Learn how the ga-zillion dollar Global Positioning Satellite System Works.

So how does that little hand-held GPS receiver you have actually figure out where you are, wherever you are? Magic? Yup. Or something like it.

The GPS receiver you have (or may have) is not all that accurate or all that expensive all things considered. There are million dollar satellites in orbit as part of this GPS system. There are expensive super-accurate atomic clocks on the satellites and ground stations where personnel work to help monitor and correct the data. Yet your cheapy $100-$500 (or so) GPS has a not so accurate quartz clock in it and it still can take precise measurements of your position, time and speed. Those three things are the key to finding out where you are and where you want to go.

To find out where you are you need to know what time it is! At least the GPS receiver needs to know what time it is. The super-precise atomic clocks on the orbiting satellites send out what time it is when they send their signal. Each satellite has a unique signature so the receivers can identify which satellite is which.

This is important because if the signals got mixed up your GPS receiver wouldn't be able to make sense of anything let alone plot your position and speed. The GPS is hard coded with each signature so it know which satellite is which because that's the way it's built.

When it receives a signal from a satellite the signal will have the time that signal was sent encoded into it. This is important because you need to know how much time has passed to find the speed.

The GPS receiver's clock subtracts it's current time from the time the signal was sent from the satellite and that time (is very small) and is how long it took the satellite to send the signal.

Now that it's gotten the time it took, it can get the distance that signal is from the satellite to the GPS receiver by knowing that the signal travels at the speed of light. Since you have the speed of light (which is approximately 186,000 Miles a Second) and the time it took the signal to get to the GPS you can get the distance from that.

Speed is equal to distance divided by time. So distance is equal to speed times time. So the GPS gets the distance from that satellite.

The satellite is moving so the satellite also sends data on where its moving so the GPS can compensate for the change in distance to minimize that error.

Now the GPS does this with more than one satellite. It does it with at least 3 to get an approximate fix on your location. It's like finding landmarks on map and drawling a line that matches the angle you see them at. You get the angle you see them at with your compass. Where the lines cross is where you are at. You need at least 2 points to see this but since there is likely a small error to where you actually are, then more lines makes the error less. Three lines would be like a triangle and you would be in that triangle somewhere. The area of the triangle is your error range. The more sights/landmarks you have to compare it to, the closer the error is likely to your actual location.

The GPS does this as well, but instead of a line it is a sphere. Each calculated distance is a radius to that sphere and the more spheres you have the closer and more accurate the numbers are.

The signal, however, is prone to errors that small at first can add up to a substantial amount. The signal get bounced all over the place from buildings and other structures; the satellites themselves may only number a few (there may only be a couple in the sky at a time) and so there are less corrections to be made which means a bigger error on where your GPS thinks it (and you) are.

The clock in your GPS receiver is not as accurate as the clocks in the GPS Satellites themselves (they're atomic, the most accurate clocks in the world!). The GPS satellite send the correct time in the signal that the receiver gets and the receiver can adjust for small errors due to the clock in the GPS receiver. The more satellites there are the better the error corrections and the better the accuracy.

Now once the GPS has a fix (lock) it can figure out your direction (if you're moving) by comparing the position from now and the position from a second ago (or so). It compares these against the north direction to give you a heading (and angle away from north that you're going. If you were going north this would be 0 degrees).

The GPS receiver only knows your direction if you're moving (unless it has an electronic compass) because if your just standing still it only knows where you are and not where you're going.

It also knows the distance you traveled and that time it took you to travel and can plot your speed based on that information. If it knows your speed it can estimate your time (ETA) to a place you want to go if it knows where you want to go as well.

It can track all other sorts of data as well, like average speed, trip time and other thing.

That is the basics of how the GPS system works. For a more advanced view see these sites:

How Your GPS Works

How GPS Receivers Work

A GPS Tutorial

GPS Tutor

Global Positioning System Overview

All About GPS (shockwave)