FINAL REPORT - Stakeholders - Ofcom

Figure 3-16 – Current (and future – dotted red line) Loran-C coverage as predicted
by NELS
3.4.3.2 NDB
Non-directional beacons have been used as a primary airborne navigation aid since
World War II. Non-directional signals are transmitted via a low or medium frequency,
whereby a properly equipped receiver can determine bearings and ‘home in’ on the
station. Alternatively, the opposite radial can be used to track away from the beacon.
The radio signal is broadcast in every direction at once; hence its name, non-directional
beacon. Ranges can vary from a few miles to hundreds of miles, depending on the
antenna power. The NDB station consists of a radio transmitter, an antenna coupling
device, and an antenna. NDBs are allocated frequencies by the ITU from 130 – 535 kHz,
with Europe (ICAO Region 1) actually using a narrower band between 255-526.5 kHz.
Mobile aeronautical off-route NDBs (used by the military) are assigned frequencies up to
979 kHz in the UK. Channelisation is 1 kHz.
NDBs are omnidirectional. Transmission power depends on the role of the beacon –
short-range locators may transmit at 10W, whereas en-route beacons can transmit at
powers up to 1kW. Typical aviation NDBs transmit at between 25 and 200W.
In order to track a signal, the aircraft must be fitted with an Automatic Direction Finder
(ADF). The ADF set is used in an aircraft, and consists of a receiver that will pick up a
radio signal in the 190 kHz to 1800 kHz radio band. The ADF receiver utilizes two
antennas to intercept the radio signal and determine the direction of that signal. This
directional information is displayed on an instrument that points to a compass heading
indicating the direction from the receiver to the source of the radio signal. No distance
measurements are included.
3.4.3.3 VHF VOR
One of the reasons for the unreliability and inaccuracy of airborne direction finding
systems operating with ground medium frequency beacons (NDBs) and public broadcast
stations is that medium frequencies (i.e. longer wavelengths) can be badly affected by
atmospheric interference, skywave interference and coastline refraction. VOR (VHF
Omni-Range) uses frequencies that are high enough to avoid these problems as their
primary signal propagation is by space waves (or line-of-sight).
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