AIS aids to navigation
1The lesson in plain words
AIS aids to navigation send a mark’s identity, position and status to your display. A virtual aid represents a mark with no physical buoy or beacon.
What it is
An AIS AtoN (aid to navigation) uses AIS (Automatic Identification System), the VHF system ships use, to broadcast information about a mark: its name, type, position and status. It sends this as Message 21, the aid-to-navigation report, normally every 3 minutes. AIS itself, its classes and its limits, is in the Electronic navigation unit.
Three kinds
A physical (real) AIS AtoN is a transmitter fitted on the mark itself. A synthetic AIS AtoN is transmitted from another station for a real mark. Monitored synthetic reports use monitored information about that mark; predicted synthetic reports use its expected position and status without monitoring. A virtual AIS AtoN is broadcast for a mark that does not physically exist.
What it tells you
Message 21 carries the mark’s MMSI (Maritime Mobile Service Identity), its nine-digit AIS identity beginning 99; its name, type, position and size. For a floating mark it includes an off-position flag, warning that the buoy is not at its assigned position. A virtual flag says whether the mark is virtual.
On the screen
On ECDIS, the ship’s official electronic chart system, and on radar, IMO symbols show an AIS AtoN as a diamond centred on its reported position. Physical and synthetic AtoN have a solid outline; a virtual AtoN has a dashed one. Leisure plotters and apps vary: some use the same symbols, some show only a label. Learn how yours draws them.
A cross-check
A physical AIS AtoN reporting off-position, or a buoy you see in a different place from its AIS symbol, tells you something is wrong. Report it to the coastguard or the buoyage authority. But AIS can fail or be faked, so an AIS symbol never replaces the mark itself, the chart and a good lookout.
2Key data
- Message 21
- AIS aid-to-navigation report, normally every 3 minutes
- Physical
- Transmitter on the mark itself
- Synthetic
- Sent from another station for a real mark
- Virtual
- Sent for a mark that does not physically exist
- ECDIS symbol
- Diamond; dashed outline when virtual
- MMSI pattern
- Nine-digit identity beginning 99; check the virtual flag.
3Key points
- 1
AIS AtoN send Message 21: the name, type, position and status of a mark.
- 2
Physical: on the mark. Synthetic: remote report for a real mark. Virtual: no physical mark.
- 3
On ECDIS an AIS AtoN is a diamond; dashed when virtual.
- 4
An off-position flag warns that a floating mark has moved.
4Frequent mistakes
Wrong: A synthetic AIS AtoN is the same as a virtual one.
Right: A synthetic AtoN stands for a real mark; a virtual one has no mark in the water.
Wrong: An AIS AtoN symbol proves the buoy is there.
Right: A physical AIS transmitter is on the mark. A monitored synthetic report comes from another station using monitored information. AIS data alone does not prove the buoy is there. Cross-check the actual mark by eye or radar.
5Practice case & exam keys
Your plotter shows a cardinal buoy’s physical AIS symbol 0.3 nautical miles (NM) east of its charted position, with an off-position flag. Your visual observations agree with the reported position. What has probably happened, and what do you do?
Show the answer
The buoy has probably dragged. Find the danger it marks on the chart; the buoy’s charted position is not necessarily the hazard’s position. Keep clear of both the charted danger and the displaced buoy. Use independent position checks, and report the discrepancy to the coastguard or buoyage authority.
Exam keys
- AIS AtoN report with Message 21.
- Physical, synthetic, virtual: on the mark, remote for a real mark, no physical mark.
- Diamond symbol; dashed outline for virtual.
- Off-position flag = floating mark not where it should be.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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