Electronic navigation: GPS, plotters and AIS

Satellite position: GPS and GNSS

Lesson 32 ended with a passage plan: waypoints, legs, timings and the dangers to avoid. This lesson is about the electronics that help you sail it — the satellite receiver, the chart plotter and AIS — and, just as important, how to tell when they are wrong.

GNSS (Global Navigation Satellite System) is the general name for satellite positioning. GPS is the American system, and the name most people use for all of them; others include Europe’s Galileo, Russia’s GLONASS and China’s BeiDou, and many receivers use several at once. The receiver times signals from the satellites it can see and needs at least four to work out its position. From the way that position changes, it also works out your course and speed over the ground.

With a clear view of the sky, a modern receiver is usually accurate to within a few metres. That is often better than the chart itself: a chart is only as good as the survey behind it, and some areas were last surveyed long ago. Many charts show where their survey data came from, and how old it is, in a source data diagram or note.

Which datum? WGS84 and the chart note

A latitude and longitude only means something against a model of the Earth’s shape, called a horizontal datum. GNSS positions are worked out on WGS84 (the World Geodetic System 1984). Most modern charts, paper and electronic, use WGS84 or a datum that is the same at chart scales, so a GPS position can be plotted directly. Older charts, and some charts of other areas, use other datums: the same latitude and longitude then lands in a slightly different place — often by tens of metres, and sometimes by much more on old charts of remote areas.

Charts carry a note about this; on Admiralty charts it is headed Satellite-derived positions. It either says that positions referred to WGS84 can be plotted directly, or tells you how far to move them — so many minutes northward or southward, and eastward or westward — to agree with the chart. Apply the shift before you plot. If your set can be switched to another datum, know which one it is using.

A course assumption: the Isle of Kerran practice chart is fictional and has no datum note, so in this course treat it as drawn on WGS84 and plot GPS positions directly. On a real chart, never assume — read the note.

Heading and speed through the water vs COG and SOG

Lesson 7 showed that the log measures speed through the water and GPS measures speed over the ground. The same is true of direction:

COG and SOG are the ground track of the vector triangle you drew in Lesson 29: water track (heading, pushed downwind by any leeway) plus tidal stream. The GPS measures the result directly, so it already includes them. A GPS on its own knows only how its position is changing, so it gives COG, not heading; heading needs a compass or a heading sensor. At very low speeds COG wanders about and means little.

Heading is where the bow points. COG is where you are actually going.

Waypoints and routes

A waypoint is a position stored in the set, typed in as latitude and longitude or placed with the cursor on the plotter. A route is a chain of waypoints, joined by legs. Here is the route used in this lesson, from Port Kerran west-about to Kerran Roads:

Good waypoints follow a few simple rules:

• Never on a mark or a hazard. GPS is accurate enough to take you straight into it, and other boats using the same mark as a waypoint converge on the same spot.
• Off, in clear water, on the safe side — the safe quadrant of a cardinal, the channel side of a lateral mark — with room for error and for the next leg.
• Check every position. Read it back, check N/S and E/W, and measure it on the chart as a bearing and distance from something charted.
• Check every leg on the chart. The set joins waypoints with straight lines, whatever lies between them. Make sure each leg clears every danger, with enough depth at the state of tide you expect.

Bearing, distance and cross-track error

Under way, the set compares your GPS position with the active leg and shows:

• BTW, bearing to waypoint: the direction of the waypoint from where you are now, in °T or °M depending on a setting (with 3°W variation, the °M figure is 3° more).
• DTW, distance to waypoint, in miles.
• XTE, cross-track error: how far you are from the planned track — the line from the previous waypoint to the next — measured at right angles to it, with L or R for the side. Sets differ in whether the arrow shows the side you are on or the way to steer, so find out which yours does.

Two ways to deal with a cross-track error: if the track was chosen to keep you clear of a danger, regain it — alter towards it until the XTE is near zero, then steer a heading that holds COG on the track bearing. If the water between you and the waypoint is open and clear, you can instead make good a new straight line from where you are: steer so that COG equals BTW. Check that new line on the chart first.

What you must not do in a cross-stream is simply point the bow at the waypoint. The stream sets you down-tide, the bearing of the waypoint changes, you turn to follow it, and you sail a curving hook that ends up heading into the stream. On leg WP2–WP3, with the stream setting 075°T at 2.0 kn, the hook takes 1 h 06 min instead of 1 h 01 min, the XTE reaches 0.72 M R, and the yacht passes about a cable inside the West Ledge buoy — on the danger side of a west cardinal, the side it is there to keep you off — within 0.4 M of Head Ledge. Steering the course to steer, 337°T, she stays on the track, well to the west of the buoy.

Don’t steer the bow at the waypoint. Steer so that COG matches the track, and keep the XTE near zero.

Using a chart plotter safely

• Zoomed out, things disappear. To keep the screen readable, a plotter may leave out soundings, small dangers and even buoys at small scales, or show a less detailed chart. A route can look clear when it isn’t. Check every leg zoomed in to the most detailed chart available.
• Zoomed in too far, nothing is added. Beyond the scale the chart was made for, the plotter only magnifies it — coastline, soundings and errors alike. Blank water between soundings is not proof of clear water, and a boat icon a few metres from a rock is no guarantee of anything.
• Know your settings: °T or °M, the datum, units, north-up or course-up. Keep electronic charts up to date, as you would correct paper ones.
• Keep the paper going. Every hour, and at each alteration, log the time, GPS position, log reading and COG/SOG (Lesson 15) and plot the position on the paper chart, as a fix labelled with the time and “GPS”. If the screen goes dark, you are never more than an hour of dead reckoning from a known position.

AIS: seeing other ships

AIS (Automatic Identification System) transmitters broadcast a vessel’s identity (name, call sign and MMSI number), its position from its own GNSS, its COG and SOG, heading and other details over VHF radio. Your set receives them and draws each vessel as a target on the plotter, with its data a tap away.

From both vessels’ COG and SOG, the set works out the CPA (closest point of approach: the nearest the target will come) and the TCPA (time to CPA), assuming both keep straight courses at steady speeds. A CPA near zero is the steady compass bearing of Lesson 20, worked out for you. Many sets can sound an alarm when a target’s CPA and TCPA fall inside limits you choose.

AIS is a superb aid, but know its limits:

• Many vessels carry no AIS, or have it switched off — small craft, many fishing boats, and others.
• The data can be wrong: details typed in by the crew, a navigational status left unchanged, or a position from a faulty GNSS.
• Targets update only every few seconds or more, and CPA and TCPA are out of date the moment either vessel alters.
• AIS tells you nothing about what the other vessel intends, and doesn’t show its lights or shapes.
• Don’t assume a ship has seen your Class B signal, or you.

Cross-check everything

Treat a GPS position like any other fix: one clue, to be checked against the others.

• Bearings. A hand-bearing compass bearing of a charted landmark should pass through the GPS position; a three-point fix (Lesson 12) is a full check.
• Transits. When two charted objects come in line, you are on that line (Lesson 13). Does the plotter agree? Clearing bearings keep you safe whatever the screen says.
• Depth. Compare the echo sounder with the charted depth plus the height of tide (Module 4), and watch for contours where you expect them.
• Marks. When a buoy is abeam by eye, the plotter should show it abeam too.
• Compass and log. The difference between heading and STW and COG and SOG should match the stream and leeway you expect.

Radar, if fitted, gives ranges of the coast and of other vessels; its basics are in Lesson 34 and radar plotting in Lesson 37.