How does GPS work?

What is GPS, and how does it tell you where you are?
18 November 2016

Interview with 

Professor Terry Moore, University of Nottingham


These days, probably rely heavily on GPS. This stands for Global Positioning System, but how many of us actually know how it works? It was actually created by the US Department of Defence, and it's one of several branches of GNSS - or global navigation satellite systems. Terry Moore is Professor of Satellite Navigation at the University of Nottingham, and he got Georgia Mills up to speed on how having satellites around the world can tell me - and my smartphone - where I am...

Terry - You are positioning yourself with respect to the satellites. Now there's a general misconception about this, and this is sometimes popularised by the press and films and things, that it's a tracking system - the sort of "spies in the sky" are watching you all the time! When you think, currently, at the moment, there are four billion receivers in the world, so it clearly doesn't work like that!

So, what happens is, signals leave the satellites and they're transmitted down, just like a radio signal on the radio or the TV. So they're just transmitter signals and then your unit in your smartphone or your satnav in your car receives those signals. And what we actually do - it's all based on time. So the satellites have very precise clocks and they transmit coded signals that basically say when the signal left the satellite. And what your receiver does in your smartphone is it times when those signals arrive, and by knowing when it arrived, and when it left, we can work out the distance using the speed of light. So, by measuring the distance to all these different satellites, we can just do some geometry and work out where we are.

Georgia - Right. So I guess it's very important that the clocks are accurate then?

Terry - Well, this is one of the really neat tricks, yeah. Because, if we think about the speed of light, if we go back to that, and we think about something that we use every day - an A4 sheet of paper. Light will travel from one end of an A4 sheet of paper to the other in one nanosecond - one billionth of a second.

So if we are wanting to position and measure the distance, let's say, to a metre, then all this timing I've talked about needs to be syncronised to three nanosecond - three billionths of a second. So we've got to have clocks on the satellite, clocks in you phone which are all going to be synchronised - that sounds impossible. Now we can get very good synchronisation on the satellites because they're using very precise atomic clocks and they're constantly monitored so we know that they are on a stable timebase.

Georgia - And time, according to Einstein, is relative. So how do you deal with the fact that how time may be moving out in space and how it may not be quite the same as how it's moving here?

Terry - Yeah, this is a very important point and it's designed into the systems. We have to take relativity into account in two ways because these clocks, as we've said, the satellites are flying along at four kilometres a second. So that has a special relativistic effect and the time appears slower. But also the clocks are flying in a different gravitational field, a much lower gravitation field than on the surface of the Earth, and so we get a general relativistic correction and that makes the clocks appear to run faster. So combine the special relativity and the general relativity together, we would get the wrong time on the clock on the satellite.

So what happens is they are actually run at the wrong rate. We seem to have them running at the rate we expect the time to be running, but they are deliberately skewed so they are running at a different rate so that by the time we see it, we're seeing the correct rate - if that makes sense.

Georgia - Without going too much into the physics, yeah. In terms of the satellites, how many are up there and how long do they last?

Terry - They are designed to have around, let's say, seven to ten years. It varies from which constellation and which generation of spacecraft, but that sort of age of about seven to ten years is a good number to work with.

And how many do you need? Well, this depends on how far away the satellites are, okay. So the satellites we use for GPS, and for the European Galileo, and the Russian Glonass, they're all about 20,000 kilometres above the Earth in what we call medium Earth orbit. And so, how many do you need for that?

Well, you need about 24, 27, 30 satellites and the different constellation of those is their key numbers. Now GPS, at the moment, currently has 31 satellites of various generations and various ages, and it's slightly overpopulated because some of the older satellites are getting quite old and they're coming to the end of their their lives. So there are newer satellites there just in case the old ones happen to have problems.

Georgia - I suppose eight to ten years doesn't actually seem that long to me when you're actually having to send something into space. I mean who picks up the bill for this?

Terry - Yes, it's an expensive business. So it depends where you are in terms of who picks up the bill. If we're talking about GPS, then the US taxpayers have been very generously doing that for us for the last thirty odd years now. So GPS is owned and operated by the United States, and it's operated as a dual-use system for civil and military uses, but it's operated by the US Air Force Space Command, so they get the money from the federal budget within the United States and that's what pays for the system.  And, as you say, it is expensive. A satellite launch may cost in the order of 60/70 million dollars.

If you are in Europe, then you and I are paying for it. Our taxes are paying for the Galileo programme, which is paid out of the European Commission's budget or the European Union's budget. And the current bill that we are paying is 900 million euros per year is being spent out of the European budget to pay for the development and deployment of Galileo.

Georgia - So GPS is already available to us in Europe, why do we need Galileo?

Terry - The European Union is very interested in developing our own system so that then we are building our own spacecraft. We are building the ground segment, we are building the receiver segment, we are building the location services industries all based on that, and that's all jobs within Europe. So you can make an argument based on market penetration. You can make arguments on what we call sovereign control.

The way that GPS is owned and operated, as I said, by a single state, the United States of America and it is there as a military force enhancer. But it is now so critical in so many aspects of our lives in terms of the positioning and navigation, but also timing. If you remember what I said earlier on, when we're solving for the way we use GPS, we solve for time all the time and that means we have a very precise way of disseminating time around. And that's now used throughout what we call critical infrastructures, so telecommunications, banking, the internet, digital radio even are all based on timing provided by GPS and other satellite navigation systems.

But that's all under the control of another state as a defence tool. So in terms of sovereign control of critical infrastructure, we need to have our hands on the system which is keeping those critical services.


It was never explained how the GPS devices synchronise with the satellites...

The GPS unit is not syncronising with the satellite. Instead it is comparing the relative timing delays between the arriving satellite transmissions that reach it. These transmissions are themselves synchronised between satellites in space, and the machine knows this, but all the GPS receiver does is to decode the signals passively. The time code is written into each signal, so the machine counts when each arrives and can work out how much longer each took to arrive at the receiver. From this, because the speed of light is constant, the distance out to each relevant satellite can be calculated and from this the surface position computed.

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