Why does the week affect GPS location?
Its like saying my garage, depending on the day, is either on the west of my home, separated only by a wall, or 10,000 miles south of my home, due to this rollover error.
No, its still one wall west of my home.
GPS devices determine their location by calculating the distance from multiple GPS satellites based on the time of arrival of timestamped messages. So, if you were really far away from one or more of the satellites relative to the others, then in theory their timestamped packets could arrive with a time difference more than a few seconds.
Of course that can't actually happen in normal circumstances since the entire set of GPS satellites is close together in earth orbit.
But it is possible that some extraterrestrials with a super sensitive GPS receiver that have a black hole between us and them could see radically different apparent path lengths to their receiver. No doubt they would figure out the discrepancy and not get lost on the way to Galactiburger in their hovercars.
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.
I'm not exactly sure how the GPS specs require that the time be stored, but apparently the week is part of that data structure.
Read the article