Editing Signals intelligence (section)

===Traffic analysis===
{{Main|Traffic analysis}}
When locations are known, usage patterns may emerge, from which inferences may be drawn. Traffic analysis is the discipline of drawing patterns from information flow among a set of senders and receivers, whether those senders and receivers are designated by location determined through [[direction finding]], by addressee and sender identifications in the message, or even [[MASINT]] techniques for "fingerprinting" transmitters or operators. Message content other than the sender and receiver is not necessary to do traffic analysis, although more information can be helpful.

For example, if a certain type of radio is known to be used only by tank units, even if the position is not precisely determined by direction finding, it may be assumed that a tank unit is in the general area of the signal. The owner of the transmitter can assume someone is listening, so might set up tank radios in an area where he wants the other side to believe he has actual tanks. As part of [[Operation Quicksilver (WWII)|Operation Quicksilver]], part of the [[deception]] plan for the invasion of Europe at the [[Invasion of Normandy|Battle of Normandy]], radio transmissions simulated the headquarters and subordinate units of the fictitious [[First United States Army Group]] (FUSAG), commanded by [[George S. Patton]], to make the German defense think that the main invasion was to come at another location. In like manner, fake radio transmissions from Japanese aircraft carriers, before the [[Battle of Pearl Harbor]], were made from Japanese local waters, while the attacking ships moved under strict radio silence.

Traffic analysis need not focus on human communications.  For example, a sequence of a radar signal, followed by an exchange of targeting data and a confirmation, followed by observation of artillery fire, may identify an automated [[counterbattery fire]] system.  A radio signal that triggers navigational beacons could be a radio landing aid for an airstrip or helicopter pad that is intended to be low-profile.

Patterns do emerge.  A radio signal with certain characteristics, originating from a fixed headquarters, may strongly suggest that a particular unit will soon move out of its regular base.  The contents of the message need not be known to infer the movement.

There is an art as well as science of traffic analysis. Expert analysts develop a sense for what is real and what is deceptive. [[Harry Kidder]],<ref>{{Cite web|url=https://www.nsa.gov/about/cryptologic-heritage/historical-figures-publications/hall-of-honor/Article/2014217/chief-harry-kidder-usn/|title=About|website=nsa.gov}}</ref> for example, was one of the star cryptanalysts of World War II, a star hidden behind the secret curtain of SIGINT.<ref>{{cite journal|last=Whitlock|first=Duane|title=The Silent War against the Japanese Navy|journal=Naval War College Review|volume=48|issue=4 |pages=43–52 |date=Autumn 1995|url=http://www.ibiblio.org/pha/ultra/nwc-01.html|access-date=30 September 2007}}</ref>