Radar, AIS, and Electronic Navigation

Prerequisites

Before this lesson you should:

Be able to fix position and plan passages by traditional methods (see Advanced Chartwork and Coastal Passage Planning).
Know the COLREGs, especially Rules 5 to 8 (lookout, safe speed, risk of collision, action to avoid collision), Rules 13 to 17 and Rule 19 (restricted visibility).
Hold, or be studying for, the RYA SRC (VHF/DSC) certificate, which the Yachtmaster Coastal exam requires.

The RYA one-day Radar course is strongly recommended for Coastal Skipper candidates, and you may be asked on the practical exam to use radar for collision avoidance or pilotage if the yacht has one. The Yachtmaster Coastal candidate is expected to use electronic aids competently while never depending on them alone.

Learning Objectives

By the end of this lesson you will be able to:

Set up a radar for the conditions: range, gain, sea and rain clutter, and tuning (syllabus: radar).
Identify radar targets and understand why some are hard to detect.
Use VRM, EBL and plotting to assess risk of collision, CPA and TCPA, in line with COLREG Rule 7.
Understand AIS classes, data and limitations, and correlate AIS with radar.
Use parallel indexing for pilotage.
Configure a GNSS chartplotter: datum, alarms, MOB, charts and layers (syllabus: electronic navigation).
Explain GNSS vulnerabilities (jamming, spoofing, multipath, datum errors) and how to mitigate them.
Integrate electronic and traditional navigation with a paper backup.

RYA syllabus items covered (Yachtmaster Coastal / Coastal Skipper, sections "Radar and AIS usage" and "Electronic navigation", with the RYA Radar course content they build on):

Radar controls: gain, tuning, sea and rain clutter; display modes (head-up, course-up, north-up, relative and true motion); range scale selection; guard zones and alarms.
Target identification and radar cross-section; radar reflectors and their limits.
Radar-assisted pilotage: parallel indexing, radar-conspicuous features, VRM ranges and EBL bearings, and heading-input errors.
Collision avoidance: action thresholds, CPA and TCPA, substantial and readily apparent action (Rule 8), verification of the result, VHF coordination.
AIS: Class A and Class B limits, CPA/TCPA monitoring and alarms, correlation with radar and visual lookout, AIS MOB devices.
GPS and chartplotter proficiency: waypoints and routes, XTE, anchor alarm, data overlays; WGS84 datum awareness, chart edition and update checks.
Backup requirements: paper chart competency, an independent handheld GNSS, and the redundancy philosophy of never relying on one electronic source.

Part 1: Radar

How radar works

A rotating antenna transmits short microwave pulses (X-band, around 9 GHz, on small craft) and measures the time for echoes to return. Range comes from the echo time, bearing from the antenna direction. Range is therefore much more accurate than bearing: a small-boat radar may have a horizontal beam width of 4 to 6 degrees, which smears targets sideways. Radar is line of sight, so the horizon limits detection: with the scanner 4 m up and a target 10 m high, the radar horizon is roughly 2.2 × (√4 + √10) ≈ 11 nm.

The photograph below shows a typical bridge radar: range rings, a heading line, an electronic bearing line, vector-bearing targets and the data panel with gain, rain and sea controls at the top left. A yacht set is smaller, but the controls and the picture are the same in principle.

A marine radar display on a ship's bridge, showing the range rings, heading line, EBL and target echoes

Image: Hervé Cozanet, CC BY-SA 3.0, via Wikimedia Commons

Display modes: orientation and motion

Choose the display orientation deliberately, because it decides how easily you can plot and how you interpret bearings.

Mode	What is at the top of the screen	Bearings read	Best use
Head-up	Your heading, always	Relative to your bow	Simple, matches what you see ahead, but the picture smears when you yaw, and bearings need heading added
Course-up	Your intended course (set when you select it)	Relative to course; true if you add the course	Steadier picture than head-up; the usual choice for pilotage
North-up	True north	True bearings read directly	Best for plotting, comparing with the chart and parallel indexing; needs a compass or heading sensor input

Relative motion shows your own boat fixed at the centre and everything else moving past it: the echo of a stationary buoy moves in the opposite direction to your own course. True motion shows your own ship moving across the screen with the land fixed, which needs speed and heading inputs and is less common on small craft. For collision avoidance, the relative motion picture is what tells you the CPA directly, because the line a target traces relative to you is the line along which it will pass.

A stabilised display (north-up or course-up) needs a good heading feed. A sluggish or wrongly aligned fluxgate compass produces smeared echoes, wrong bearings and a wrong radar overlay, so check heading alignment by comparing a charted headland or pier on the radar with the chart.

Range selection

Use the shortest range that shows all relevant targets, and switch periodically to a longer range to pick up approaching ships early. Typical: 1.5 to 3 nm in confined waters and pilotage; 6 to 12 nm in open water for collision avoidance. A ship closing at 20 knots covers 6 nm in 18 minutes, so a 3 nm range alone gives too little warning.

Gain

Gain is the receiver's amplification. Too low and weak echoes (small boats, buoys) vanish; too high and the screen fills with noise. Set it on a longer range so that a light background speckle is just visible, then back off slightly. Readjust whenever the range, weather or target changes.

Sea clutter

Waves near the boat reflect radar energy, producing a bright clutter around the centre of the screen. Sea clutter control (STC) reduces gain at short range. Increase it gradually until the wave returns just disappear but small targets remain. Too much and you will remove the yacht or buoy you most need to see. Automatic modes help but are not infallible.

Rain clutter

Rain and showers appear as amorphous blobs that can hide ships inside them. Rain clutter control (FTC) shortens echoes so that hard targets stand out. Periodically vary it to check whether a vessel is hiding in a squall.

Tuning

Most modern sets tune automatically. If echoes seem weak or inconsistent, select a known target on a medium range and adjust the tuning for the strongest echo.

Identifying targets

Radar target types: land, vessel, buoy and rain squall

Target	Appearance
Land	Solid, stable, matches the charted coastline (but low beaches may not show; the radar sees the cliffs behind)
Ship	Moves relative to land; steel ships give strong echoes
Yacht	Weak, intermittent; GRP and wood reflect poorly unless a radar reflector or active target enhancer is fitted
Buoy	Small, sometimes intermittent; racon buoys show a Morse coded flash outward from the echo
Rain/squall	Fuzzy, changing shape, moving with the wind

A ship's aspect affects its echo strength.

Vessel aspect and radar return: beam-on versus bow-on

Beam-on, a ship presents a large reflecting area; bow-on or stern-on it can be surprisingly faint. This is one reason a small yacht heading towards you may not appear until close.

Radar reflectors and being seen

Your own visibility on other vessels' radars matters as much as your use of your own set. SOLAS Chapter V Regulation 19 (applied to small craft through MCA guidance, MGN 599) expects vessels under 150 GT to carry a radar reflector where practicable, one that works at both 9 GHz (X-band) and 3 GHz (S-band), mounted as high as possible. The classic octahedral metal reflector is shown below. Hung in the "catch-rain" orientation it presents the best response; a tilted or sagging reflector, or one shadowed by the mast or sails, loses much of its effect.

An octahedral radar reflector hung in a yacht's rigging

Image: Tim Sheerman-Chase, CC BY 2.0, via Wikimedia Commons

Practical points:

Passive reflectors do not make a yacht easy to see. Tests show many "reflectors" give an echo no better than a small plastic boat, especially when the yacht heels and the reflector tilts. Never assume a ship has seen you.
An active radar target enhancer (RTE) amplifies and returns the pulse, giving a much stronger echo, and is a worthwhile upgrade for offshore and fog.
A radar reflector does not replace lights, a fog signal or a visual lookout. In fog, keep the lookout, sound the signal and keep clear of shipping lanes.
AIS Class B on your own yacht also makes you visible, on the ship's AIS display, and is the best defence in a crowded shipping lane.

Guard zones and alarms

A guard zone is a sector or ring around your boat. Any target entering it triggers an audible alarm. Set it to a ring or arc covering the area from which a threat could reach you, usually from about 0.5 nm out to 3 nm on a yacht, wide enough to give warning but not so tight that every wave or buoy sets it off. Use the zone to give yourself a safety net while you concentrate elsewhere, never as an alternative to a visual lookout. Alarm fatigue is real: if the alarm sounds constantly, adjust sea clutter or the zone rather than switching it off.

Remember an alarm only tells you a target is there. You still have to look, plot and decide.

VRM, EBL and the cursor

VRM and EBL for measuring range and bearing

The Variable Range Marker measures range; the Electronic Bearing Line measures bearing. On a head-up display, bearings are relative to the ship's head: add your heading to get a true (or magnetic) bearing. North-up or course-up displays, stabilised by a heading sensor, give true bearings directly and make plotting much easier.

Systematic scanning

Systematic radar scanning pattern

Sweep the whole display every 30 to 60 seconds, including the edges and the clutter area near the centre. Pause on each contact to assess its bearing movement and range. Rule 5 requires a proper lookout by "all available means", which includes using the radar properly if you have one, but the radar does not replace the eyes on deck.

Collision avoidance with radar

Rule 7 says risk of collision exists if the compass bearing of an approaching vessel does not appreciably change, and warns against "assumptions made on scanty information, especially scanty radar information". Rule 19 governs restricted visibility, where you cannot see the other vessel.

CPA and TCPA, and alarm thresholds

CPA (Closest Point of Approach): how close the target will pass.
TCPA (Time to CPA): how long until it does.
Constant bearing and decreasing range means risk of collision.

A sensible yacht alarm setting in open water is CPA under 0.5 nm and TCPA under 12 minutes, tighter in confined waters. Respond to every alarm; do not silence warnings just because they are annoying.

Manual plotting

ARPA/MARPA does the work automatically but needs a stable heading input and can lose small targets. Every radar user should be able to plot by hand.

Manual radar plotting: plotting echoes and deriving relative motion

Mark the target's position (range and bearing) at regular intervals, usually every 3 minutes, on a plotting sheet or a chinagraph on the screen.
Join the plots: this is the relative motion line. Extend it past the centre; the closest it passes to the centre is the CPA.
The time to CPA follows from the speed along the line.
To find the target's true course and speed, draw your own vessel's movement for the same interval backwards from the first plot (the "O-W-A" triangle: O is the first plot, W is where the target would be had only you moved, A is the actual later plot). W to A is the target's true movement.

Worked example. At 1000 a target bears 045°T at 6.0 nm. At 1006 it bears 045°T at 4.8 nm. Bearing steady, range closing 1.2 nm in 6 minutes, i.e. 12 knots of closing speed. TCPA is 24 minutes, CPA is zero. Risk of collision exists.

MARPA and ARPA tracking

Most modern yacht radars offer MARPA (Mini Automatic Radar Plotting Aid) or an ARPA-like function: you acquire a target with the cursor and the set calculates its true course, speed, CPA and TCPA, and draws a vector. Points to know:

Vectors can be relative or true. Relative vectors show where the target moves relative to you (and their intersection with your own position shows CPA). True vectors show the target's actual course and speed, which helps you apply the COLREGs.
Tracking needs several scans to settle. Do not trust the first numbers; wait for a stable vector.
Targets can be lost or swapped when two echoes pass close together, especially in clutter. Check that the tracked target still matches the echo.
The calculation depends on accurate heading and speed input. A wrong log or heading sensor corrupts every vector.
Use ARPA information as an aid. Rule 7 specifically warns against assumptions on scanty information, and plotting by hand remains the examinable check.

Action thresholds and verifying the result

Rule 8 requires action to avoid collision to be positive, made in ample time and with due regard to good seamanship, large enough to be readily apparent to another vessel observing visually or by radar, and a succession of small alterations should be avoided. For a yacht in open water:

Decide your action threshold in advance, for example act if CPA is less than 1 nm and TCPA is below 20 minutes in open water, tighter (0.3 to 0.5 nm) in confined waters.
Make a bold alteration of at least 30 degrees, preferably 60, or reduce speed substantially, as the circumstances and the rules allow.
Continue to plot after the manoeuvre. Check that the CPA is opening and the bearing is drawing. Rule 8 requires you to check that the action is effective until the other vessel is finally past and clear.
If you are the stand-on vessel in clear visibility, you must keep your course and speed but may act under Rule 17 when it becomes apparent the give-way vessel is not acting appropriately.
If uncertain of intentions, a VHF call (preferably using the identity from AIS) can clarify. Keep it short and avoid agreeing manoeuvres you cannot see executed. Do not use VHF as a substitute for the Rules.

Rule 6 on safe speed expressly lists radar factors: the characteristics, efficiency and limitations of the radar equipment, the radar range scale in use, the effect of sea state, weather and other interference, and the possibility that small vessels, ice and other floating objects may not be detected at adequate range. A safe speed in fog is one at which you can stop in half the distance of visibility, and certainly one that gives you time to take effective action.

Aspect and COLREG decisions

Aspect determination and the give-way decision

In clear visibility, the aspect of a target (which side and how much of it you see) tells you which crossing rule applies. In the case shown, the target is on your starboard bow on a steady bearing: you are the give-way vessel under Rule 15 and must take early, substantial action, normally by altering to starboard to pass astern of it.

In restricted visibility, Rule 19 replaces Rules 11 to 18: there is no stand-on or give-way vessel. If you detect a vessel by radar alone and a close-quarters situation is developing, you must take avoiding action in ample time, and, if the action is an alteration of course, avoid:

altering to port for a vessel forward of the beam, other than one being overtaken;
altering towards a vessel abeam or abaft the beam.

In practice this usually means altering to starboard for a vessel ahead or on the starboard bow, and away from a vessel abeam or abaft the beam. Make alterations large (at least 30 to 60 degrees) so they are obvious on the other ship's radar. Consider reducing speed: Rule 19 requires vessels to slow to bare steerageway if they hear a fog signal forward of the beam.

Parallel indexing

Parallel indexing: keeping a safe offset from a coastline

Measure on the chart the perpendicular distance from your intended track to a radar-conspicuous object. On the radar (north-up or course-up), set an index line parallel to the track at that offset. If the object's echo moves along the line, you are on track; if it drifts inside, you are being set towards it. Set up the lines before visibility closes in.

Part 2: AIS

Classes

	Class A	Class B
Carriage	Mandatory for SOLAS ships of 300 GT and over on international voyages, all passenger ships, and many fishing vessels over 15 m in EU waters	Voluntary, leisure and small craft
Report interval when moving	2 to 10 seconds (faster when turning)	30 seconds (CS); faster for SO units
Power	12.5 W	2 W (CS) or 5 W (SO)
Data	MMSI, name, call sign, type, dimensions, draught, destination, ETA, navigational status, COG, SOG, heading, rate of turn	MMSI, name, type, dimensions, COG, SOG, heading

Busy areas may cause Class B (CS) transmissions to be dropped to make room for Class A traffic, so a nearby yacht may disappear from your screen.

Limitations

Not all vessels carry AIS: many small fishing boats, yachts, warships and some leisure vessels do not, or switch it off.
AIS position comes from the other vessel's GNSS, not your measurement; errors and even deliberate falsification are possible.
Equipment, antenna or power failures make vessels vanish.
Data latency: a 30-second Class B update on a 25-knot RIB means 400 m of movement between reports.
Static data (destination, status) is entered by hand and often wrong.

AIS is a superb supplement, giving identity (useful for a direct VHF or DSC call) and early warning beyond radar range, but radar and eyes remain primary. The COLREGs do not mention AIS, and using it alone for collision avoidance is an assumption on scanty information.

Using AIS for CPA and TCPA, and personal AIS devices

A plotter or AIS receiver calculates CPA and TCPA for each AIS target from its reported COG and SOG and yours. Set the alarm thresholds on the AIS display (CPA 0.5 to 1 nm, TCPA 12 to 20 minutes offshore, tighter inshore) and review them as conditions change. Remember that the numbers use the other vessel's GNSS-based COG and SOG, so a vessel manoeuvring or turning gives a misleading CPA, and a ship that has altered course will take a scan or two to show it.

Beyond the identity you get a ready-made VHF call: the MMSI can be used for a DSC individual call (which then moves to a working channel), or you can call the vessel by name on a working channel agreed or listed for the area, avoiding Channel 16 for routine traffic. In busy lanes such as the Dover Strait, a Class B transponder on your own yacht lets the ship's officer see and avoid you.

Personal AIS devices for man overboard recovery are small, lifejacket-mounted beacons that transmit on activation, appearing as an alarm on the yacht's AIS plotter and on nearby vessels' displays. Their range is typically 2 to 4 nm to a vessel at similar height and they are a useful addition to a lifejacket, but they do not replace the MOB button on the plotter, the DSC alert or a proper recovery drill. Know how a device appears on your own plotter, and test it according to the manufacturer's guidance, never by activating it in a way that might provoke a false distress. AIS-SART is a different device, used in a liferaft, and shows on AIS displays as a distinctive symbol; it is covered in the survival lessons.

Correlating radar and AIS

Radar and AIS correlation: matched, radar-only and AIS-only targets

Matched: radar echo with AIS symbol on it. Confirmed target with identity.
Radar only: a vessel without AIS, or a buoy or rocks. Treat as a real hazard and investigate.
AIS only: probably beyond radar range or hidden in clutter; or a virtual AIS aid to navigation; or false data.

Note any conflicts in the log for later review.

Part 3: GNSS and Chartplotters

Configuration

Before each passage, check:

Datum: WGS84 for virtually all modern charts; old charts may be on OSGB36 or ED50.
Units and bearings: True or Magnetic, knots and nautical miles, the correct variation.
Satellite status and HDOP/accuracy figure.
Alarms set and audible.
The route checked waypoint by waypoint against the paper chart at large scale.

Waypoints and route management

Create each waypoint from a position taken off the paper chart (or a large-scale electronic chart), not by tapping the screen at a small scale. Name waypoints meaningfully (for example "NAB-E" rather than "WPT 007"), and check the route leg by leg for:

the bearing and distance of each leg against your own chartwork;
shoals, wrecks, overfalls and cardinal marks within a safe offset of the leg (a safe margin is typically at least 0.5 nm offshore, more with tidal set);
tidal gates and the times at which you will pass them, so the route is practical for the tide you will actually have;
the next leg after each waypoint, so that the autopilot turn does not sweep you across a hazard.

A plotter shows course over ground (COG) and speed over ground (SOG), which already include the tide. Compare them with course steered and log speed: the difference is the tidal vector, which is useful for an estimated position check and for tidal stream confirmation.

Cross-track error in a curved channel can mislead. On a bending buoyed channel a straight-line XTE alarm may sound while you are in the middle of the channel, or fail to sound while you cut a corner. In such water, steer by the buoys and confirm on the chart.

Data overlays and information overload

Plotters can display radar, AIS, weather, depth, tide and satellite imagery together. The skill examiners look for is selecting what you need. Overlay radar and AIS for traffic work and pilotage, remove weather layers when concentrating on a harbour approach, and keep the chart readable at a scale where soundings and hazards are displayed. Check that the chart you are viewing has been updated for the current season, and that you know the chart's source and edition date.

Datum errors

Datum mismatch error between GNSS and chart

A position on the wrong datum can be displaced by 100 to 200 m in UK waters, enough to put you on a rock in a narrow entrance. Paper charts show their datum and give a "satellite-derived positions" note with any correction to apply. Check, and record in the passage plan.

Digital chart management

Digital chart layers

Vector charts let you switch layers on and off. Turning off too much can hide hazards: some plotters hide spot soundings, rocks or light sectors at small zoom levels. Always zoom in to check a route; never plan on a small-scale view. Update charts before the season and download areas for offline use. Crowd-sourced data can be valuable but must be checked against official sources.

Alarms

Cross-track error (XTE): set from the width of safe water. In a buoyed channel 100 m wide, perhaps 25 m; offshore, 0.2 to 0.5 nm.

Waypoint arrival alarm zones

Waypoint arrival: a radius of 0.1 to 0.2 nm warns you of the next alteration. Do not let the autopilot "follow route" blindly through the turn without checking the new course is clear.

Depth alarm: minimum safe depth with tide allowance

Depth (shallow) alarm: set from your draught plus the minimum clearance you will accept, allowing for the tide and swell. Know whether the sounder shows depth below transducer, keel or surface.

Anchor alarm radius from swing circle and scope

Anchor alarm: set on the anchor's position, not the boat's, with a radius of the chain or rode length plus boat length plus a small margin. Example: 30 m of chain in 6 m of water on a 10 m boat gives roughly 40 m radius; set 45 m.

MOB function

Plotter MOB function

Pressing (or pressing and holding) the MOB button records the position and gives range and bearing back to it. Integrated systems may also trigger a DSC distress alert via the VHF; know whether yours does. Practise until every crew member can find and operate it without looking. Remember the MOB mark is the position where the person went in; in a tidal stream they drift with the water, as does the boat, so the steer back is along the bearing in the water, not over the ground.

Part 4: GNSS Vulnerabilities

Jamming

GNSS signals are extremely weak and easily overwhelmed. Military exercises (published as navigational warnings), illegal jammers in vehicles and conflict zones can wipe out positions. Symptoms: loss of fix, wildly wrong positions or speeds, many receivers failing at once (plotter, AIS, DSC radio).

Spoofing

Spoofing transmits false signals so the receiver shows a confident but wrong position. It is more dangerous than jamming because nothing appears wrong. The only defence is cross-checking with independent methods: bearings, radar ranges, depth.

Multipath

Signals reflected from cliffs, harbour walls and large ships arrive late and cause jumps of tens of metres and erratic speed readings, exactly when you are close to hazards.

Mitigation

Keep and use paper charts for the passage and harbour plans.
Keep a log with hourly positions, so a DR/EP can be resumed instantly.
Cross-check GNSS regularly against visual and radar fixes.
Have a handheld GNSS with spare batteries.
Watch for navigational warnings of GNSS interference.

Part 5: Integration

Radar overlay

Overlaying radar on the chart is powerful, but any heading sensor error or datum shift misaligns the picture. Check that fixed objects (a pier, a headland) line up with their charted positions at several ranges. If they do not, trust the raw radar picture over the chart overlay; then recalibrate.

Best practice

Safe navigation sits where electronic fixes, visual fixes, radar ranges and depth soundings agree. When one disagrees, the others tell you which is wrong. Log both electronic and traditional fixes so there is an audit trail.

Worked Example: Fog in the Approaches

You are 5 nm from a harbour at dusk when visibility drops to 200 m. You have radar, AIS and a plotter.

Sound the fog signal (under power, including motor sailing, one prolonged blast at intervals of not more than 2 minutes; under sail alone, one prolonged and two short). Reduce to a safe speed, lifejackets on, crew warned, radar reflector checked, navigation lights on.
Radar on 3 nm range, switching to 6 nm every few minutes. Gain and clutter set. Log a position from GNSS and confirm with radar ranges of two headlands: agreement within 0.1 nm.
A target appears at 040° relative, 2.8 nm. You plot it every 3 minutes: bearing steady, range closing 0.3 nm per 3 minutes, so 6 knots of closing speed. It has no AIS.
Rule 19 applies. The target is forward of the beam on the starboard side. You must not alter to port. You make a bold 60° alteration to starboard and slow down. After two more plots the bearing is drawing aft and CPA is 0.8 nm.
For the entry, you use parallel index lines prepared earlier off the breakwater, ranges by VRM, and depth checks at each waypoint. You call the harbour office on VHF to report your entry.

Worked Example: Radar Target, CPA 0.2 nm, No AIS

This is the kind of scenario the RYA examiner may pose verbally. Your radar shows a target at 3 nm, CPA 0.2 nm, TCPA 18 minutes, with no AIS. Visibility is poor, about 1 nm, and you are motoring at 6 knots on a course of 090 degrees true in open water.

Do not accept the numbers at face value. Check the target is being tracked correctly, then plot by hand: three plots at 3-minute intervals. If the bearing is steady and the range closing, risk of collision exists (Rule 7).
Determine which rule applies. With visibility 1 nm and the vessel not yet seen, Rule 19 applies, not the crossing rules.
Slow to a safe speed and have the crew put on lifejackets and keep a lookout and listen for fog signals.
Make a bold alteration. If the target is forward of the beam on your starboard side, alter to starboard by 60 degrees or more. Altering to port would be wrong under Rule 19(d)(i).
Check the result. Re-plot for at least 3 to 4 scans. CPA should increase beyond 1 nm and the bearing should begin to draw aft.
Resume course only when the target is clear and past, then record the event in the log.

The point examiners listen for is the sequence: plot, identify the rule, act early, make a large alteration, verify, and record. Naming the right sector of Rule 19(d) matters.

Worked Example: Datum Mismatch

Your GPS position differs from a visual fix by 0.5 nm. The chart's datum note says WGS84, but your plotter is set to OSGB36. A 0.5 nm error is more than a typical datum shift in UK waters (about 100 to 200 m between OSGB36 and WGS84), so datum alone does not fully explain it, and you should look for more than one cause: a wrong datum setting, a mis-identified visual mark, a position typed in wrongly, or poor satellite geometry. Corrective action: set the plotter to WGS84, recheck against the visual fix and a second independent method such as a radar range and a depth check, and record both in the log. Until the discrepancy is resolved, navigate by the most conservative position.

Exam-Style Tips: What the Instructor Expects

Show a methodical routine: lookout, radar on, range changed, clutter set, plot, decide.
Name the COLREG rule you are applying and explain why it applies.
Say "I would plot it by hand" even if you have ARPA, and know how to do so.
When asked about electronics, always include the backup: paper chart, handheld GNSS, DR and log.
Mention VHF contact only as a supplement, never instead of action.
Explain what you would do if the radar fails: reduce speed, increase lookout, use the sounder, use the paper chart and sound signals.

Safety Notes

Radar scanner emissions can cause burns and eye damage near the aerial at close range. Keep clear of the scanner in transmit mode and switch to standby when crew work aloft.
Do not rely on a radar overlay in a narrow entrance until you have verified alignment at several fixed objects.
Treat any GNSS inconsistency as a possible interference event and report it to the coastguard if you suspect jamming.
Keep the log. Radar and AIS information vanish when the power is lost; the log is the record.

Common Mistakes

Over-suppressing clutter and removing small targets with the waves.
Relying on AIS as if every vessel had it.
Using bearings from a head-up display without adding heading.
Small alterations that the other vessel cannot detect on its radar.
Wrong datum or uncorrected digital charts.
Planning on a zoomed-out plotter view that hides hazards.
Silencing alarms rather than responding.
No paper backup when the electronics fail.

Summary

Set radar range, gain, sea clutter, rain clutter and tuning for the conditions; check longer ranges regularly.
Radar range is more accurate than bearing; target aspect, material and size affect detection.
Steady bearing with closing range means risk of collision; plot to find CPA and TCPA.
In restricted visibility, Rule 19 applies; make large alterations, normally to starboard for vessels ahead, and never towards vessels abeam or abaft.
AIS supplements radar but many vessels have none; correlate both and treat radar-only contacts as real.
Configure the plotter carefully: datum, alarms, MOB, layers, route checks.
GNSS can be jammed, spoofed or reflected; cross-check with traditional methods and keep a paper backup.

Check Your Understanding

How should you set the radar gain?

Answer: On a longer range, increase gain until a light background speckle is just visible, then reduce slightly. Readjust when range or conditions change.

Why might a small GRP yacht not show on radar until close?

Answer: GRP reflects radar energy poorly, the yacht is small and low, and its echo may be lost in sea clutter. A radar reflector or active target enhancer greatly improves detection.

A target's bearing stays at 330°T while its range falls from 4 nm to 3 nm in 6 minutes. What does this tell you?

Answer: Steady bearing and closing range: risk of collision. Closing speed is 10 knots, so TCPA is about 18 minutes and CPA is near zero. Action is required.

In fog, a radar-only target is closing on a steady bearing on your port bow. Which way should you alter?

Answer: Rule 19 says avoid altering to port for a vessel forward of the beam (other than one being overtaken), so make a substantial alteration to starboard and consider reducing speed.

Give three limitations of AIS.

Answer: Not all vessels transmit; positions come from the other vessel's GNSS and may be wrong; Class B updates are slow (30 seconds) and may be dropped in busy areas; equipment or antenna failures; manually entered data is often wrong.

What is a radar-only contact and how should you treat it?

Answer: A radar echo with no AIS data. It could be a vessel without AIS, a buoy or a hazard. Treat it as a real target, plot it and investigate.

How would you set an anchor alarm?

Answer: Set the centre at the anchor's position, with a radius equal to the length of chain or rode let out plus the boat's length plus a small safety margin.

Why is spoofing more dangerous than jamming?

Answer: Jamming usually causes obvious loss of position. Spoofing gives a confident but false position, so nothing appears wrong unless you cross-check with independent methods.

Your radar overlay shows the pier 100 m from its charted position. What do you do?

Answer: Treat the overlay as misaligned (heading sensor or datum error). Navigate from the raw radar picture and independent fixes, and recalibrate the overlay before relying on it.

What is the purpose of parallel indexing?

Answer: To monitor cross-track position continuously on radar without plotting, by watching a fixed object's echo move along a line set parallel to the track at the planned offset.

A yacht's radar overlay is slightly rotated relative to the chart. What are the likely causes, and what is the effect on pilotage?

Answer: Heading sensor misalignment, a wrong magnetic variation or heading offset, or a datum difference. Charted features such as piers and headlands appear offset, so ranges and clearances taken from the overlay are wrong. Verify with a fixed object at several ranges, navigate from the raw radar and independent fixes, and recalibrate.

Compare the three radar display orientations and say which suits plotting and parallel indexing.

Answer: Head-up shows your bow at the top and gives relative bearings; course-up is steadier and suits pilotage; north-up gives true bearings directly and is best for plotting, chart comparison and parallel indexing, given a reliable heading input.