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)).

[ukeng]

Mode A and B differences as I can see it is the pulse spacing (Mode A 8 micro seconds, Mode B 17 micro seconds) that's between the first interrogation pulse (P1) and the second interrogation pulse (P2)
For info Mode C has 21 microsecond pulse spacing and Mode D 25 microseconds.
As mode S is backwards compatable with all the other modes I'd suggest Mode B transponders could slot into the place of a Mode A one (same interrogation and reply frequencies).

Mode D i've got as not used and unassigned (think this is still the case?)

Mode S - yes you still need to enter the Squawk code manually. It's normally either a dial (as per Most mode A's) or a numerical keypad with display.

[Blacksheep]

With apologies to Pronto who is probably just another innocent young fellow.

I have a healthy suspicion of people wanting to know how things such as transponders are operated. Those who need to know have access to the relevant documents and/or training. There has to be a doubt as to why anyone else would need to know whether or not you need to enter a Mode A squawk into a mode S system. Fortunately the reinforced cockpit doors keep people armed with anything less than a rocket propelled grenade outside.

This is a public forum and these days we must all learn to think 'security'.

[ukeng]

Mmm, think you're over reacting mate.
You're correct in that some information needs to be handled carefully but this isn't one of those cases.
If you feel i'm missing something send me a PM and I can always ammend the post.

[Pronto]

UKEng

Thanks for your input. Inevitably, shortly after posting I found some of the answers, including a schematic which gives the spacing between pulses for all modes.

So far as I can ascertain, it seems that Mode D was either experimental or simply not adopted internationally.

With regard to Mode S, accepting that it continuously sends a 24-bit unique address, I assume that (so far as you are aware) the Squawk code is there to show what ATSU is providing a service or what the aircraft is doing (ie general VFR on a 7000 squawk, aeros, para dropping etc) or have I understood the rationale behind the unique address, it's there simply for the interrogator "roll call" function?

So far as Blacksheep is concerned, you need a reality check. A quick internet search will tell you quite a lot more about transponders than I'm asking and, to my suprise, about the military equivalent (IFF).

If you'd like to be horrified some more, go here: http://www.tpub.com/content/combat/14308/index.htm and then read chapter 8.

P

[Blacksheep]

No, I'm afraid I don't need a reality check. In case you hadn't noticed, the world changed a couple of years ago. You can Google the instructions for just about anything but that doesn't make it right.

I said it before and I'll say it again. The operating details for aircraft systems are made available to all those with a legitimate need for them in the course of their training. There has to be a doubt as to why anyone else needs to know. In an environment where a civil airliner can be legally shot out of the sky if it deviates from flight plan and can't identify itself, I can think of plenty of reasons why non-aircrew might want to know the proper use of mode S transponder controls, what codes to enter and the interoperability of those ATC installations with military IFF systems.

We'll soon have people posting questions about the new aircraft surveillance equipment that will shortly be mandated and no doubt there will be people daft enough to provide detailed answers.

[ukeng]

Thanks for the message Blacksheep ;)

[Pronto]

No - for reasons I'm not going to share with you - I have very good reasons for noticing that "things" changed on 11 Sep 01. However, I'm still a little bewildered as to how you think anyone knowing how a transponder is operated is going to compromise anyones security. After all, the entire system is, by definition insecure - ie Unclassified. It must be or else you wouldn't be able to obtain a transponder and you certainly wouldn't be allowed to leave one in an aircraft which can be opened with a piece of bent wire, parked on an unlit and remote aerodrome overnight. (The significance being, that the ungodly could simply remove one from a parked light aircraft and be away. Pick the right aircraft and the theft might not be noticed for weeks. They could simply take it home and stick it on a test rig - presto!)

Your assertion that "The operating details for aircraft systems are made available to all those with a legitimate need for them in the course of their training." is risible. If proof is needed, have a look at this - http://www.caa.co.uk/docs/33/SRG_GAD_GASIL1OF2004.PDF - the item you want is at the bottom of page 6. Interestingly, this chap didn't know what the A prefix meant, nor did I until I started researching it. I don't know how I'd have reacted to the instruction either, and it's now over 20 years and 650 flying hours since the CAA issued me with a PPL. You may also like to have a quick peek in the Private Flying forum - you'll find a number of threads which are somewhat critical of how flying clubs/hour building instructors teach (and what they teach). I can show you a number of other GASILs, all of them telling tales of a lack of knowledge. If you'd like a quick look, try here: http://www.caa.co.uk/publications/publications.asp

Bearing in mind that I can - subject to being able to afford the landing fees - take my Warrior into virtually any UK aerodrome including some handling international flights, don't you think that if I don't know how something operates or think I need further information to operate safely, then it's beholden on me to obtain it?

History indicates that the only time that you have all the information you should have had is when you read the report of the Board of Enquiry. If you're very unlucky, it's when St Peter debriefs you.

May I say that I find Blacksheep's comments about RPGs in aircraft both distasteful and offensive in an aviation forum.

Finally, thanks to all those who have taken the time and trouble to post and broaden my knowledge.