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Why long-wave infrared threatens fighter stealth and survivability

This week, The Central Blue welcomes ADFA graduate Jack Ryan as he examines the implications of emerging Infrared sensors. While IR sensor use in air-to-air combat is not new, Ryan contends that the continuing development of Long-Wave Infrared (LWIR) sensors poses a significant issue for Australia and its allies. In particular, he explores how advances in such sensors could severely undermine the technological edge held by Western forces that could see stealth assets such as the F35 lose their ‘surprise’ advantages – a situation for which Ryan recommends urgent joint R&D action.

Militaries have come a long way in reducing the radio frequency (RF) signatures of their advanced fighter aircraft, but advancements in infrared (IR) detection threaten that stealth. New 5th generation platforms such as the F-35 have been applauded for their enhanced survivability, which is due in large part to a significant reduction in radar signature. Developments in airframe design, material composition and production quality give the F-35 an “unmatched ability to evade enemy detection” according to Lockheed Martin. Despite all this, advancements in IR detection threaten to erode the stealth advantage. Advances in IR technology, specifically the threat that Long Wave IR (LWIR) sensors pose, may be one that cannot be easily rectified. Resultantly, we could be entering an age never before seen, where passive IR systems outperform active RF radars.

A Theory Refresh

To understand why LWIR advancements may be a game changer, specifically in relation to air combat, it is necessary to first review fundamental electro-magnetic (EM) theory. RF and IR energy both form part of the broader EM spectrum. RF is typically talked about in terms of frequency (GHz, MHz etc), and is associated with radars. The IR component of the spectrum is measured by wavelengths (λ), or microns (µm), and in a military context, is commonly associated with Infra-Red Search and Track (IRST) systems.

Figure 1. The EM Spectrum

Radar is an inherently active sensor. That is, radars actively transmit an RF signal to be reflected by a target. The reflected signal, captured by a receiver, is able to determine the target’s range, azimuth or velocity. Air intercept (AI) radars on fighter jets typically operate in the X band (between 8-12 GHz). For example,the F-16 APG-68 operating envelope starts at 9.86GHz. This frequency selection offers a favourable compromise between radar size, cost, accuracy and fidelity – all of which are crucial characteristics for air combat.

Alternately, an IRST is a passive sensor. Unlike radar, an IRST does not transmit any form of EM energy, it just receives all sources of heat energy (think of a thermographic (infrared) camera), and is sensitive in the 1 – 14µm range. As the IRST detects an IR heat signature, it is capable of tracking it to potentially determine the target's azimuth. In conjunction with other aircraft sensors, it can then also calculate range or velocity. Within the IR spectrum there are two major ‘dead zones’ (illustrated in Figure 2). Within these zones, wavelengths are heavily absorbed by the atmosphere, thus making target detection near impossible. Due to these characteristics, to optimise the use of the IR spectrum, practitioners are best using Mid-Wave IR (MWIR) or LWIR.

MWIR has traditionally been the most utilised part of the IR spectrum - it detects heat sources that transmit between 3 – 5µm. Consequently, any such IRST system has been limited to detecting the hottest part of a jet - the area with the most energy and shortest wavelength. This is typically the engine and exhaust fumes. As Figure 3 illustrates, these regions are typically between 2 – 5µm. The ability for a MWIR tracking sensor to detect therefore relies heavily on the target aspect. Targets approaching head on block their engine and exhaust, subsequently preventing IR detection and tracking. Legacy missiles (such as the SA-7) which utilise MWIR sensors to lock and track targets, and are known as ‘tailpipe chasers’ – they can only see and hone in on the hot signatures to the rear of an aircraft.

Developments in LWIR sensor systems negate these limitations. Their ability to detect energy in the 8 – 12µm band enables the sensor to see, and potentially track, much cooler targets (illustrated in Figure 3). LWIR provides the most signal for a given difference in temperature between target and background. Skin heat produced by the friction of air over the fuselage and leading edges during flight emits within the LWIR band. The ability to detect these cooler elements offers a solution to the problem of aspect inherent to MWIR discussed above. The use of LWIR enables far greater flexibility in air-to-air tracking, while also countering any RF stealth properties the target might have. Furthermore, LWIR increases the lethality of passive engagement sequences through increased range and fidelity.

The application of IRSTs in Air-to-Air Combat

Utilising IR sensors in air-to-air combat is not new - the United States have been incorporating IRST technology since the F-101 Voodoo in 1954, while European aircraft have done so since the Saab Draken in 1965 and Russia since the MiG-23 in 1967. Initial MWIR systems were typically used to cue other, more accurate, sensors – namely the AI radar. The engagement process in the early days required the transmission of a radar to successfully employ an active missile, with the use of emissions control (EMCON) tactics not typically relied on.

As technology increasingly developed, the desire for stealth also increased. If an aircraft could remain invisible to enemy radars, they would be able to penetrate deeper into hostile territory - possibly preventing an adversary engagement altogether. An aircraft utilising EMCON silent tactics (that is no active RF signals are emitted), forces the adversary to ‘find’ them rather than homing in on radar emissions. For a stealth aircraft, awareness of adversary EMCON posture is vital to maintaining its survivability. Advanced IRST systems are crucial in this regard - as LWIR IRST systems proliferate, situational awareness will improve, and potentially outperform the traditional radar detection of stealth aircraft.

Beyond providing increased awareness of enemy movements, LWIR also complicates air-to-air weapon employment. Along with aspect deficiencies, legacy IR missiles also had poor range. Early AIM-9 sidewinder variants for example, had a maximum target detection range of 2.6nm meaning they could only be used strictly for dogfights within visual range (WVR). These legacy missiles and MWIR IRSTs complimented each other. Both sensors had poor range and resolution that restricted completely passive engagements to unrealistically close scenarios. However, alongside the development of LWIR has also come the development of longer-range IR missiles. One of China’s newest IR missiles, the PL-10, allegedly has a range of up to 11nm. If paired with a LWIR IRST which could provide cueing data at extended ranges, a missile such as the PL-10 may now be capable of targeting beyond visual range (BVR) – all while maintaining a completely EMCON silent.

5th Generation Relevance

As 5th generation platforms refine their EMCON and RF stealth properties, it is highly likely that adversaries will seek to develop and leverage LWIR as a means of defeat. Russia’s PAK-FA T-50 fighter is known to contain an advanced IRST system that can allegedly detect an F-22 at 13nm. China’s J-20 fighter utilizes a Distributed Aperture System (DAS), similar to the F-35’s, that combines IR detection along with electro-optical capabilities. It is unclear if either of these systems have LWIR capabilities, but it is a logical assumption that both Russia and China would desire this advantage.

US reporting suggests that both jets appear to be inferior to platforms such as the F-22 and F-35 in terms of technology and stealth. President of Sukhoi Mikhail Pogosyan said of the J-20’s 5th generation credentials that “China obviously has a long way to go”, while the RAND corporation estimated that the PAK-FA would have “attributes of 4th generation heavy fighter bomber” while “lacking the LO features of the F-35”.

Despite this assessment, both aircraft have made considerable improvements over previous 4th generation airframes, with the gap between East and West reducing. This indicates that the previously dramatic technological edge held by the US, and subsequently Australia, is eroding. Practically, this means that extremely permissive environments, such as those encountered in Operation OKRA in which RAAF F/A-18’s participated, are unlikely to exist in the future battlespace.

As a standalone sensor, LWIR IRSTs challenge the survivability of the F-35 by mitigating its RF stealth. Detection of F-35 skin returns, which are difficult to engineer against given persistent friction during flight, at significant ranges could allow a J-20 the ability to; manoeuvre away, shoot a PL-10 or call for backup – all of which mitigate the ‘surprise’ factor stealth.

Resultantly, the F-35 may transmit its powerful APG-81 AI radar to achieve a ‘first look, first shot’ engagement (illustrated in figure 4). Using an active sensor helps detect the enemy, but coincidently broadcasts the F-35’s location to any system utilising an RWR or passive direction finding. China increasingly employs various advanced radars, which degrade the RF stealth of an F-35. As surface threats become increasingly networked it is possible that an F-35 could be detected from a ground platform which informs airborne J-20s where to target.

Figure 2. F-35 Conceptual Engagements against air and surface targets

When integrated into a broader sensor suite, the LWIR capability is able to contribute to the immense task of data fusion. A core aspect of being ‘5th Gen’ is an aircraft's ability to collect data, process that data and disseminate as much credible and usable information as possible. The ‘fusion engines’ of advanced fighters require onboard sensors to provide as much data as possible, in order to reduce pilot workload as well as increase the lethality of its other systems. As discussed, LWIR offers these ‘engines’ high-quality data at greater ranges than a similar MWIR sensor. The result is an increase in decision superiority.

In Conclusion

As China continues to deploy advanced aircraft to surveil Taiwanese airspace and the East China Sea, key Australian national security personnel have indicated that the “drums of war” are starting to echo. The ADF’s ability to maintain a ‘secure, prosperous and inclusive Indo-Pacific’ will likely be challenged by the rise of advanced technologies - LWIR being one of them. LWIR sensors will affect all aircraft, however most seriously threaten the RAAFs latest and greatest weapon - the F-35. As the RAAF’s premier air combat platform, the possible degradation of the F-35s stealth characteristics reduce overall survivability. Increased detect and launch ranges, combined with EMCON silent tactics, would improve the lethality of regional threats such as the J-20 - to the detriment of Australian and allied airpower. Moving forward the ADF, in conjunction with the US and industry, should commit to research that aims to provide a solution to LWIR detection. In the interim, previously held assumptions of tactical warfighting will need to be re-considered, taking into consideration the possibility that RF stealth may have, for now, been countered.

About the author: Jack Ryan is a junior officer in the Royal Australian Air Force. He is a Distinguished Graduate of the Australian Defence Force Academy where he studied politics and history. You can follow him on twitter @justjackryan. The views expressed are his alone and do not reflect the opinion of the Royal Australian Air Force, the Department of Defence, or the Australian Government.

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