Airspace restrictions and procedures implemented around Washington, D.C., after the 9/11 terrorist attacks are now permanent under a final rule issued by the FAA. The move was opposed by many in the general aviation community. It is now the only such zone within U.S. borders. The secure airspace is comprised of two concentric rings. The interior ring, called the Flight Restricted Zone (FRZ), describes a 15-nautical-mile radius around Ronald Reagan Washington National Airport (DCA). The outer ring, called the Special Flight Rules Area (SFRA), describes a 30-nautical-mile radius around DCA. Flight operations within the FRZ are restricted to flights authorized by the FAA and the Transportation Security Administration (TSA). Within the SFRA, pilots must file a flight plan, establish two-way radio communications with air traffic control and operate the aircraft transponder on the transponder code assigned by air traffic.