The 1090MHz Riddle

An open-access book about decoding Mode-S and ADS-B data

ADS-B versions

In this advanced chapter, we are going to look into different versions and evolution of the ADS-B.

Since the beginning of ADS-B, there have been three different versions (to my knowledge) implemented. The major reason for these updates is to enable more information (types of data) in ADS-B. Documentations on these versions and differences are quite far from user friendly. They are always presented in a very scattered fashion. Even the official ICAO_9871 document is confusing to read. I am going to try my best to put the pieces together in this chapter.

There are three versions implemented so far, starting from Version 0, then Version 1 around 2008 and Version 2 around 2012. Major changes in Version 1 and Version 2 are listed as follows:

From Version 0 to Version 1:

  • Added Type Code 28, 29, and 31 messages

    • TC=28: Aircraft status - Emergency/priority status and ACAS RA Broadcast

    • TC=29: Target state and status

    • TC=31: Operational status

  • Introduced the “Navigation integrity category (NIC)” and “Surveillance integrity level (SIL)” in addition to the “Navigation accuracy category (NAC)” from the Version 0

    • Type Code and an NIC Supplement bit (NICs) is used to define the NIC

    • NIC Supplement bit included in TC=31 messages

  • The ADS-B version number is now indicated in operation status message TC=31

From Version 1 to Version 2:

  • Re-defined the structure and content of TC=28, TC=29, and TC=31 messages.

  • Introduced two additional NIC Supplement Bit

  • NICa is defined in operational status messages (TC=31)

  • NICb is defined in airborne position messages (TC=9-18)

  • NICc is defined in operational status messages (TC=31)

  • Introduced an additional “Horizontal Containment Radius (Rc)” within NIC=6 / TC=13

Identify the ADS-B Version

There are two steps to check the ADS-B version, this is due to the fact that ADS-B Version 0 is not included in any message.

  1. Step 1: Check whether an aircraft is broadcasting ADS-B messages with TC=31 at all. If no message is ever reported, it is safe to assume that the version is Version 0

  2. Step 2: If messages with TC=31 are received, check the version numbers located in the 41-43 bit of the payload (or 73-75 bit of the message).

After identifying the right ADS-B version for an aircraft (which does not change often), one can decode related TC=28, TC=29, and TC=31 messages accordingly.

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