I've been looking for a detailed explanation for how the transponder works and the importance of a squawk code.
$\begingroup$ What have you found out so far? Are you looking for instructions for a specific make and model of transponder? Do you want to know what are the common squawk codes and their meanings? Do you want to know how ATC reassign squawk codes? Please clarify - I am voting to close in meantime. $\endgroup$– RedGrittyBrickMay 5, 2015 at 9:59
1$\begingroup$ Actually, I was hoping for an answer that addressed all of that, or at least an answer that talks about the way ATCs reassign squawk codes and how they are made visible to ATCs in the first place. $\endgroup$– Madhav SudarshanMay 5, 2015 at 10:04
$\begingroup$ Start at Wikipedia - In my opinion, that's a lot of material to reproduce here. $\endgroup$– RedGrittyBrickMay 5, 2015 at 10:08
The squawk code is assigned by Air Traffic Control to the aircraft and is used for radar identification purposes. It is a 4 digit octal number (each digit has a value in the range [0-7]).
The flight crew enters the assigned squawk code into the transponder control panel and the radar extracts the code using a Mode-A interrogation (or UF5 / UF21 interrogation in case of a Mode S radar/ transponder).
The extracted squawk code can then be used to correlate the radar reply with an aircraft's flight plan. A number of squawk codes are reserved for generic purposes and emergencies. These codes are used in emergency, or when no flight plan correlation is needed. Some generic codes are:
1000 - Conspicuity code signalling the Flight Data Processing system to use Mode-S Aircraft ID for flight plan correlation. (Primarily in Europe)
1200 - Visual Flight Rules standard squawk code (USA & Canada)
2000 - Used when entering a Secondary Surveillance Area and no code has yet been assigned.
7000 - Visual Flight Rules standard squawk code (ICAO, USA & Canada use 1200 instead)
7500 - Unlawful interference / hijack (ICAO, worldwide)
7600 - Radio communication lost (ICAO, worldwide)
7700 - General emergency (ICAO, worldwide)
$\begingroup$ Also, ATC squawk codes while seemingly random may have a meaning within that particular facility, but there is not exactly a standard or meaning useful to anyone but ATC. For example, "local" flights may be assigned codes in the 0xxx range, but in the US, the squawk code is automatically determined by the ATC system. Occasionally conflicts can occur, and you'll hear ATC direct to "reset transponder, squawk xxxx". $\endgroup$ May 5, 2015 at 22:10
It allows Secondary radar to identify each aircraft. The response ping from the aircraft contains the 12 bit squawk code.
Each plane is assigned a different squawk code as they enter an airspace and the software compares that to the flight plans filed and then can display the relevant information on the display.
If you fly VFR without a flight plan you will squawk the general VFR code until asked to change it when entering controlled airspace. When leaving tghe controlled airspace you will be asked to "squawk VFR" again.
They can also be used to declare emergency by squawking 7700, 7600 or 7500 (for general emergency, radio loss and hijack resp.)
75, taken alive. 76, in a fix. 77, going to heaven$\endgroup$– SimonMay 5, 2015 at 17:43