As noted on the Critical Dates page, the weeks counters in the Global Positioning System clocks reset on August 22, 1999. While all the satellites in orbit have been corrected to continue service, it now turns out some GPS receivers, especially those made before pre-1994, may have difficulty in managing the revised protocol. However, many of the manufacturers of those devices have firmware upgrades available to to allow the units to continue functioning.
The following links will take you into several GPS support areas -
An especially interesting and detailed article on the global positioning system is the following -
The author goes into a lot of detail about the methodology (and the several levels of approximation that take place before you are given the final display) of deriving the location, and presents quite a few examples of applications the original planners probably never thought of.