A NDB is a radio transmitter at a terrestrial location that is used to obtain the course or position of an aircraft. Due to the fact that NDB uses radiogoniometry, its signals are affected (more than other aids) by atmospheric conditions, mountainous terrain, coastal refraction, and electrical storms, particularly at long range. The navigation based on NDB aids consists of two fundamental parts: the automatic direction finder (ADF), which is the equipment on-board the aircraft that detects the NDB’s signal, and the NDB transmitter. ADF equipment determines the direction to the NDB station relative to the aircraft, which is presented to the pilot on a Radio Magnetic Indicator (RMI). In this way, in a simple, intuitive manner pilots know if the aircraft is addressing towards an NDB; if not, they now de deviation and can correct the course.
Figure 11.11: Non Directional Beacon.
NDBs are also used to determine airways of fixes. NDB bearings16 provide a method for defining a network of routes aircraft can fly. In this way, the network of terrestrial NDB stations (also VORs) can uniquely define a network of fixes (connected by airways, i.e., the bearings) in the sky. Indeed, 20-30 years ago, the routes aircraft followed to complete a flight plan were only based on NDB/VOR stations. In a navigation chart a NDB is designated by a symbol as in Figure 11.11.a. More recently, another way of navigation has arisen: the so-called RNAV. It is based on calculating fixes based on the information provided by two aids. For instance, using the information coming from two NDBs, fixes are computed by extending lines through known navigational reference points until they intersect. In this manner, many fictitious (in the sense that are not related to an existing terrestrial station) fixes or waypoints have been defined, increasing the network of routes and thus the capacity and efficiency of the system. See Figure 11.11.b.
16. A bearing is a line passing through the station that points in a specific direction.