Penetrating the “Stealth” Barrier

January 1991.

A massive armada of military aircraft from a wide array of nations fill the skies over the Persian Gulf to begin the mission to oust Saddam Hussein’s army from Kuwait. Iraq’s air defences are amongst the densest in the world especially around the capital city of Baghdad where Saddam’s regime believed they are safest. And yet for nearly a month coalition bombs rain down on them with impunity by an unseen enemy that prowls the skies almost totally undetected.

F-117 Nighthawk

F-117A Nighthawk

The F-117A Nighthawk captured the world’s imagination during and after the 1991 Gulf War. It’s rakish looks and ability to evade radar detection made it feel like something out of Star Trek and both it and the even more advanced B-2A Spirit stealth bomber enjoyed a reputation of being unstoppable throughout the 1990s. Then on March 27th 1999, this golden age of stealth technology seemed to come to an abrupt end when an F-117A Nighthawk was shot down by Serbian forces during Operation: Allied Force. Despite the circumstances surrounding the loss still being the source of some debate it is clear that US confidence in stealth was shaken and the missions following the incident were flown with more extensive support from conventional combat aircraft. Even the term “stealth” fell out of favour being replaced by “low-observability” indicating that the veil that once protected these aircraft had holes in it.

F-35 Lightning II 1

F-35 Lightning II

The legacy of those missions over Baghdad in 1991 lives on however with the latest combat aircraft possessing some degree of low-observability technology. The goal now is to have a frontline force of combat aircraft that have the same level of protection the F-117A enjoyed in 1991 and nowhere is this more obvious than in the F-35 Lightning II Joint Strike Fighter. The F-35 relies heavily on its ability to remain undetected in air combat sacrificing traditional performance for low-observability and advanced weaponry. This has raised more than a few eyebrows from observers and analysts around the world especially in countries where the future of their respective air forces weighs heavily on a fleet of F-35s. The problem is that as the F-117A proved (to an extent), today’s stealth technology may not be of any use tomorrow. Therefore, when tomorrow comes the aircraft has to fall back on its weapons and more traditional air combat tactics.

So how difficult will it be to detect the F-35 and future stealth or low-observability aircraft in the future?

To address this question we must first look at just how these aircraft achieve their low observability. Radar works by sending out a radio signal that bounces off the target aircraft back to the radar dish. By examining the angle of the reflection and the time it takes to travel to and from the target the aircraft’s position can be determined. Aircraft like the F-22 Raptor and F-35 Lightning II are designed to deflect the radar waves away from the transmitting radar meaning the radar does not get the return signal it needs to identify the location of the aircraft. Additionally, the aircraft have Radar Absorbent Material (RAM) on key parts of the airframe where it is not so easy to deflect the radar signal away. Despite these measures the aircraft are not entirely invisible to radar since it is impossible to completely deflect or absorb the signal but the remaining radar return is so small that only the most sophisticated radar sets can identify it. In this instance the aircraft utilise active electronic countermeasures (ECM) to jam the radar set.

The problem is as low-observability technology becomes more prevalent in military aviation, efforts have been ramped up to defeat it. With such consideration given to defeating radar some militaries and research establishments are looking at alternate ways to detect low-observable aircraft. Here are just some of the options that could render the F-22 Raptor and the F-35 Lightning II detectable and subject to attack.


  1. Multistatic Radar
    Since stealth aircraft work primarily by deflecting radar signals then the immediate answer for detecting them to is to capture those deflected signals. A Multistatic radar system works by utilising numerous emitters/receivers all networked together. If a radar signal sent out by one emitter is deflected, then a second receiver can capture it and use that information to get the aircraft’s location. Post-war analysis showed that this happened by accident in 1991 with Iraqi radars operating on similar frequencies picking up returns from another radar set that had been deflected by the F-117’s design but because there was no networked computer system to analyse the signals they were unable to make use of this advantage.There are numerous problems with this method however. Firstly, the radar sets are themselves vulnerable to attack by anti-radar missiles such as the AGM-88 HARM and ALARM carried by the intruding aircraft. It is also a very costly method whose effectiveness is significantly reduced if a set is destroyed or malfunctioning. It also requires a great deal of processing power.

  2. Low Frequency Radar
    A low frequency radar set is a much clumsier form of detection than a high frequency set. The wavelength of a low frequency radar is significantly larger meaning the signal the stealth aircraft can’t deflect will be larger in its return making it easier for the receiver to capture. Unfortunately, low frequency radars are not as accurate and can only give a rough estimation of the target’s location. They are also susceptible to background noise from other radio sources and like all radars it reveals its location leaving it susceptible to attack.

  3. Passive radar
    The European Aeronautic Defence and Space Company (EADS) has proposed utilising a form of “passive radar” to detect stealth aircraft. This works by harnessing the many thousands of radio and microwave signals that already fill the air from sources such as mobile phones and television signals. Passive radar monitors how these signals move through the air and identifies a disruption to them such as the passing of a physical object (the target aircraft). The system has the advantage of not sending out any signals itself that can be detected and attacked like a traditional radar station meaning the intruding aircraft is unaware it’s been detected. It would also be very hard to effectively jam. Passive radar technology remains quite infantile but development work continues with EADS proposing both military and civilian applications in the future.

  4. Infra-Red Search and Track Systems (IRST)
    An IRST is another passive system that works by detecting the infra-red energy given off by a heat source such as the exhaust of a jet engine or the friction heated fuselage of an aircraft flying at high speed. Using infra-red to detect aircraft is nothing new to the world of air combat with primitive sets being used by German night fighters during World War II to find and destroy RAF bombers. Back then the sets were so large they could only be mounted in converted bombers such as the Dornier Do.17 but they are now so small that they can be mounted on fast jets with relatively little modification.

    Eurofighter 2000 Typhoon FGR.4

    IRST scanner ahead of cockpit

    During the Cold War, aircraft designers in the Soviet Union had far more interest in IRST devices than their counterparts in the west with every major fighter aircraft having an IRST device at the very least optional since the MiG-23 “Flogger” in the 1960s. The F-4 Phantom II initially adopted an IRST as did the F-14A Tomcat before deleting them but the F-15 Eagle or Britain’s Tornado F.3 had no such device. Now however the RAF’s Typhoon FGR.4 has the option to utilise the PIRATE (Passive Infra-Red Airborne Track Equipment) sensor mounted on the port side of the nose just forward of the cockpit which can track up to 200 targets simultaneously.

    Traditional low-observability technology would be ineffective against an IRST since it is impossible to keep an aircraft’s temperature low enough to avoid detection. However, the system is far from perfect with even the most advanced sets being very susceptible to atmospheric conditions. It is also rather short ranged technology compared to traditional radar sets and chaff/flare decoys can temporarily blind them.

  5. Ultraviolet (UV) detectors
    The next generation of small surface-to-air missiles will feature UV filters or rely entirely on UV guidance. The sun gives off huge amounts of UV radiation which an aircraft can block as it flies over head. UV guided weapons detect the UV radiation and effectively lock on to the “hole in the sky” as the seeker would view it and have the advantage of being practically impossible to jam or decoy away. It is impossible for any aircraft to avoid being detected by this method although it’s effectiveness is reduced somewhat at lower levels where excess UV can get in the way of the “hole”.

  6. Acoustic Detection
    One thing all aircraft do is make noise and at altitude that sound can carry for many miles especially when travelling supersonic. The first supersonic airliner, Concorde, found that its biggest selling point – it’s speed – was its biggest drawback in sales because few countries allowed such a large aircraft to fly at supersonic speeds over their territory because of the noise of it’s sonic boom which could be heard up to 50 miles from its location.

    Acoustic mirror

    Acoustic mirror

    Acoustic detection devices work by detecting, amplifying and then identifying the direction a sound came from. Like IRST technology it is not a new concept with devices known as acoustic mirrors being established along the south east of the UK during and after World War I to give warning of an incoming bomber force. While the technology couldn’t locate the exact position of a bomber force it could give enough warning to launch aircraft in to the direction they were coming from as well as allow time for the civilian population to flee to their air raid shelters.

    The advent of radar technology largely negated its effectiveness and use but modern sets using sophisticated computer processing could localise a sound pattern from an intruding aircraft and zero in a fighter which could then use its IRST to prosecute the intercept. The more acoustic devices there are in operation the more accurate the system would be. In principle its operation is not too dissimilar to how submarines use passive acoustic devices to locate an enemy submarine.

  7. Magnetic Anomaly Detection
    Magnetic Anomaly Detectors (MAD) have been used by aircraft to locate submarines since World War II. They work by measuring minute variations in the Earth’s magnetic field caused by a metallic object passing through. Computer processing then tries to narrow down the centre of the disruption thus locating the object. Similar devices have been proposed for detecting stealth aircraft. There has even been research carried out in to radar devices that can monitor the gravimetric field over great distances and by establishing a base reading can identify when a large metal object disrupts it. The technology is far from being perfected but continues to be researched.

  8. Schlieren Photography
    Schlieren Photography

    A Schlieren photogtaph of shockwaves from a gun

    An aircraft flying through the air causes a great deal of disturbance in the atmosphere. It is possible to detect this disturbance using Schlieren photography; a rather complex process involving the observation of deflected light compared to undeflected light in a viewing screen. This process has already been used by aircraft manufacturers to investigate airflow in wind tunnels. If a system could be devised to repeatedly photograph large areas of sky quickly then in theory it could detect a stealth aircraft.

    In the wake of the 1999 shootdown of the F-117A over Serbia many conspiracy theorists suggested that the Chinese had tested an early form of this technology which is what allowed the Serbians to locate the Nighthawk. They believe this is why the Chinese embassy was bombed by the US Air Force in order to destroy the device since it seemed inconceivable that the USAF had carried out the attack by mistake as they claimed.

It’s clear the technology is out there to detect low-observability aircraft. Through one or a combination of these methods an advanced adversary could nullify the advantage of stealth that the US has enjoyed for the past two decades. This is why so many are concerned about the F-35. If the aircraft is effectively reduced to being as visible as an F/A-18 then how well will it perform as traditional fighter?

Given the level of secrecy surrounding the aircraft it is hard to answer this question. Certainly the story of an F-16 Fighting Falcon, an aircraft from the 1970s, defeating it in a mock visual range dogfight doesn’t help its case but how representative of a real battle were these tests? If detected the F-35 will have to fall back on its weapon system which few would deny is an advanced piece of kit. While the Sukhoi Su-35BM is claimed to be far more manoeuvrable and faster, if the F-35’s weapon system is as advanced as promised then both these advantages are completely nullified. If further proof of how important weapon technology is over aircraft performance were required, then one need only look to the Falklands War where British Harriers using advanced AIM-9L Sidewinder missiles proved unbeatable in air combat against Argentine Mirages that on paper had the advantage of speed.

F-35 Lightning II 3


One final important note; unless a major global conflict breaks out the F-35 like the aircraft it is replacing (F-15, F-16, F/A-18, Tornado) may never face a truly modern enemy with the very latest detection equipment meaning it will remain “stealthy”. It may be fashionable to criticise the F-35 for being a poor aeroplane but it remains to be seen what kind of weapon it will be.


5 responses to “Penetrating the “Stealth” Barrier

  1. A fascinating insight into this incredibly sophisticated area of technology. On board detections systems are only good if they can detect and destroy a target before visual contact is made. Once the target is in sight, more traditional methods would come into play, as you say. Many A-A rockets track and kill their targets without pilot intervention even in close proximity. Can an aircraft ever be truly 100% ‘stealth’ I doubt it, what one side develops, the other develops counter measure for, but that said, who knows what is round the next corner. Really interesting Tony.

    Liked by 1 person

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