GNSS and the chart plotter, honestly

GNSS — GPS and its siblings — works by timing signals from satellites: each timing fixes your distance from one satellite, several distances fix your position, typically to a boat-length or two. From successive positions the receiver derives course and speed OVER THE GROUND (COGCOGCourse over ground, from GNSS — where you are actually going, tide included.full glossary →, SOG) — the tide-included truth of where you are actually going, as distinct from the heading the compass shows and the through-water speed the log shows. Accuracy degrades gracefully but really: poor satellite geometry, a masted antenna shadowed by sails, heavy rain, and — far more often — the human errors around it. The receiver even reports that geometry as a single figure, the HDOPHDOPHorizontal dilution of precision — how satellite geometry sharpens a GNSS fix: satellites spread round the sky give a low HDOP (sharp), bunched ones a high HDOP (smeared).full glossary → (horizontal dilution of precision): low means the satellites are well spread and the fix is tight, high means they are bunched and the position soft. And because the whole system can be jammed or spoofed — increasingly a fact of life in some seas — the discipline is to treat a satellite fix as one position line among several: cross-check it now and then against a transit, a depth contour or a bearing, so the day it lies to you, you already know.

The waypoint is the working unit of plotter navigation: a chosen position you ask the machine to guide you to. Choose them like a navigator, not a darts player: NEVER on top of a buoy (everyone else picked it too, and converging boats meet at marks), but a sensible offset to the safe side; off a danger, not at its edge; and at course-change points your pilotagepilotageEyeball navigation in confined waters, from a prepared plan of marks, bearings and depths.full glossary → plan already wanted. A chain of waypoints is a route; the plotter then reports bearing and distance to the next one, and your progress along the chain.

Cross-track error — XTEXTECross-track error — how far you sit left or right of the straight line between waypoints.full glossary → — is the plotter telling you how far you sit LEFT or RIGHT of the straight line between waypoints. Think of it as a highway: XTE is your distance from the centreline. In a cross-stream, the lazy way is to keep steering AT the waypoint — you crab down-tide in a long curve, XTE growing, ground wasted. The seamanlike way is the course-to-steer you already know from chartwork: offset the heading up-stream and the boat tracks the line, XTE near zero, heading visibly different from COG. The trainer below lets you feel both.

AIS — the other screen

Alongside the position screen usually sits AISAISAutomatic Identification System — transmitting vessels broadcast identity, position and course. A clear screen is not a clear sea.full glossary → — vessels broadcasting identity, position, course and speed over VHF. Ships carry the full Class A; many leisure boats carry the lighter Class B (or receive-only). Used well it names the radar blip, hands you a CPACPAClosest point of approach — how near a radar contact will pass if nobody changes anything.full glossary → for a converging ship, and makes YOU visible to bridge watchkeepers if you transmit. Its one rule: AIS shows only vessels that transmit — most small craft do not — so a clear AIS screen is not a clear sea, and the Mark One Eyeball remains the primary sensor.

What the plotter cannot promise

The screen’s confidence is its danger. The cartography behind it can be old survey data dressed in modern graphics; detail appears and DISAPPEARS with zoom level, so a rock can hide one zoom step out; a receiver set to the wrong chart datum offsets every position; and the whole stack — antenna, receiver, screen, power — is one electrical fault from blank. None of this argues against using it; everything argues against using ONLY it. The cross-check habit from the fixes lesson is the answer: a visual bearing, a depth-contour comparison, a transit — anything independent. The plotter is a brilliant first opinion. Keep a second.