What GPS Actually Is?
What is GPS?
At over 20,000 Kilometres above sea level is a constellation of satellites, each orbiting earth every 11 hours and 58 minutes. These satellites are continuously beaming data down to us on earth, which in turn is received by devices such as your phone or navigational units in your cars, allowing you to see where you are on the planet. Alot of misunderstanding about how GPS actually works, an example being that your phone and the GPS satellites are both talking to each other.
How do GPS’s work?
GPS is the abbreviation of Global Positioning System, which works through trilateration, not triangulation, which is common misconceived. There are many different types of navigational satellite systems from countries across the world. But the most popular and commonly used system is Navistar, which is the USA system. There are however Russian, Indian, Chinese and European equivalent systems, although the Indian and Chinese systems sit in a geosynchronous orbit above their countries which means they are not worldwide systems. The Navistar system, which is simply referred to as GPS, is what we will be focusing more on. Although most phones and devices tend to have the capabilities to use both GPS and GLONASS. GPS satellites are set up in such a way that from almost anywhere on the surface of earth you should have a direct line of sight of at least four GPS satellite. This is quite important on the basis that GPS point positioning requires at least four satellites to calculate three position coordinates and the clock deviation. As GPS units are receivers, there needs to be something sending some sort of signal to devices, such as your phone to receive. Each GPS satellite broadcasts a navigational message towards earth which contains an extremely accurate timestamp (obtained through atomic clocks onboard the satellites), and the satellites also broadcast their position at the time of broadcast, with all GPS signals broadcasting at 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The information in these two bits allows you to begin to work out your position on earth. With the satellites all sending exceptionally accurate time down to earth, your phone or GPS receiver can compare the difference of time between the signal being sent and received to work out the distance between you and the satellite. By multiplying this time difference with the speed of light (as the signal is sent as the speed of light), you can get the distance you are from the satellite. As the satellites are also sending whereabouts they are, you can begin to draw spheres around the satellites, with you being somewhere on the outside border of the sphere, As we introduce more GPS satellites into the mix, we begin to get closer to where we are. By calculating the time differences between these satellites, we move from having no idea where we are, to be able to pinpoint where we are, typically down to five to ten minutes on average, with the potential error being around 15 meters. There are a lot of factors which escalate the potential error, but the most significant is due to the ionosphere, a part of the upper atmosphere extending from 60 km to 2,000 km, where free electrons occur frequently enough to have an appreciable influence on the proportion of electromagnetic waves passing through this layer. This error is substantially smaller when satellites are directly overhead, compared to being larger, the closer satellites are to the horizon relative to you as the path between you and the GPS satellite goes through more of the atmosphere compared to being directly overhead. Even things such as small variation in the atomic clocks found on board these satellites can cause a major error. This is compensated by the GPS’s clocks frequencies being slightly slowed down from 10.23 MHz to 10.22999999543 MHZ to cancel out the effects of relativity. So next time you turn on your GPS, you should realize just how much physics and maths is going into you finding your own location.