Saturday, January 24, 2009


Here’s one of those FYI postings… Even if you aren’t tech savvy, you’ll still probably find this interesting.

Have you ever wondered how the GPS receiver in your car or phone is able to figure out where you are? You and I navigate using street signs and landmarks, but GPS devices don’t have that luxury, so they use a totally different technique to calculate their location. We kind of take them for granted because they have become so pervasive, but the technology behind them is quite sophisticated. How about a peek inside?

The way that it is done is very interesting. Signals are transmitted from a number of satellites in space (between 24 and 32 of them) containing information about the location of each satellite, and a very accurate time stamp. The GPS satellites have atomic clocks on board (not just one, but three, to guarantee reliability) to maintain accurate time, and this is critical to the way the system works. Why? Because GPS receivers calculate their position based on their distances from the GPS satellites, and they calculate that distance based on the amount of time it takes for the signals to travel from the satellite’s transmitter to the receiver’s antenna. (Distance = rate x time.) Considering that radio signals travel at the speed of light, the clocks must be extremely accurate. For every billionth of a second that a clock may be off, the calculated position of that satellite the GPS is going to be off by about one foot. Much more than a few billionths of a second and the system breaks entirely.

GPS receiver lock on to signals from at least 3 satellites (preferably more) and use the data in their signals (the satellite’s position and time stamp) to calculate the distances between the receiver and the satellites, and once that is known, figuring out the location of the receiver isn’t too terribly difficult.

But since the GPS receiver doesn't have its starting position and its own atomic clock as a reference point, it can’t calculate directly the distance from the GPS satellite. It has to calculate when the signal was sent in kind of a roundabout way based on the signals from other satellites. The receiver does know the time difference for the signals from the different satellites to arrive relative to one another, and from there it is then able to figure out how long it took for all of the signals to arrive. All down to a few billionths of a second.

Imagine a large room where four of your friends pick a random location to stand. Each one snaps their fingers once every second, all at the same time. And each time they snap they also announce a clue to their exact location in the room (but not all of the information about their location; there isn’t time in one second to do so). You then put on a blindfold and insert one earplug, and walk into the room, and attempt to ascertain your location as you move around the room (down to a millimeter) based on how long it takes for the sound of their fingers snapping to reach your ears. Sound difficult? Yep. But that’s how GPS works.

Add to this the fact that the GPS satellites are constantly moving, travelling at roughly 17,000 MPH and it sounds like an impossible task. But GPS devices are doing it, and these days the chips to make them work are a fraction of a square inch in size. Amazing.

1 comment:

Rob said...

My theory is that GPS is throwing us all off what would be the normal space-time continuum. Navigation actually changes while you use GPS. I know it sounds crazy, but I think it's easier to get places without GPS because your destination remains fixed.

(When you're poor, it's fun to come up with crazy ideas why you don't have cool tech stuff.)

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