Sea navigation before GPS

Backstaff from Lancaster Maritime Museum © Lancaster Maritime Museum

Lancaster physicist Dr Joe Kinrade is talking about navigation and GPS as part of a series of local history podcasts and exhibitions to mark a hundred years of Lancaster City Museums.

To mark the centenary, a hundred objects housed across the city’s museums are being featured in weekly podcasts featuring local people, experts and museum staff.

Dr Kinrade’ s chosen object is a backstaff from the collection of Lancaster Maritime Museum. This measures the elevation of the sun above the horizon by the projection of a shadow and was vital for navigation at sea hundreds of years ago before satellite navigation.

His expertise is in GNSS (Global Navigation Satellite Systems) such as the Global Positioning System (GPS), the satellite age bringing in a modern age of navigation.

“This signalled the end of things like foghorns, sadly no longer required - I grew up near a foghorn and still miss the sound on misty days- although lighthouses and radio beacons still provide a critical level of assurance to mariners in the fog.”

Before satellite navigation, the backstaff allowed a navigator to find a ship's latitude at sea, either reading the sun's height above the horizon, or cross-referencing star positions with specialised catalogues originating from astronomer observations in port cities like London and Lisbon.

“From about the 16th century, the world was opening up to exploration and trade. Shipwrecks or even just prolonged voyages became costlier, so the need for more accurate navigation drove an age of instrumentation. Ocean navigation shifted from the realm of astrology and 'dead reckoning' - timing how long it took an object to float past the ship in the water - to astronomy and trigonometry.

“The sextant dealt with the problem of sighting the Sun at sea, using Newtonian optics to focus light to a point despite the lilting of a moving ship and horizon. Most ships still carry a sextant as a backup to modern satellite-based navigation systems.”

Dr Kinrade’s early research career looked at how auroral activity in polar latitudes affects GNSS signals as they travel through the ionosphere, the charged layer of the atmosphere. Turbulence and scattering effects can cause GNSS signals to 'twinkle' and refract much like a star observed on a warm evening, and it can be a problem for high precision service users like ocean-based drilling platforms, air traffic control, and even timing applications like automated bank transfers. A major space weather event could cause major disruption to anything that uses GPS positioning and timing.

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