I live near an airbase and use an ADS-B receiver that picks up aircraft using regular ICAO assignments. They do not include flight path details or position info, but can be identified by the ICAO number which is a civilian aircraft identifier. Sometimes, it captures aircraft SSR returns with what appear to be invalid numbers, beginning with FF followed by 4 digits, such as FF0024 FF7777.

Are these drones? cruise missiles? Aurora? Or some other military aircraft identity?

  • 1
    $\begingroup$ What receiver do you use? What software do you use? $\endgroup$ – DeltaLima Dec 22 '14 at 14:51

I am going to assume you use Flightaware's variety of dump1090 as a decoder. When that receives a Mode A code, it publishes it as if it were a 24 bit address. The first two hexadecimal digits are filled with 'FF'. I have no idea why this may be a good plan.

In mode_ac.c, lines 368-370:

// Fudge an ICAO address based on Mode A (remove the Ident bit) // Use an upper address byte of FF, since this is ICAO unallocated mm->addr = 0x00FF0000 | (ModeA & 0x0000FF7F);

These may be drones but most likely they are just simple aircraft replying to Mode A/C interrogations.

If you are in the USA, FF7777 is a field test transponder (Range & Azimuth Beacon Monitor, RABM) , usually located not far from a radar to verify the correct working of the radar.


If these are 24-bit ICAO aircraft addresses, then they belong to an unallocated part of the address space. Several places on the net (1, 2) document that addresses starting with F are reserved for future use by ICAO.

The referenced documents, however, pre-date the widespread development and deployment of UAVs, and other sources on the net (e.g. here, here) indicate that "high-order" 24-bit addresses (which apparently means ones starting with F) are to be used for UAVs equipped with "ACAS Xu", a collision avoidance system (currently under development? testing?) which uses the same ADS-B compatible radio protocol as ordinary ACAS/TCAS systems, but cannot send interrogations.

The point of using addresses starting with F (rather than allocating addresses out of the nationally assigned range for the drone's country of origin) seems to be that the ACAS protocol uses "which aircraft has the higher address" as one of the inputs in coordinating which resolution advisory to issue. If a drone and an ordinary ACAS II equipped aircraft are converging, there's a risk that both ACAS systems would issue "climb" advisories (or both would issue "descend") and be technically unable to change it unless the drone has a higher address. At least, that's what I get from the sources linked above.

So your FFxxxx traffic, if you're sure they are 24-bit addresses, are probably drones exchanging ACAS reports with nearby aircraft. (DeltaLima's answer looks more convincing.)

  • $\begingroup$ ACAS X is the new generation of TCAS skybrary.aero/index.php/ACAS_X. ACAS Xu is the UAV variation. I assume it has no interrogation capabilities to prevent foes from detecting its position and shooting it down. $\endgroup$ – rbp Dec 22 '14 at 15:30
  • $\begingroup$ @rbp not every UAV is military. Originally Xu was thought to be entirely passive, now interrogation capabilities have been added to the scope. $\endgroup$ – DeltaLima Dec 22 '14 at 15:40
  • $\begingroup$ @DeltaLima i will further assume that interrogation got added to the scope as more civilian UAVs took to the skies. i'm sure the ones circling war zones will have interrogation turned off. $\endgroup$ – rbp Dec 22 '14 at 17:29
  • $\begingroup$ @rbp That's correct; if capable of interrogating, military UAV will have interrogation off in war zones, but active while operating in civil airspace. $\endgroup$ – DeltaLima Dec 22 '14 at 17:56

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