Marine Radar and AIS Explained: What Live Vessel Maps Show You

Fog lifts slowly off a harbour entrance. On the screen, a bright echo sits two miles off the starboard bow, and beside it a small triangle labelled with a vessel name, a course and a speed. Both refer to the same ship. One is a measurement, the other is a radio message. Knowing which is which changes the way you approach a narrow channel on a grey morning, and it is the single most useful distinction in coastal navigation today.

Two Different Questions: Where Is It, and Who Is It?

Radar asks the sea a physical question: is there something out there that reflects my pulse, and exactly how far and in which direction is it? AIS asks a social one: which vessel is broadcasting on this frequency, where does its receiver say it is, and what does it say it plans to do?

The first answer is evidence. The second is a report. Most of the confusion around live vessel maps, whether on a bridge display or a public website, comes from treating the report as if it were evidence.

What a Radar Echo Really Tells You

A marine radar sends out short pulses of microwave energy and listens for what bounces back. The strength of that return depends on the target's size, its material, and the angle it presents. A steel coaster broadside gives a fat, unmistakable paint. A wooden dinghy, a kayak, a slack sail edge-on or a floating pallet may give almost nothing until you are close enough for the sea clutter to swallow it anyway.

That is the honest limitation, and it is worth respecting. Radar works in darkness and fog, needs no cooperation from the other vessel, and gives you true bearing and range to whatever it can paint. It also shows things that never transmit anything at all: pot buoys, drifting timber, a breakwater, the unlit hull of a fishing boat.

• Paints well: steel ships, large trawlers, breakwaters, buoys with radar reflectors, anything with a big flat vertical surface.
• Paints poorly: small open boats, fibreglass hulls with no reflector, low floating debris, sails seen edge-on.
• Disappears into: sea clutter in a chop, rain clutter in a squall, and the shadow sectors behind your own mast or superstructure.

Your radar horizon is another hard boundary. A target low in the water stays hidden until it lifts above the curve, however powerful the set. Interpretation matters as much as hardware: gain, sea clutter and rain clutter controls set badly will either flood the screen or erase the small stuff you most need to see.

What AIS Adds, and What It Cannot Do

AIS is a VHF data link. Each equipped vessel broadcasts its identity, type, dimensions and a position taken from a satellite navigation receiver, along with course and speed over ground, heading, and often a destination and estimated arrival. Commercial ships carry Class A units; many leisure craft carry simpler Class B units with slower update rates.

What you gain is enormous in one specific area: identification. You get a name to call on the radio instead of "the vessel off my starboard bow". You get a reported course and speed you can use to estimate closest point of approach, and often a destination that hints at what the other watchkeeper is trying to do.

What you do not get is a guarantee. AIS only shows vessels that have a working, switched-on transponder. Position comes from a receiver that can be misconfigured or offset, and the text fields are typed by humans who make mistakes — a destination left over from the last voyage, a wrong vessel type, a status still set to moored. Transponders fail, get switched off, or are fed by an antenna that is not where the position was calibrated. And plenty of things at sea carry no AIS at all.

Where the Two Pictures Disagree

Heading is not course over ground

AIS heading comes from a compass; course over ground comes from the track the receiver has actually travelled. In a cross-current or a stiff beam wind, the two diverge, sometimes by a lot. The direction a symbol appears to point is not always the direction the hull is aimed, and neither is automatically the direction that matters for collision avoidance. The echo, plotted over time, tells you what the vessel is really doing relative to you.

Stale and misconfigured data

Treat the text as a claim and the echo as proof. If the AIS says a ship is doing twelve knots and the echo has not moved relative to a fixed point in ten minutes, believe the echo. If the reported position sits a cable off the visible hull, the antenna offset or the position source is wrong.

Targets that appear on only one display

Radar-only targets are common near the coast: small fishing boats, ribs, dinghies, unlit floats. AIS-only targets happen too — a vessel masked by your own superstructure, one sitting in a clutter patch, or one just beyond radar range whose VHF signal still reaches you. Absence on one screen is never proof of absence at sea.

Reading a Public Live Vessel Map With the Right Expectations

The vessel maps most people meet first are shore-based networks: receivers on headlands, harbours and rooftops feeding positions to a website. Coverage depends on where those receivers are, so a boat can vanish behind a headland and reappear a mile later. Updates arrive with a delay that varies with the service and the vessel's class, and a slow Class B craft may report only every half minute or so.

Useful for understanding traffic patterns, timing a departure around a ferry schedule, or identifying a ship in a photograph. Not a navigation instrument, and not something to plan a close-quarters passage with. A symbol that jumps or draws a suspiciously straight line is usually a coverage or timing artefact, not a vessel behaving strangely.

Practical Habits for Coastal Navigation

1. Set the radar up properly before you need it. Adjust gain, sea and rain clutter in clear conditions so you know what normal looks like, and learn your blind sectors.
2. Identify with AIS, verify with radar. Confirm range and bearing on the echo before you act on a symbol.
3. Compare the plots. If the AIS closest-point-of-approach and your own radar or ARPA plot disagree, find out why before you alter course.
4. Use the name. A short VHF call using the vessel's AIS name resolves most ambiguous encounters in seconds.
5. Look for what is not transmitting. In fog near anchorages, slipways or pot-buoy fields, slow down for the targets that will never appear as a symbol.
6. Note mismatches. A symbol with no echo at close range, or an echo with no symbol in open water, deserves a second look and, where it matters, a report.

The Rule to Carry With You

Let the echo lead and the symbol support. Radar tells you what is out there and where it is; AIS tells you who it is and what they say they are doing. When the two agree, you can act with confidence. When they disagree, slow down, keep watching, and trust the measurement over the message. That habit costs nothing and pays off in exactly the conditions where a live vessel map is least reliable: fog, rain, a crowded harbour mouth, and the small boat that never had a transponder in the first place.

Photo: Ninh Tien Dat / Pexels