A Non Directional Beacon (NDB) is a radio station placed at an identified location and it is used as an aviation or maritime navigation aid. In aviation, the NDB use is regulated by the annex 10 of ICAO which specifies that NDBs are exploited over a frequency range between 190 and 1750 kHz (in Europe this range is reduced within 255 and 525 kHz).
These beacons are mostly located near airports since they provide the simplest navigation information to reach an airport. A NDB is characterized by its range:
The signals from a NDB are received by an instrument in the airplane called ADF (Automatic Direction Finder).
Read more about the Non-directional beacon - NDB (Beacon)
The Automatic Direction Finder -- ADF - when tuned to a selected NDB frequency, is the on-board equipment that determines the relative bearing (RB) from the aircraft to the ground beacon or station.
The relative bearing (RB) is the number of degrees measured clockwise between the heading of the aircraft and the direction from which the bearing is taken. The relative bearing must be corrected with the aircraft position's variation and heading to obtain the magnetic bearing (MB) (the variation is the difference between the true north and the magnetic north).
Typical ADF equipment includes:
The NDB transmits signals in all directions that reach airborne ADF's loop and sense antennas. When both inputs are processed together, the equipment is able to display the relative bearing on the indicator instrument.
The ADF receiver is the control unit where pilots select NDB frequencies from which they want to obtain the bearing.
ADF equipment has its own controls:
Depending on the equipment, there could also be two displays showing the ACTIVE and STANDBY ('SBY') frequencies. In this type of control units, the frequency is first selected on the STANDBY frequency display and then transferred to the ACTIVE frequency using the transfer switch (XFR) located normally between the displays.
Relative bearing from NDBs can be shown in two different instruments:
Basic instruments consist of a compass rose with one needle that may indicate Relative Bearing (RB) or Magnetic Bearing (MB), depending on the instrument. The head of the needle indicates bearing TO the station and the tail of the needle indicates bearing FROM the station.
It only has one needle and a fixed (not movable) compass rose. It always indicates heading North at the top of the instrument.
The moving needle indicates the Relative Bearing (RB) to the station, relative to the longitudinal axis of the aircraft (fore and aft axis). That Relative Bearing (RB) is read clockwise from 0o until the value the needle is pointing at.
This is the simplest instrument and so its usage is not easy since the Magnetic Bearing (MB) is not shown at first glance. Relating to the current heading, the pilot will need to check the relative bearing and calculate the Magnetic Bearing (MB) each time.
Magnetic Bearing (MB) = Relative Bearing (RB) + aircraft heading.
Similar to the radio bearing indicator with fixed card, this instrument has the advantage to have a compass rose which can be rotated manually by the pilot.
So the aircraft's current heading can be set on the top. This allows the instrument to show directly the Magnetic Bearing (MB), which eases the pilot's work.
In the figure, the movable card was put at a heading 345°. As a consequence the NDB magnetic heading is 060°.
This is an advanced instrument as it automatically rotates the compass rose to represent the current aircraft heading at the top. The Magnetic Bearing (MB) can be obtained easily.
The RMI has one or two needles which can be used to indicate navigation information from either the ADF or the VOR receivers. Both needles are different in appearance, one of them operating with NAV 1 or ADF 1 radio and the other one operating with NAV 2 or ADF 2 radio.
There are two switches that allow the pilot to change each needle source from VOR to ADF or from ADF to VOR.
In more complex instruments mounted on bigger or commercial airplanes, the ADF might be integrated into glass cockpits EHSI where NDB bearings may be shown in the Navigation Display.
The head of the needle indicates bearing TO the station and the tail of the needle indicates bearing FROM the station. The needle is not in function of aircraft heading.
The needle on the ADF is pointing north. Then the NDB is in front of the aircraft.
The needle on the ADF is pointing south. Then the NDB is behind the aircraft.
The needle on the ADF is pointing 90°. Then the NDB is on the right of the aircraft.
The needle on the ADF is pointing 45°. Then the NDB is on the right of the aircraft.
When following a NDB navigation aid with presence of cross wind, if you maintain only the ADF at the top position of the instrument, your heading will increase or decrease like the image below.
In this case, you do not make a direct and follow the same NDB track.