A proposed 4G network by wireless broadband firm LightSquared would cause harmful interference to Global Positioning System devices, according to the government organization that deals with GPS matters. LightSquared’s original and modified plan to mitigate interference to GPS receivers would be harmful to “many” of them, federal officials on the National Executive Committee for Space-Based Positioning, Navigation, and Timing, said in a letter Friday. The letter to the administrator of the National Telecommunications and Information Administration (NTIA) was in response to the agency’s request for further tests of LightSquared’s proposed mobile network. NTIA  is responsible for advising the president on telecommunications and information…