BBC Focus Magazine

[Graham Ogle]

Hello,
Been puzzling over a problem. How does a GPS know which direction you are facing or which way to point the direction of travel needle? GPS will tell you where you are, but it doesn't know in which direction you are facing. If you are moving, it can work out your direction of travel and hence north, but standing still, it has no idea.

Now it may be that some GPS units have an electronic compass in them which controls the direction of travel needle, but what happens between Arctic Canada and the North Geographical Pole? As you leave Northern Canada, the North Magnetic Pole is somewhere north and east of you. As you travel further, it is east of you and at the Geographic Pole, it is to your south ( and east - before someone points out all points are south of you at the North Pole). Under these conditions, even an electronic compass will fail unless it has a lookup table of corrections for each position.

Just wondering if anyone has any ideas please?

[M Paul Lloyd]

As I understand it based on the early military units I came across in the 1970-80's (and I admit I may not be correct ) the signals from the satellites that triangulate your position arrive at your gps unit at frequences relative to their respective directions and velocities and a clever bit of software in your sat nav uses this information coupled with its own digital compass to calculate which way it, and therefore you are facing.

But I stand ready to be corrected on this.

[Jamie]

If you were stationary, I think it would have difficulty. However, if you are moving it would be simple trigonometry to calculate where you were heading.

[M Paul Lloyd]

Done some checking and Jamie is right and I was only partly so as your basic road navigating gps doesn't have an internal compass.
So you have to be moving for the gps to determine which direction you are going which is calculated from the satellite signals.

[Graham Ogle]

Certainly my car Garmin satnav knows which way I am pointing if I stop, i.e. have been travelling, but it's not so good when I turn it on for the first time.
Handheld GPS's have a direction of travel arrow so I'm assuming they have an electronic compass in them. Waiting for a friend to get back to me with an answer on their use at high latitudes.

[Graham Ogle]

Although a sample of one doesn't cover all options, a friend has been up in the Arctic and he uses the sun's shadow or a sundial to identify north. If using a compass or a GPS, he uses a look up table to correct the north facing arrow according to his latitude. Sounds as if the GPS uses an electronic compass for knowing which way is north.

[Colm]

As you leave Northern Canada, the North Magnetic Pole is somewhere north and east of you

Well if it knows where you are, then it can use Maths to figure out where the North Magnetic and Geographic poles are relative to you. It is fairly simple then for a computer to use that info to figure out which way you are facing, relative to the North Geographic pole, if it knows which way you're facing relative to the Magnetic pole.