Philltowns,
This confusion about the issue of whether TCAS II needs mode S to work or not results from the following:
In order for TCAS II to work in an aircraft, that aircraft must be equipped with at least 1 functioning Mode S transponder.
Once an aircraft is equipped, the TCAS can generate various advisories against any intruder that is equipped with an ICAO compliant transponder. In all cases, traffic advisories (TAs) can be generated. When the intruder is altitude reporting (be it Mode S or Mode C) then a Resolution Advisory (RA) is issued when necessary. When the intruder is TCAS equipped, the RAs generated are coordinated, by datalinking via the mode S transponders, to ensure that the avoiding manoeuvres are coordinated (e.g. to make sure one aircraft pitches up, and the other down, rather than both up!).
I.E. *You* have to have mode S for your TCAS to work. The intruder doesn't.
The other area of confusion about this issue is what is meant by a Mode C transponder versus a Mode A transponder.
Mode A is an interrogation sequence that prompts the transponder to reply by encoding its identifying 4096 code.
Mode C is an interrogation sequence that prompts the transponder to reply by encoding its altitude.
Hence, most people (and most textbooks) take the view that when an aircraft is not equipped with altitude encoding it is equipped with a Mode A rather than a Mode C transponder. This is incorrect.
ALL non mode S transponders reply to both Mode A and Mode C interrogations. ATC radars send out both a Mode A and a Mode C interrogation. When the aircraft it not equipped with Altitude reporting, the transponder still replies to the mode C call, but with a pulse frame only, and the encoded section of the reply is omitted.
I.E.
What is popularly referred to as a 'Mode A' transponder is in fact a 'Mode A and C without Altitude Reporting transponder'.
What is popularly referred to as a 'Mode C' transponder is in fact a 'Mode A and C with Altitude Reporting Transponder'.
This incorrect designation even goes as far as many transponder controllers, where the mode selecting knob is labelled something like 'Off', 'STBY', 'A', 'A+C', when it should be 'Off', 'STBY','On', 'On+Alt'.
Nevertheless, I accept that this is a fine distinction that does not really affect a typical line pilot. However, if you want to understand how TCAS works, then it becomes an issue......
Because practically every nuance of how TCAS and Mode S function is driven by the engineering need to avoid any unnecessary transmissions in the secondary radar bands. Whereas pre-TCAS, the only transmissions were from ground radar heads to aircraft, now every aircraft is also keeping track of every other aircraft within the vicinity (something like 40 NM). Therefore, without care, the entire secondary radar bands would be swamped!
Mode S includes a lot of tricks to achieve this, but I don't propose to talk about it in this post but will expand if you wish. As far as other transponders go, appreciate that TCAS doesn't care what 4096 ident code the intruder is wearing. It only cares about the range to the intruder, and the intruders altitude.
To find the the range, a time delay versus speed of light calculation is required. This simply needs the intruders transponder to reply, and this could be achieved by either a mode A or a Mode C interrogation.
To find the altitude, obviously a Mode C interrogation is required.
From this we can see that if TCAS sends a Mode C interrogation, it can achieve both objectives, both range and Altitude. Therefore there is no need for TCAS to send a Mode A interrogation, so to avoid garbling the secondary radar frequencies it doesn't.
I.E. TCAS only sends mode C, and not mode A, interrogations for dealing with non mode S intruders,
So, technically, TCAS will only see Mode C and Mode S transponders.
So, technically, TCAS will not see Mode A transponders, and you will not get any advisories.
However, what people think are Mode A transponders are in fact Mode A and C transponders, just without altitude encoding.
I hope that helps...
CPB
TCAS instructor