Farmpond, 

I am splitting hairs here I realize - I stand corrected on my "not NASA" comment. 

In 2001 NavCom partnered with NASA's Real Time GISPSY network of reference stations, to add to the list of reference stations they had started in 1999.  So yes, NavCom uses correction signals from the GISPSY network, puts them through processing hubs (which may be part of GISPSY as I couldn't find a reference that explained who ran them) and then broadcasts the signals to Inmarsat's satellites, and back to earth.