Originally Posted by
Captain Charisma
@ Lord Spandex Masher
TCAS and Transponder are two totally separate pieces of equipment.
TCAS receives the transponder signal of other aircraft and then displays them on its own display or an integrated display. It does not show the height (altitude) of an aircraft but rather the relative altitude difference between the TCAS receiver and the other aircraft.
The transponder operates on a generally ATC allocated code. It is totally independent of the TCAS. Without a transponder you are invisible to the TCAS hence the need for transponder mandatory airspace.
So yes the one can operate without the other.
If you were going to piggy-back you wouldn't need more than the relative altitude difference between yourself and the other aircraft. To identify the other aircraft you could monitor the ATC frequency being used.
As an aside, if a transponder is only operating in mode A there will never be a TCAS RA only a TA as the TCAS receiver has no altitude information to work with.
They are NOT totally separate. It is an integrated unit.
The semantics of height versus altitude in the case of TCAS are irrelevant, but it is height above or below you and it will also display absolute height of a target aircraft.
One cannot operate without the other. Even if you select 'transponder' only, TCAS still operates normally except you will not have a TCAS display.