NON-DIRECTIONAL BEACON NDB

Nondirectional Beacon The oldest (and simplest) navigation system in general use – a simple transmitter radiating in an omidirectional radiation pattern Frequency of Operation: 200 to 500 kHz Power: 20 W to several kW Modulation: 1020Hz tone + Morse Code Identifier Propagation: Ground Wave

GROUND BASED EQUIPMENT NDB’s are rarely found in the US, But still common in Canada. Used in busier areas as a means of identifying approach fixes (ILS), and as a sole means of navigation and approach in remote areas. (unless equipped with GPS) NDB facilities are relatively inexpensive to build and maintain. NDB simplicity is it’s greatest strength. Essentially an AM radio transmitter.

AIRCRAFT EQUIPMENT Receiver Control Head Indicator (Automatic Direction Finder) Fixed Card – relative bearing or magnetic bearing Radio Magnetic Indicator (RMI) – magnetic bearing The head of the needle always points to the station. Two Antennas Sense – non-directional signal Loop – directional signal (180° ambiguity without sense) Note: see propilot simulation

OPERATION The desired frequency is tuned. The station is identified. ANT setting may be used to identify station, but no directional information will be available, ADF must be selected prior to navigation. ADF equipment has no flag system to indicate reception, therefore pilots must monitor the audible identifier of the station anytime ADF is the sole means of navigation and throughout the approach phase of flight. Note: some EFIS displays incorporate a flag warning system, making audio monitoring unnecessary.

SELKIRK AVIATION SOP 2.8.11 Monitoring ADF Audio When IFR or simulated IFR and navigating by ADF, the ADF audio identifier will be monitored during all NDB instrument departures and approaches. When navigating enroute with ADF only, pilots should check the Morse code identifier frequently to confirm the station is still working. When flying GPS overlay approaches that have been properly setup from the database it is not necessary to monitor ADF audio.

ERRORS Bank Error – needle will point down to the station when aircraft is banked. Quadrantal error – Caused by the refraction (bending) of the incoming radio waves by the aircraft structure. Minimum when station is off cardinal points (nose, tail, or wingtip). Maximum in between.

LIMITATIONS Skip Zone: an area of erratic indication or signal loss between the points where the ground wave ends and the sky wave hits the ground. Night Effect: occurs at night due to the reception of sky waves reflected by the ionosphere. It is most pronounced at sunrise and sunset. Has a greater effect on higher frequencies. More pronounced further from the station. Fading: similar to night effect although the erratic indications are caused by out of phase signals.

Shoreline Effect: water and land have different levels of conductivity causing the signal to bend as it crosses a shoreline. (most pronounced at angles of less than 30° with the shoreline; accounted for on published airways.) Terrain: mountains and natural magnetic disturbances effect signals. (accounted for on published airways.)

Interference: if an ADF receives two signals it will alternate back and forth between them until one becomes dominant. (needle will swing back and forth.) Thunderstorms: electrical disturbances cause static interference and erroneous or erratic indications. The needle will sometimes point towards the storm.

ACCURACY Systems are flight checked to an accuracy of at least +/- 5° for approach and +/- 10° for en route. The accuracy of an NDB at any given time is difficult to determine when considering all the factors creating errors. However the accuracy will be adequate for the purpose intended. (en-route 50-75nm range, approach 25nm range.)