GPS stands for Global Positioning System. It's a network of 31 satellites orbiting Earth about 20,200km above the surface, each continuously broadcasting a radio signal containing one piece of information: exactly what time it is and where the satellite is right now. Your phone uses those signals to figure out where it is. The maths is elegant.
Triangulation from satellites
Radio signals travel at the speed of light. By measuring how long a signal took to arrive from a satellite, your GPS receiver can calculate how far it is from that satellite — distance equals speed multiplied by time. One satellite gives you a sphere of possible locations. A second satellite narrows it to a circle. A third satellite narrows it to two points (usually one of which is obviously wrong — in space or underground). A fourth satellite pins down your exact position in three dimensions.
Imagine you're blindfolded in an unfamiliar city. Someone calls you and says "I'm 500 metres away." That could be anywhere in a huge circle around them. A second person calls: "I'm 300 metres away." Now you're at the intersection of two circles — still two possible spots. A third person calls from 400 metres — and there's only one point that's 500m from person 1, 300m from person 2, AND 400m from person 3. That's you. GPS satellites are those callers, and the distances are calculated using signal travel time instead of guessing.
Why does GPS need such precise clocks?
Light travels 30cm in one nanosecond (one billionth of a second). An error of just one microsecond (one millionth of a second) in timing introduces 300 metres of position error. GPS satellites carry atomic clocks accurate to one nanosecond. Your phone's clock is far less accurate, which is why the fourth satellite is needed — it's used to correct your receiver's clock error as well as to fix your position.
What's the civilian accuracy?
Modern consumer GPS is typically accurate to 2–5 metres. The US military, which operates the GPS system, has access to more precise signals. Augmentation systems (like the European Galileo network) can increase precision to under a metre for applications like precision agriculture and autonomous vehicles. GPS has become so fundamental to modern life — not just navigation, but timing signals for financial transactions, mobile networks, and power grids — that a GPS outage would be genuinely catastrophic.
GPS stands for Global Positioning System. It's a network of 31 satellites flying around Earth. They fly about 20,200km above us. Each satellite sends out radio signals all the time. The signals tell us exactly what time it is. They also tell us where the satellite is right now. Your phone uses these signals to work out where you are. The maths is really clever.
Finding your position using satellites
Radio signals travel at the speed of light. Your phone measures how long a signal takes to arrive. Then it can work out how far away the satellite is. Distance equals speed times time. One satellite tells you that you're somewhere on a big circle. A second satellite makes the circle smaller. A third satellite narrows it down to just two spots. Usually one spot is silly - like in space or underground. A fourth satellite tells you exactly where you are.
Imagine you're playing hide and seek with a blindfold on. Your friend calls out "I'm 10 steps away from you!" You could be anywhere in a big circle around them. Then another friend shouts "I'm 6 steps away!" Now you know you're at one of two places where the circles meet. A third friend calls "I'm 8 steps away!" Now there's only one spot that's exactly 10 steps from friend 1, 6 steps from friend 2, and 8 steps from friend 3. That spot is where you are! GPS satellites work like your friends calling out. But they use the time signals take to travel instead of counting steps.
Why does GPS need really good clocks?
Light travels 30cm in one nanosecond. A nanosecond is one billionth of a second. If the time is wrong by just one microsecond, your position is wrong by 300 metres. A microsecond is one millionth of a second. GPS satellites have special atomic clocks. These clocks are accurate to one nanosecond. Your phone's clock isn't as good. That's why we need the fourth satellite. It helps fix any mistakes in your phone's clock.
How accurate is GPS for normal people?
Normal GPS is usually accurate to 2-5 metres. The US military runs the GPS system. They have access to more precise signals. Special systems like the European Galileo network can be even more accurate. They can pinpoint locations to less than one metre. This helps with farming machines and self-driving cars. GPS is really important for modern life. We don't just use it for directions. Banks use it for timing money transfers. Phone networks need it too. Power stations use GPS timing as well. If GPS stopped working, it would cause huge problems everywhere.