Transponders

[Pronto]

As we all know, the transponder modes used by light aircraft in the UK are Mode A (aircraft identity), Mode C (altitude reporting) and – in the near future – Mode S (Selective reporting).

Mode B is mentioned by one internet source as being used in some countries instead of Mode A, while the sources all appear to agree that Mode D is not internationally established. Arising from that, I have several questions for the technical types:

· What is the difference between Mode A and Mode B?
· Are 3/A4096 transponders interoperable with Mode B interrogators (and vice versa)?
· What is Mode D?

So far as Mode S is concerned, can someone confirm for me that, despite the transponder automatically sending a unique address indicator to ATC, the pilot still has to enter a Mode A squawk code (possibly merely to identify who is controlling the aircraft or its task)? The photo’s I’ve seen of Mode S transponders still have the Mode A setting dials. Some of the articles on the internet are a little unclear and imply that a squawk still has to be input.

Can anyone assist, please? If anybody can suggest where I can find out more (particularly on the Mode B and Mode D points) I’d be grateful.

P

(PS I’m aware that the civil transponder system is a development of the military IFF system; that the military use Modes 1-4 inclusive and that the only Mode that IFF and transponders have in common is Mode 3 which is similar to Mode A (hence the oft seen 3/A prefix)).

[Flyin'Dutch']

P,

I think you may have more joy in asking this question on the ATC forum as I think the level of expertise regarding this subject is likely to be higher there.

You can either cross post or ask BRL to move.

Interesting subject

FD

[Pronto]

FD

Thanks for your advice. I must admit that I hadn't thought of the ATC forum. I've already cross posted to the Engineering and Tech forum (in the hope that an avionics tech sees it and can supply an answer) but posted here too just to cast the net wider.

Regards

P

[skydriller]

Good Question,

I would also like to know the advantages of Mode S over Mode C, in plain and informative English - after all the Authorities are pretty keen to have us go to Mode S in the near future.

Regards, SD.

[Pronto]

SD

As I understand it (and I'm still researching this) one of the problems with the current Mode A and C transponders is that if two aircraft are close together and both are operating transponders, then it is possible for the replies from each aircraft to overlap and corrupt each other. (All transponders are interrogated and reply on the same frequencies - one interrogation transmit, one interrogation reply).

Mode S gets round this by interrogating all transponders then, on the next scan of the antenna, interrogating specific Mode S addresses so that only that aircraft will answer.

You might like to have a look at http://www.dfs.de/dfs/internet/deutsch/default/download/ortung_im_luftverkehr_engl.pdf . This (despite its title) is in english and is on the DFS (which is, I think, the German ATC authority) website.

P

[Keef]

There are some very long and technical explanations of the workings of Mode A, C, and S on the various techie websites. It's fascinating stuff if you like that sort of thing.

Basically, there is a finite maximum number of Mode A/C transponders that can be handled by any radar head, before it all starts to go silly. Mode S allows the radar unit to select which aircraft to interrogate, so cuts down unnecessary/unwanted replies and hence the garbling and other problems that go with Mode A/C.

The problem is that Mode S, which is now becoming mandatory around Europe, is early-1980s technology and already obsolescent. The later generation is ADS-B which is being introduced in the USA. No doubt we'll all buy and install our mandatory Mode S transponders, then a few years later be compelled to spring for ADS-B units.

[WorkingHard]

Of course we will Keef - because the pathetic brain-dead "regulators" have absolutely no concept of what they are legislating for. It is well rumoured (ATC comment anyone) that mode s will simply be filtered out for almost anything except CAT or near controlled airspace. Is it also a forerunner of Eurocontrol charging for ALL flight; IFR or VFR at any a/c weight?

[MLS-12D]

No aircraft that I've flown recently has a transponder of any description. I consider myself well-equipped when I have a radio. ;)

I would suggest that a policy of "stay away from clouds, keep a good look-out, and if possible wear a parachute" is a better way of addressing collision risk than reliance upon mandatory expensive electronic gadgets ... but that would be so old-fashioned of me. :rolleyes:

[bookworm]

I would suggest that a policy of "stay away from clouds, keep a good look-out, and if possible wear a parachute" is a better way of addressing collision risk than reliance upon mandatory expensive electronic gadgets ... but that would be so old-fashioned of me.

It might also be considered rather selfish, since the other guy and his passengers are unlikely to be wearing a parachute. But you'll be fine...

[bar shaker]

How can using available safety equipment be considered selfish?

[FNG]

Hey, I've bought a second parachute for my passengers to wear; do I have to out and buy everyone else one now? Can people let me know their sizes?

[bookworm]

Cheaper just to buy a transponder, FNG. ;)

[robin]

Maybe - but I'd feel happier in my glider wearing a parachute. I've never been able to fully rely on a transponder. If some idiot rams me from behind in the open FIR, if still conscious, I don't think clutching the transponder would help, as compared with the 'chute.

[IO540]

This debate is a bit of a farce, as usually happens when pilots are asked for their views on transponders.

The reason we are getting mandatory transponders is not to avoid GA-GA mid-airs; they are already extremely rare and anyway very few owners are able/willing to spend the £20k+ on the other half of the TCAS kit.

The reason for transponders is to do with separation between GA and commercial traffic.

I think it would be nice, one day, if the TCAS price came down but it isn't likely to come down much unless the decrepit GA fleet is scrapped and replaced en-masse, which isn't likely :O And then people on PP will still be posting messages saying that the Mk1 eyeball is the proper way to detect traffic.

[MLS-12D]

What's really a farce is the whole transponder concept ... :p

[IO540]

That's probably true from the GA point of view. But I think that if there were no CAS infringements, Mode S would not be being made mandatory.

What strikes me as slightly bizzare is that we all have to spend £3k (current cost of a Garmin or Honeywell Mode S XP), while we are expected by the "safety is no accident" crowd to navigate around UK airspace with a compass and a stopwatch.

[GroundBound]

WorkingHard

just a small clarification :)

Eurocontrol does not charge you for flying (a popular misconception). The route charges part of Eurocontrol is a centralised debt collector for all Member States. Each Member State sets its rules, amounts and conditions - Eurocontrol merely applies them according to an agreed formula, based on weight, and distance flown, countries crossed etc.

The money collected by Eurocontrol is retured to the Member States, less a trivial charge for the cost of running the service.


GB.

[LowNSlow]

IO540 I think it's more likely to be the dead hand of European harmonisation that is dictating Mode S introduction rather than CAS violations. Does the number of CAS incursions alone justify the huge expense and disruption that introducing Mode S is going to cause? Don't think so.

Mandatory Mode C would be sufficient to cover every complaint I've heard about VFR traffic. I talked to a chap at the PFA Rally and for 1,500 quid I can buy a Mode C transponder (1,000) and a new 760 Radio (500). Another 500 squids to fit them (they are both instrument hole mounted) and my little Auster would be seeable on everybodies TCAS / Radar / Crystal Ball etc. I might have to get batteries with more oomph than the pair currently powering my radio though but then again, maybe not. He did say that the units were very low consumption but he would say that wouldn't he!

IO540 reckons that if I have to fit Mode S then it is going to cost me 3k plus installation. The extra 2,000 for the Mode S system will provide no significant improvement over Mode C as my steam age (literally) aeroplane does not have the electronics to transmit heading, height etc. from the aircraft systems unless the CAA approve connection of things like a Pilot III to the Mode S.

[Brooklands]

FNG,
Hey, I've bought a second parachute for my passengers to wear; do I have to out and buy everyone else one now? Can people let me know their sizes? If you do go out and buy us all parachutes, don't forget that one of them will have to be PINK :)

Brooklands

[WorkingHard]

GroundBound - Thank you for pointing that out, I was aware of this and simply tried to "be brief". However the point I was making was that this may well be part of an agenda about which we can only guess at this time. But it will come to pass!
Where are the results of the cost/benefit analysis that is supposed to be required before such changes?
IO540 says:
"That's probably true from the GA point of view. But I think that if there were no CAS infringements, Mode S would not be being made mandatory.

What strikes me as slightly bizzare is that we all have to spend £3k (current cost of a Garmin or Honeywell Mode S XP), while we are expected by the "safety is no accident" crowd to navigate around UK airspace with a compass and a stopwatch."
I am sure most of us would agree with the second paragraph but as for CAS infringements one needs to look at the statistics and it is not just GA that gets it wrong occasionally. Considering the number of flights and hours flown GA actually has a very good record. Military on the other hand are "infringers" and they are not required to carry transponders of any kind. Those that do are not required to have them active. CAT also infringe by level busts etc. and so on and so on.
SOOOOO - put these questions to your MP.
1) Why is GA being singled out for a very expensive piece of kit that is of little benefit to GA?
2) Where is the cost benefit analysis?
3) Why are not all airspace users treated the same?

and anything else relevant that better brains than I have may think of.

Educate me someone please

[IO540]

LowNSlow

I agree that Mode C would be as good for the TCAS purpose.

However an elementary Mode S XP doesn't need the inputs you mention. It is a straight replacement for the existing XP, if you have one.

Only an enhanced Mode S XP needs them, and they don't apply to GA. Which is just as well since they cost 10x as much and would be completely impractical for 99% of GA piston aircraft.