Modern smartphones have many useful features, but probably the single most helpful is how they know exactly where you are at any given moment. This allows you to call up maps of the local area, let the phone direct you to destinations and more. All of these are made possible due to the magic of the ‘Global Positioning System’ or GPS. Today we’re going to take a look at exactly how this system uses satellites and clocks to tell exactly where you are in the world!
A Brief History Of Not Getting Lost
If we use myself as a reasonable example of a regular person, then humanity has spent 75-80% of it's time on earth confidently going in the wrong direction, turning around in circles with a confused look, and generally getting lost. People have tried many ideas to fix this issue: maps are always a good solution, but they can require a high degree of skill to work out where you are. Telescopes are great, but don't work too well in the dark. Signal flares are helpful if you get too lost, but Apple hasn't worked out a way to make the iPhone function as a emergency flare just yet… at least more than once.
During the Great Wars, radio navigation was used to determine the position of aircraft. In effect, it used radio waves to determine the direction of the source - either by using a directional antenna on the plane or by a rotating beacon on the ground - and calculating the relative bearing from where the signal was strongest (i.e - pointing directly towards the aircraft). While these systems did allow for geolocation, they were slow and inaccurate, and a better solution was needed.
Then in 1973 the US military started a project to create a new navigation system for their forces. It built off previous radio navigation technologies, but used satellites as the basis for triangulating position. This was significantly more powerful and accurate - allowing geolocation to be accurately pinpointed from anywhere on the planet. While this system was originally designed solely for the military, President Regan opened it up to civilian use in the 1980s. This measure allowed for civilian GPS systems to be designed, leading to the arrival of SatNavs, GPS watches and GPS-enabled smartphones. Finally, no more getting lost!
Alternate GPS systems
While the system is publicly available, GPS is still owned, maintained and controlled by the US government. This means it has the ability to restrict access if it chooses - which has led to several countries developing alternate GPS systems to break America’s monopoly. Other systems include the Russian ‘Global Navigation Satellite System’ (GLONASS). the European Union ‘Galileo’, and China’s ‘BeiDou Navigation Satellite System’.
While independent, some of these systems can run alongside GPS - using the additional satellite coverage to enable faster and more accurate positioning.
How GPS Works
So the question on your mind is likely: how exactly does Global Positioning work? Well as with many technologies, the underlying idea is surprisingly simple. It sends a request to several satellites, and uses the speed of their replies to work out your current position. However let’s go into a bit more detail:
As we’ve seen, most navigation systems work by sending a radio signal to a known signal tower, and then measuring the speed and/or direction of the return from that point. One of these measurements will allow you to tell which direction you are facing (your ‘heading’) but not a specific distance. However by taking several measurements from different towers, they can be plotted together to ascertain your exact current position. This concept is known as ‘triangulation’ or ‘trilateration’.
GPS differs from previous navigation systems because it uses space-based satellites rather than ground-based towers. There are currently 27 GPS satellites in orbit around the earth - 24 in active use, and 3 backups just in case. Each of these satellites has a unique identity and a precise atomic clock to provide a very accurate time measurement. When trying to find a position, a GPS system runs through these steps:
The GPS receiver in your phone or car determines which of the satellites are closest and unobstructed, and sends signals to them. To achieve an accurate reading, the system needs to use 3 satellites at once.
The satellites receive the signal, and beam back a reply. This reply takes the form of the current time, as measured by an atomic clock inside the satellite.
The GPS receiver measures the difference between the time encoded in the reply, and the time it received the signal. With this difference, and the known position of the satellite, it works out how far the satellite is away from you.
By taking readings from 3 satellites at once, the GPS system maps out a radius from each satellite. The point where these intersect is your current position.
When GPS works, it’s highly accurate - able to pinpoint your location to within a matter of meters. However it has several disadvantages. The first is that you need a clear line-of-sight to the satellites - meaning buildings or mountains could block the signal path. Secondly, sending GPS signals requires a lot of power, and can take a long time - up to a minute per signal.
Therefore, smartphones use
Assisted GPS (A-GPS) to make things easier for the satellites. They augment the GPS signal with the location data from your phone network - your network knows approximately where you are because of the
cellar towers being accessed by your phone. By using a mixture of the GPS satellite data and the cellular data, the GPS receiver in your phone is able to pinpoint your location much faster, with far less power usage.
That is the basics of how the Global Positioning System works! I hope it’s helped de-mystify what GPS is, and make it seem less like magic. So next time you get lost, pull out your smartphone and the wonders of satellites and math will help direct you home!