How GPS Works
By Cynthia Mills
Just in case you didn't already know, here's a primer on how satellites can tell you where you are.
It's all in the timing, but first, let's play a game.
Say you don't know where you are, but someone else does but doesn't want to tell you. (It's a game, remember.) You can make him tell you by asking some questions. How far am I from Minneapolis? He has to answer. XX miles he says.
You draw a circle with a radius of XX miles with the center of the circle being Minneapolis. You could be anywhere on that circle.
How far am I from Denver? YY miles, he answers.
Another circle. Only this time the circle crosses the Minneapolis circle in two places. You have to be at one of those two places.
How far am I from Oklahoma City? ZZ miles.
This last circle will cross in two places as well, but one of the places must be where the first two circles cross. Viola! There you are.
With GPS, the satellites replace the cities, and they tell you how far they are away from you.
They do so with a microwave signal and timing. Even though the microwaves travel as fast as light (186,000 miles per second) the satellites are far enough away (11,000 to 13,000 miles) to take a measurable amount of time — a few thousandths of a second — to get to you. The satellite signal is sent out as something called "pseudo-random noise," which for our purposes is like a melody. Your receiver and the satellite play the same melody, synchronized by super-accurate atomic clocks to the same time. But when the satellite sends its melody to the receiver, it arrives out of synch. The receiver measures the timing difference and calculates the distance the satellite's melody traveled to get there.
The receiver takes these measurements from three satellites and has the three distances it needs to find its locality. With four measurements, it can even tell you your altitude. So instead of thinking of location as flat circles on our map, 3-D-bubbles might be more accurate.
There are lots of calculations made to correct for errors, like the fact that signals don't travel as fast through Earth's bumpy atmosphere compared to the smooth vacuum of space. And the receiver has to double-check its timing with all the satellites because it only has a quartz clock ( atomic clocks being very expensive).
You can make GPS even more accurate by adding a land station. Any station on Earth is closer to a receiver than a space-stationed satellite, so there is less room for error when receiving, comparing and correcting melodies. This system is called differential GPS, and can place you in geographical space as closely as the distance between your thumb and index finger.
