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It's been 30 years since my college job where I worked on the first generation of these things but this is what I remember. The GPS signal emitted by the satellites is actually two different types of signals. The "CA" code which is what we use in everyday GPS technology. Originally this was meant as only a guide for the receiver so it could pick up and interpret the "P" code which was military/government only. The "CA" code was designed to only be accurate for civilian use down to about 100 meters. Later some mathematicians and other bright people figured out how to use it to get down to a few feet. Pres Clinton, in one of the few things he did right signed a document allowing the "CA" code to be non degraded thus all civilians could have fairly accurate GPS positioning to within a few feet.

The military "P" code is accurate to within fractions of a millimeter. It however is encoded. The encoding algorithm works on a week by week basis. There were 37 different algorithms created thus you could have 37 different weeks. This allowed for a full constellation of 24 satellites and a handful of ground stations.

Now you may ask why does this affect the non military "CA" code receivers. It has to do with time keeping in the ephemeris file. Each satellite sends out an ephemeris block of data. The ephemeris data block details how the satellites orbit varies from the ideal orbit it should be in. All satellites are affected by things like Solar wind and cosmic radiation. Thus they are not exactly where the should be. The week is broadcast along with other data in the ephemeris file and your receiver gets that information and uses it to calculate time as well as position. Hope this helps.