electromagnetic energy for the purposes of immediate threat recognition (such as warning…
A German message intercepted by the British during World War II, signaling Germany's unconditional surrender
Zimmermann Telegram, as decoded by Room 40 in 1917
A Mark 2 Colossus computer. The ten Colossi were the world's first programmable electronic computers, and were built to break the German codes.