What is a ship’s call sign?
· 3 min read
In short
A call sign is the international radio identifier of a ship station, a short combination of letters and digits such as H3RC or A8AB9. It is issued by the flag state’s radio administration from the call sign series the International Telecommunication Union allocates to that country, so its first characters identify the nationality. Like the MMSI, it belongs to the ship’s radio licence and usually changes when the ship changes flag.
How a ship’s call sign is formed
Article 19 of the ITU Radio Regulations sets the rules. Every call sign in the international series starts with two characters (two letters, or a letter and a digit, or a digit and a letter), and those first two characters, or sometimes just the first, identify the nationality. For ship stations, the Regulations allow four shapes:
| Shape | Example of the pattern |
|---|---|
| Two characters and two letters | XXAA |
| Two characters, two letters and one digit (not 0 or 1) | XXAA2 |
| Two characters (the second a letter) and four digits | XL2345 |
| Two characters, one letter and four digits | XXA2345 |
The digits 0 and 1 are not used straight after a letter, because they are easily confused with the letters O and I. A ship’s lifeboats and rafts use the parent ship’s call sign followed by two digits.
What the prefix tells you
The ITU allocates call sign series to countries in Appendix 42 of the Radio Regulations. H3A–H3Z, for example, belongs to Panama and A8A–A8Z to Liberia, so a call sign starting H3 was issued by Panama and one starting A8 by Liberia. Large registries hold several series. The ITU publishes the full table.
The prefix tells you who issued the licence, which is the flag state, not where the owner is. For the difference, see flags of convenience.
Why a call sign changes
When a ship changes flag, its old licence lapses and the new flag state issues a new call sign from its own series, together with a new MMSI. The ship’s IMO number is the only identifier that stays. The live table below lists the call signs Voydar has received with one IMO number; after a change of flag, the new call sign starts with the new flag’s prefix.
One IMO number, 2 MMSIs
Live from VoydarWhat Voydar has received over AIS from EVER GIVEN (IMO 9811000). Each row is a radio identity the ship transmitted under; the country is decoded from the MMSI’s Maritime Identification Digits.
| MMSI | MID country | Call sign | First heard | Last heard |
|---|---|---|---|---|
| 636026627 | Liberia | A8AB9 | Sep 15, 2026, 11:36 UTC | Sep 29, 2026, 05:35 UTC |
| 353136000 | Panama | H3RC | Jun 19, 2026, 18:49 UTC | Sep 15, 2026, 09:45 UTC |
Voydar AIS reception. First and last heard are when Voydar’s feeds first and last received that MMSI with this IMO number, not official registration dates.
Where you see it
- In voice radio traffic, where a ship identifies itself by name and call sign.
- In the ship’s AIS static data, which carries up to seven characters of call sign.
- In the ITU’s list of ship stations (the MARS database) and on the ship’s radio licence.
- As the ship’s signal letters, which in general agree with its call sign.
On Voydar the call sign is listed next to the IMO number and MMSI at the top of each vessel page, and the call signs a ship used before are part of its identity history.
Questions and answers
- Is a call sign the same as an MMSI?
- No. Both identify the ship’s radio station and both come from the flag state, but the call sign is letters and digits for voice and signalling, while the MMSI is nine digits for digital calling and AIS.
- Can I find a ship by its call sign?
- Often, yes, but call signs are reissued after a ship leaves a flag, so the same call sign can belong to different ships over the years. Confirm with the IMO number.
- What is the call sign of a lifeboat?
- The parent ship’s call sign followed by two digits, under the ITU Radio Regulations.
Look up a ship
Search any vessel by name, IMO number or MMSI for its free Voydar profile: live position, identity history, ownership, inspections and sanctions.
Sources
Primary sources for the facts in this guide. Figures labelled as Voydar data come from Voydar’s own records, with their method stated next to them.
- ITU Radio Regulations, Article 19, Section III: Formation of call signs
- ITU: Table of International Call Sign Series (Appendix 42)
- ITU: List of ship stations and MMSI assignments (MARS)
Published Sep 28, 2026 · For information only. Not for navigation, and not a substitute for the official sources cited.