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GPS -

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Paul Willis

As with so many of my modern gadgets, I've only got a vague idea how GPS works.

GPS

Go straight ahead. You have reached your destination

NARRATION

Today, I'm on a journey of understanding.

Paul Willis

Thorston.

Dr Thorston Kostulski

Hello Paul.

Paul Willis

Nice to meet you.

NARRATION

Dr Thorston Kostulski is a telecommunications engineer he'll know how it works.

Paul Willis

Now how does this thing actually work? How does it know where we are?

Dr Thorston Kostulski

Well it receives signals from CPS satellites which are organised in six orbital planes in an
inclination of 55 degrees around the earth.

NARRATION

Let's see if we can break this down. Here's our car with a GPS receiver and here's a satellite. The
satellite sends a radio signal at the speed of light, containing a code that identifies the
satellite, along with its precise orbital location and the time the message was sent. Using this
information, the receiver can calculate the distance to the satellite. But this gives a circle of
possible locations on earth. A second satellite reduces this to two possible locations, where the
circles intersect.A third satellite fixes one unique location. And this is how your GPS unit knows
where you are, wherever you travel on the planet.

Dr Thorston Kostulski

But there's a little problem.

Paul Willis

There's a problem?

Dr Thorston Kostulski

The problem is if your clock is off by only one microsecond this multiplied by the speed of light
you could be as far as 300 metres away from where you think you are.

NARRATION

What we need is a very, very accurate clock.

Dr Thorston Kostulski

This is an atomic clock which is the most accurate way of measuring time. There is one of these
aboard each of the GPS satellites and they cost around $100,000 each.

Paul Willis

Well, at that price you don't get one in every mobile phone.

Dr Thorston Kostulski

That's right, that's why you need the fourth satellite.

NARRATION

A fourth satellite reduces the error of the inferior clock in our cheap GPS device giving a more
accurate location. That's why there must always be at least four GPS satellites in view. But that's
not always possible.

GPS

"Recalculating, recalculating, recalculating"

Topics: Technology

Reporter: Dr Paul Willis

Producer: Anja Taylor

Researcher: Faye Welborn

Camera: Jeff Malouf

Sound: Anthony Frisina

Editor: Dylan Behan

Related Info

How GPS works

Story Contacts

Dr Thorsten Kostulski

Senior Research Engineer

Faculty of Engineering and Information Technology

University of Technology, Sydney