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Any aircraft traversing through the air would impart velocity, pressure, and temperature gradients to the surrounding air, then there are some phenomenon which are unique to all winged bodies such as wing tip vortices. These gradients do not occur naturally and develop in a very distinct manner (say in a straight line)

If there is a doppler radar which is attuned to ignore all naturally occurring gradients and only look for gradients that have man made origins then it can tell that there is something flying through the space. It doesn't even need to be trained on stealth aircraft as it is not even looking at the aircraft but the flow field around it and such data can be readily generated by any conventional fighter jet.

If it is working in conjunction with "regular" radar and it sees something that the regular radar doesn't then it implies presence of a stealth aircraft and that area in space can be investigated with more energy and other sensors.

Also, why can't a surface-to-air missile be launched and be guided by such a radar? After all, it can clearly determine how the source of disturbances is traversing in space because disturbances follow the source (duh!), when the missile reaches close to the aircraft, hopefully its own radar would be able to lock on its target.

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To a limited extent, yes, it's possible to detect the air disturbances left by aircraft. This NASA article studies the radar reflectivity of vortex wakes exhaustively. It's being proposed as a safety measure for civil aviation, to avoid wake turbulence incidents.

This is a 2000s follow-up to the 1992 Gilson study. Gilson is much easier to read, but was more focused on assessing wake detectability with present sensors than on looking for the best one.

Gilson

Besides the wake vortex itself, the higher temperature and humidity of engine exhaust also contribute to radar detectability to a small extent. All this makes building a completely invisible aircraft physically impossible. But this is not likely to be the best method for detecting stealth aircraft.

The newer studies against a small target have measured a typical wake RCS of about -62 dBsm, but it goes higher with optimized equipment.

enter image description here enter image description here

This is smaller than most stealth aircraft' RCS itself (-40 to -20 dBsm). But it scales with size, as can be seen in the Gilson study, where the C-5A produced up to -30 dBsm.

Jet fighters are in between the C-5A and the test target in size. Wake RCS also cannot be defeated by any kind of stealth shaping, RAM, or RAS. As long as it's flying, an aircraft's wake will be more detectable the larger and the faster it is. The wake is also persistent, greatly improving the sensitivity gains from continuous scanning.

But the utility of this can be overestimated without accounting for the finer points of how radar and stealth works.

  • First of all, an air-to-air missile can be launched without a guiding radar at all, as a "mad dog". It will acquire a stealth aircraft at a closer, but still usable range.
  • Second, radars can be made significantly more sensitive and selective, if you use a narrow beam at low scan speed and know where to point it.
  • Third, the radar cross-section of an airplane is not a static number. It's a complex 4D function, which depends non-linearly on the angle of observation and radar frequency. Add flight controls position and polarization, then it gets really complex.

The RCS of a fighter can be very roughly visualized like this:

enter image description here

Some combination of the 3 factors above is how the Serbs have shot down an F-117 using only an S-125 - a 1961 system, predating Ufimtsev's seminal work on radar stealth by a year, so clearly not meant for the job.

Now to the fourth factor: the role of a radar isn't simply to detect targets. It's to detect, classify, measure their range, velocity and motion vector, track, and optionally illuminate them for missile lock.

There are many ways to detect the presence of a stealth aircraft. Multistatic radars, satellite photos, good old espionage. None of them, however, give the tracking ability of a direct radar beam.

Wake tracking is one more way to detect aircraft, but it won't be very effective for tracking them. This is because the wake's RCS increases with time, making older detection events more visible than new ones.

For tracking, most systems would still have to rely on a traditional radar. Can it do it? Here's the modeled X-band RCS for an F-35:

enter image description here

A stealth aircraft looks to a radar a bit like a disco ball: its reflections are all specular and concentrated in specific directions. Except, unlike a disco ball, the mirrors are arranged such as not to hit the observer's eye as often, and are tinted dark by using RAM.

So how does it look to the pilot, say if it's an F-14 trying to detect and track an F-22?

It's not a complete absence of signal. Due to the many spikes on the charts, the stealth aircraft will still be detected, even by an old, pre-stealth radar. But it will look like intermittent blips on the screen, rather than a continuous track. The F-14 pilot will not be able to set his systems to track the aircraft, and will have to rely on a mad dog launch or a visual to have a shot at it.

For older radars, it wasn't trivial to get a track on a regular aircraft. They aren't designed to deal with stealth ones, which give occasional blinks rather than a solid reflection. While at that, many countries don't even have F-14 level tech.

Modern radar systems are nowhere near as stealth-naive as the S-125 or the AWG-9. It took the Soviets a decade to realize its importance, but the development of counter-stealth technology started once they learned of the F-117.

These include IRST systems, against which radar stealth has no effect, very high output radars, which counter it directly, but not by much, and renewed development of radars functioning outside the X-band, where stealth is much less effective. Here is the statistically modeled RCS of the F-22 at VHF, namely 230 MHz:

enter image description here

It's still pretty good, still better than a 4th-gen fighter, like in the charts above. However, this frequency band is only generally used by ground radars, and then only non-NATO ones.

However, the US has installed a dual-band radar in its two newest warships, and Russia in its latest fighter, the Su-57. Dual-band radars are more effective against stealth than disjointed sensors, being able to focus the high-frequency beam to track the blips spotted by the low-frequency one. Stealth and counterstealth are complementary games that both sides are playing now.

Where an F-14 pilot would only see occasional radar blips, 4.5-5 gen fighters can at least target a stealth aircraft. The track will be inaccurate and much jumpier than usual, but better than nothing. IRST can also reach over 100 km by now, but it's more weather-dependent than radar.

In high-intensity warfare, with ground radars, IADS, AEW, stand-off jamming, countermeasures, many flights in the air, it's not just a jousting duel. Both stealth and non-stealth aircraft can be detected or avoid detection, and stealth acts like setting your opponent's radar back by 20 years.

But it's not set back by 100 years. Successful covert missions are much more achievable with a stealth aircraft, but still require planning and avoidance.

enter image description here

Summary: there are easier ways to detect and track stealth aircraft than chasing their wake. The best-case RCS of an F-22 is still higher than that of its wake, at least in subsonic flight. But if radar and IR stealth were to improve greatly, this could be useful.

There's inconclusive evidence on whether wake detection might work better with higher-frequency radars. These potentially allow for stealth detection with much smaller sensors. However, wake tracking is not useful for missiles, since the wake's signature gets larger as it spreads - like a built-in chaff dispenser.

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  • $\begingroup$ "A stealth aircraft looks to a radar a bit like a disco ball: its reflections are all specular and concentrated in specific directions. Except, unlike a disco ball, the mirrors are arranged such as not to hit the observer's eye as often," In other words, an anti-retroflector. $\endgroup$ Dec 3, 2023 at 6:48

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