GPS satellites regularly broadcast their designed orbit (the "Almanac") and even their current deviation from their defined orbit (that's part of WAAS I think).
BP, that's not quite right. Ground stations track all the GPS satellites and the information is used to predict the orbits (ephemerides) of the satellites. This information is transferred to the satellites and broadcast to GPS receivers.
More information here:
http://is.gd/b7l7k
GPS Division personnel process daily satellite data from NGA, USAF, and International GPS Service GPS monitor stations. This data is used to produce daily precise (cm level to WGS 84) ephemerides (orbit), predicted orbits, and clock solutions for all GPS satellites.
WAAS (Wide Area Augmentation System) was developed by the FAA to provide corrections to GPS signals to improve the accuracy of the signal to allow Cat 1 precision approaches. Ground reference stations are used to calculate correction factors which are broadcast from a geo-stationary satellite. WAAS-enabled GPS receivers use this information to refine the calculated location to an accuracy of typically 1 m horizontally and 1.5 m vertically.