Countermeasures against SAMs and AAMs

Ever since the first operational deployment of chaff (then codenamed: ‘Window’) by the RAF over Hamburg in July 1943, aircraft and the technical mechanisms designed to kill them have been locked in a lethal cycle of measure/countermeasure. This cycle has included the introduction of new threats, and the production of a variety of means of defeating them.

The missile threat to aircraft consists mainly of systems guided by radar or infrared (IR), with both posing a different challenge in terms of detection, evasion techniques and countermeasure. As a survivability specialist, I always cast air platform protection (APP) in terms of the ‘Survivability Onion’, so termed as it consists of a number of layers. I intend to ‘peel back’ some of the layers to discuss the threat to aircraft from missiles, and, crucially, how they can be counteracted.

‘Left of launch’ approaches

The first layer is: ‘Don’t be there’ – accurate information is the most effective countermeasure. If operations are restricted to where enemy surface-to-air missiles (SAMs) and fighters are not, you’ll be safe. However, this tactic is only as good as the intelligence you base it on. Moreover, in the case of highly mobile and easily concealed weapons, such as man portable air defence systems (MANPADS), it’s increasingly difficult to ensure that any area is truly ‘clear’ of a threat.

The next layer is: ‘Don’t be seen’ – and in terms of all guidance systems, this continues to be relevant. Terrain masking is the classic countermeasure ­– exploiting the natural folds of the environment and man-made infrastructure to obscure the platform, preventing acquisition by any of the usual guidance systems. A more modern aspect is low observability (LO) – ‘stealth’. Designing a platform with a low radar cross section (RCS) will make detection by radar either impossible, or too late to have an effect. Likewise, IR suppression can be highly effective in preventing an IR missile from ‘seeing’ the target thermally. Radar LO has to be ‘baked into’ the design at a very early stage, but IR ‘stealth’ can be applied to an existing design.

IR suppression is most usually associated with transport aircraft and, in particular, helicopters. For the former, the extra weight and impact upon performance has to be weighed against the amount of time spent in the ‘threat envelope’, whereas for the latter, it’s increasingly essential as part of any synergistic APP fit. For rotorcraft, there are simple and complex methods of reducing their IR signature – especially in Band I IR, classically the thermal plume of a jet exhaust. The simplest method is to divert the hot exhaust gasses up into the rotor system to both shield the jet pipe exit from below the aircraft, and to cool the exhaust by mixing it with the toroidal airflow. More complex IR suppression systems (IRSS), such as the Davis system fitted to many aircraft and helicopters (including the CH-47), mix exhaust gas with ambient air, resulting in a cool efflux that is almost undetectable by Band I seekers.

As a countermeasure, MANPADS designers started producing more complex, cooled IR seekers that could detect the Band IV signature, the ‘hot metal’ of the engines and transmissions. Therefore, modern IRSS don’t just consist of engine suppressors and diffusers, but line-of-sight ‘blockers’ and thermal ‘blankets’ around the ‘hot metal’ components.

 

If a platform cannot avoid being seen, the next layer is: ‘Don’t be targeted/engaged’. At this stage, the ‘fight’ starts to migrate from passive to more active means. As a Tactics Instructor, we often called this phase ‘keeping the missile on the rail or in the tube’. RCS and IRSS can help here, but it’s where jammers start to play a key role. Against radars, jamming can introduce hundreds of false targets into an enemy’s radar screens, causing a huge ‘sort’ problem. Radars, by their nature, have to be quite sensitive to receive the long-range pulses they have emitted, after they are reflected back off the target. It is this need to be sensitive that enables stand-off jammers (SOJ) to generate false returns in a manner the radar is ‘expecting’ to see or use the ‘brute force’ by transmitting so much power that the ‘noise floor’ is raised in such a way that the enemy radar display is saturated. Since radiated power decreases as a function of the square of the range (in physics termed the ‘inverse square law’), achieving a convincing effect on the host radar requires either a lot of power at long range (as with SOJ), or, if stand-in jamming (SIJ) is employed, lower power at short range.

Tactical aircraft can fit SOJ/SIJ capability in a pod, at the cost of losing a pylon for weapons or fuel. Dedicated jamming aircraft also exist, tasked with escorting packages of non-LO aircraft through a hostile integrated air defence system (IADS). The US Navy EF-18G ‘Growler’ is probably the best-known example, but the Tornado electronic combat and reconnaissance (ECR) also specialises in this role. The B-52 still carries immensely powerful jammers. During the first night of the 2003 Second Gulf War, our assault package was escorted by a B-52 tasked with suppressing any fire control radars (FCRs) that ‘lit up’ – J2 (intelligence) had state Roland, Crotale and ZSU-23/4 air defence systems were present in the target area. I had a long chat with the B-52 EW Officer (EWO) about the frequencies I didn’t want him to jam, as they might impact systems on our own aircraft.

SIJ is where unmanned aerial vehicles (UAVs) have an increasingly large role to play, especially if fitted with relatively cheap digital radio frequency memory (DRFM) jammers and carried to the target area by aircraft, then deployed as ‘Launched Effects’ (LEs). DRFM detects the radar pulse, records it digitally, samples it, then processes and replays it back to the emitting radar to create a credible false target. As DRFM mainly relies on exploiting received energy, any mission data can be placed into volatile memory that zeroises when power is removed, enabling their use without risk of compromise if the platform is downed.

 

Traditional RF evasion tactics still play a part. There is a finite time required for the tracking radar to generate enough information to create a firing solution. During this time there is still the opportunity to attempt to escape the Radar Resolution Cell (RRC) before the shot is taken. To conduct effective RF tactics requires the use of a Radar Warning Receiver (RWR) to detect, characterise and provide a bearing to the threat. Once detected and locked, the well-timed use of manoeuvre and chaff is key – the tactic being to ‘seduce’ the radar onto the chaff as the targeted platform manoeuvres to exit the RRC. For fast jets, this can include a steep descent, while for helicopters it’s inevitably a ‘flatter’ tactic. These ‘Breaklock’ manoeuvres are unique to each aircraft type, as they exploit aspect changes of RCS, turning rates and chaff effectiveness – and sometimes involve multiple manoeuvre chains, since the FCR may reacquire as chaff rapidly dissipates, and many modern radars have algorithms to predict where the target will reappear.

Against air-to-air radars, it is possible to ‘fade’ from the displays of older generation Pulse Doppler radars by the use of a ‘notch’. This exploits a ‘clutter gate’ in such radars that is normally selected to 148–185 km/h (80–100 kn) and is designed to stop the radar from detecting surface features and slow-moving vehicles. The critical aspect is the relative Closure Velocity (V^C). For example, if a jet flying straight towards another jet has a V^C of 889 km/h (480 kn), taking a 45° ‘cut’ halves this to 444 km/h (240 kn), while turning to 90° to the threat radar reduces the V^C to zero – effectively hiding the aircraft in the ‘Main Beam Clutter’. For helicopters, with low V^C to start with, it’s very easy to fly a good enough manoeuvre without having to go all the way to 90°. More modern, electronically scanned (E-Scan) radars are not as susceptible to this technique.

Unlike RF jamming, proactive IR jamming (IRJ) to prevent a lock-and-launch seems to be in something of a decline. During the 1970s and 80s, the AN/ALQ-144 ‘Disco Ball’ was a familiar sight on many helicopters, with the larger AN/ALQ-157 often fitted to heavy-lift helicopters (CH-47/MH-53) and transport aircraft (C-130). IRJ essentially used plasma lamps to ‘pulse’ IR energy into the seeker heads of early generation MANPADS and AAMs – especially those that relied upon a ‘Spin Scan’ or ‘Conical Scan’ for guidance. The IRJs introduced false targets into the seekers by pulsing broadly in line with the spin rate of the reticules, making it impossible for the then relatively simple missile guidance to ‘null’ the signal into a guidance command, and, in effect, preventing the user from launching. This was always one of the issues when trying to justify the funding for IRJs, as it was impossible to prove how many ‘shots’ they had prevented, only which ones they hadn’t. As the sophistication of MANPADS has grown, and other technologies emerged, the IRJ came to be seen as something of a heavy and expensive ‘one-trick pony’ as it was only useful against early-generation threats.

Surviving ‘right of launch’

Despite best efforts, the threat sometimes acquires enough information to achieve a lock-and-shoot – the missile is in the air. What’s crucial now is: ‘Don’t be hit’. The first part of survival is to know you’ve been shot at.

For a radar SAM/AAM, the RWR should, if capable of detecting the frequencies involved, provide warning of when the ‘shot’ is taken. Older semi-automatic radar homing (SARH) missiles required the ‘host’ radar to ‘paint’ the target with its continuous wave (CW) illuminator – the screeching ‘tone’ seen in so many movies – with the missile homing to the reflected energy. More modern missiles with active radar homing (ARH) seekers may be launched ‘cold’ and steered via datalink until a point where they activate their own onboard radar to illuminate the target and then home. This kill chain is problematic to defeat ­– especially if the target has been detected passively via an IR search & track (IRST) sensor or cued by a third-party sensor, as the seeker only goes active very late in the engagement, and there is no ‘host radar’ to defeat.

Against IR threats, missile warning systems (MWS) are used to detect the inbound threat. MWS are usually passive, ultraviolet (UV) or IR based – the latter increasingly capable of imaging the scene around the aircraft. The most ubiquitous MWS are the US-produced AAR-47 and AAR-57, the latter also known as the Common MWS (CMWS), in use across many platforms. Both are UV-based, while the Thales Elix-IR, as the name suggests, uses IR technology. The benefit of IR over UV is that the greater sensitivity of IR allows it to see additional threats such as ball ammunition, not just tracer rounds. Active MWS, using doppler radar, are still available. Examples include the ALQ-156, which is now obsolete but can still be encountered, and the Typhoon’s Pretorian system, which includes a Doppler MWS.

It is also important to realise that no MWS has a Percentage Declaration (PDec) of 100%. MWS performance is often ‘tuned’ to achieve the highest PDec balanced against the lowest false alarm rate (FAR) – a high FAR can result in users ignoring the warnings and, potentially, a high expenditure rate of IR countermeasures.

 

The defeat mechanisms for radar threats are a continuation of previous methods; decoy and manoeuvre. Chaff is increasingly of limited effect against modern missiles and radars, so the ante has been raised by the increasing use of offboard jamming – either a towed radar decoy (TRD), usually streamed from a pod and designed to create a more attractive false target behind the aircraft or, more recently, an expendable active decoy (EAD). The Selex (now Leonardo) ‘BriteCloud’ EAD, developed in collaboration with the UK MoD, uses DRFM technology to confuse and seduce an incoming threat away from the aircraft. It’s important to appreciate that the RF ‘fight’ is complex, with often a blend of manoeuvre and on/offboard jamming required.

Against IR missiles, protection is effectively a choice between decoy flares and a directional IR counter measure (DIRCM). Flares have the advantage of being cheap to buy and are often able to be rapidly modified in terms of type and dispense pattern to react to emerging threats. However, the increase in the use of counter-counter measures (CCMs) by IR missiles, especially MANPADS, has had an impact upon flare effectiveness. Moreover, in a tactical situation where the platform is operating at low level and trying to avoid detection, the use of flares either pre-emptively (as is being seen a lot in Ukraine) or FAR release, risk revealing the location of the platform to hostiles – especially at night.

To overcome these shortcomings, designers turned to DIRCM. Originally a directional IR lamp, modern DIRCM are equipped with laser turrets. The defeat mechanism is either a ‘jam code’ using the modulated IR laser to input false targets or steering information into the seeker head, or to simply saturate the seeker with energy such that it is unable to guide. For DIRCM to be effective, the platform installation needs to be highly accurate, as the hand-off from the MWS to the fine track sensor (FTS) needs to be exact to permit it to steer the transmit head onto the missile. It also all has to happen quickly and is dependent on effective jam codes being developed and loaded.

Closing thoughts

The cycle of measure/countermeasure continues. The imminent arrival of directed energy APP systems will, likely, once more tip the balance in favour of the defending platform. No doubt new threats and countermeasures are already being conceived.

Author: Paul ‘Foo’ Kennard is a former UK RAF Helicopter pilot, Tactics & Electronic Warfare Instructor and Operational Evaluation pilot. He has seen operational service in Northern Ireland, Bosnia, Kosovo, Iraq and Afghanistan. He now runs his own independent consultancy company, Ascalon, providing specialist technical and user input into a wide variety of defence & aerospace programmes for governments, NATO and the broader defence industry; and operates as a freelance journalist.