Talk:Transponder (aeronautics)

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Opening heading[edit]

On March 15, 2007, Germany will use 7000 for VFR flights, just as the rest of Europe does, instead of 0021 and 0022. See - we need to change the article accordingly 12:20, 19 February 2007 (UTC) Wouter

Changing squawk codes[edit]

Australian procedure is to select STBY first, then select the new code, then reselect ALT, to avoid inadvertently selecting an emergency code, or another code already in use. This is from the AIPs —The preceding unsigned comment was added by Cabdude (talkcontribs) 11:23, 1 May 2007 (UTC).

Canadians are specifically warned not to do this, so as to not lose the protection afforded by TCAS during the code switch. —Preceding unsigned comment added by (talk) 14:34, 8 October 2007 (UTC)
I saw lately in ICAO 4444 Pans OPS a certain phraséology guideline to avoid this kind of confusion, if the controller estimates that the pilot could pass over an emergency squawk, he will use the term: RECYCLE ( put transponder on stby, the tune new code then react transponder) else he should use the term : Squawk XXX ; however with the new technology on comercial planes this is less and less a factor due to the fact that transponders are digital and no more manual inputs. —Preceding unsigned comment added by Luckyglider (talkcontribs) 11:47, 12 May 2008 (UTC)
The no-standby directions in the article need to be moderated or modified. Why do you suppose engineers designed transponders with a "standby" function? That kind of stuff costs money and increases equipment complexity. If it wasn't meant to be used, it would not be included. There are many self-styled "experts" who write nonsense on various topics, since they lack proper technical grounding in the subject; this is most often evident in subjects concerning electronics and electricity, and can even include widely published authors.
Having taking pilot training for several years, I can't recall a single instance of an instructor telling me not to turn the selector knob to the "standby" position, but I do remember being chided several times for failing to do so.
I suggest revising the section to say that "some authorities" recommend using the standby function, while others recommend against it, and give the reasons for each. I don't have access to the cited reference by Peppler, so I'm not prepared to pass judgment yet. —QuicksilverT @ 20:23, 1 June 2011 (UTC)
In this case the self-styled "experts" are the engineers at Transport Canada. The Transport Canada AIM cited in the article says: "Do not select “STANDBY” while changing codes as this will cause the target to be lost on the ATC radar screen. Pilots should adjust transponders to “STANDBY” while taxiing for takeoff, to “ON” (or “NORMAL”) as late as practicable before takeoff, and to “STANDBY” or “OFF” as soon as practicable after landing. In practice, transponders should be turned on only upon entering the active runway for departure and turned off as soon as the aircraft exits the runway after landing." This seems pretty clear and the Peppler ref says pretty much exactly the same thing: "However do not select "standby" while changing codes as this action will cause the target to be lost on the radar screen at ATC". In a recent tour of the Nav Canada Air Traffic Control Dev Center they indicated the same thing - modern computer-based radar will track a target while you change squawks on the fly, but not if you select stand-by. So, according to TC, it seems that the standby mode is there to allow the set to "idle" while on the ground and be ready for use once in the air, probably more of a carry-over from the days when they had tubes that needed to warmed up, than modern transistorized sets. If other jurisdictions have other opinions on this subject and refs can be found to show that they disagree, then this can certainly be added as "disagreement". - Ahunt (talk) 21:04, 1 June 2011 (UTC)

Adding a note for future reference[edit]

The confusion about the issue of whether TCAS II needs mode S to work or not results from the following:

In order for TCAS II to work in an aircraft, that aircraft must be equipped with at least 1 functioning Mode S transponder.

Once an aircraft is equipped, the TCAS can generate various advisories against any intruder that is equipped with an ICAO compliant transponder. In all cases, traffic advisories (TAs) can be generated. When the intruder is altitude reporting (be it Mode S or Mode C) then a Resolution Advisory (RA) is issued when necessary. When the intruder is TCAS equipped, the RAs generated are coordinated, by datalinking via the mode S transponders, to ensure that the avoiding manoeuvres are coordinated (e.g. to make sure one aircraft pitches up, and the other down, rather than both up!).

I.E. *You* have to have mode S for your TCAS to work. The intruder doesn't.

The other area of confusion about this issue is what is meant by a Mode C transponder versus a Mode A transponder.

Mode A is an interrogation sequence that prompts the transponder to reply by encoding its identifying 4096 code.

Mode C is an interrogation sequence that prompts the transponder to reply by encoding its altitude.

Hence, most people (and most textbooks) take the view that when an aircraft is not equipped with altitude encoding it is equipped with a Mode A rather than a Mode C transponder. This is incorrect.

ALL non mode S transponders reply to both Mode A and Mode C interrogations. ATC radars send out both a Mode A and a Mode C interrogation. When the aircraft it not equipped with Altitude reporting, the transponder still replies to the mode C call, but with a pulse frame only, and the encoded section of the reply is omitted.

I.E. What is popularly referred to as a 'Mode A' transponder is in fact a 'Mode A and C without Altitude Reporting transponder'.

What is popularly referred to as a 'Mode C' transponder is in fact a 'Mode A and C with Altitude Reporting Transponder'.

This incorrect designation even goes as far as many transponder controllers, where the mode selecting knob is labelled something like 'Off', 'STBY', 'A', 'A+C', when it should be 'Off', 'STBY','On', 'On+Alt'.

Nevertheless, I accept that this is a fine distinction that does not really affect a typical line pilot. However, if you want to understand how TCAS works, then it becomes an issue......

Because practically every nuance of how TCAS and Mode S function is driven by the engineering need to avoid any unnecessary transmissions in the secondary radar bands. Whereas pre-TCAS, the only transmissions were from ground radar heads to aircraft, now every aircraft is also keeping track of every other aircraft within the vicinity (something like 40 NM). Therefore, without care, the entire secondary radar bands would be swamped!

Mode S includes a lot of tricks to achieve this, but I don't propose to talk about it in this post but will expand if you wish. As far as other transponders go, appreciate that TCAS doesn't care what 4096 ident code the intruder is wearing. It only cares about the range to the intruder, and the intruders altitude.

To find the the range, a time delay versus speed of light calculation is required. This simply needs the intruders transponder to reply, and this could be achieved by either a mode A or a Mode C interrogation.

To find the altitude, obviously a Mode C interrogation is required.

From this we can see that if TCAS sends a Mode C interrogation, it can achieve both objectives, both range and Altitude. Therefore there is no need for TCAS to send a Mode A interrogation, so to avoid garbling the secondary radar frequencies it doesn't.

I.E. TCAS only sends mode C, and not mode A, interrogations for dealing with non mode S intruders,

So, technically, TCAS will only see Mode C and Mode S transponders.

So, technically, TCAS will not see Mode A transponders, and you will not get any advisories.

However, what people think are Mode A transponders are in fact Mode A and C transponders, just without altitude encoding.

CPB TCAS instructor


--Natasha2006 18:21, 14 June 2007 (UTC)

Mode A and Mode C[edit]

THE STRAIGHT SCOOP ON HOW IT WORKS by Darryl Phillips [2] [3]

--Natasha2006 18:29, 14 June 2007 (UTC)

Deletion of mnemonics[edit] I have for a second time reverted the inclusion of mnemonics for people learning how to memorize transponder codes as part of pilot ground school. As is made very clear in Wikipedia Policy Wikipedia is not a how to manual and this information does not belong here.

As for the personal attack that went with the reversion of my edit, this is also not permitted under Wikipedia policy Wikipedia:No_personal_attacks. If you would like to discuss the issue civilly then this talk page is the forum to do that.

- Ahunt 22:14, 1 November 2007 (UTC)

Proposed merger of Transponder (aviation) with Secondary surveillance radar[edit]

This article was tagged for consideration for merger on 11 December 2007 by PEHowland. The tag was incorrectly constructed and led to the wrong talk page, so that may have slowed down comment on this issue!

I will start off the debate by indicating that I am against merging these two articles. I don't see that they are one subject. - Ahunt (talk) 23:21, 1 January 2008 (UTC)

Against merger - the transponder is an avionics system fitted to an aircraft, SSR is a ground based radar system. Just because the SSR interogates the transponder is not motive for one article. MilborneOne (talk) 13:08, 2 January 2008 (UTC)

This matter has been up for discussion for 5 weeks now and not one editor has come out in favour of merging the two articles. I think this puts the matter to rest and so I will remove the proposed merger tags from both articles. - Ahunt (talk) 19:00, 5 January 2008 (UTC)

I think think that this page should have been merged with the Secondary Surveillance Radar. SSR isn't a topic germane to anything other than aviation, and the transponder (beacon) is a necessary component of SSR. SSR doesn't work without it and the transponder is useless except when it's part of an SSR system Therefore, I think that the transponder should be considered part of the SSR system, and therefore included as part of the SSR article. Conortodd (talk) 23:12, 26 April 2012 (UTC)

That debate was closed four years ago. If you want to start over then you will have to re-propose it. Personally I don't see anything has changed in the intervening time. - Ahunt (talk) 00:21, 27 April 2012 (UTC)

Unreferenced text[edit]

I have removed some unsourced information from this article on the apparent history of the term "squawk and "parrot check". Parts of this paragraph have been tagged as unreferenced for five months. The information seem doubtful or apocryphal at best and my attempts to finds refs for it have not been successful. Since no one else has found any refs for it in five months I have removed it. The response from an IP address and (same person) has been to edit war over it, including uncivil remarks and attempts to bully over this issue with such statements as "no one elected you sherrif (sic), move along".

The policies on this sort of issue are very clear:

"If a claim is doubtful but not harmful to the whole article or to Wikipedia, use the {{Fact}} tag, but remember to go back and remove the claim if no source is produced within a reasonable time."

I think five months is reasonable time.

If any one has a reference for this removed para then by all means please re-instate it. If you don't have a ref then it will remain out of the article.

For anyone who doesn't think it is important for Wikipedia to be scrupulously referenced, I suggest that you read this CBC article. A lack of references can cause real damage in some cases. - Ahunt (talk) 18:58, 13 March 2008 (UTC)

Transponder codes[edit]

This article should be about the item of equipment called the Transponder the large section on codes and how they are used should be deleted/moved to Secondary Surveillance Radar or similar. Any comments? MilborneOne (talk) 20:28, 7 May 2008 (UTC)

Selection of standby to change codes[edit]

An IP editor has now twice removed the word "not" from this sentence in the article:

"Pilots are instructed not to place the transponder in "standby mode" while changing the codes as it causes the loss of target information on the ATC radar screen, but instead to carefully change codes to avoid inadvertently selecting an emergency code."

The Transport Canada ref cited says: "Do not select “STANDBY” while changing codes as this will cause the target to be lost on the ATC radar screen."

Does anyone have any other information on this subject? Many years ago it was the practice to select "stby" when changing codes, but with radar modernization in the 1980s this was changed.

Besides that the removal of the one word makes the sentence non-sensible. Ahunt (talk) 20:56, 7 May 2008 (UTC)

The "standby" function mutes the transmitter, but leaves the rest of the transponder's electronics, including the timebase circuits, operating in a stable fashion. This prevents spurious squawks from being transmitted while the code is being changed, especially on older equipment with rotary selector switches. By leaving the circuits active, it also prevents "chirping of the timebase reference. —QuicksilverT @ 20:23, 1 June 2011 (UTC)

Finding references[edit]


just some problems to find some references , i have added two SSr 3/A codes allocation but unfortunately I haven't got the latest official Icao/Nato MODE 3 allocation table however if someone could give me a hand on this this could be nice but for french paradrop I am certain of the squawks used by FR FIC also for Belgians I have no doubt over them .

have a nice day —Preceding unsigned comment added by Luckyglider (talkcontribs) 11:41, 12 May 2008 (UTC)

Deletion of hijack code 7500[edit]

Note to all those people who have been told by their flight instructors that the hijack transponder code of 7500 is a state secret and they are not to divulge it even under torture - that is a myth, so please stop deleting content from this article! There is no reason to remove the hijack or any other transponder code from this article. Many government websites, such as Transport Canada publish this information and it is very clearly public and not classified. See also WP:NOTCENSORED. - Ahunt (talk) 18:49, 27 January 2009 (UTC)


True, the hijack ('Unlawful Interference') transponder code is no state secret. It is well known among the aviation community as one of the ways to communicate distress to ATC. That being said, it is intended to be used in situations that preclude the use of verbal conversation, i.e. when the pilots are under observation by potentially dangerous people. Given that the code is intended to be used discreetly, I believe it is in the best interest of the aviation community to limit where this code is published. True, determined and informed hijackers will most likely be able to recognize the code. However, most members of the public can not, and it is common sense to keep it that way. This (obviously sensitive) information is of no use to most people, but could put possibly put them, the pilots and other passengers in danger should they find themselves in a hijacking situation.

Pcbene (talk) 19:34, 15 May 2009 (UTC)

Transport Canada and the FAA both disagree with you and publish the information on the internet. My own discussions with senior Transport Canada policy staff on this issue, when I was involved in the drafting of the CARs, indicate that it was never intended to be a secret and that a hijacked aircraft squawking or not squawking this code will not affect its ATC handling or other response. Any aircraft deviating from clearance and unresponsive is treated as a hijacking. The use of the code is a procedural convenience for ATC and that is all. If it were intended to be limited in the dissemination of it, it would be only taught to airline captains and not 100,000s of student pilots each year. It would also not be in every ground school text book. I specifically asked TC staff if this shouldn't be kept secret and they indicated that it should be widely disseminated as possible. They freely acknowledged that most hijackers would know about it, but that doesn't affect its use. It also doesn't put crews at risk, in many past hijackings the hijackers have asked the pilot to select 7500. You are going to have to come up with some better reason than your opinion "I believe it is in the best interest of the aviation community to limit where this code is published" to delete it from the article. - Ahunt (talk) 20:31, 15 May 2009 (UTC)

i agree —Preceding unsigned comment added by (talk) 00:54, 19 June 2009 (UTC)

'I fly helicopters. We are often the target of "oportunist" hijackings. EG: Break my friend out of jail, bank robber on the run needs a getaway, that sort of thing.They are often not to bright and have done no homework prior to the act. The hijack code becoming public knowledge is dangerous as the information might well have filtered down to them. If anyone is serious such as a terrorist, they will have done a great deal of homework and would know it anyway. Who does this information serve?...The public?..I am all for freedom of information but I bet they would rather it stay out of the public realm if they knew it could cost them their life. I wil continue to edit the pages with the code on them. I feel it is in my best interest and the publics to do so.'As for not putting the pilot at risk by using it if the hijacker knows it...Well that's just crazy....Not all hijackings occur in 747's..many that don't make the news are the type cited above in small aircraft....They will often tell you to fly below 500feet "to avoid the radar" because they've seen it in the movies...How do you think they would react if they found the pilot advertising the hijacking via the transponder?...Leave the code out..It serves no purpose to make it public knowledge and it might cost me or one of my mates a life one day - — Preceding unsigned comment added by (talk)

I flew helicopters too for several decades and what you have written is ill-informed. Read the thread above. - Ahunt (talk) 02:48, 1 April 2010 (UTC)

Mate, I dissagree, if I may ask...What purpose, as you see it, does publishing the code serve?? Why would you..A pilot, want to publish the code so badly?? I don't know any pilots who think this should be made public knowledge. In fact I discussed it with fellow pilots after seeing it on Wiki. The unanomous opinion was that it was reckless to do so...(the language was a little more colourful). My point is that it serves no purpose. The general public simply don't need to know it. The "aviosexuals" and REAL bad guys will already know it. The possible ramifications for making it "common" knowledge, however remote the possibility, are simply not worth the risk. If you go to the government website looking for codes you will find them...true..But you have to know they exist to want to find them. The oportunist hijacker won't have done that. Obviously others have been editing the article and I will be doing my part. I notice you "flew" choppers..Well I still fly them...The socio-economic demographics of many of the areas I operate make hijack a real threat and until you or anyone else can give me a good reason to publish the code I will continue to edit. Cheers —Preceding unsigned comment added by (talk) 23:05, 1 April 2010 (UTC)

Read the section above where I discuss the Transport Canada opinion on this subject. TC insists that disseminating the information as widely as possible puts no one at risk. Despite your insistence that Wikipedia be censored, Wikipedia is not censored. Your continued attempts to censor Wikipedia against consensus here and official government policy that this information be widely distributed is misguided personal opinion. - Ahunt (talk) 23:13, 1 April 2010 (UTC)

Actually mate if you read the posts above..the consensus is in fact to leave it out....It is in fact YOUR misguided opinion that seems to be the odd one out......Removing the codes is not censoring it's comon sense..The public is not served by it being in there..Look up something rude on Wiki and see what comes up...Not censored?? No..but the public is not served by adding the nitty gritty stuff....An overview seems to suffice there...:) —Preceding unsigned comment added by (talk) 05:58, 17 May 2010 (UTC)

Actually, there's no clear concensus to remove the code from the article, your own multiple posts not withstanding. If TC doesn't have a problem with it, why should we? - BilCat (talk) 06:23, 17 May 2010 (UTC)
Bill is quite right: Not only is there no consensus to delete this information, but the publication of this information as "unclassified" by TC and the FAA just proves that your fears and prejudices are unfounded. Removing this text is nothing more than censorship. - Ahunt (talk) 12:35, 17 May 2010 (UTC)
I knew about this code when I was nine years old. Not only is Wikipedia not censored, removing it is, in a word, silly. - The Bushranger Return fireFlank speed 13:51, 17 May 2010 (UTC)
I am a commercial pilot and flight instructor. I fly approximately five days a week and I always find it funny when someone acts like the 7500 code is something other than public information. As a private, commercial, or airline transport pilot you are not privy to ANY classified or secret information (on any level) because of your license. To believe that you are is simply misinformed, arrogant, and setting yourself up to be the butt of a good few jokes at the airport lounge. The part of all this which was sad and not funny at all, was that because one individual thought he was entitled to knowledge that he felt others should not be entitled to, he was editing the article despite the consensus that the information should have stayed. One person's ignorance was preventing others from becoming educated on the fact. I'm glad we were talking about something as simple as a transponder code that is rarely used; I hope that guy doesn't behave that way when more important knowledge is involved. — Preceding unsigned comment added by (talk) 00:45, 28 September 2011 (UTC)
👍 Like - Ahunt (talk) 11:39, 28 September 2011 (UTC)

──────────────────────────────────────────────────────────────────────────────────────────────────── *Admin hat on* Consensus is clear that the code is relevant and should be included in the article. I've created an edit notice to that effect. Further removal of the code against consensus will result in administrative action being taken. Mjroots (talk) 08:59, 28 November 2011 (UTC)

The editnotice doesn't seem to have been moved to match the article itself. --SoledadKabocha (talk) 02:00, 19 February 2015 (UTC)

Move "Transponder modes" section[edit]

There seems to have been an attempt last February to move the Transponder Modes section into a main article. The section in this article has since grown considerably. I have flagged the page proposing a move of the section to the Aviation_transponder_interrogation_modes article. A summary of the section, moving the text over, and a format check will have to be done. Given the size of the section, I believe it deserves its own article.--Fbfree (talk) 05:37, 6 August 2009 (UTC)

Check a summary provided in Secondary_surveillance_radar#Modes. This doubling up may need reconciliation. Perhaps the listing of mode types should only be provided in the main article. --Fbfree (talk) 05:57, 6 August 2009 (UTC)
Sounds like a good move to me. it will reduce overlap and also some clutter in this article. - Ahunt (talk) 00:58, 8 August 2009 (UTC)

It's moved. I did not leave any content on this page as there did not seem to be any information uniquely relevant to this topic on that page, or any part of that page not relevant to this one. --Fbfree (talk) 01:23, 8 August 2009 (UTC)

I checked over what you did on all three articles and it looks good - I agree that there isn't much point leaving a short section there. - Ahunt (talk) 18:30, 8 August 2009 (UTC)

Avoidance of dual code assignment[edit]

I am not aware of the actual numbers involved, but the number of less than 4096 available squawk codes looks a bit low. How does ATC avoid assigning the same code to two aircrafts at the same time that might then fly into the same airspace?--Cancun771 (talk) 17:17, 23 December 2009 (UTC)

There are four Mode 3 digits available and you can select 0-7 on each digit, which gives (8X8X8X8) 4096 combinations, including ones that can't be assigned for daily use, such as 0000, 7500, etc. In many cases aircraft are assigned the same code in the same airspace, such as 1200 for transiting VFR traffic. Otherwise codes are assigned from a computer database list for each locality to avoid duplication of codes in the same airspace when that is required. - Ahunt (talk) 23:54, 1 April 2010 (UTC)


The introduction of a table for US transponder codes has made a bit of a mess of the article. There are still US codes in the second section and the two sections should probably both be put into table format, or else neither. - Ahunt (talk) 00:18, 27 April 2012 (UTC)

I agree. I think that including the codes at all on this page is excessive and unnecessary. Why not just reference the documents where the assignments can be found, and be done with it? Conortodd (talk) 00:12, 10 May 2012 (UTC)
That makes some sense. I think a list of commonly used codes is okay, but the formatting needs to be better than it is right now. - Ahunt (talk) 11:45, 10 May 2012 (UTC)
In reading over the information and the refs I think the best solution is to combine both into the existing table. At least this will eliminate duplication and the US-centric look to the article right now. Let me give it a try over several edits and then see how it looks. - Ahunt (talk) 15:51, 10 May 2012 (UTC)
Okay, I have combined them. See if that at least makes some sense now. - Ahunt (talk) 19:53, 10 May 2012 (UTC)
That looks dandy. I hope they pay you well :) Conortodd (talk) 18:53, 18 May 2012 (UTC)
Glad you think that looks better. It's all free! - Ahunt (talk) 19:12, 18 May 2012 (UTC)
Ahunt and whomever else is interested, what about changing "to" to a dash "-". I am not sure there is a standard here but to me it sould look a bit better and flow a little better. Just wanted to get your oppinion. You did a really good job on it, by the way.ErikkFriberg (talk) 13:09, 5 December 2012 (UTC)
Yes check.svg Done - Ahunt (talk) 13:57, 5 December 2012 (UTC)

SFRA rules on squawking 1200[edit]

From "Navigating the DC SFRA Effective 0500 Z, 02/17/2009"


6. Keep your discrete code to the ground. squawking 1200 for as little as ten seconds while in SFRA airspace will result in an FAA violation !!!! 7. Never squawk 1200 in SFRA !!!! — Preceding unsigned comment added by Wamnet (talkcontribs) 20:36, 18 June 2012 (UTC)

0001 no longer in use?[edit]

In past it was used for Air Force One, no? Flightsoffancy (talk) 16:47, 10 August 2012 (UTC)

It isn't listed in any of the refs cited in the article either way. Do you have a reference? - Ahunt (talk) 23:09, 10 August 2012 (UTC)
If 0001 use was abandoned then current refs wont show it. This site lists 0001 as "0001: Military code for high speed uncontrolled (non ATC directed) flight (US)", which could be Air Force One, but that is speculative. This site note 0000 used for drones , which article does not mention. I am beginning to think 0001 for AF1 is just "hangar talk" and not current policy (although it might have in distant past). Flightsoffancy (talk) 16:49, 13 August 2012 (UTC)


"Around busy airspace there is often a regulatory requirement that all aircraft be equipped with an altitude-reporting mode C or mode S transponders. In the United States, this is known as a Mode C veil."

I think there should be a difference distinguished between the Mode C Veil and Class B and C transponder (Mode C) requirements. This is with the understanding that the Mode C Veil and Class B airspace are almost the same airspace, but there is a slight difference. Look at DFW, you can be in the Mode C Veil in some places but not in Class B, and on the flip side, you can be in class B but not in the Mode C veil. I know this is nit picking but that what the FAA does.

I feel it should be distinguished, but I just wanted to see what others thought. --ErikkFriberg (talk) 15:19, 4 December 2012 (UTC)

[1] Note:§91.215 (b)(1) and (b)(2) denote two autonomous areas, that share common airports and airspace.

Invention and Earliest Adoption for Use[edit]

This information is missing and would be a beneficial inclusion at the top of the article. — Preceding unsigned comment added by (talk) 16:54, 12 July 2016 (UTC)

AIM References[edit]

Noticed that this article still references AIM RAC for transponder operation whereas it is now covered in COM.Dwt2 (talk) 22:40, 29 July 2016 (UTC)

  1. ^ 14 CFR Part 91.215