AIS shows less than everyone: what's missing from the screen

AIS is convenient enough that you get used to it fast: names, headings, speeds on screen, and it starts to feel like what's drawn is exactly what's out there. The International Maritime Organization's guidelines on the operational use of AIS (resolution A.1106(29)) open with a warning about exactly that.

Verbatim, it says: not all vessels carry AIS, and the user must bear in mind that AIS information may not present a complete or accurate picture of the traffic around.

Who won't show up on screen

The list from that same document, and it's uncomfortably wide.

• Pleasure craft. The carriage requirement doesn't apply to everyone, and the yacht next to you may be invisible.
• Fishing vessels. The very ones that manoeuvre unpredictably and sit anchored with nets out.
• Warships.
• Some shore stations, including certain vessel traffic service centers.

A separate point: a vessel required to carry AIS may switch it off on the master's professional judgment, if continuous operation would compromise the safety or security of the vessel or of navigation. In other words, an empty sector on the screen never means empty water.

Why a target appears before its name

This isn't a glitch, it's how the system is built. The Australian Maritime Safety Authority describes the transmission rhythm like this:

• position reports go out often — every 2–10 seconds underway depending on speed, and once every 3 minutes at anchor;
• static and voyage data — name, call sign, dimensions, vessel type, draft, destination — go out once every 6 minutes.

The usual result: a return is already tracking toward you, and who it actually is only becomes clear a few minutes later. You give way to the return, not to the name.

The reverse holds too: your own return on someone else's bridge updates on the same delay, and once every three minutes at anchor.

AIS is a supplement to lookout, not a replacement

The IMO guidelines define the system's purpose: to help identify vessels, assist in target tracking, assist in search and rescue, simplify information exchange, and support situational awareness. And in the same breath, it states that AIS is a useful source of supplementary information to what navigation equipment, including radar, already provides.

The practical difference is simple. Radar sees what reflects. AIS sees what reports itself. Eyes see what's there. A fishing boat without a transponder, a buoy, a floating container, a dinghy — all of these exist only for the last two methods.

A separate note: vessel-tracking websites

Public websites and apps that show vessels on a map are built from data collected by amateur receivers. The IMO does not endorse this kind of AIS display: the receivers aren't always certified, and the data may be inaccurate or unreliable.

For curiosity's sake — fine. For resolving a close-quarters situation in a strait — no.

Another common mix-up: the Long Range Identification and Tracking system (LRIT) is not AIS and doesn't replace it. It's a different system, working over satellites and under different rules.

What to do on watch

• Keep lookout by sight and hearing as the primary method, not a backup.
• Treat an empty sector on the screen as unknown, not clear.
• Give way by bearing and range, not by the other vessel's name and type.
• In fog and at night — radar together with AIS, not instead of it.
• Remember that your own vessel appears on someone else's screen with the same delay, especially at anchor.

AIS made the sea easier to read. It didn't make it fully known.

Sources: International Maritime Organization, resolution A.1106(29) "Revised Guidelines for the Onboard Operational Use of Shipborne Automatic Identification Systems (AIS)" (copy hosted on the U.S. Coast Guard NAVCEN server); Australian Maritime Safety Authority, description of AIS operation.