What is a squawk code?

Or better how it generated? I can't come up with a better way of asking the question. I know it's octal, but is there a type of modulation that is used to generate the code? Like PCM or what? I asked this question in my sim forum and got no answer and so much as told me to read Wikipedia. Well, I have, but it doesn't say what modulates (for lack of a better word) the squawk code.

I'm eager to learn please be respectful. Thanks in advance!

Squawk Codes

Squawk, or better yet, transponder codes are indeed pulse code modulated. In the standard FAA Mode A (military mode 3/A), the ground based secondary radar (sometimes referred to as the IFF/SIF system) sends out a series of pulses that are timed to trigger specific responses from the aircraft transponder. In the case of a squawk code assigned by an air traffic controller, the pilot dials the four octal digits into a box in his instrument panel. The transponder then will respond to the correct pulse-timed interrogations on 1030 MHz, with a pulse coded reply on 1090 MHz. In almost all cases, the ground based radar interrogates more than one mode at a time, using an interlaced set of pulses. In the case of the simple mode (not more modern Mode S) interrogation, the radar usually requests mode 3/A and mode C. Usually in a train like 3/A, 3/A, C, then repeating. Mode C is the aircraft altitude, in a different code, called a Gray code, that represents the uncorrected barometric altitude of the plane. You can Google Gray codes to find out what is so special about it.

Anyway, I've made a short story long, but in any case, the squawk code is usually assigned by air traffic control, unless the aircraft is flying under Visual Flight Rules, in which case the code would be 1200.

Hope that helped (I spent about 15 of my 22 years in the U.S. Air Force maintaining air traffic control radar).

Edit: Take a look at this link: http://www.airsport-corp.com/modec.htm

73,
Greg

Oh wow! Thank you very much! That link has some good info. I know about 1200, 7500 and 7600 and such, but just wanted the low done on how the code is modulated. If I have more questions about what I have read I'll post. LOL

That was a great answer. I'm a fan of any response that includes PRI.

After reading that link I now have a headache. LOL! But Now I know that when I input my squawk code it is indeed PCM.

I am an ex-FAA Controller and thought I'd throw in a bit of info on how beacon codes are generated. I doubt it has changed since I was at Dayton International Airport in Ohio. The controlling ARTCC assigns a bank of four digit codes to each Terminal ATC facility. That facility issues the computer generated codes to the aircraft when they give clearances to IFR and VFR aircraft that receive flight following. In addition, each ARTCC sector has allotted transponder codes they give out.

Alan - KE4AH