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Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.

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Radar Navigation
Gain and Tune
The two basic controls that set receiver sensitivity and peak performance
Gain sets receiver amplification and should be adjusted on a long range scale with clutter controls off, until a light speckled background noise is just visible; less than that means weak targets are being lost. Tune matches the receiver to the transmitter frequency in magnetron sets and drifts as the magnetron ages, so a set that has lost range often only needs retuning. Solid-state radars are inherently tuned and offer no such control.
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Meteorology for Navigators
Gale Warning
An official alert that winds of force 8 or more are expected
Issued when mean winds of Beaufort force 8, that is 34 knots or more, or gusts of 43 to 51 knots, are expected. Severe gale is force 9 or gusts 52 to 60 knots, storm force 10 or gusts 61 to 68, violent storm force 11 or gusts 69 knots and above, and hurricane force is force 12. Timing terms are strictly defined: imminent within 6 hours, soon within 6 to 12, later beyond 12.
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GNSS & Satellite Navigation

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Galileo

The European civil satellite navigation system, free to all users

Galileo is a civil-controlled European constellation orbiting at roughly 23,200 km in three planes. Its Open Service is free and interoperable with GPS on L1, and its signal design supports better multipath rejection. Galileo also offers navigation message authentication, which lets a receiver verify that the data it is using genuinely came from the satellites, an increasingly useful defence against spoofing in areas of known interference.

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Chart Projections & Datums

Geodetic Datum

The reference model of the Earth's shape that positions are measured from

A geodetic datum defines a reference ellipsoid and its orientation and origin relative to the Earth, fixing what a stated latitude and longitude actually means. Local datums such as OSGB36 or ED50 were fitted to a region and can differ from global datums by hundreds of metres. Because a satellite receiver reports on a global datum, positions must be converted before plotting on a chart drawn to a different datum, or the plotted position will be systematically wrong.

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Celestial Navigation Concepts

Geographical Position (GP)

The spot on Earth directly beneath a star, sun, moon or planet

The point where a line from the body's centre to the Earth's centre pierces the surface. Its latitude equals the body's declination and its longitude corresponds to the Greenwich Hour Angle, converted by subtracting from 360 degrees when GHA exceeds 180. Because the Earth rotates about 15 degrees of longitude per hour, the sun's GP moves roughly 15 minutes of arc westward each minute of time, which is why timing a sight to the second matters.

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Lights & Light Characteristics

Geographical Range

Distance at which the Earth's curve hides a light

Geographical range depends purely on the height of the light and the observer's eye. A common working formula is range in nautical miles equals 2.08 times the sum of the square roots of the two heights in metres, often simplified to 2.1 or, using feet, 1.17 times the square roots. For a light 40 metres high seen from an eye height of 2.25 metres, 2.08 times 6.32 plus 1.5 gives about 16.3 nautical miles.

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Chart Projections & Datums

Geoid

The Earth's true gravity-defined sea level surface

The geoid is the equipotential surface of the Earth's gravity field that best matches undisturbed mean sea level, and it undulates by up to roughly a hundred metres relative to the reference ellipsoid because of density variations. The separation between geoid and ellipsoid, called geoid height or N, is why GNSS altitude differs from height above sea level, and why vertical datum transformations require a geoid model as well as a horizontal datum.

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Meteorology for Navigators

Geostrophic Wind

The theoretical wind from balanced pressure and Earth rotation forces

The flow that results when the pressure gradient force exactly balances the Coriolis force, giving wind parallel to straight isobars. It is what a geostrophic scale on a synoptic chart measures from isobar spacing and latitude. Real surface wind over the open sea is typically about two thirds of the geostrophic value and backed 10 to 20 degrees towards low pressure, with a greater reduction and larger backing over land.

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Emergency Navigation & Distress

Global Maritime Distress and Safety System (GMDSS)

The worldwide framework of radio and satellite alerting at sea

Adopted under SOLAS, GMDSS replaced manual radio watchkeeping with automated alerting, defining carriage requirements by sea area rather than ship size. Its functions include ship-to-shore, shore-to-ship and ship-to-ship distress alerting, search and rescue coordination communications, locating signals, maritime safety information broadcasts and general communications. Equipment typically comprises DSC radios, satellite EPIRB, radar or AIS SART, NAVTEX and satellite terminals with prescribed battery reserves.

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GNSS & Satellite Navigation

Global Navigation Satellite System (GNSS)

The general name for any satellite system that fixes your position

GNSS is the umbrella term for constellations that broadcast ranging signals allowing a receiver to compute position, velocity and time anywhere on Earth. It covers GPS, GLONASS, Galileo and BeiDou, together with regional systems and augmentation services. A modern multi-constellation marine receiver tracks several systems at once, which improves satellite availability in harbours and under rig shadowing, and typically yields horizontal accuracy of a few metres without any augmentation.

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GNSS & Satellite Navigation

Global Positioning System (GPS)

The American satellite constellation most yachts use for position

GPS is operated by the United States Space Force with a nominal constellation of at least twenty-four satellites in six orbital planes at roughly 20,200 km altitude, each orbiting in about twelve hours. Civil signals are broadcast on L1 at 1575.42 MHz, with L5 at 1176.45 MHz on newer satellites. Positions are referenced to the WGS84 datum, so charts on other datums require a stated shift before plotting.

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GNSS & Satellite Navigation

GLONASS

Russia's satellite navigation constellation, often combined with GPS in receivers

GLONASS uses a nominal twenty-four satellites in three orbital planes at about 19,100 km, inclined more steeply than GPS, which gives good coverage at high latitudes. Older satellites separate signals by frequency rather than by code, so receivers need dedicated hardware. Positions are computed in the PZ-90 reference frame and transformed to WGS84 for chart use; the residual difference is well under a metre and negligible for coastal work.

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Emergency Navigation & Distress

GMDSS Sea Areas (A1 to A4)

Zones defining what distress equipment a ship must carry

Area A1 lies within VHF DSC range of at least one coast station, commonly some twenty to thirty miles offshore. Area A2 extends to MF DSC coverage, often around one hundred miles. Area A3 is the region covered by recognised geostationary satellite services, approximately between 70 degrees North and 70 degrees South, and area A4 is everything remaining, essentially the polar regions, where HF equipment is required.

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Chart Projections & Datums

Gnomonic Projection

The chart where a straight line is the shortest route across the ocean

Projected from the centre of the Earth onto a tangent plane, the gnomonic renders every great circle as a straight line, which is why ocean planning charts and polar charts use it. Angles and shapes are badly distorted away from the tangent point and no more than a hemisphere can be shown. Practical use is to draw the great circle track as a straight line, read off intermediate positions at convenient longitude intervals, and transfer those to a Mercator chart as a series of rhumb legs.

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Position Fixing

GNSS Fix

A position calculated automatically from satellite signals

A satellite receiver resolves position by measuring ranges to four or more satellites, solving for latitude, longitude, altitude and clock offset. Typical horizontal accuracy for civil single-frequency GPS is within about 10 metres, improved to a few metres with SBAS augmentation. The fix must be checked against the chart datum in use, as receivers output WGS84 while older charts may be on a local datum. Good practice records GNSS position in the log at regular intervals to support dead reckoning if signal is lost.

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GNSS & Satellite Navigation

GNSS Jamming

Radio noise that drowns out weak satellite signals and kills the fix

Satellite signals reach the surface at extremely low power, well below the natural noise floor, so even a low-power interfering transmitter can deny reception over many miles. Symptoms include a sudden loss of fix, wildly reduced satellite count, or a frozen position, often affecting several vessels in the same area simultaneously. Prudent practice is to keep a parallel fix by radar, visual bearings and a running echo sounder, and to report the outage.

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GNSS & Satellite Navigation

GNSS Spoofing

Fake satellite signals that make a receiver show a false position

Spoofing transmits counterfeit signals that a receiver accepts as genuine, producing a plausible but wrong position or time; unlike jamming, the display looks perfectly healthy. Warning signs include a position jumping to a distant location, GNSS speed disagreeing with the log, heading conflicting with the compass, or the clock stepping. Cross-checking against radar ranges, visual transits and depth remains the only reliable defence on most yachts.

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Points of Sail & Sailing Manoeuvres

Goose-winged (Wing-on-Wing)

Running with mainsail one side and headsail the other

On a dead run the headsail is set opposite the mainsail so it is not blanketed, usually held out with a whisker or spinnaker pole to stop it collapsing as the boat rolls. The rig becomes laterally balanced and drive improves, but the configuration is sensitive to steering: a small wander to leeward invites sailing by the lee and an accidental gybe, so a preventer on the boom is standard.

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Chart Projections & Datums

Graticule

The grid of latitude and longitude lines printed on a chart

The graticule is the network of meridians and parallels drawn according to the projection, distinguished from a rectangular grid which is a plane coordinate overlay. On a Mercator chart the graticule is rectangular, with meridians evenly spaced and parallels widening poleward. Subdivided borders allow positions to be read to a tenth of a minute on large scale charts. Every chart states the projection and the datum to which its graticule refers.

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Chart Projections & Datums

Great Circle

The shortest path between two points on the globe

A great circle is the intersection of the Earth's surface with a plane through its centre, and it gives the minimum distance between two positions. Its true course changes continuously along the track, being most northerly or southerly at the vertex. On a Mercator chart it appears as a curve bowing toward the nearer pole; on a gnomonic it is straight. The saving over a rhumb line grows with distance, with east-west orientation and with latitude.

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Great Circle & Ocean Routing

Great Circle (GC)

The shortest path between two places on the round Earth

A great circle is any circle on the sphere whose plane passes through the Earth's centre, so its arc gives the shortest surface distance between two points. On a Mercator chart it appears as a curve bowing towards the nearer pole, and the course changes continuously along it. Savings over a rhumb line grow with latitude and with east-west separation; on a 3,000 mile North Atlantic crossing the gain is roughly 100 miles.

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Great Circle & Ocean Routing

Great Circle Waypoint Interval

Chopping a curved ocean track into short straight steering legs

A great circle is sailed in practice as a chain of rhumb line legs joined at intermediate waypoints, conventionally every five degrees of longitude on ocean crossings, or every ten degrees where latitudes are low and the curvature slight. Each leg is steered on a constant course, giving a stepped approximation that adds only a fraction of a mile. Waypoints are entered into the plotter or plotted on a Mercator chart for monitoring.

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Celestial Navigation Concepts

Greenwich Hour Angle (GHA)

How far west of Greenwich a body currently lies, measured as an angle

Measured westward from the Greenwich meridian from 0 to 360 degrees, GHA is tabulated hourly in the almanac and interpolated with the increments and corrections pages using minutes and seconds of GMT. For stars, GHA is found by adding the Sidereal Hour Angle of the star to the GHA of Aries. Local Hour Angle is GHA minus west longitude or plus east longitude, again normalised into the range 0 to 360 degrees.

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Meteorology for Navigators

GRIB File

Compact computer weather data downloaded for use on a chart plotter

GRIdded Binary is the World Meteorological Organization format for numerical model output, giving values such as wind, pressure, precipitation and wave height at regular grid points and time steps. Files are small enough for satellite or SSB download and can be displayed as arrows or contours in navigation software. Crucially GRIB data is raw unmoderated model output with no forecaster input, so it typically understates fronts, squalls and local acceleration effects.

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Anchoring & Mooring

Ground Tackle

The whole anchoring outfit: anchor, chain or rope, and their fittings

Collective term for the anchor, cable, swivel, shackles, snubber, chain stopper and windlass that together hold a vessel to the seabed. Sizing is driven by displacement and windage rather than length alone; a cruising yacht typically carries a bower anchor of about one kilogram per metre of hull length as a rough starting point, plus a second anchor of different type as a kedge or storm backup.

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Dead Reckoning & Estimated Position

Ground Track (Course Over the Ground, COG)

The path the boat actually traces across the seabed

The ground track is the vector sum of the water track and the tidal stream or current, and is the line that matters for clearing hazards. It is drawn with a double arrowhead from the starting fix to the EP. GNSS reports it directly as course over ground, which is the resultant of all influences including any steering error, and comparing reported COG with the predicted ground track is a quick check that the tidal assumptions are behaving.

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Wind, Waves & Sea State

Gust and Lull

Short bursts of stronger wind and the calmer patches between

Short-lived departures from the mean wind caused by turbulent mixing bringing faster-moving air down to the surface. Over open water gusts commonly reach 1.3 to 1.4 times the mean speed, and considerably more in unstable air behind a cold front or downwind of high land. Because forecast forces state means, a force 6 forecast implies gusts near force 8, which is the level a reefing decision should be based on.

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Points of Sail & Sailing Manoeuvres

Gybe (Jibe)

Turning the stern through the wind so the boom swings across

Sailing downwind, the boat is borne away until the wind crosses the stern and the mainsail swings to the other side. Because the boom travels fast and under load, the controlled technique is to sheet the main hard in beforehand, turn steadily, then ease it out on the new gybe. An accidental or crash gybe is a common cause of rig damage and head injury, which a preventer guards against.

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Compass & Magnetism

Gyrocompass

A powered compass that finds true north without magnetism

A gyrocompass exploits the gyroscopic properties of rigidity in space and precession, combined with a gravity-based control device, so that the spin axis settles along the meridian and indicates true north. Settling takes from tens of minutes to several hours after starting. It is unaffected by variation, deviation and local anomalies, but suffers speed, course and latitude errors requiring correction, and becomes unreliable at very high latitudes. It is standard on ships and rare on yachts because of cost and power draw.

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Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.