GPS in asymmetric geometry

[Loren Booda]

How does the Global Positioning System compensate for mapping the irregular surface of the Earth relative to its center of mass?

[russ_watters]

If I understand what you are asking, it doesn't - it gives a lat, long, and altitude above msl (so the model of earth it has is smooth and based on msl). If the surface of the earth happens to be at that altitude, great. If not...hope you have wings...

[mgb_phys]

Inherently it calculates a height above the earth barycentre, essentially a distance below it's orbit.
It then converts that to a height based on whichever spheroid model you selected (for GPS=WGS84), then if you have selected a particular map grid (eg OSGB) it converts the height into the mean sea level for the default for that coordinate system.
This is all necessary to deal with bumps - the coast of the Britain is several metres below sea level according to WGS84.

[nucleus]

In order to get better results you can use Differential GPS (DGPS).