Mode S. Selective interrogation mode

[alexious85]

Can someone explain the mode S selective interrogation mode principle in simple words as initial search in internet tends to be quite technical?
thank you

[alexious85]

I always took it as giving the ability for the controller to decide when and how many information will be displayed from a given transponder , but reading more it now gives me a different impression

[ahramin]

Mode C secondary radars interrogate in a sweep. The Mode C transponder replies when the radar is pointing at it and doesn't reply when the antenna is pointing elsewhere.

Since Mode S transponders have a unique address, it allows the interrogation of a specific transponder.

[Equivocal]

A 'classic' SSR (mode A/C) sends an interrogation to all aircraft within radar cover, and all aircraft reply. This can overload the rx'er and radar data processor. Mode S Selective addressing enables the radar system to identify aircraft that it knows about and to individually request a reply from specific aircraft (rather than triggering a response each time the radar head turns) and reduces the processing load. It also enables the update of the aircraft position symbol to be more frequent and accurate if required, in a high density traffic area for example, and reduces workload where this is not necessary. I've never seen a radar system where the controller has any involvement with selective addressing, it's a technical configuration issue.On the other hand, where Mode S Enhanced Surveillance is implemented, the controller may have access to additional information about the aircraft, for example selected altitude in the FMS and ground speed (I can't recall the other parameters that might be available), but exactly what is available again relies on the local configuration.

I hope that helps with the assignment :).

[kontrolor]

to add to that - each mode s radar has its unique address,

[ShyTorque]

Some time ago I was the sole pilot of a Mode S equipped helicopter. The transponder developed a fault and was replaced by the maintenance organisation. A few weeks after that I was on the ground and happened to check out one of the “Apps” showing aircraft in flight and to my surprise it showed that our helicopter was airborne, or at least its registration was. Our transponder had been repaired and was now flying in an aircraft owned by the maintenance company but no-one had thought to change the aircraft registration attached to that particular box. I prompted them to sort that out asap because I would have been implicated if anything untoward had happened.

[Glamdring]

Traditional Mode-C Interrogation: Hey! Anybody there?

Selective Mode-S Interrogation: Hey Frank!

:)

[kontrolor]

not only that, also - Frank, you are lying about your speed again

[ShyTorque]

Traditional Mode-C Interrogation: Hey! Anybody there?

Selective Mode-S Interrogation: Hey Frank!

:)

Surely to be more correct it’s: Mode C - “Knock, knock……who’s there?”

Mode S: “Hey Frank!”

[Dont Hang Up]

Mode S does actually have a "whose there" interrogation known as the "All Call". Sent out continuously through the whole antenna rotation.

However when it finds a new target (aircraft) it then starts sending out selective (i.e addressed).interrogations (also known as Roll Call) to that target and only in the direction it knows the target to be. Furthermore it tells the target "please stop replying to my 'whose there' interrogations" using a protocol known as "lockout". It is this lockout protocol that allows the Mode S radar to be much less polluting of the SSR frequency band than the classical Mode A/C, even though it is asking for much more information.

[zonoma]

I was told during the early stages of Mode S introduction that the amount of data that could be downlinked was in the 10's of thousands, but obviously most of it would be useless to ATC.

[DaveReidUK]

A Mode S packet can be either 56 or 112 bits, so that's a good guide to the amount of data that's involved.