How does a GPS work?

I try and challenge myself when I am in unfamiliar territory to figure out where I am and how to get home. I did this last Friday and miraculously made it all the way home. This being said, I do always have a GPS in the car with me, whether it be my actual four year old GPS or something that refuses to talk to me, like say, an iPhone. Why won't it say something to me? I can't read a map while driving Steve! Anyways, I guess what still bothers me is; how does it work? 

I asked Ben.

There are two parts to GPS (Global Positioning System): the satellites up in space and the receiver in your hand (e.g. your GPS enabled smart phone).

There are a whole bunch of GPS satellites (Wikipedia says 31) so that at any point in time, on any point on earth, at least 4 of them are in the sky overhead.  Each satellite carries a ridiculously accurate atomic clock and basically just sits in orbit constantly transmitting the time and its exact location in orbit.  

So if, for example, you are in an unfamiliar city and you are in an emergency situation where you absolutely must find the location of the nearest frozen custard shop, then all you have to do is pull out your smart phone, which is a GPS receiver.  The receiver also has a very accurate clock (although not as accurate as the satellites have) and it establishes a link with as many satellites as possible.  Then it compares its own time to the time that each of the satellites is broadcasting.  Since the signal has to travel through thousands of miles of space, the time broadcast by the satellites is going to be a little bit behind the time that the receiver is keeping.  The receiver uses this difference in time along with a little bit of math to calculate how far the broadcast signal has traveled, and thus how far away each individual satellite is.  At that point, the receiver knows the exact orbital location of several satellites and exactly how far away each one of them is, and it uses that information to triangulate its position.  To get a location that is accurate enough to be usable the receiver has to calculate out to 7 or 8 decimal places, so it's pretty heavy stuff.