Signals Intelligence (SIGINT)

Intelligence gathering by interception of communications, lasers, or radio signals. Signals intelligence (SIGINT) became a critical tool for military intelligence gathering with the development of wireless communications in the late 19th century. Nearly all militaries deploy units that specialize in SIGINT, and the U.S. National Security Agency (NSA) specializes in this field of intelligence. SIGINT often involves encryption (coding) and cryptanalysis (decoding), since sensitive diplomatic and military communications are usually encrypted.

Types of Sigint

There are five types of signals intelligence. Communications intelligence (COMINT) is the interception, [Page 658]processing, and analysis of communications from foreign sources. These include voice messages, Morse code, or Teletype messages that may or may not be encrypted. COMINT often involves interception of diplomatic communications.

Electronic intelligence (ELINT) is the interception of signals from military and civilian hardware that is not used for traditional forms of communication, such as radar. Interception of signals from air defense radars allows analysts to discern their operating characteristics and devise ways for friendly aircraft to avoid detection. ELINT may also involve interception of signals from navigation and weapons tracking systems on aircraft, vehicles, or ships.

Radar intelligence is the detection of enemy aircraft or missiles and their operational characteristics by means of radar. Whereas ELINT scans enemy radar signals to gather information, radar intelligence collects data by sending out radar signals of its own.

Nonimaging infrared intelligence employs sensors that can detect the presence and movement of an object by its temperature. This type of intelligence gathering may be used at night or in other situations in which the detection of enemy forces by visual means is difficult.

Laser intelligence involves the interception and analysis of laser communications. Laser intelligence has been critical in the development of precision-guided munitions, or smart bombs. A laser called a target designator shoots a laser beam at a target and sensors in the smart bomb pick up and follow the beam to the target. Because this is a relatively new field of SIGINT, many activities remain classified.

History of Sigint

Signals intelligence is as old as the history of human conflict. Leaders in the American War of Independence, for example, employed relatively simple codes and ciphers as well as invisible ink in their communications. Encrypted British communications, intercepted and deciphered by the Americans, helped seal the fate of the British at the Battle of Yorktown in 1781.

However, the ability to act quickly on signals intelligence became possible only with the advent of wireless communications toward the end of the 1800s. Letters or battle plans inadvertently captured in combat may have been days or weeks out of date. Radio or telephone intercepts, on the other hand, provide current information and the technology to communicate them instantly to military commanders. As a result, SIGINT came to occupy a central role in the wars of the 20th century.

In World War I, the failure of the czar's forces to protect their communications led to a catastrophic Russian defeat at the hands of the Germans at the Battle of Tannenburg in 1914. On the other side of the Atlantic, the 1917 interception of the German Zimmerman telegram by the United States contributed significantly to the U.S. decision to enter World War I on the side of the Allies. The telegram outlined a German offer to annex large parts of the United States to Mexico if Mexico entered the war on the side of Germany.

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