82

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 it's actually a 12-bit binary number and octal works really well since can be used as a "shortcut" for entering groups of three binary digits at a time (000 - ...


71

If you make a radio call, unless you are on 121.5 (or 243 military), then only the station you are talking to will initially know about the emergency. Initial calls should always be with the unit you are working with unless you are VFR. If you squawk 7700, then all stations in transponder range, including possible airborne stations such as AWACS and SAR ...


62

Basically everything that consumes power on an aircraft can potentially cause interference, short-circuits, or otherwise jeopardize the safety of flight and therefore must be switchable. Sometimes the switch is in the form a button, otherwise by a fuse. There are several particular reasons that the transponder can be turned off. If the transponder ...


62

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. Today, such transponders use 12 pulses (mode S transponders use a 24-bit address which is different): Source Pulses are labeled A4-A2-A1, B4-B2-B1, C4-C2-C1 ...


46

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 performs calculations to determine the azimuth and range of everything it receives. The interrogator sends out radio pulses which transponders in aircraft ...


37

Aircraft squawking 7700 are getting a special indication (e.g highlighted, distinct colour, boxed) on Air Traffic Control (ATC) displays. This helps to identify the emergency aircraft not only to the air traffic controller who is controlling the airspace the aircraft is in, but also to controllers of other (nearby) airspaces. This helps to improve awareness ...


34

A transponder in most planes look something like this (source) The code is changed by turning the knobs beneath the numbers. Lets say you are assigned 5700 as your code but your previous code was 7456. If you are careless or start from the right most knob your transponder will briefly be set in 7700 until the most significant digit is changed. If the ...


28

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 will not be detected by secondary radar and active multilateration. However, the aircraft can still be detected by primary radar because that does not rely on ...


27

(Note: This answer is tailored to the U.S. / FAA, because that is what I'm familiar with. All the citations and references are FAA materials. If anyone has any tips on possible international differences, please let me know and I'll edit them in. That said, I believe the descriptions I give are fairly consistent across the world, even if the FAA reg citations ...


26

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 from combat missions over Germany and mainland Europe, the (brand new technology) radar controllers had no way of knowing "who" the aircraft were. To solve ...


26

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 range is proportional to the time difference, minus a constant that allows for the aircraft transponder to perform processing. This would not work if ...


25

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 uncovered a number of switch failures. I figure that someone decided that for TCAS it was better to have a transponder squawking any code than not squawking. But ...


20

The class of ATC radars that you are talking about is secondary radar. Secondary radar works by transmitting an interrogation signal on 1030 MHz to which the transponder replies on 1090MHz. There are several modes of interrogation. For civil ATC, modes A,C and S are important. The uplink signal (interrogation) is a simple pulse encoding (Mode A/C) or a ...


20

Everything including the flight recorder has a power switch or circuit breaker. Electrical devices occasionally malfunction, and you don't want a sparky transponder setting the plane on fire when you can just turn it off and use the other one.


20

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 indicated altitude may include errors, such as the pressure ATC provides, but all aircraft in the area will presumably be using the same barometric pressure, and will ...


18

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 that the ATC system can interpret and display. Older transponders only support the Mode A identifier code (the squawk code we're familiar with selecting) - this ...


18

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 active air defense missions without ATC clearance. This would mean that the interceptor aircraft would change it's squawk to 7777 for the military/civilian air ...


17

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 should be). This also insures proper separation in the airspace system. FWIW Transponders generally report the altitude calibrated to a set 29.92 and then ...


17

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 for the Class B airspace and ask for permission to make the flight. Upon agreeing to conditions (including direction of flight and altitude), the pilot will ...


16

At least in the United States the FAA recently (I think in 2012?) changed the recommendation for transponder operation in the AIM. It now reads: Civil and military transponders should be turned to the “on" or normal altitude reporting position prior to moving on the airport surface to ensure the aircraft is visible to ATC surveillance systems. IN ALL ...


15

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 frequencies of the radar transmitter. Secondary radar shows the replies from transponders. A transponder responds to an "interrogation" signal transmitted ...


14

This FAA webpage has a list of the various transponder codes and what they are used for, which will give you an idea of what the problem is. Unless there is an actual aircraft incident or accident which requires the FAA to investigate though, you won't be able to find a record of it. In fact, pilots aren't even required to file a report unless requested, ...


14

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)(2) of this section, any aircraft which was not originally certificated with an engine-driven electrical system or which has not subsequently been ...


13

The transponder usually uses its own pressure reading, not what is set in the pilot's altimeter. So to prevent cheating as you describe, it is inspected and calibrated every 24 months. Tampering with it would be difficult to do on the fly because you'd have to adjust it based on the current atmospheric conditions and what altitude you want to seem to fly at. ...


13

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). Also, GPS SA (Selective Availability) was turned off in May 2000 to make available the required accuracy for civilian aviation. ADS-B relies on the aircraft ...


12

The purpose of squawk code 2000 is to prevent aircraft entering a Secondary Surveillance Radar (SSR) area from transmitting a code that is the same as a discrete code assigned by ATC to an individual aircraft. If you are flying in the USA under Visual Flight Rules (VFR), you will be assigned (implicitly) code 1200. Upon entering a SSR area you might get ...


12

Yes these codes are used. Only 7700 is used to denote emergencies, and usually only if you aren't in contact with ATC, or can't get their attention(frequency is packed). Codes in this range are allocated according to the National Beacon Assignment Plan in the US. There are other codes that are assigned for various issues, 7777 as mentioned in the comments is ...


11

Because it gets highlighted immediately on ATC's equipment, without needing a lengthy radio message to describe the problem and identify one's position.


11

For navigation and communication radios the feature of having a second readout that can be pre-set to the next frequency is useful since the next frequency is usually known in advance. Your next transponder code is a somewhat random number and the moment that you will get it is not as predictable as a frequency change. Therefore there is no need for a pre-...


11

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 modulation in the name is a misnomer, PCM doesn't relate to modulation in any way. PCM is used in audio processing to take an analog signal and encode it into ...


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