Transponder Mode A and TA

[Dont Hang Up]

A TA can occur against a non-Mode C target due to the closing range as Graybeard says.

However if the true height difference is large (eg your aircraft is at FL200 and the non-Mode C is at FL50) the closing rate of lateral separation has to be pretty high, and slant-range geometry dictates that the TA will anyway tend to cancel as the lateral separation reduces. This is because the vertical separation will then predominate and the closing rate will appear to be dropping (ie the supposedly co-height threat will ultimately appear to be passing safely to the side).