X marks the self


The satellite system PHOTO/PCMag

Pinpoint: How GPS Is Changing Our World by Greg Milner (Granta)

In August, a man with a sword was arrested near Buckingham Palace on suspicion of preparing to commit an act of terrorism. Westminster Magistrates Court heard that the man, an Uber driver from Luton, had intended to go to Windsor Castle but his satnav directed him to a pub called The Windsor Castle instead. Without stopping for a drink, he drove on to Buckingham Palace. It isn’t clear if he was still relying on the satnav for the final stage of his journey, or whether rage at the mistake was a motivating factor in his alleged offence. Three police officers were said to have received minor injuries; presumably he hadn’t stopped to ask them for directions.

Greg Milner includes a few stories about satnav fails in Pinpoint, his lively history of satellite navigation technology – his central chapter is called ‘Death by GPS’ – but one of the eye-opening things about his book is quite how far-reaching the tech is. As well as guiding missiles and encouraging motorists not to pay attention to road signs or even to the road ahead of them, GPS is used in crop management, high frequency trading, weather forecasting, earthquake measurement, nuclear-detonation detection and space exploration, as well as the smooth running of countless infrastructure networks, from electricity grids to the internet.


GPS, which stands for Global Positioning System, was developed by the American military. The US Department of Defence currently spends more than a billion dollars a year maintaining it. There are 31 GPS satellites orbiting the earth, all monitored, along with hundreds of other military satellites, from Schriever Air Force Base in Colorado. For the system to work, a receiver on the ground – your mobile phone, for example – needs to have a ‘line of sight’ to at least four of the satellites (there are very few places on earth where it wouldn’t). Each satellite continously broadcasts its position, along with the time the signal left the satellite. The time it takes for the signal to reach you (measured in milliseconds) will tell you exactly how far away it is. Three of these signals provide enough information to pinpoint your position; the fourth confirms the time used in the calculations. GPS satellites, unlike mobile phones, carry super-accurate atomic clocks, which are continually synchronised with one another. This is necessary for the precision of the positioning system, but many of the applications of GPS make use of it primarily as a timekeeping device.

Since 1967, the second has been defined as ‘the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom’. A pendulum clock uses gravity to make a pendulum oscillate at a measurable frequency; a quartz clock uses electricity to make a quartz crystal oscillate at a measurable frequency; an atomic clock uses microwaves to make caesium (or similar) atoms oscillate at a measurable frequency. In the 1970s, the only way to synchronise your atomic clock with the one at the International Bureau of Weights and Measures was to take it to Paris with you and compare them side by side. Now it’s all done by satellite signals. GPS time is also what enables stocks and shares to change hands in microseconds, prevents power surges in vast electrical grids and keeps the internet ticking smoothly.

London Review of Books for more

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