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NDB and ADF

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3 NDB AND ADF

3.1 NDB; principle of operationNon Directional Beacons (NDB) are ground-based transmitters which transmitradio energy equally in all directions; hence their name. The airborne system iscalled the Automatic Direction Finder (ADF). Its indicator (theoretically) alwayspoints towards the tuned NDB. NDBs transmit radio signals omni- directionallyin a wave pattern from the station.

The NDB transmitter is very simple: A RF oscillator provides the carrier wave.The carrier wave is the NDB signal used by the airborne equipment (ADF) todetermine the direction of the transmitting station. A low frequency oscillatorprovides the identification signal of the transmitting station or «ident».

The low frequency signal modulates the carrier wave in the modulator.

The types of modulation normally used by NDB are:

• NON and AlA for long range NDBs,• NON and A2A for short and medium range NDBs.

The modulation class of the NDB is usually referred to as AlA or A2A only butNDB stations actually use two different signals together: NON (the unmodulatedsignal or carrier wave) is used by the airborne equipment to determine the direc-tion of the signal. AlA or A2A (the modulated signal) is used to transmit theNDB’s identification.

After being modulated, the signal is driven to the power amplifier, where it isboosted to the final transmission power. The transmitter power of an NDB isclosely allied to its intended use and may vary from as low as 15 watts to severalkilowatts. The nominal maximum range of an NDB can be determined from theformula Rmax = 3√ P where

Rmax is the maximum range in NM P is the radiated power in watts.

Types, typical associated power outputs and uses are as follows:

• Locator Beacon - 15 to 40 watts. - Used for intermediate approachguidance towards establishing the final approach path of an ILS. Thesebeacons are short range and are normally NON/A2A.

• Airways/Route Beacons - up to 200 watts. - Used for track guidanceand general navigation. These beacons are normally NON/A2A

• Long-Range Beacons - up to 4 kilowatts. - Generally located on islandsor oceanic coastlines, these are intended to provide guidance andnavigation resource to transoceanic flights. These beacons arenormally NON/A1A.

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The amplified signal finally reaches the transmission aerial where it is radiatedomni-directionally. The transmission mast may be either a single mast or a largeT-aerial strung between two masts. (Fig. RA 3.1).

Figure: RA 3.1 ‘T’ shaped NDB Aerial

These aerial arrangements produce a vertically polarised signal. The polar dia-gram for the aerial is omni directional in the horizontal plane but exhibits direc-tional properties in the vertical plane, as shown in figure RA 1.2.

Figure: RA 3.2

Another sketch, showing the immediate vicinity of the NDB, is shown in figure RA3.2. Above the station, marked by the points at which the radiated power hasfallen to 0.5 of its maximum value, is a conical space in which signal strength maybe too low to be used. This volume of space is called the ‘cone of silence’.

The frequency band chosen to produce surface ranges of intermediate order areupper LF and lower MF. The bands are ideally placed to produce the ground/surface wave range required. The frequencies assigned by the ICAO for NDB trans-missions are from 190 kHz to 1750 kHz. In Europe, NDB frequencies are nor-mally found between 255 kHz and 455 kHz. It should be noted that many othertransmitters operate within the NDB band of frequencies and can be detected bythe aircraft’s receiver. These include broadcast stations (i.e. those carrying enter-tainment, news, etc.) and Marine Beacons. Stations must not be used if theirdetails are not published in the AIP or appropriate Flight Guides. Where details

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NDB and ADF

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of Marine Beacons are published, users should note that a number of such bea-cons will be grouped together to serve an area. These beacons will share a singletransmission frequency, each transmitting for a period of 60 seconds in a cycle ofsix minutes. Make sure that the bearing you are reading is for the ‘correct’ beacon.

The use of signals from such published stations guarantees that, within the pub-lished range by day, the signal from the desired station will be at least three timesstronger than any other signal on the same or near frequency.The use of transmissions from non-published sources may lead to errors, as theyare not protected from such harmful interference.

3.2 ADF Principle of OperationThe Automatic Direction Finder (ADF) consists of a receiver, a sense aerial, a loopaerial and an indicator. The receiver control panel and the indicator are located onthe instrument panel, the loop and the sense aerial are normally combined in asingle aerial unit, normally mounted under the fuselage. The pilot uses the re-ceiver control panel to dial the frequency corresponding to the NDB for intendeduse.

The ADF indicator consists of a needle, which indicates the direction from whichthe signals of the selected NDB ground station are being received. In its most basicform, the needle moves against a scale calibrated in degrees from 0° - 359°. Thisis known as a Radio Compass. The datum for the direction measurement is takenfrom the nose of the aircraft and therefore, the radio compass indications arerelative bearings.

3.2.1 Bearing determinationThe loop, a directional aerial, is rotated electronically and, by combining informa-tion from the loop and sense aerials, the bearing to the station is internally de-rived in the ADF.

One of the basic principles of electricity says that if a variable number of electro-magnetic field lines pass through a coil, a voltage will be induced in the coil. Whena looped conductor, such as the loop aerial, is hit by electromagnetic waves, voltageswill be induced in the loop. These voltages depend on the angular position of theloop relative to the incoming electromagnetic (EM) waves. The voltage induced inthe loop is at it`s maximum when the loop`s plane is parallel to the receivedsignal. Thus the receiver will detect the greatest voltage when the plane of theloop`s plane is PARALLEL to the direction of propagation of the radio waves. If theloop aerial is rotated until it`s perpendicular to the radio to the direction of move-ment of the radio waves, none of the EM waves will pass through the loop and theresultant signal will be a null. For one 360o rotation of the loop aerial, the receiverwill detect two maximums and two nulls. Small angular deflections of the loopaerial near its null position produce larger changes in voltage than similar angularchanges near the loop’s maximum position. For this reason a null position is usedfor direction finding purposes.

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The horizontal polar diagram of a loop aerial will have the shape of a figure “8”, asshown in figure RA 3.3. In this figure the plane of the loop is indicated, as well asthe loop`s axis which is perpendicular to the loop`s plane. Beware of these twoways of describing the loops direction: the direction of the plane and the directionof the axis. Many tricky multiple choice questions may be constructed playingwith these two expressions!

Since the NULL occurs in two positions during the 360° rotation, there is a 180°ambiguity in the BEARING INDICATION. This ambiguity is resolved by the use ofa sense aerial. The sense aerial takes many different forms but they all have thesame function, each acting as if a straight piece of wire were hanging verticallyfrom the aircraft. The function of the SENSE aerial, so far as automatic directionfinding is concerned, is to eliminate the ambiguity of a loop by distinguishingbetween the signals received from one side of the loop and the signals receivedfrom the other side of the loop.

As we have seen, the sense aerial receives equally well from all directions and thusits polar diagram is a circle. If a polar diagram is traced out in order to show thesignal strength produced by the loop at different angles through 360°, the result isa figure eight. In adding the steady signal from the sense aerial to the alternatingsignal from the loop signal, the resultant polar diagram is a heart shaped figure,called a cardioid. (Figure RA 3.3).

Figure: RA 3.3

To resolve the 180° ambiguity problem, the polar diagram of the loop antenna iselectronically switched back and forth some 30 to 120 times a second after havingbeen fed to the ADF. This results in the combined cardioid polar diagram beingswitched between two diagrams, one been a mirror image of the other. In thisprocess the total signal strength reaching the ADF receiver will vary with a fre-quency of 30 to 120 Hz. The variation in strength will be according to the strength

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NDB and ADF

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received from each of the cardioids from the direction of the NDB. There will betwo directions, along the loop`s axis, from which there will be no change in strengthwhen switching between the cardioids. The ADF will automatically direct the loopaerial into such a position. In this position of the loop aerial the axis of the loopwill point to the NDB. An automatic comparison of the phases of the signals fromthe loop and sense antennas enables the ADF to solve the 180° ambiguity, and theADF indicator will hence point unambiguously toward the NDB been received.

3.2.2 Control Panels and Indicators

Figure: RA 3.4

There are different types of ADF CONTROL PANELS, but their operational use isalmost the same and an example is shown in figure 3.4. The mode selector, orfunction switch, has several positions, enabling the pilot to select the function hewants to use. Typical markings are:- OFF, ANT, ADF, and LOOP.

«ADF» is the normal position when the pilot wants bearing information to bedisplayed automatically by the needle.

“ANT” is the abbreviation of antenna and, in this position, only the signal from thesense aerial is used. This results in no satisfactory directional information to theADF needle. The reason for selecting the ANT position is that it gives the bestaudio reception. This allows for easier identification of the NDB station and alsobetter understanding of any voice messages.

In the LOOP position only the loop aerial is connected to the ADF, and the strengthof signal presented in the headphones is dependent on the polar diagram of theloop antenna. In this position the direction of the loop antenna may be changedaurally by operation of the LOOP RIGHT/LEFT switch shown on the panels lowerright. The operator will rotate the loop until an aural null is reached. An observa-tion of the ADF instrument needle will then provide a bearing to the NDB. Unfor-tunately this bearing has a 180° ambiguity which may be solved by the operator,based on other information.

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In modern ADFs the LOOP function is only available in the most sophisticatedsets. In difficult receiving conditions, the accuracy of an aural bearing may be moreaccurate than the bearing obtained using the automatic function.

The BFO stands for Beat Frequency Oscillator. Sometimes this position is la-belled CW, which is the abbreviation for Continuous Wave, which is the same ascarrier wave. It is necessary to select the BFO “ON” position when identifyingNDBs that use AlA transmissions. The BFO circuit imposes a tone onto the car-rier wave signal to make it audible to the pilot, so that the NDB signal can beidentified.

Once the station has been properly tuned and identified, the Mode Selector shouldbe switched from ANT to ADF. This is very important, since bearing informationwill not be displayed unless the switch is in the ADF position. Never leave themode selector in ANT or Loop position if you are navigating using the ADF.

In the ANT and LOOP modes the needle will remain stationary and not corre-spond to direction to the NDB.

In order to avoid this dangerous problem, it is possible to identify most NDBstransmitting on A2A with the mode selector in the ADF position, so that the ANTposition can be avoided. Since there is no failure flag on an ADF receiver or indica-tor, the only way to be sure that the instrument is receiving a valid signal from theNDB is to continuously monitor the station’s identification.

Each NDB is identifiable by a two or three lettered Morse code identification sig-nal, which is transmitted together with its normal signal. This is known as itsIDENT. When tuning an NDB it is absolutely essential that the facility is correctlyidentified before being used for navigation.

In modern ADFs the NDB carrier frequency in kHz is selected digitally with highelectronic accuracy. For the operator it only remains to check that the correctdigits are set on the control panel.

Some sophisticated ADFs may have a bandwidth selector, marked BROAD/SHARPin figure RA 3.4. Sharp or narrow bandwidth should be chosen when interferencefrom other stations is experienced, and when strong static, such as from CBs, ispresent. Broad or wide bandwidth should be chosen to receive voice or music.

Many ADF units incorporate a TEST switch.

Normally, the only thing the test function does is to turn the needle. If the needledoes not turn, the unit is not working properly. If it does turn but doesn’t returnto its previous position, then the signal is too weak to be used for navigation. If itturns and returns to its previous position, then the system is working properlyand the received signal is good.

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NDB and ADF

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3.2.3 Bearing IndicatorsBearings to the station are displayed on an indicator consisting of a bearing scale(calibrated in degrees) and a pointer. There are three types of bearing scale withvarying degrees of sophistication. They are:

• Fixed card indicator or RBI (figure RA 3.5)• the manually rotatable card,• the radio magnetic indicator (RMI).

The bearing displayed on a FIXED CARD indicator is a RELATIVE BEARING;therefore it is called a Relative Bearing Indicator (RBI). Since the Card is fixed,zero is always at the top and 180° is always at the bottom. RELATIVE BEARINGSare measured CLOCKWISE. It is sometimes convenient, however, to describe thebearing of the NDB in relation to the NOSE or TAIL of the aircraft.

The RELATIVE BEARING indicates the position of the station relative to the longi-tudinal axis of the aircraft. If the needle points to 90 degrees, for instance, itmeans that the station is 90° to the right of the nose, off the right wing tip. If theneedle points to 330 degrees, it means that the station is 30 degrees to the left ofthe nose.

Since the card is fixed, the indicated relative bearing has to be combined with themagnetic heading of the aircraft in order to obtain the magnetic bearing to thestation, QDM. If the result of this addition exceeds 360, 360 has to be subtractedfrom the result in order to obtain a meaningful bearing.

The MAGNETIC BEARING of the aircraft FROM the station, the (QDR), is theRECIPROCAL of the QDM. A quicker way to determine the QDM is tomentally superimpose the RBI needle onto the directional gyro. This is not veryaccurate, but it is a good double check on your calculations. The QDR can bevisualised as the tail of the needle when it is mentally transferred from the RBIonto the directional gyro.

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Figure: RA 3.5

An easier method for finding the reciprocal than adding or subtracting 180°, is toeither:

• ADD 200 and SUBTRACT 20 or• SUBTRACT 200 and ADD 20

Each time the aircraft changes its heading, it will carry the fixed card with it.Therefore, with each change in heading, the RBI needle will indicate a differentrelative bearing. But remember that the MAGNETIC BEARING to the station isalways the sum of the magnetic heading and the relative bearing.

3.2.4 Rotatable cardA rotatable card type of indicator is exactly like a fixed card indicator, except thatthe card can be rotated to reflect the aircraft’s heading. When the card is alignedwith the Directional Gyro, the needle will indicate QDM and the tail of the needlewill indicate QDR. This eliminates any need for mental arithmetic but does re-quire constant manual realignment. This instrument is seldom seen now exceptin some older aeroplanes.

3.2.5 The Radio Magnetic Indicator (RMI)This combines the Relative Bearing Indicator and Remote Indicating Gyro Com-pass into one instrument, with the compass card being aligned automatically withMagnetic North. The RMI normally has two indicators formed as arrows, onemade thin and the other wise as shown in figure RA 3.6. The indicators may beselectable to indicate ADF or VOR information. In figure RA 3.6 the thin arrow isindicating ADF information, the relative bearing to the NDB is 225°. This will alsobe magnetic (or compass bearing) because the heading is 360°. A rough reading ofthe relative bearing may be made using the markings for every 45° on the outsideof the compass scale. These markings are fixed.

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NDB and ADF

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The QDM is continuously indicated under the pointer.The QDR is continuously indicated under the tail.This is now the most common type of presentation.

Figure: RA 3.6

3.3 NDB navigationProcedure for obtaining an ADF bearing.

1. Determine the frequency, identification and modulation of the required beacon and ensure that your aircraft is within the published(promulgated) range.

2. Switch on the ADF and adjust volume.3. Tune the frequency and identify the station using “ANT” and BFO” as

necessary.4. Select ADF on the control panel and read the bearing on the indicator.

3.3.1 Line of Position (LOP) using the RBIWith the help of the information we get from our instruments, we are now able todetermine the line of position along which our aircraft is positioned. To draw thisLOP on the chart we need the QDR or the QTE.

In figure RA 3.5 the relative bearing, as read under the pointer, is 270°. Thismeans that the NDB is 90° to the left of the aircraft nose.

With the fixed card indicator, the only way to find the accurate QDM is to add therelative bearing of 270° to the magnetic heading of 360°.This gives QDM270°. In order to obtain the QDR (the magnetic bearing from the NDB to the air-craft), we need to add or subtract 180° to the QDM. In the above case this givesQDR 180°.

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As a cross check, always remember to mentally superimpose the RBI needle onthe directional gyro.

An RMI solves these calculations automatically. The RMI provides continuouslyQDMs and QDRs. Magnetic Bearings can only be used on charts that are orientedto Magnetic North. Most instrument charts do show the direction towards Mag-netic North.

Since most VFR charts are oriented to True North, always remember to convertMagnetic Bearings to True Bearings before plotting them on the chart.True North is shown by the direction of the meridians; and lines through places ofequal variation, so called Isogonic lines, indicate the value of variation.

If the compass deviation values are small and may be ignored, the heading read onthe RMI may be treated as Magnetic Heading. If deviation is known, it should beused to transform the Compass Heading indicated to Magnetic heading. In thesamples shown above, any difference in variation and any convergence between theNDB and the aircraft position have been disregarded. Exercise as well as FinalExam questions involving these factors are included in the ATPL syllabus.

3.3.2 Homing and NDBSince the ADF needle always points towards the station, the easiest way to reachthe beacon is to constantly fly with the needle pointing to the top of the indicator.This procedure is known as HOMING.

The easiest way to home to a station is to turn the aircraft in the direction of theneedle until the needle points to the top of the indicator. This points the nose ofthe aircraft directly towards the station.Once aimed at the station, any crosswind component will displace the aircraft toeither side of the straight track to the station and the ADF needle will swing awayfrom the top of the indicator.The pilot will then have to make a correction of the heading towards the needle inorder to continue heading to the station.

This process will have to be repeated again and again since the crosswind pushesthe aircraft away from the straight track. The resulting path to the station willthus be a curved one. (Fig. RA 3.7)

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Figure: RA 3.7 Homing: Aeroplane’s heading and the track followed by theaeroplane

The crosswind component forces the aircraft to turn further and further into thewind in order to continue toward the station. The aircraft must turn until a pointis eventually reached where the aircraft is headed directly into the wind. At thatpoint, the aircraft will no longer drift off the direct track but is now heading straightto the station. The actual curved path that results will be different for each combi-nation of crosswind and TAS; strong crosswind component and low TAS willresult in a large deviation. A weak crosswind component and a high TAS willresult in a small deviation. Since the actual track over the ground will vary withevery wind and airspeed combination, there is no way to ensure that any givenaircraft will stay within the boundaries of an airway or approach path when hom-ing. Homing is a very simple but extremely inefficient procedure. Because of itsuncertain demands on airspace, it is not commonly used.

3.3.3 Intercepting a trackThe correct way to navigate with the help of ADF and NDB can be divided in threesteps: first visualise your position, second intercept the desired track and thirdmaintain the track to or from the station. In figure RA 3.8 a fixed card type indica-tor is shown. The magnetic heading at this time is 075°, and the desired inboundtrack to the NDB is 035°. The first step is to visualise your position. You shouldfind yourself south-west of the NDB, heading 075°, as shown on the sketch to theright in figure RA 3.8. (This is a rough sketch; the directions are not correctlypresented).The second step is to turn to a heading that gives you a suitable intercept. Observethe instrument readings during the turn.

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Figure: RA 3.8

Now look at the corresponding plan view.The heading of 075° gives you an intercept angle of 40°. Since the desired QDM is035°, the RBI indication of 320° will indicate that you have intercepted the desiredtrack.

When the needle is reaching the desired relative bearing, in this case 320°, startyour turn towards the station and your aircraft will be on the desired inboundtrack. Compare with the instrument indications.Look at the plan view. Now you are on track.To intercept a track outbound, follow the same procedures. First of all look at theradio compass and visualise your position. Consider Fig. RA 3.9

Figure: RA 3.9

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The relative bearing of 100° combined with the magnetic heading of 125°, indicatesthat you are North and East of the NDB.

The desired track is 085 OUTBOUND. Our intercept angle is 40°. When the rela-tive bearing is 140° we will have reached our outbound track. Observe the instru-ment indications.When the needle has reached 130° degrees, start turning to intercept the out-bound track. Look at the instrument.Heading 085° with relative bearing 180°, now you are on track.

As we have seen, in order to intercept a specific course, first you have to know yourposition relative to the desired track, then you establish a suitable interceptionangle. Consider the following situation:You are on a heading of 340°. The relative bearing to the NDB is 080°. The requiredtrack to the NDB is 090°.To help you to visualise the situation, it is a good idea to draw a plan and theinstrument indications.

By maintaining a heading of 340°, the aircraft will eventually intercept the 090track. This would be a rather untidy intercept, however, in that a turn of 110°would be required.A tidier and more efficient intercept could be achieved by turning onto an initialheading of 360°, for a 90-degree intercept.A heading of 030° will lead to a 60-degree intercept with the required inboundtrack.Since the aircraft is on QDR 240, a heading of 060° would turn the aircraft directlytowards the station, and the QDM 090 will not be intercepted.Once you have turned to a correct intercept heading, the rule is a simple one.When the angle formed by the aircraft’s heading and the desired track is the sameas the angle between the zero mark at the top of the indicator and the pointer, thenthe aircraft is on the desired track (QDR or QDM).If you are intercepting OUTBOUND, the aircraft is on the desired track when theintercept angle is the same as the angle between the zero mark at the top of theindicator and the TAIL of the needle.

To intercept a specific track from an assigned heading with this technique, youhave to know the interception angle. For instance, on heading 220° and a clearanceto intercept QDM 180, the intercept angle is 40 degrees. When the needle is 40degrees to the left of zero, the track has been intercepted.

Another Situation: Your heading is 265° and the RBI indicates 005°. You require tojoin QDM 240 at an intercept angle of 60°.The first step is always to visualise your position.What is the QDR? You are east of the station,Which way do you have to turn to make the intercept, left or right?The track is to the right of the aircraft, so a right turn has to be made for theinterception.

Which heading will you need in order to intercept the QDM 240 with an interceptangle of 60°? To intercept QDM 240 at 60 degrees, the aircraft should be turned toa heading of 300°.

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Maintain a 300° heading, and observe the RBI needle. Remember that when inter-cepting inbound, the needle falls.

Since it is a 60° intercept, wait until the pointer falls to 60° left of the zero indica-tion on RBI. Remember that to mentally superimpose the RBI needle on the direc-tional gyro is always a good crosscheck. Since an aircraft needs some distance toturn, the pilot should start the turn onto the track a few degrees before the de-sired QDM is reached. Observe the instruments and see that the turn is initiateda few degrees before reaching QDM 240. The RMI eliminates the need to do anymental calculation. It always displays the QDM under the pointer and the QDRunder the tail.

The procedure of intercepting QDRs and QDMs is made a lot easier if you main-tain a mental picture of where the aircraft is and where you want it to be.

3.3.4 TrackingWith no crosswind, a direct inbound track can be achieved by heading the aircraftdirectly at the NDB, and maintaining the ADF needle on the nose of the aircraft. Ifthere is no crosswind to blow the aircraft off track, then everything will remainconstant.

If you point your aeroplane’s nose at the station any crosswind will cause theaircraft to be blown off track. In the cockpit, this is indicated by the ADF needle asit starts to move away from the top of the indicator.The only way to fly a straight track to the station is to use the procedure calledTRACKING. Tracking means to establish a wind correction angle (WCA) that com-pensates the drift caused by the crosswind.

If the exact W/V is not known, then use an estimated WCA obtained from theavailable information, (forecasts, pilots’ reports, etc.). Remember that higher cross-wind requires greater WCA and, for the same crosswind, slower aircraft will needto establish a greater WCA than faster aircraft.

After having estimated the WCA you will apply this to the desired track and findthe required heading.After having turned to this heading the ADF needle should indicate a bearingequal to the WCA, right or left of the aircraft nose.

If the ADF needle indicates a constant relative bearing while you are maintaining aconstant magnetic heading, your wind correction angle is correct and the aircraftis tracking directly to or from the station.

A wind correction angle that does not correctly compensate for the present windwill cause the aircraft to drift off track and the ADF needle to show a graduallychanging relative bearing.

If the head of the ADF needle moves to the right, it indicates that a turn to the righthas to be made to maintain the track to the NDB and, conversely, if the head of theneedle moves to the left, a left turn has to be made.

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How large each correcting turn should be depends upon the deviation from thetrack. A simple method is to double the angle of bearing change. Observe that ifthe aircraft has deviated 10 degrees to the left, the needle will have moved 10degrees to the right.To double the angle of bearing change simply means that you alter your heading 20degrees to the right.Having regained the track, turn left by only half the correcting turn of 20 degrees.That is to say, turn left 10 degrees to maintain the track. This WCA should pro-vide reasonable tracking.

In real life an absolutely perfect track is difficult to achieve and the pilot will makea number of minor corrections to the heading. This technique is known as brack-eting the track. If an RMI is used, tracking a QDM or QDR is simplified. Firstestimate the WCA and apply this to the QDM to get MH. After having turned tothis MH the ADF needle will indicate the desired magnetic track (QDM) on theRMI scale. As soon as the ADF needle start drifting right or left from this indica-tion, turn left or right in order to maintain a steady reading on the RMI scaleunder the ADF needle.

The ADF needle will become more and more sensitive as the NDB station is ap-proached. Minor displacements to the left or right of the track will cause largerand larger changes in the relative bearings and the QDM. When passing overheadthe NDB, the ADF needle will oscillate then move toward the bottom of the dialand settle down.

To facilitate the QDR calculations when tracking outbound, you should rememberthat the QDR is equal to the Magnetic Heading plus or minus the deflection of thetail of the needle.

Suppose that the desired course outbound from an NDB is QDR 040 and the pilotestimates a WCA of 10 degrees to the right to counteract the wind from the right.Flying the QDR 040 in a no wind condition is achieved by flying heading 040. Sincewe have a right crosswind that requires a l0° WCA, the heading in this case is050°.

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Figure: RA 3.10

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3.3.5 NDB ApproachThis chart (plate) in figure RA 3.10 shows the let down procedure when using theNDB with the ident ‘DA’ on frequency 404 kHz.To carry out this procedure, as with any other procedure, you need to brief your-self well on:

• What the procedure is; studying the holding, approach path planview and the vertical profile (at the bottom of the plate).

• The sector safety altitudes.• The missed approach procedure.

Having studied the plate you should now build a mental picture on how you aregoing to fly the approach, the headings you anticipate flying, the speeds and, mostimportant, the times. Let’s assume the wind, reported by the approach controller,is 270/20 and that your manoeuvring speed is 120 kts.

Now let’s plan:You are approaching Dalen from the Northeast and you are “cleared the approach”,by ATC, Now, look at the plate. What is the minimum sector altitude in our area?(3000ft, on QNH) What is initial altitude overhead DA? (2500ft, on QNH)

What bearing on the RMI would indicate that you had reached the inbound trackafter finished outbound track. (WDM 180°)

What heading would you use to maintain that track? Come on, either guessti-mate or get your navigation computer out. OK it’s 190° and, by the way, you haveto start your descent. You do know where you are don’t you? — because if youdon’t, do not descend.

Now, here comes your first checkpoint. What should your altitude be at the DANDB? Don’t let it be any lower than 1570 ft.OK, continue your let down but if you can’t see the runway by the time you reachyour decision height you must overshoot. You should know the overshoot proce-dure if you have properly briefed yourself.

3.4 NDB and ADF - Limitations and accuracy

3.4.1 LimitationsNearly all the limitations common to NDB navigation are a direct function of itsoperating in the LOW and MEDIUM frequency band.The signal from an NDB transmitter in the LF/MF band actually propagates alongthree paths: GROUND WAVE, DIRECT WAVE, and SKY WAVE.

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Ground wave’s limitationsAt the frequency bands used, the radio signals ‘bend’ readily around the surface ofthe earth. This results in a ground wave which is very stable and reliable and, atthe NDB frequency, may travel for several hundred miles. The distance dependson station power, frequency and type of surface over which the signal propagates.The lower the frequency, the lower the attenuation will be.

High power, over-water NDBs usually transmit at the low end of the assignedband in order to take advantage of the weaker attenuation. Furthermore, due tobetter conductivity, the ground wave has greater range over water than over drysoil.

Direct wave’s limitationsThe Direct Wave follows the line of sight and its range can be determined from theformula given in chapter 1. In most cases the direct wave range will be consider-ably less than that of the Ground Wave. Height may become significant when it isdesirable to receive the direct wave, such as when endeavouring to minimise therisk of ADF error if flying in mountainous areas or when using coastal NDBs.

Figure: RA 3.11 Dead Space

Sky wave’s limitationsAt some frequencies there will be a gap in coverage between the ground wave andthe first return of the sky wave. The ground wave coverage might extend out to 300miles, while the first skywave returns at 1000 miles. This gap is called the deadspace or Skip Zone. (Fig. RA 3.11)

The exact size of the dead space depends on frequency and the state of ionisationof the atmosphere. At frequencies in the lower MF and the LF bands, intenseionisation by day attenuates (absorbs) RF signals and no sky wave return is no-ticeable. By night the ionisation levels fall and returning sky waves will be de-tected.

At ranges from about 80 nm these sky waves will mix with the ground wave signal(there won’t be a dead space) and, because they will have arrived over a differentpath, will be at a different phase from the ground wave. This will have the effect ofsuppressing or displacing the aerial ‘null’ signal, in a random way, and the needlewill appear to wander. This effect is at its most variable during twilight, and iscalled “Night effect”.

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NDB and ADF

©NAR and TFHS

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 ran-domly shifted so that a horizontally polarised component may be randomly intro-duced into the loop aerial. This will cause the null signal to be displaced.

It should also be noted that sky wave signals from distant transmitters operatingon the same or near frequencies might well be detected at night.In summary, the airborne ADF is designed and optimised to be used in conjunc-tion with the more predictable ‘ground wave’ signal from the selected NDB.

3.4.2 Errors of the ADFThe ADF bearing is subject to a number of error sources including any or all of thefollowing:

Quadrantal Error - The metal components of the aeroplane’s structure behave asan aerial. They absorb signals at all frequencies but more readily so at frequenciesin 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. (Fig. RA 3.12)

The effect of this signal is a displacement of the measured null towards the majorelectrical axis of the aeroplane creating an error that is maximum on relative bear-ings 045,135, 225, 315 (the quadrantals). This error is minimised by calibrationand electro-mechanical compensation at installation.

Radio Navigation


Figure: RA 3.12

Figure: RA 3.13

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NDB and ADF

©NAR and TFHS

Dip (Bank) Error - During turns, the horizontal member of the loop aerial willdetect a signal. This will cause the null to be displaced and a ‘short-term’ errone-ous bearing to be displayed.

Coastal Refraction - When flying over the sea and using a land based beacon, thechanges in propagation properties of the signal as it passes from land to sea willcause the ‘wave front’ to be displaced. (Fig. RA 3.13)This will result in a bearing error.

Such bearing errors may be minimised by any or all of the following:

1. Do not use beacons unless they are situated on islands or nearto the coast.

2. If using an inland NDB only use bearings at or near to 90° to thecoast.

3. Remember that coastal refraction is less as height is increased.4. A position line plotted without correction for Coastal Refraction

will indicate a position closer to the shore than the real position.

Multipath Signals - When flying in mountainous regions, signals may be refracted(bent) around and/or reflected from mountains. The ADF may be affected by suchmultipath signals and the bearings will be unreliable.

Sky wave (night) Effect - As the loop aerial is receiving sky wave signals from thesame NDB at the same time as ground wave signal are being received, the null willbe suppressed or displaced in a random manner. Some of the displacements maygive stable (but wrong) bearing indications for a period of time and are thereforevery hazardous. At dawn and dusk, as the state of ionisation changes, these er-rors are particularly unpredictable.

Noise - This is defined as any signal detected at the receiver other than the desiredsignal.

Man-made Noise - Each published NDB has an associated published range. Ifuse of that NDB is restricted to that range, the desired signal is protected from theharmful interference of ground waves from other known transmitters on the sameor near frequencies. It should be remembered that, from sunset to sunrise, skywave propagation of signals in the LF and MF bands is possible. This will causethe signal to noise ratio to be reduced and will result in errors as the null isdisplaced, usually randomly.

Another localised source of man-made noise is overhead power cables. Many ofthese cables carry not only electrical power but also modulated signals used bythe power companies for communication. These modulated signals radiate fromthe power cables and create mini NDBs. Such emissions are monitored but, insome states, monitoring may not be carried out. The rule is – if unsure, use withextreme caution.

Atmospheric Noise - There are an average of 44,000 thunderstorms over theearth’s surface in every period of 24 hours and more than half of these occur overor near land surfaces within 30° latitude of the Equator.

Radio Navigation


Each thunderstorm generates electro-magnetic signals and these radiate in alldirections 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 deflectedtowards that storm. Such noise levels are normally quite low but they will in-crease

• In temperate latitudes in the summer• As you move towards the tropics• At night as a result of sky wave propagation.

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 thenotified range. You could be at half the published range before a reliable signal isreceived.

Accuracy - When used within the published range by day in good conditions, awell Calibrated ADF should give a bearing accuracy within ± 5°.

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NDB and ADF

©NAR and TFHS

Can you answer these?

1 The basic information given by the ADF is:

a) The magnetic bearing from the aircraft to the NDB.b) The relative bearing from the aircraft to the NDB.c) The true great circle track from the NDB to the aircraft.d) The magnetic direction of the loop aerial with reference to the sense

aerial

2 Using an ADF indicator of the manually rotateable card type:

a) Relative bearing is normally indicated under the pointer needle;

b) The aircraft heading may be marked on the indicator with amanually controlled “bug”;

c) May be combined with a VOR indicator;d) The card should be rotated so that the aircraft heading is at the top

of the indicator.

3 The basic information given by the ADF is:

a) the relative bearing from the aircraft to the NDB;b) the magnetic bearing from the aircraft to the NDB;c) the true great circle track from the NDB to the aircraft;d) the magnetic direction of the loop aerial with reference to the sense

aerial.

4 ilst correctly tuned to an NDB transmitting a NONA2A signal, with theBFO switched off, you should hear:

a) the identification but not a tone;b) no signal;c) both a tone and the identification;d) a tone but not the identification.

SA-RN 1.2

Radio Navigation


5 Which of the following statements regarding an aeronautical NDB iscorrect?

a) It operates in the MF/HF band.b) To overcome the limitations caused by “line of sight” propagation,

high power transmitters must be used.c) It is very simple, being required to transmit only a carrier wave and

identification.d) In Europe, most NDB’s operate in the frequency band 455 – 1750

kHz.

6 Long range NDB’s normally employ the following emissioncharacteristics:

a) N0NA2A.b) N0NA1A.c) A3W.d) A9E.

7 The purpose of the BFO in the ADF receiver is to:

a) Manufacture a signal within the ADF receiver that when mixed withan incoming unmodulated transmission renders it audible.

b) Manufacture a signal of perhaps 5 Khz.c) Improve the range of the equipment.d) Minimise precipitation static.

8 Which of the following is the ICAO allocated frequency band for ADFreceivers.

a) 108,0 MHz – 117,9 MHz.b) 200 – 1759 MHz.c) 200 – 1759 Hz.d) 200 – 1759 kHz.

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NDB and ADF

©NAR and TFHS

SA-RN 1.3

9 Homing on an NDB

a) Calls for an assessment of the driftb) Is most effective in strong windsc) Will in most situations result in frequent heading changes when

approaching the NDBd) Will result in passing the NDB along the planned track

10 Flying in the vicinity of CB clouds and using ADF

a) The ANT position of the function switch should be used whenlistening for NDB ID

b) Strong static emitted from the CB may cause the ADF needle todeflect towards the CB

c) The static emitted from the CB will fade soon after you have passedit

d) All 3 answers are correct

11 An aircraft is flying on heading 330°, and relative bearing to an NDB is190°. Calculate QDR.

a) 360°.b) 160°.c) 340°.d) 140°.

12 An aircraft is flying on heading 300°, variation in the area 13°W and therelative bearing is 350°. Calculate QDM.

a) 110°.b) 290°.c) 300°.d) 150°.

Radio Navigation


13 Aircraft ADF systems use both a loop and an omni-directional (sense)aerial, this is to measure the bearing of the beacon:

a) By differential phase comparison.b) By phase comparison.c) By producing lobes to establish an equi-signal.d) By producing two cardioids to find an unambiguous null.

14 The bearings from NDB’s are the least accurate at:

a) Midnight.b) Midday.c) Dawn and dusk.d) The accuracy does not change during the day or night.

15 Fading of an ADF signal, together with a hunting needle, is indication of:

a) Quadrantal error.b) Thunderstorm effect.c) Night effect.d) Mountain effect.

16 The range that may be expected from a NDB of 10 KW power when usedover the sea in average conditions is:

a) 100 nm.b) 300 nm.c) 500 nm.d) 1000 nm.

3 ndb and adf - horizon sfadaa.horizon-sfa.ch/.../radionavigation/nar_ndb_and_… · ·...

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