Spooky traffic from ADS-B

[Bill Watson]

Yesterday on a leg from Pittsburgh's KAGC to White Plains KHPN, I had traffic appear on my display that was exactly at my altitude and at one point, exactly at my position. Does anyone else experience that with ADS-B traffic?

I've been using the Navworx transceiver hooked up to GRT's HX display (both experimental) for over 2 years.

Occasionally I will get a traffic shadow where it appears that another plane is at my altitude flying formation. It appears for a minute or two and then disappears. It's obvious that it is not another plane but rather some sort of shadow. No Problem.

What made this notable is that this looked like real traffic that appeared several miles in trail at my 7 o'clock. It slowly overtook me, crossed my path at my altitude and then proceeded to fly past me at 1 o'clock until 10+ miles away. Fortunately I was <500' above a solid undercast and could clearly see any conflicting traffic

You can imagine I was asking the controller about this but they assured me that no other traffic was in the vicinity. They said this happens occassionaly and mumbled something about a false return or something. This was a Wilkes Barre approach controller and the incident happened over Williamsport.

 

[Jim Logajan]

That is indeed an unusual "shadow" problem, assuming it was such. Shadowing appears to be common enough that it is discussed in the AIM, so it is worthwhile that anyone having ADS-B out read section 4-5-7, which can be found on this page:

https://www.faa.gov/air_traffic/publications/atpubs/aim/aim0405.html

2. While TIS-B is a useful aid to visual traffic avoidance, its inherent system limitations must be understood to ensure proper use.
(a) A pilot may receive an intermittent TIS-B target of themselves, typically when maneuvering (e.g., climbing turns) due to the radar not tracking the aircraft as quickly as ADS-B.
(b) The ADS-B-to-radar association process within the ground system may at times have difficulty correlating an ADS-B report with corresponding radar returns from the same aircraft. When this happens the pilot may see duplicate traffic symbols (i.e., “TIS-B shadows”) on the cockpit display.
(c) Updates of TIS-B traffic reports will occur less often than ADS-B traffic updates. TIS-B position updates will occur approximately once every 3-13 seconds depending on the type of radar system in use within the coverage area. In comparison, the update rate for ADS-B is nominally once per second.
(d) The TIS-B system only uplinks data pertaining to transponder-equipped aircraft. Aircraft without a transponder will not be displayed as TIS-B traffic.
(e) There is no indication provided when any aircraft is operating inside or outside the TIS-B service volume, therefore it is difficult to know if one is receiving uplinked TIS-B traffic information.