Transponder signals - mode differences

[radian]

Hello there!
Could anybody explain to me what's the difference between signals from:
- A mode only capable transponder
and:
- A/C mode capable transponder set to A mode only ?

TCAS is able to track A/C transponders without altitude information but it's unable to track mode A signals. Why? What's the difference between the signals?

[Capt Pit Bull]

Difference between

- Mode A only transponder

and

- Mode A and C transponder selected to Mode A only.

No difference... IF thats really whats going on.

But it almost certainly isn't whats going on.

More likely, you have a mode A and C transponder with 'altitude reporting' turned off.

Note that in this case the transponder still replies to mode C interrogations (Tcas does interrogate Mode C all call, but not Mode A all call becasue it doesn't give a stuff what 4096 code you are wearing). However it sends an empty pulse frame, (no altitude date).

But, since it is still replying, TCAS can calculate time delay versus speed of light and calculate the range, and hence range rate, and issue a TA.

pb