In the past decade, millions have come to depend on the seeming magic of the global positioning system (GPS) to guide them to their destination. The navigational gadgets in cars, cell phones and other hand-held devices can even be a lifesaver. Now the system may be undermined by a Federal Communications Commission (FCC) decision last month to allow a well-connected company to exploit a slice of the airwaves in a way that potentially blocks GPS signals. The FCC bent the rules so the Reston-based firm LightSquared could offer a new wireless Internet service that fulfills President Obama‘s high-profile push for...