In the coming decade, the Indian Air Force (IAF) will confront the problem of detection of stealth aircraft. Our Northern neighbour has developed a fifth generation air dominance fighter J-20 with stealth characteristics.

The J-20, a midair-refuelable, missile-capable jet designed to fly far beyond China’s borders, was for years kept in top-secret development by the Chinese. It was first showcased to the world with a test flight during US defence secretary, Robert M. Gate’s visit of China in January 2011. It has been operationalised since then and 20 plus are in service. After solving the engine related issue, the Chinese are expected to undertake the serial production of the aircraft. The Chinese are known to be engaged in developing stealth long-range bomber (H-20) which, too, could materialise soon from their stable.  Analysts forecast the H-20’s first flight in the early 2020s, with production possibly beginning around 2025.

An intruding aircraft can be detected by its radar, infrared (IR) and acoustic signatures. For radar detection, reflected radio waves from the aircraft are also picked up by the source radar (airborne or ground) which leads to its radar detection. The range depends on the transmitted power, height of the aircraft, the radar horizon of the ground radar, and effect of extraneous clutter/ interference.

The IR signatures, too, can lead to detection of the airborne/ground targets. This mode of detection is normally employed by the aircraft. Aircraft generate heat from their engines as well through frictional heating with the air. The nose and leading edges become hot, more so when flying supersonically. This can make detection easier. Therefore, stealth planes stay subsonic even if they can fly supersonically or have super-cruise capability without switching reheat. For example, Forward Looking Infrared (FLIR) is available in most combat aircraft for forward-looking to all-round situation awareness. Such systems use a passive thermographic camera, so that they do not give out any radiation of their own, unlike radar. As the IR attenuates in the atmosphere, the range of detection by this means is not very large.

Relevant literature suggests that acoustic signatures, too, can be processed to detect the aircraft. This is possible as all aircraft emit specific acoustic signatures. This is a passive system; hence, it would not be prone to the threats of anti-radiation missiles But, acoustic detection has not developed to an extent that this can be universally used to detect the combat aircraft. In the US, however, customs and border protection record excursions at low level across the northern borders for illegal activities like smuggling of drugs/criminal transit / human trafficking etc. The sound emitted by the low flying aircraft is picked up by the Acoustic Seismic Detection System (ASDS) which consists of several nodes. The single node detects the presence of the aircraft by more than one giveaway direction. The detection by more than one node helps in determining the bearing and localisation. Logically it is not suited for detecting the fighter aircraft unless these are approaching at low level.

Stealth aircraft have been designed to reduce the radar, IR signatures to avoid detections. A look at stealth fighter like F-22 reveals its designs adhere to the shaping principles of a stealthy design to reduce the radar cross-section. The leading and trailing edges of the wing and tail have identical sweep angles (a design technique called plan form alignment). The fuselage and canopy have sloping sides. The canopy seam, bay doors, and other surface interfaces are saw-toothed. The vertical tails are canted. The engine face is deeply hidden by a serpentine inlet duct and weapons are carried internally. The result is that for the size of the aircraft its radar cross-section, which for the size of the aircraft should be more that 3-4 metres, is just reduced to from certain angle -40dB sm equivalent to the reflection from a size of a steel marble. F22 is known to be an example of perfect stealth. This has raised its cost to astronomical level which even America has found it unaffordable and therefore, has developed F-35, a fewer stealth aircraft but a cheaper version. Likewise, the Northrop B-2 Spirit, also known as the Stealth Bomber, is an American heavy penetration strategic bomber, featuring stealth technology designed for penetrating dense anti-aircraft defences. Other aircraft under development such as Chinese J 20, J 31 and Russian SU 57(Pak-FA) may not be as stealth as F-22 but, these aircraft still would be difficult to detect and even if detected, the ranges would be so low to enable credible response by the air defence. Russia and China are also developing stealth bombers, the details are classified. The US company, the Northrop, is developing a new and more capable B-21 Raider, a long strategic bomber, capable of delivering conventional and thermo-nuclear weapons. Few details are known but claim to survive enemy air defences while operating from all heights unlike B-2 which flies at high altitude at subsonic speeds. The stealth and height of operation shield B-2 from detection as well as from the destructive ranges of air defence weapons.