SIGINT (signals intelligence) is intelligence obtained from the interception and analysis of electromagnetic signals. It divides into two primary subdisciplines: COMINT (communications intelligence), which intercepts messages between parties, and ELINT (electronic intelligence), which intercepts non-communication emissions such as radar signals, weapons guidance systems, and telemetry.
SIGINT’s operational advantage is scale. Unlike HUMINT, which depends on individual relationships developed over months, SIGINT can collect against many targets simultaneously through automated systems. The NSA’s Project SHAMROCK, exposed by the Church Committee, intercepted international telegrams at scale for decades. Contemporary SIGINT collects across the electromagnetic spectrum — radio, microwave, satellite, fiber-optic, and digital network traffic — using both ground-based and space-based systems.
The discipline depends on the adversary communicating electronically and on the collector’s ability to intercept, decrypt, or interpret what is collected. Encrypted communications resist collection unless the encryption is broken or circumvented. The adversary’s use of communication security — changing codes, using couriers instead of radios, employing burst transmissions — is a direct countermeasure to SIGINT. Metadata analysis (who communicated with whom, when, and from where) can produce intelligence even when message content is unrecoverable.
SIGINT’s role in surveillance of domestic populations — particularly through programs authorized under the PATRIOT Act and Section 702 of FISA — represents the convergence of military collection capabilities and political surveillance that the intelligence discipline must analytically distinguish even as it acknowledges their institutional entanglement.
Related terms
- HUMINT — human collection that provides context SIGINT cannot
- MASINT — measurement of physical signatures that complements electronic intercepts
- OSINT — open-source collection that can provide targeting data for SIGINT
- counterintelligence — the detection of adversary SIGINT operations against one’s own communications