[Nrsolis]

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.

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.