A rough guide: The GPS signal works on two codes: coarse acquisition (C/A) and precision (P) codes. The P codes are 1 week long and require a secret key even to this day. C/A is I think a few hundred milliseconds and easily locked onto.
They used to use C/A for what it says: coarse acquisition of where you are (space and time), to work out where you are in the P code signal to start looking. Apparently a difficult problem without the C/A to start you in the right place.
To stop baddies using GPS for anything evil, the C/A had an error built in. This error in C/A is what Bill Clinton removed.
To this day military still can use P codes and it is still possible for the C/A to go out to lunch. But practically this is unlikely to happen given the amount of civil applications dependent on accurate gps.
They used to use C/A for what it says: coarse acquisition of where you are (space and time), to work out where you are in the P code signal to start looking. Apparently a difficult problem without the C/A to start you in the right place.
To stop baddies using GPS for anything evil, the C/A had an error built in. This error in C/A is what Bill Clinton removed.
To this day military still can use P codes and it is still possible for the C/A to go out to lunch. But practically this is unlikely to happen given the amount of civil applications dependent on accurate gps.