Every radar that is looking for you has to shout to do it. That is the whole weakness of the thing. A fire-control radar cannot find a target without flooding the sky with energy, and any aircraft carrying the right antennas can hear that energy long before the radar operator has worked out what he is looking at.

The radar warning receiver is the box that listens. It sits behind a handful of wideband antennas spread around the airframe, sorts the wash of incoming pulses into individual emitters, works out roughly where each one is and how dangerous it is, and then tells the pilot in two ways at once: a symbol on a small circular display, and a tone in the headset.

It is one of the least glamorous systems on a combat aircraft and one of the very few that regularly decides whether the crew comes home. It exists because in 1965 American aircraft started falling out of the sky over North Vietnam and nobody in the cockpit knew why until it was too late.

Rolling Thunder and the problem of the invisible shooter

When Soviet-supplied surface-to-air missiles began downing American strike aircraft over North Vietnam during Operation Rolling Thunder, the USAF had no systematic way of telling a crew that an SA-2 Guideline battery had them. The response, in 1965, was a development programme headed by General Kenneth Dempster and given the code name Wild Weasel: a dedicated aircraft for detecting and suppressing SAM sites. It had originally been called Project Ferret, after the animal that goes into its prey’s den, until somebody pointed out that Ferret had already been used in the Second World War for radar-countermeasures bombers.

The suppression missions themselves were authorised on 12 August 1965 under the operational code Iron Hand. The aircraft was the two-seat F model of the F-100 Super Sabre, and the first squadron was the 354th Tactical Fighter Squadron at Takhli Royal Thai Air Base.

The first mission flew on 20 December 1965.

What they were hunting looked, from directly above, like nothing else on earth: six launchers arranged around a central guidance radar, connected by access roads, unmistakable once you knew the shape.

Forty-five days, one aeroplane left

The F-100F did not have the performance to survive where it was being asked to work. After 45 days of operations against North Vietnamese targets the 354th had one aircraft left. Of the 16 aircrew, four had been killed, two were prisoners of war, three had been wounded and two had quit.

It is worth pausing on the job description. The Weasel’s method was to make itself the most attractive target in the sky, wait for the site to light up, and then follow the radar beam back to its source. The F-105F that took over the role in the summer of 1966 deliberately did not carry radar jamming, because its purpose was to be a decoy: get the SAMs launched, watch for the bright smoke of the boosters, and dive-bomb the site that produced it. Miss the missiles coming at three times fighter cruise speed and the mission ends with you.

The man who put this most memorably was Jack Donovan, a former B-52 electronic warfare officer, on being told what he had volunteered for.

Donovan flew on the first mission anyway, in the back of an F-100F with Captain Al Lamb in front, and the pair took out a site during a Rolling Thunder strike on the railyard at Yên Bái.

What the box actually does

A typical airborne RWR is several wideband antennas positioned around the aircraft so that between them they cover every bearing, feeding a receiver that periodically scans across the frequency band. For every signal it catches, it measures a set of parameters: frequency, the shape of the signal, the direction it arrived from, and the pulse repetition frequency.

That produces a mess. In a busy electromagnetic environment the receiver is not hearing one radar, it is hearing a dozen overlapping pulse trains chopped into each other. The first real piece of work is deinterleaving: untangling that mixture back into separate emitters, so that the system can say these pulses belong to one radar and those belong to another.

Only then does the interesting part happen. The sorted emitters are matched against what the system knows about radar types and ranked by how much of a threat each one poses, using signal strength, phase and waveform detail. A simple RWR might do no more than announce that there is energy in a band associated with known SAM systems. A modern one classifies the source and tells you what kind of vehicle is probably carrying it.

Reading the scope

The display is usually a circle, and the geometry is refreshingly literal. A radar straight ahead appears at the top. One directly behind appears at the bottom. Everything else is drawn at its bearing relative to the current heading of the aircraft, which means the picture rotates around you as you turn.

What the distance from the centre means depends on the unit. On some systems it represents the estimated distance to the emitter. On others it is a measure of how badly the emitter wants to kill you, with tracking radars drawn closer to the centre than search radars. That second scheme is less intuitive and more useful: a search radar 10 miles away is a smaller problem than a tracking radar 30 miles away.

The symbol itself encodes what the emitter is, usually with a clear distinction between ground-based and airborne radars. Modern aircraft often show the same information in more than one place in the cockpit rather than on a single dedicated scope.

The sound of being locked up

The audio channel is the half of the system that most people underestimate. The RWR feeds tones directly into the headsets of the pilot and, in a two-seat aircraft, the back-seater. This matters because a fighter pilot in a high-workload moment is not staring at a small circular display. The ears are the only sense not already fully committed.

The critical event is a change of mode. A semi-active radar homing missile needs its launching radar to keep illuminating the target, and that guidance mode looks different on the air from a search sweep. A capable RWR can detect that change and escalate: much more insistent warning tones, and bracketed symbols flashing on the display. The pilot may then be able to visually acquire the missile, having been told where to look.

From there the RWR stops being a warning device and becomes a tactical instrument. Bearing and closure let the crew work out whether diving away will out-run the missile or whether it is time to jettison external stores and turn into it. The system can also signal a countermeasures dispenser to put chaff into the air without waiting to be asked.

Avoid, evade, or kill

Used offensively, the RWR is a sensor in its own right. A fighter on combat air patrol may see enemy fighters appear on the RWR, then use its own radar to find and engage them. Because different fighter types carry different radars, the moment an opponent switches on and sweeps in your direction, the RWR can tell you which of the contacts on your own scope is which.

Used defensively, it is a map of where not to go. An aircraft trying to avoid a fight can turn its own radar off entirely and steer around everything the RWR shows. Above 30,000 feet there are very few threats that do not emit, so as long as somebody is watching for an aircraft creeping up under AWACS or ground control without using its radar, an RWR picture plus a cool head will keep you clear of SAMs, fighters and radar-directed anti-aircraft artillery.

At the top end sit the SEAD and ELINT aircraft, which carry far more sensitive equipment than a standard warning receiver. The US HARM targeting system pod finds and classifies emitters much further out than a typical RWR, overlays threat circles on a map on a cockpit display, and can store what it hears for later analysis or transmit it to the ground to help plan the next mission. That is the direct descendant of the F-105F sitting there being shot at, only now it does the finding without being the bait.

The family tree

The American AN/ALR series maps neatly onto the aircraft that carried it. The AN/ALR-46 went into the F-4 and RF-4 Phantom, the F-5 and the B-52. The AN/ALR-56 serves the F-15 and F-16 Block 50 and 52. The AN/ALR-67 is the naval line: AV-8B Harrier II, F-14 Tomcat, F/A-18 Hornet and Super Hornet, EA-6B Prowler, and the Canadian CF-18. The AN/ALR-69 covers a wide spread including the A-10, AC-130, F-16 and B-52H, with a major revision as the ALR-69A(V) in 2005. Rotary-wing and transport aircraft largely use the AN/APR-39, whose latest revision, the APR-39E(V)2, entered production in 2024.

Britain had the Marconi ARI 18223 on aircraft such as the Jaguar and the ARI 18228 on British Phantoms, the latter developing into the Sky Guardian series. Sweden and Germany use the BOW-21 on the Gripen and the Tornado. France built the RWR function into Thales Spectra on the Rafale. India fields Tarang on the MiG-27, Tejas, Jaguar and Su-30MKI. Israel’s Elbit sells the SPS family, in Portuguese service on the F-16 and the KC-390.

On the Soviet and Russian side the Sirena series gave way to the SPO-15 Beryoza, which turns up on the MiG-29, the Su-27SK and the Chinese J-11, and then to the SPO-150 Pastel. And on the F-35 the separate warning receiver has disappeared as a discrete box altogether: the AN/ASQ-239 is a full electronic warfare and countermeasures system that happens to include the RWR function.

What it cannot tell you

The limitation is built into the principle. An RWR only knows about things that transmit. A fighter closing in under ground-controlled interception with its radar off, guided by somebody else’s picture, is invisible to it. So is an infrared-guided missile, which needs no illumination at all. The warning receiver is a superb instrument for a world of radar-guided threats and completely blind to the ones that stay quiet.

That is why the RWR has never been a system on its own. It sits inside a defensive suite with jammers, chaff and flare dispensers, missile approach warners and, increasingly, the aircraft’s own sensors feeding the same threat picture. But it remains the first voice in the chain. Everything else in the suite is waiting to be told that somebody out there has started shouting.

Sources: Wikipedia (Radar warning receiver; Wild Weasel), Walter J. Boyne, Beyond the Wild Blue: A History of the U.S. Air Force 1947–1997, and Dan Hampton, Viper Pilot: A Memoir of Air Combat.