Editing Signals intelligence (section)

===Need for multiple, coordinated receivers===
First, atmospheric conditions, [[sunspot]]s, the target's transmission schedule and antenna characteristics, and other factors create uncertainty that a given signal intercept sensor will be able to "hear" the signal of interest, even with a geographically fixed target and an opponent making no attempt to evade interception. Basic countermeasures against interception include frequent changing of [[radio frequency]], [[Polarization (waves)|polarization]], and other transmission characteristics. An intercept aircraft could not get off the ground if it had to carry antennas and receivers for every possible frequency and signal type to deal with such countermeasures.

Second, locating the transmitter's position is usually part of SIGINT. [[Triangulation]] and more sophisticated [[radiolocation|radio location]] techniques, such as [[time of arrival]] methods, require multiple receiving points at different locations. These receivers send location-relevant information to a central point, or perhaps to a distributed system in which all participate, such that the information can be correlated and a location computed.