Radar Contact

Over the years, I've had the occasion to talk to many aircraft flying outside of radar coverage. Nothing unusual in itself. But there is the odd pilot every now and then who will get ornery if he's not seen on radar. I can appreciate the idea of a watchful eye as much as the next pilot, but sometimes it's kind of funny. These ones will ask, "Do you have me on radar yet?" When I say "no", the most common response is, "Well, I'm showing interrogation on my transponder. Are you sure you don't see me?" Yup. I'm pretty sure.

In the olden days, when radar was scarce, you were more likely to be right in saying ATC could see you. In fact, even now, ATC somewhere might, even if it's not the one you're talking to. For example, in the northwestern portion of the Moncton FIR, if you're flying low enough, Moncton ATC can't see you, but if you're far enough northwest, Boston may be able to. They have radar s that we don't get the data from, so your transponder may be showing interrogation but not from a Moncton radar source. Cross-border sharing agreements are in place, but we are not yet getting the radar data. Other centers across Canada and the US are already sharing radar data, but it's not a high priority on this end of the country right now, apparently.

In any case, there is another possibility. You may not be seen by any civilian radar at all. It might be a radar from a military installation that is interrogating your trusty transponder. If necessary, it may be possible for you to contact these individuals for assistance, or even to relay through civilian ATC to the owners of the radar. Don't count on it, though.

Lastly, there's one more source of interrogation that may be the culprit: TCAS. TCAS on other aircraft do send out signals for replies from transponders, and that's how they "see" other aircraft and receive Mode C altitudes for traffic detection.

So it's not only ATC you're talking to watching you. There are other eyes out there, too.