A reference receiver, or a station, computes corrections for each satellite signals and gives these corrections to the remote, or rover, receiver which apply these individual pseudorange corrections to each satellite used in the navigation solution. It has a lot of animation which able to understand the way the GPS works. It can be downloaded here. There is a readme file here. The geodesy is the study of the shape and dimensions of the Earth.
Classic geodesy makes a difference between altimetry and planimetry.
In this last case, the points on the Earth must be visible to each others in order to determine their respective position. With the launch of the first artificial satellite, Sputnik I, in 1957, scientists have found that artificial satellites can be used for determining position. The space geodesy overtakes the limited visibility of the classic geodesy. Indeed, satellites are now simultaneously visible from several different terrestrial points. We can get all the coordinates in a common reference system which allows positioning at the international level.