- NDBs are older radio navigation systems that transmit low frequency signals used for en route and approach navigation. They provide reasonably accurate guidance at costs lower than other systems. - An aircraft's ADF system uses a loop antenna to determine the direction to an NDB station based on signal strength. It has a fixed or movable indicator showing the relative bearing. - Pilots can navigate to or from an NDB station by homing directly toward it or tracking a specific course using wind correction. Tracking involves adjusting the aircraft's heading to keep the needle deflection equal to the intercept angle in order to stay on course. - ADF guidance can be affected by nighttime skywaves, signal fading, terrain interference, shore

1. ADF/NDB Navigation

2. NDB Background
• Oldest form of radio navigation still in use
• Operate in the LF/MF range (190 – 535 kHz- or
between 100 and 10,000 meters)
• Still in use because of its simplicity and low cost
to set up and operate + decent accuracy
• Little used for enroute navigation in the US but
still used for approaches
• Used for enroute nav. In much of the world

3. NDB background
• NDB’s are also useful at ranges up to 600
N.M.
• Ground equipment consists of a simple
a.m. radio station
• Aircraft receiver is little more complicated
than an AM radio.

4. Principles of NDB Navigation
• Transmitters vary in power from 25 watts for
Locator Outer Markers (LOM’s) on an ILS
approach to as much as 2000 watts.
• The range of the most powerful transmitters is
listed as 75 miles but is usually much greater.
• 4 types of NDB’s; Compas locators, Approach
aids (25 NM), Enroute beacon, High power
beacons- used in some costal areas- largely
obsolete

5. Aircraft Equipment
• The Automatic Direction Finder (ADF)
consists of:
– AM receiver
– Sense Antenna
– Loop Antenna (directional antenna)
– Indicator (fixed or movable card)

6. Aircraft Equipment Ctn.
• The Automatic in ADF comes from the fact
that the directional antenna is rotated
electronically rather than manually by
rotating the antenna or turning the aircraft
to make signal strength determinations.

7. ADF Operation
• There is a switch on the receiver marked:
– OFF/ADF/ANT/BFO
– ANT- gives maximum receiver sensitivity and
should be used for tuning and identifying (3
letter coded I.D. except for LOM’s which have
2 letter I.D.’s)
– BFO (ContinuousWave) position is used for
better identifying unmodulated signals which
are used in other parts of the world.

8. ADF Operation Ctn.
• After identifying station be sure to place selector
switch in the “ADF” mode as the indicator will
not display any bearing information in the ANT
mode
• Leave volume up continuously when using the
ADF as NDB frequencies can shift
• Most units have a “Test” function which swings
the needle; don’t use if the needle doesn’t swing.
This verifies proper signal reception.

9. ADF indicators
• Fixed card
– With a fixed card there are two ways to
determine magnetic bearing to the station:
• Turn the aircraft toward the station and note
heading
• MH + RB = MB (to the station)
– MH = aircraft magnetic heading
– RB = Relative bearing
– MB = Magnetic Bearing (to the station)
– (If the result is more than 360 then subtract 360

10. ADF Indicators
• Movable Card Indicators:
– Set aircraft heading at top of the indicator and
read the relative bearing to the station.

11. NDB Navigation Techniques
• Homing:
– Turing the aircraft such that the needle always
points directly toward the station- easy but
inefficient as it does not compensate for
crosswinds
– Results in a curved path to the station in a
crosswind situation

12. NDB Navigation Techniques
• Tracking:
– Establishing a wind correction angle that negates the
drift caused by the crosswind
– Principle: WHEN THE ANGLE OF DEFLECTION =
THE ANGLE OF INTERCEPTION YOU’RE ON
COURSE
– Or: when the angle formed by the aircraft heading and
the desired course is the same as the angle between
either the 0 or 180 mark on the indicator and the
pointer, the aircraft is on course.

13. Tracking ctn.
• Tracking tip- When tracking inbound on an NDB
bearing and you note you’re off course:
– Note the angle of deflection, double it, and use that as
your intercept heading. When that angle of deflection
is then noted on the indicator (from the nose or tail)
the aircraft is back on course
– Establish an initial track by taking half of that
intercept angle out

14. Tracking Ctn.
• Example:
– You want to track inbound on the 360 bearing
to the station. There is a wind from the west.
– While tracking (heading north) unaware of the
wind you note a left needle deflection of 10
degrees.
– Double that to 20 degrees for your intercept
heading and turn the aircraft left to 340
degrees.

15. Tracking Ctn.
• As you turn to 340 you notice the needle move
toward the top of the indicator.
• As you continue on 340 the needle will drift
toward the right. When it indicates 20 degrees,
the aircraft is back on course.
• Take half your calculated intercept heading and
use that as your initial track heading (350
degrees). This results in a 010 degree indication
on the indicator.
• Repeat the process until track correction and
needle deflection are equal.

16. NDB/ADF Limitations
• Night Effect-
– NDB’s emit three signals, among these are the ground
wave and the sky wave. Normally the ground wave is
the strongest which is the one use for navigation.
– The skip zone is the difference or gap between the
ground wave and the reflected sky wave.
– Light passing through the ionosphere causes height
fluctuations within it varying the angle of the reflected
sky wave and possibly causing signal confusion
during this time between the two signals. The needle
may wander. Resolves shortly after sunrise or sunset.

17. Fading
• Usually occurs at night when ground wave and
sky wave interact going in and out of “phase”
causing the signals to be either canceled or
reinforced as the atmosphere changes.
• Pilots will notice a rythmic swinging of the
needle and a volume fluctuation of the identifier.
• Average the fluctuations and note the bearing
when the signal seems strong.

18. Shoreline Effect
• Ground waves change direction as they
pass from land to water and visa versa;
they are bent slightly.
• Pilots should note potential bearing
indication errors when flying in the vicinity
of costal areas. NDB’s used primarily for
oceanic navigation have been designed to
minimize this error.

19. Other sources of Error
• Terrain- mountains, areas with high
concentrations of iron
• Interference- between two stations may
cause the ADF to oscillate between one
station and the other- indicated by
receiving two simultaneous I.D. codes.
Usually resolves itself with proximity.

20. Thunderstorms
• Lightening can create signal disturbances
causing the needle to momentarily swing in
the direction of the storm. As such ADF is
sometimes referred to as a “Poor man’s
Storm Scope.”
• Pilots should not rely on ADF indications
in the vicinity of thunderstorms.

21. ADF Accuracy
• Variable- not possible to determine. As
such, back up ADF indications whenever
possible.

22. The End
Questions?