AND
Non Directional Beacons (NDB) are ground-based transmitters which transmit radio energy equally in all directions; hence their name. The airborne system is called the Automatic Direction Finder (ADF). Its indicator (theoretically) always points towards the tuned NDB. NDBs transmit radio signals omni- directionally in 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) 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.
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 but
NDB stations actually use two different signals together: NON (the unmodulated signal or carrier wave) is used by the airborne equipment to determine the direction of the signal. AlA or A2A (the modulated signal) is used to transmit the
NDB’s identification.
After being modulated, the signal is driven to the power amplifier, where it is boosted to the final transmission power. The transmitter power of an NDB is closely allied to its intended use and may vary from as low as 15 watts to several kilowatts. The nominal maximum range of an NDB can be determined from the formula R max
= 3
√
P where
R max
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 approach guidance towards establishing the final approach path of an ILS. These beacons are short range and are normally NON/A2A.
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 kilowatts. - 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.
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The amplified signal finally reaches the transmission aerial where it is radiated omni-directionally. The transmission mast may be either a single mast or a large
T-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 diagram for the aerial is omni directional in the horizontal plane but exhibits directional 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 RA
3.2. Above the station, marked by the points at which the radiated power has fallen to 0.5 of its maximum value, is a conical space in which signal strength may be 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 are upper 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 transmissions are from 190 kHz to 1750 kHz. In Europe, NDB frequencies are normally found between 255 kHz and 455 kHz. It should be noted that many other 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.) and Marine Beacons. Stations must not be used if their details are not published in the AIP or appropriate Flight Guides. Where details
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of Marine Beacons are published, users should note that a number of such beacons will be grouped together to serve an area. These beacons will share a single transmission frequency, each transmitting for a period of 60 seconds in a cycle of six 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 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.
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 dial 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, a directional aerial, is rotated electronically and, by combining information from the loop and sense aerials, the bearing to the station is internally derived in the ADF.
One of the basic principles of electricity says that if a variable number of electromagnetic field lines pass through a coil, a voltage will be induced in the coil. When a looped conductor, such as the loop aerial, is hit by electromagnetic waves, voltages will be induced in the loop. These voltages depend on the angular position of the loop relative to the incoming electromagnetic (EM) waves. The voltage induced in the loop is at it`s maximum when the loop`s plane is parallel to the received signal. Thus the receiver will detect the greatest voltage when the plane of the loop`s plane is PARALLEL to the direction of propagation of the radio waves. If the loop aerial is rotated until it`s perpendicular to the radio to the direction of movement of the radio waves, none of the EM waves will pass through the loop and the resultant signal will be a null. For one 360 o rotation of the loop aerial, the receiver will detect two maximums and two nulls. Small angular deflections of the loop aerial near its null position produce larger changes in voltage than similar angular changes near the loop’s maximum position. For this reason a null position is used for direction finding purposes.
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The horizontal polar diagram of a loop aerial will have the shape of a figure “8”, as shown in figure RA 3.3. In this figure the plane of the loop is indicated, as well as the loop`s axis which is perpendicular to the loop`s plane. Beware of these two ways of describing the loops direction: the direction of the plane and the direction of the axis. Many tricky multiple choice questions may be constructed playing with 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 of a sense aerial. The sense aerial takes many different forms but they all have the same function, each acting as if a straight piece of wire were hanging vertically from the aircraft. The function of the SENSE aerial, so far as automatic direction finding is concerned, is to eliminate the ambiguity of a loop by distinguishing between the signals received from one side of the loop and the signals received from the other side of the loop.
As we have seen, the sense aerial receives equally well from all directions and thus its polar diagram is a circle. If a polar diagram is traced out in order to show the signal strength produced by the loop at different angles through 360°, the result is a figure eight. In adding the steady signal from the sense aerial to the alternating signal 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 is electronically switched back and forth some 30 to 120 times a second after having been fed to the ADF. This results in the combined cardioid polar diagram being switched between two diagrams, one been a mirror image of the other. In this process the total signal strength reaching the ADF receiver will vary with a frequency of 30 to 120 Hz. The variation in strength will be according to the strength
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received from each of the cardioids from the direction of the NDB. There will be two directions, along the loop`s axis, from which there will be no change in strength when switching between the cardioids. The ADF will automatically direct the loop aerial into such a position. In this position of the loop aerial the axis of the loop will point to the NDB. An automatic comparison of the phases of the signals from the loop and sense antennas enables the ADF to solve the 180° ambiguity, and the
ADF indicator will hence point unambiguously toward the NDB been received.
Figure: RA 3.4
There are different types of ADF CONTROL PANELS, but their operational use is almost the same and an example is shown in figure 3.4. The mode selector, or function switch, has several positions, enabling the pilot to select the function he wants to use.
Typical markings are:- OFF, ANT, ADF, and LOOP.
«ADF» is the normal position when the pilot wants bearing information to be displayed automatically by the needle.
“ANT” is 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. The reason for selecting the ANT position is that it gives the best audio reception. This allows for easier identification of the NDB station and also better understanding of any voice messages.
In the LOOP position only the loop aerial is connected to the ADF, and the strength of signal presented in the headphones is dependent on the polar diagram of the loop antenna. In this position the direction of the loop antenna may be changed aurally by operation of the LOOP RIGHT/LEFT switch shown on the panels lower right. The operator will rotate the loop until an aural null is reached. An observation of the ADF instrument needle will then provide a bearing to the NDB. Unfortunately 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 sophisticated sets. In difficult receiving conditions, the accuracy of an aural bearing may be more accurate than the bearing obtained using the automatic function.
The BFO stands for Beat Frequency Oscillator. Sometimes this position is labelled CW, which is the abbreviation for Continuous Wave, which is the same as carrier wave. It is necessary to select the BFO “ON” position when identifying
NDBs that use AlA transmissions. 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.
Once the station has been properly tuned and identified, the Mode Selector should be switched from ANT to ADF. This is very important, since bearing information will not be displayed unless the switch is in the ADF position. Never leave the mode 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 correspond to direction to the NDB.
In order to avoid this dangerous problem, it is possible to identify most NDBs transmitting on A2A with the mode selector in the ADF position, so that the ANT position can be avoided. Since 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 is correctly identified before being used for navigation.
In modern ADFs the NDB carrier frequency in kHz is selected digitally with high electronic accuracy. For the operator it only remains to check that the correct digits are set on the control panel.
Some sophisticated ADFs may have a bandwidth selector, marked BROAD/SHARP in figure RA 3.4. Sharp or narrow bandwidth should be chosen when interference from other stations is experienced, and when strong static, such as from CBs, is present. 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 needle does not turn, the unit is not working properly. If it does turn but doesn’t return to its previous position, then the signal is too weak to be used for navigation. If it turns and returns to its previous position, then the system is working properly and the received signal is good.
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Bearings 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 with varying 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 BEARINGS are measured CLOCKWISE. It is sometimes convenient, however, to describe the bearing 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 longitudinal axis of the aircraft. If the needle points to 90 degrees, for instance, it means that the station is 90° to the right of the nose, off the right wing tip. If the needle points to 330 degrees, it means that the station is 30 degrees to the left of the nose.
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.
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.
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Figure: RA 3.5
An easier method for finding the reciprocal than adding or subtracting 180°, is to either:
•
•
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 different relative bearing. But remember that the MAGNETIC BEARING to the station is always the sum of the magnetic heading and the relative bearing.
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. When the card is aligned with the Directional Gyro, the needle will indicate QDM and the tail of the needle will indicate QDR. This eliminates any need for mental arithmetic but does require constant manual realignment. This instrument is seldom seen now except in some older aeroplanes.
This combines the Relative Bearing Indicator and Remote Indicating Gyro Compass into one instrument, with the compass card being aligned automatically with
Magnetic North. The RMI normally has two indicators formed as arrows, one made thin and the other wise as shown in figure RA 3.6. The indicators may be selectable to indicate ADF or VOR information. In figure RA 3.6 the thin arrow is indicating ADF information, the relative bearing to the NDB is 225°. This will also be magnetic (or compass bearing) because the heading is 360°. A rough reading of the relative bearing may be made using the markings for every 45° on the outside of the compass scale. These markings are fixed.
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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
Procedure for obtaining an ADF bearing.
1.
Determine the frequency, identification and modulation of the re quired 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.
With the help of the information we get from our instruments, we are now able to determine the line of position along which our aircraft is positioned. To draw this
LOP 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°. This means 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 the relative bearing of 270° to the magnetic heading of 360°.
This gives QDM
270°. In order to obtain the QDR (the magnetic bearing from the NDB to the aircraft), we need to add or subtract 180° to the QDM. In the above case this gives
QDR 180°.
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As a cross check, always remember to mentally superimpose the RBI needle on the directional gyro.
An RMI solves these calculations automatically. The RMI provides continuously
QDMs and QDRs. Magnetic Bearings can only be used on charts that are oriented to Magnetic North. Most instrument charts do show the direction towards Magnetic North.
Since most VFR charts are oriented to True North, always remember to convert
Magnetic Bearings to True Bearings before plotting them on the chart.
True North is shown by the direction of the meridians; and lines through places of equal variation, so called Isogonic lines, indicate the value of variation.
If the compass deviation values are small and may be ignored, the heading read on the RMI may be treated as Magnetic Heading. If deviation is known, it should be used to transform the Compass Heading indicated to Magnetic heading. In the samples shown above, any difference in variation and any convergence between the
NDB and the aircraft position have been disregarded. Exercise as well as Final
Exam questions involving these factors are included in the ATPL syllabus.
Since the ADF needle always points towards the station, the easiest way to reach the 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 the needle until the needle points to the top of the indicator. This points the nose of the aircraft directly towards the station.
Once aimed at the station, any crosswind component will displace the aircraft to either side of the straight track to the station and the ADF needle will swing away from the top of the indicator.
The pilot will then have to make a correction of the heading towards the needle in order to continue heading to the station.
This process will have to be repeated again and again since the crosswind pushes the aircraft away from the straight track. The resulting path to the station will thus 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 the aeroplane
The crosswind component forces the aircraft to turn further and further into the wind in order to continue toward the station. The aircraft must turn until a point is eventually reached where the aircraft is headed directly into the wind. At that point, the aircraft will no longer drift off the direct track but is now heading straight to the station. The actual curved path that results will be different for each combination of crosswind and TAS; strong crosswind component and low TAS will result in a large deviation. A weak crosswind component and a high TAS will result in a small deviation. Since the actual track over the ground will vary with every wind and airspeed combination, there is no way to ensure that any given aircraft will stay within the boundaries of an airway or approach path when homing. Homing is a very simple but extremely inefficient procedure. Because of its uncertain demands on airspace, it is not commonly used.
The correct way to navigate with the help of ADF and NDB can be divided in three steps: first visualise your position, second intercept the desired track and third maintain the track to or from the station. In figure RA 3.8 a fixed card type indicator is shown. The magnetic heading at this time is 075°, and the desired inbound track to the NDB is 035°. The first step is to visualise your position. You should find yourself south-west of the NDB, heading 075°, as shown on the sketch to the right in figure RA 3.8. (This is a rough sketch; the directions are not correctly presented).
The second step is to turn to a heading that gives you a suitable intercept. Observe the 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 is
035°, the RBI indication of 320° will indicate that you have intercepted the desired track.
When the needle is reaching the desired relative bearing, in this case 320°, start your turn towards the station and your aircraft will be on the desired inbound track. 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 the radio 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°, indicates that you are North and East of the NDB.
The desired track is 085 OUTBOUND. Our intercept angle is 40°. When the relative bearing is 140° we will have reached our outbound track. Observe the instrument indications.
When the needle has reached 130° degrees, start turning to intercept the outbound 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 your position relative to the desired track, then you establish a suitable interception angle. Consider the following situation:
You are on a heading of 340°. The relative bearing to the NDB is 080°. The required track to the NDB is 090°.
To help you to visualise the situation, it is a good idea to draw a plan and the instrument indications.
By maintaining a heading of 340°, the aircraft will eventually intercept the 090 track. 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 initial heading of 360°, for a 90-degree intercept.
A heading of 030° will lead to a 60-degree intercept with the required inbound track.
Since the aircraft is on QDR 240, a heading of 060° would turn the aircraft directly towards 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 same as the angle between the zero mark at the top of the indicator and the pointer, then the aircraft is on the desired track (QDR or QDM).
If you are intercepting OUTBOUND, the aircraft is on the desired track when the intercept angle is the same as the angle between the zero mark at the top of the indicator and the TAIL of the needle.
To intercept a specific track from an assigned heading with this technique, you have to know the interception angle. For instance, on heading 220° and a clearance to intercept QDM 180, the intercept angle is 40 degrees. When the needle is 40 degrees to the left of zero, the track has been intercepted.
Another Situation: Your heading is 265° and the RBI indicates 005°. You require to join 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 the interception.
Which heading will you need in order to intercept the QDM 240 with an intercept angle of 60°? To intercept QDM 240 at 60 degrees, the aircraft should be turned to a heading of 300°.
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Maintain a 300° heading, and observe the RBI needle. Remember that when intercepting inbound, the needle falls.
Since it is a 60° intercept, wait until the pointer falls to 60° left of the zero indication on RBI. Remember that to mentally superimpose the RBI needle on the directional gyro is always a good crosscheck. Since an aircraft needs some distance to turn, the pilot should start the turn onto the track a few degrees before the desired QDM is reached. Observe the instruments and see that the turn is initiated a few degrees before reaching QDM 240. The RMI eliminates the need to do any mental calculation. It always displays the QDM under the pointer and the QDR under the tail.
The procedure of intercepting QDRs and QDMs is made a lot easier if you maintain a mental picture of where the aircraft is and where you want it to be.
With no crosswind, a direct inbound track can be achieved by heading the aircraft directly at the NDB, and maintaining the ADF needle on the nose of the aircraft. If there is no crosswind to blow the aircraft off track, then everything will remain constant.
If you point your aeroplane’s nose at the station any crosswind will cause the aircraft to be blown off track. In the cockpit, this is indicated by the ADF needle as it 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 called
TRACKING. Tracking means to establish a wind correction angle (WCA) that compensates the drift caused by the crosswind.
If the exact W/V is not known, then use an estimated WCA obtained from the available information, (forecasts, pilots’ reports, etc.). Remember that higher crosswind requires greater WCA and, for the same crosswind, slower aircraft will need to establish a greater WCA than faster aircraft.
After having estimated the WCA you will apply this to the desired track and find the required heading.
After having turned to this heading the ADF needle should indicate a bearing equal to the WCA, right or left of the aircraft nose.
If the ADF needle indicates a constant relative bearing while you are maintaining a constant magnetic heading, your wind correction angle is correct and the aircraft is tracking directly to or from the station.
A wind correction angle that does not correctly compensate for the present wind will cause the aircraft to drift off track and the ADF needle to show a gradually changing relative bearing.
If the head of the ADF needle moves to the right, it indicates that a turn to the right has to be made to maintain the track to the NDB and, conversely, if the head of the needle 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 the track. A simple method is to double the angle of bearing change. Observe that if the aircraft has deviated 10 degrees to the left, the needle will have moved 10 degrees to the right.
To double the angle of bearing change simply means that you alter your heading 20 degrees 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 provide reasonable tracking.
In real life an absolutely perfect track is difficult to achieve and the pilot will make a number of minor corrections to the heading. This technique is known as bracketing the track. If an RMI is used, tracking a QDM or QDR is simplified. First estimate the WCA and apply this to the QDM to get MH. After having turned to this MH the ADF needle will indicate the desired magnetic track (QDM) on the
RMI scale. As soon as the ADF needle start drifting right or left from this indication, turn left or right in order to maintain a steady reading on the RMI scale under the ADF needle.
The ADF needle will become more and more sensitive as the NDB station is approached. Minor displacements to the left or right of the track will cause larger and larger changes in the relative bearings and the QDM. When passing overhead the NDB, the ADF needle will oscillate then move toward the bottom of the dial and settle down.
To facilitate the QDR calculations when tracking outbound, you should remember that the QDR is equal to the Magnetic Heading plus or minus the deflection of the tail of the needle.
Suppose that the desired course outbound from an NDB is QDR 040 and the pilot estimates 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. Since we have a right crosswind that requires a l0° WCA, the heading in this case is
050°.
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Figure: RA 3.10
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This chart (plate) in figure RA 3.10 shows the let down procedure when using the
NDB with the ident ‘DA’ on frequency 404 kHz.
To carry out this procedure, as with any other procedure, you need to brief yourself well on:
•
•
•
What the procedure is; studying the holding, approach path plan view 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 are going to fly the approach, the headings you anticipate flying, the speeds and, most important, 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 track after finished outbound track. (WDM 180°)
What heading would you use to maintain that track?
Come on, either guesstimate or get your navigation computer out. OK it’s 190° and, by the way, you have to start your descent.
You do know where you are don’t you? — because if you don’t, do not descend.
Now, here comes your first checkpoint. What should your altitude be at the DA
NDB? 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 reach your decision height you must overshoot. You should know the overshoot procedure if you have properly briefed yourself.
Nearly all the limitations common to NDB navigation are a direct function of its operating in the LOW and MEDIUM frequency band.
The signal from an NDB transmitter in the LF/MF band actually propagates along three paths: GROUND WAVE, DIRECT WAVE, and SKY WAVE.
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Ground wave’s limitations
At the frequency bands used, the radio signals ‘bend’ readily around the surface of the earth. This results in a ground wave which is very stable and reliable and, at the NDB frequency, may travel for several hundred miles. The distance depends on 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 assigned band in order to take advantage of the weaker attenuation. Furthermore, due to better conductivity, the ground wave has greater range over water than over dry soil.
Direct wave’s limitations
The Direct Wave follows the line of sight and its range can be determined from the formula given in chapter 1. 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, such as when endeavouring to minimise the risk of ADF error if flying in mountainous areas or when using coastal NDBs.
Figure: RA 3.11 Dead Space
Sky wave’s 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 or Skip Zone. (Fig. RA 3.11)
The exact size 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 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 different path, will be at a different phase from the ground wave. This will have the effect of suppressing or displacing the aerial ‘null’ signal, in a random way, and the needle will appear to wander. This effect is at its most variable during twilight, and is called “Night effect”.
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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.
It should also be noted that sky wave signals from distant transmitters operating on the same or near frequencies might well be detected at night.
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.
The ADF bearing is subject to a number of error sources including any or all of the following:
Quadrantal Error 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. (Fig. RA 3.12)
The effect of this signal is a displacement of 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.
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Figure: RA 3.12
Figure: RA 3.13
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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.
Coastal Refraction 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. (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.
2.
3.
4.
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 90° to the coast.
Remember that coastal refraction is less as height is increased.
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 such multipath signals and the bearings will be unreliable.
Sky wave (night) Effect As the loop aerial is receiving sky wave signals from the same NDB at the same time as ground wave signal are being received, the null will be suppressed or displaced in a random manner. Some of the displacements may give stable (but wrong) bearing indications for a period of time and are therefore very hazardous. At dawn and dusk, as the state of ionisation changes, these errors are particularly unpredictable.
Noise - 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.
Atmospheric Noise 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 30° latitude of the Equator.
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•
•
•
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
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 the notified range. You could be at half the published range before a reliable signal is received.
Accuracy When used within the published range by day in good conditions, a well Calibrated ADF should give a bearing accuracy within ± 5°.
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SA-RN 1.2
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 a manually 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 the
BFO 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.
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5 Which of the following statements regarding an aeronautical NDB is correct?
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 emission characteristics: 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 with an 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 ADF receivers.
a) 108,0 MHz – 117,9 MHz.
b) 200 – 1759 MHz.
c) 200 – 1759 Hz.
d) 200 – 1759 kHz.
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SA-RN 1.3
9 Homing on an NDB a) Calls for an assessment of the drift b) Is most effective in strong winds c) Will in most situations result in frequent heading changes when approaching the NDB d) 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 when listening for NDB ID b) Strong static emitted from the CB may cause the ADF needle to deflect towards the CB c) The static emitted from the CB will fade soon after you have passed it d) All 3 answers are correct
11 An aircraft is flying on heading 330°, and relative bearing to an NDB is
190°. 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 the relative bearing is 350°. Calculate QDM.
a) 110°.
b) 290°.
c) 300°.
d) 150°.
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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 used over the sea in average conditions is: a) 100 nm.
b) 300 nm.
c) 500 nm.
d) 1000 nm.
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