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How does GPS work?

Around 1500-1600 a major advance was made in dead-reckoning with the invention of a thing called the ‘chip log’.

A piece of light rope with knots at regular intervals was weighted so that it would drag in the water. As it was gradually released over the stern (back) of the boat, someone would count the knots that could be let out over a given period of time. From this you could work out the speed at which the ship was moving.

The faster the speed, the more ‘knots’. That’s why today, instead of miles/ kilometres per hour, sailors still measure speed in knots.

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