It listens for signals from whole constellations of satellites in orbit,

and by triangulating all those signals and using a lot of maths, it works out where you are and shows you a little dot on a map.

But how do the satellites know where they are?

There aren't any landmarks in orbit for them to refer to.

If satellites are in a high enough orbit, well away from the atmosphere, then they can predict their position.

They'll just keep orbiting in about the same path: there's no atmosphere to slow them down.

But that's not precise enough.

There will be distortions caused by gravity from the sun and moon, or even from mountain ranges down on Earth.

So maybe they could use the other satellite signals, like GPS.

But if they're all relying on each other, that could steadily drift apart from reality.

Maybe they could track the stars or the features down on Earth, but that's not precise enough either.

At some point, something down here on Earth has to look up in the sky and work out the position precisely.

And one of those somethings is hidden away in the countryside of southern England.

It looks like a regular observatory, but it's not looking at the stars.

We track manmade satellites in various orbits doing various jobs.

They all have to have special reflectors on that enables the light to be returned to us.

Uh, and so we can measure the time of flight, and from the time of flight, derive the distance.

You send out a very short pulse of laser light. You do that a thousand times a second.

Each of those shots, because you're using such a tiny short-pulsed laser, is 10 picoseconds long, which is about 3mm.

That's the limitation on your precision. So each shot is about 3-4mm precision.

This station works around the clock, whenever the sky is clear.

That's why I'm here today: there will be some genuine observations when the clouds break, but right now, I'm not actually disrupting anything.

But it does look significantly better after dark.

Because it's a laser, it's a very specific wavelength, so you can filter, and that's what lets you work in day and night.

The main problem with the atmosphere is: the speed of light in a vacuum is a very well known constant.

The speed of light through the atmosphere is variable according to the mainly the pressure, but also temperature and humidity.

So we measure those things on site, as well.

There's probably about 35 active stations scattered around the globe, located predominantly in the northern hemisphere, which is a slight problem from a science point of view.

You'd like them evenly distributed all around the globe.

There's a couple in Australia which are really important because they're providing a bulk of the data from southern hemisphere.

Ideally, we'd have a more even distribution.

Of course, as the light fades, there's one assumption that we've been working on throughout this video: that the earth, the ground we're standing on, doesn't move.

And that's not strictly true.

In the early days of laser ranging, in the 60s, it was the primary method by which they were deriving those early measurements of the drift between us and North America.

So you can see long-term trends, like the ice caps melting and the redistribution of mass from the ice caps to the equator.

All the data goes off to international databases, from all the sites all around the world.

Anyone, any researcher, anywhere can go and grab the international data set, the data from us, from Australia, from North America, from China, from Russia,

and can use that data to derive the orbits and do the research they need to do.

Thanks very much to all the team at the Space Geodesy Facility. You can find out more about them and their work at the link in the description.