A lot of people think GPS works by triangulation, but it doesn’t. It uses trilateration. Triangulation is measuring the angle to or from several known locations to your location. Trilateration is measuring the distance. You know the location of each GPS satellite because each satellite is broadcasting its precise orbital parameters, along with an extremely precise time signal. By taking the difference between the timestamp that the satellite broadcast and the time at your location and multiplying by the speed of light, you can draw a sphere around the satellite, the surface of which contains your location. Doing this for a second satellite gives you a second sphere, narrowing your location to somewhere the surfaces of the two spheres intersect. A third satellite should narrow your position down to a single point, but because of uncertainty in the signal, you typically get more of a blob. Typically, you need 3 satellites to get a good 2D fix (lattitude & longitude), but 4 or more to get a good 3D fix (lattitude, longitude and altitude).

Now consider this. In order to figure your location to within 10 meters, you need to know the satellite’s position even more precisely than that. Anecdotal reports (by a guy sitting at the next table at No-Name) put the precision of the satellite’s at under half a meter. Now, I rarely know where I am to within half a meter, and I’m not hurtling through space at 4 km/sec, having been shot there atop a rocket in the first place.

I’ve always wondered about the details of the GPS signal, and in particular the satellite orbits, which my more modern Bluetooth GPS doesn’t support. If only there were some way to decode the raw satellite signal, so I could play with it…