Chartplotters, GPS and Introduction to Radar

Prerequisites

Before this lesson you should:

• Be able to plot a position by latitude and longitude, and by bearing and distance, on a paper chart.
• Understand course, bearing, true and magnetic, variation and deviation, and be able to work out a course to steer including tidal stream.
• Know how to estimate position by dead reckoning (DR) and estimated position (EP), and how to fix position by visual bearings, transits and depth.
• Understand chart symbols and datum, and know the meaning of knots, nautical miles and cables.
• Have done the Day Skipper lessons on charts, passage planning and collision regulations. You will need the rules of the road when the radar shows another vessel.

The RYA Day Skipper shorebased syllabus includes the use of electronic navigation aids, and the practical course expects you to use GPS and a chartplotter as part of everyday navigation. The RYA Electronic Navigation handbook and the Introduction to Radar handbook (G34) go into further detail on both subjects, and this lesson covers the same ground at Day Skipper level. Radar operation to full competence is taught on the separate RYA Radar course.

Learning Objectives

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

• Explain in plain terms how GPS works, what it gives you and how accurate it is.
• Set up a GPS or chartplotter correctly, including the datum, units, and position format.
• Create, use and manage waypoints and routes, and read the navigation data screen.
• Define and use cross-track error (XTE), COG, SOG, bearing to waypoint (BTW), distance to waypoint (DTW), VMG and ETA.
• Recognise the limitations and failure modes of GPS and chartplotters, and cross-check them.
• Describe how radar works and what its main controls do: range, gain, tune, sea clutter, rain clutter, brilliance.
• Use the electronic bearing line (EBL), variable range marker (VRM) and fixed range rings to measure range and bearing.
• Interpret a radar picture, including radar shadows, false echoes and targets that are missed.
• Understand heading-up, north-up and course-up displays, and relative or true motion.
• Understand what AIS is, what it shows, and what it does not.
• State the limitations of electronic aids and the need to keep a proper visual lookout (COLREGs Rule 5 and Rule 7).

Electronics: A Servant, Not a Master

Electronic navigation has changed how we sail. A modern chartplotter shows your position on a chart to within a few metres, updates every second, and can warn you of shallows, off-course drift and approaching ships. This is a wonderful tool, and a very dangerous one when it is trusted blindly. Each year several yachts in UK waters run aground while following a plotter confidently across a rock that was displayed but not looked at, or because the chart was zoomed out too far to show it, or because the plotter chart was out of date.

The RYA's advice is simple. Use electronics as one source of information among several. Cross-check your position regularly against something independent: a visual bearing, a transit, a depth contour, a radar range. Keep a paper chart or a second device available, and keep up a written log so that you can still navigate if the electronics fail.

Click to zoom

How GPS Works

The Global Positioning System (GPS) is operated by the United States. A constellation of at least 24 satellites orbits at about 20,000 km altitude. Each satellite continuously transmits its position and an extremely accurate time signal from onboard atomic clocks. A receiver measures the time delay of the signal from each satellite it can see, converts the delays into distances, and finds where those distances intersect.

• With signals from three satellites a receiver can in principle fix a position in two dimensions, if its own clock is perfect.
• Receivers have cheap clocks, so they need a fourth satellite to solve for position and for the clock error. In practice a receiver tracks eight or more.
• The more satellites in good geometry, the better the fix. A measure called HDOP (horizontal dilution of precision) tells you how good the geometry is: a low figure, such as 1 to 2, is excellent; above 5 is poor.

Other systems exist: GLONASS (Russia), Galileo (the European Union) and BeiDou (China). Most modern chartplotters and handheld receivers use several, which improves availability. This lesson uses the term GPS to include all of them.

Accuracy

A modern marine GPS receiver is typically accurate to within 3 to 10 metres horizontally, 95 percent of the time. With satellite-based augmentation (EGNOS in Europe, WAAS in North America), accuracy is often better than 3 metres. This is far more accurate than the accuracy of most paper charts, whose position is only as good as the survey that made them, and which in some old surveys may be tens or hundreds of metres out.

A fix from GPS is not exact. In the narrow entrance of a harbour, 10 metres can be the difference between the channel and the rock. Allow for it.

What GPS gives you

GPS provides:

• Position: latitude and longitude, plus the time.
• COG: course over the ground, the direction in which you are actually moving across the seabed, in degrees.
• SOG: speed over the ground, in knots.
• Time: UTC to a fraction of a second.

It does not give your heading (the direction the bow points), nor your speed through the water, unless sensors for these are connected. If you are crabbing sideways in a tidal stream the COG and the heading will be different; if the boat is motionless, the COG becomes meaningless and will jump about.

Datum

A datum is the mathematical model of the Earth against which latitude and longitude are measured. GPS works in WGS 84 (World Geodetic System 1984). Charts that were based on older surveys were drawn with other datums, such as OSGB 36 in Great Britain or ED 50 in Europe. The same latitude and longitude differ between datums: the differences can be 100 metres or more.

Modern Admiralty charts are in WGS 84, and the chart says so in a note near the title. Where a chart is based on another datum, a note tells you how much to shift a GPS position (in minutes of latitude and longitude, for example "move positions 0.05 minutes north and 0.03 minutes east") before plotting it.

• Set your GPS and chartplotter to WGS 84, matching the chart in use.
• Read the datum note on every paper chart before plotting a GPS position.
• On electronic charts the software handles the datum, but only if the plotter datum is set correctly.

An unnoticed datum error is exactly the kind of mistake that has put yachts on the rocks. It is a very good reason to compare your plotted GPS position with a visual check.

Setting Up a Chartplotter

Configure your plotter before you sail, not when you need it. The key settings:

• Datum: WGS 84 (check the chart datum note).
• Position format: degrees and decimal minutes (for example 50 48.12 N). This matches the paper chart scale, where one minute of latitude is one nautical mile, and is the format the lifeboat and coastguard use on VHF.
• Units: nautical miles, knots, metres for depth, degrees. Do not mix up feet and fathoms.
• North reference: true or magnetic. Most plotters can show courses and bearings either way. Decide which and stick to it. A common convention is to work in true on the chart and convert to magnetic only for the helm.
• Heading source: the plotter may use COG (from GPS), a fluxgate compass or an autopilot. At low speed COG is unreliable.
• Time: UTC or local, set clearly.
• Alarms: depth, shallow water, arrival, anchor drag, off-course and low battery. Set sensible limits.
• Chart orientation: north-up, heading-up or course-up.
• Safety contour and safety depth: set to your draught plus a margin, so that the plotter shades shallow water. In shaded water treat the picture with caution.

Click to zoom

Raster and vector charts

Electronic charts come in two forms.

• Raster charts are scanned images of the paper chart (Admiralty Raster Chart Service, ARCS). They look just like the paper chart, and carry the same information, but you cannot interrogate objects or switch layers, and they can become hard to read when zoomed.
• Vector charts (ENC, or private systems such as C-MAP and Navionics) are databases of objects: depths, lights, buoys, rocks. You can touch an object to see its data, switch layers off, and sound alarms when your route crosses a danger. But the display depends on the chart settings: at some zoom levels small objects (a lone rock, a wreck, a cable) are removed to avoid clutter, and may be hidden.

A vector chartplotter will happily let you zoom out until a dangerous rock disappears. Always zoom in to the scale of your position when close to land, and cross-check with paper charts and the pilot book. Keep the charts updated: an out-of-date chart can show a buoy that has moved or been removed.

Official ENCs (electronic navigational charts) and Admiralty vector charts are corrected weekly. Cheaper private charts have limited correction and may contain their own errors. Ensure the plotter carries the right version for the area, and don't forget that no electronic chart system is a legal substitute for official paper charts on a ship subject to SOLAS; yachts should nevertheless carry paper charts as a backup.

Waypoints and Routes

A waypoint is a named position stored in the GPS, normally entered as latitude and longitude or placed on the chart screen. A route is a list of waypoints in sequence, with legs between them. The plotter navigates you from each waypoint to the next and gives the bearing and distance.

Good waypoint practice

• Place waypoints in safe water and so that the straight line between them clears all dangers by a comfortable margin. Check every leg of the route on the screen and, preferably, on paper.
• Do not put a waypoint on a buoy or beacon. In congested waters you may collide with it, or with others heading for the same point. Place it a cable or so to the safe side of the mark.
• Name waypoints clearly, with a scheme that you understand (for example "Needles ch" for the Needles channel entrance).
• Write each waypoint's latitude and longitude in the log, and check the position against the chart, to catch typing errors. Waypoints entered as numbers can be wrong by a degree or a minute without any indication.
• Check the bearing and distance that the plotter gives against the figures on your chart for the leg.
• Pay special attention to the arrival radius and the auto-advance setting, which switches to the next leg when you come within a set distance. Turn this off if you do not want the plotter to sequence on its own through a hazardous section.
• Make a pre-planned route and save it, but do not follow it as if it were a track on rails. Review it against the actual tide and wind.

A route displayed on a chartplotter is only a line between two points. It does not know about the tide, the wind or the rock sitting 50 m to the side. Keep checking.

Passage planning with the plotter

The plotter is a very good way to plan: you can create the route, read the distances, work out total passage time at your cruising speed, and add the ETA at each waypoint. But the planning principles, set out in the passage planning lesson, still apply: tidal heights, tidal streams, weather, a harbour of refuge and an escape plan for each leg. Print or note the main details in a paper plan.

The Navigation Display

The data displayed when you are navigating to a waypoint are the same on almost any GPS or plotter. Learn what each means.

Cross-track error (XTE)

The route from one waypoint to the next is a straight line, the track. The cross-track error is the perpendicular distance from your position to that line, shown in nautical miles (or cables) with a direction (left or right of track, port or starboard). If the XTE is 0.2 nm to starboard, you are two cables to the right of the planned line.

XTE is a measure of the distance you are off the planned line, not of how well you are steering. It is the key figure for staying on a safe line past hazards. If your planned track runs half a mile off a headland, an XTE alarm of 0.2 nm gives you a margin.

Watch what XTE does as you steer. The classical error is to chase the bearing to waypoint: steer straight at the waypoint from wherever you are after being swept sideways by a tidal stream. You then follow a curve, and approach the waypoint along the line of the stream rather than along the planned track. Steer to get back on the line instead, using the XTE reading and a course to steer that includes tide.

COG versus heading

In tidal water your COG differs from your heading, the angle between them being the leeway and tidal stream effect. This is useful. When you steer a compass course through a tidal stream, the GPS COG shows you the actual track being made. If you are making the intended track, COG equals the bearing of the planned leg. If COG is, say, 10 degrees to the right of the planned track, you can see you are being set to the right, and you can correct.

A useful routine: every half hour, plot the GPS position, and compare the actual track with the planned track. Compare the estimated tidal stream, set and drift, with what the GPS indicates. The difference between your DR position (based on log and compass) and the GPS fix is exactly the effect of the tidal stream and leeway.

Putting XTE to work: an example

You are sailing from waypoint A to waypoint B, 12 nm away, on a track of 090 degrees true. After an hour the plotter shows: BTW 084, DTW 6.3 nm, XTE 0.4 nm to port (left), COG 088, SOG 5.8. You are being set north of the track by the stream.

You know you must steer to the right of the track line, to bring XTE back to zero. Alter your heading by about 10 degrees to the south of the intended heading, watch the XTE decrease, and bring it back to the track bearing as it reaches zero. With the tidal set now known, you can plan the new heading to hold the line.

Do not steer a heading equal to the BTW from where you are when the tidal set is strong. At the XTE of 0.4 nm and 6.3 nm to run, the BTW of 084 is only a short-term correction; you also need to allow for continuing tide.

Limitations and Failure Modes of GPS

GPS is reliable, but it can be wrong or lost. Know the modes of failure.

• Loss of signal. Satellites hidden by high cliffs, sea walls or tall buildings, or in an electrical storm. The plotter's position freezes or drifts; the screen may display "no fix". Know what a loss looks like on your set.
• Jamming and spoofing. Radio-frequency jammers can block the GPS signal, and in some regions of the world, including parts of the eastern Mediterranean and Baltic, GPS has been deliberately jammed or spoofed with false positions. If the plotter shows a position that does not match what you can see, believe your eyes.
• Datum errors, as described above.
• Multipath errors. Signals reflected from buildings or structures near the aerial give a false position.
• Aerial problems. A faulty cable or connector, water in a connector, or masking by metal nearby.
• Power failure. A flat battery, a blown fuse or a loose connection. Plotters take power from the same supply as the instruments, so a single failure can switch off several systems at once.
• Operator errors. Wrong datum, wrong waypoint (a transposed digit), wrong unit, waiting at the helm in the dark, zoomed out to the wrong scale, using a worn-out chart, or misreading the screen in sunlight.
• Chart limitations. The chart may be poorly surveyed. GPS accuracy exceeds that of many charts, so the plotted boat may be shown on a rock that is actually 50 m away, or the other way round.
• Display problems. Screens that wash out in bright light, mist up inside, or fail in a splash of water.

Click to zoom

Defences

• Cross-check the plotter against independent information every 15 to 30 minutes, more often in pilotage: a compass bearing of a charted object, a transit, a radar range, a depth that matches the chart.
• Keep a written log with the GPS position, time, log reading and compass course, so that you can fall back on DR and EP if the electronics fail.
• Carry paper charts and the means to plot on them, and a handheld GPS or a hand bearing compass as a backup. Keep spare batteries for a handheld.
• Keep the plotter's alarms on, and know how to silence them without turning them off.
• Do not look at the screen when you should be looking out of the window. The commonest cause of collisions in clear weather is a crew member staring at a screen.
• Practise: cover the screen and navigate for a leg using only paper and a compass.

Man Overboard and Other Functions

Most plotters have a MOB button that records your position at once and shows bearing and distance back to that point. Press it the moment someone falls in; the position it saves is the best datum for the search. Check this function before you sail, as the settings differ between sets. Other useful functions include:

• Anchor alarm: sounds if the boat moves outside a set radius from the anchored position. Set the radius to allow for the swinging circle and GPS wander.
• Track recording: stores your past track, handy for retracing your way into a harbour in fog.
• Tidal data and stream overlays: some systems superimpose tidal arrows from predictions. Treat them as indicative.
• AIS overlay: shows targets on the chart.
• Depth alarms: from a connected echo sounder.

Introduction to Radar

Radar (RAdio Detection And Ranging) was once the equipment of ships and big motor yachts. Compact scanners and screens are now common on cruising yachts. The Day Skipper does not need to be a qualified radar user, but should understand how radar works, what it shows, and where it is weak. The RYA Radar course, and the RYA handbook Introduction to Radar (G34), cover the subject fully.

How radar works

A scanner on the mast sends out pulses of microwave energy in a narrow beam, rotating through 360 degrees about 20 to 30 times a minute. Each pulse spreads out, strikes objects and a small fraction of its energy is reflected back. The set times the delay between the pulse and the return. Radio waves travel at a known speed, so the delay gives the range to the target. The direction of the scanner at the moment of return gives the bearing. The display builds up a picture: bright spots (echoes) at the right range and bearing, with your boat at the centre.

Most small-craft radars operate at X-band (about 3 cm wavelength, 9.4 GHz), which gives a good picture of small targets. Some use S-band (10 cm), which handles rain better but needs a larger scanner. Open array scanners (a rotating bar) give a narrower beam and better definition than dome scanners (a small enclosed rotating antenna); a narrow beam is more accurate for bearings.

What radar shows well, and badly

Radar echoes depend on the size, shape, material and angle of the target, not just its size. Good reflectors:

• Steel ships and metal structures.
• Buoys with radar reflectors, racons and lighthouses.
• Steep cliffs and harbour walls facing you.
• Rain squalls and thunderclouds.

Poor reflectors:

• Small GRP and wooden boats. A GRP yacht gives a weak and variable echo, especially in waves. Pass the word: a small yacht may be invisible on the radar of a ship.
• Low shores, sandy beaches and mudflats, which slope gently and reflect energy away.
• Anything behind something else (see radar shadow).
• Navigation marks without reflectors.

Radar reflectors

Fitting a radar reflector improves the chance that a ship sees you. A passive reflector, mounted as high as possible and in the correct orientation (usually with a corner of the octahedral reflector pointing up), is better than none. The performance of small reflectors is mixed, and many fall well short of the figure that the Maritime and Coastguard Agency recommended. Look for one that meets the ISO 8729 standard, and consider an active reflector or a radar target enhancer. Even then, no reflector can guarantee detection, so keep your own visual watch and do not rely on being seen.

The Radar Controls

Learn the basic controls. Different sets use different names and menus, but the functions are the same.

Range

The range selector sets the distance represented by the radius of the screen, for example 0.75, 1.5, 3, 6, 12 or 24 nm. Fixed range rings are evenly spaced circles that help you estimate distance.

Choose the shortest range that shows what you need. In confined waters, use 0.75 to 3 nm to see detail. In open water use 6 to 12 nm for early detection of ships. Switch periodically to a longer range to look ahead, then back for detail. A target that is dangerous at 3 nm is already visible at 12 nm.

Click to zoom

Gain

Gain controls the receiver's sensitivity. Too low and weak echoes disappear. Too high and the screen fills with speckle noise, hiding real targets. Set the gain so that a light background speckle is just visible, then turn it back slightly.

Tune

On some sets tune adjusts the receiver to the exact frequency of the transmitter. Adjust for the strongest echoes of a distant target, using the tuning indicator. Many modern sets tune automatically. A badly tuned set shows weak echoes and misses targets.

Click to zoom

Sea clutter

Waves near your boat reflect the radar pulses and fill the centre of the screen with a bright mass of echoes that can hide small targets (including another yacht, buoys and small boats). The sea clutter (STC, sensitivity time control) control reduces gain at short range. Increase it only until the clutter breaks up into specks that no longer hide targets; excessive use hides real targets near the centre. Too much STC can remove a small boat at 0.5 nm. Rough seas upwind produce more clutter than downwind.

Rain clutter

Rain, snow and hail return echoes in blotchy areas that mask targets behind them. The rain clutter (FTC, fast time constant) control breaks up the rain returns so that targets stand out. Use it sparingly, and turn it off when it is not needed. It is also a useful way to see a rain squall: you can use the radar to see where the heavy rain is and steer around it.

Other controls

• Brilliance and contrast adjust screen brightness. At night, turn the screen down so that you keep your night vision.
• Interference rejection reduces the speckle patterns caused by other radars nearby.
• Guard zones (alarms) sound an alarm when a target enters a zone set round your boat. Useful on a night watch, but they are an aid to lookout, not a substitute.
• Echo trails show the past tracks of targets, as a trail of afterglow, which helps in judging their motion.
• Standby/transmit. Radar radiates microwave energy, so switch to standby when not needed and never stand close to a scanner when it is transmitting. Stay clear of the beam.

Display Orientation and Motion

Orientation

• Heading-up (head-up): the boat's bow is always at the top of the screen. Bearings seen are relative to the boat. This is simple, but the picture swings as you yaw or change course.
• North-up: north is at the top, and the heading line rotates. It matches the chart, so is easy to compare with it. It needs a compass input to the radar.
• Course-up: the set course is held at the top. It gives a stable picture, matching the way you are going.

Relative and true motion

• Relative motion: your boat stays fixed in the centre and everything moves relative to you. A stationary buoy appears to move down the screen, at your speed.
• True motion: land stays still on the screen and moving targets move in their real tracks. Your boat moves across the screen. It needs speed and compass inputs.

Day Skipper users will mostly see relative motion, head-up or north-up.

Taking Ranges and Bearings: EBL and VRM

The two measuring tools:

• EBL (electronic bearing line): a line from the centre of the screen that you rotate to line up with a target. The set displays its bearing, either relative to your bow or true (or magnetic), depending on the set up.
• VRM (variable range marker): a circle, adjustable in size, that you expand until it touches the target's near edge. The set displays the range.

The set shows range and bearing to the target, and you can plot them as a position line.

Which gives the better position line

Radar is a good rangefinder and a poor direction finder. Range is accurate to roughly one or two percent of the range scale in use, but bearing is limited by the beam width (typically 2 to 5 degrees), which makes bearings less precise by a few degrees, and by the heading marker alignment. So:

• Ranges are more reliable than radar bearings. Use radar ranges, not bearings, for fixing, and use visual compass bearings when you can.
• A fix from two or three radar ranges of different objects (circles of position) is better than a fix from radar bearings.
• A fix by radar range and visual bearing of the same object is also good.
• Measure the range to the near edge of the echo, because the edge of the echo is where the pulse first met the target. The bearing is best taken to the centre of the target.
• Use charted objects that give a clear echo: a lighthouse, a steep headland, or a harbour breakwater. A low sloping coast may return only from some distance inland. Identify the object by comparing the radar picture with the chart.

Plotting a radar fix

Radar ranges are plotted on the chart with dividers: set the dividers to the range scale, using the latitude scale at the side of the chart, then draw an arc about the object. Two or three arcs from different objects intersect at the fix. If the arcs form a small triangle, take the centre as the position, or the point nearest danger, as with visual bearings.

Worked example: a three-range radar fix

At 1430, a yacht on passage along the south coast takes radar ranges:

• Needles lighthouse, 3.8 nm.
• St Catherine's Point, 8.2 nm.
• A steep headland on the coast, 5.4 nm.

Set dividers to 3.8 nm using the latitude scale, centre on the lighthouse, and draw an arc towards the boat. Repeat for each of the others. The three arcs meet in a small triangle. Take the centre of the triangle, label the position with the time (1430) and the log reading, and compare it with the DR and GPS positions. Agreement within a few cables, in a place where the accuracy of all three is limited, confirms that all is well; a larger difference tells you to find out why before you rely on any of them.

Interpreting the Radar Picture

Relating the radar screen to the chart is a skill that takes practice. Practise on clear days in familiar waters, with visual confirmation.

• Compare the picture with the chart: identify headlands, islands, buoys and harbours. Look at the shape first and match with the chart.
• Landfall at a distance shows the highest land first, and the lower coast later.
• Land echoes may be from hills inland rather than the coast.
• Moored vessels and buoys appear as small dots. Several dots may be a moored fleet.
• Moving vessels appear as dots that change position between sweeps.

Radar shadow and blind arcs

A target behind a bigger one is hidden in the radar shadow, just as in the lee of a building. A small boat behind a ship, a ship behind an island, or a buoy behind a headland will be absent from the screen. Parts of the scanner's field of view can also be blocked by the mast, the rigging or the sail. Keep watch for blind arcs, and find out where they are on your boat.

False and spurious echoes

• Multiple echoes: a strong target nearby may show a second echo further out, from a reflection of the signal between the target and your own vessel.
• Indirect echoes: reflected from a part of your boat, such as the mast, or a ship, producing a false echo in a different direction.
• Side-lobe echoes: arcs of echoes at the same range on either side of a big nearby target.
• Second trace echoes: in certain atmospheric conditions (ducting), a distant target appears at the wrong range.

Treat any echo that appears and disappears, or that is in an unlikely position, with suspicion.

Radar in poor visibility

In fog the radar will allow you to see ships and land which you cannot see. But the COLREGs still apply. Rule 19 requires every vessel to proceed at a safe speed for the conditions, with the engine ready for manoeuvre. Radar does not tell you what the other vessel will do. The RYA Radar course teaches manual plotting and collision avoidance in detail.

Radar for collision avoidance: basic idea

A target on a steady bearing and decreasing range is on a collision course. On a radar screen, if the bearing of a target does not change and the range closes, there is a risk of collision. This is the same principle as with visual lookout, set out in Rule 7 (risk of collision) and Rule 19 (conduct in restricted visibility). Look for a target that is moving along a line to the centre of the screen. Early and substantial action is essential, as set out in Rule 8. Use of the EBL on a target is a handy check: if the target remains on the EBL as the range closes, the bearing is constant.

Rule 7(b) specifically requires proper use of radar equipment, including long-range scanning and systematic observation of detected objects. Rule 19(d) says that a vessel which detects by radar alone the presence of another vessel, and determines that a close-quarters situation is developing or risk of collision exists, shall take avoiding action in ample time. Avoid altering course to port for a vessel forward of the beam, other than for a vessel being overtaken, and avoid altering course towards a vessel abeam or abaft the beam.

Click to zoom

AIS Basics

The Automatic Identification System (AIS) is a VHF data system that broadcasts a vessel's identity, position, course, speed and other data to nearby vessels and shore stations. SOLAS ships and many fishing vessels carry a Class A transponder. Many yachts carry the cheaper Class B transponder, or a receive-only unit.

What AIS tells you

For a target, AIS can show:

• The vessel's name, MMSI (identity number) and type.
• Position, COG, SOG and heading.
• Dimensions.
• Destination and ETA (not always reliable).
• CPA (closest point of approach) and TCPA (time to CPA) calculated by your equipment.

Used well, AIS is a great help: you can call a ship by name on VHF or DSC, and you can see the CPA and TCPA without plotting.

Classes of equipment

• Class A: fitted to SOLAS ships. 12.5 W transmit power, reports every 2 to 10 seconds under way, and has priority in the system.
• Class B: fitted to leisure craft. 2 W (SOTDMA types) or 5 W, reports less often (every 30 seconds when over 2 knots) and slightly lower priority.
• Receive-only: shows targets but does not transmit your position. Cheaper, but it does not let others see you.
• AIS-SART and AIS MOB devices: transmit an alert and position in an emergency.

AIS limitations

• Not everyone has AIS. Small craft, fishing boats, pleasure boats, naval ships and buoys may not transmit. A screen with few AIS targets does not mean few vessels.
• It depends on the other vessel's input. Data is only as good as the vessel's sensors and crew; the wrong heading, an old destination or an incorrectly entered status are all common.
• It is not a collision-avoidance system in itself. A CPA warning is a prompt to look, not an instruction for action. COLREGs apply regardless of what the other vessel's AIS says.
• The screen is not the sea. Targets lag by a few seconds, may be miscorrelated, and positions share all the limitations of GPS.
• Radar is the primary sensor. AIS supplements it. An unmatched radar echo is a hazard, with or without AIS.

For these reasons the RYA says AIS should be used in addition to radar and visual lookout, never instead of them.

Safety and Limitations of Electronic Aids

COLREGs Rule 5 requires every vessel to keep a proper lookout by sight and hearing, as well as by all available means appropriate to the circumstances. Electronic aids are included in the "all available means". They add to the lookout, they do not replace it.

• Keep looking out. A lookout with eyes, ears and common sense will see a fishing boat with no AIS and no radar echo.
• Know your equipment. Practise operating it before you need it in fog or at night, in the way you will use it. Set it up with the right controls and know where the menus are.
• Do not become dependent. Practise navigating with traditional tools. Plot a fix every hour even if the plotter shows your position continuously.
• Maintain it. Keep spare fuses and batteries, check the connectors, and protect screens from the weather.
• Beware of complacency. The more reliable the system, the more likely it is that the crew stops checking. A wrong waypoint or a frozen plotter display can be very convincing.
• Power. Make sure the batteries can supply the electronics for the whole passage, and know what you will lose first if the power runs low. Radar uses a significant current; do not leave it running when it is not needed.
• Brief the crew. Every crew member who stands a watch should know what the screens mean, what to ignore, and when to wake the skipper.

Worked Example: A Passage with Electronics

You are skipper of a 36 ft yacht on a passage from Yarmouth, Isle of Wight, to Poole through the Needles Channel, in moderate visibility. You have a GPS chartplotter, a depth sounder and a small radar.

Before sailing. Check that the plotter datum is WGS 84, units are nautical miles and knots, and the position format is degrees and decimal minutes. Enter a route with waypoints in safe water, check each leg on the chart screen and on paper for dangers, check the bearing and distance from each leg, and note the waypoint positions in the log. Set the XTE alarm at 0.3 nm, the shallow depth alarm at 3 m (draught 1.7 m plus margin), the anchor alarm off, and the arrival alarm on. Test the MOB function. Make sure the radar is working on standby, with the scanner clear of people.

On passage. Every 30 minutes you record in the log the plotter position, COG and SOG, the log reading, the compass heading and the depth. Twice you compare a visual bearing of a lighthouse with the plotter position and plot it on the paper chart; they agree to within 0.1 nm.

Approaching the Needles Channel. Visibility drops. You turn on the radar and set it to 3 nm. The echo of the Needles headland is clear, and you identify the cliffs. You take a VRM range of 1.6 nm to the near edge of the cliff and a second range of 2.4 nm to a buoy, and plot the arcs. Your fix agrees with the plotter to within two cables. You see a large echo approaching from the south west at 6 nm; the AIS shows it as a 180 m ferry on a bearing that does not change, CPA 0.1 nm. You act early, per Rule 8, with a large alteration to starboard, and agree the plan by VHF.

Lessons. The electronics helped, but the position was confirmed independently. The skipper made decisions in good time. When the plotter froze for several minutes in the Needles channel, the paper chart and a hand bearing compass allowed the skipper to continue safely.

Common Mistakes

• Wrong datum or units, causing an offset in the plotted position.
• Trusting the plotter without cross-checks. Always confirm with a bearing, transit, radar range or depth.
• A waypoint typed wrongly. Always check bearing and distance against the chart.
• Waypoints on the buoy, or routes that pass over unnoticed hazards at the scale in use.
• Zoomed out too far for the chart to show rocks, wrecks and cables. Zoom in when close to land.
• Chasing BTW instead of watching XTE in a tidal stream.
• Ignoring COG versus heading. Not using COG to see the effect of stream and leeway.
• No backup: no paper chart, no handheld, flat batteries.
• Radar gain and clutter set badly, hiding small targets.
• Using radar bearings for fixes instead of ranges.
• Believing that an empty radar or AIS screen means empty water.
• Not keeping a proper lookout because of the screen.
• Overuse of the sea clutter control, which removes small boats.
• Leaving radar in transmit when people are working near the scanner.

Summary

• GPS gives position, COG, SOG and time with an accuracy of around 3 to 10 metres, but not heading. Set the datum to WGS 84 and match it to the chart.
• Configure the plotter before sailing: datum, units, alarms, safety contour, and test the MOB function.
• Waypoints should be in safe water, not on buoys. Check every typed position and every leg.
• XTE is the perpendicular distance from the planned track; use it, rather than BTW, to stay on the line in a tidal stream. COG shows the actual track over the ground.
• GPS can fail, be jammed or give wrong positions through errors in setup. Cross-check regularly against independent information and keep paper charts as a backup.
• Radar measures range accurately, bearing less so. Use ranges for fixes, EBL and VRM for measuring, and the shortest range scale that shows what you need.
• Gain, tune, sea clutter and rain clutter must be set correctly. Radar shadows, false echoes and poor reflectors such as small GRP boats mean the picture is incomplete.
• AIS supplements radar and lookout. Not all vessels transmit AIS, and CPA/TCPA warnings do not replace the rules of the road.
• Electronics support a proper lookout (Rule 5) and safe speed and avoidance in restricted visibility (Rules 6, 7, 8 and 19). They never replace them.

Check Your Understanding

1. What datum should your GPS be set to, and why does it matter?

Answer: WGS 84, matching the chart. If the plotter datum differs from the chart datum, plotted positions may be wrong by up to 100 metres or more. Always read the datum note on the chart and apply any correction shown.

2. What is the difference between heading and COG?

Answer: Heading is the direction the bow points. COG is the direction in which the boat is actually travelling over the seabed, the sum of heading, leeway and tidal stream. In a cross-tide they can differ by many degrees.

3. Define cross-track error and explain how you use it.

Answer: XTE is the perpendicular distance from the planned track between two waypoints, shown with a direction. Use it to see how far off the line you are and to steer back on to it, and set an XTE alarm so that you do not stray into danger. Do not simply steer towards the waypoint's bearing from an off-track position.

4. Give four ways in which a chartplotter can mislead you.

Answer: Wrong datum; wrong or mistyped waypoint; zoomed out so dangers are not shown; out-of-date or inaccurate chart; signal loss, jamming or multipath; frozen display or power failure. Any four of these.

5. Why are radar ranges preferred to radar bearings for fixing?

Answer: Range is measured by the time of the pulse return and is accurate to one or two percent of the scale. Bearings depend on beam width (several degrees) and heading marker alignment, and are less accurate.

6. What does the sea clutter control do, and what is the danger of too much?

Answer: It reduces receiver sensitivity at short range to suppress echoes from waves near the boat. Too much removes small real targets, such as a small boat or a buoy, close to you.

7. Name three targets that a radar may fail to show.

Answer: A small GRP or wooden yacht, low sandy shores or mudflats, an object in the radar shadow of a larger one, a buoy without a reflector, or a target hidden in sea or rain clutter or a blind arc.

8. What is the difference between relative motion and true motion displays?

Answer: In relative motion your boat stays fixed on the screen and everything else moves relative to you, so a stationary buoy appears to approach at your speed. In true motion land stays still and each vessel moves according to its actual course and speed. True motion needs speed and compass input.

9. What can AIS show, and what are three of its limitations?

Answer: It shows vessel name, MMSI, type, position, COG, SOG, heading, dimensions, and calculated CPA and TCPA. Limitations: not all vessels carry it, the data may be wrong, it lags, it relies on GPS, and it is no substitute for lookout or radar.

10. How does COLREGs Rule 5 relate to the use of electronic aids?

Answer: Rule 5 requires a proper lookout by sight and hearing and by all available means appropriate to the circumstances. Electronic aids are part of those means but do not replace sight and hearing. You must still look out of the window, and your decisions must follow the rules of the road.