SIGINT is abbreviated from SIGnal INTelligence. It is one of the major forms of intelligence. Signals Intelligence satellites are designed to detect transmissions from broadcast communications systems such as radios, as well as radars and other electronic systems. The Interception of such transmissions can provide information on the type and location of even low power transmitters, such as hand-held radios. However, these satellites are not capable of intercepting communications carried over land lines, such as undersea fiber optic cables.
Signals Intelligence (SIGINT) comprises of Communications Intelligence (COMINT), which is directed at the analysis of the source and content of message traffic. While most military communications are protected by encryption techniques, computer processing can be used to decrypt some traffic, and additional intelligence can be derived from analysis of patterns of transmissions over time. Electronic Intelligence (ELINT) is a devoted analysis of non-communication electronic transmissions. This would include telemetry from missile tests (TELINT), or radar transmitters (RADINT).
Even if a signal is human communications (e.g., a radio), the intelligence collection specialists have to know it exists. If the targeting function described above learns that a country has a radar that operates in a certain frequency range, the first step is to use a sensitive receiver, with one or more antennas that capture signals in every direction, to find an area where such a radar is operating. Once the radar is known to be in the area, the next step is to find its location.
If operators know the probable frequencies of transmissions of interest, they may use a set of receivers, preset to the frequencies of interest. These are the frequency (horizontal axis) versus power (vertical axis) produced at the transmitter, before any filtering of signals that do not add to the information being transmitted. Received energy on a particular frequency may start a recorder and alert a human to listen to the signals if they are intelligible (i.e., COMINT). If the frequency is not known, the operators may look for power on primary or sideband frequencies using a spectrum analyzer signals. Information from the spectrum analyzer is then used to tune receivers to the signals of interest. For example, in this simplified spectrum, the actual information is at 800 KHz and 1.2 MHz.
The earliest, and still common, means of direction finding is to use directional antennas as goniometers so that a line can be drawn from the receiver through the position of the signal of interest. HF/DF knowing the compass bearing from a single point to the transmitter does not locate it. Where the bearings from multiple points, using goniometry, are plotted on a map, the transmitter will be located at the point where the bearings intersect. This is the simplest case; a target may try to confuse listeners by having multiple transmitters, giving the same signal from different locations, switching on and off in a pattern known to their user but apparently random to the listener.
Individual directional antennas have to be manually or automatically turned to find the signal direction, which may be too slow when the signal is of short duration.
It is the study of who is signalling whom and in what quantity. When locations are known, usage patterns may emerge, and inferences are 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.