Data Hacking

Appendix A — GPS Primer
Knowing your distance from Point C further refines the positional information (see Figure A-4).
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FIGURE A-4: Knowing your distance from three points provides a lot of information.
This is an example of two-dimensional trilateration (2-D trilateration). What GPS does is take
this into three dimensions (3-D trilateration or triangulation).
In principle, three-dimensional trilateration doesn’t differ much from two-dimensional trilateration,
but it is trickier to grasp. What you need to do is imagine the radii of the circles from the
preceding examples going off in all directions, so instead of a getting a series of circles, you get
a series of spheres.
If you know you are fifteen miles from Point A (or satellite A in the sky), you could be anywhere
on the surface of a huge, imaginary sphere with a fifteen-mile radius. If you also know you are
eighteen miles from satellite B, you can overlap the first sphere with second, larger sphere. These
spheres all intersect in a perfect circle. Finally, if you know the distance to satellite C, you get a
third sphere, which will intersect with the other circles at two points, as shown in Figure A-5.
The Earth itself acts as another sphere. It is assumed that you are on the Earth, so you can
eliminate the other point in outer space, as shown in Figure A-6.
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