As PAXboy noted the earth is an oblate spheroid - but only just. 
WGS-84, used as the default spheroid by GPS, has an equatorial diameter of 12,756 km. The polar diameter is 43 km less - a difference of 0.3%.
Originally Posted by
Mogwi
We discovered that it hadn’t been programmed to cope with the southern hemisphere and always updated to the corresponding position in the northern hemisphere!
Made us larf!
Mog
I remembered a similar problem with a flight of F-22s crossing the dateline on their way to Japan:
F-22 Software glitch
I once worked for a Canadian oil company that started doing exploration work in places other than North America. Their computer-mapping system not only couldn't cope with anywhere south of the equator, but it assumed, for convenience, that all positive Longitudes were west of Greenwich!
A colleague of mine who was assigned to fix this problem, created a test file to make sure the new program was always correct, even after modifications. It consisted of four short seismic lines with True azimuths of 360, 045, 090 and 135. They intersected each other, at the Equator and the Greenwich Meridian.