Updated: 2023-05-18 --- # ESM Principles ESM or electronic support measures is a collective term for systems that detect and localize electromagnetic signals. Any emitter that sends out electromagnetic energy such as radars and communication devices can be detected and localized by ESM equipment. ESM equipment can detect signals at very long range. Radars can generally be detected at between 1.5 and 2 times the radars own detection range. Communication equipment uses weaker signals, but can still generally be detected at extended ranges. By measuring the time of arrival of a signal to antennas at multiple point on an aircraft, the direction the signal is being received from. Over time, this information can be used to determine the emitters exact position. # Lines of Bearing One way to determine the position of an emitter is to measure the direction of arrival of a signal and plot a line of bearing (LOB) along that heading, then move the aircraft and do this again from another position. This process is repeated until enough lines of bearing have been created that a area of probability can be created that will have the emitter somewhere in it. This process is done automatically by ESM software very quickly, the longer the emitter is active and the faster the aircraft is moving, the faster and more accurate the localization of the emitter. The counter to this localization tactic is to turn the emitter on only for short bursts. In addition this tactic is not very effective for moving contacts. ![[Pasted image 20230811235050.png|400]] # Cross Fix Another tactic to localize an emitter is called a cross fix. It requires at least two entities to have ESM information on an emitter at the same time. By comparing the LOB from two or more different ESM systems at the exact same time, the exact position of the emitter can be determined instantly. This tactic enables short burst emitters or emitters from moving contacts to be localized. ![[Pasted image 20230811235124.png]] # ESM vs RWR RWRs or RADAR warning receivers are specific type of ESM equipment with a different purpose. Whereas general ESM equipment is designed to detect and localize emitters, RWR equipment is specifically designed to provide threat warning to enable the entity to take defensive action. RWRs are equipped to fighters and are limited in their ability to provide emitter position data. Full ESM suites are generally only present on either larger aircraft such as the E-2 or RC-135 or dedicated EW aircraft like the EA-18G. However, modern datalink aircraft can transmit RWR information to C2 aircraft to enable their RWR data to be used to generate crossfixes.