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What is the minimum set of information (for ex.: aircraft registration, position, altitude ... and what else) that a transponder in an aircraft must provide after being interrogated by a ground station ? Is this defined somewhere ? Is there information about the airport of departure/arrival ?

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    $\begingroup$ Take a look at this: Decoding ADS-B messages. $\endgroup$ – Ron Beyer Jan 9 at 15:24
  • $\begingroup$ @RonBeyer, this is pretty much what I was searching for. If you post it as an answer I will accept it $\endgroup$ – adl Jan 9 at 15:32
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    $\begingroup$ ADS-B messages are not replies to Mode-S interrogations, they are automatic. When interrogated, a Mode-S transponder replies with the data requested by the interrogation defined by the uplink format (UF) message. Some of that is discussed in the Enhanced Mode-S section of the document @RonBeyer linked. The same data broadcast by ADS-B can be requested by EHS interrogations, but the formats are slightly different and only the data requested is sent. $\endgroup$ – Gerry Jan 9 at 16:24
  • $\begingroup$ very helpful @Gerry. So if interrogated for all of the fields a transponder must reply to all of the requests and give all the info ? $\endgroup$ – adl Jan 10 at 10:27
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    $\begingroup$ A qualified yes. It will take multiple interrogations and replies and not all interrogations result in replies (for many reasons.) Here's a good primer on Mode S Surveillance Principle. The data exchange info starts at slide 16. $\endgroup$ – Gerry Jan 10 at 12:30
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From Mode-S.org

An ADS-B message is 112 bits long:

+--------+--------+-----------+--------------------------+---------+
|  DF 5  |  ** 3  |  ICAO 24  |          DATA 56         |  PI 24  |
+--------+--------+-----------+--------------------------+---------+

DF is the downlink format, ICAO is the 24-bit ICAO identifier of the aircraft, DATA is the message being transmitted, and PI is the parity/interrogator ID.

There are a number of different message types available:

enter image description here

The messages transmit things like position, altitude, turns, track, airspeed, roll angle, turn rate, etc. But they do not contain any information about the originating airport or the destination.

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