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In a typical situation, where an unknown aircraft appears on radar, and doesn't respond to any radio message, sooner or later fighter aircraft are scrambled to take a closer look.

However, can it be reliably determined whether the unknown aircraft in question is a military aircraft or a civilian one, before the pilots of the scrambled jets can take a closer look with their own eyes?

By armed I don't mean deciding whether it carries real missiles or just harmless training dummies, but whether it's a military aircraft or a civilian one. Do we know of cases when this was determined before making visual contact (assuming the UFO didn't respond to any messages)

For example, is it a fighter jet or a small private business jet? Is it an airliner or a strategic bomber? From how far can you tell the difference?

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  • $\begingroup$ I'm not sure I have an answer to this question, but there are cases of airliners (with passengers) being shot down over mistaken identity (not all these, but some). $\endgroup$ – Ron Beyer Feb 3 '16 at 16:19
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    $\begingroup$ It depends. You're essentially asking how to obtain a positive ID. Radar can sometimes ID the a/c model, eg by counting fan blades. See also IRST (essentially a telescope) and IFF. $\endgroup$ – Hephaestus Aetnaean Feb 3 '16 at 16:25
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    $\begingroup$ Sorry to edit your title but i think this way it will be draw less attention from vote-to-close people. Feel free to roll back. $\endgroup$ – vasin1987 Feb 3 '16 at 16:40
  • $\begingroup$ Still at this time, the question should be clarified, there are interesting answers that are said to not match the question (so difficult to +1), but to me they should, unless the question is rewritten. $\endgroup$ – mins Feb 4 '16 at 10:11
  • $\begingroup$ I'm not qualified to post an answer about this, but there are electronic countermeasure capabilities that military forces utilize that can identify various types of "things" based on electronic signatures. This could be identifying a radar's frequency, pulse repetition rate, etc., for example. It's possible to identify a specific contact (not type of contact, but the specific contact if the signature is known). I suspect this can be performed on signals that are "leaking" from the contact as well (i.e., radar turned off, but still some emissions present). $\endgroup$ – Steve Feb 4 '16 at 14:02
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Only if the airplane wants you to know it is armed.

It has several ways at its discretion:

  • If it has a powerful radar, it can switch it on at altitude and announce its presence to other aircraft up to 150 km away.
  • It can send IFF squawks which are reserved for military aircraft. Note that IFF is used by both military and civilian aircraft.

Both methods are only indirectly indicating whether an aircraft is armed. The armament itself will not show up at BVR distances. In the end, if you want to be sure, you have to move some sensors close to the aircraft if the weapons are carried externally, or even wait until they are deployed when they are carried internally. Especially many small aircraft are used both in a civilian and a military role.

A BVR radar is a dead giveaway for a military aircraft - such equipment is not fitted with civilian aircraft, and removed or disabled when an ex-military aircraft is converted to civilian use. Similarly, supersonic speed, which can reliably be detected with Doppler radars, is a telltale sign of military aircraft approaching.

Transport aircraft are much harder to distinguish from bombers on the basis of radar signals alone, and in the end you cannot exclude that the hostile aircraft tries to spoof your equipment into classifying it as a civilian aircraft.

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  • $\begingroup$ The question was not meant to be as specific as to identify the ammunition inside the gun of a fighter, but more about identifying the type of the aircraft. Is it a fighter jet or a small private business jet? Is it an airliner or a strategic bomber? From how far can you tell the difference? $\endgroup$ – vsz Feb 3 '16 at 18:36
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    $\begingroup$ @vsz: Depends on the weather and flight altitude. With clear skies and good binoculars maybe from 10 km (6 miles). With overcast skies only when the bombs are falling. $\endgroup$ – Peter Kämpf Feb 3 '16 at 22:32
  • $\begingroup$ I wouldn't be so sure that armament, at least externally mounted armament, is undetectable outside BVR. It can have a significant affect on the radar cross-section, and while it's just a guess, I wouldn't be surprised to hear that we can even tell a bomb from a missile with certain systems (when mounted on the wing). $\endgroup$ – BryKKan Apr 4 at 23:14
  • $\begingroup$ @BryKKan: Yes, you are right about the radar echo of external weapons. If you have a database of radar returns of hostile aircraft with all possible variation in external weapons and at all aspect angles, then yes, you can detect external weapons BVR. I think this is rather sophisticated and needs a lot of preparation. $\endgroup$ – Peter Kämpf Apr 5 at 7:02
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No, this is not possible.

Imagine, FedEx flying a plane into Russia with FedEx parcels on it. A week later, the exact same plane, now hired by the US Army enters Russian airspace.

Last week, it was a civilian plane, this week, it is a military plane. The only difference is who pays the bill. How is radar going to know who pays the bill?

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    $\begingroup$ My question was more about identifying the type of the aircraft. Is it a fighter jet or a small private business jet? Is it an airliner or a strategic bomber? $\endgroup$ – vsz Feb 3 '16 at 17:17
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    $\begingroup$ I think you are misunderstanding the question. $\endgroup$ – curious_cat Feb 3 '16 at 20:01
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    $\begingroup$ @curious_cat: Actually, the more I read the OP's comments, it seems to me that he doesn't know what the question is. I should probably have voted to close as "unclear" instead of answering. $\endgroup$ – Jörg W Mittag Feb 3 '16 at 20:44
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The only system designed to do this (other than the radars that Jörg mentioned that try to identify radar signatures) is IFF, which stands for Identification Friend or Foe.

The system was developed during WWII and has gone through a number of refinements. The identification is determined from the mode in which the transponder replies to interrogation. Modes 1, 2, 4 and 5 are designated for military use only. 4 and 5 are used by NATO forces. Modes A, C and S can be used for military or civilian.

The system has a lot of holes in it, though. It depends on cooperation from the aircraft to squawk the correct mode to identify itself as military. If they squawk mode A, C and S or don't squawk at all then you don't know anything for certain.

The system was part of the problem in the shootdown of Iran Air flight 655. A mistake made by the radar operator caused him to misidentify the airliner as an Iranian fighter plane.

Sometimes even visual contact doesn't help. Korean Air flight 007, a 747 was visually mistaken for a military RC-135 by a Soviet pilot and shot down.

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There are methods beyond the IFF itself, although what's available publicly isn't all that comprehensive.

This book on Amazon is a start; this Wikipedia article goes into more depth:

Radar offers the potential of non-cooperative target recognition (NCTR). These techniques, which could work if IFF systems fail, have been especially secret. No one has yet proposed, however, NCTR that will be effective if a coalition partner is flying the same aircraft type as the enemy, as in Desert Storm. IFF, presumably with encryption, probably is the only answer to that problem.

One open-literature study combined several pieces of radar information: cross-section, range, and Doppler measurements.[22] A 1997 Defense Department report mentions "Air Force and Navy combat identification efforts focus on noncooperative target recognition technologies, including inverse synthetic aperture radar imaging, jet engine modulation (JEM), and unintentional modulation on pulse-based specific emitters".[23]

NCTR on JEM specifically depends on the periodic rotation of the blades of a turbine, with variations caused by the geometry of the elements of the engine (e.g., multiple rotors, the cowling, exhaust, and stators). More generally, the idea of "micro-Doppler" mechanisms, from any mechanical movements in the target structure ("micro-motion dynamics"), extends the problem to cover more than rotating aircraft structures, but also automatic gait recognition of human beings.[24] The micro-Doppler idea is more general than those used in JEM alone to consider objects that have vibrational or other kinds of mechanical movement. The basisc of JEM is described in .[25][26] One non-rotational effect would be the surface vibrations of a ground vehicle, caused by the engine, which would be different for gas turbines of tanks and diesel engines of trucks. ISAR is especially useful for NCTR, since it can provide a two-dimensional map of the micromovements.

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All of the answers here do address aspects of your question, albeit from different perspectives. I think you may be missing that the finer points of the answer are probably classified and not publicly available. You're also confusing the question of whether an aircraft is armed and whether its operators intend to use it to attack you.

As Jörg and TomMcW pointed out, the specific airframe or even general type - even assuming you can reliably identify it BVR - is not necessarily an indication of its belligerent or benign intentions. Even balloons have been used in "modern" warfare. Nothing in particular (other than cost and fear of having its legitimate civilian aircraft be targeted) would prevent a hostile force from packing say, a Gulfstream G650, with explosives and using it for kamikaze style attack. No military aircraft were used in the September 11 attacks, but that didn't prevent them from causing a great deal of destruction.

The closest answer to your technical question is probably Ralph J's, but as mentioned, the details are mostly classified (for obvious reasons). That being said, identifying key characteristics of the target aircraft is definitely possible BVR (with unknown, but likely fairly high reliability). Whether those characteristics are enough to clearly differentiate a combat aircraft from a civilian aircraft really depends on how your enemy defines "combat aircraft".

As to "how far out", that's likely not going to have a simple (non-probabilistic) answer. Target identification is generally based upon a fusion of data from multiple sensors with varying operating characteristics. The specific answer almost certainly depends upon a number of variables such as terrain, weather, the size and configuration of the aircraft, its flight profile, and aspect to target.

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