Questions tagged [transponder]

A radar transponder is a radio beacon on an aircraft that replies to a radar interrogation. The transponder encodes information into the reply, allowing air traffic control to know the aircraft's altitude and identity.

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46
votes
9answers
171k views

What are the advantages of squawking 7700?

I was reading this question about how do pilots send distress signals in emergency scenarios, and that got me thinking... In voretaq7's answer, he states that there are three ways to send distress ...
44
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6answers
17k views

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

As a controller on IVAO I sometimes assign codes like 0168 and find out later that the code isn’t valid. For me, the range 0-9 seems more logical. Why do we have to use octal numbers and not decimal?
43
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3answers
35k views

Why do pilots have the ability to turn off the transponder in-flight?

These days, when reading news about missing Malaysia Airlines Boeing 777-200, I keep coming across a scenario where pilot might have deliberately turned off the transponder which is used for the ...
35
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9answers
12k views

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

I'm writing a story about the pilot of the Austrian Air Force, who uses his Eurofighter Typhoon aircraft for unintended purposes. I've read stories in the internets that civil pilots allegedly can ...
32
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2answers
5k views

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

The flight school I usually rent from is under the shelf of class C airspace. If you're going out to a practice area, the standard procedure is to contact approach control and get a squawk code, so ...
30
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3answers
35k views

How does IDENT work?

In order to help ATC identify an aircraft on their screen, they may ask the aircraft to "ident." The pilot presses a button on their transponder, and the controller can verify that the aircraft they ...
27
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3answers
4k views

What could cause an inadvertent squawking of 7700?

I get alerts whenever a plane squawks 7700, using a popular flight tracker application. It happens quite frequently that the aircraft, reported to squawk ...
22
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2answers
3k views

Should I switch the transponder to STBY or not while changing codes?

I´m now reading Bob Gardner’s book “Say again, please” and there is this instruction on page 54: Do not turn the transponder function switch to STBY while changing codes. What is the reason for ...
21
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1answer
6k views

How does ATC radar/ADS-B ground stations not get confused with signals sent from multiple aircraft?

I understand that the radar sends a 1030 MHz interrogation signal to an aircraft and the aircraft's transpoder responds using 1090 MHz. When it comes to Mode S squitter, the transmissions are ...
18
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3answers
3k views

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

(A non-pilot here, just a Aviation.SE reader, recently into listening to ATC. I am wondering...) Why do the transponders use their own codes? (squawk) It seems to me, it would be safer and less of a ...
17
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2answers
2k views

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

While flying from KSNA to KVCB VFR at night, Norcal Approach asked for my current altitude. I was somewhere between 12.0 and 12.5 depending on which reading I was going to use. ATC had me confirm. ...
16
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2answers
5k views

When should a transponder be used?

When should a transponder be turned turn on? On the ground? Always in flight? Only when required: during radar service (flight following), above 10,000ft MSL, inside the Mode C Veil, or on IFR flight?...
15
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7answers
4k views

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

An aircraft’s transponder transmits signals that announce (at the very least) the aircraft’s identity, altitude, and direction of motion (plus, for most newer aircraft, a whole slew of other useful ...
15
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2answers
2k views

How are airspace violations detected?

Are airspace violations (e.g. entry to class B without clearance) based on primary radar and/or Mode C transponder, or something else? I read that Mode C altitude is based on pressure altitude, i.e., ...
14
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3answers
21k views

What is Transponder Code 2000 actually used for?

On What is the significance of a squawk code? @DeltaLima gave a fairly concise answer. In it he listed several 'standard' codes used for things like emergencies. Also in the list was Code 2000, for ...
14
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2answers
58k views

Can I find out why an aircraft squawked an emergency code (7x00)?

I have recently been using a mobile app to track flights, which is really cool. I live in the rural heartland of America, so it's an event for me to see an A380 actually flying. Every once in a ...
14
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2answers
9k views

Are transponder codes 7701 - 7777 ever used?

I understand that blocks of transponder codes are assigned to different area controls. I am curious if the block 7701 to 7777 is in use anywhere? Maybe the emergency code 7700 needs some extra ...
13
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2answers
2k views

Are transponders required for hot-air balloons in the US when operating within a Mode C Veil?

I've taken some people flying recently for fun, and they noticed some hot-air balloons and asked me if they had transponders. In San Diego (Southern California) I often see hot air balloons well ...
12
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3answers
9k views

How do I tell ATC that I don't have a transponder?

If my plane doesn't have a transponder and ATC tells me to squawk some transponder code, how should I respond? Is it just as simple as "I don't have a transponder"? This is for the United States and ...
11
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3answers
2k views

Does ATC use mode S addresses to identify planes?

By design, Mode S surveillance radar equipment uses global 24-bit airframe addresses to keep track of the aircraft it is tracking. Are these addresses also used by higher levels of the ATC system, ...
11
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3answers
1k views

In a part 91 formation flight within a mode C veil, are all the aircraft required to have their transponders on?

Two aircraft are planning on conducting a standard formation flight (aircraft remaining within 1 mile of each other) as follows in the U.S.: Maintaining VFR below 10,000 MSL; Remaining outside/clear ...
10
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3answers
3k views

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

Stealth aircraft are built to have near-zero radar visibility, so that, in a warzone, it'll be harder for the enemy to see it on their radars to shoot it down. However, air traffic control also use ...
10
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2answers
11k views

What is the significance of a squawk code?

I've been looking for a detailed explanation for how the transponder works and the importance of a squawk code.
10
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3answers
1k views

Why don't transponders have active and stand-by value like radios?

This answer explains how pilots when switching code on transponder should either carefully avoid temporarily squawking one of the emergency codes, or should temporarily put the transponder in stand-by....
9
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1answer
3k views

What is Mode B on the transponder?

Mode A selected on the transponder provides identification of the aircraft to the secondary radar; Mode C will add an automatic height read-out of an aircraft. Mode S is a selective addressing. But ...
9
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2answers
1k views

What do you squawk when multiple conditions apply?

There are three standard transponder codes for exceptional situations: 7500 (hijacking), 7600 (radio failure), and 7700 (general emergency). Which code do you use if multiple situations apply? For ...
9
votes
2answers
537 views

Do interceptor squawk codes suppress TCAS RAs?

Some parts of the world reserve a discrete squawk code for use by military interceptor operations (in the NBCAP, code 7777 is used for this). Does this squawk code, or some equivalent value, suppress ...
8
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2answers
13k views

What is the purpose of squawking 7777?

Aircraft use transponders to communicate to ATC their position and status. Some squawk codes are reserved, such as 7700 (emergency), 7600 (communication failure), 7500 (hijacking), 1202 (glider), 1200 ...
8
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2answers
14k views

Why are there so many 7700 squawks?

I installed an app some months ago, which gives a push message when an aircraft squawks 7700. The first time it happened, I thought the plane was about to crash, ...
7
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3answers
9k views

What is the ICAO 24-bit code used for?

I know that each transponder is linked to a unique 24-bit code assigned by ICAO. But to be seen by the Secondary surveillance radar the pilot set the squawk code (4 digit octal code) on the ...
7
votes
1answer
1k views

Is it recommended to have an ADS-B transponder in ALT mode on the ground? [duplicate]

Early on in training I was told that unless the airport specifically noted it on the chart, that the transponder was turned to ALT (altitude) mode just prior to taking the runway for take-off (lights, ...
6
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3answers
2k views

How does the modulation of the transponder work?

The transponder works in PCM (pulse code modulation): how does it work in the transponder? I can find explanation of what PCM is, but not if the signal sent from the transponder antenna is digital or ...
6
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2answers
396 views

Does the transponder need to be on in the upper limit of Class B airspace?

Is a transponder needed to be turned on to operate above the upper limit of class B airspace?
6
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1answer
134 views

Why bits are mixed up (interleaved) during ATC transponder mode A and mode C reply format?

In the image above from radartutorial.eu, we can see that the sequence of bits are c1 a1 c2 a2 c4 a4 during the reply of ATC transponder mode A. Why are the bits not transmitted like c1 c2 c4 then a1 ...
6
votes
1answer
294 views

How are both squawk and altitude transmited when operating a Transponder in Mode C?

I know that in Mode $3$ (which includes Mode A and Mode C) the aircraft transmits $12$ information pulses. That corresponds to $2^{12}=4096$ squawk combinations in Mode A which are sent to ATC to ...
6
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3answers
235 views

Would FAR 91.227, or any other regulation, forbid use of ADS-B-out in an unregistered ultralight?

In the US, would FAR 91.227, or any other regulation or law, forbid use of ADS-B-out in an unregistered ultralight? Could the N-number field simply be left blank, without violating any regulation or ...
6
votes
1answer
1k views

What's required for the ADS-B to function? A GPS or Mode S Transponder?

Jeppesen defines the ADS-B as a surveillance system broadcasting GPS-derived positions and other information such as velocity over the data link. Yet, the Wiki article on ADS-B relates it to the Mode ...
6
votes
1answer
2k views

How can I ensure that I have a mode C transponder?

How can I make sure my aircraft is equipped with a mode C transponder? ====================== If a transponder has an ALT button does that mean that the aircraft is equipped with MODE C?
5
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2answers
950 views

What happens to the transponder, FDR and CVR in case of an explosion?

Will the above mentioned equipment still work and transmit data/location information if an explosion occurs inside the aircraft?
5
votes
1answer
4k views

Which antenna on an commercial airline airplane is used for the transponder (SSR)?

On the 767-200 image I see no SSR antenna, but since that TCAS uses the transponder I guess that are sharing the same antenna. Am I right ?]1
5
votes
2answers
1k views

Why does the 1090 ES use the same frequency for both uplink and downlink?

The 1090 ES is the transponder system used in ADS-B. Why does the 1090 ES use the same frequency (1090MHz) for uplink as well as downlink? 1090 ES is an extension of Mode S. The Mode S used 1030MHz ...
5
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3answers
2k views

Is a mode C transponder required underneath Class C airspace?

FAR 91.215 states that a mode C transponder is required for... all aircraft in all airspace above the ceiling and within the lateral boundaries of a Class B or Class C airspace area designated ...
5
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1answer
1k views

Is there a relation between the Mode 3/A Code and the ICAO aircraft address?

I would like to know is there is any way to match mode 3/a code (which is assigned by ATC to aircraft) and ICAO address of the aircraft. I mean how mode 3/a code is assigned to each aircraft by ATC? ...
5
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2answers
2k views

When may I change my transponder code to 1200 when being handled by ATC?

When am I allowed to change my transponder code to 1200? Is it after clearing the boundary of regulatory airspace (ie. airspace requiring radio contact and an operating transponder)?
5
votes
1answer
1k views

Why would a Mode-C transponder report a different altitude than an altimeter/GPS?

This is related to a small single-engined plane. On a recent flight, the controller kept telling me that I was 300 ft higher than it appeared on my altimeter and GPS (which were in sync). This was ...
5
votes
1answer
671 views

How does FlightAware correlate flights to Part 91 flight plan data?

I'm looking at some flights in FlightAware. With the scheduled air carrier flights, their call-sign can be crossreferenced with a flight schedule database to determine the origin and destination. How ...
4
votes
3answers
758 views

What aircraft would have these tail numbers?

Last night I was looking at a flight radar on my phone, and I saw two blips flying close to each other over Camden, NJ. They were heading northeast at around 250 knots, and the only information ...
4
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2answers
1k views

How far can a plane squawking 7700 be heard and how quickly will the news spread?

This is just an idle question I have after watching Non-Stop (2014), but suppose a plane (let's assume a Boeing 767-400) is over the Atlantic and sets its transponder to indicate it's in an emergency. ...
4
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2answers
3k views

When is a transponder required below class B airspace?

I just wanted to clear up questions I have about flying within the 30 mile veil of Class B. My understanding is that the Class B is the inverted wedding cake, having tiers as the airspace moves ...
3
votes
1answer
2k views

After purchasing an aircraft, how do I put the transponder in my name?

I just purchased an aircraft and the prior owner said that I need to put the transponder to my name. What is the procedure to do that?