Transponder Interrogation

Every now and then, a pilot won't believe me when I tell him I don't have him on radar. He'll look at the little light on his is transponder and say, "Well, Moncton, I'm showing interrogation," as if to say, "look a little harder." I have difficulty in seeing why I might try to lie to a pilot and tell him I don't have him on radar since it's most often to my advantage to see an airplane, but anyway, there are several possible explanations as to why you see evidence of interrogation but I don't see you.

First, there is the radar source issue. I have radars across the region. So do other ATC units like adjacent ACCs/ARTCCs and Terminal Control Units. As most people know, the TCUs in Canada are, for the most part, integrated into the parent ACCs now. For example, when you talk to Halifax Terminal, you're talking to someone working within Moncton ACC. They will see the same radar sources that the parent ACC sees. An example where this is not so is a military TCU. These are often located right on the military airfield, and they will normally have their own radar antenna as well. What all this means is that you could be seeing interrogation from a radar antenna that is not fed into the unit that you're talking to, whether it be an ACC and you're receiving interrogation from a military TCU's radar, or the other way around.

Also, TCAS works by broadcasting an interrogation signal and listening for responses, then processing the returns as a direction finding antenna would and calculating range. This means that there is an airborne source for interrogation which may be showing up on your transponder that certainly no ground source will see.

So, showing interrogation doesn't necessarily mean that the ATC unit you're speaking with can see you. Maybe another one can, but that's not necessarily the case, either.