How Satellite Navigation Works

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Satellite navigational systems, or sat navs, have changed the way we navigate our way around the high roads. Gone are the days when travellers had to have a glove box full of maps and gone too is the need to stop and ask a local for directions.

Satellite navigation means that we an now go from point A to point B confident our systems will take us there and while sat nav systems are not fool proof (we must have all read the stories of people driving over cliffs and into rivers etc), it has certainly revolutionised our wayfinding.

Currently there is only one Global Navigational Satellite System (GNSS) the American run Global Positioning System (GPS). Although, a rival European System (Galileo) is set to go online sometime after 2012 and a both a Russian (GLONASS) and Chinese (COMPASS) system are being developed.

However, all these GNSS networks will operate using the same technology as employed by GPS, and in fact, current GPS systems should be able to utilise these future systems without much alteration.

The GPS system is basically a constellation of satellites (currently there are 27). These satellites each contain onboard an atomic clock (actually two are on most GPS satellites but for the purpose of this explanation only one need be considered). The signals that are transmitted from the GPS satellite contain several pieces of information sent as one integer:

* 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)

A satellite navigation receiver, the kind found on the dashbopard of your car, receives this information and using the timing information works out the exact distance from the receiver to the satellite. By using three or more of these signals the exact position can be triangulated (four signals are actually required as height above sea level has to be worked out too).

Because the triangulation works out when the time signal was sent and how long it took to arrive at the receiver, the signals have to be incredibly accurate. Even a second of inaccuracy could see the navigational information out but thousands of kilometres as light, and therefore radio signals, can travel nearly 300,000 km each second.

Currently the GPS satellite network can provide navigational accuracy to within 5 metres which goes to show just how accurate atomic clocks can be.

The Sat Nav How it Works

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The ‘sat-nav’ has revolutionised the way we travel. From taxi drivers, couriers and the family car to airliners and tanks, satellite navigation devices are now fitted in almost every vehicle as it comes off the production line. While GPS systems certainly have their flaws, they have several uses too. Navigation is just one of the main uses of GPS but it is also employed as a source of time for GPS NTP time servers.

Being able to pin point locations from space has saved countless lives as well as making travelling to unfamiliar destinations trouble free. Satellite navigation relies on a constellation of satellites known as GNSS (Global Navigational Satellite Systems). Currently there is only one fully functioning GNSS in the world which is the Global Positioning System (GPS).

GPS is owned and run by the US military. The satellites broadcast two signals, one for the American military and one for civilian use. Originally, GPS was meant solely for the US armed forces but following an accidental shooting down of an airliner, the then President of the US Ronald Reagan opened the GPS system to the world’s population to prevent future tragedies.

GPS has a constellation of over 30 satellites. At any one time at least four of these satellites are overhead, which is the minimum number required for accurate navigation.

The GPS satellites each have onboard an atomic clock. Atomic clocks use the resonance of an atom (the vibration or frequency at particular energy states) which makes them highly accurate, not losing as much as a second in time over a million years. This incredible precision is what makes satellite navigation possible.

The satellites broadcast a signal from the onboard clock. This signal consists of the time and the position of the satellite. This signal is beamed back to earth where your car’s sat nav retrieves it. By working out how long this signal took to reach the car and triangulating four of these signals the computer in your GPS system will work out exactly where you are on the face of the world.  (Four signals are used because of elevation changes – on a ‘flat’ earth only three would be required).

GPS systems can only work because of the highly precise accuracy of the atomic clocks. Because the signals are broadcast at the speed of light and accuracy of even a millisecond (a thousandth of a second) could alter the positioning calculations by 100 kilometres as light can travel nearly 100,00km each and every second –currently GPS systems are accurate to about five metres.

The atomic clocks onboard GPS systems are not just used for navigation either. Because atomic clocks are so accurate GPS makes a good source of time. NTP time servers use GPS signals to synchronize computers networks to. A NTP GPS server will receive the time signal from the GPS satellite then convert it to UTC (Coordinated Universal Time) and distribute it to all devices on a network providing highly accurate time synchronization.