GPS was developed by the Defense
Department primarily for military purposes. And even though it's been estimated that there
are ten times as many civilian receivers as military ones the system still has
considerable military significance.
To that end the military maintains exclusive access to the more accurate
"P-code" pseudo random code. It's ten times the frequency of the civilian C/A
code (and so potentially much more accurate) and much harder to jam. When it's encrypted
it's called "Y-code" and only military receivers with the encryption key can
receive it. Because this code is modulated on two carriers, sophisticated games can be
played with the frequencies to help eliminate errors caused by the atmosphere.
It almost makes you want to enlist, doesn't it?