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On condition that the pilot reports to ATC, an aircraft's TCAS sees a drone in the vicinity.

How can the pilot be sure that it was a drone on TCAS? What kind of symbol is shown on TCAS that can be identified as a drone? Or is it just a primary radar target pick up by TCAS?

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    $\begingroup$ Are you asking about big drones like the predator or small, multi rotor drones? $\endgroup$ – GdD Apr 15 at 8:35
  • $\begingroup$ Do you have any reason to think that a pilot would ever make such a report? $\endgroup$ – David K Apr 15 at 21:26
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No, a drone will normally not be visible on an aircraft TCAS. The TCAS system is based entirely on direct communication between aircraft transponders, so both parties (aircraft and drone) must have a transponder in order for TCAS to work. Normal drones do not have transponders, so they will not be visible on TCAS.

If a pilot reports a drone near the aircraft, what he means is that he (visually) has seen a drone out the window, not that it is indicated on an instrument anywhere. Besides, TCAS does not know different aircraft types, so even if a drone had a transponder and it was presented on a TCAS, the pilot would have no way of knowing that it was a drone (as opposed to any other aircraft).

Or just primary radar target pick up by TCAS.

TCAS is NOT a radar. It is not able to display anything without a transponder. And even ground based primary radars will not necessarily pick up a small drone.

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    $\begingroup$ "he has seen a drone" - believes that he has seen a drone $\endgroup$ – Chris Charles Apr 15 at 12:58
  • $\begingroup$ @ChrisCharles - He has seen something which is believed to be a drone. $\endgroup$ – J... Apr 15 at 14:30
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    $\begingroup$ @J... no - he may not have seen anything at all - it may have been an optical illusion, reflection or a moment of insanity. If you wanted to highlight that maybe the pilot could be reporting without believing there was a drone then that would be fair - the pilot could be reporting it because of some other motive - but your correction is wrong. $\endgroup$ – UKMonkey Apr 15 at 16:35
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    $\begingroup$ @UKMonkey All of those things (illusion, reflection, insanity) are "seeing something". If a pilot reports a drone, we can be quite certain that the pilot saw something. We have variable certainty that the thing is a drone. In any case, I was being deliberately pedantic to point out the ridiculousness of being overly pedantic on this issue. $\endgroup$ – J... Apr 15 at 16:41
  • $\begingroup$ @J... I believe part of UKMonkey's point is that if we're going to that level of pedantry, there's no reason to believe the pilot saw anything at all. Perhaps they are reporting a drone because they're a jerk and want to mess with people... $\endgroup$ – mbrig Apr 15 at 20:07
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TCAS is based on aircraft transponders, so unless the drone has one installed it cannot be seen by TCAS.

At least no commercial drones intended for consumers have transponders installed. On the other hand drones are not allowed in airspaces used by airliners so there should not be a need for one.

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  • $\begingroup$ ... unless they have ATC approval, though I don't know how often that is given. $\endgroup$ – StephenS Apr 15 at 16:23
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It's already been said TCAS is not a radar, but how come it knows the bearing, and how does it work with the different transponder types?

Radar

A basic ground secondary surveillance radar sends an interrogation signal as it sweeps the sky, and the transponders in the direction of sending reply.

TCAS

A TCAS system is similar, by having an on board directional antenna, the plane sends interrogation signals, and airborne transponders reply to it.

The reply delay would indicate the distance, the reply will contain the barometric pressure (which is used to discern the altitude difference), and the directional antenna will display the relative bearing.

The bearing is informative only (for display), and its resolution is not used in the decision making.

enter image description here
White diamonds show other traffic with +/- relative altitude in hundreds of feet and arrows would indicate climb/descent (flickr.com).

Cooperative

The TCAS' decision (conflict resolution advice) can be cooperative as well in Mode S. So if a TCAS of one plane says "climb", this information is sent, so the other TCAS in conflict would order the opposite, for example.

Once an RA has been issued, the vertical sense (direction) of the RA is coordinated with other ACAS II equipped aircraft via a mode S link, so that two aircraft choose complementary manoeuvres. (SKYbrary)

Drones

So if the unmanned aerial vehicle does not have at least a barometric transponder (Mode C) system, TCAS equipped aircraft won't offer conflict resolution.

ACAS II will not detect non-transponder-equipped aircraft and will not issue any resolution advice for traffic without altitude reporting transponder. (SKYbrary)

ADS-B

But what about having only an ADS-B transponder on the UAV? TCAS still needs Mode C/S on the other plane, ADS-B only permits fewer interrogations.

Hybrid surveillance does not make use of ADS–B's aircraft flight information in the TCAS conflict detection algorithms; ADS–B is used only to identify aircraft that can safely be interrogated at a lower rate. (Wikipedia)


Further reading: https://www.skybrary.aero/index.php/Airborne_Collision_Avoidance_System_(ACAS)

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