How Invisible is Stealth?
Iraqi air-defense radar actually picked up F-117 stealth fighters but could not support attacks against them. [America's Inadvertent Empire, 69]While the Iraqi regime fell without much of an organized defense, much less an integrated air defense, it is still relevant to ask just how stealthy our low-observable aircraft remain. Especially interesting (and relevant here) is the origin of the Iraqi air-defense radar mentioned above--China. If China can detect our B-2 Spirit or F-117 Nighthawk; how much does that impair the USAF's ability to conduct (or threaten) air strikes against China in the event of a crisis? It could also seriously affect our defense procurement plans, most notably the F-22 and F-35, in planning for wars over the horizon.
The desire to be able to detect and down America's stealth planes is certainly present within the People Liberation Army:
As a result of the Yugoslav War, and particularly as a result of the lessons derived from the PLA's study of the air war, a new program known as the 'three attacks and three defenses' (san da san fang) was initiated throughout the military beginning in late 1999. The 'three attacks' are attacking stealth aircraft, cruise missiles, and armed helicopters. The 'three defenses' are defending against enemy reconnaissance and surveillance, precision strikes, and electronic interference. [Emphasis added. Modernizing China's Military, 87-88]
The PLA watches closely all actions taken by the American military, so it is no secret that America likes to blind its opponents in the early stages of a conflict.
Before getting into the finer points of detecting stealth aircraft, I feel a refresher course on how radar and stealth work is in order, but, of course, if you are familiar with the technology, you can skip ahead. From Wikipedia:
[Radar] is a system used to detect, range (determine the distance of), and map objects such as aircraft and rain. Strong radio waves are transmitted, and a receiver listens for any echoes. By analysing the reflected signal, the reflector can be located, and sometimes identified. Although the amount of signal returned is tiny, radio signals can easily be detected and amplified.
Stealth, as described in this Wikipedia article, uses vehicle shape, non-metallic materials, radar absorbing paint, and other technologies to reduce a plane's radar cross section (RCS).
One possible method of detecting low-observable aircraft is "Passive Radar" (also known as "Passive Coherent Location," or PCL). One conspiracy theory attributes the bombing of the Chinese embassy in Belgrade to an attempt to prevent Chinese from using a PCL system to collect information on American aircraft.
Basically PCL processes the target reflections of already existant transmitters, such as cellular tower and television transmitters. The interpretation of the signals that are returned are more complex than with designated radar transmitters, but modern computing has reached a level where such interpretation is possible and has been demonstrated.
The best non-technical explanation of passive radar, complete with a graphic depicting it in action, is here, just scroll down. (For those interested in the technical details, a search for "passive radar" or Lockheed's "Silent Sentry" should provide you all the details you seek.) A more general explanation of the concept, as it applies to tracking more than just aircraft, can be found here.
Low Frequency Radar
Another possible method of detecting stealth aircraft is using low frequency radar. It works the same as traditional radar, but has a longer wavelength and its performance is not degraded by stealth technologies to the same degree as normal radar. As a reader explained it to me:
Conventional radar operating frequencies are in the range of 10 GHz, so a wavelength is on the order of 3 cm meaning that just about any surface on the aircraft represents many many wavelengths, and so there is no resonance and the absorptive properties of the material come into play.
Low frequency radar operates in the vicinity of 150 MHz, so a wavelength is about 2 meters - roughly the diameter of the fuselage for a typical aircraft, and so even though the aircraft uses non-metallic materials on the surface, there are probably internal structural pieces that represent multiples of a half-wavelength, and so there may be an enhanced return.
Stealth is no panacea. No aircraft ever designed, no matter how advanced its stealth design and technology, is completely invisible. The idea behind low-observable aircraft is to make their RCS small enough for them to blend in with the background noise on the opponent's radar and thus preventing the enemy from noticing them. When that fails, an aircraft is still safe as long as the information delivered by the radar is not accurate enough for anti-aircraft artillery (AAA) to down the plane. This was the case in 1991, when the Iraqis were able to see the American Nighthawks on their radar but were unable to shoot them down. A country with a more formidable air force (eg: China), however, could use such radar notification to scramble fighters to down the aircraft. (The location handed off to the interceptors would not be specific and the fighters' radar would have a harder time finding the low-observable aircraft than the ground-based system, but manned fighters would have a better chance hitting their target than simply launching AAA into the night sky in the stealth craft's general direction.)
Rather than take the chance that an opponent's radar can detect America's aircraft and provide a target solution to triple-A, America often opts to take out the country's radar installations with cruise missiles before the air strikes begin. This technique would work against LF radar just as well as its traditional cousin, but PCL would not be so easily overcome. PCL does not have a limited number of transmitters for America to attack, but instead uses every television station, cellular phone tower, and radio station to paint the target. It is unlikely that America could, or would attempt to, wipe out a nation's airwaves one tower at a time. Therefore wiping out such a system would require finding the computers that analyze the signals or attack the military's C4ISR writ large.
Conclusion - Questions, Not Answers
The PLA is known to be researching PCL and other methods of stealth detection, but the extent of their research, effectiveness of their processing, and status of their deployment is unknown.
Douglas MacGregor notes in Transformation Under Fire that
Since 1941, no American soldier has set foot on foreign soil or entered an enemy capital without the USAF in control of the skies overhead. Do China's possible advances in the technology to counter stealth, when combined with the growing PLAAF, undermine the ability of the USAF to guarantee air superiority to the point that the US Army should begin planning for the possibility that it may have to enter combat without air superiority?