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A recent theaviationist.com article speculates that the reason behind the 'Combat Sent' and other SIGINT aircraft escorting the F-35's over Estonia two days ago was to deter Russia from using its radars—as their locations would be exposed.

Thinking about what harm could the radars have done, I have the following hypothetical questions not related to a particular aircraft / current world events.

  • Can an air defense system uncover the radar signature of a stealth fighter and use the findings to easily spot that stealth fighter in the future?

  • Can this information be passed on to the fighters so they can easily engage in beyond-visual-range (BVR) combat with that stealth fighter?

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  • $\begingroup$ This is not related your main questions, but those big RC-135s should be easily identified by conventional radars. Would they need to broadcast ADS-B to be sure Russians understand? $\endgroup$ – mins Apr 27 '17 at 19:43
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I would like to make some general comments, as having worked on programs stood up to build sensors for various military threats. In these programs we were always attempting to get data on relevant threats. The more challenging the signature the more it drove the sensor technology, hinting at areas of improvement. In the sensor research we often met with the target community. At times we used their services to look at sensor sensitivities. Perhaps not surprisingly, even though they were on our side, they were at times reluctant to talk about certain sensitive areas.

What I learned from this experience is that, if you are the target you hide your signature as much as possible, and if you are the sensor you hide its characteristics as much as possible. Whenever you give away information you risk exposing your vulnerabilities, and each has its vulnerabilities. An example might be controlling creation of photographic images of sensor optics on an air-to-air missile. This is why some "black" programs hold their technology in war reserve (think hidden in hangers), and are only utilized when the threat warrants exposing this new hardware to the adversary.

As another example, I can point out missions during my cruises aboard the USS Nimitz flying light attack. During this time period the battle group often passed by Libya. The carrier stayed out of the contested waters while the destroyers breached the line drawn in the sand by Gaddafi. Other missions included flying electronic surveillance platforms with fighter escorts close aboard the coast. The radars would light up the surveillance aircraft and that data was captured for further analysis.

My assessment would be that, whenever you bring out a new technology you try to cloak it as much as possible. The other side is out there collecting data on you. Sensors improve, and targets find better ways of hiding. A good example of the evolution of threat can be found looking through the literature on surface-to-air missiles. As soon as the Russians came out with a new SAM we came out with a new countermeasure, which drove them to better sensor technologies.

The findings that a collection activity might obtain will more than likely not allow them to "easily" spot the target. This sort of technology is on the "bleeding edge" and gains are typically hard won and incremental. As we get better sensors, they get better at masking their targets.

One last observation which might not be obvious, but definitely quite important, is that the target might use information about the sensor to develop tactical procedures to minimize its effectiveness. If I learn that your sensor is using microwave electromagnetic radiation I will think about hiding in water vapor or rain storms to minimize its effectiveness. It is truly a cat and mouse game.

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Simple answers:

First stealth means two things: 1. Reduced Radar Cross Section (RCS) or reflective target as might be detected by primary radars. Primary radars are ones which rely on reflected energy (as opposed to transponders). And, 2. Stealth also implements other passive and active measures. The passive ones include reducing ALL emissions from the aircraft and systems, and the active ones are subtle and may be designed to interfere with various detection techniques. The active measures do not necessarily amount to emitted energy.

So in theory there are not radar signatures of stealth aircraft, at least as common radars go. However, there are many techniques for detecting which are not conventional radar. For example quantum techniques relying on polarization and/or entanglement may be useful where conventional radars fail. Another example is utilization of spread spectrum emissions. Yet another is the use of multistatic radar and processing the signals from multiple sources concurrently received by multiple receivers. Just examples, and there are lots and lots of demonstrated techniques. And countermeasures. Kind of like Spy vs Spy in Mad Magazine.

So when you refer to a radar signature, I assume that you mean the current detection signals of a target at hand. And then the question is can the attributes of that signature be communicated to a fighter that might engage with the initial stealth target. Yes, to a very limited degree. But mostly no, because the detection hardware enhanced detection of stealth aircraft is involved, and not commonly known to exist in common fighter aircraft.

With respect to visual range, it is assumed that when we apply apply the term stealth we are talking of targets which are not visually acquired. Typically they operate at night, or in adverse weather, so that they are not detectable. There are no large scale demonstrated visual stealth, sometimes called cloaking, techniques.

What can readily happen is that ground sensor data from multiple sites can be fused into composite information which is then uplinked to a fighter aircraft. This can be done near real time, but in this example, there is no real detection on the stealth seeking fighter, rather just a situation display utilizing ground sensor and processed data.

There are many more complex answers, and several texts and copious open literature papers are available on this topic. I have just provided a simplified and generalized answer.

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  • $\begingroup$ A more interesting question might be how do these lads fly around at night, in the clouds, and not crash into one another under radio silence....... $\endgroup$ – Trevor_G Apr 27 '17 at 19:24
  • $\begingroup$ That question was not asked by @ymb1. However I partially answered it. Visual, lasercom and navigational (spatial, temporal) separation. $\endgroup$ – mongo Apr 27 '17 at 19:26
  • $\begingroup$ Indeed, but visual and lasers wont work in clouds. that leaves spacial, since I assume active radar would be a bit of a give-away.. $\endgroup$ – Trevor_G Apr 27 '17 at 19:29
  • $\begingroup$ Many techniques have been proposed over the years, but the unclassified data I have been exposed to is that visual and optical link are the mechanisms. Keep in mind that while the clouds may be visually opaque, they are not as much at certain spectral regions. $\endgroup$ – mongo Apr 27 '17 at 19:32
  • $\begingroup$ Optical Depth: spiff.rit.edu/classes/phys440/lectures/optd/optd.html $\endgroup$ – mongo Apr 27 '17 at 19:35
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I am surprised no one mentioned it but yes it can be done and has been done. F-117 Nighthawk shot down by Colonel Zoltán Dani of Army of Yugoslavia in 1999. But to do so he modified his targeting radar to improve its detection capability also he had to track the F-117 for quite a period of time. The moment F-117 opened its bay to drop bomb, its RCS increased and it gave enough evidence to Dani to launch SAM shot it down. Stealth doesn't make you invisible, it just makes you less visible. For a skilled radar operator and if you know what you are looking for. US did same mistake with U-2 "dragonlady", they thought that Russian were not tracking it (they were tracking it 15 miles before its border) but instead they were and shot it down when they made sure it was a reconnaissance aircraft.

Reference : https://web.archive.org/web/20090415224243/http://www.defenceaviation.com/2007/02/how-was-f-117-shot-down-part-2.html

https://www.strategypage.com/htmw/htada/articles/20051121.aspx

Now onto your second question, pass the information to another aircraft for BVR doesn't make much sense when you can launch attack from ground itself. The question contradicts itself, if its stealth you cannot detect it in air not your radar can. If you have already discovered it then an attack from ground where you are tracking it make much more sense rather passing the information and loosing precious time. In the F-117 scenario Dani didn't had much time of radar signature, it all depended on bay opening and closing which give or take is 2 minutes max. So to pass this information and then act on it by another aircraft you lose precious time and by the time aircraft modifies its Radar sensitivity (If its is possible) to detect the stealth aircraft its RCS might have changed.

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