Talk:Aviation transponder interrogation modes

Mode C description

To contributor Ahunt: kindly refrain from absolutely wanting to have the last word. The change by the ip-contributor was totally correct and justified, there was no need for you to revert it. Jan olieslagers (talk) 15:47, 18 September 2012 (UTC)

It is not a question of having the last word - it is a question that the text should be verifiable against reliable sources. As it currently stands the text says "Mode C – provides 4-digit octal code and aircraft's pressure altitude. (military and civilian)" The cited ref, Peppler, which I have here, makes it clear that the information in the statement is incorrect. Additional references, such as Transport Canada's AIM and the FAA AIM concur with Peppler. Both refs show that Mode C provides only pressure altitude encoding and nothing more. It does not provide a "4-digit octal code". This is provided by Mode 3A, with which Mode C is usually paired, so that together Mode 3A and C provide "4-digit octal code and aircraft's pressure altitude". But as it is written the current statement is wrong and not in accordance with the refs, so unless you can provide another ref that casts doubt on the text from these three refs, this text needs to be fixed. - Ahunt (talk) 17:51, 18 September 2012 (UTC)

Well, again, I am sorry to disagree. As already pointed out, I know for a fact that a mode C transponder does give both the 12-bit squawk code and the altitude. And no, there is no such thing as an "altitude only" mode C, nor is there a mode "A+C", not on my transponder nor on any other that I have ever seen. I challenge you to show a weblink to a definition of mode C that excludes the 12-bit squawk code. As already said I do not have the time to search for web references, so if you really want to have it your way I shall have to leave you at it. Nor do I have access to the cited reference book. But could you cite where any reference positively states mode C does not include the squawk code?

At the very least I again request you should leave the article as it was left by the IP contributor, until we get this sorted out, either between the two of us or with additional input from other editors, which I would greatly welcome.

And, sorry to sound harsh, but "for altitude information" was as little supported by reliable references as "and altitude information", rather it DOES suggest that both the altitude and the squawk are given - and yet you wanted to revert that change - which started all our present argument. Jan olieslagers (talk) 18:02, 18 September 2012 (UTC)

I couldn't resist searching a bit, and voila: "Mode C takes it one step further. If the aircraft is equipped with an encoder and altimeter, ATC will actually see the flight level altitude. So, if you have a Mode A/C, transponder, which most of you do, ATC will be able to see your coded number and altitude information." (from http://www.swaviator.com/html/issueAS99/modes.html You might well be quick to question the authority of one website among a zillion, but this is just the first I came across. Again, I am confident more and more solid may be found.) Jan olieslagers (talk) 18:19, 18 September 2012 (UTC)

Your ref there supports exactly what the other refs I cited say, that mode 3A provides the code and Mode C provides the altitude, so that Mode A and C together provide both. To quote from the article "When Mode A is used, it transmits a group of coded pulses. These codes consist of four digit identification numbers that have been assigned by ATC. ATC will announce that code to you and you will enter it in to your transponder. Once you enter it in, it is sent back to ATC as a Mode A reply. ATC can then identify each aircraft that has an operational transponder by its distinct, coded number. Mode C takes it one step further. If the aircraft is equipped with an encoder and altimeter, ATC will actually see the flight level altitude. So, if you have a Mode A/C, transponder, which most of you do, ATC will be able to see your coded number and altitude information."

The key is in the last line "So, if you have a Mode A/C, transponder, which most of you do, ATC will be able to see your coded number and altitude information", which shows that Mode A provides the code and Mode C the altitude.

All the references I have cited above, plus your ref too state that Mode C is pressure altitude reporting only. So far we have none that say that it provides anything else. You are not going to find a ref that says what it doesn't do. You can disagree all you want, but we don't go by your opinion or mine for that matter, we go with verifiable sources. So here is another reliable source, the Garmin GTX-327 Transponder installation manual which says on page 13:

• "ON  Turns the transponder ON in Mode A, the identification mode or selects Mode A. At power on the last active identification code will be selected. In addition to the aircraft’s identification code, the transponder will also reply to altitude interrogations (mode C) with signals that do not contain altitude information. The Reply Symbol "®" will be displayed when the transponder replies to ground interrogation.
• ALT  Turns the transponder on in Mode A and Mode C, or, if already on, selects Mode A and Mode C. This is the identification and altitude-reporting modes to respond to ATC aircraft identification interrogations and altitude interrogations with standard pressure altitude (29.92 inches Hg.) received from an external altitude digital encoder. The ALT position may be used in aircraft that are not equipped with the optional altitude encoder, however, the only response will be discreet signals that do not contain altitude information. The Reply Symbol "®" will be displayed when the transponder replies to ground interrogation."

It also further clarifies on page 2:

"Mode A Capability 4096 Identification Codes, Mode C Capability 100 Foot Increments from -1000 to 63,000 feet"

What references do you have that show that all these four refs I have cited are wrong? You will also now need to show that the ref you cited is also wrong as they all agree. - Ahunt (talk) 18:39, 18 September 2012 (UTC)

Well, I think I may begin to understand where it all goes vague. There do seem to be mode A replies to interrogation, giving the 12-bit squawk, plus the ident bit, and mode C replies giving the Gray-encoded altitude. Thus far, taking things very literally, you might be on the right side. But practically, NO transponder will ever send out a mode C reply while not also sending out a mode A reply. That is what got me so hooked. I must avow I was rather considering the operational mode of the transponder, than the stream of data that it squatters in the aether. The full story might be something like "In civil aviation, a minimal transponder will respond with class A replies only. More elaborate types will also, if so interrogated, transmit mode C (altitude) information, or mode S data". Could we reflect this in the article? Jan olieslagers (talk) 18:50, 18 September 2012 (UTC)

I think you are getting closer there. What tends to add to the confusion is that a lot of people in the avionics business tend to refer to Mode 3A and 3C together as "Mode C". Technically this is incorrect, but it is used as a shorthand, even though subtracts from clarity, rather than adds to it.

Sure that would be useful to add to the article. You are quite right, while it would be possible for a xpdr to reply only in Mode 3C and not Mode 3A, in practice it never happens and would be very non-helpful to ATC as they would have an altitude for an aircraft, but no location or squawk. As the manual for the GTX-327 explains for that model, usually with all xpdrs you have a choice of selecting Mode 3A on or Mode 3A+C on, never just Mode 3C. - Ahunt (talk) 19:15, 18 September 2012 (UTC)

I added this into the article text, have a look and see if that makes sense now. - Ahunt (talk) 11:59, 19 September 2012 (UTC)

Well done sir, and thanks for bearing with me. Jan olieslagers (talk) 22:33, 19 September 2012 (UTC)

Not going to take this apart today, but you're both mistaken. Non-Mode-S ATCRBS transponders will respond with 12 bits corresponding to the squawk code when interrogated with a Mode A pulse train, and with 12 bits corresponding to pressure altitude when interrogated with a Mode C pulse train. See the simplified explanation — Preceding unsigned comment added by Altaphon (talk • contribs) 04:48, 13 August 2017 (UTC)

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- Ahunt (talk) 14:42, 22 October 2016 (UTC)