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Non-Directional Beacons (NDB) &
Automatic Direction Finding (ADF)
General Principles
NDB Radiation Pattern
Omni-directional
polar diagram
Types of Modulation
 NØNA1A = Unmodulated identification
 NØNA2A = Modulated identification
NØNA1A – Carrier Wave only
NØNA2A – Modulated Carrier Wave
Range of an NDB
Max. Range = 3 Power (Watts)
e.g. A 100W transmitter will have an
approximate range of 30NM
An NDB ground
station
An NDB ground
station
An NDB ground
station
The “cone of silence” above
the NDB station in which
the signal strength may
be too low to be used.
Radio Navigation – Chapter 3
ADF Principle of Operation
- Antenna System
• 2 aerials are required:
– Loop antenna (to detect null directions)
– Sense antenna (to resolve ambiguity of null
directions)
– Fixed Loop system
Loop antenna
polar diagram
Null
Sense antenna
(Omni-directional)
Null
The resultant cardioid
polar diagram of the
ADF aerial.
Radio Navigation – Chapter 3
Typical ADF control panel.
Radio Navigation – Chapter 3
Frequency
range
NDB frequency range
190 - 1750 kHz (ICAO)
Aeronautical Range
MF 315 - 405 kHz
LF 255 - 285 kHz
Marine 285 - 315 kHz
Broadcast
LF 153 - 279 kHz
MF 531 - 1602 kHz
A rotatable card type of indicator is exactly like a fixed card indicator,
except that the card can be rotated to reflect the aircraft’s heading.
Radio Navigation – Chapter 3
The most common
type of presentation
is the RMI - Radio
Magnetic Indicator
Radio Navigation – Chapter 3
Homing will point the nose of the aircraft directly to the station as
long as the ADF needle is kept pointing to the top of the indicator.
Radio Navigation – Chapter 3
Relative Bearing Indicator
RB
= 345°
HDG
= 075°
(= –15°)
QDM = HDG + RB = 075 + 345 = 420°
420 – 360 = 060°;
or 075 – 015 = 060°
Relative Bearing Indicator
 Required QDM
= 035°
 HDG = 075°
Required RB to
intercept radial
= QDM – HDG
= 035 - 075
= – 040;
or
360 - 040 = 320°
Intercept of an outbound track 085° from a NDB.
HDG
125ºM
RB
100º
QDM = 225°M
QDR = 045°M
Radio Navigation – Chapter 3
What
RB =relative
(085+180)
bearing
- 125do
= 140°
we
require to intercept radial 085?
NDB & ADF
Limitations & Accuracy
• Radio wave propagation
– LF/MF
• Ground wave - Day/Night
• Sky wave - Night
• Direct wave - line of sight only
The ground wave coverage might extend out to 300 nm,
while the first skywave returns at 1000 nm.
This gap is called the skip zone.
Radio Navigation – Chapter 3
Quadrantal Error
The quadrantal error is
maximum on relative
bearings 045°, 135°,
225° and 315°
No error
Radio Navigation – Chapter 3
Coastal Refraction
The effect of coastal
refraction is least
when bearings are
90° to the coastline
Radio Navigation – Chapter 3
Coastal Refraction
Note: The further inland the
NDB’s are, the worse is the
error
NDB2
NDB1
Apparent
position of
a/c
Other ADF errors
• Dip (Bank) Error
• Mountain effect - multipath signals
• Noise:
– Static Interference (worse at LF)
– Electric storms
(increasing in summer & towards the equator)
Electric Storms (Thunderstorms)
• A lightning discharge near the aircraft may
give erroneous readings
• Can result in a complete reversal of the ADF
needle
Effect will fade with
increasing distance
from cloud
Other ADF errors
• Noise: S/N ratio 3:1
– Man-made, e.g. from power lines, also:
electronic equipment, especially cellphones,
laptop computers, electronic games, etc.
– Station Interference due to sky waves at night
time
Skywave problems - Night Effect
Skywave problems - Night Effect
• Symptoms - hunting of the indicator needle and
signals fading or fluctuating in strength
• Condition is most noticeable around dawn or
dusk (the “Twilight Zone” or Transition Period)
Accuracy of an NDB
(ICAO) ± 5° — Daytime only
Questions
• What is the basic information given by ADF?
– The relative bearing from the aircraft to the
NDB
• On a manually rotateable card type indicator,
what should be indicated at the top?
– The aircraft heading
More Questions
• Whilst correctly tuned to an NDB transmitting
a N0NA2A signal, with the BFO off, what
should you hear?
– The station identification only
• What is the ICAO frequency allocation for
ADF receivers?
– 190 to 1750 kHz
More Questions
• An aircraft is heading 330°M, the relative
bearing to an NDB is 190°, what is the QDM?
330 + 190 = 520 (-360) = 160°M
• What is the usual effect of coastal refraction?
– It puts the apparent position of the aircraft
closer to the coast than the real position
• When will bearings be most affected by
coastal refraction?
– When the NDB is further inland
More Questions
• What aerials are used with ADF?
– Loop and sense aerials
• At what time of day are bearings likely to be
least accurate?
– Around dawn or dusk
• What symptoms does night effect produce?
– Hunting of the needle and fading of the signal