User's Guide

Teletrac, Inc. - Prism TM Information and Installation Guide
1622-0300 B1 3/18/04 41
Step 1: Triangulating from Satellites
Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference
points for locations here on earth. That's right, by very, very accurately measuring our distance
from three satellites we can "triangulate" our position anywhere on earth. Forget for a moment
how our receiver measures this distance. We'll get to that later. First consider how distance
measurements from three satellites can pinpoint you in space.
THE BIG IDEA GEOMETRICALLY:
Suppose we measure our distance from a satellite and find it to be 11,000 miles.
Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations
we could be in the whole universe to the surface of a sphere that is centered on this satellite and
has a radius of 11,000 miles.
Next, say we measure our distance to a second satellite and find out that it's 12,000 miles away.
That tells us that we're not only on the first sphere but we're also on a sphere that's 12,000 miles
from the second satellite. Or in other words, we're somewhere on the circle where these two
spheres intersect.
If we then make a measurement from a third satellite and find that we're 13,000 miles from that
one, that narrows our position down even farther, to the two points where the 13,000 mile sphere
cuts through the circle that's the intersection of the first two spheres.