How Does a GPS System Work?
For the non-geek and non-techie, Global Positioning System technology is mystifying. Worse yet, ask your friendly neighborhood geocacher to explain it, and they'll bury you in hyper-space buzzwords like Mr. Spock explaining the Romulan cloaking device. So, let's try to explain what's going on, this time in clear, plain, simple English. That isn't very easy to do, because it requires thinking in 3-dimensional space and understanding a thing or two about trigonometry, two things which daunt the average person. But we're going to try anyway!
OK, picture any beacon, say one bobbing on the surface of the ocean. It sends out a signal - perhaps it just says 'beep' and lights up red. Now start out somewhere on the face of the Earth, on that ocean, and knowing nothing else. If you know exactly where the beacon is, and you (in a boat) are where you can see and hear the beacon, you know you are somewhere within a circle around the beacon at X/Y location on the map. But that's all.
Now, how about a way to measure exactly how far you are from the beacon? Let's say that you can look at it through a telescope or compare the sound to a graph or whatever, you can determine that you're 500 kilometers from the beacon. Now, instead of being inside a circle of unknown size, you know you're on the perimeter of a circle that is 1000 kilometers across with the beacon in the middle. If you add a second beacon, which goes 'bong' and shines with a green light, and you happen to be where you can see both of them, you'd have two intersecting circles. Knowing that you were 200 kilometers from the green beacon would mean that you were at one of two points - there are always two points where two circles overlap.
You'd have to add a third beacon, a blue one, say, to triangulate on your exact location by eliminating one of those two points. And that is only on a flat, 2-dimensional surface! With a 3-dimensional world, you have intersecting spheres instead of circles. So with one beacon, you're on the surface of a sphere, with two beacons you're on the circle where two spheres intersect, with three beacons, you have one of two points in space you could be. We know enough to eliminate the point in outer space and use the Earth itself as the fourth sphere, pinpointing your location to a point on the surface of the Earth.
But is it really pinpointed? Let's go back to how we figure out how far you are from one beacon - is there such a way? Remember, you can't just measure it with a ruler, you don't know how big the beacon is, and in the case of being lost, you don't even know which direction you are facing when you're looking at the beacon. Well, there is a way, but it takes advantage of the speed of light and sound. It is related to same way you can tell how far a lightning bolt is by counting the seconds between when you see the flash and when you hear the boom.
The GPS satellite, then, broadcasts much more data than just a 'beep'; it broadcasts the exact time according to an on-board atomic clock. The signal is traveling at the speed of light. If you, too, have an atomic clock handy, you can then compare the difference between the time you have and the time broadcast to you. You have 3:04:06.004 and the satellite says it's 3:04:06.005. That means that the satellite is 299,792 meters away, to simplify for the purposes of illustration. Neat, huh?
Just one problem - an atomic clock that is accurate to thousandths of a second isn't something that you can lug around, and even if you could it would be too expensive. Instead, we add in a fourth satellite and check all four against the time which we think it should be. Thus, the GPS system is designed so that at any time, there are four positioning satellites visible above the horizon at any point on Earth. We then leave it up to computers to do all the calculating and trigonometry for us, and then compare the location (at latitude/longitude/altitude) to existing maps that we have already drawn to give a pointed location.
Even then, we don't have perfect accuracy. Current systems can peg a location to within a few feet. We have extra satellites in case one goes dark. Atmospheric disturbance can fuzz our signal - remember, the speed of light is different in atmosphere than it is in the vacuum of space! Finally, gravitational tides from the moon and sun warp the orbits of satellites slightly just like they cause the ocean tides, so there's an adjustment for that. Plus, if you have a thunderstorm over your head or obstructions such as mountains or skyscrapers which either block or bounce the signal, you have even more inaccuracies.
As you can see, this is complicated business! The system corrects for errors whenever it can and also double-checks with stationary ground towers whenever there is one handy, so we have a system that's about as reliable as any we could invent. But keep in mind that GPS systems aren't intended to be more accurate than a few feet, so don't be one of those news stories we read about somebody who blindly followed their GPS navigator as they drove off into a creek.
Filed Under: Mobile Computing
