You've used GPS countless times, probably without a second thought about the constellation of spacecraft making it possible. Behind that little blue dot on a map is a beautifully simple idea executed with great precision: a set of satellites broadcasting signals that let a receiver work out exactly where it is. Let's open up how it really works.

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GPS Is One of Several Navigation Systems

GPS belongs to a broader family called Global Navigation Satellite Systems, or GNSS - the general term for navigation systems that use signals broadcast from global satellite constellations for positioning and navigation. GPS, whose full name is the Navigation System with Timing and Ranging Global Positioning System, is the best-known example.

It works on the principle of radio-navigation, which has a specific meaning: the users receive signals to determine their own position. That word "own" is important. Your receiver isn't being tracked from above - it's listening to broadcasts and figuring out its own location.

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What a Receiver Actually Works Out

From the signals it receives, a smart receiver can compute several things at once:

• Its three-dimensional location - longitude, latitude and altitude.
• Precise time.
• Its velocity.

That combination of position, navigation and timing is the whole point of the system. And because the receiver does the processing itself, an essentially unlimited number of users can use the service simultaneously, anywhere in the world, in any weather, day or night.

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The Constellation Above You

For this to work everywhere, you need satellites well distributed around the planet. The GPS constellation is built from 24 satellites arranged in six orbital planes, inclined at 55 degrees, orbiting at an altitude of about 20,200 kilometres with a period of roughly 12 hours.

That altitude is chosen with care. From 20,200 kilometres up, each satellite can "see" about a third of the Earth's surface at once. Spread the right number of satellites across the right planes, and you ensure that a receiver almost anywhere on Earth can pick up signals from enough of them at any given time.

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The Signals That Carry the Information

Each GPS satellite broadcasts on shared carrier frequencies - designated L1 at 1575.42 MHz and L2 at 1227.6 MHz. While the frequencies are common to the constellation, each satellite transmits its own specific codes, which lets a receiver tell the satellites apart.

Crucially, each satellite broadcasts not just an identifying code but also data, including the system time and information about the satellites' positions. So every signal arriving at your receiver effectively says "this is which satellite I am, this is the time, and this is where I am." That's the raw material the receiver needs.

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The Maths Behind Your Position

Here's where it comes together. Because each satellite tells the receiver the precise time its signal was sent, and the receiver knows when the signal arrived, it can work out how far away each satellite is - the signal's travel time corresponds to a distance. Knowing the distance to one satellite places you somewhere on a sphere around it. Add a second satellite and the possibilities narrow. Bring in more satellites and their combined distance information pins down your single location in three dimensions, along with precise time.

This is why timing is at the heart of GPS. The system is as much about precise time as it is about position - the two are inseparable, which is why the field is often described together as positioning, navigation and timing.

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A Tip for Grasping the Concept

Imagine friends standing at known spots, each shouting "I'm here, and it's exactly this time!" If you know how fast sound travels and exactly when each shout was made, the delay before you hear each one tells you how far you are from each friend. Combine several and you can pinpoint your own spot. GPS does the same thing with radio signals from satellites instead of shouts from friends - and the satellites' precise clocks and known positions are what make the trick work.

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Conclusion

GPS works by having a constellation of 24 satellites, orbiting around 20,200 kilometres up, each broadcasting its own coded signal carrying the time and its position. A smart receiver listens to several at once, uses the signal travel times to work out its distance from each, and from those distances computes its own three-dimensional location, velocity and precise time. It's a member of the wider GNSS family, built on the elegant principle of receiving signals to find yourself - and timing is every bit as central to it as position.