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ADF-Automatic Direction Finder Basics

ADF avionic system is a short range navigational aids and it stands for Automatic Direction Finder. It provides aircraft bearing information with respect to a ground station known as NDB (Non Directional Beacon).

ADF,Automatic Direction Finder

The bearing is measured clockwise starting from the aircraft longitudinal axis and stopping at the segment of the air craft NDB. Note that the reading obtained in the Flight Compartment indicator is a relative bearing known as the ADF bearing. To obtain the NDB magnetic bearing we need to perform the following calculations as depicted in the figure-1.
{NDB Magnetic Bearing with respect to Aircraft = Aircraft Magnetic Heading + ADF Relative Bearing information }

Applications

Following are ADF applications:
•  •  Track Intercept: To be able to get back to the track due to cross wind.
•  •  Station Homing: Travel toward or away from an NDB. Wrong Station Homing can result when ADF bearing is aligned to the aircraft longitudinal axis. Hence a dramatic curve will occur due to crosswind.

•  •   Correct Station Homing:
•  Approximate wind Drift Angle to get back to track is about 5 Degree
•  Approximate an extra angle to overcome future crosswind once track is regained: Extra3 Degree
•  Aircrfat Heading is corrected to fly at 90 Degree + 5 Degree + 3Degree = 98Degree
Path correction is achieved when both the ADF and the heading needles are constant.

Triangulation Position Fix: To know where the aircraft is located.
•  Tune ADF receiver to an NDB
•  Obtain the ADF bearing.
•  Calculate the NDB magnetic bearing
•  Calculate the NDB true bearing
•  Place a corresponding line on the map
•  Do the same by tuning to another NDB station
•  Extend the lines on the map
•  The intersection of the lines provides the 2D position fix of the aircraft.

On the ground ADF has following specifications:
•  Frequency of operation: 220 to 550 Hz
•  NDB sends a morse ID code to the Aircraft ADF receive

In the Aircraft ADF has following specifications:
•  Frequency of operation: MF from 550 to 1750 KHz, commercial AM band
•  ADF bearing needle points only to the tuned NDB.
•  When the Aircraft flies toward an NDB the ADF bearing pointer indicates 000 Degree.
•  When the Aircraft flies above an NDB the ADF bearing pointer indicates 180 Degree.

Advantages

•  ADF does not suffer from LOS since it operates at the MF band.

Disadvantages

•   Error in ground wave is approx. +/- 5 Degree and in sky wave is +/- 30 Degree
•   Quadrantal error based on the bending of radio waves due to Aircraft metal structure is approx. +/- 10 Degree(Max.)
•   During night time, ionosphere layer is low and hence strong sky waves lead to large error. To minimize the errors at this point of time it is recommended to limit the use of ground stations.
•  Terrain has effect on the use of this technology due to reflection from mountains as well as coastal region.
•  Rain, snow and thunderstone will affect operation of antenna and hence this ADF system.

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