AIS (Class A)

What sets Class A apart from Class B

Class A is the mandatory SOLAS transponder: it transmits at up to 12.5 watts, updates position as often as every two seconds when the vessel is manoeuvring, and carries a fixed, mandated set of static and voyage data. Class B, fitted voluntarily on smaller craft, transmits at lower power, updates less frequently and can be deprioritised by shore stations under heavy traffic. A Class A set integrates directly with the ship's ECDIS and radar to overlay other vessels' AIS targets on the plotted picture, which is the main reason it functions as a collision-avoidance tool rather than just a tracking beacon.

Main components

VHF transceiver

Transmits and receives on two dedicated marine VHF channels, using time-division multiple access so many vessels can share the channel without colliding transmissions.

GPS receiver

Provides position, course over ground and speed over ground; many units use an internal receiver but also accept an external GNSS input for redundancy.

Minimum keyboard display or integration unit

Lets the crew enter voyage data such as destination, ETA and draught, and view nearby targets, though most bridges display AIS through the ECDIS or radar rather than the display unit itself.

Pilot plug

A standard connector giving a boarding pilot's portable pilot unit direct access to the ship's AIS and navigation data.

Sensor interfaces

Inputs from the gyro compass for heading and a rate of turn indicator, plus the ship's own static data, all broadcast as part of the AIS message set.

Selection and installation

Selection is largely about integration: compatibility with the existing ECDIS and radar's AIS overlay, the interface standard commonly known as NMEA, and whether the unit supports the long-range AIS polling function some flag administrations require. Antenna placement matters more than the unit's own specification. VHF antenna separation from other transmitting antennas and a clear GPS antenna view both affect real-world range and position accuracy more than any setting in the unit itself.

Regulations and class

SOLAS V/19 requires AIS on all ships of 300 GT and upwards on international voyages, all cargo ships of 500 GT and upwards not on international voyages, and all passenger ships regardless of size. The set must be kept in continuous operation at sea, with switching off only permitted where the master judges continued operation would compromise the safety or security of the ship, an entry that has to be logged. Static and voyage data such as MMSI, call sign, IMO number, ship type, destination, ETA and draught must be kept current; an outdated destination or draught is a common port state control observation, not a technical fault.

Typical faults

• Static data left from the previous voyage or a prior owner's identity: flagged at port state control and creates confusion for VTS and other vessels.
• GPS antenna cable degraded or antenna shadowed by superstructure: intermittent position dropouts, target shown jumping on other vessels' displays.
• VHF antenna too close to radar or other VHF antennas: self-interference, reduced effective range.
• Heading input from gyro lost or uncalibrated: AIS transmits without heading, degrading other vessels' ability to judge aspect and CPA.
• Unit switched off and left off after a security transit, log entry not reversed: non-compliance discovered at the next inspection.

What to look for in a supplier

• Type approval matching the flag administration's accepted list and the ship's SOLAS category.
• Confirmed interface compatibility with the ship's existing ECDIS, radar and VDR.
• Support for the pilot plug standard and long-range AIS reporting where the trading pattern requires it.
• Firmware update and support history, since AIS message formats have been revised over time and an unsupported unit can fall out of step with shore systems.

Check the static and voyage data every time the vessel changes trade or owner, not just when something looks obviously wrong - a stale destination field is one of the most common findings in a routine PSC inspection.