Mode S & latest GASIL
 

In the first CAA consultation on Mode S when they were in their "compulsory Mode S for all flying objects" phase, one of the objections raised was that a large number of aircraft transponding in close proximity such as a gaggle of gliders in the same thermal would blot out or otherwise interfere with radar returns etc.

That was, if I remember correctly, pooh poohed in the CAA's response.


However in the latest GASIL (3/2008) - a CAA document - we have a section labelled "Formation Flying" which states

"Two or more transponders transmitting in close proximity will garble (interfere) with other transmissions and present an air traffic controller (or an aircraft fitted with a collision avoidance system) with confusing or sometimes false information"

The GASIL article goes on to specify that only one aircraft in a formation should transpond and that the other aircraft should switch theirs off until they separate.

Is this the CAA getting their wires crossed or is this a Mode A/C problem only?

Formation flying has always been a limitation on transponder technology because both the interrogation and the response are each on fixed frequencies.

Mode S does not help in this case because the initial sweep (which generates a list of targets to interrogate selectively on subsequent sweeps) will see a garbled return.

So, a formation should have just one plane with transponder on.

Thanks.

Any idea what constitutes "close proximity"?

“Any idea what constitutes "close proximity"?”

This is a good question. I have done quite a bit of flying in combine (very loose formation) with other aircraft. Some ATC units have told us to switch all but one transponder off and others have not. A combine can take up quite a bit of airspace, so a TCAS equipped aircraft could assume that it was avoiding one aircraft when there were many in its way.

Rod1

Maybe we could ask the Dutch what their experience is as Mode-S is already mandatory there.

My guess is that a gaggle of 15 - 20 gliders all circling in a thermal, with constant variations in relative height, speed and direction is going to provide the technology with quite a challenge. Wonder how the problem of which glider should transpond for everybody would work ? "Glider XYZ please stop transponding as you're near to somebody else who's trasnponding already"

I don't think that's the case. During the All-Call period, Mode S transponders can be told to reply with a certain probability. On some interrogations, one aircraft will reply while the other is silent. Once the reply is received, the interrogator instructs that transponder not to reply to All-Call interrogations, leaving only the other aircraft replying, sometimes, to the All-Call. Over a number of cycles, all the aircraft will be able to reply, and subsequently will be included in the selective Roll-Call.

Mode S radars should have no problem at all with Mode S transponders in formation.

“Mode S radars should have no problem at all with Mode S transponders in formation.”

But there are only 2 mode s radars in the UK and according to a military LARS controller who gave my flying club a talk a few weeks ago, the military have no plan to upgrade in the next 20 years.

Rod1

Thank you bookworm - very interesting :ok:

Thanks also Bookworm

Will Mode S collision avoidance systems be as clever?

I'm not sure what you mean by "Mode S collision avoidance systems". TCAS II? It certainly can use those principles but I don't know how sophisticated TCAS interrogators are.

Hi Rod1,
NATS is refitting all its radars to Mode "S". Currently I can think of the Debden and the Pease, I am not sure if the Clee is mode S but i am sure someone can enlighten us. But with the two that I mentioned, most of the London FIR is covered by Mode S.
I think NATS also has a feed of mode S in the west country area, but I can't remember from where :)

Hi Bookworm

I am working from deep levels of ignorance here, which is why I'm seeking enlightenment! Please excuse.

Correct me if I'm wrong but:- Collision avodance systems e.g. TCAS rely on a transponder return - no transponder, no collision warning triggering?

So if everyone has to change to Mode S, can the current TCAS systems which presumably work with Modes A & C also work with Mode S or do they all have change to TCAS II or whatever it is and will TCAS II still work with any residual older modes?

Ta

A mode s unit is also mode a and c compatible.

Nats may be upgrading, but the LARS is mostly military, and appears not to be.

Rod1

All mandatory-carriage ACAS is TCAS II, and thus in effect all modern installed TCAS systems are already TCAS II. It uses Mode S or Mode C (not Mode A).

I think a lot of people get confused by certain Mode S transponders (GTX330?) supposedly supporting ADS-B over 1090ES.

But anything like this is many years away here in Europe.

I am sure that Eurocontrol will mandate a triple INS as mandatory carriage for IFR, before then :)

I flew in the USA with the Garmin GTX330 with ADS-B over 1090ES. I can tell you it works amazingly well.

It was coupled to a 430. Very sad we can't have that technology over here.

I doubt it was ADS-B over 1090-ES. My understanding is the ADS-B rollout uses UDA to exchange data air to air and to upload data from the radar mosaic. This is deployed in a relatively small are. However, much of the US has TIS (displayed by a 330 and 430/530 typically) and this would be common on US aircraft. Which is why, with no mandate to go mode-s, over half the US fleet went out and installed Mode-S.

A few radar units may have software which automatically detects when SSR labels overlap and 'moves' one of them to a position where they don't garble, hence the reason why some units don't insist on only one aircraft in a formation squawking.