It seems that nearly every car dashboard has a GPS receiver perched on the top. They have become incredibly popular as a navigational tool with many people relying on them solely to work their way around the road networks.
The Global Positioning System has been around for quite a few years now but was originally designed and built for US military applications but was extended for civilian use following an airline disaster.
Whilst it is incredibly useful and convenient a tool, the GPS systems is relatively simple in its operation. The navigation works using a constellation of 30 or so satellites (there are quite a few more that are orbiting but no longer operational).
The signals sent from the satellites contain three pieces of information that are received by the sat nav devices in our cars.
That information includes:
* The time the message was sent
* The orbital position of the satellite (known as the ephemeris)
* The general system health and orbits of the other GPS satellites (known as the almanac)
The way the navigational information is worked out is by using the information from four satellites. The time the signals left the each of the satellites is recorded by the sat nav receiver and the distance from each satellite is then worked out using this information. By using the information from four satellites it possible to work out exactly where the satellite receiver is, this process is known as triangulation.
However, working out exactly where you are in the world does rely on complete accuracy in the time signals that are broadcast by the satellites. As signals such as the GPS travel at the speed of light (approximately 300,000 km a second through a vacuum) even a one second inaccuracy could see positioning information out by 300 kilometres! Currently the GPS system is accurate to five metres which demonstrates just how accurate the timing information broadcast by the satellites is.
This high level of accuracy is possible because each GPS satellite contains atomic clocks. Atomic clocks are incredibly accurate relying on the unwavering oscillations of atoms to keep time – in fact each GPS satellite will run for over a million years before it will drift by as much as a second (compared to the average electronic watch which will drift by a second in a week or two)
Because of this high level of accuracy the atomic clocks on board GPS satellites can be used as a source of accurate time for the synchronization of computer networks and other devices that require synchronization.
Receiving this time signal requires the use of a NTP GPS server that will synchronize with the satellite and distribute the time to all devices on a network.