Notes on the AN/ALR-67 RWR

Whoever has flown a mission or practised against hostile aircraft, either human or AI, has noticed how the Radar Warning Receiver seems very inaccurate in terms of direction and number of radars detected.

The AN/ALR-67 mounted on the F-14B is not that imprecise and to understand why sometimes it is less accurate than any one of us after a couple of dozens of pints, we need to go a bit deeper into how the RWR works.

RWR – aka: when it yells, it’s kind of late

Simply put, a number of antennas are placed on the aircraft. Depending on the aircraft they can or cannot cover the entire “space” around it. If the antennas do not cover the entire area, then there can be a blindspot and a threat (such as an ARH missile) coming from that direction may hit the target without triggering the RWR.

To understand even better the AN/ALR-67, let’s consider the following video (source), released in mid 2018 by Heatblur during the development of the F-14:

As you can see, since the antennas are placed on moving parts, when the F-14 is manoeuvring, those sensors moves as well, resulting in wrong indications on the RWR.

The position of the antenna is not the only factor. As usual, Heatblur outdone themselves by recreating a realistic simulation of the RWR. This post by IronMike, quoted below, describes how realistic is the representation of the AN/ALR-67 mounted on the F-14B:

The procedures/logic:

Four sensors/antennas for the radar bands of tracking radars and airborne radars.

Each antenna FOV is ~180° (or slightly more), and almost a perfect cone.

The sensitivity at the edges of the cone is significantly lower than in the centre.

When we get a message from DCS about being radiated, we simulate the signal it produces in each sensor. This includes factors such as the distance from the emitter (attenuation), the angle of arrival for each antenna, noise and other random signal amplitude fluctuations.

From this moment we treat the signal as if we didn’t know about the true parameters of the emitter, and we only use the information from the emulated sensors (the previous step).

We take the amplitude of the signal from each sensor, apply signal-to-noise cuts, combine and reconstruct the threat direction.

Then, the reconstructed direction together with the signal signature is compared with the list of threats already being displayed. If we find one that correlates, we update its direction. Otherwise, we create a new threat and inform about it with the ‘new guy’ sound.

Some consequences of the procedure described above and a bunch of other features:

No blind spots. However, if directly above or below, the threat has to be significantly closer (compared to the horizontal plane) to pass the SNR threshold.

The direction is reconstructed in the 2D plane (the local aircraft frame of reference). For threats significantly outside that plane, their reconstructed direction may be inaccurate, and it usually shifts towards the 12, 3, 6, or 9 o’clock from the true position.

The direction reconstruction accuracy improves as the distance from the emitter decreases. For the scan modes of the emitter (RWS/TWS), it’s somewhere around 10-15° RMS.

For the emitters in scan modes, a misassociation of a known-threat with a new signal can happen, and it occurs quite often, especially at long ranges. It can result in:

ghosts (fake threats) appearing on the display – more probable if you or the threat do some manoeuvres;

merging a group of two or more threats of the same type into one threat. For example, a group of two Su-27 flying in close formation, both scanning with their radars, can appear on the screen as one ’29’ until they get closer.

A malfunction/damage of one antenna/sensor doesn’t make you completely blind in that direction, as the two adjacent antennas should still cover that area. However, the lack of that sensor makes the direction reconstruction procedure very inaccurate, and it’s very likely that some threats will be displaced by more than 90°.

Compared with the default RWR from DCS:

An entirely new dedicated code, written from the grounds up.

Antenna/electronics emulation.

Threat reconstruction using the emulated signals.

Enhance information obtained from the engine with more details (radar modes, missile guidance, noise etc.).

No blind spots.

Imperfect like a real device should be, and not a god’s eye.

Some weak radars can appear late.

The directions will be inaccurate.

It will be harder to estimate the number of threats of one type when they form a group.

You’ll receive launch warnings not only when you are the target of the missile. For example when flying in a close formation with your buddy; if an enemy launches a weapon such as AIM-7 or SA-6 at your buddy, you may receive a launch warning from that threat as well.

Detailed failures/damages.

Another thing to consider with the RWR is that the antennas move with the control surfaces, which means that this will roll your RWR picture, just as when you are manoeuvring the aircraft, the RWR picture will roll with it (and might display erroneous contacts). This requires additional pilot skill to take RWR readings at the proper (level) moment in the manoeuvre in order to keep up an accurate SA as well as an eye to spot wrong readings in between.

Too good to be realistic?

More advanced (or less realistic?) RWR can be used to nail perfect notches on enemy radars. We have no such luxury on the F-14 (and I’m honestly happy about it), so, how do you use it combat? Well, I usually put it this way: if you are relying on the RWR to know when you are in danger, then the problem is not the RWR. The AN/ALR-67 is, in fact, only one of the many tools available to the crew to build and maintain Situation Awareness:

The AWG-9 WCS is an old yet incredible radar suite.
The F-14 has two forms of datalink (LINK4A and LINK4C) so it can share information to other Tomcats (up to 4) even when no AWACS is available.
At longer distances, when not manoeuvring, the RWR is precise enough to have an idea of where the threat is.

The list could be longer, but those three points should pass the concept clearly enough.

More info about the AN/ALR-67, as usual, are available in the ~~bible~~ F-14B Manual.