Low probability of intercept
Low probability of intercept electromagnetic emitters, such as radio and radar transmitters, use a number of mechanisms to minimize the probability they will be detected by electronic intelligence sensors, and, if detected, precisely location.
Methods to do this include:
- Frequency agility
- Spread spectrum: Ultra-wide band signals distribute a given amount of energy over frequency, with the same effect of hardening an ELINT receiver's task of detecting them. The radar itself knows which signal it is looking for, and the receiver circuitry has been matched to the signal properties.  The inherent wide bandwidth of spread spectrum can be combined with rapidly changing frequency changing within the band, not putting equal power in all frequencies at any one time
- Ultra-low Sidelobe Antennae: reduce the energy in the parasitic sidelobes of the useful radar beam to 10-5 of the beam energy; these are highly directional antennas
- Encrypted and highly variable pulse trains
- Multistatic systems: locating and destroying one transmitter does not stop the system from working'
- BBC H2G2, Low Probability of Intercept Radar