Originally Posted by
jonkster
I rang and the controller thanked me for calling saying he just wanted to apologise and explain - he said had glanced at his screen just before clearing us, (this D tower gets a feed from the radar centre), and there was another aircraft slightly lower than us, on final about 100m in front and although he could only eyeball our aircraft from the cab he thought had stuffed up his sequencing and immediately acted to separate us from the other aircraft. As we went round, that aircraft then disappeared from the screen. Turned out it was a ghost.
I asked him if this was something that had ever happened before and he said every now and again they do get spurious aircraft that suddenly appear on their display. Normally he said it is obvious it is not legit but in this case with the busy circuit he thought he had made a mistake and stuffed up the sequence so acted to try and avoid a collision.
I have no problem with technology but if it can falsely add aircraft, I suspect it can also falsely remove them. If we rely on the new gizmos do be our eyeballs and common sense we will stop being pilots and just be video game operators and trust the machine rather than focusing outside on the real world.
Rather than being an additional spurious aircraft they were likely to both have been you - one your SSR return and the other your ADS-B. For a coupled track (as in a surveillance return linked to a flight data record) we usually see just one track - the computer combines both returns into one but if one drops out briefly it might be an update or two before the tracks are recombined, so we'll see two tracks in close proximity.
For uncoupled tracks (your typical VFR squawking 1200) we see both SSR and ADS-B, usually very close together but not always, particularly during sharp turns as ADS-B is your GPS derived position while SSR use track smoothing to estimate where you are - great for when you're moving more or less in a straight line but not so hot in a tight turn.