View Full Version : How GPS works by professor Want A. Fanta

11-04-07, 22:58
GPS is a system of navigation that lets you find out where you are any place on the earth. You need a special radio called a GPS receiver. GPS stands for Global Positioning System. It uses 24 orbiting satellites to gain a "fix" on a GPS receiver on the earth's surface. It's amazingly simple to understand.
Each satellite is in a geo-synchronous orbit. That means the satellite's angular speed around the earth matches the earth's angular speed. If it takes 24 hours for the earth to turn once, it must take exactly 24 hours for the satellite to complete one orbit. That way, the satellite is always in the same place in the sky, just like the overhead light on your ceiling.

This is where it gets fun. If you know where Satellite 1 is, and if you know how far away it is from measuring its radio signal, then you know that your position could lie anywhere on a circle with Satellite 1 at the center and above.

Now, if you also know that your distance from Stallite No. 2 is x miles, then your position also lies on a circle x amount of miles from satellite No. 2! Now you know that your position lies on either of two points where the circles intersect.

And if you measure your distance from yet a 3rd Satellite, you now will know which of those 2 points your position lies on. Now you know exactly where you are. This is how modern airliners navigate across the ocean. In fact, they even use it on common land routes because of its precise accuracy.

In the figure below, your position is at point b, where the 3 circles intersect one another.


12-04-07, 23:24
In the figure below, your position is at point b, where the 3 circles intersect one another.

http://home.att.net/~dalibrul/gps.gifThanks so much Prof wantafanta:D, this is really interesting. I knew a little about this triangulation, but your explanation is clear!

Can you imagine: I've just wasted time pretending the centres of the circles (so I mean the satellites themselves) are above "...Berlin, Paris and Tallinn" [Scooch lyrics:jmp:]. So I imagined a circle encompassing those places, and its centre, and the distance of that centre from your point in the diagram called b... And I forgot the world is not flat. So my calcs are so wrong:vlol:! Oh well, never mind, I love your demo.:tmb:

H4L:wve: [Jessika]

13-04-07, 00:15
I use a GPS for my job. It tracks on average 7-9 satellites at any given time. It has to use that many to determine approximate speed and elevation so it can calculate my ETA. Very handy since I don't have to bother with maps.

13-04-07, 20:08
I have satellite navigation ( GPS) in my car, i couldn't live without it, i regularly drive the length of the country, and never have a clue where I'm going, i just do as I'm told by Mr sat nav " at the roundabout take the 4th exit"