by Paul Ducklin
I bet you’ve heard of GPS, short for Global Positioning System.
It’s owned and operated by the US government but it’s available for free to anyone in the world – and, boy, is it widely used.
GPS is a fantastic feat of science and engineering that is anything but simple in implementation, but that is fairly simply explained.
A number of orbiting satellites (31 are active at the moment) continuously broadcast both their position in space and the current time.
Radio receivers on earth listen out for these broadcasts, and as long as they can “hear” the signals from three different satellites at the same time, and have their own reliable way of measuring the time, they can solve a system of mathematical equations to compute their own position.
The calculations rely on the fact that the time it takes for the signal to travel from the satellite to the receiver determines its distance, and with three distances you can lock in your position uniquely in three dimensions.
The time from satellite to receiver pinpoints the distance reliably because radio waves travel at a constant speed, and distance = speed × time.
Radio waves, known collectively as EMR, short for electromagnetic radiation, travel at what’s commonly called the speed of light, because light is just a special type of radio wave in the right frequency range to set off the detectors in the human retina. This speed is denoted by c, as in the famous equation E = mc2, and is defined in the GPS standard as 299,792,458 metres per second.
Fascinatingly, GPS positional calculations need to take Einstein’s theories of relativity into account.
The satellites are moving very fast relative to a receiver on earth, which makes their clocks seem to run slowly to us.
Author: Paul Ducklin