# What information an aircraft responder must give to mode S ground receiver

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 ?

• Take a look at this: Decoding ADS-B messages. Jan 9 '20 at 15:24
• @RonBeyer, this is pretty much what I was searching for. If you post it as an answer I will accept it
Jan 9 '20 at 15:32
• 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. Jan 9 '20 at 16:24
• 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 ?
Jan 10 '20 at 10:27
• 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. Jan 10 '20 at 12:30

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:

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.