AIS Aid to Navigation (AtoN) Types: Physical (Real) | Virtual | Synthetic

An AIS AtoN is a digital aid to navigation that is broadcast by an authorized service provider using the AIS Message 21 (Aids-to-navigation report) and is displayed on navigation equipment such as the Electronic Chart Display Information System ( ECDIS), radar, or an Integrated Navigation System ( INS) ( International Maritime Organization ( IMO) MSC.1/Circ.1473 ).

The IMO, considering the mariner’s perspective, has recognized three types of AIS AtoN:

• A Physical AIS AtoN (more commonly known as a Real AIS AtoN) is an AIS Message 21 transmitted from an AtoN that physically exists; and
• A Virtual AIS AtoN is transmitted as a Message 21 for an AtoN that does not physically exist.
• A Synthetic AIS ATON is a physical AtoN without an AIS transmitter, but for which AIS messages are broadcast from another AIS unit, usually land-based.

The International Association of Marine Aids to Navigation and Lighthouse ( IALA), having assessed the possibilities of AIS technology according to AtoN service providers (national authorities), has proposed the addition of a third type: the Synthetic AIS AtoN.

In accordance with IALA Recommendation A-126, Synthetic AIS AtoN are divided into two types: monitored and predicted.

• A Monitored Synthetic AIS AtoN is transmitted as a Message 21 from an AIS station that is located at a distance from the AtoN. The AtoN physically exists and there is a communication link between the AIS station and the AtoN. This communication link confirms the position and status of the AtoN. A Monitored Synthetic AIS AtoN ensures the integrity of the Message 21 and can be considered an alternative to the Real AIS AtoN.
• A Predicted Synthetic AIS AtoN is transmitted as a Message 21 from an AIS station located at a distance from the AtoN. The AtoN physically exists, but it is not equipped with a monitoring device to confirm its position and status. A Predicted Synthetic AIS AtoN cannot ensure the integrity of the Message 21 (position and status), and therefore it is not recommended for use on a floating aid. The use of a Predicted Synthetic AIS AtoN on a fixed aid is acceptable, as the position will not change, but the status of the AtoN is not verified. .

Because information is transmitted using radio frequencies, each type of AIS AtoN must include Maritime Mobile Service Identity ( MMSI) number, which is obtained from Innovation, Science and Economic Development Canada ( ISED). As there is currently no recognized symbol for Synthetic AIS AtoN, the distinction between each type is made only by the MMSI number, the AIS AtoN coding parameters (virtual: yes/no), and the positioning mode.

A diamond shaped symbol is used to represent an AIS AtoN on the chart and radar systems that are interfaced with the AIS. Real and Synthetic AIS AtoN use a diamond shape with a solid line, while Virtual AIS AtoN use a dotted line (see Figure 2-1). The AIS AtoN information is displayed when you click the AIS AtoN symbol or move the cursor over it. The displayed description is the only way to distinguish a Real AIS AtoN from a Synthetic or Virtual AIS AtoN (see Figure 2-2).