Anti-stealth countermeasures are now "proliferating". Whereas most radars operate between 2GHz and 40GHz, a low-band equivalent such as VHF radar operates between 1MHz and 2MHz and is able to pick out most stealth planes that are known to be flying today.
The Russians persevered with low-band radar due to their technological conservativism.
VHF can pick up "noise" such as clouds and rain, which was a reason why the West abandoned it – it does have basic physics on its side: its wavelength is the same magnitude as the prominent features on many stealth planes, so that its signal bounces back.
Russia and China (UK, USA, Australia, Israel, South Korea and basically any country with a modern air force) have VHF-Aesa (Actively scanned electronic array) radars.
Aesa (Actively scanned electronic array) radars like those supposedly on China's Divine Eagle drone are made up of a large number of solid state, chip-like modules that each emit an individual radio wave; these meet in front of the antenna to form a beam that can be easily aimed at a very specific target – and, combined with VHF, are an effective stealth-hunting tool.
Three main determinants dictate the maximum number of transmit receiver modules a fighter radar can accommodate:
1. the volume of the aircraft’s nose,
2. the technological maturity of the firm/country’s T/R module packaging technology, and
3. the effectiveness of the radar's thermal management system(s).
The volume of the nose is a fairly intuitive constraint, the larger an aircraft’s nose is, the larger the radar can be. For example, the F-15C’s nose cone is able to accommodate the much larger 1,500 T/R element APG-63V(3) radar vs. the F-16C Block 60 with its comparatively smaller nose cone and its 1,000 T/R element APG-80 AESA.
Packaging technology refers to how many individual T/R modules can be installed within the finite space usually accomplished by reductions in size of the individual T/R modules. The more technologically advanced a firm’s T/R packaging technology is, the smaller the individual T/R modules will be resulting in an increase density of the layout of T/R modules within the array.
Thermal management systems are instrumental for the operation of high power AESA radars. Unlike MSA systems, air cooling systems are insufficient to prevent heat related system failures and frequent maintenance issues.
Chinese defense forums have posted copies of the image above which claim to cite the J-20’s AESA T/R module count at 1,856, the J-16’s at 1,760, and the J-10B at 1,200 T/R modules. It is likely the J-10B is the first Chinese fighter aircraft to feature an AESA; J-10B units achieved initial operational capability (IOC) in October of 2014. The volume of the J-10s nose cone is not substantially different from that of the F-16 or the Israeli Lavi from which the J-10 is partially based. Therefore, if one were to assume China had reached parity with the United States in packaging technology, the 1,200 T/R module figure would be plausible but slightly high.
Russia’s first fighter mounted AESA radar, the Zhuk-AE, contained 652 T/R modules and was unveiled in 2007. The Israeli ELM-2052 AESA radar, which has been marketed for both the F-16 and the FA-50 – a joint Korean Aerospace Industry and Lockheed Martin F-16 derivative, has roughly 512 T/R modules.
With high quality AESA radar detection ranges go up to 50-300 miles.
Combining different radar and multiple large drones with radar to blanket an area
The final trick is "to combine together different radars into an integrated air-defence system and a central information-processing centre that can make life very difficult for any stealth fighter or bomber", Sutyagin says. And stealth planes are not always stealthy from every angle (it costs too much money). So if you have radar in front, at the sides and above – with a high-altitude drone such as the Divine Eagle – along with satellite tracking of any target, then it might well be a case of RIP stealth.
Popular Science has coverage of China's stealth hunting drones and radar
Divine Eagle has 7 radars include a X/UHF AMTI Active Electronically Scanned Array (AESA) radar on the front, two X/UHF AMTI/SAR/GMTI AESA radars on the twin booms, two X/UHF AMTI AESA radars on either side of the engine nozzles, and two more radars on the end of the booms. AMTI and GMTI radars are used for tracking air and surface targets, respectively, while SAR is used to provide detailed imagines of ground targets like bases and infrastructure.
Sina Defense. The JY-26 "Skywatch" AESA Radar, operates in the long wave band to detect stealth aircraft, which are often optimized against detection by shorter wavelenghts. The JY-26 is claimed to have a range of 500km and Chinese media claimed it detected F-22 Raptor fighters off the South Korean coast in mid 2014. The Divine Eagle is likely to have similar radar technology to detect stealth bomber and fighters at long range.
Countering the counter measures with next level stealth
However, not everyone has these capabilities and centralised air defences are also vulnerable to hacking, or even special forces. While one response might be to reduce the profile of the plane – using electronic warfare systems and stand-off weapons to increase survivability instead of stealth – there is, according to Sweetman, another option: develop weapon systems like that of the Dassault nEURon drone, which can be programmed to visually seek out large radar arrays.
Not yet, though, Sweetman says. "You can take stealth to the next level," he says, meaning a large, flat, tailless subsonic flying wing and active stealth technology. "In theory, by transmitting a signal just half a wavelength off the wavelength of the radar, your plane can disappear."
The US might have already reached the next level, even though the decision on its new stealth Long Range Strike Bomber is still some way away.