Electronics intelligence (ELINT) is a branch of signals intelligence, which targets non-communications signals intelligence, such as radar and navigation signals. The Joint Chiefs of Staff define it as "Technical and geolocation intelligence derived from foreign noncommunications electromagnetic radiations emanating from other than nuclear detonations or radioactive sources."
Both ELINT and SIGINT were critical in the Battle of Britain, and some of the ELINT details were declassified before the 1975 declassification of the COMINT success against the German Enigma machine. See SIGINT in the Second World War - Battle of Britain. One of the first and critical aspects has been called the "battle of the beams", in which the British scientific intelligence organization, under R.V. Jones, determined that the German night bombing attacks were guided by electronic navigation signals sent from occupied Europe. Once the systems were understood, it was possible to mislead the bombers, such that they bombed open country, or, on a few wondrous occasions, became so disoriented that they landed at airfields in the U.K. 
"During the human struggle between the British and the German Air Forces, between pilot and pilot, between AAA batteries and aircraft, between ruthless bombing and fortitude of the British people, another conflict was going on, step by step, month by month. This was a secret war, whose battles were lost or won unknown to the public, and only with difficulty comprehended, even now, to those outside the small scientific circles concerned. Unless British science had proven superior to German, and unless its strange, sinister resources had been brought to bear in the struggle for survival, we might well have been defeated, and defeated, destroyed." — Winston Churchill
Signal identification is performed by analyzing the collected parameters of a specific signal, and either matching it to known criteria, or recording it as a possible new emitter. ELINT data is usually highly classified information, and is protected as such. In the US and its allies, rather confusingly, ELINT that is categorized as sensitive compartmented information is handled not through a compartmented control system named after SIGINT or ELINT, but is marked CCO: Handle through COMINT Channels Only.
The data gathered is typically pertinent to the electronics of an opponent's defense network, especially the electronic parts such as radars, surface-to-air missile systems, aircraft, etc. ELINT can be used to detect ships and aircraft by their radar and other electromagnetic radiation; commanders have to make choices between not using radar (EMCON), intermittently using it, or using it and expecting to avoid defenses. ELINT can be collected from ground stations near the opponent's territory, ships off their coast, aircraft near or in their airspace, or by satellite.
Radar warning and intercept receivers
Military radar ELINT receivers have a much more difficult challenge than a civilian police radar detector, because the radar receiver in the car can make simplifying assumptions about the greatest radar threat axis being in the direction of travel. It is true that mobile radar in police cars can come from behind, but that is relatively rare.
Against a sophisticated enemy threat, a more common method would be to start with a set of directional antennas, or elements of a mechanically scanned phased array radar, or active electronically scanned array (AESA), and observe the directions from which a signal is strongest. Especially for the warning case, simply remembering signal level, and correcting for the motion of the platform, can give warning before the hostile radar can get useful returned signals.
Of course, this is most likely to work against a stable frequency of significant power. frequency agility and spread spectrum techniques make it much less likely that a changing signal will be recognized. If the opponent's radar is multistatic with multiple transmitters, in different places, pseudo-randomly switching on and off, again, it will be much harder to detect the signal.
Signals intelligence methods are complementary
During the Battle of the Atlantic in World War II, communications intelligence was not always available because Bletchley Park was not always able to read the U-Boat Enigma cryptosystem traffic. But "HF/DF" High Frequency Direction Finder) was still able to find where the U-Boats were by analysis of radio transmissions and the positions through triangulation from the direction located by two or more Huff-Duff systems. The Admiralty was able to use this information to plot courses which took convoys away from high concentrations of U-Boats.
Yet other ELINT disciplines include intercepting and analyzing enemy weapons control signals, or the Identification, friend or foe responses from transponders in aircraft used to distinguish enemy craft from friendly ones.
Role in Air Warfare
A very common area of ELINT is intercepting radars and learning their locations and operating procedures. Attacking forces may be able to avoid the coverage of certain radars, or, knowing their characteristics, electronic warfare units may jam radars or send them deceptive signals. Confusing a radar electronically is called a "soft kill", but military units will also send specialized missiles at radars, or bomb them, to get a "hard kill". See Suppression of enemy air defense.
ELINT collection, in military aircraft, ranges from basic radar warning receivers, to tactical analysis aboard electronic warfare airplanes such as the EA-6B Prowler and EF-18 Growler, to large converted transports dedicated to strategic radar collection analysis (e.g., RC-135U COMBAT SENT). National-level ELINT aircraft may work, as a team, with a COMINT platform such as the RC-135V/W RIVET JOINT.
Knowing where each surface-to-air missile and anti-aircraft artillery system is and its type means that air raids can be plotted to avoid the most heavily defended areas and to fly on a flight profile which will give the aircraft the best chance of evading ground fire and fighter patrols. It also allows for the jamming or "spoofing" or deceptive jamming of the enemy's defence network (see electronic warfare). Good electronic intelligence can be very important to stealth operations; stealth aircraft are not totally undetectable and need to know which areas to avoid. Similarly, conventional aircraft need to know where fixed or semi-mobile air defense systems are so that they can shut them down or fly around them.
ELINT and ESM
Electronic Support Measures (ESM) are really ELINT techniques, but the term is used in the specific context of tactical warfare. ESM give the information needed for Electronic Attack (EA) such as jamming. EA is also called Electronic Counter-Measures. ESM provides information needed for Electronic Counter-Counter Measures (ECCM), such as understanding a spoofing or jamming mode so one can change one's radar characteristics to avoid them.
ELINT for Meaconing
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Low probability of intercept radar
To analyze radar, the ELINT operator must first recognize the transmitter's signals, or, through techniques other than ELINT, learn about the receiver. Traditional radar systems have a colocated transmitter and receiver, but that is changing. Before going to more complex transmitter-receiver models, however, consider how the transmitter may be made harder to detect.
Frequency agility and spread spectrum
Detecting a transmitter means that the ELINT receiver can intercept a radar transmission that is a sufficiently long and persistent signal to be recognized. Several techniques make this more difficult. One of the most basic is frequency agility, in which the transmitter and receiver are time-synchronized, the transmitted shifts pseudo-randomly among frequencies, and the receiver shifts to detect the frequency of short pulse trains or even individual pulses. The energy of the transmitted signal, at any given time, is sufficiently strong that if an ELINT receiver were listening on the right frequency, it would recognize the signal as a radar pulse, but how is it to find the right frequency? Without knowing the pseudo-random frequency shift pattern, the main hope is having a large number of receivers simultaneously monitoring multiple frequencies.
Even more difficult is the spread spectrum situation, where the transmitter simultaneously sends on multiple frequencies, but at very low power. The power level at any given frequency is intended to be sufficiently low that an ELINT receiver listening on that frequency will not recognize a radar signal there. The intended receiving system, however, has a receiver for each of the frequencies over which the signal is distributed, and, by correlating inputs of mixed signals and noise from the many receivers, finds the common pattern of the real signal.
Spread spectrum can be combined with frequency agility, so only a subset of the frequencies might, at any given time, have the energy spread among them. The unused channels have only noise, but the synchronized receiving system knows which channels to ignore, and thus correlates only from meaningful channels. Even if the ELINT system could listen on all the frequencies, it will include the null/noisy channels and have a much harder time in correlating to find the real signal.
Defeating combined MASINT-ELINT techniques
Radiofrequency MASINT techniques might be able to detect a sidelobe pattern from a frequency-agile, spread-spectrum radar, but another counter-ELINT technique is to make the antennas extremely directional, perhaps using shielding to suppress sidelobes. Conceivably, a MASINT sensor might listen for the intermediate frequencies (IF) of a radar receiver, but a wise receiver designer shields these very well.
ELINT on bistatic, multistatic, and passive radar
Bistatic systems have a paired transmitter and receiver, but in different physical locations. Multistatic systems have more than 2 receivers and more than 2. An especially tricky technique is passive radar, in which no dedicated transmitter exist; the receivers look for reflections of energy from some known "innocent" electromagnetic signal, such as a television station.
One bistatic technique, commonly used with surface-to-air missiles (SAM), is to form a bistatic system from an illuminator (i.e., transmitter) that, perhaps coupled with a search radar, is causing radar reflections to bounce off the target and into a receiver in a SAM's seeker. Coupled missile guidance illuminators and search radars are a feature of modern air defense systems such as the U.S. Navy's AEGIS battle management system.
Since transmitters tend to be cheaper than receivers with their processing system, the opponent may use more transmitters than receivers, and pseudo-randomly switch among the active transmitters, which, in turn, may be pseudo-randomly varying their transmitting frequency, or frequencies in the case of spread spectrum. If your side detects a transmitter and kills it with an anti-radiation missile, a sufficient number of transmitters will make his system quite tolerant of losing a few transmitters.
With passive radar, "As this type of radar doesn't have any transmitter, it is obvious that an enemy's ELINT assets are confronted with the problem of finding something that doesn't actually exist."  To have any chance, the ELINT operator will almost certainly need to complement ELINT with other collection disciplines. MASINT, as mentioned, might intercept IF. IMINT might reveal the antennas, perhaps with their axis pointing at the "innocent" transmitter. Of course, if HUMINT can get someone on the other side to reveal the existence and location of the system components, ELINT might not be needed at all to attack the processing station.
Still at the research level are techniques that can only be described as counter-ELINT, which would be part of a SEAD campaign. It may be informative to compare and contrast counter-ELINT with electronic protection/electronic counter-countermeasures (ECCM}.
- US Department of Defense (12 July 2007), Joint Publication 1-02 Department of Defense Dictionary of Military and Associated Terms
- Jones, R. V. (1978), The Wizard War: British Scientific Intelligence 1939-1945, Hamish Hamilton.
- Churchill, Winston (2005). The Second World War, Volume 2: Their Finest Hour. Penguin Books Ltd. ISBN 0141441739.
- BBC H2G2, Low Probability of Intercept Radar