The Automatic Direction Finding (ADF) equipment on-board of aircraft is used together with the Non Directional Beacon (NDB) transmitters installed on the ground. The on-board ADF is able to receive and decode NDB signals through an antenna system and a HF receiver to provide pilots with an indication of the relative position shown in the instrument.
A Non Directional Beacon, abbreviated 'NDB', is a ground installation consisting of a LF transmitter which transmits vertically polarized radio signals continuously and in all directions.
This radio navigation system is used mainly for Instrument Flight Rules flights (IFR). However, during VFR flights it can be quite useful in order to check aircraft's position.
This system is an old radio-aid system which is getting replaced gradually with more modern and accurate systems such as VOR or even GPS systems.
NDBs transmit their signals in the Low Frequency (LF) and Medium Frequency (MF) bands, operating from 190 to 1750 KHz. The radio signal propagates as a surface wave does, which provides quite a big amount of errors on the ADF which may sometimes cause the instrument indication not to be reliable. With the purpose of suppressing those errors as much as possible, they normally transmit between 250 and 450 KHz.
NDB errors may not be found on our perfect simulators. Each NDB has its own frequency and all of them are identified by names consisting of two or three letters or an alphanumeric combination. There are two types of NDBs, regarding the purpose for which they have been installed:
The NDB power class are :
|Class||Power||Estimated Distance (Radius)|
|MH||25W - 50W||25 NM|
|H||50W - 2kW||50 NM|
NDB can transmit other information for local aircraft like:
Ground NDB installation
The symbol of NDB radio navigation beacons on charts can be like the figures below.
Associated with the NDB figure, you can have additional information written in a rectangle: