Radio navigation is used by all commercial pilots, as well as by most other pilots. For this type of navigation, the pilot tunes the radio navigation equipment so as to receive a signal from a ground stationed NAVAID (navigation aid). A needle on the equipment tells the pilot when he is flying on a direct course to or from the station. It also shows when the aircraft drifts off course so that its direction can be corrected. The most common system designed for civil aircraft is called VOR (very-high frequency omni-directional radio beacon). In con junction with the VOR, airlines use another special device called DME (distance measuring equipment). This combined system is known as VOR/DME. A similar system used almost exclusively for military aircraft is called TACAN (tactical air navigation). A combined system, called VORTAC, can be used by both civil and military aircraft. Prior to the evolution of these systems (VOR, TACAN, and VORTAC) the radio compass low- frequency radio receiver in conjunction with the non-directional beacon (NDB) was the mainstay of air naviga tion. The low-frequency radio receiver allows the pilot to home on signals from the radio station. By applying certain procedural steps, the pilot is then able to navigate off of the station. These non-directional radio beacons, however, are subject to disturbances such as fading signals and interference from distant stations during night operations, which can cause the instrument to deviate drastically. There are three transoceanic navigation system commonly used by civilian air transports throughout the world:(1) Inertial Navigation Systems，(2) LORAN (long range navigation)，(3) Decca. Inertial navigation equipment is especially suitable for unaided operation during long flights over water or land areas without adequate radio stations on the ground; its functions naturally complement those of radio navigation systems. The LORAN (long range navigation) system was developed in the United States during World War II to overcome the accuracy limitations of older systems. Several LORAN sys tems cover most of the important trade routes of the world. They are used by marine naviga tors as well as air navigators. The Decca system was developed in Great Britain, also during World War II. It has since come into extensive use in many parts of the world. Decca is like LORAN in principle, the main difference being the type of signals used. Planes which use LORAN or Decca have equipment for receiving special signals, sent out continuously from transmitting stations, to indicate the plane’s exact position.1. According to the passage, the most common system for civil aircraft is .