Welll.....that's not strictly true. If you've got an environment free of multipath, you use differential or WAAS-enabled GPS receivers, you can get pretty good accuracy.
Please note that all of these things depend on many factors and can be considered nearly "heroic" measures when it comes to position accuracy, but the Navy has historically been pretty damn concerned about knowing where their assets are and is somewhat obsessive about error-factors where that's concerned.
You can receive RTK corrections over IP (via a cellular connection). Government bodies across the US push the data over IP to you if you register for a free account.
In agriculture, it is not uncommon to get correction data over mobile data connections, where the static receiver is provided by a third-party service provider.
Source: http://www.oc.nps.edu/oc2902w/gps/gpsacc.html
Please note that all of these things depend on many factors and can be considered nearly "heroic" measures when it comes to position accuracy, but the Navy has historically been pretty damn concerned about knowing where their assets are and is somewhat obsessive about error-factors where that's concerned.