Mankind has a long rich history of travelling and exploring. Land based communities have long used well worn paths, trade routes and landmarks as a means of navigating.
Island communities wanting to go out of sight of the coast line have had a harder time. Navigation by the stars has been a harder task given the movement through the night or being obscured by cloud.
Britain had a long history of sea faring and its sea farers had long known of the troubles navigating long ocean voyages using astronomical navigation. Cumulative errors resulted in them missing their targets with many ship wrecks.
It had been long known that one of the sources of error was the accurate measuring of time. Pendulum and escarpment based time keepers of the day were adversely affected by the movement of a ship. This had become such an acute problem that in 1714 the UK parliament passed the Longitude Act, a bill administered by the Board of Longitude with incremental awards being offered for simple and precise methods to establish longitude. Britain was by no means the first to offer awards for this.
The use of astronomical data for navigation meant the Royal Observatory at Greenwich was the base line for astronomical observations used by sea farers and thus the ‘zero’. This ultimately lead to Greenwich being the zero in GMT (Greenwich Mean Time) or UTC (Coordinated Universal Time).
Early space exploration revealed some interesting and useful methods as scientists developed tracking abilities of deployed satellites. Whilst many names are associated with the development of GPS such as Roger L. Easton, Ivan Getting and Bradford Parkinson to name a few, I am going to single out Dr Gladys West for her work on the math for an extremely accurate geodetic Earth model, a part that is crucial to the accuracy of GPS.
In 1973, the US department of defence started the Global Positioning System project. The full constellation of 24 satellites entered service in 1993 with the first prototype launched in 1978. Although initially only available for military use, in 1983 the use of the system become available for everyone although the US military still had a better class of service than the public. This changed on May 1, 2000 with full public availability being made.
Although GPS is the most well known, there are other satellite based navigation systems with modern silicon devices being able to make use of them.
GPS works by using trilateration, deriving a position by measuring distance from a number of satellite signals and involves an accurate measure of time, something I am sure John Harrison would have liked to see.