NavList:

The GPS signal is extermely weak and is actually below the noise level. The signal can only be used is by extensive signal processing. Here is an analogy. Suppose you were looking at a billboard covered with a million lightbulbs that are all flashing on and off in a random fashion, this is the signal with the background noise. The real signals are no brighter than the other bulbs. Now hold a sheet of paper up that is punched with a grid of a number of punched  holes that only line up with the GPS signal light bulbs, which flash on and off to convey the information. This is how the civilian GPS works.

The military, encrypted, GPS is similar except that all of the holes in the grid have covers that open and close in an almost random fashion as determined by the code variables from the very long key. You only look at the lights that shine through those open holes. This of course requires that the receiver be syncronized with the GPS. If you have just a standard GPS (that is also able to receive that channel from the satelites) it won't know which lights are conveying GPS information.

gl