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This is with reference to a News Article that I read today. I find it scary that radars would not be able to detect an aircraft. Is it really possible? If yes, how can ATCs manage this situation?

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  • $\begingroup$ Note that even non-stealth military aircraft can collide with civilian aircraft. $\endgroup$ Commented Sep 7, 2014 at 12:51
  • $\begingroup$ Reading through avherald.com I've got the impression that Indian news outlets, including Times of India, are rather sloppy checking facts when reporting aviation events. $\endgroup$
    – Jan Hudec
    Commented Sep 8, 2014 at 5:18

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Some military aircraft reflect little enough radar energy to be effectively undetectable. Note that the actual details are likely classified, but even the modern F-22 still has a radar cross-section of about the size of a steel marble. Still, the B-2, an older stealth aircraft, is claimed to disappear from air traffic radar when in stealth mode.

The same article on the B-2 also notes that it has "deployable radio antennas for navigation in civilian skies". When flying during peacetime and outside of an MOA, military aircraft generally have some level of coordination with ATC. If the pilots of a stealth aircraft don't want to be seen, then presumably they would be responsible for maintaining separation from other aircraft, as they cannot be tracked on radar. Of course, there's also plenty of airspace where aircraft don't need to talk to ATC at all (E and G in the US), and the pilot of each aircraft is responsible for maintaining separation.

That said, there are some things in this article that are unclear:

  • The Etihad aircraft received a TCAS alert for an aircraft that didn't show up for ATC. TCAS intentionally uses the same frequencies and signalling as ground-based secondary radar, so if an aircraft receives a TCAS response it should also show up for ATC. The article does note that the aircraft were "450 to 480km from Mumbai's coastline", so it's possible that the mystery aircraft just had a weaker TCAS response that wasn't received by a distant ground-based receiver.
  • The article suggests that fighter aircraft don't fly as high as 36,500 feet. A ground attack aircraft might not be able to fly that high, but most, if not all, air superiority fighters have service ceilings of at least 50,000 feet.
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  • $\begingroup$ Thanks for the explanation! But as you rightly suggested the article does not bring facts well. $\endgroup$
    – xrasch
    Commented Sep 7, 2014 at 5:31
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    $\begingroup$ Stealth doesn't make aircraft undetectable, it only reduces detection distance. If the stealth aircraft leaves the transponder on, it is perfectly visible to ATC. $\endgroup$ Commented Sep 7, 2014 at 5:38
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This is possible, and actually fairly common and expected.

Primary radars, as you most likely know, works by picking up reflected EM waves from an object. But many people do not realize that in engineering (as opposed to science), things are never perfect. Radars always pick up "ghost images", caused by reflections of clouds in the atmosphere, a vessel, terrain etc. I've yet to seen a raw radar image that has absolutely zero noise.

Moreover, in reality a plane is not a infinite small point in space, but a large object which reflects a certain amount of rays. These rays form a "patch" on the scope, and depending on the plane's orientation, this "patch" varies a lot.

If this sounds abstract, think of recording with a microphone. Even professional studio recordings has noise, and it is present in every single take. They are digitally suppressed in post production.

Likewise, a radar has to be tuned to "see" the specific type of reflections it is built for. In the case of civilian ATC radar, it is tuned to see large and slow moving objects, and filters out "noise" data which does not correspond to this profile. A fast moving military jet may be invisible to radar. A fast moving missile is more than likely invisible to controllers.

Partly due to their inaccuracy, controllers seldom use primary radar to track planes. They use "secondary radar", which in simple terms receives data like location, airspeed, heading and altitude broadcasted from the plane's transponder.


As for the article you linked, it states wrongly that fighter jets cannot fly at 36,000 feet, as pointed out by NathanG. Even propeller planes can fly at 25,000 feet nowadays. I also agree that probably the aircraft has a week ADB-S transponder that is not received by ground stations.

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