Ship Call Signs

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Ship Call Signs

A ship’s call sign identifies a radio station in maritime radio communications. It is used to address or identify the station over the radio; it is neither the ship’s name nor a number from the MMSI or IMO number fields. These details may appear side by side in technical ship data because they serve different identification purposes.

Distinguishing call signs, MMSIs and IMO numbers

The International Telecommunication Union (ITU) lists ship name, call sign and MMSI as separate details in its List of Ship Stations. A call sign is formed from the international call sign series in accordance with the Radio Regulations. In the database, an equals sign indicates that the ship’s name is used to identify the station in radiotelephony. This shows the connection between the name and the radio station, but does not make the name an additional call sign.

• Call sign: The identifier for the radio station that is given in radio communications. The ITU lists it separately from the name and the MMSI.
• MMSI: The “Maritime Mobile Service Identity” is a nine-digit identifier transmitted by radio for maritime stations. For a ship station, the first three digits form the Maritime Identification Digits (MID); they identify the administration responsible for the station.
• IMO number: Identifies the ship as a separate identification field. It is not the station’s radio identifier and should not be confused with the MMSI.

The article on ship registries and flag states explains how ship identity and registration are connected.

Separate fields in AIS too

The IMO guidelines for shipborne automatic identification systems (AIS) list MMSI, “Call sign and name,” and “IMO Number” as separate static information fields. AIS transmits these details along with other data so that a received target can be identified. The IMO names the ship’s name or call sign as possible details for identifying a target. An AIS display therefore brings the values together without combining them into a single identifier.

The guidelines also distinguish between static, dynamic and voyage-related information. Static details are entered into AIS during installation; dynamic data are updated by connected ship sensors, except for navigational status. Voyage-related data can be updated manually during a voyage. The call sign is therefore among the static details, not the movement data continuously refreshed by sensors.

For static data, the IMO specifies particular reasons for updating them: a change to the ship’s name or MMSI, a change in the position of the electronic position-fixing system’s antenna, or a major conversion to another ship type. The table adds that the MMSI and “Call sign and name” may need to be adjusted when ownership changes. This describes a possible update, not automatic reassignment after every change of ownership. The IMO number remains a separate field in the same table.

Further background on the structure and purpose of the MMSI and on AIS in relation to radar and nautical charts can help put these identifiers in context within the relevant ship data.