Jacinta Bowler
Twenty thousand kilometres above our heads, 31 satellites are whizzing past each other so that we can understand exactly where we are on the planet. This group of satellites is called GPS, or Global Positioning System. But have you ever wondered how these satellites actually work? Or if they could suddenly stop working and you’d have to pull out the old street directory?
Let’s dive into everything you ever wanted to know about the science of GPS.
How does GPS work?
Say your watch wants to know where it is when you’re out for a run – it’ll listen out for radio signals from the GPS, and once it has clocked at least four of them it can give you a pretty good estimate (down to a metre or two) of where in the world you’re located.
It also gives you the precise time, which we’ll come back to later.
The satellites for GPS – then called Navstar GPS – were first launched back in 1978, and a constellation of 24 satellites was operational by 1993. They’re positioned in ‘medium’ Earth orbit – above the rabble of low orbit satellites, but below the geostationary satellites. This is to ensure that they can see enough of the Earth that only 20-30 satellites are needed, but not so high that the radio waves can’t reach us back on Earth.
It was created by the US Department of Defence for military use but has been used by civilians for almost its entire operation.
Unlike phone towers or WiFi, the signals that GPS emit are completely one way – this means there’s no way to track who is using the system.
Professor Samuel Drake, a Flinders University researcher focusing on electromagnetic systems security, explains that GPS is a bit like a bunch of people shouting at you.
“If you shout and I know exactly where you are and at what time you shouted, I can work out how far away I am from you. That gives me a circle,” he tells Cosmos.
“And if somebody else shouts, I get the same information. If I know exactly where they were, and exactly what time they shouted, I can do an intersection of circles.”
GPS doesn’t just do this on a 2D plane either – it works in 3D, meaning your phone knows if you’re at the top of a hill or underneath it.
Is GPS the only version?
Although many of us use the word GPS to mean the system that helps us know where we are, there’s actually more than one ‘Global Positioning System’. Collectively they’re known as Global Navigation Satellite Systems (GNSS).
Paulsava/Wikimedia
Currently the US owns GPS, but Russia, China and the European Union all have their own versions that work much the same way.
“If you’ve got a running watch it will switch without you realising,” said Drake.
“I think predominantly, it’s still GPS and GLONASS [the Russian version], but it can and does switch to picking up whatever satellite is working best.”
GLONASS was developed at the same time as the GPS, but the others came later, after the US degraded the accuracy of GPS for civilians from a few metres to up to 100 metres.
The way they did this is pretty fascinating. When the satellites beam out their shouty signals, what they’re shouting is also important. Embedded in their 1.5 GhZ signal is information on where the satellite is and what time it was when the signal was sent.
The military added a ‘selective availability error’, which caused the GPS satellites to broadcast the time slightly wrong. Only those who knew the classified ‘key’ of how much the time was out could use the GPS accurately. Fixed stations popped up to be able to try and minimise the error.
“From a lot of pressure from the car market, they essentially switched [selective availability] off but at any time they’d like they can switch that back on and just deny access,” says Drake.
“The US military owns the GPS system, and they are perfectly within their rights to switch it off at any time.”
Can GPS go down?
Aside from the US government deciding to stop GPS from working, is it likely that it, or another GNSS, could stop functioning?
Drake says that’s unlikely. And even if it does, there’s some failsafes.
“It would not be instant. If we lost GPS, and we couldn’t go to GLONASS or Galileo, or one of the other ones that’s around, you’d still have a bit of time before things started to degrade.
“That clock would, what we call, ‘drift’. So instead of 1000 people being able to call at the same time, it would drop down to 100 and then 10. It would be a steady degradation.”
That’s not to say that parts of the process can’t have issues. In the last few weeks, a satellite called Inmarsat I-4 F1, which enhances GPS to accuracies of a few centimetres, went down for 12 hours, severely affecting farming equipment.
But the GPS section of the process was fine. The service failed because the Inmarsat I-4 F1 lost power.
Another problem that can happen to GPS is what’s known as ‘jamming’. GPS transmits at a measly 50 watts or less – about the same as a regular lightbulb – and by the time it reaches the Earth, it’s the equivalent of 0.0000000000000000001 watts. This makes the signal pretty easy to interfere with.
“Jammers just transmit a signal at that same frequency but at a higher power,” says Drake.
“[Satellites are] 20,000 kilometres above your head, and you’re still seeing the signal, so it’s very, very weak.”
Although GPS jammers are illegal in Australia, they do still slip into the country. It can also accidently happen, but this happens less now than it used to.
And finally, why didn’t we know any of this stuff before?
“Probably in the same way that I’m not really aware of how all my internal organs work, but they’re really important to me,” says Drake. “We just sort of use it.”