AIS Transponder

What sets AIS apart from other navigation equipment

Radar and ECDIS build a picture of the world around the ship from reflected energy or stored charts. AIS works the other way: the transponder transmits the vessel's own identity, position, course, speed and voyage data over VHF at regular intervals, and receives the same broadcasts from every other AIS-fitted vessel and shore station within range. It is a cooperative system -- it depends on other ships transmitting honestly and correctly -- which is why it supplements rather than replaces radar and visual lookout, not the other way round.

Main components

Transceiver unit

Transmits and receives on two dedicated VHF channels simultaneously, using Self-Organising Time Division Multiple Access to share the channel with every other AIS unit in range without collisions.

GNSS position input

A dedicated internal or external GPS receiver providing the position and time reference the transponder broadcasts; loss of this input degrades the AIS message quality even if transmission continues.

Pilot plug and display

A standard connector giving a visiting pilot direct access to AIS data on a laptop, and a minimum keyboard and display unit on the bridge for entering voyage-specific data such as destination and ETA.

Selection criteria

• Class A versus Class B: SOLAS vessels require Class A with its higher transmit power and reporting rate; Class B suits smaller craft not subject to SOLAS carriage requirements.
• Integration with ECDIS and VDR, since AIS targets are normally overlaid on the electronic chart and its messages are logged by the voyage data recorder.
• Antenna separation from other VHF and radar equipment to avoid mutual interference, a common installation oversight on retrofits.

Regulations and class

SOLAS Chapter V requires AIS on all ships of 300 gross tonnage and above on international voyages, cargo ships of 500 gross tonnage and above not on international voyages, and all passenger ships regardless of size. The transponder must transmit continuously except where the master judges it a security risk to do so, a provision intended for high-risk piracy areas rather than routine discretion. Class surveys check the unit's static data programming matches the vessel's actual particulars and that voyage data is updated each passage.

Typical faults

• Incorrect or stale static data such as destination, draught or ship type, which the crew forgot to update and which then misleads other traffic and VTS.
• GNSS antenna fault degrading position accuracy silently, since the transponder keeps transmitting even when its own position input has drifted.
• VHF antenna or cable degradation reducing transmit range, often only discovered when a nearby vessel or VTS reports poor reception.
• Overlapping MMSI numbers from a duplicated or incorrectly programmed identity, which corrupts the target picture for everyone tracking the vessel.

What to look for in a supplier

• Type approval matching flag state requirements and confirmed interoperability with the vessel's existing ECDIS and VDR interfaces.
• Straightforward static and voyage data entry at the bridge unit, since this is updated by the crew every passage and a clumsy interface leads to skipped updates.
• Documented antenna separation guidance to avoid interference with existing VHF and radar installations during retrofit.

Check the static data programmed into the transponder against the vessel's actual particulars at every port call, not just at commissioning -- a wrong ship type or an outdated draught quietly undermines the whole reason AIS exists.