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 located.
Methods to do this include:
- Frequency agility; varying one or more transmitted signal frequencies
- 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 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
- Directional antennas: focus as much energy as possible in a direction where it is needed, in as narrow a beam as the application permits. Search radars that have to be omnidirectional cannot use this technique.
- Ultra-low sidelobe antennas: highly directional antennas that minimize the energy in the parasitic sidelobes of the useful radar beam to 10-5 of the beam energy
- Encrypted and highly variable pulse trains
- Multistatic systems: locating and destroying one transmitter does not stop the system from working
- Passive radar: receivers that listen to electromagnetic energy sources not associated with radar, such as television
Another approach to the same problem is to keep high-energy radio or radar transitter completely separate from the tactical platform in harm's way, sending the processed information via laser, satellite radio coming from angles outside the field of potential interception, or, with ground platforms, separating the transmitter and user by a long cable.