explain aviation transponder operation

The latest word on Malaysia Air Flight 370 is that the transponder stopped working shortly before the airraft turned and flew several hundred miles away from its planned route.

After reading the Wikipedia link (see above), I’m looking for more detail about how a transponder works:

-The “Code Assignments” list at that link lists a bunch of possible codes, but they mostly seem to be related to broad categories of aircraft depending on their operating conditions. Can multiple aircraft in the same area use the same transponder code?

-Does the traffic controller’s radar screen display just the transponder code for each aircraft, or does it display information about each aircraft that has been manually associated with that code? For example, if MA370’s transponder code were 1234, would the controller’s screen have “1234” next to that plane’s location, or does it have “MA370” (or something similar) listed there instead?

-Does a transponder shout out its info on a continuous/unsolicited basis, or only in response to each time the aircraft is painted by ATC radar?

The answers I can provide are true of the US airspace system, as I’ve been an air traffic controller in the US for many years.

Only one aircraft in each facility’s airspace may occupy each code. The US is covered by 22 enroute centers and each one has its own computer keeping track of all the codes. So, for example, an aircraft flying along in New York Center’s (ZNY) airspace may be assigned code 1234 and an aircraft in Boston Center’s (ZBW) airspace may also be on code 1234. That’s fine because the two computers are independent from each other. However, if the flight from ZNY is going to enter ZBW’s airspace, the ZBW computer will generate a new code for the flight once it enters the airspace because 1234 is already taken.

It displays a datablock that has information about the flight on it. Exactly what is on the datablock will vary based on the computer system that the ATC facility has, but at a minimum generally it will have the flight number, altitude, and groundspeed.

Before the flight departs, the computer has already assigned a transponder (“squawk”) code and this code is delivered to the pilot as part of their initial IFR clearance. When the plane takes off, the radar will recognize the code that the plane is squawking and the datablock will appear on the radar. Usually it takes about two or three radar sweeps for that to happen, so right after the plane takes off there will be just a radar target and the transponder code next to it until the computer recognizes the code and generates the datablock. If the plane takes off and the pilot has set the wrong transponder code and the computer doesn’t recognize the code, then no datablock will acquire and it will just be a target with the incorrect code next to it.

As far as I know, it’s only when the radar sweeps and “talks” to the transponder. Here is a picture of a rudimentary transponder. The yellow “reply” light in the upper left will light up when the ATC radar sweeps by and the responder broadcasts to it. In center airspace that will be about every 10 seconds; in terminal airspace it will be about every 3 seconds.

Yes, a transponder only transmits/responds (hence the name) when “queried”. When that happens, there will be an indicator (usually a light) on the instrument face, but the pilot really doesn’t need to know that.

If it has “Mode C” capability, as most do (and are required to in some places, and when IFR), then it also transmits the airplane’s altitude divided by 100 feet, via an electrical connection to an altimeter. “Mode S”, now coming into broader use, transmits longitude/latitude/altitude from GPS. But altitude reporting can be disabled if the system is acting up and ATC requests it. ATC can also request an Ident, which the pilot will respond to by pushing a button that sends another code that makes the code temporarily brighter on the display.

I was an air force radar tech (and instructor) in the '80s & '90s.
Radar and transponder are two separate things, with very different technology and therefore equipment (Google airport radar and you see images with 2 antennas - the rectangle one is the transponder system’s). The controllers screen combines them for the controller.
Military aircraft can have multiple codes - some of which can be changed in flight by the flightcrew (usually the pilot).
Transponders only send out their codes (transpond = transmit respond) when they are queried, which is called an interrogation - otherwise the ground receiver would have no way of telling where the transponder is.

Note that planes flying VFR use 1200 (unless they are using flight following or in certain air spaces) so multiple planes would be using the same code


Very true. 1200 (and anything else ending in “00”) is what’s known as a non-discrete code. It is displayed, but not connected to a particular aircraft.

It’s been over a decade since I’ve flown but what about the 7500, 7600, and 7700 codes, which if I remember correctly, correspond to emergency, communications failure, and hijacking? Wouldn’t those be connected to a particular aircraft?

The aircraft I used to learn was a Cessna 172. The transponder would be left on 1200 (VFR) unless entering controlled airspace. Then you would contact ATC and be assigned a code. In my limited experience they would usually respond with ‘Code 1234 and ident’. You would dial in 1234 on your transponder and press the squawk button. That would force the transponder to transmit a pulse without being queried. I was taught it would make our blips flash on the ATC screen making us easier to identify.

It depends. If an aircraft is squawking 1200 and then starts squawking 7600, the radar scope will just display “RDOF” on the VFR datablock. The controller will still not know the callsign, type, etc of the aircraft, since that information was not entered and the computer has no idea who that aircraft is; it’s just a 1200 code that was out flying around.

If it’s an aircraft that was previously radar identified (had been assigned a squawk code), the computer will “recognize” that even though it’s no longer on that code, it’s still that aircraft even though it’s changed to a non-discrete squawk (in this case, 7600). What the controller would see would be the datablock along with a blinking “RDOF.” Even though the aircraft isn’t still on the assigned squawk code, the computer is still tracking the primary radar response (the raw radar bouncing off the aircraft) and talking to the Mode C. The computer figures that even though the code was changed, that’s the same aircraft.

There are a few ways for a controller to establish radar contact. One of the ways is to observe a code change from 1200 to the assigned code. Another is to tell an aircraft to “ident”. By ident-ing, the aircraft blinks “ID” next to the VFR datablock (at least that’s what it does with the computer system we have at my facility; other systems have different displays). A controller can provide radar contact to a 1200 code by knowing their approximate location and altitude and then observing the ident. Sometimes if they’re busy they will tell the aircraft to squawk a certain code AND ident so they don’t have to wait to see the code change from 1200 to the assigned code, or they might do that if they see a whole gaggle of aircraft in an area and they want to quickly identify which one they’re talking to.