Non Directional Beacons and Automatic Direction Finding
Non Directional Beacons (NDB) are ground-based transmitters which transmit radio energy equally in all directions. The airborne system in the aircraft is called the Automatic Direction Finder (ADF). The indicator in the aircraft always points towards the tuned NDB (exceptions to this will be discussed later in this chapter).
Principles of Operation
The NDB transmitter is very simple. A RF oscillator provides a carrier wave. This carrier wave is the NDB signal that is used by the airborne equipment (ADF) to determine the direction of the transmitting station. A low frequency oscillator provides the identification signal of the transmitting station or “ident”. The low frequency signal modulates the carrier wave in the modulator.
LF/MF – 190 to 1750 KHz. In Europe the frequencies are normally between 225 to 455 KHz.
- Long Range Beacons N0N A1A
- Short Range Beacons N0N A2A
An amplified signal is radiated omni-directionally. The transmission mast may be either a single mast or a large T-aerial strung between two masts. These aerial arrangements produce a vertically polarised signal. The polar diagram for the aerial is omnidirectional in the horizontal plane but, as shown below, exhibits directional properties in the vertical plane.
Above the station, marked by the points at which the radiated power has fallen to 0.5 of its maximum value, is a conical area in which signal strength may be too low to be used. This volume of space is called the ‘cone of silence’. For a NDB this angle is 40° from the vertical.
The Automatic Direction Finder (ADF) consists of a receiver, a sense aerial, a loop aerial and an indicator. The receiver control panel and the indicator are located on the instrument panel, the loop and the sense aerial are normally combined in a single aerial unit, normally mounted under the fuselage. The pilot uses the receiver control panel to enter the frequency corresponding to the NDB for intended use.
The ADF indicator consists of a needle, which indicates the direction from which the signals of the selected NDB ground station are being received. In its most basic form, the needle moves against a scale calibrated in degrees from 0° - 359°. This is known as a Radio Compass. The datum for the direction measurement is taken from the nose of the aircraft and therefore, the radio compass indications are relative bearings.
The loop, or directional, aerial is rotated electronically and, by combining information from the loop and sense aerials, the bearing to the station is internally derived. When a looped conductor, such as the loop aerial, is hit by electromagnetic waves, voltages are induced in the two halves of the loop. These voltages depend on the angular position of the loop relative to the incoming electromagnetic (EM) waves. The total voltage induced in the loop is the algebraic difference between the voltages from the two halves. This total voltage is the signal output from the loop aerial.
Typical associated power outputs and uses are as follows:
- Locator Beacon: 15 to 40 watts - Used for intermediate approach guidance towards establishing the final approach path of an ILS. These beacons are short range and are normally NON/A2A. Maximum range 15 – 25 nm.
- Homing: Primarily an approach and holding aid in the vicinity of an aerodrome. Medium range beacon normally N0N A2A. Maximum range 50 nm.
- Airways/Route Beacons: up to 200 watts - Used for track guidance and general navigation. These beacons are normally NON/A2A
- Long-Range Beacons: up to 4 kilowatt -Generally located on islands or oceanic coastlines, these are intended to provide guidance and navigation resource to transoceanic flights. These beacons are normally NON/A1A.
It should be noted that different transmitters operate within the NDB band of frequencies and can be detected by the aircraft’s receiver. These include:
- Broadcast stations (i.e. those carrying entertainment, news, etc.)
- Marine Beacons.
Stations must not be used if their details are not published in the AIP or appropriate Flight Guides. Where details of Marine Beacons are published, users should note that a number of beacons are grouped together on the same frequency. Each beacon transmitting for a period of 60 seconds in a cycle of six minutes.
The use of signals from such published stations guarantees that, within the published range by day, the signal from the desired station will be at least three times stronger than any other signal on the same or near frequency. The use of transmissions from non-published sources may lead to errors, as they are not protected from such harmful interference.
Control Panels and Indicators
There are different types of ADF control panels, but their operational use is almost the same and an example is shown below. The mode selector, or function switch, has several positions, enabling the pilot to select the function he wants to use. Typical markings are - OFF, ADF, ANT, and LOOP.
- ADF: The position when the pilot wants bearing information to be displayed automatically by the needle.
- ANT: The abbreviation of antenna and, in this position, only the signal from the sense aerial is used. This results in no satisfactory directional information to the ADF needle.
There are two reasons for selecting the ANT position:
- Easier identification of the NDB station
- Better understanding of voice transmissions
BFO stands for Beat Frequency Oscillator. This position can be labelled CW, the abbreviation for Carrier Wave. The BFO circuit imposes a tone onto the carrier wave signal to make it audible to the pilot, so that the NDB signal can be identified.
The emission characteristics determine the position of the BFO switch:
- Tuning = on
- Identification = on
- Tuning = on
- Identification = off
Once the station has been properly tuned and identified, the Mode Selector should be switched back to ADF. This is important, as no bearing information will be displayed unless the switch is in the ADF position. When BFO or ANT are selected some ADFs automatically default to the 180° position, others remain on the last bearing computed. So never leave the mode selector in ANT or BFO position if you are navigating using the ADF.
In order to avoid the dangers of this problem, NDBs transmitting on A2A can be identified with the mode selector in the ADF position, so the ANT position can be avoided. There is no failure flag on an ADF receiver or indicator, the only way to be sure that the instrument is receiving a valid signal from the NDB is to continuously monitor the station’s identification.
Each NDB is identifiable by a two or three lettered Morse code identification signal, which is transmitted together with its normal signal. This is known as its IDENT. When tuning an NDB it is absolutely essential that the facility be correctly identified before using.
If a test switch is incorporated, pressing test swings the needle:
- If the needle does not swing, the unit is not working properly.
- If the needle does swing but doesn’t return to its previous position, the signal is too weak to be used for navigation.
- If it swings and returns to its previous position, then the system is working properly and the received signal is good.
Bearings to the station are displayed on an indicator consisting of a bearing scale (calibrated in degrees) and a pointer. There are four types of bearing scale with varying degrees of sophistication. They are:
- The fixed card.
- The manually rotatable card.
- The radio magnetic indicator (RMI).
- Fixed card indicator or RBI.
Only two systems will be discussed, the RBI and the RMI.
Relative Bearing Indicator (RBI)
The bearing displayed on a fixed card indicator is a relative bearing; thus the name Relative Bearing Indicator (RBI). Since the Card is fixed, zero is always at the top and 180° is always at the bottom.
A relative bearing is always measured clockwise from the nose of the aircraft. In the diagram above the needle is pointing to 100°. This means that the station is 100° to the right of the aircraft nose.
In the diagram below the relative bearing is 340°. The NDB is 340° right of the nose.
A more convenient way of expressing this is that the station is 20° left of the nose.
It is sometimes convenient to describe the bearing of the NDB in relation to the NOSE or TAIL of the aircraft.
Since the card is fixed, the indicated relative bearing has to be combined with the magnetic heading of the aircraft in order to obtain the magnetic bearing to the station (QDM). If the result of this addition exceeds 360°, 360° has to be subtracted from the result in order to obtain a meaningful bearing.
Assume for the diagram above that our aircraft is heading 230°M. The bearing to the NDB is:
230° + 340° = 570°
Because this is more than 360 then we must subtract 360 from 570 = 210°
- The QDM is 210°
- the QDR is 030°
The magnetic bearing of the aircraft from the station, the (QDR), is the reciprocal of the QDM. A quicker way to determine the QDM is to mentally superimpose the RBI needle onto the directional gyro. This is not very accurate, but it is a good double check on your calculations. The QDR can be visualised as the tail of the needle when it is mentally transferred from the RBI onto the directional gyro indicator.
Radio Magnetic Indicator (RMI)
This combines the Relative Bearing Indicator and Remote Indicating Gyro Compass into a single instrument, with the compass card being aligned automatically with Magnetic North. In the diagram below:
- The heading is 332°M.
- The VOR or ADF can be indicated by either pointer depending upon the switching.
- The QDM is continuously indicated by the arrow head of the pointer.
- The QDR is continuously indicated under the tail.
This is now the most common type of presentation.
If the double pointer represents the ADF then:
- The QDM is 300°
- The QDR is 120°
Direct Wave Limitations
The Direct Wave follows the line of sight and its range can be determined from the line of sight formula. In most cases the direct wave range will be considerably less than that of the Ground Wave. Height may become significant when it is desirable to receive the direct wave, so as to minimise the risk of ADF error when flying in mountainous areas or when using coastal NDBs.
Sky Wave Limitations
At some frequencies there will be a gap in coverage between the ground wave and the first return of the sky wave. The ground wave coverage might extend out to 300 miles, while the first skywave returns at 1000 miles. This gap is called the dead space.
The exact length of the dead space depends on frequency and the state of ionisation of the atmosphere. At frequencies in the lower MF and the LF bands, intense ionisation by day attenuates (absorbs) RF signals and no sky wave return is noticeable. By night the ionisation levels fall and returning sky waves will be detected.
At short range (30 to 80 miles) the sky waves will mix with the ground wave signal (there is no dead space). Because the returning sky waves have travelled over a different path they have a different phase from the ground wave. This will have the effect of suppressing or displacing the aerial ‘null’ signal, in a random manner. The needle on the RMI or RBI will wander. This effect is at its most variable during twilight at dusk and dawn.
A further effect is due to the design of the loop aerial system. The loop uses a vertically polarised signal. As the radio wave travels through the ionosphere the vertical polarisation is changed as the wave is refracted back towards the earth, so the returning wave has a horizontal polarisation component.
A current is now induced in the horizontal members of the loop:
- The horizontal member AB.
- The two smaller feeds to the bottom of the aerial.
The resultant current flow further degrades the null position and an accurate reading is impossible.
At longer ranges the sky wave signal will become progressively stronger. However, ionospheric refraction may cause the plane of polarisation of the signal to be randomly shifted so that a horizontally polarised component may be randomly introduced into the loop aerial. This will cause the null signal to be displaced.
In summary, the airborne ADF is designed and optimised to be used in conjunction with the more predictable ‘ground wave’ signal from the selected NDB.
Errors of the ADF
The ADF bearing is subject to a number of error sources including any or all of the following.
The metal components of the aeroplane’s structure behave as an aerial. They absorb signals at all frequencies but more readily so at frequencies in the MF band. Once absorbed, these are then re-radiated as weak signals but, being close to the ADF aerial, are strong enough to be detected.
The effect of this signal is to displace the measured null towards the major electrical axis of the aeroplane creating an error that is maximum on relative bearings 045°,135°, 225°, 315° (the quadrantals). This error is minimised by calibration and electro-mechanical compensation at installation.
Dip (Bank) Error
During turns, the horizontal member of the loop aerial will detect a signal. This will cause the null to be displaced and a ‘short-term’ erroneous bearing to be displayed.
When flying over the sea and using a land based beacon, the changes in propagation properties of the signal as it passes from land to sea will cause the ‘wave front’ to be displaced. This will result in a bearing error.
Such bearing errors may be minimised by any or all of the following:
- Do not use beacons unless they are situated on islands or near to the coast.
- If using an inland NDB only use bearings at or near to 90º to the coast.
- Remember that coastal refraction is less as height is increased.
When flying in mountainous regions, signals may be refracted (bent) around and/or reflected from mountains. The ADF may be affected by such multipath signals and the bearings will be unreliable.
This is defined as any signal detected at the receiver other than the desired signal.
- Man Made Noise: Each published NDB has an associated published range. If use of that NDB is restricted to that range, the desired signal is protected from the harmful interference of ground waves from other known transmitters on the same or near frequencies. It should be remembered that, from sunset to sunrise, sky wave
propagation of signals in the LF and MF bands is possible. This will cause the signal to noise ratio to be reduced and will result in errors as the null is displaced, usually randomly.
Another localised source of man-made noise is overhead power cables. Many of these cables carry not only electrical power but also modulated signals used by the power companies for communication. These modulated signals radiate from the power cables and create mini NDBs. Such emissions are monitored but, in some states, monitoring may not be carried out. The rule is – if unsure, use with extreme caution.
- Lightning: There are an average of 44 000 thunderstorms over the earth’s surface in every period of 24 hours and more than half of these occur over or near land surfaces within 30º latitude of the Equator.
Each thunderstorm generates electro-magnetic signals and these radiate in all directions from that storm. If you happen to be flying near one of these storms, your ADF will detect the signal and the bearing indication may well be deflected towards that storm. Such noise levels are normally quite low but they will increase with:
- In temperate latitudes in the summer.
- As you move towards the tropics.
- At night as a result of sky wave propagation.
- Charged Water Droplets: Water droplets held in a cloud have an electric charge. As an aircraft flies through the cloud the water droplets that contact the aircraft will discharge on the metal surface. The collective effort of the water droplet discharge can distort and blur the polar diagram such that the null position is displaced.
Noise effects can be indicated by:
- Seeing the bearing indication randomly wandering.
- Using the audio output and noting audible signals such as voice/music/static.
If ‘noise effect’ is suspected, only use the published NDBs when well within the notified range. You could be at half the published range before a reliable signal is found.
Where two or more beacons are transmitting on the same frequency then the measured bearing becomes the resultant of the two received signals.
As long as the NDB is used within its promulgated range the effects of synchronous transmission should be a minimum.
Most NDBs are given a daytime only protection range where the unwanted signals are limited to ± 5°. Outside this range the error will increase. The propagation conditions at night also increase the bearing errors.
Absence of Failure Warning
There is no visible indication to the user that there is a system failure.
When used within the published range by day the ADF should give a bearing accuracy within ± 6.