Editing Signals intelligence (section)

===Direction-finding===
{{Main|Direction finding}}
The earliest, and still common, means of direction finding is to use directional antennas as [[goniometer]]s, so that a line can be drawn from the receiver through the position of the signal of interest. (See [[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. One alternative is the [[Wullenweber]] array technique. In this method, several concentric rings of antenna elements simultaneously receive the signal, so that the best bearing will ideally be clearly on a single antenna or a small set. Wullenweber arrays for high-frequency signals are enormous, referred to as "elephant cages" by their users.

A more advance approach is [[Amplitude-comparison monopulse|Amplitude comparison]]. An alternative to tunable directional antennas or large omnidirectional arrays such as the Wullenweber is to measure the [[time of arrival]] of the signal at multiple points, using [[GPS]] or a similar method to have precise time synchronization. Receivers can be on ground stations, ships, aircraft, or satellites, giving great flexibility. 

A more accurate approach is [[Interferometry|Interferometer.]] 

Modern [[anti-radiation missile]]s can home in on and attack transmitters; military antennas are rarely a safe distance from the user of the transmitter.