Radar, AIS and chart: build the picture before you go aboard

A chartplotter, a radar and an AIS screen can each make you feel well informed. The real skill is to look at all three and at the sea, and build one picture of what is going on around you. Good navigators call it situational awareness. It comes with practice, and most of that practice can be done long before a first watch.

What each one tells you, and what it hides

So never trust one alone. A yacht or a fishing boat may have no AIS. A ship on AIS may be sailing a course other than what you expect. The picture comes from comparing.

Four questions for each target

1. What is it? Does the echo match something on the AIS, on the chart, or something you can see?
2. Where is it going? Read its course and speed, or watch its echo.
3. Will it come close? CPA is the closest point of approach, how close it will pass. TCPA is the time until then. A CPA of 0.1 NM in 40 minutes is a different problem from the same CPA in 5.
4. What will I do, and when? Decide while there is plenty of time, and check that it worked.

The old rule still applies: if the compass bearing does not noticeably change as the range closes, a risk of collision exists. Check it a few times; one reading proves nothing.

Choosing the range

Beginners often stay on a single range. Look out at a long range to see what is coming, then in at a shorter one to see what matters now. Changing range is a habit worth building early.

Where to practise each piece

Everything here runs in the browser, and we want you to know exactly what is in which part:

• In the race: the radar and the chart are part of the race window. They show the course, the marks and the line to the next one, and you can switch between them. Other boats and passing ships appear as targets, so you can compare the screen with what you see out of the 3-D view, by day and at night. It is a good, low-pressure place to get used to reading a radar. Ships keep clear of you and the race boats pass through each other, so this is for looking, not for collision avoidance.
• In Free Mode: the sea has radar and AIS, the chart with CPA and TCPA on every target, and COLREG traffic that gives way as the Rules say. Fog is where the radar really earns its place. It is open to everyone on two boats, with no account needed: see Free Mode.
• In the COLREG exercises: 54 exercises check what you actually did, including a CPA/TCPA recognition task and seven restricted-visibility exercises under Rule 19. One in each module is open without a subscription.
• On the Maneuvering Board: to understand relative motion on paper, try our Maneuvering Board trainer. After it the CPA number stops being magic.

A first exercise

In the race, at night, wait for a ship to come by. Before reading any numbers, look out and name what you can see. Then find her on the radar. Note her range and bearing, then check again five minutes later. Has the bearing changed? Which way is she going? Where would she pass you?

Doing this ten times teaches you more than reading about it, and it also shows you how long it takes to get a reliable idea of another ship's movement.

What real equipment adds

The lights, the radar picture and the chart look and behave much as they do at sea. But each real set has its own controls, and tuning a real radar (gain, sea and rain clutter) takes hands-on time on board or with an instructor. A simulator is a way to practise the thinking, and no substitute for proper radar training or for the look-out required by Rule 5. When you first step aboard, you won't be starting from zero, and it will show.

FAQ

Is AIS enough without radar?

No. Only vessels that transmit appear on AIS, and small craft often do not. Radar also has limits, so use all available means, including your eyes.

What is a safe CPA?

It depends on the sea, the traffic, the vessels and your own experience. In open water many mariners want a good margin of a mile or more, but there is no single number. Decide your limit before you need it.

Related reading

• Reading ship lights at night: how to practise what you learned for the exam
• Practise passage planning from home: forecast, route and options in a real-time race
• Think like a skipper: how an online regatta becomes virtual practice for real decisions
• COLREG simulator — 54 collision-avoidance exercises on a radar, AIS and bridge-view console, in the browser
• "Chart plotter (ECDIS) simulator — plan a passage on real depths, then prove it for your boat, in the browser"
• All articles · Open the simulator

Boat types are described generically; SkipperCheck is not affiliated with any boat builder or class association.