Block Diagram

Tracking to an NDB

Tracking to an NDB

A useful ADF application in visual navigation is to locate a particular NDB and then track – or home – directly to it. The ADF receiver is tuned to the NDB frequency, and the audio volume turned up, so that the NDB can be identified as soon as the aircraft comes within range. The ADF needle indicates the bearing to the NDB and the wind correction angle necessary to maintain that track is then ascertained by bracketing, a technique which bears some similarity to the double track error method. The term 'bracketing' is derived from the artillery technique for ranging the target by deliberately placing initial rounds behind and in front of it.

Note: this sequence is best performed if the heading being flown is positioned on the ADF card at TDC, the diagrams in the left column below indicate the readings with those settings.

procedure

Needle position	Compass heading	Event sequence
	060°	Position A. When receiving the NDB signal turn the aircraft so that the head of the ADF needle is pointing to TDC, then check the heading from the compass. That heading is the track required to home directly to the NDB, for our example 060° magnetic. Rotate the ADF compass card to set 060° at TDC and the needle head will also indicate 060°. Remember that all heading changes should be logged.
	060°	Position B. As the flight progresses, holding the 060° heading, the crosswind causes the aircraft to drift to the south of the required track and the ADF needle has moved left about 5° to 055°.
	030°	Position C. We now have to make a first rough cut at the track error – it is best to initially overestimate so let's choose 15° and, applying the double track error technique, we turn left 30° on to an intercept heading of 030° magnetic. Positioning the 030° heading at TDC, the head of the needle will still initially indicate 055° but will move towards 060° as we close with the required track.
	045°	Position D. When the needle reaches 060° the 060° track to the NDB has been regained. Now halve the intercept angle (i.e. subtract the track error) and turn right onto an initial wind correction heading of 045° magnetic, i.e. the estimated track error was 15°, we turned left 30° onto the intercept heading of 030° and now, having regained the required track, we turn right 15° onto a wind correction heading of 045°. Now rotate the card to the 045° heading and the needle remains at the 060° bearing.
	045°	Position E. If the 15° WCA is correct then the ADF needle will remain at the 015° position whilst the 045° heading is maintained. However it is most likely that we have overcorrected, the aircraft will drift north of track, shown by the needle moving clockwise a few degrees from the 015° position so we now have to refine the wind correction angle.
	055°	Position F. We might guess that we have overestimated the WCA by about 5° so, applying the double track error technique, we turn right 10° on to an intercept heading of 055° magnetic. Positioning the 055° heading at TDC, the head of the needle will still initially indicate something greater than 060°, say 063°, but will move towards 060° as we close with the required track.
	050°	Position G. When the needle reaches 060° the 060° track to the NDB has been regained. Now halve the intercept angle and turn 5° left onto a wind correction heading of 050° magnetic, rotate the card to the 050° heading and, if we've estimated correctly, the needle will remain at the 060° bearing, maintaining a 10° WCA, while we continue along the required 060° track to the NDB.

Tracking from an NDB

Another useful application for the ADF in visual navigation is in determining track error when departing from an airfield equipped with an NDB or when overflying an NDB.

For example: the flight plan calls for a departure – from overhead an NDB – on a track of 240° magnetic with any necessary wind correction to be assessed after departure – using the ADF – with the track recovery and heading correction to be made by a slightly modified double track error method. (The modification is that rather than timing the intercept leg to estimate track recovery we will use the ADF needle to indicate when we are back over the required track.)

In this ADF application the ADF card may be used with the 0° position set at TDC or your personal preference may be to set the 240° heading at TDC. The diagrams and the text below indicate the procedure and the readings with 0° positioned at TDC but the additional text in italics is the procedure when rotating the card to the new heading for every change. Hopefully you will be able to see that the latter method is easier to handle.

Note that when tracking away from an NDB we use the tail of the ADF needle, rather than the head, as the indicator.

Needle position	Compass heading	Event sequence
	240°	Departing from overhead the NDB to track 240° magnetic. The magnetic compass heading is 240° ( i.e. no wind correction provision) and the tail of the ADF needle swings to the 0° position. With the 240° magnetic heading set at TDC the position of the needle relative to TDC is exactly the same as in the diagram but, on the background card, the needle tail indicates the 240° heading.
	240°	Position B. As the flight progresses holding the 240° heading the crosswind causes the aircraft to drift to the south of the required track. The tail of the ADF needle has moved about 15° to 345° and is in the left half of the card. Thus the opening angle, or track error, is 15° and the tail of the needle represents the track made good, which is 15° to the left of the required track. With the 240° magnetic heading set at TDC the tail of the needle will indicate the track made good, 225° or an opening angle, or track error, of 15°.
	270°	Position C. Use the double track error method to intercept the required track. The aircraft is turned 30° [2 × 15] onto a heading of 270° magnetic. The ADF needle tail initially moves 30° to 315° then commences to reverse direction as the 270° heading is maintained and the aircraft is closing the 240° track out. The aircraft is turned 30° [2 × 15] onto a heading of 270° magnetic and 270° magnetic is now set at TDC, the tail of the needle will then initially still indicate 225° but will move towards 240° as you close with the required track.
	270°	Position D. When the needle has moved through a 15° arc and is back to the 30° left position [330°], on a heading of 270°, the 240° track out from the NDB has been regained. With the 270° magnetic heading set at TDC the 240° track out from the NDB has been regained when the tail of the needle reaches 240°.
	255°	Position E. Subtract the track error [15°] and turn left onto the new heading of 255° which will then maintain the necessary 15° wind correction angle. The ADF needle moves 15° clockwise and the aircraft should hold the required track – if the heading is maintained and the needle kept at the 345° position. Subtract the track error [15°] and turn left onto the new heading of 255° which will then maintain the necessary 15° wind correction angle. Set the 255° magnetic heading at TDC, the tail of the needle now indicates 255°. After flying this heading for a while you may find that you still have some drift – indicated by movement of the needle. In this case a small heading correction is usually enough compensation.

Running fix / distance from NDB

Whenever your track will pass abeam an NDB it is quite easy to obtain a running fix, using the 1-in-60 rule and a little mental arithmetic, providing you have a reasonable idea of your groundspeed. The technique is illustrated in the diagram:

Procedure

1. Tune and identify the NDB, set your heading at TDC and watch the ADF needle, the NDB is directly abeam when it moves to 90° either side of TDC, position A in the diagram.

2. Note the time and continue flying your heading, for example 040° magnetic.

3. When the needle has moved a sufficient amount to get a good reading (position B on the diagram), note the time and the bearing from the NDB, indicated by the tail of the needle. Let's say the needle has moved 10°, the elapsed time is 8 minutes, the bearing from the NDB is 110° magnetic and you reckon your groundspeed at 70 knots.

4. Now we calculate the distance we are along that bearing using the 1-in-60 rule:
i.e the distance [nm] from the NDB = elapsed time [minutes] × ground speed [knots] / degrees traversed = 8 × 70/10 = 56 nm. The aircraft's position at time B is then 110°/56 nm from the NDB. The real difficulty now is to measure and plot that position on the navigation chart whilst flying the aircraft (not forgetting to convert the bearing to °true) – so best to get the passenger to do it.

If you are wondering what happened to the '60' in the 1-in-60 application the answer is it is negated by the usage of minutes in one factor and nautical miles per hour in another. In the diagram the dashed red line outlines the right angle triangle on which the calculation is based – the distance from the NDB to position B forms the hypotenuse.

ADF applications

There are several applications for the ADF in light aircraft cross country VMC navigation – remembering the Visual Flight Rules require that the pilot must be able to navigate by reference to the ground and position fixes must be taken at least every 30 minutes.

Position fixes. If two (or better - three) transmitters are in range then the bearing from each can be ascertained, the lines of position roughly plotted on the chart (after converting to true bearings) and the aircraft position will be close to the intersection point. In most of Australia to have two NDBs in range at the same time is not so common and three would be most unlikely, so the most likely position fixing use is to combine a surface line feature with an NDB bearing.

Running fix / distance from NDB. The 1-in-60 rule can be applied when the aircraft is within range of a transmitter by turning the aircraft so that the station is abeam and then measuring the degrees traversed against time. This is a form of running fix in that two bearings are taken, at an interval, from one source and the aircraft's position is the distance along the second LOP from the NDB. For example:

Distance [nm] to NDB = elapsed time [minutes] × ground speed [knots] / degrees traversed

Homing & tracking to or from an NDB. If there is no crosswind component then tracking toward an NDB is quite simple, just keep the head of the ADF needle at TDC and you will arrive overhead; the track over the ground will be straight and the magnetic heading consistent. However if there is a crosswind component and you just endeavour to keep the head of the ADF needle at TDC, you will eventually arrive but, due to the drift, the track followed will be curved and the magnetic heading will need to be consistently changing. This is called homing, and you will arrive at the NDB on an into-wind heading. Thus tracking, or flying directly towards, or from, an NDB is exactly the same as tracking from A to B – you have to calculate a wind correction angle. Passage overhead an NDB is signified by a "cone of silence" (if the ident volume has been turned up beforehand) and the needle then swinging to the reciprocal bearing.

Using the ADF probably appears to be fairly simple, which it is, but there will be difficulties, for the uninitiated in perceiving, from the position of the needle, the headings to fly when attempting to intercept and then track along a particular magnetic bearing to or from the ground station.

As in all navigation you should always maintain an awareness of the aircraft's position in terms of being north, south, east or west of the NDB and, when initiating a turn, think in the same terms e.g. a left turn will take you further east.

NDB/ADF errors

Electrical interference. Radio waves are emitted by the aircraft alternator in the frequency band of the ADF. An alternator suppressor is fitted to contain those emissions but this component does not have a long life and it is wise to test the ADF for correct operation during pre-flight checks. The test is made by selecting a transmitter – which must be a reasonable distance away, say 30 nm – then watch the ADF needle during the engine run up. If the needle moves as rpm increase there is electrical interference and probably the alternator suppressor should be replaced. Magnetos may also interfere with the ADF.

Thunderstorms emit electrical energy in the NDB band and will deflect the ADF needle towards the storm.

Twilight/night effect. Radio waves arriving at a receiver come both directly from the transmitter – the ground wave – and indirectly as a wave reflected from the ionosphere – the sky wave. The sky wave is affected by the daily changes in the ionosphere, read the ionisation layers section in the Aviation Meteorology Guide. Twilight effect is minimal on transmissions at frequencies below 350 kHz.

Terrain and coastal effects. In mountainous areas NDB signals may be reflected by the terrain which can cause the bearing indications to fluctuate. Some NDBs located in conditions where mountain effect is troublesome transmit at the higher frequency of 1655 kHz. Ground waves are refracted when passing across coast lines at low angles and this will affect the indicated bearing for an aircraft tracking to seaward and following the shore line.

Attitude effects. The indicated bearing will not be accurate whilst the aircraft is banked.
 

Components

C. ADF COMPONENTS
The NDB Control Panels figure, on the right, shows the major ADF components except the receiving antenna, which on most light aircraft is a length of wire running from an insulator on the cabin to the vertical stabilizer.
l . RECEIVER: Controls on the ADF receiver permit the pilot to tune the station desired and to select the mode of operation. \When tuning the receiver the pilot must positively identify the station. The low or medium frequency radio beacons transmit a signal with 1,020 Hertz (cycles per second [Hz]) modification keyed to provide continuous identification except during voice communications. All air facilities radio beacons transmit a continuous two- or three-unit identification in Morse code, except for ILS front course radio beacons which normally transmit a continuous one letter identifier in Morse code. The signal is received, amplified, and converted to audible voice or Morse code transmission. The signal also powers the bearing indicator.
Tuning the ADF- To tune the ADF receiver, the pilot should follow these steps:

1. turn the function knob to the RECEIVE mode. This turns the set on and selects the mode that provides the best reception. Use the RECEIVE mode for tuning the ADF and for continuous listening when the ADF function is not required;
2. select the desired frequency band and adjust the volume until background noise is heard;
3. with the tuning controls, tune the desired frequency and then re-adjust volume for best listening level and identify the station;
4. to operate the radio as an automatic direction finder, switch the function knob to ADF; and
5. the pointer on the bearing indicator shows the bearing to the station in relation to the nose of the aircraft. A loop switch aids in checking the indicator for proper operation. Close the switch. The pointer should move away from the bearing of the selected station. Then release the switch; the pointer should return promptly to the bearing of the selected station. A sluggish return or no return indicates malfunctioning of the equipment or a signal too weak to use.

2. CONTROL BOX - DIGITAL READOUT TYPE: Most modern aircraft have this type of control in the cockpit. In this equipment the frequency tuned is displayed as a digital readout of numbers rather than tuning a frequency band.
a) Function Selector (Mode Control). Allows selection of OFF, ADF, ANT or TEST Position.

• ADF - Automatically determines bearing to selected station and displays it on the RMI. Uses sense and loop antennae.
• ANT - Reception of Radio signals using the sense antenna. Recommended for tuning.
• TEST - Performs ADF system self-test. RMI needle moves to 315°.

b) Frequency Selector Switches. Three concentric knobs, permit selection of operating frequency. Two frequencies can be preselected. Only one can be used at a time. The transfer switch indicates the frequency in use.
c) Selected Frequency Indicators. Provides a visual read-out of the frequencies selected. The numbers can be printed on drums that rotate vertically or, in more modern sets, they arc displayed by light emitting diodes.
3. ANTENNAE: The ADF receives signals on both loop and sense antennae. The loop antenna in common use today is a small flat antenna without moving parts. Within the antenna are several coils spaced at various angles. The loop antenna senses the direction of the station by the strength of the signal on each coil but cannot determine whether the bearing is TO or FROM the station. The sense antenna provides this latter information, and also voice reception when the ADF function is not required.
4. BEARING INDICATOR: As mentioned above, the bearing indicator (see Fixed Card Bearing Indicator figure, on the right) displays the bearing to the station relative to the nose of the aircraft. If the pilot is flying directly to the station, the bearing indicator points to 0°. An ADF with a fixed card bearing indicator always represents the nose of the aircraft as 0° and the tail as 180°.
Relative bearing (see NDB Bearings figure, on the left) is the angle formed by the intersection of a line drawn through the centerline of the aircraft and a line drawn from the aircraft to the radio station. This angle is always measured clockwise from the nose of the aircraft and is indicated directly by the pointer on the bearing indicator.
Magnetic bearing (see NDB Bearings figure, on the left) is the angle formed by the intersection of a line drawn from the aircraft to the radio station and a line drawn from the aircraft to magnetic north. The pilot calculates the magnetic bearing by adding the relative bearing shown on the indicator to the magnetic heading of the aircraft. For example, if the magnetic heading of the aircraft is 40° and the relative bearing 210°, the magnetic bearing to the station is 250°. Reciprocal bearing is the opposite of the magnetic bearing, obtained by adding or subtracting 180° from the magnetic bearing. The pilot calculates it when tracking outbound and when plotting fixes.
D. ADF OPERATIONS
1. MONITORING: Since the ADF receiver normally has no system failure or "OFF warning flags to provide the pilot with immediate indication of a beacon failure or receiver failure, the ADF audio must be monitored. The "idents" should be monitored anytime the ADF is used as a sole means of en route navigation. During the critical phases of approach, missed approach and holding, at least one pilot or flight crew member shall aurally monitor the beacon "idents" unless the aircraft instruments automatically advise the pilots of ADF or receiver failure.
2. HOMING: One of the most common ADF uses is "homing to a station". When using this procedure, the pilot flies to a station by keeping the bearing indicator needle on 0° when using a fixed-card ADF (See Homing to an NDB figure, below right). The pilot should follow these steps:

1. tune the desired frequency and identify the station. Set the function selector knob to ADF and note the relative bearing;
2. turn the aircraft toward the relative bearing until the bearing indicator pointer is 0°; and
3. continue flight to the station by maintaining a relative bearing of 0°.

The figure Homing to an NDB, on the right, shows that if the pilot must change the magnetic heading to bold the aircraft on 0° the aircraft is drifting due to a crosswind. If the pilot does not make crosswind corrections, the aircraft flies a curved path to the station while the bearing indicator pointer remains at zero. The aircraft in position 2 must keep changing its heading to maintain the 0° relative bearing while flying to the station.
The bracketing method used here is basically the same as that explained elsewhere. The major difference is that bracketing a VOR requires the pilot to bracket a radial identified by the TB needle, whereas bracketing an ADF magnetic bearing requires the pilot to identify it by using both the bearing indicator and the heading indicator.
Assume the pilot of the aircraft in position l (see Bracketing an NDB Magnetic Bearing figure, on the right) desires to intercept the 090° magnetic bearing to the non-directional beacon. The pilot then sets up an intercept angle of 30° which is shown by the 120° heading of the aircraft. The ADF pointer indicates 340°. Because the magnetic bearing equals the magnetic heading of the aircraft and the relative bearing, the pilot adds 120° (the relative bearing) and finds that the aircraft is on the 100° magnetic bearing.
NOTE:
Whenever the aircraft heading and relative bearing equal more than 360° the pilot should subtract 360° from the resulting figure. The pilot then follows the rest of the bracketing procedure.
3. TRACKING FROM A STATION: A pilot can use ADF to track from a station by employing the principles of bracketing a magnetic bearing. The Tracking from an NDB figure, below left, illustrates an aircraft tracking outbound from a station with a crosswind from the north. The reciprocal bearing is 090°, and the pilot tracks this bearing by flying the aircraft with 10° of wind correction. The pilot knows that the aircraft is tracking a reciprocal bearing because the heading indicator (080°) and relative bearing (190°) equal the magnetic bearing (270°).
4. POSITION FIX BY ADF: The ADF receiver can help the pilot to make a definite position fix by using two or more stations and the process of triangulation. To determine the exact location of the aircraft, the pilot should use these procedures:

1. locate two stations in the vicinity of the aircraft. Tune and identify each;
2. set the function selector knob to ADF, then note the magnetic heading of the aircraft as read on the heading of the aircraft as read on the heading indicator. Continue to fly this heading and tune in the stations previously identified, recording the relative bearing for each station;
3. add the relative bearing of each station to the magnetic heading to obtain the magnetic bearing. Correct the magnetic bearing for east-west variation to obtain the true bearing; and
4. plot the reciprocal for each true bearing on the chart. The aircraft is located at the intersection of the bearing lines (see Position Fix by NDB figure, on the right).

5. TIME COMPUTATION TO FLY TO A STATION: Computing time to the station is basically the same for ADF as it is for VOR (refer to Article 2.2.3. E (2)) therefore, a brief example is sufficient here. The basic procedure is to:

1. turn the aircraft until the ADF pointer is either at 090°s or 270°s and note the time; and
2. fly a constant magnetic heading until the ADF pointer indicates a bearing change of 10°. Note the time again and apply the following formula:

(TIME IN SECONDS BETWEEN BEARING CHANGE)/(DEGREES OF BEARING CHANGE) equals TIME TO STATION IN MINUTES.
For example, if it takes 45 seconds to fly a bearing change of 10°, the aircraft is:
45 / 10 = 4.5 min from the station.
To find distance to a station multiply time by distance covered in one minute using TAS or preferably G/S.
As with VOR procedures, a 10° bearing change is the simplest and easiest to use in making this calculation. If the pointer moves so rapidly that a satisfactory time check cannot be obtained during a 10° bearing change, this rapid movement indicates that the aircraft is very close to the station

Description and Limitation

A. DESCRIPTION
One of the older types of radio navigation is the automatic direction finder (ADF) or non-directional beacon (NDB). The ADF receiver, a "backup" system for the VHF equipment, can be used when line-of-sight transmission becomes unreliable or when there is no VOR equipment on the ground or in the aircraft. It is used as a means of identifying positions, receiving low and medium frequency voice communications, homing, tracking, and for navigation on instrument approach procedures.
The low/medium frequency navigation stations used by ADF include non-directional beacons, ILS radio beacon locators, and commercial broadcast stations. Because commercial broadcast stations normally are not used in navigation, this section will deal only with the non-directional beacon and ILS radio beacon.
A non-directional radio beacon (NDB) is classed according to its power output and usage:

1. the L radio beacon has a power of less than 50 watts (W),
2. the M classification of radio, beacon has a power of 50 watts up to 2,000 W;
3. the H radio beacon has a power output of 2,000 W or more;
4. the ILS radio beacon is a beacon which is placed at the same position as the outer marker of an ILS system (or replaces the OM).

B. LIMITATIONS AND BENEFITS
Pilots using ADF should be aware of the following limitations:
Radio waves reflected by the ionosphere return to the earth 30 to 60 miles from the station and may cause the ADF pointer to fluctuate. The twilight effect is most pronounced during the period just before and after sunrise/sunset. Generally, the greater the distance from the station the greater the effect. The effect can be minimized by averaging the fluctuation, by flying at a higher altitude, or by selecting a station with a lower frequency (NDB transmissions on frequencies lower than 350 kHz have very little twilight effect).
Mountains or cliffs can reflect radio waves, producing a terrain effect. Furthermore, some of these slopes may have magnetic deposits that cause indefinite indications. Pilots flying near mountains should use only strong stations that give definite directional indications, and should not use stations obstructed by mountains.
Shorelines can refract or bend low frequency radio waves as they pass from land to water. Pilots flying over water should not use an NDB signal that crosses over the shoreline to the aircraft at an angle less than 30°. The shoreline has little or no effect on radio waves reaching the aircraft at angles greater than 30°.
When an electrical storm is nearby, the ADF needle points to the source of lightning rather than to the selected station because the lighting sends out radio waves. The pilot should note the flashes and not use the indications caused by them.
The ADF is subject to errors when the aircraft is banked. Bank error is present in all turns because the loop antenna which rotates to sense the direction of the incoming signal is mounted so that its axis is parallel to the normal axis of the aircraft. Bank error is a significant factor during NDB approaches.
While the ADF has drawbacks in special situations, the system does have some general advantages. Two of these benefits are the low cost of installation of NDBs and their relatively low degree of maintenance. Because of this, NDBs provide homing and navigational facilities in terminal areas and en route navigation on low-level airways and air routes without VOR coverage. Through the installation of NDBs many smaller airports are able to provide an instrument approach that otherwise would not be economically feasible.
The NDBs transmit in the frequency band of 200 to 415 kHz. The signal is not transmitted in a line of sight as VHF or UHF, but rather follows the curvature of the earth; this permits reception at low altitudes over great distances.
The ADF is used for primary navigation over long distances in remote areas of Canada.