24 Sep 2016

How GPS is changing our world

From This Way Up, 12:03 pm on 24 September 2016
GPS constellation

Photo: (By U.S. Govt Source - NOAA, Public Domain)

If you've used your cell phone, EFTPOS card, the internet or your car today then you've used GPS – the global positioning system developed in the '70s and owned by the US military.

Greg Milner

Photo: (Deborah Zeolla)

Greg Milner is the author of Pinpoint: How GPS is Changing our World.

Read an excerpt of the interview:

Simon Morton: Greg, take me through the data trial how does my phone work out exactly where I am using the GPS system?

Greg Milner: Every spot on earth – or almost every spot on earth – has a direct line of sight to at least four GPS satellites at any given movement. When you flip on your phone and turn on your map programme, it searches for the four strongest satellite signals. These satellites are broadcasting a constant radio signal and there’s information in this signal that tells the GPS chip in your phone exactly where the satellite was when the signal was released and the exact amount of time it’s taken to reach your phone from the satellite. If the phone can make these calculations with at least four satellites simultaneously it can extrapolate that data and translate it into latitude and longitude – and altitude data, if necessary – and then that is translated on to your map programme. So really it’s all about measuring the transmission time of these signals. That’s how it figures how far away it is from the satellites – and by doing that it can figure out where it is on earth.

Simon Morton: How long does a signal take to move from my GPS chip to the satellite? There have to be four satellites for it to be functional. That’s the critical bit here, isn’t it, the four?

Greg Milner: This is an important thing to remember about GPS because it’s one of the reasons why it was so valued by the military. Your phone is not transmitting anything. GPS is what’s called a passive navigation system, meaning all you have to do is receive the signal from the satellite. This was good for the military because it meant that it couldn’t be used by some hostile force to track where you are. It’s purely just about receiving that signal from the satellite and figuring how where you are based on how long it’s taken for those signals to reach you.