Before revision: In 1981, acoustic positioning was proposed as part of the MX missile system.MX Missile Basing, pages 173-175, 1981, ISBN 1-4289-2450-7, ISBN 978-1-4289-2450-5 A network of 150 covert transponder fields was envisioned.
After revision: U.S. military'sIn 1981, acoustic positioning was proposed as part of the MX missile system.MX Missile Basing, pages 173-175, 1981, ISBN 1-4289-2450-7, ISBN 978-1-4289-2450-5 A network of 150 covert transponder fields was envisioned.