Airbus ATC AUTO mode
 

good day
FCOM says:
AUTO: In flight : Selected transponder operates.
AUTO: On ground : Selected transponder only operates in mode S (Selective aircraft
interrogation mode).

could anyone explain what is the difference between "operates" and "only operates in mode S"? what modes are operating else rather than mode S in AUTO (in flight) or ON?
did not find anything either in OM/TM/FM or in the web.

thanks in advance

No TA/RA on ground?

While I have no direct experience with these systems as implemented on large aircraft, my understanding with respect to mode S transponders is that:
-on the ground, mode S selective responses should normally be active, and carry coded data including aircraft ident...mode S does some other neat things, including data exchange for TCAS and ADS-B
-modes A and C should not be active on the ground

I'd read that as saying that "Auto" in flight gives you modes A and C as well as S, whereas on the ground it's S only.

Mode S (known as extended squitter - ES) adds a significant amount of detail to the basic secondary radar transponder response see (http://www.garmin.com/us/intheair/ads-b/squit/ )

Why Airbus would only supply this information on the ground is a little puzzling.

Err, it doesn't! With AUTO selected, the transponder operates in ALL modes when the aircraft is airborne, but on the ground it only operates in Mode S. That's because the Advanced Surface Movement Guidance and Control Systems (A-SMGCS) used by ATC at some airports uses the Mode S 'squitter' to obtain aircraft identification and position. The other modes are disabled on the ground so the transponder doesn't respond to transmissions from the Mode A/C Terminal Area Radars.

so the purpose is to be visible only for Ground ATC, right?

Mode S responses also include a data bit which tells the surveillance system if the aircraft is on the ground or not, which can then be filtered by the surveillance system. Mode A/C does not.

Most importantly Mode S allows the radar to interrogate each aircraft selectively (using the aircraft's unique address) and can therefore provide useful ranging on the ground.

Mode A/C is non selective so everyone in the radar-beam replies - and at ranges commensurate with the size of an airport that means massive garble.

Just to clarify on the comment from Ian W.
Mode S is not Extended Squitter.
Extended Squitter is a sub-function of Mode S which carries ADS-B data. A Mode S transponder will 'squitter' whether or not it is seen by a radar. If it is seen by a radar it will also 'reply' to the radar interrogations.

The "Auto" behaviour will be the same for all aircraft types.

On the ground:

A&C responses are inhibited
No responses to Mode S "All calls" (but, as 'Dont Hang Up' says, the aircraft can be selectively called with Mode S)

Another aircraft's TCAS can selectively call your aircraft. It knows your TCAS/Mode S-equipped aircraft is on the ground because of the Mode S data being sent. The other aircraft computes if a Mode C aircraft is on the ground by other methods.

You can still make it sqwawk mode A and C on the ground by selecting "ON" instead on "Auto".

is this correct?
I was under the impression that a transponder will only reply to an interrogation unless it is ADS-B.

Transponders reply to radar. That is what they are for.

Mode S was invented as a replacement for the somewhat limited Mode A/C. Operating on the same frequencies and with Mode A/C backward compatibility built into its design, Mode S was a means of transferring much more data between the air and ground over these interrogation/reply transactions. It also provided the additional benefit of selective interrogation based a unique address assigned to each aircraft. "Selective" is actually what the "S" in Mode S stands for. This eliminates many of the problems of ambiguity and message garbling of the old Mode A and Mode C.

However, in the quite early stages of Mode S development it was recognised that its use could be significantly extended. Transmissions which contained the unique aircraft address did not necessarily have to be triggered only in response to a radar interrogation. Instead, information could be spontaneously broadcast for the benefit of anyone who wanted to listed. Thus the concept of "squitter" was born.

Initially this was only a "short" or "ID" squitter. Containing little more than the address, this was intended to make the aircraft's existence known to the TCAS of nearby aircraft.

The concept was then extended - literally "Extended Squitter". These longer and more frequent transmissions are used to carry ADS-B (provided the transponder has a suitable on-board navigation source fed into it).

In summary, a Mode S transponder looks like:
- an ADS-B target to an ADS-B receiver;
- a Mode S target to a Mode S radar;
- a Mode A/C target to a Mode A/C radar

It does the whole job. And all at the same time.

Well that's lovely but it doesn't answer the question does it?

Well in that case I have to admit I don't understand the question.

Possibly a stupid question but why have the "ON" position at all if "AUTO" does it all?
Is there ever a need to use mode A or C on the ground?
The airline I fly with now on the Bus use AUTO on the ground if required by ATC and turn it ON lining up and AUTO/OFF after landing.
Seems leaving it AUTO from pushback to chocks-on simplifies things.

One would say so. No info in the FCOM re ON vs AUTO.

A330 standard fit

We turn ours to Auto before pushback and then later switch back to Standby after shutdown.

AUTO is AUTO, no need to select it to On.

In a slightly similar vein, why do we leave tcas off until line up?

We dont need to do that either, TCAS is always on.

It is recommended to select TCAS off while on ground (and not on a runway) because otherwise mode A and C will also be active and lots of returns from many aircraft on ground in close proximity creates problems for ATC secondary radar.