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What is:

  • ADS-B ICAO addresses
  • TIS-B ICAO addresses
  • ADS-B self-assigned addresses
  • TIS-B track file id

How likely is a name collision between the self-assigned address and track file id?

How likely is a name collision between the two gld90 ICAO addresses?

How likely is a name collision between the two ADS-B addresses?

How likely is a name collision between the two TIS-B addresses?

Can you give me examples of the four types?

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  • $\begingroup$ I think you are correct. I'm processing GLD90 traffic packets and trying to understand the packet. The GLD90 protocol has separate address types for those four things listed. $\endgroup$
    – codeMetis
    Commented May 22 at 1:36

1 Answer 1

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Direct answers to your questions

  • What are ADS-B ICAO addresses 🠪 What GLD 90 calls "ICAO addresses", are the permanent addresses assigned by registries from ICAO blocks. See sections ICAO 24-bit address overall and Permanent addresses.

  • What are ADS-B self-assigned addresses 🠪 The anonymous addresses used only on UAT channel. See section Anonymous address (for UAT only)

  • What are TIS-B addresses 🠪 They are permanent ICAO addresses already mentioned. Permanent addresses are used both by ADS-B and TIS-B.

  • What are TIS-B trackfile ID 🠪 The 12 random bits assigned to tracks by a ground radars for Mode A/C targets which don't use Mode S (thus don't disclose a 24-bit address). These bits are added to the 12 bits of the squawk code to build a pseudo Mode S address which can be used in TIS-B messages. See TIS-B pseudo-address section.

  • Collision between the two GLD 90 ICAO addresses. 🠪 Permanent addresses are unique for aircraft but can collide because of errors when configuring the equipment. In addition ground vehicles are allowed to share the same permanent address when they are distant by more than 1,000km. See section Duplicates.

  • Collision between addresses of all kinds 🠪 See section Duplicates, as it cannot be summarized here.

  • Examples of the four types 🠪 permanent ICAO addresses are part of blocks allocated to a State registry, and not all blocks are currently allocated. Uruguay is allocated the block of addresses starting with 0xE90 (1110 10 010 000), so 0xE90AAA (1110 10 010 000 1010 1010 1010) is a permanent address for an aircraft registered in Uruguay.

    For the other types, any valid 24-bit combination can be found as they are random. Nothing distinguishes them.

I'm adding detailed explanations below. Most of the information comes from ICAO Annex 10 volume III - Communication Systems.


ICAO 24-bit address overall

  • Aircraft, aerodrome vehicles, surface obstacles and Mode S surface equipments used for surveillance and radar monitoring can be assigned an address.

  • An aircraft is assigned a unique address. Surface vehicles and devices can share the same address, provided they are distant by more than 1,000km.

  • The address is assigned e.g. to the aircraft. It is not assigned to a particular equipment, all equipments aboard the aircraft use the same ICAO address (1090 ES and 978 UAT).

  • Valid addresses are all combinations of 24 bits, excluding all 0s and all 1s.

TIS-B pseudo-address

Addresses mentioned above are used by aircraft working with Mode S. In particular ADS-B periodic messages allow aircraft to track other aircraft positions in real time. This excludes aircraft in Mode A/C from the tracking. TIS-B is used to fill the gap.

The ground station tracking the Mode A/C aircraft sends TIS-B position reports to Mode S aircraft, related to the Mode A/C aircraft.

But in Mode S all aircraft are identified by their ICAO address, not by their Mode A/C squawk code. The ground station must temporarily identify the Mode A/C aircraft with a pseudo-address made of the 12 bits of the squawk code and 12 random bits assigned to the track.

See this question for details (with a bit of luck, @DeltaLima will post an answer here, he's a specialist in this field)

Permanent address

A permanent address is assigned by the State registry. This is a two-step process:

  • Address blocks are allocated by ICAO to State registries, per table 9.1 in Annex 10.

  • The State registry then assigns addresses to aircraft by picking in its allocated block.

Special blocks are allocated to ICAO and not available to State registries.

  • 1000-10-011-001-00 (1000-address block)
  • 1111-00-001-001-00 (1000-address block)
  • 1111-00-000 (32,000-address block)

Temporary address

ICAO block 1111-00-001-001-00 is used for temporary addresses, when users are waiting for the registering procedure to complete. Assignment is administered by ICAO, not by a State registry.

Anonymous address (for UAT only)

For privacy purposes, a pseudo-random self-assigned address can be built by the aircraft equipment, starting from the permanent address if there is one, else from the current time, see this question for details.

Duplicates

The probability to see the same address in a given area is very low, but not null, because:

  • The same permanent address can be re-used by surface equipments distant by more than 1,000km.

  • Errors can be made while configuring the permanent address in the equipment (some questions on the site relate to flight trackers identifying the wrong aircraft due to address configuration mistakes).

  • Self-assigned UAT addresses conflicting either with a permanent address or another self-assigned address. The probability is lower than the one of a permanent address duplicate.

  • Perhaps the same for self-assigned TIS-B target addresses.

GDL 90

All address types can be found in field 'aa aa aa' of a GDL 90 message. The uniqueness of this field, even combined with field 't' cannot be guaranteed. How addresses call collide has been explained in section Duplicates

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  • $\begingroup$ t=0 is any normal aircraft emitting ads-b? t=2 is any normal aircraft emitting tis-b? t=1 is what? t=3 is what? $\endgroup$
    – codeMetis
    Commented May 23 at 6:17
  • $\begingroup$ any chance you could reframe the answer around the specific address types I listed? ie t=0 through to t=4. $\endgroup$
    – codeMetis
    Commented May 23 at 6:18
  • $\begingroup$ @codeMetis: This is a bit remote from the current question focused on address assignment. You should post another question, focused on GDL 90 fields. Whatever, messages can be ADS-B or TIS-B and the address used can be from a registry (ICAO) or self-assigned according to the principles described in the answer, thus types 0-3. See §3.5.1.2. Types 4 and 5 identify a surface equipment regardless of the datalink used. I've updated the answer to explain what are TIS-B target addresses. $\endgroup$
    – mins
    Commented May 23 at 8:53
  • $\begingroup$ my apologies, I meant to ask about the 4 specific kinds of address types that I listed in my question. Sorry if that wasn't clear. $\endgroup$
    – codeMetis
    Commented May 26 at 23:35
  • $\begingroup$ @codeMetis. All your previous questions were already answered, I answered the new trackfile ID question. I also added a summary at the beginning to link the explanations to your questions. $\endgroup$
    – mins
    Commented May 27 at 8:22

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