Transponder Mode A and TA

[ron83]

Hello guys,

Here is situatuion, VFR traffic circling in uncontrolled airspace squawking mode A only, another traffic descending with shallow rate, separation more than 1000' reports TA...question is would any transponder squawking A only would generate TA for other traffic?

[Dont Hang Up]

TCAS will assume co-height in the case of no Mode C. And thereby may well give TA based on rate of closing range.

[Graybeard]

TCAS measures slant range to a mode A target, just like DME. Assuming they are not at same altitude, the rate of closure will decrease, and the target may disappear, or actually move around at the slant range distance when crossing over.

GB

[ron83]

so if no mode C it assumes it's at the same height and because of rate of closure issues TA is that correct? Can it issue RA?

[Denti]

Correct, and no, it cannot issue an RA, for that it needs Mode C.

[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).