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Departing VFR from class D airspace, either you can request flight following after you leave the airspace or you may ask the airport ground/tower and they assign you a squawk code and setup the flight following for you.

For example, on a short flight from KCNO to KSNA, I usually get my squawk code from tower and after departure I switch to SOCAL and let them know I departed KCNO and usually follows by "ident".

How does ATC know if you already have a squawk code assigned to you or not?

How does initial communication with SOCAL differ from when you've been assigned squawk on ground before take off from when you want to request one?

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    $\begingroup$ When you hit "ident" you will start blinking on the screen. The number will be assigned in "the system" which ATC uses to track flights, just like if it were IFR. Sometimes (or a lot of times) when getting handed off I'll get a revised squawk code instead of them using the one that the local tower assigned me. $\endgroup$ – Ron Beyer Jul 29 '16 at 21:24
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    $\begingroup$ also, the Class D tower doesn't assign you the squawk code. The tower contacts TRACON ("departure"), which assigns the code. The tower just relays the message. $\endgroup$ – rbp Jul 29 '16 at 21:37
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    $\begingroup$ My tower almost always declines to help with a Squawk or Flight Following. They tell me to contact Center. I don't even bother asking the tower. $\endgroup$ – abelenky Sep 21 '16 at 17:54
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@J.Hougaard's answer is excellent (as usual), but it doesn't address the last part of your question:

How does initial communication with SOCAL differ from when you've been assigned squawk on ground before take off from when you want to request one?

If you have a discrete code, then you are "handed off" from one facility or sector to another. This means that the sending controller does something on his console to transfer responsibility for your flight, which prompts the receiving controller (who may be at the next scope or hundreds of miles away) to accept the transfer. After the transfer is accepted, the sender tells you by radio to change frequencies and call your new controller who, having just accepted you, will be expecting your call. This call, commonly referred to as "checking in", needs only contain your tail number (or callsign/flight number, if applicable) and altitude; the main purpose is to let the receiving controller know you're now on their frequency.

If you don't have a discrete code, then you're just an anonymous blip on the controllers' screens, so you are not just checking in. You will need to request flight following and give all the information (type, destination, cruise altitude, etc.) that you would have given on the ground, and they will give you back a code and start looking for you on their scope.

Either way, a controller may ask you to ident because it makes your target stand out on their scope, which saves them time. If you gave an approximate position when you called, this is less likely since they'll know where to be looking. Don't ident unless they ask, though, since its not always needed and may distract them when they're looking for someone else.

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    $\begingroup$ Are you an US ATCO? Is this really how it works over there? I am ATCO in Germany and what you write is not quite the case here. $\endgroup$ – pcfreakxx Mar 7 at 9:52
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    $\begingroup$ We have the same thing called Flight Information Service (FIS) by dedicated personell on dedicated frequencies - which makes sense because I think the VFR traffic in Central Europe is probably much more crowded than in the US. FIS stations here have 40-50 aircraft on frequency sometimes. But that‘s not what I meant: You wrote that the sending Controller has to do sth to initiate transfer and only after transfer was accepted he may send the aircraft to the next frequency. That’s nothing we are used to over here. We just send the aircraft to the next freq., no coordination necessary. $\endgroup$ – pcfreakxx Mar 7 at 17:20
  • $\begingroup$ OK, I see. So your systems are not trajectory based? European systems are really modern today. In Germany for example only aircraft who are planned for your sector are blue because the system calculates a 4D-trajectory and knows exactly when an aircraft enters which sectors. Once the previous controller sends the acft it will change into send state on my screen. I can see that it is send, but I won‘t assume the label until the aircraft called on my freq. All aircraft on my frequency are black and the others outside my sector are white so you can see them but they don’t distract you. $\endgroup$ – pcfreakxx Mar 7 at 19:28
  • $\begingroup$ Oh this sounds like it‘s limiting your ability to grow with the traffic increase as well? Or it requires hard work by the ATCOS to compensate? In upper airspace in the meantime we work roughly 60 aircraft per hour in a sector extending from FL320-FL360 with a 70nm by 70nm shape. $\endgroup$ – pcfreakxx Mar 7 at 21:27
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – StephenS Mar 8 at 1:57
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Let's remember what a transponder code is used for in the first place.

When transmitting a non-discrete code, such as 1200, ATC has no way1 of telling you apart from other aircraft transmitting the same code. On the ATC radar, you will simply show up as something like "1200/A030" where 1200 is the code transmitted and A030 (3000FT) is you mode C altitude.
To provide you with radar service, ATC has to be able to identify you on the radar, which means that they have to be able to distinctly associate your flight with a specific track on the radar. They do this using transponder codes2. An assigned (discrete) transponder code will be unique for a flight within a given area. Say you are instructed to squawk 2174, you will now show up on the screen as "2174/A030", and because ATC knows that you, and no one else in the area, is squawking 2174, they know that they are you looking at you.

Your question is, how does ATC keep track of the assigned transponder codes? Today this is mostly an automated process. A central flight data processing system (or the US equivalent - sorry for my European terminology) contains a database of all active flightplans as well as their assigned transponder codes. When a discrete transponder code is received by the ATM system, it will automatically query this database, asking for any flightplan information associated with this specific transponder code. It will then show the received information on the radar screen. Essentially, the transponder code is a database key, allowing the system to look up information about your flight and show it to the controller.

Now you are no longer shown on the radar with just your transponder code and mode C altitude, because the system has a lot more information about you. You might be shown as something like "N12345/A030/C172" (callsign/altitude/aircraft type). The exact label varies greatly from system to system - the point here being that the system is now able to show specific information from your flightplan rather than just generic information submitted by your transponder.

So how does a controller know if you have been assigned a transponder code or not? If you have, you will be shown on their radar screen as a full label, with callsign etc. If not, your callsign is no where to be found on the radar, and they will have to guess that you are probably one of the (potentially many) 1200-tracks on the screen.

1: They can in fact identify you by other means, such as requesting your exact position and heading, but identification with the help of transponder is a lot simpler.

2: Normal 4-digit transponder codes are slowly being replaced by mode-S transponders, which transmit a unique 24-bit aircraft identifier at all times to the radar system. But that's probably best left for another question.

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