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Stealth aircraft are built to have near-zero radar visibility, so that, in a warzone, it'll be harder for the enemy to see it on their radars to shoot it down.

However, air traffic control also use radars.

Normally, this wouldn't be a problem, since the aircraft is carrying a transponder that continually broadcasts positional and movement data (so-called "secondary" radar returns), not just to ATC, but also to other nearby aircraft (so that they can avoid running into it).

If an aircraft's transponder fails, though, then ATC is reduced to tracking it via "primary" radar returns (literally bouncing radio waves off of it and seeing where they're reflecting from)... but stealth aircraft are specifically built so that they can't be picked up by most primary radars (at least without a lot of trouble).

So how does ATC - and, for that matter, all the other planes in the air who would rather not run into it - know the position of a stealth aircraft if its transponder fails? Do they have the flight crew of the stealth aircraft continually read off map position, height, speed, direction, etc. data and try to use this to steer other planes away from it?

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  • $\begingroup$ so that they can avoid running into it I think that task is one-sided (receive-only TCAS of sorts). But then again a TCAS antenna would make it less stealthy. $\endgroup$
    – user14897
    Commented Mar 12, 2018 at 23:35
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    $\begingroup$ ATC primary radar isn’t built with the idea of detecting military-type targets, so it probably just wouldn’t see a low-observable aircraft. They would have to rely on pilots to report their position. $\endgroup$
    – TomMcW
    Commented Mar 13, 2018 at 0:30
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    $\begingroup$ It would be up to the military pilot to "see and avoid", ATC has no authority over military flights anyway, they usually call in/do routing as a courtesy to local ATC, but are not bound by it. $\endgroup$
    – Ron Beyer
    Commented Mar 13, 2018 at 1:40
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    $\begingroup$ @RonBeyer- FAA-ATC controls almost all IFR aircraft military, civilian, etc. operating in the NAS (excluding MOA's, other military training space etc.). When operating at a busy military base the military will often operate, in addition to a tower, Radar Approach Control for local ops. At many locations the FAA will operate a TRACON for mil ops into military airports. FAA ARTCC's (Centers) cover 100% of U.S. airspace and handle all IFR aircraft (mil/civ) within the confines of its area of responsibility (higher altitudes and where no other ATC facilities are avail). $\endgroup$
    – user22445
    Commented Mar 13, 2018 at 15:10
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    $\begingroup$ It's entirely possible and legal to fly a plane without working transponder. Here's a great video by Boldmethod. Their transponder failed when they were still on ground, it's ok for ATC. More communication is needed of course. $\endgroup$
    – Noah Krasser
    Commented Mar 13, 2018 at 20:56

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It depends on the aircraft. One example was on the F-117, which had removable radar reflectors on the sides of the aircraft, allowing the jet to be detected by primary ATC radars.

Front three-quarters view of an F-117 Nighthawk in flight with the Luneburg radar reflectors on either side of the aircraft circled in red

As to more modern stealth aircraft, it’s possible they use similar systems to ensure ATC tracking in peacetime.

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    $\begingroup$ Seriously? Them little things? Impressive. $\endgroup$
    – Notts90
    Commented Mar 13, 2018 at 16:45
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    $\begingroup$ A screw not properly seated on a maintenance panel can make the plane show up like a barn door on a radar. Not the size, it’s the reflectivity that matters $\endgroup$
    – Romeo_4808N
    Commented Mar 13, 2018 at 17:00
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    $\begingroup$ These are called Luneburg reflectors, and are indeed still used by the F22 and F35. $\endgroup$
    – MSalters
    Commented May 13, 2018 at 19:48
  • $\begingroup$ It doesn't sound like those are deployable in flight should the transponder fail, unless I misunderstood. $\endgroup$
    – FreeMan
    Commented Aug 25, 2021 at 17:37
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    $\begingroup$ @FreeMan: They are normally installed when flying anywhere in ATC controlled airspace. They are only removed for stealthy combat missions, or stealth training exercises. $\endgroup$
    – abelenky
    Commented Aug 25, 2021 at 19:16
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I think you are overthinking things a bit, since it doesn't really matter if it's a stealth aircraft or not.

Many places in the world do not have primary radar coverage at all, meaning that if a transponder fails - stealth aircraft or not - it will disappear from the radar screen. Controllers will then revert to providing procedural (non-radar) separation, based on position reports transmitted by the pilot.

Providing separation based on position reports is fairly normal, and procedures to use are established, so it's not a matter of "trying" to steer other planes away - it's a routine task.

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    $\begingroup$ @Sean, primary radar is standard ATC equipment almost anywhere, but it is nearly useless when the secondary fails both because it is not able to determine altitude to any useful precision and because most of them have a lot of ground clutter that the interesting target will be quickly lost in. $\endgroup$
    – Jan Hudec
    Commented Mar 13, 2018 at 18:16
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    $\begingroup$ @JanHudec - ground clutter is not a problem in ATC radar because of the use of "Moving Target Indicator" (MTI) technology. Essentially, objects that are not moving (or going extremely slow) such as mountains, buildings, etc., are removed from the radar display allowing for the normal radar return of moving targets (e.g., aircraft). This technology has been around for decades and without it ATC radar, especially in Approach Control airspace where aircraft are tracked all the way to the runway surface, would be effectively useless. $\endgroup$
    – user22445
    Commented Mar 14, 2018 at 0:44
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    $\begingroup$ @757toga - That's assuming that MTI works as intended - that's not always the case. I've worked sectors that habitually picked up ground clutter from highways, watercraft, and other objects which should've been filtered out. Primary can pick up a LOT of things controllers don't want. The major civilian (FAA) radar systems in the US have a toggle to turn off primary returns, and standard operating procedure is to work with it turned off unless needed to track a transponderless aircraft (or at least it was where I worked). $\endgroup$
    – Aluminum Showers
    Commented Mar 14, 2018 at 4:09
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    $\begingroup$ @JanHudec- Also, you will find some excellent info beginning in Chp 5, Sec 1, of the FAA Air Traffic Handbook (faa.gov/documentLibrary/media/Order/…). This info (focus on para. 5-1-3) shows the extremely limited authority of using Secondary radar only (transponders) for ATC. With some exceptions, Primary radar is required everywhere outside of Class A airspace. The ability to see primary targets is a core requirement and vitally important in providing ATC. This may change one day - but not today. $\endgroup$
    – user22445
    Commented Mar 14, 2018 at 23:31
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    $\begingroup$ @JanHudec “Cessna 123AB, traffic over the train tracks, primary only, slow moving, could be a helicopter. [pause] Could be a train.” $\endgroup$
    – StephenS
    Commented Aug 25, 2021 at 17:28
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When a B2 visited the UK for the Fairford air show a few years ago, it was always escorted by a pair of F15s, one on each wing-tip. So radar simply had to look for the 2 F15s, and the B2 was the hole in between them!

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    $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$
    – Romeo_4808N
    Commented Mar 13, 2018 at 13:19
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    $\begingroup$ @CarloFelicione looks mixed, +3/-2 on votes and 2/3 reviews said looks OK. It certainly demonstrates a way to deal with the issue so I think it is valid and does provide an answer, though a reference would improve it considerably. That said, your answer has no reference either. $\endgroup$
    – Notts90
    Commented Mar 13, 2018 at 16:50
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Actually, we can locate and track them the same way as if our radar fails. Get position reports, as needed, from the aircraft. Not as many trained non-radar controllers in the US as there used to be, though, and the best non-radar tracking device, the flight progress strip, has been supplanted by EDST.

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As others have said, the most basic level of enroute air traffic control (i.e. not "near the airport" control) is what is called non-radar control in the USA and procedural control in the rest of the world. The idea in any case is to prevent collisions between aircraft using the air traffic control services, and this can be accomplished in two different ways:

  • If the aircraft can be depicted as targets on a radar scope, the controller issues instructions to keep the targets at least some minimum distance apart. If the targets don't get close on the scope it means the aircraft didn't get close in the air.
  • If the aircraft cannot be depicted on a radar scope, the controller issues instructions to ensure the aircraft are procedurally separated—they do not cross the same fix at the same time, or they are on different airways, or at different altitudes.

In the United States this is becoming very much a lost art, as radar sites and radar scope systems have become more and more reliable—certainly I have never had to do it. And operating without radar contact limits pilots in several ways, most notably the pilots cannot operate on "random" (point-to-point) RNAV routes—they must be on defined airways and report when they cross various points along the airway. Many instrument approach procedures are not usable unless the aircraft is in radar contact (indicated by "RADAR REQUIRED" in the approach plate), and of course other services like vectors-to-final and radar traffic advisories are impossible. But it is possible for ATC to ensure safe separation even if the aircraft is not shown on radar.

So to answer your direct question:

So how does ATC - and, for that matter, all the other planes in the air who would rather not run into it - know the position of a stealth aircraft if its transponder fails?

The pilots report their position to ATC, who update the flight progress strip with that information and use that to mentally track the aircraft. For more information see this answer about position report phraseology.

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