Definition
Transmitted in message 21, giving the mark's name, type, position, dimensions and an off-position indicator. A real AtoN carries a physical transponder. A synthetic AtoN is transmitted on behalf of an existing mark from elsewhere, while a virtual AtoN has no physical object at all and is used to mark new wrecks or temporary hazards rapidly. Displays show them with a distinctive diamond-based symbol rather than a ship triangle.
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