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Supplementary navigation techniques

Rev. 13 — page content was last changed November 18, 2009
Flight Planning and Navigation
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The ground-based radio beacons and omni-directional radio ranges utilised by aircraft navigating under the Instrument Flight Rules can be used for back-up navigation if the aircraft is equipped with the on-board instrumentation and the navigator is familiar with basic operating technique.

9.1 Non-directional beacon

In Australia the non-directional beacons [NDBs] are the oldest established and, though technically obsolescent, still the most common radio navigation aid. The beacons are usually located at or near an airfield (they were originally called homing beacons), although a very few are still sited to mark waypoints along air routes. The reason they are called 'non-directional' is that the aural radio ranges they originally replaced had directional antennas. The NDBs transmit an omni-directional carrier signal in the low frequency band between 190 and 535 kHz. Their effective range is primarily dependent on the operating power. Most inland NDBs have a transmitter power between 100 and 500 watts providing a range, during daylight hours, usually between 40–100 nm but tending toward the lesser figure. The rated coverage of each NDB is shown in the ERSA entry for the airfield or waypoint.

Low-power NDBs, known as 'locators' with a range of 30 nm or less, are sited around major airports and are associated with their instrument landing systems [ILS]. There are also high-power (2–3 kW) NDBs sited near the coast to provide guidance for overwater routes, their over water range being much greater than their inland range.

The carrier wave is transmitted on a specific frequency, but a two- or three-letter Morse code signal is continually superimposed on the carrier for NDB identification. The frequency and identification for each beacon is given in ERSA and shown on VNCs, ERC-Ls and VTCs. Some NDBs may provide an intermittent 'voice-over' facility for airfield information. If you want to practise, the morse code dots and dashes are rendered phonetically as 'dits' and 'dahs'.

If a Bureau of Meteorology automatic weather station [AWS] is located at an aerodrome that also has an NDB then an aerodrome weather information service [AWIS] may be available at that station. The NDB installation includes voice modulation on the carrier to broadcast the actual, or recent, weather conditions. For further information consult AIP GEN 3.5 para 7.4.
AM broadcast stations
The companion airborne system which makes use of the NDBs — known as automatic direction finding [ADF] equipment — can also receive transmissions in the 520 to 1611 kHz AM broadcast band. The power output of the broadcast stations is usually considerably greater than NDBs — typically 2, 5, 10 or 50 kilowatts — so their signal can be received at a greater distance. The transmission frequency, power and latitude/longitude of broadcast station transmitters are shown in the NAV/COMM section of ERSA.

Broadcast stations must be used with caution because of identification problems. There are long intervals between station identification calls and even then the transmitter to which you are tuned may be relaying programs from another station. The information contained in ERSA may not be up to date.

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9.2 ADF equipment

The ADF, or radio compass, equipment consists of an antenna system, a receiver/control box system and a panel-mounted indicator instrument. The antenna system comprises a loop antenna and a sense antenna which, depending on the age of any particular unit, may be completely separate or combined into one unit. The ADF receiver includes the frequency selector (probably 190 to 1799 kHz) and usually some test capability.

The loop antenna nowadays may be a fixed square ferrite core with two perpendicular windings and may be coupled with a goniometer — (a device for measuring angles, with a great number of scientific applications) — in the receiver. Such a system automatically ascertains the direction of the transmitter relative to the longitudinal axis of the aircraft. Hence the reason for the term "Automatic" DF because in earlier days the loop antenna was a physical loop (mounted on top of, or beneath, the fuselage and often enclosed in an egg-shaped fairing) which, simply put, had to be manually rotated by the operator to find the direction of the transmitter, which was read off a scale. At that time, and later, the sense antenna was a wire from the top of the tail fin to a fuselage connection.

The output from the receiver is fed to the panel-mounted instrument, which is a needle indicating the direction to the NDB, or broadcast station, as an angle relative to the aircraft's longitudinal axis. Behind the needle is a circular card marked off in 5-degree azimuth divisions from 0° to 355°, with a mark at the top dead centre [TDC] indicating the aircraft's nose. Depending on the age of the instrument, that card may be fixed — in which case 0° is always at TDC — or, more commonly, manually rotatable by turning a heading knob on the instrument.

ADF If the card is rotated so that the aircraft's current magnetic heading is situated at TDC then the head of the needle indicates the magnetic track to the transmitter and the tail of the needle indicates the reciprocal bearing — the aircraft's magnetic bearing from the station. When using the ADF indicator it should be normal practice to adjust the card whenever the aircraft's heading is changed. The illustration shows the ADF instrument with the heading knob [HDG] rotated so that the aircraft's heading of 350° magnetic is at TDC; the needle head indicates the track to the NDB is 155° magnetic while the bearing from the transmitter is 335°.

However, whether the card is fixed or rotatable, the head of the needle should always point directly to the transmitter and the angle (the number of degrees) between TDC and the head of the needle is always the angle between the fore and aft axis, and the direction of the transmitter. In the illustration that angle is 10 + 155 = 165°.

Heavier aircraft are usually fitted with a more complex and very expensive form of ADF called a Radio Magnetic Indicator [RMI], which incorporates, or is slaved to, a directional gyro. It may also have a two-needle display, the second needle being tuned to another navigation aid which, of course, makes position fixing remarkably easy.

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9.3 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. The applications briefly described below will be detailed in the 'Using the ADF' module.

Position fixes. If two (three is best) 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 (mins) × ground speed (kn) / degrees traversed

Homing and 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 constant. 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. Passing overhead an NDB is signified by a "cone of silence" (if the 'ident' volume has been turned up beforehand) and the needle then swings 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.

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9.4 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 increases 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 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 while the aircraft is banked.

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9.5 VHF Omni Range

In Australia the VHF omni-directional radio ranges [VORs] operate in the Very High Frequency aviation navigation [NAV] band between 112.1 and 117.9 MHz. As VHF transmissions are line-of-sight, the ground-to-air range depends on the elevation of the beacon site, the height of the aircraft and the power output. The VOR beacons are usually located at airfields but as they serve to define designated air routes (airways) some are located away from airfields, often on high ground.

A simplified concept of the ground beacon is that it simultaneously transmits two signals, a constant omni-directional signal called the reference phase and a directional signal which rotates through 360°, during a 0.03 second system cycle, and consistently varies in phase through each rotation. The two signals are only exactly in phase once during each rotation — when the directional signal is aligned to magnetic north.

Imagine a wheel with 360 spokes, at one degree azimuth spacing, with the VOR beacon being the hub. The spokes are numbered clockwise from one to 360 and each spoke or radial represents a magnetic bearing from the VOR beacon. The airborne navigation circuitry measures the phase angle difference between the directional signal phase received and the reference signal phase, and interprets that as the angular, or 'radial', indication currently being received. Radials are identified by magnetic bearing — e.g. the 30° radial — and thus form the basis for VOR and designated air route navigation. Essentially the system indicates a line of position from the selected VOR, on which the aircraft is located at any time.

The beacon also transmits a Morse code aural identification signal at about 10 second intervals.

The airborne system utilising the VOR beacon transmissions usually consists of an antenna (probably a V-type dipole mounted horizontally on the fin or fuselage, but could be the more expensive 'blade' or 'towel rail' types), a conventional VHF receiver (if combined with the VHF communications transceiver it is then called a NAV/COMM unit), navigation circuitry and the separate panel-mounted navigation indicator or 'omni bearing indicator' [OBI].

Some hand-held aviation COMMS transceivers can also receive the NAV band VOR transmissions and appear to have some navigation circuitry but, from all reports, their VOR navigation capability, if it exists at all, is limited.

Bendix-King omni bearing indicator

A basic OBI , like the above Bendix-King model, has a manually operated radial or 'omni bearing' selector [OBS] that rotates an azimuth ring marked from 0° to 355°. The OBS selected radial is indicated by the arrow at top dead centre and the reciprocal bearing is indicated by the bottom arrow. The other features of a basic OBI are the TO–FROM indicators, a deviation bar, a deviation indicator needle and a NAV/OFF alarm flag.
The TO–FROM logic
VOR baseline logicThe TO–FROM indications on the OBI are dependent on the aircraft's position relative to a notional ground baseline, formed perpendicular to the selected radial and passing through the beacon site. Unlike the NDB the indication is completely independent of the aircraft's heading. The navigation circuitry compares the difference between the radial being received and the radial selected. If the aircraft is located anywhere within range on the radial side of the baseline, the 'FROM' indication will be displayed on the OBI and, if located within range on the reciprocal side, the 'TO' indication will be displayed. For example if the 030° radial is selected on the OBI, the ground baseline is established between 300° and 120°. If the radial received indicates the aircraft is anywhere in the blue shaded area of the diagram and no matter whether it is headed towards or away from the VOR, or in any direction whatsoever, the OBI will display 'FROM'. Similarly if it is in the yellow area the OBI will display 'TO' no matter which direction the aircraft is headed. There are two areas of ambiguity – near bearings at right angles to the radial (e.g. shown at 120° and 300°) – where the OBI will give fluctuating indications, or display the 'OFF' flag.
The course deviation indicator
VOR baseline logicThe deviation bar and the deviation indicator needle together form the course deviation indicator [CDI]. If the needle is over the centre point the aircraft is then located at some position along the selected radial — or its reciprocal. The five division marks or dots either side of the centre point are spaced at two-degree intervals, thus if the needle is over the third mark, left or right of centre, the aircraft is positioned at a radial six degrees in azimuth from the selected radial, or its reciprocal. (Actually the aircraft is at the centre mark and the needle indicates the position of the selected radial). Full travel of the needle from the centre to either side represents 10° or more of azimuth. The ambiguity of whether the OBS selection is the radial or the reciprocal is determined by the TO/FROM indication; in the diagram at left 030 must be the radial, as the aircraft is in the FROM area. When the aircraft passes overhead the beacon, the needle will swing from side to side, the alarm flag may temporarily indicate that navigation is 'OFF' and the TO/FROM indication will reverse.

A difficulty for a non-IFR trained pilot using the VOR is a lack of perception of which way to turn the aircraft to fly to a selected radial, using the CDI indications. However, for VFR purposes, this is easily ascertained if the pilot follows two simple rules:
1. To track FROM a VOR select the radial required and ensure FROM is indicated.
2. To track TO a VOR rotate the OBS until the CDI is centred and TO is indicated.
In both cases as wind effect drifts the aircraft off track the deviation indicator needle will move to one side and that movement indicates the direction to turn to regain track. i.e. turn towards the needle.

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9.6 VOR applications

Like the NDB/ADF there are several applications for the VOR in light aircraft cross-country VMC navigation. The applications are briefly described below.

Homing and tracking to a VOR. Even with a crosswind component, tracking toward a VOR is quite simple — rotate the OBS until the CDI is centred and TO is indicated, turn onto that magnetic heading and then just keep the CDI centred and you will track more or less directly to the VOR.

Tracking from a VOR. Rotate the OBS to the required track (radial), ensure FROM is indicated, turn onto that magnetic heading and just keep the CDI centred and you will maintain the track.

Position fixes. If two VORs are in range then the bearing from each can be ascertained, roughly plotted on the chart (after converting to true bearings) and the aircraft position will be close to the intersection point of the LOPs. Alternatively a VOR bearing and a NDB bearing can be used, or a VOR bearing and a line feature on the chart — the latter technique being the most frequently used.

Running fix/distance from VOR. 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, as in the NDB running fix application above. The advantage with the VOR is that the CDI needle indicates the degrees traversed. As in the NDB application, the position fix is the distance along the second radial from the beacon.
VOR simulator
If your browser is Java enabled then I suggest a visit to www.fergworld/training/ and try out the single VOR trainer applet. Drag the aircraft symbol to position your aircraft, set the aircraft's heading on the directional gyro, rotate the OBS to a desired radial and check the CDI and flag. Then try to intercept the radial and track to or from the VOR, by changing heading and dragging the symbol along the flight path.Try the quiz.

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9.7 VOR errors

Standard VOR systems are more accurate than NDB/ADF but are still subject to errors at the ground station, bending distortion of signals caused by terrain effect and avionics errors. The aggregation of all errors is very unlikely to exceed 5°.

Though very thin on the ground in the outback areas of Australia, NDB and VOR can be very useful, provided the aircraft is within range, but not the best value for money. That distinction now belongs to another, and more advanced, supplementary navigation tool — the Global Positioning System.

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Groundschool – Flight Planning & Navigation Guide

| Guide content | 1. Australian airspace regulations | 2. Charts & compass | 3. Route planning | 4. Effect of wind |

| 5. Flight plan completion | 6. Safety audit | 7. Airmanship & flight discipline | 8. Enroute adjustments |

| [9. Supplementary navigation techniques] | 10. Global Positioning System | 11. Using the ADF |

| 12. Electronic planning & navigation | 13. ADS-B surveillance technology |

Supplementary documents

| Operations at non-controlled airfields | Safety during take-off & landing |

Next - other navigation techniques - GPS The next section of the Flight Planning & Navigation Guide discusses the use of the Global Positioning System as another supplementary navigation technique

Copyright © 2001–2009 John Brandon     [contact information]