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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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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.
Read definition →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.
Read definition →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.
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.