Non-Directional Beacon (NDB) is ground-based transmitter which is used for direction finding by the Automatic Direction Finder(ADF)
NDB is transmits a Vertically polarised interrupted carrier wave signal in the LF or MF bands
Transmission is omni-directional or equally spread in 360 degrees around the ariel
Frequencies allocated to the NDB are from 190 to 1750 kHz in LF and MF bands
NDB Ariels are huge ‘T’ shaped due to their high wave length
Emission designators of NDB are NON-A1A or NON-A2A depending on the requirement
Range of NDB could be from 25 to 500 NM depending on their purpose
Each NDB transmits a unique three letter identifier in morse code
Principle of ADF
ADF equipment in the aircraft uses loop antenna principle to locate the NDB
Automatic Direction Finder (ADF) uses NDB beacon to aid in navigation
ADF equipment is capable of selecting from 190 to 1750 kHz
Loop antenna of ADF has two vertical elements receiving the signal
Simultaneous receipt in verticals would result in no phase difference
Detection of same phase in the verticals causes no current to flow in circuit
Zero current indicates that NDB is located perpendicular to the loop
NDBs located in front as well as behind the loop would indicate Null position
Measurement of current in other positions can be resolved to provide directions
Resolution of Null Position in ADF by using Cardioid Polar Diagram
Loop antenna of the ADF identifies a null position of zero current, but is insufficient to distinguish between Ground Stations ahead and behind the aircraft
Polar diagram of a loop ariel is in the form of a figure of 8 created by rotation of loop through 360 degrees
Datum point of ADF is its null position of zero current, at which the NDB could be either in front or behind the aircraft
A sense antenna with a circular polar diagram is used to resolve this ambiguity
The combined polar diagram by mixing Circular (Sense Ariel)and Figure of ‘8’ (Loop Ariel) is heart shaped called as Cardioid
Cardioid polar diagram has only one null position which perpendicularly ahead of the loop antenna
Polarity of Sense Antenna is frequently reversed for obtaining better accuracy
ADF Antenna Housing
Loop and sense antenna are located together in tear drop shaped housing fixed to the underside of fuselage
Modern ADF equipments have two non-rotating loop antennas at right angles
Antenna is connected to a goniometer to measure angles which has a search coil that identifies the null position
The current produced by error signal moves the search coil until it reaches null position to show relative bearing of ground station
Control Panel of ADF
ADF is controlled using an analogue or digital control panel located in the cockpit with these features
Frequency selector is used to select the required NDB by changing frequency
Antenna (ANT) button is used to check morse code identification of the NDB
Bearings have to be ignored when ANT button is pressed due to disengagement of loop antenna
Test button (TEST) button is a spring loaded switch to check the indicator by moving the needle of RBI by 90 degrees
Digital control panel has a facility to set main and standby frequencies
Beat Frequency Oscillator in ADF
Beat frequency oscillator (BFO) labelled as TONE / VOICE is provided to obtain an audible output from NON-A1A NDB transmissions
Each NDB transmits a three letter unique identification code for positive identification
NON-A1A NDB transmissions are used in long range NDBs since the amplitude modulation of A2A transmissions tends to reduce range
Unlike NON A2A signals, NON-A1A NDB transmissions require BFO for obtaining the transmitted audible identification code
BFO in the ADF has a Heterodyne unit which produces a beat frequency, which is combined with received signal to produce the audible tone
BFO should be selected ON for Tuning, Identification and Monitoring of NON-A1A NDB transmissions but OFF during Bearing Measurement
Relative Bearing Indicator (RBI)
RBI indicates position of NDB in relation to the longitudinal axis of aircraft
QDM is the magnetic bearing to the station obtained by applying aircraft’s heading to the relative bearing
If the QDM obtained is more than 360, subtract 360 to get the actual QDM
QDR is the magnetic bearing from the station obtained by adding or subtracting 180 for QDM is lesser or greater than than 180
Moving compass card is useful to set the heading manually to get QDM or QDR in the head or tail of the pointer
Radio Magnetic Indicator (RMI)
RMI converts relative bearings to magnetic bearings on a moving compass dial by combining aircraft heading with relative bearing
Aircraft’s heading is indicated on the moving compass dial
While head of the RMI pointer indicates QDM its tail indicates QDR
RMI has two pointers, which can be selected to two NDBs, or two VORs, or a combination of NDB and VOR
True bearings can be obtained from RMI indications by application of Variation and Deviation
Error of Automatic Direction Finder (ADF)
ADF suffers from errors due to Terrain Effect, Static Interference, Quadrantal Error, Cone of Silence, Dip Error, Station Interference, Coastal Refraction and Night Effect
Terrain effect is caused due to interference from reflected waves which can be minimised by flying higher in mountainous terrain
Static interference is caused due to static and electric discharge from clouds, hence ADF readings have to be treated with caution during thunder storm activity
Quadrantal error is caused due to distortion of radio waves by airframe which can be reduced by electronic devices
Maximum error is observed on quadrantal headings relative to the aircraft’s longitudinal axis
Cone of silence is located overhead the Non Directional Beacon when the ADF pointer would fluctuates rapidly
Dip error is caused due to tilt of loop antenna during manoeuvres indicating higher value on the side of tilt of loop antenna
Coastal refraction causes an error in NDBs located close to coast line due to bending of radio waves while crossing the coast
Since the speed of radio waves is faster over sea due to change in medium radio waves bend towards land which is the slower speed medium
Coastal refraction is lesser in higher frequencies and therefore coastal NDBs use higher frequency transmissions
Coastal NDBs are designed to transmit crossing perpendicular to coast to minimise Coastal Refraction
Night effect affects accuracy of bearings of ADF at night due to reception and interference between Ground and Sky waves of the same NDB
During day, D layer of ionosphere attenuates sky waves while the D layer disappears a causing sky waves to reach earth
Returning sky waves are out of phase and change in polarity which induce current in the horizontal elements
Night effect is caused due to the interference of sky waves with ground waves when ADF wanders at dawn and dusk time
Designated Operational Coverage (DOC) ensures bearing accuracy during day but are not applicable during night due to night effect
Station interference is caused due to nearby NDB stations transmitting on similar frequencies which can be reduced by judicious frequency allocation
ADF does not have any failure warning mechanism and hence positive identification is mandatory before using ADF Indications
Range and Accuracy of ADF
ADF Range of is affected by the Power, Frequency, Type of Transmission and Receiver Quality
Minimum signal to noise ratio of three to one is essential for ADF operations
Hilly terrain and Precipitation would reduce the effective range of ADF due to reduction in signal quality
Bearing accuracy of ADF is five degrees within the Designated Operational Coverage (DOC) during day time