Electronic countermeasures (ECM) are a subsection of electronic warfare which includes any sort of electrical or electronic device designed to prevent or disrupt electromagnetic signalling. It may be used both offensively and defensively. ECM often takes the form of jamming, whereby a jamming signal is transmitted by an ECM system to block the reception of other signals within the bandwidth of the jamming signal. However, in general the jamming signal has a finite physical area of effect determined by the area distribution of its radiated energy. Outside this area the jamming may be only partially effective.
Some types of ECM jamming systems are commonly used as part of a convoy of vehicles to protect against hostile use of the electromagnetic spectrum. In these cases, the ECM jamming system provides a mobile area of protection against, for example, remote detonation of an explosive device on or near the convoy's path, while the convoy passes by. Because microwave energy cannot be sensed by involved personnel and ECM operators, determining the real-time, in-situ protection offered by an ECM system is a problem endemic to the use of ECM.