85 votes
Accepted

Why is the range of transponder digits 0 to 7 rather than 0 to 9?

The four-digit transponder code that is entered by a pilot is an octal number rather than a decimal number, and in the octal numbering system, only the digits 0-7 are valid. As far as why, internally ...
Lnafziger's user avatar
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64 votes

Why is the range of transponder digits 0 to 7 rather than 0 to 9?

Octal representation is a cheap way to deal with small binary numbers Modes A and C transponders send their ID using a train of pulses of unmodulated carrier, which can be seen as a sequence of bits. ...
mins's user avatar
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51 votes
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Can a military pilot make his airplane invisible for the ATCs?

Radar systems have 2 aspects, the primary and secondary systems, or the Radar and the interrogator. The radar sends out a RF signal and receives everything that is returned on that same frequency, and ...
alex's user avatar
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50 votes
Accepted

Should I contact ATC if I don't have a transponder?

Every US controller I've ever heard from says they'd prefer to be talking to every aircraft in their airspace, period. This lets them know your intentions and allows them to move you around if needed, ...
StephenS's user avatar
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31 votes

Can a military pilot make his airplane invisible for the ATCs?

The Eurofighter has a transponder that can be switched off, similar to a Cessna. By switching off the transponder, the aircraft becomes invisible to cooperative surveillance. This means the aircraft ...
DeltaLima's user avatar
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29 votes

Why do (or did, until very recently) aircraft transponders wait to be interrogated before broadcasting beacon signals?

The Secondary Surveillance Radar system provides range in a method similar to primary radar: by measuring the time between the transmission of the interrogation signal and reception of the reply. The ...
user71659's user avatar
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27 votes

Should I contact ATC if I don't have a transponder?

They'd rather talk to you, even if they can't do much for you. If you report over XYZ at 4500 heading east, they can correlate that to a primary target & note that it's N1234S "over there". They ...
Ralph J's user avatar
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26 votes
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Why do the transponders introduce codes instead of using aircraft registration numbers?

To expand upon voretaq7's answer, understanding the history and evolution of transponders can more deeply explain why this is the way it is (for now). When Allied (US/UK/Others) WWII planes returned ...
Jimmy's user avatar
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26 votes
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Should I switch the transponder to STBY or not while changing codes?

The FAA started advocating not selecting standby for code changes after it was found that more often than one would expect, the transponder doesn't always come back on after standby. Seems they ...
mongo's user avatar
  • 17.7k
25 votes
Accepted

Which source to use when reporting the current altitude to ATC?

When asked for altitude, you report the altimeter reading, utilizing the correct barometric pressure entered into the Kollsman window. ATC separates traffic based upon indicated altitude. The ...
mongo's user avatar
  • 17.7k
23 votes
Accepted

What is the purpose of squawking 7777?

In countries outside of the US, 7777 may be used by test transponders (RABMs) to check correctness of radar stations (BITE). e.g. on top of a mountain. In the US, it seems that it is used as well on ...
blackswan's user avatar
  • 370
23 votes
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Do airliners have a "switch" or other means of rapidly squawking emergency?

In the 737, it's just the 4 dials (well, 2 sets of concentric knobs). Since we're basically always talking to ATC, the need to squawk 7700 is quite rare; all the emergencies I've declared have been ...
Ralph J's user avatar
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20 votes
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How do I tell ATC that I don't have a transponder?

From the AOPA: Flying into a Mode C Veil Without a Transponder For flying into a Mode C veil without an operable transponder, the pilot needs to telephone the appropriate radar facility ...
DLH's user avatar
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19 votes
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How does AWACS pick up enemy info like speed and altitude without transponder interrogation?

Primary radar doesn't require a transponder or IFF module. It works by actually bouncing a radio signal off the skin of the airplane. It doesn't require the cooperation of the plane being detected, so ...
HiddenWindshield's user avatar
18 votes

Why do the transponders introduce codes instead of using aircraft registration numbers?

The short answer is that they often do both. Mode S transponders (which you'll find on most airliners, and are now becoming common on light aircraft as well) transmit a unique aircraft identifier ...
voretaq7's user avatar
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18 votes

Which source to use when reporting the current altitude to ATC?

You should generally report what you see on your altimeter (at least that is what i was always instructed to do) but this presumes that you are up to date on your local pressure settings (which you ...
Dave's user avatar
  • 101k
17 votes

Can a military pilot make his airplane invisible for the ATCs?

ATC radar has two modes: Primary and Secondary. Primary shows radar echo returns. Normally this is from metal aircraft, precipitation, birds and anything which reflects radio frequency energy at the ...
mongo's user avatar
  • 17.7k
17 votes

What does prevent installing an ADS-B transponder on a vehicle?

Use of ADB-S transmitters is limited by regulation: in the US, frequency use is limited to FCC license holders; FCC licenses are delivered only after approval by FAA, the aviation agency; FAA approves ...
mins's user avatar
  • 71.8k
16 votes

Why do (or did, until very recently) aircraft transponders wait to be interrogated before broadcasting beacon signals?

I don’t get why most (or, until very recently, all) transponders don’t broadcast unbidden GNSS systems have only been commercially available relatively recently (compared to the timeline of aviation)....
Greg Hewgill's user avatar
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16 votes
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How far can a plane squawking 7700 be heard and how quickly will the news spread?

Terrestrial radar systems only cover so much area. The latest Terminal radar system the FAA uses, the ASR-11, has a working range of 60 nautical miles. The Enroute ARSR-4 has a range of 250 nautical ...
randomhead's user avatar
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15 votes
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Are transponders required for hot-air balloons in the US when operating within a Mode C Veil?

I've gotten this question enough times to look up the regulations and answer my own question :) 14 CFR 91.215 lays it out quite nicely. Particularly section B(3): (3) Notwithstanding paragraph (b)...
Canuk's user avatar
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15 votes
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What is Mode B on the transponder?

All modes From a 1984 edition of Doc 9426 (ATS): For civil aviation purposes, ICAO has designated four, differently spaced, pairs of interrogator pulses known as Modes A, B, C and D [...] Currently ...
mins's user avatar
  • 71.8k
15 votes

How does ATC locate a stealth aircraft if its transponder fails?

It depends on the aircraft. One example was on the F-117, which had removable radar reflectors on the sides of the aircraft, allowing the jet to be detected by primary ATC radars. As to more modern ...
Romeo_4808N's user avatar
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15 votes
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Why is 5 seconds the time to respond to a TCAS RA?

If the pilot reacts at impact it is too late to avoid collision If the pilot reacts one second before impact it is too late to make any significant altitude change and to avoid collision. If the pilot ...
DeltaLima's user avatar
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13 votes
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What is the ICAO 24-bit code used for?

In short: the 24 bit address is critical on the technical layer of Air Traffic Control, but it is not used in the operational level. For identification of aircraft and association to flight plans, the ...
DeltaLima's user avatar
  • 83.1k
13 votes
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How does the modulation of the transponder work?

About PCM I do not understand which are the advantages of using PCM [in a transponder] Pulse Code Modulation (PCM) is a method used in codecs, while modulation is a method used in modems. PCM with ...
mins's user avatar
  • 71.8k
13 votes
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How does ATC locate a stealth aircraft if its transponder fails?

I think you are overthinking things a bit, since it doesn't really matter if it's a stealth aircraft or not. Many places in the world do not have primary radar coverage at all, meaning that if a ...
60levelchange's user avatar
12 votes

Do airliners have a "switch" or other means of rapidly squawking emergency?

It's been a few years, but in the 9 years or so I developed Comm/Nav/Surv equipment for the air transport market I never saw any quick select buttons. If you wanted a code, you had to dial it in. I ...
Gerry's user avatar
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11 votes

How does ATC locate a stealth aircraft if its transponder fails?

When a B2 visited the UK for the Fairford air show a few years ago, it was always escorted by a pair of F15s, one on each wing-tip. So radar simply had to look for the 2 F15s, and the B2 was the hole ...
RAC's user avatar
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