wantafanta

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.

http://home.att.net/~dalibrul/gps.gif

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.

http://home.att.net/~dalibrul/gps.gif