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About Global Positioning System (GPS)

Here is a pictorial explanation of how GPS works (from Robert Pepper) and here is a question by me (Frank). How is the altitude determined by the GPS system. As I understand it, the 4 satellites required to establish position give longitude, latitude and some sort of altitude. I say some sort because it seems to me that the altitude is relative to the center of the orbial paths - i.e. near the center of the earth. As I move around the earth, my altitude goes up or down, relative to the center of the earth - but how does the GPS system know where the surface of the earth is - in order to provide accurate altitude information?? I simply don't get it. If you have some ideas, email them to me

GPS Orbits

Position locus derived from two satellites

Position locus derived from three satellites (actually, four are required for accurate location, with the 4th required to reduce the error in time associated the three satellites

Austrailia by latitude and longitude