The successors to the old commercial stations are the aeronautical NDBs transmitting AM signals in the upper LF and lower MF bands, between 190 kHz and 1750 kHz. The advantages of these frequencies lie in the diffraction they suffer close to the earth’s surface. Aircraft can receive surface waves if the direct waves are disrupted by obstructions or the curvature of the earth. In fact, coastal NDBs can be used by both ships and aircraft. There are of course disadvantages also, as readers can deduce. We shall look at these disadvantages later.
Beacons are situated along airways to guide controlled air traffic, on ocean coastlines to provide navigation assistance far out to sea, on airfields to provide homing, and under runway approach paths to lead aircraft safely and speedily on to the Instrument Landing System (1ILS). Each of these positions requires different characteristics from their transmissions, mainly in the signal strength and consequent maximum reception range.
We have seen that modulating a signal requires power, for an AM signal up to 50% above the basic carrier transmission power. For that reason, some NDBs, mainly those used for long-range navigation at ocean coasts, are unmodulated, with a short period of keying used to provide an identification message. These emissions are coded NON A1A (the second part of the code refers to the keyed part). Unfortunately, to receive any part of the signal, the airborne equipment must employ a beat frequency oscillator (BFO) (described later). In addition, during the breaks in the transmission required for the keying, there is no signal for direction finding. This type of emission is not recommended unless there is no alternative.
To reduce the NON A1A disadvantages, many NDBs use a different modulation to provide the identification message. The carrier wave continues unmodulated for most of its duration, but when the identification is needed some of the power is used to amplitude modulate the carrier for identification. The resultant emission code is NON A2A. The BFO is needed only for initial tuning. NDBs which do not need the maximum signal strength can have their carrier wave amplitude modulated continuously. A BFO is not required to receive or identify this signal, whose emission code is A2A.
NDBs are designed for particular purposes. The area within which their signals must be receivable is calculated, and within that ‘rated coverage the signal strength must be sufficient to give good reception and DirectionFinding. ICAO recommends a particular field strength and signal/noise ratio that should be receivable at the edges of that area.
Types of NDB
Locators – These are low-powered NDBs, usually installed as a supplement to ILS and located together with middle or outer markers. A locator has an average rated coverage of between 10 and 25 nm. Emission is usually NON A2A, and they send identification signals every 10 seconds.
Homing and holding NDBs These are intended primarily as approach and holding aids in the vicinity of aerodromes, with rated coverage of about 50 nm. Their emission and ident characteristics are similar to locators.
Old airborne equipment required an operator to turn the loop and determine the original null, then switch in the sense antenna and turn it again to discover the correct direction. Progressively, equipment has improved to allow it to earn the correct expansion of the initials ADF “Automatic Direction Finder.
The total signal from the cardiod can be amplified and fed to a motor which can turn the loop. The direction of turn depends on the phase of the total signal, which will automatically turn the loop towards the null position.
When there is no total signal, the loop and its associated pointer indicate the direction of the incoming signal relative to the datum of the aircrafts longitudinal axis (fuselage).
The beat frequency oscillator (BFO)
In order for a pilot or operator to actually hear a signal, to either tune it or for identification, it must be at an audio frequency, between 300 and 3000 kHz. The NDB carrier waves are, of course, at a much higher frequency than that. A receiver can be fitted with a BFO, which the pilot can switch in to the receiver circuit when required. The BFO is a device which produces a signal inside the receiver at a frequency of about 1000 Hz removed from the received wave. The received wave is compared with the produced signal, and the difference in frequency is converted to an actual audio signal at the same frequency which the pilot or operator can hear. The BFO must be switched on when manually tuning a NON A1A or NON A2A signal, and when identifying a NON A1A signal. For direction finding, it should be switched off. In many equipments, the BFO is controlled by a ‘tone’ switch.
RBI (Relative Bearing Indicator)
Old equipments use a fixed circular instrument face, marked in 360° from the nose of the aircraft. The pointer indicating the direction of the signal moves around the dial to indicate the direction relative to the aircrafť’s heading. The pilot or operator has to then make mathematical calculations to determine the magnetic or true bearing of the aircraft from the beacon in order to plot a position line. This instrument is called the ‘Relative Bearing Indicator, or RBI. It is also sometimes called the ‘Fixed Card Indicator’, or ‘Radio Compass.
RMI (Radio magnetic indicator)
The common display is now the Remote Magnetic Indicator or RMI. The scale is automatically orientated, like a remote indicatng compass, to the earth’s magnetic field. Again, the needle indicates the actual bearing of the beacon from the aircraft, the tail indicates the bearing of the aircraft from the beacon, and the relative bearing of the beacon can be assessed from the position of the needle relative to the top of the instrument. Most RMIs have two needles, each of which can be selected to show information from an ADF or a VOR equipment.
To use the RBI to fly to a beacon, the pilot must tune and check the identification of the NDB. Once the ADF has locked on to the NDB the relative bearing can be measured. The relative bearing can be added to the current heading to find the magnetic track towards the beacon (QDM), then he should turn his aircraft on to a heading which will take him towards the beacon. That heading must allow for the expected drift.
Factors affecting ADF range
Night effect– Sky wave interference reduces the maximum safe (protected) range to 70 nm.
Transmission power– Range is proportional to the square root of the power.
Frequency– Low frequency means less attenuation of surtace wave so greater range.
Emission A2A has least range, NON A1A greatest, because of the transmission power required for modulation.
Terrain– Smooth terrain and especially a sea surface gives less attenuation so greater range.
Most airborne equipment is capable of accuracies in the region of +-2, but the combined accuracy of the svstem, including the NDBs, is +- 5° within the beacon’s protected range.