Have you ever found yourself asking "How do Sat Navs work?" Well, there's more to the little gadget that comfortably sits in it's holder, affixed to your window, than there may seem.
So Sat Nav, short for Satellite Navigation, does exactly as it says on the tin. It navigates you on your journey using satellites that orbit the Earth - and trust me there's a fair few. In total, there are around 9,494 satellites currently in orbit around the Earth which report weather, research and perform numerous other tasks.
The main GPS satellites are as follows, and are split into regions:
- 35-38 usable satellites are in the GNSS and 28 of those are active GPS satellites controlled by the USA, with the first launch happening in 1978. The GNSS was also known as "NAVSTAR", which was the USA's original satellite system used in the Cold War to guide intercontinental missiles across the world (fun fact there!).
- GLONASS is formed up of 28 satellites, which covers the Russian area of the world.
- The Galileo system of satellites contains 30 MEO satellites, with 3 of those being spares. Galileo covers the majority of Europe.
- BeiDou-1 & BeiDou-2 covers China, and respectively have 4 satellites and 35 satellites.
Signal Transmission
Each satellite transmits signals that include:
- The time the signal was sent
- The position of the satellite when the signal was sent
These signals are electromagnetic waves that travel at the speed of light and are called Ephemeris data.
Sat Nav Signal Reception
To determine your position, a Sat Nav device must receive signals from at least four satellites. A Sat Nav uses its built-in receiver to capture these signals, which take time to travel from the satellites to the receiver on Earth. The delay in receiving each signal helps the device determine how far away each satellite is.
Triangulation
Using the distance measurements to at least three satellites, the device can perform a process called triangulation to calculate its precise location. Here’s how it works:
- Each distance measurement places you somewhere on a spherical shell at a specific radius around each satellite.
- The intersection of these three spherical shells narrows your location down to two points.
- The inclusion of a fourth satellite provides additional data to resolve ambiguities (like which of the two points is your actual location) and to calculate your altitude.
- In actual fact, most systems will use at least 10 satellites - to ensure the most accurate result possible.
From here, your location is super imposed onto a digital "map" which allows you to see where you're in comparison to where you need to be. Using these digital maps with your location on from the satellites allows your Sat Nav device to measure turns or changes in the road, and can direct you safely to where you need to be!
Some Fun Facts About Sat Navs
- Early GPS receivers could only calculate to within 100 meters of your location due to being downgraded by the US military.
- In the 2000s, they upgraded sat navs to within 15 meters of your location.
- The US military can still scramble or completely turn off select satellites to perform certain tasks, such as covert operations.
- Vehicle Sat Navs incorporated compasses, accelerometers and other sensors to help with travel accuracy.
- Vehicle Sat Navs will always presume that you're on a road (so if signal is lost it will still continue to work based on other sensors and data) if the reading is slightly out.
