GPS signals are somewhere around the 1.4-1.5GHz mark if memory serves; that is basically line of sight only and the curvature of the earth solves the rest. As

GPS signals are somewhere around the 1.4-1.5GHz mark if memory serves; that is basically line of sight only and the curvature of the earth solves the rest. As for 50ft and up, I think - mind, think - that NOTAMs are limited in scope to issues not on the ground; hence, 50ft and up is just a way to let pilots know the problem persists at any altitude; you will not see a NOTAM referring to something at sea level. Again, I /think/ this is why it is worded the way it is, but I am not a pilot.