Airman 1st Class Robert Alvies, 28th Communications Squadron airfield systems technician, tests the voltage of air traffic control equipment at Ellsworth Air Force Base, S.D., Aug. 13, 2013. Airfield systems personnel are responsible for maintaining navigational aids and radio communication between aircraft and the air traffic control facility. (U.S. Air Force photo by Airman 1st Class Alystria Maurer/Released)