What is an NDB?
NDB stands for Non-Directional Beacon, the ground transmitter.
What is ADF?
ADF stands for Automatic Direction Finder, the airborne receiver. (Magnetic)
Operating principle of an NDB?
NDBs transmit an omnidirectional signal in the LF/MF frequency bands, between 190-1750 kHz ICAO frequency band, which is also used by some commercial radio stations.
Normal NDB frequencies.
200 to 455kHz
NDB ground station?
- NDBs have an automatic monitoring ground station.
- An NDB station emits a NON/A1A(Requires beat frequency occilator) or a NON/A2A signal.
- An NDB station is identified by means of a two or three letter Morse code identification signal. broadcast 2-3 times every 30 seconds.
En route and long range NDBs?
- 3 letter Ident
- Range greater then 50nm (up to 300nm more over water)
- Tend to be in lower part of the LF range
- Used for route guidence along airways.
Long range NDBs restricted to daylight hours only.
NON A1A – CW or Continuous Wave:
The A1A part of the signal is a keyed, un-modulated sub-carrier wave, which is mixed with an additional audio signal at the receiver, by manual or automatic activation of a Beat Frequency Oscillator (BFO) circuit, to produce an amplitude modulated audible tone, at least once during every one minute period.
NON A2A – MCW or Modulated Continuous Wave:
The A2A signal is a keyed, amplitude modulated (AM) sub-carrier wave, transmitted at least once during every 30 seconds period. You can hear this is ADF mode.
Main things effecting the range of NDBs?
- Transmit power
- The Earth’s surface over which the ground wave travels also has an impact on the range of an NDB.
What is the main type of NDBs?
NDB with a range of 50nm or which may be used for approach procedures or en route navigation.
The Beat Frequency Oscillator (BFO)?
The Beat Frequency Oscillator (BFO) – automatically activated on modern aircraft – must be activated to hear the identification audio tone on NON/A1A NDBs.
What are locators (LCTR beacons)?
Locators are low-powered NDBs, associated with instrument approach procedures, with a range of 10–25 nm.
NDB range is limited by transmitter power output and calculated using the following formulae:
- Range (Over land) = 2 × √Tx Power (Watts).
- Range (Over water) = 3 × √Tx Power (Watts).
Equipment for using NDBs
Airborne equipment - ADF
Groudn equipment - NDB
How do NDBs propagate?
NDBs mainly propagate diffracted, surface (ground) waves.
NDB signals refract from the ionosphere at night, returning to Earth as sky waves, which mix with ground waves, causing signal fading.
Cockpit display equipment includes?
- Relative Bearing Indicator (RBI) – Fixed Card and Moving Card.
- Radio Magnetic Indicator (RMI).
- Electronic display
The cockpit displays indicate compass bearing to and from an NDB:
TO – at the point of the needle.
FROM – at the tail of the needle.
Convert realtive bearings to and from NDBs?
Heading + Relative Bearing = Bearing To (NDB).
Homing in corsswind conditions?
Homing to an NDB (along a curved path in crosswind conditions) can be achieved by maintaining a constant relative bearing of 000°.
Homing can only be used inbound to NDBs
Effects in heading when when homing on an NDB?
Right crosswind = left drift = heading increasing.
Left crosswind = right drift = heading decreasing.
How can direct tracking to an NDB be achived?
Direct tracking to an NDB (along a direct path in crosswind conditions) can be achieved by adjusting the aircraft heading for the correct wind correction angle.
Can to be used TO and FROM NDBs
Needle may point slightly left or right of top of instrument.
Nav requirements for NDB tracking procedure?
All NDB tracking procedures, including procedural turns and holding patterns, must be flown within ±5° of track bearing.
SNR of 3:1 required.
NDBs are susceptible to a number of error:
- Absence of warning indication.
- Static from cumulonimbus clouds (Static and TCU).
- Coastal refraction. Signal refraction caused by differeing speeds of surface wave over land and sea.
- Night/twilight effect – Skywave distortion of the null position and is maximum at dawn and dusk.
- Mountain effect – multipath propagation and weakening of signals.
- Dip error – needle dips towards down-going wing in turn, caused by the sense aerial being masked by the loop aerial. Worse when NDB is at 45degrees and 135degrees relative bearing.
- Station Interfence - Interference from stations on similar frequencies overlapping. (Symptoms eratic indications and 2 idents)
You must listen to the ident constaintly in the approach.
How can the effects of coastal refraction be reduced?
- Flying higher - a higher altitude minimises this coastal refraction effect.
- Using NDBs close to the coast - this reduces the effect of the error to the pilot, even though the waves bend just as much.
- Using signals that cross the coast at or near to 90° - where refraction would not take place.
