Luftkrieg im 21. Jahrhundert

[Quintus Fabius]

Ein Artikel von Juni 2015 über IRST, dass meiner Meinung nach die Zukunft im Luftkrieg darstellt weshalb Stealth im IR Bereich und Sensorfähigkeiten in selbigem von größter Bedeutung sein werden, deutlich wichtiger als alle Fragen von Stealth gegen Radar usw

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IRST is a sensory device which uses IR (infrared) radiation for detection and targeting purposes. IR radiation has wavelength of 0,75 to 1.000 microns (micrometers), longer than wavelengths of color red in the visible spectrum (visible spectrum ranges from 0,39 to 0,7 microns, with violet at 0,4 and red at 0,7 microns). It is given off by all objects above absolute zero, though objects that are below average temperature of their surroundings will absorb far more IR radiation than they will give out. Unlike FLIR which is a targeting device, IRST can be used for initial detection as well.

R bands are also associated with temperatures of bodies producing radiation. Visible light is associated with temperatures above 1.000 *C. Next come IR bands: 0,7-4 microns (1.000 – 400 *C); 5-25 microns (400 – -150 *C) and 25-350 microns (this is a civilian, not military, division). Bodies at room temperature have radiation peak at around 10 microns, while 3-5 micron band is used in civilian applications for its effectiveness in tropical conditions.

In civillian purposes, astronomers use IR telescopes to penetrate dusty regions of space that block off visible light. NASA also has an airborne IR system – SOFIA (Stratospheric Observatory for Infrared Astronomy) which is flown at altitudes of over 41.000 ft, allowing 85% of the entire IR spectrum to reach it. Its service ceilling is 45.000 ft.

Due to relatively shorter wavelength, IRST is more sensitive than radar to adverse weather conditions. Much of the infrared radiation is absorbed by water vapor, carbon dioxide, methane and ozone. However, there are two wavelength “windows” in which very little infrared radiation is absorbed by the atmosphere. These windows are at 3-5 and 8-12 microns. Both modern IRSTs and modern IR missile seekers typically operate in both bands. 3-5 mM band is optimized for detection of aircraft in afterburner, while 8-12 mM band is better suited for detection of subsonic or supercruising aircraft through aerodynamic heating of skin. More specifically, afterburner exhaust plume is more prominent in midwave than in logwave band, with most emissions being in 2-8 mM wavelength band, while emissions from nonafterburning plume are only useful in 4,15-4,2 mM band. Blackbody radiation from a warm object is most prominent in 10-15 mM band, and only objects above ~300 K give appreciable MWIR emissions (still inferior to to LWIR band). As a result, LWIR detectors have good sensitivity against targets at ambient temperatures.

While IRSTs can detect even relatively cool targets through thin cloud cover, detection range is reduced (more than it is in case of radar), and thicker clouds can significantly degrade detection range. As a result, IRST is most useful for air superiority fighters, which typically operate at 30.000 ft and above – well above normal cloud cover and in relatively thin atmosphere.* ** Only clouds typically present at altitudes above 8 km (~26.000 ft) are those of cirrus variety, which are IR transparent. While dense cumulonimbus clouds can reach extreme heights (60.000-75.000 ft), it is very rare; vast majority does not reach above 20.000 ft. They are also very hazardous to aircraft (especially those of stealth variety), with frequent lightning discharges and large hailstones ranging from 0,5 to 5 cm in diameter, which can damage aircraft’s skin.

Major advantage over the radar is that it cannot be easily jammed. As a result, actual tracking and engagement range of IRST can be expected to be greater than that of radar, even if latter has a major advantage in initial detection range.

Being a passive sensor, IRST alone has issues with range finding. There are some workarounds. Obvious one is laser rangefinder, but being an active sensor it means that the target is warned of the impending attack (IRST still retains its passive surveillance advantage over the radar). Second one is triangulation, which can be done in several ways: datalinking two or more aircraft together, flying in a zig-zag / weaving pattern and measuring apparent target shift, or flying in straight line perpendicular to the target while doing the same. First two are usable against aircraft, while last one is only practical against ground-based targets.

It is a general wisdom that IRST is of limited usefulness due to its sensitivity to adverse weather conditions. However, most modern stealth fighters are intended to operate at high altitudes – above 50.000 ft – where ambient temperatures range from -30 to -60 degrees Celsius, which helps provide excellent contrast. Air at this altitude is also very dry, with 99,8% of the atmospheric water being below 45.000 ft. Combined with low air density and low aerosoil content, this means that there is very little atmospheric absorption of IR radiation.

Stealth aircraft are designed to have certain IR signature reduction measures, but effectiveness of these is rather limited due to basic physics. To fly, aircraft has to overcome two basic forces: gravity and drag. Drag is created due to friction with air, compressibility effects and lift. To overcome gravity, aircraft needs lift. To generate lift, aircraft has to move forward and overcome drag. As a result, aircraft has to perform work – which creates heat.

Other than the engines themselves and their exhaust, there are other sources of IR radiation. Any moving objects have to push the air out of the way. If object is fast – for example, an aircraft flying at high subsonic or supersonic speeds – air cannot move out of the way quickly enough. This leads to compression of the air in front of the aircraft, which in turn leads to heating of said air. At Mach 1,7, a supercruising fighter generates shock cones with stagnation temperature of 87 degrees Celsius. As the air moves out of the way for the aircraft, it also creates significant friction with the aircraft itself, leading to heating of the aircraft’s Skin.

While historically IRST had major performance issues, modern IRST systems, especially Western ones, have mostly solved these issues. As a result, IRST can be expected to become a primary sensor in any air war between competent opponents, for the same reasons as those that led to night vision googles being used for night fighting in place of flashlights.