APA

United States, . (1977). Modifications to ground facilities, systems, & equipment in the natl. airspace system. [Washington, D.C: .

Chicago

United States, . 1977. Modifications to ground facilities, systems, & equipment in the natl. airspace system. [Washington, D.C: .

Harvard

United States, . (1977). Modifications to ground facilities, systems, & equipment in the natl. airspace system. [Washington, D.C: .

MLA

United States, . Modifications to ground facilities, systems, & equipment in the natl. airspace system. [Washington, D.C: . 1977.