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

Max. Range = 3 Power (Watts) 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. Frequency range Typical ADF control panel. Radio Navigation – Chapter 3

190 - 1750 kHz (ICAO) Aeronautical Range MF 315 - 405 kHz 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 HDG = 075° RB = 345° (= –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°

What relative bearing do we require to intercept radial 085? Intercept of an outbound track 085° from a NDB. HDG 125ºM RB 100º QDM = 225°M QDR = 045°M RB = (085+180) - 125 = 140° What relative bearing do we require to intercept radial 085? Radio Navigation – Chapter 3

NDB & ADF Limitations & Accuracy Radio wave propagation LF/MF Ground wave - Day/Night Sky wave - Night Direct wave - line of sight only

This gap is called the skip zone. 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 No 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 NDB1 NDB2 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)

(ICAO) ± 5° — Daytime only 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