Collision Avoidance with Radar and AIS

A modern helm can tell you that a ship four miles away is called Pacific Aria, that it is making 18.4 knots on a course of 284°, and that if nothing changes it will pass 0.3 NM ahead of you in eleven minutes.

None of that tells you what to do.

The decision belongs to the International Regulations for Preventing Collisions at Sea — the COLREGs — and to the judgement of the person on watch. Electronics change how early and how well you can see the problem. They do not change the rules, and they introduce a few new ways to be confidently wrong.

#What each sensor actually contributes

The useful habit is to treat these as four opinions that should agree. When radar shows a strong target and AIS shows nothing there, that is information: it is probably a vessel without AIS, and it deserves more attention rather than less.

#Detecting risk of collision

The regulations describe the process in a way that maps neatly onto modern equipment: use all available means appropriate to the conditions, and assume risk of collision exists if you cannot be sure it does not.

In practice, on a boat with radar and AIS, that means:

1. maintain a visual and aural lookout that is not interrupted by screen work
2. keep radar running in poor visibility and at night, not just when you feel uncertain
3. watch compass bearing trend on visual targets — a steady bearing with decreasing range is the classic collision signature
4. use radar plotting or MARPA to derive course, speed, CPA and TCPA for targets that matter
5. read AIS for identity and vector on cooperating traffic
6. decide early, act decisively, and verify the result

Step six is where most electronics-equipped boats go wrong. Screens invite monitoring. A developing close-quarters situation wants a manoeuvre.

#CPA and TCPA are predictions, not facts

Target:  bulk carrier, 4.6 NM
CPA:     0.2 NM
TCPA:    11 min

That is a target requiring action. But the number was produced by assuming both vessels maintain approximately their present vectors, and it is only as good as its inputs:

• your own heading, from a sensor that may be badly sited or poorly calibrated
• your own speed over ground
• a stable radar return, which sea clutter and rain can spoil
• the target's transmitted or observed vector

Change your own course and every CPA on the screen recalculates. That is correct behaviour, but it means a CPA you improved by turning is a plan, not an outcome. Watch the target until the situation is genuinely resolved.

Automated vectors also degrade quietly. A target that manoeuvres invalidates its own prediction, and a vessel altering slowly can hold an apparently safe CPA while steadily worsening the real situation.

#The traps specific to electronic aids

#Target swap

Two targets crossing close together can exchange identities in a tracker. The label stays; the vector becomes nonsense. If a tracked target's behaviour suddenly changes character, re-acquire it rather than trusting the history.

#Heading error propagating into everything

A heading offset rotates the radar overlay, biases MARPA solutions and corrupts true wind. This is why heading sensors belong in the collision-avoidance conversation at all: a 6° heading error is invisible on the display and material in a CPA calculation.

#AIS alarm fatigue

The fastest way to make AIS useless is to leave offshore alarm thresholds running through a busy harbour, get flooded, and switch the alarms off permanently. Build environment profiles instead.

#Trusting a clear screen

Radar can miss kayaks, small timber boats, low fibreglass dinghies, floating debris and partially submerged objects. A clear screen is weak evidence, not proof.

#Internet AIS

Web-based vessel tracking is useful for planning and shoreside awareness, and it can be delayed or incomplete. It is not a substitute for onboard AIS for collision avoidance.

#Restricted visibility changes the problem

In or near restricted visibility the regulations impose specific obligations: a safe speed, engines ready for manoeuvre, sound signals, and the requirement to take action when a close-quarters situation is developing with a vessel detected by radar alone.

Two points matter for an electronics-equipped recreational boat.

First, the crossing, overtaking and head-on rules that apply to vessels in sight of one another do not apply the same way when you have not seen the other vessel. A radar contact in fog is not a "give-way vessel" you can expect to behave predictably — you are obliged to act.

Second, safe speed is a real constraint, not a formality. Your stopping distance, radar detection range, sea state and traffic density all feed into it. A yacht under autopilot making passage speed in fog with nobody watching the radar has already made the important mistake.

Read the night and restricted visibility guide for the watchkeeping side of this.

#Manoeuvring in a way radar can read

Because the other vessel may be assessing you by radar plot or ARPA, the quality of your manoeuvre matters:

• Make it early. Small ships take minutes to respond and their crews need time to interpret
• Make it large. A 5° alteration is hard to detect on a radar plot. A 30° alteration is unambiguous
• Make it one manoeuvre. A sequence of small adjustments produces a confusing track
• Avoid altering to port for a vessel on your port side in a crossing situation — it is exactly the manoeuvre the rules discourage, and it closes the range.
• Hold the new course long enough for the change to be visible and for the CPA to actually open

Sailing yachts have an extra obligation here: if you are the stand-on vessel but the situation is deteriorating, the rules still require action when collision cannot be avoided by the other vessel's action alone.

#A workable watch routine

For a shorthanded offshore boat at night:

1. visual sweep, including astern, every few minutes
2. radar sweep on a short range and a medium range
3. check AIS list for new targets and any CPA under your threshold
4. confirm depth and position against the plan
5. note anything requiring a decision inside the next half hour

The cycle should take under two minutes and should not require sitting at the chart table.

#What to carry, and what to practise

Equipment that genuinely helps:

• solid-state Doppler radar, which highlights closing targets automatically
• a transmitting Class B AIS, preferably SOTDMA offshore
• an accurate heading source feeding both
• DSC VHF with the correct MMSI programmed
• thermal imaging on fast night-running vessels

Skill that genuinely helps, and costs nothing:

• identify headlands, buoys, yachts, ships and rain on radar in clear daylight
• compare visual, chart, radar and AIS on the same target until the mental model is automatic
• practise a MARPA acquisition before you need one in fog
• rehearse the harbour, coastal and offshore alarm profiles

#Bottom line

Radar and AIS give a recreational skipper detection and identification that would have been extraordinary a generation ago. They do not transfer the decision. Use them to see the situation earlier, then apply the collision regulations deliberately, early and visibly — and keep someone looking out of the window.