United States. Government Accountability Office. Responses to Questions for the Record; Hearing on The Future of Air Traffic Control: The R&D Agenda, text, May 17, 2006; (https://digital.library.unt.edu/ark:/67531/metadc302188/m1/1/: accessed January 19, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .