The Basics - GPS and Satellite Navigation

02 May 2010

Interview with

Chaz Dixon, E.A.D.S. Astrium and Colin Lee, Garmin

Meera: - This week I'm on a mission to find out how the Global Positioning System, or GPS actually works.  My first stop is European space company, E.A.D.S. Astrium down in Portsmouth and with me here is Dr Chaz Dixon, Head of Future Programmes Navigation.  Let's kick things off with, what is GPS?

Chaz: -   Most people will be aware of GPS through things like their in-car sat navs.  Sitting behind that is a constellation of 24 satellites.  They were created by the U.S. military and through the generosity of the U.S. tax payer, there is a civilian service released worldwide for essentially free use for the rest of the world.

Meera: -   How does it actually work in order for the people relying on it to know where they are in the world?

Chaz: -   To understand this, we have to realise what "where you are" means.  We don't know where we are on a map.  We typically don't know what our height is either.  So we have three uncertainties: our position in latitude, longitude and height.  From the satellites - each of which has a precisely known location based on its ephemeris which is a data set telling you where it is - we can take a ranging measurement.  We now know our range from a satellite at a precisely known location.  That gives us this sphere of positions.  If we then take measurements from two and three satellites, we now have the intercept of two and three spheres which intercept at a single point which is the user's derived position.  Unfortunately, it's fractionally more difficult than that because the measurements are timing measurements.  We have to measure our position relative to the satellite transmitted time.  That means we have to make our fourth measurement to solve for our local time relative to system time.  So the satellite includes data that lets us know the time of transmission. If we then measure the time of reception, speed of light being constant, then we know the distance.  So four measurements gives you user position, and coincidentally for most people, time as well.

Meera: -   Now, as you've mentioned, GPS is a US system and Galileo is a European version which is in production at the moment, and is hoped to be in operation by 2014. Gallileo satellite The actual navigation components of Galileo are being made here at Astrium.

Chaz: -   That's exactly right.  Galileo comprises 27 satellites baseline plus three operational spares which means that the user will see 30 transmitting space craft.  Here in Portsmouth, we only deal with the nav part, with the payload.  The reason for the constellation is essentially to augment the GPS coverage.  To make GPS and Galileo truly compatible and therefore, the problems you see with the famous urban canyon where you may lose GPS coverage will be a thing of the past.

Meera: -   This will hopefully result in more accuracy as well.

Chaz: -   More accuracy and even more significantly more availability.

Meera: -   What's the current accuracy of GPS?

Chaz: -   The accuracy the people will see is typically five metres.

Meera: -   Now this ability to position yourself globally is extremely important today.  How important would you say this technology has become?

Chaz: - Really, it's been revolutionary and obviously the thing that many people will be familiar with is in-car sat nav.  On the back of that are commercial fleet management systems, another interesting community is aviation, immensely increasing the accuracy of positioning.  And another interesting community is the survey community who have much more demanding needs in terms of the accuracy that lets them do ultra precise positioning for trigger points for buildings, for oil exploration et cetera that never existed in the past.

GPS receivers from Trimble, Garmin und LeicaMeera: -   Now, as you mentioned, the one thing that most people are commonly aware of is the in-car sat nav.  So I'm off to see this in action and find out a bit more about how that works...


...I've now come down to Southampton, to the headquarters of satellite navigation company Garmin, to see some GPS devices in action.  With me is Colin Lee, product manager here at Garmin.  Colin, we're inside your car and you've got a lot of sat navs set up inside this car.  How do these actually work in order to get us to our destination?

Colin: -   Well basically, these are multichannel receivers which receive radio waves from the satellites that are orbiting above us.  They all beam a signal at the same time, telling us the position of where they are and also, the delay when they come down to us.  We can then determine where we are in relationship with them.

Meera: -   So now, we're going for a drive in order to see theis sat nav in action.  So where are we going?

Colin: -   We're going to the Southampton Maritime Museum.

Meera: -   Okay, great.  Let's set off then.  I've never been there...

GPS Audio -   "Please drive to highlighted route..."

Meera: -   How is this working, Colin, in order to just know that we're exactly on this road and that we're heading in the right direction towards the Maritime Museum?

GPS Audio -   "Turn left..."

Colin: -   The satellites are beaming their position and location so we can then plot that data onto a map which is inside the unit - latitude and longitude positioning.  So we take that data, we then put that data onto a map and we then show that on a graphic display so you can see where you are, in relationship to the satellites.

Meera: -   It's quite clear roads here actually.  So it looks like it will be quite a pleasant journey getting there, but if it were rush hour, how would the traffic predictions within these sat navs work in order to help you find a quicker route home?

Colin: -   We've got two types of way you can deal with traffic.  One of them is the live traffic that comes off with these sensors next to the road which then get sent back via fm signals to the car; and the other way doing it is you have data - historic data which we can refer to so we can avoid things like road blocks and so on.

Meera: -   And what about if signals are suddenly lost?  What could happen in order for you not receive a GPS signal anymore?

Colin: -   The signal is the same as any radio signal and any radio signal could be affected by rain, it could be affected by buildings, and so, if you're in a forest where it's been raining, then it could affect your signal.  If you're between buildings in London or something like that, then the reflections from the buildings can cancel signals.  But as soon as you come out of that area, it's only a small area normally, then the signals come back to normal and you're up and running.

GPS Audio -   "Turn left on Bugle Street..."

Meera -   Oh!  And we've reached our destination then.  That didn't take very long.

GPS Audio -   "Arriving at Southampton Maritime Museum on right..."

Meera: -   Now what could be coming up in the future of this technology in order to make journeys like that even smoother?

Colin: -   We'll try to get better accuracy by using the Galileo system which is going to be all over Europe soon.  We will try to personalise sat navs more, learn more about you and your car, and we'll be looking more into the environment.  We'll be improving 3D technology so it makes it easy for you to see your environment around you.  And by the way, the reason why I brought you to the Maritime Museum is because 100 years ago, they used sextants and stars to actually navigate whereas today, you're in my car using a sat nav, using satellites instead of stars.

Helen: -   GPS certainly is changing all our lives - I'm thinking about getting a satellite navigation system for my bicycle. That was Colin Lee, from Garmin, they are one of the many manufacturers of satellite navigation systems, and before that was Chaz Dixon, E.A.D.S. Astrium talking to Meera about the science behind the latest in positioning and satellite navigation.

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