View Full Version : Transponders

20th Jul 2004, 13:10
This is a cross post from some of the other forums. It's been put here too to increase visibility and hopefully, generate a useful reply! Any assistance will be gratefully received.

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 photos Ive 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) Id be grateful.


(PS Im 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)).

22nd Jul 2004, 05:23
Yes they do like to know who's flying the aeroplane. For now the cockpit doors are all locked and they're going to stay that way. Once you qualify for a key they'll teach you all you need to know. :suspect:

Alex Whittingham
22nd Jul 2004, 13:21
Unfortunately I can't reference this but I can tell you what I was once told....

Mode B was a 'spare' set of 4096 codes available to civvy ATC, presumably when they ran out of Mode A codes. Very few aircraft had Mode B transponders, I saw them only on some of the RAF L1011-500s, I think the ex Pan Am aircraft. I have never met anyone who has ever been asked to squawk Mode B.

Mode D is listed in 1960's textbooks as 'experimental'

There is a brief description of the Modes here (http://www.dtic.mil/jcs/j6/cceb/acps/Acp160b.pdf) including the military modes.

22nd Jul 2004, 13:33

That's useful, thank you. Almost inevitably, after I'd posted, I found some of the answers to the questions I posed. Mode B is an aircraft identification mode (like Mode A). The difference is in the length of the gap between interrogation P1 and P3. (You'll recall that the SSR interrogator uses three pulses known as P1, P2 and P3. P1 and P3 are sent from the rotating array, while P2 is produced from an omnidirectional antenna. P2 is there to prevent transponders from replying to side lobes from the rotating antenna). Why they should choose to have two identification modes is beyond me. One source indicates that transponders may have Mode B as an optional extra.

The hunt for my other question - on Mode S - continues!


Alex Whittingham
22nd Jul 2004, 13:55
I've never flown with Mode S but I understand that you still only get asked to set one of the 4096 codes available to Mode A, even though the transponder is capable of sending many more number sequences. I've taught people who have flown with Mode S for several years and not realised it!

4096 numbers are not enough to give all the radar units in Canada and the US sufficient discreet codes for their needs.

Transport Canada Code Employment Plan (http://bathursted.ccnb.nb.ca/vatcan/training/SSR-EN.pdf)

22nd Jul 2004, 14:09
Pronto - I 'Googled' (like you:D ) and got PAGES, including this (http://www.caa.co.uk/docs/7/DAP_SSM_Mode_S_SSR_Factsheet.pdf) - any help?

CJ Driver
22nd Jul 2004, 22:14
The Mode S 24 bit address is quite distinct from the squawk code (and the related flight ID). The 24 bit address is "just a number", and the only official significance is that every aircraft has a unique number. If you're into computers and know anything about local area networks, then the Mode S address is the equivalent of the low level hardware address of your Ethernet card - it is used at the lowest level to make sure the packets are received by the correct destination, but has no significance for higher level protocols. In the radar environment, the 24 bit address is used purely by the interrogating radar to uniquely "paint" a single aircraft. What is then received by the interrogator (and displayed to the controller, etc) is the usual squawk code, altitude and so on. So, from an ATC operational point of view, it's still the squawk they track, not the 24 bit ID.

Thus, a Mode S transponder user inputs a squawk code in much the same way as a Mode A/C transponder user.

In answer to Alex's comment, there are still only 4096 codes for the squawk. The difference is that, because the computer that is running the radar can reliably track each individual target (because it can discriminate using the 24 bit ID) there is no problem with squawk overlaps - you could cope with two targets flying near each other, both with the same squawk, and the computer would never get confused about which was which.

A more recent innovation though is that in addition to the classic squawk code, new Mode S transponders can also transmit Flight ID. On an airliner that would be the flight code, like EZY1234. On a GA type that is usually the registration, like G-ABCD. This makes life a lot simpler for the ground radar system. For backwards compatibility though, the squawk is still always required.

Alex Whittingham
23rd Jul 2004, 08:21
Ta. That helps. Do you know the history of Mode B then, CJ driver?

23rd Jul 2004, 08:30
I can't see how that can be correct CJ Driver. The stated aims of equippage with Elementary Mode S are to reduce frequency congestion and to alleviate Mode A code shortages.

Eurocontrol's stuff on this (there was a guidance leaflet for aircrew but I can't find it now) suggests that a generic Mode A conspicuity code will be requested of aircraft that are Mode S equipped.

CJ Driver
23rd Jul 2004, 14:11
Bookworm, to avoid too much repetition, can I refer you back to this thread: threadid=134758 (http://www.pprune.org/forums/showthread.php?s=&threadid=134758)

The reason that Mode S achieves reduced frequency congestion on the surveillance frequency is because the radar head can selectively interrogate individual aircraft. For example Mode S can reliably interrogate each aircraft in a close formation, or every aircraft in a holding stack, something that Mode A/C struggles to do because of mutual interference. Indirectly, enhanced surveillance also reduces congestion on a Comm channel, because instead of the controller saying "what is your present heading?" he can just read it with Mode S.

As mentioned, all Mode S aircraft still need to squawk a traditional 4096 code. The code is required when you are being painted by an older Mode A/C ground station. It is also one of several pieces of data provided to the computer in a Mode S equipped air traffic control center. Because the computer also has access to Flight ID, aircraft address, and several other parameters, it can more easily distinguish between aircraft, and therefore two flights could carry the same squawk code in the same sector with no chance of confusion. This therefore alleviates Mode A code shortages.

As an aside, there is still a pretty interesting code allocation problem that is probably taxing the brains of a few planners and mathematicians. Although quite a few of the key ATC en-route sectors in Europe will be Mode S capable within the next year, it will be MANY years before the last of the Mode A/C sites is phased out. Until then, the computer that allocates squawk codes will still need to ensure that flights that criss-cross the patches of Mode A-only coverage do so with unique squawk codes. When only the fringes are left, it's possible that you'll need to change squawk code to a locally assigned value as you fall off the edge of Mode S coverage into the Mode A world. But, despite your comment about Eurocontrol guidance, I've not heard anyone brave enough to estimate a date for when the last Mode A ground station is switched off.:ooh:

FE Hoppy
23rd Jul 2004, 15:25
I understand that the time frame by which all new a/c must be fitted with mode s is about september this year and all older aircraft will have to be retrofitted by Sept 05. Also Enhanced mode S is on the way and will be implemented late 05. Enhanced will transmit more data for example not just heading but also pre selected heading and preselected (armed) alt. Then there is ADS-B. of which I know nothing. Anyone care to enlighten?

23rd Jul 2004, 17:12
Thank you to everyone who has taken time to post. I appreciate the time an trouble you've all taken to answer what must have seemed to be some very basic questions. Thank you again.


24th Jul 2004, 08:51
Bookworm, to avoid too much repetition, can I refer you back to this thread:

You can, but it doesn't address the issue, which I failed to identify clearly in my brief response.

You wrote:

What is then received by the interrogator (and displayed to the controller, etc) is the usual squawk code, altitude and so on. So, from an ATC operational point of view, it's still the squawk they track, not the 24 bit ID.

It's not the usual sqauwk. The Mode S reply includes the 24-bit address, and a flight identification code. Those, which are unique, are tracked, not the Mode A code, which is just there for backward compatability.

CJ Driver
24th Jul 2004, 15:21

I think we may be talking at cross-purposes here. From a practical point of view, what the controller tracks is actually a wee icon with a data block painted next to it, that includes things like aircraft type, flight ID, altitude and destination (these items are variable according to controller taste and role, and which brand of radar display they are using, but the basic principle is the same the world over).

In a conventional Mode A environment, all that information is actually associated with the icon by a computer, which achieves that minor miracle by using the Mode A code to look up the answers which it has previously been told by entering from a data strip, or whatever. If several aircraft had been allocated the same Mode A code, the computer could become confused as to which was which.

In a Mode S environment, all the information that the controller looks at is also associated with the icon by a computer, and generally speaking, they look at exactly the same stuff - flight ID, type, altitude, etc. The only difference is that now, the computer has the benefit of additional data from the aircraft, including of course the 24 bit address. Thus, as I pointed out earlier, it is quite possible for two aircraft to carry the same squawk code without any risk of confusion. In theory the computer could also automatically load the flight ID, since that is now a Mode S readable parameter, but in practice my understanding is that not enough aircraft implement flight ID correctly yet to make that worthwhile. Altitude is of course a Mode S readable parameter, but other items in the data block, like destination, are not, so there is still a data strip to make up.

I think the point I was trying to make is that at no point does an air traffic controller look at a 24 bit address (although his radar system is using the address to speak to individual aircraft). So it doesn't pop up on the screen as 0C-14-7D and the controller says "ah yes, that'll be the 07:30 from Manchester". On the other hand, the radar displays I have seen DO show the squawk code, including the usual 12 bit codes from Mode S equipped aircraft.

24th Jul 2004, 18:03
I think we may be talking at cross-purposes here

I guess we are. I've no argument with anything you write there.