NDB is a Direction Finding Aid, which can be used as a Radio Navigational aid for the purpose of navigation.
Frequency- LF, MF, 200-460 KHz
Propagation- Ground Waves
Emission- AO/A1, BFO used, (Without Modulation)
AO/A2, BFO not used (With Modulation)
BFO = Beat Frequency Oscillator, It is used to convert the radio frequency into emission that are made audible to human ears.
NDB is a vertical antenna fitted on the ground, it transmits vertically polarized signals.
The NDB has a very higher range, because it operates on LF, MF.
It is also very easy to use, and a low cost equipment.
AUTOMATIC DIRECTION FINDING
Frequency- 150 to 1750 KHz.
ADF is a Radio receiver located in the aircraft to receive the NDB signals. It has two aerials –
- Loop Aerial, It is a Fixed Aerial, It has two maximum current position and two minimum positions.
- Sense Aerial, It is an Omni Directional Aerial, It is used to remove the ambiguity created by the Loop Aerial of two minimums.
The resultant shape on combining these two aerials is a heart shaped CARDIOID.
The CARDIOID has only one null position, which is used as a datum.
The ADF is also called NULL SEEKING EQUIPMENT.
Errors-
- No failure flags or warning device- The instrument has no way to find out if the received signal is reliable.
- Loop Misalignment- This is a Technician error, it occurs if the Loop Aerial is not aligned with the Fore and Aft axis of the a/c in the fitting of the equipment.
The errors experienced by the ADF are of LF and MF waves.
Errors-
- Night Effect- Max Error occurs at Dusk and Dawn, At night when Ionosphere layer changes due to the absence of Sun rays that causes a layer consisting of charged Ionized particles, caused by the atoms of high velocity and high energy Ultra violet rays in the Sun’s rays. It is the region of free electrons consisting of positive and negative ionized particles, which surrounds the Earth. The Earth is surrounded by few layers that absorb these ultra violet rays from reaching the Earth. When sun is no longer over a particular area, it is called night, hence the direct ultra violet radiation in the night is absent and the layer that consisted charged ions in the day almost vanishes. For the Sky wave to be deflected from the atmosphere it has to travel much higher than in the day to be reflected back to the Earth. This causes the range of the Radio Aid to be increased in the night time.
- Thunderstorm Effect- ADF needle points in the direction of the Thunderstorm activity, due to being influenced by the positive charged accumulated in the Thunderstorms.
- Quadrant Error- Radio waves are re-radiated by the metallic fuselage and causes error. This error is most prominent on Quadrant directions, E.g. 045°, 135°, 225°, 315°.
- Coastal Error- Radio waves deflects when passing from different medium, when passing from denser to rarer medium (land to sea) they bend away from normal. When passing from rarer to denser medium (sea to land) they bend towards normal.
- Station Interference/Fruiting- This error occurs because of two stations overlapping each other’s frequency and causes ambiguity in the bearings. The protection range is provided in distance to guarantee reliable coverage of the signal.