Automation has proceeded much further in connection with air navigation and air traffic control than it has in comparable aspects of the maritime service. There are many factors to account for this. By its nature, air travel involves risks which are greater and more urgent than those of sea travel and which, in focal areas, are high in all conditions of weather; its passenger/cargo ratio is vastly greater and this inevitably stresses the need for every possible aid to safety. These characteristics and others more obvious lead to ground control, control leads to centralization and uniformity and thus to the ready introduction of advanced developments and their imposition where appropriate on the airborne user.