If you want to know precisely where you are, this is the way. I've got a Garmin GPS II+.
This LGPL library for communication with Garmin GPS receivers contains, since the last jeeps posting, new NMEA data capture functions and 18 new map projection functions and their inverses as well as the established Garmin Communication Protocol functions from the 0.0.x releases.
Paper with recommendations but few conclusions. Unfortunately it is at present impossible to determine causality between the use of electronic devices and their effects on aircraft navigational systems. If pilots suspect that there is a problem, they are required to demand that suspected devices be turned off. There is thus, after that point, no way to validate via the traditional scientific method of experimental testing that the device actually was at fault or has detectable effect on the navigational system.
As a result, while there are many claims made both pro- and con- the use of electronic devices aboard aircraft, none to this point have the backing of any scientific testing.
Boeing suggests further that any reasonable personal electronic device that is not an intentional radio transmitter should be permitted on their aircraft, and that specific instances of claimed EFI should be documented and sent to them.
The C/A Code is one of the key principles which make GPS possible.
If you are interested in seeing what the PRN codes look like (or knowing what PRN means), or in seeing how the GPS receiver gets a lock on a GPS satellite and how it determines the pseudorange to that satellite, the above web page has the basic information to explain the C/A Code and a Java applet to demonstrate it.