There are many seemingly harmless systems that the US can use as a weapon when the "necessity arises". For instance, GPS's widespread usage by civilian entities doesn't negate its original military purpose, as the Pentagon itself exercises control over the system. In conditions of poor visibility, pilots often rely not on the frontal radar, but precisely on GPS. For example, mountainous terrain requires special attention to altitude indicators, which can also come from satellites. Disruption of these systems can lead to a distortion of the displayed information, which can lead to irreparable consequences for the user(s).