Satellite position in space precisely known to tracking stations. 24 satellites are available in the US constellation. Each satellite sends coded messages telling when a signal left the satellite. Each receiver knows time when signal arrives. Combination of at least three Distances provides earth location.
Base station needs to be at known location.
Base station calculates what T travel should be given known location of satellites and its own location. It is an ellipsoid measuring 39,939,593.9 m pole to pole and 40,075,452.7 m at the equator. How we represent it depends on what datum is used, what coordinate system and what projection. GPS units usually provide ample numbers of them to select and 'user defined' choices so that coordinate readings are presented in the proper system to match your map.