Jeppesen-Radio-Navigation

VHF Direction Finding Chapter 2

DECODING THE CHART

The chart is in three parts:
PART 1 – ADMINISTRATION

Right Side
¾ The airfield the chart refers to
¾ The procedure VDF 210° to Aerodrome
¾ The VDF frequency in MHz

Left Side
¾ The Minimum Safe Altitude circle valid for the 4 quadrants shown out to 25 nm from

the aerodrome reference point (ARP)

Centre
¾ The four letter ICAO code for Cranfield (EGTC)
¾ The categories of aircraft that can fly this approach (A & B)
¾ The airfield data
¾ The airfield frequencies

Note that:

¾ The approach frequency is also the primary VDF frequency
¾ The tower frequency is the secondary VDF frequency

Radio Navigation 2-5

Chapter 2 VHF Direction Finding
PART 2 – THE PLAN VIEW

¾ The plan view shows a range circle out to 10 nm from the aerodrome, which is the
centre point.

¾ Inside the circle are marked airspace restrictions such as D206.
¾ Significant obstacles are also marked.
The Approach
¾ The aircraft initially homes to the Cranfield overhead, the initial approach fix (IAF),

which is co-located with the VDF.
¾ The outbound leg is 016° and the inbound leg is 210°.
¾ Overhead the VDF is the missed approach point (MAPt).
The above shows the course of the aircraft over the ground but shows no elevation information.
This is given in the bottom part of the chart.

2-6 Radio Navigation

VHF Direction Finding Chapter 2

PART 3 – THE ELEVATION VIEW

The elevation view is as a bystander would see on the ground looking at the aeroplane from the
side.

¾ Initially the aircraft homes to the IAF at an altitude of 2200 ft. This is QNH; the QFE
figures are given in the brackets.

¾ On the outbound leg, the aircraft descends to 1664 ft QNH (1300 ft QFE).
¾ After the inbound turn, the aircraft can then descend to the appropriate MDA for the

category of aircraft shown in the boxes to the bottom right of the diagram.

If the airfield is not seen by the time the aircraft is over the VDF, a missed approach must be
carried out.

REFUSAL OF SERVICE

DF stations have the authority to refuse to give bearings when conditions are unsatisfactory or
when the bearings do not fall within the calibrated limits of the station. The station will state the
reason at the time of refusal.

Full R/T procedures to be used, when requiring VDF assistance, are contained in the
Communications section of your notes.

AUTOMATIC VDF

Automatic VDF stations assist in the radar identification for ATC procedures. They do not provide
a normal VDF service to aircraft.

Radio Navigation 2-7

Chapter 2 VHF Direction Finding

RANGE AND ERRORS

Being a VHF transmission, the range is line of sight and the maximum range formula applies:

Range = 1.25(√HT + √HR)

Where: R = the maximum range between the stations
HT = the height of the transmitter
HR = the height of the receiver

Other factors that affect the expected range are:

Intervening Terrain

This can screen the transmitter/receiver path (remember that VHF is a line of site
transmission).

Atmospheric Refraction

An increased refractive index (resultant from the inversions of temperature and/or
humidity) can cause super refraction and increased ranges. Sub-refraction reduces the
expected range.

Transmitter Power
The bearing signal measured may be in error. The major sources of error are:

Ground Reflections

These can cause VHF and UHF signals to reach the DF station aerial from
multiple paths. Additional phase differences are detected which deflect the
bearing indication.

Synchronous Transmissions

The DF station detects signals from other aircraft communications equipment at
the same time as the desired signal. This causes a deflection of the measured
bearing. This is a problem in congested airspace when atmospheric conditions
favour super refraction and cause detection of transmissions from beyond the
radio horizon.

The signal quality may decrease if an aircraft is not flying straight and level. VHF radio signals are
vertically polarised and reception is only optimal when the aircraft has a small amount of pitch
and bank. To ensure good reception, avoid asking for bearings or headings to steer during steep
turns.

2-8 Radio Navigation

INTRODUCTION

Non Directional Beacons (NDB) are ground-based transmitters that transmit radio energy equally
in all directions. The airborne system in the aircraft is the Automatic Direction Finder (ADF). The
indicator in the aircraft always points toward the tuned NDB. (Exceptions to this are discussed
later in this chapter.)

PRINCIPLES OF OPERATION

The NDB transmitter is very simple. An RF oscillator provides a carrier wave. This carrier wave is
the NDB signal that the airborne equipment (ADF) uses 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.

Frequency
LF/MF – 190 to 1750 kHz. In Europe, the frequencies are normally between 225 and 455 kHz.

EMISSION CHARACTERISTICS

Long Range Beacons N0N A1A
Short Range Beacons N0N A2A

It is important to bear in mind that, although the airborne equipment only needs the bare carrier
signal to indicate the direction to the transmitter, there must be a way of identifying the selected
station. In the above emission characteristics, both the long and short range beacons transmit
N0N. This is the unmodulated carrier wave on which the indication relies. It is the A1A or A2A that
provides the identification.

The A1A emission keys the carrier wave. The paragraph in Chapter 1 on heterodyning states that
to have an audio frequency input to the headphones there must be two radio frequencies. As a
result, the BFO must be turned on to provide the second frequency. This means that the audio
tone is heard during the entire N0N phase of the incoming signal and during the active part of the
keyed A1A signal.

The A2A emission modulates the audio tone frequency directly onto the carrier and therefore the
BFO selection should be turned off. In this case, it is the demodulator within the ADF that feeds
the audio ident tone to the headphones.

The ICAO recommended emission characteristic is A2A, unless operational or environmental
considerations dictate the use of A1A, such as long range coastal installations.

Radio Navigation 3-1

Chapter 3 Non Directional Beacons and Automatic Direction Finding

LOOP THEORY

To understand direction finding, an understanding of a loop aerial is necessary. Below is a
representation of a loop aerial. The loop has two vertical elements shown as A and B.

AB

Phase Difference (AC) = AB Cosß

The NDB transmits a vertically polarised signal which induces voltage in vertical elements A and
B, but no voltage in the horizontal elements of the loop. If the loop is lying across the path of the
incoming signal, the voltages induced in elements A and B are equal, and vary with the incoming
signal at the same rate. Because an electric current only flows when a voltage difference occurs,
no current flows in the loop when it is across the path of the incoming signal.

Now consider the loop aerial aligned with the incoming signal as in the diagram above. The
magnitutde of the signal wave form is different across elements A and B, and this induces
voltages of different levels which causes an AC current to flow in the loop. By plotting the strength
of the iduced current in the loop for one complete revolution of the signal source around the loop
a figure of eight polar diagram is developed.

A β
- B

+

The polar diagram shows two ill-defined maxima (90° and 270°) and two well-defined minima at
(0° and 180°).

The minima are usually used in direction finding. Even with two well-defined minima, there is no
indication as to which side of the loop the transmitter is sited. A sensing aerial resolves the
ambiguity.

3-2 Radio Navigation

Non Directional Beacons and Automatic Direction Finding Chapter 3

SENSING

Inserting a vertical di-pole into the loop resolves the ambiguity of the polar diagram above as
shown in the diagram below.

The polar diagram of the sensing di-pole appears below.

Combining the polar diagram for the loop aerial and the sensing aerial forms a polar diagram in
the shape of a cardioid.

++ -

A cardioid diagram has only one null position, and resolves the 180° ambiguity. The principle of
the ADF is that the loop is turned to the position for minimum which corresponds to the null
position of the cardioid. The instrument’s needle indications are also relative to the position of the
loop aerial. The system is called the Automatic Direction Finder because the aerial rotation and
the interpretation of its relative signal strength are done automatically. The indicator information is
such that, by laying the instrument panel down flat, the ADF needle points directly at the
transmitting station. The system component which drives the indicator in response to the sensed
direction of the signal source is called a ‘Goniometer’.

Radio Navigation 3-3

Chapter 3 Non Directional Beacons and Automatic Direction Finding

NDB OPERATION

In this method of operation, an amplified signal radiates omni-directionally. The transmission mast
may be either a single mast or a large T-aerial strung between two masts. These aerial
arrangements produce a vertically polarised signal. The polar diagram for the aerial is omni-
directional in the horizontal plane but, as shown below, exhibits directional properties in the
vertical plane.

Above the station, marked by the points at which the radiated power has fallen to 0.5 of its
maximum value, is a conical area in which signal strength may be too low for use. This volume of
space is called the cone of silence or cone of confusion. For an NDB, this angle is 40° from the
vertical.

ADF OPERATION

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 on the instrument panel, and the loop
and sense aerials are normally combined in a single aerial unit, normally mounted under the
fuselage. The pilot uses the receiver control panel to enter the frequency corresponding to the
NDB for intended use.

The ADF indicator consists of a needle, which indicates the direction from which the signals of the
selected NDB ground station are coming. The most basic ADF indicator is known as a Radio
Compass. In this configuration, the needle moves against a scale calibrated in degrees from
0° - 359°. The datum for the direction measurement is the nose of the aircraft and therefore, the
radio compass indications are relative bearings.

3-4 Radio Navigation

Non Directional Beacons and Automatic Direction Finding Chapter 3

BEARING DETERMINATION

The loop, or directional aerial, rotates electronically and, by combining information from the loop
and sense aerials, the unit derives the bearing to the station internally. When a looped conductor,
such as the loop aerial encounters electromagnetic waves it induces voltages in the two halves of
the loop. These voltages depend on the angular position of the loop relative to the incoming
electromagnetic (EM) waves. The total voltage induced in the loop is the algebraic difference
between the voltages from the two halves. This total voltage is the signal output from the loop
aerial.

TYPES

Typical associated power outputs and uses are as follows:

Locator Beacon
Radiating between 15 and 40 watts and used for intermediate approach guidance toward
establishing the final approach path of an ILS, these beacons are short range and are
normally N0N A2A. Their maximum range is 15 - 25 nm.

Homing
These beacons primarily serve as an approach and holding aid near an aerodrome. They
are medium range beacons, normally N0N A2A. Their maximum range is 50 nm.

Airways/Route Beacons
Radiating at up to 200 watts and used for track guidance and general navigation, these
beacons are normally N0N A2A.

Long-Range Beacons
Radiating at up to 4 kilowatts, and generally located on islands or oceanic coastlines,
these beacons serve to provide guidance and navigation resources to transoceanic
flights. These beacons are normally N0N A1A.

Besides NDBs, other transmitters operate within the NDB band of frequencies and can be
detected by the aircraft’s receiver. These include:

¾ Broadcast stations (i.e. those carrying entertainment, news, etc.). Broadcast stations
can have repeater transmitters at different locations causing synchronous
transmission errors.

¾ Marine Beacons.

Do not use stations if their details are not published in the AIP or appropriate Flight Guides.
Where information on Marine Beacon is published, pilots need to be aware that a number of
beacons are grouped together on the same frequency, and each beacon transmits for a period of
60 seconds in a cycle of six minutes.

The use of signals from such published stations guarantees that, within the published range by
day, the signal from the desired station is 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.

Radio Navigation 3-5

Chapter 3 Non Directional Beacons and Automatic Direction Finding

CONTROL PANELS AND INDICATORS

CONTROL PANEL

There are different types of ADF control panels, but their operational use is almost the same. An
example appears below. The mode selector, or function switch, has several positions, enabling
the pilot to select the desired function. Typical markings are: OFF, ADF, ANT, and LOOP.

ADF is the position when the pilot wants the needle to automatically display bearing
information.

ANT is the abbreviation of antenna. In this position, only the signal from the sense aerial
is used. This results in no satisfactory directional information to the ADF needle.

There are two reasons for selecting the ANT position:

¾ Easier identification of the NDB station
¾ Better understanding of voice transmissions

BFO stands for Beat Frequency Oscillator. Sometimes this position is labeled CW, the
abbreviation for Carrier Wave. The BFO circuit imposes a tone onto the carrier wave signal to
make it audible to the pilot, enabling identification of the NDB signal.

The emission characteristics determine the position of the BFO switch:

N0N A1A Tuning Identification
N0N A2A ON ON
ON OFF

For N0N A2A, the BFO is on for tuning in order to check the integrity of the incoming signal by
providing an uninterrupted tone for an uninterrupted carrier signal.

Once the station has been properly tuned and identified, switch the Mode Selector back to ADF.
This is important, as no bearing information shows unless the switch is in the ADF position. When
a pilot selects BFO or ANT, some ADFs automatically default to the 180° position, while others
remain on the last bearing computed. Never leave the mode selector in ANT or BFO position if
navigating using the ADF.

3-6 Radio Navigation

Non Directional Beacons and Automatic Direction Finding Chapter 3

In order to avoid the dangers of this problem, NDBs transmitting on A2A are identifiable with the
mode selector in the ADF position, so it becomes possible to avoid the ANT position. 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, transmitted
together with its normal signal and known as its IDENT. When tuning an NDB it is absolutely
essential to correctly identify the facility before using it.

TEST SWITCH

If the unit has a test switch, pressing it swings the indicator needle. If the needle does not swing,
this indicates the unit is not working properly. If the needle swings, but does not return to its
previous position, the signal is too weak to be used for navigation. If it swings and returns to its
previous position, the system is working properly and the received signal is good.

BEARING INDICATORS

Bearings to the station display on an indicator consisting of a bearing scale (calibrated in
degrees) and a pointer. There are four types of bearing scale with varying degrees of
sophistication. They are:

¾ The fixed card
¾ The manually rotatable card
¾ The radio magnetic indicator (RMI)
¾ The fixed card indicator, or relative bearing indicator (RBI)

This manual discusses only two systems, the RBI and the RMI.

RELATIVE BEARING INDICATOR (RBI)
The bearing displayed on a fixed card indicator is a relative bearing; thus the name Relative
Bearing Indicator (RBI). Since the card is fixed, zero is always at the top and 180° always at the
bottom.

33 0 3

24 27 30 6 9 12

15 18 21

HDG

A relative bearing is always measured clockwise from the nose of the aircraft. In the diagram
above, the needle is pointing to 100°. This means that the station is 100° to the right of the
aircraft nose.

Radio Navigation 3-7

Chapter 3 Non Directional Beacons and Automatic Direction Finding

In the diagram below the relative bearing is 340°. The NDB is 340° right of the nose.

0 3
33

24 27 30 6 9 12

15 18 21

HDG

A more convenient way of expressing this is that the station is 20° left of the nose.

It is sometimes convenient to describe the bearing of the NDB in relation to the NOSE or TAIL of
the aircraft.

Since the card is fixed, the indicated relative bearing must 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°, it is necessary to subtract 360° from the result in order to obtain a
meaningful bearing.

Example:

Assume for the diagram above that the aircraft is heading 230°M
The bearing to the NDB is:

230° + 340° = 570°

Because this is more than 360, it is necessary to subtract 360 from 570 = 210°

The QDM is 210°.

This means the QDR is 030°.

The magnetic bearing of the aircraft from the station, the QDR, is the reciprocal of the QDM. A
quicker way to determine the QDM is to mentally superimpose the RBI needle onto the directional
gyro. This is not very accurate, but it is a good double check for calculations. Visualise the QDR
as the tail of the needle when it is mentally transferred from the RBI onto the directional gyro
indicator.

3-8 Radio Navigation

Non Directional Beacons and Automatic Direction Finding Chapter 3

RADIO MAGNETIC INDICATOR (RMI)
This combines the Relative Bearing Indicator and Remote Indicating Gyro Compass into a single
instrument, with the compass card being aligned automatically with Magnetic North. In the
diagram below:

¾ The heading is 332°M.
¾ The VOR or ADF can be indicated by either pointer depending upon the switching.
¾ The QDM is continuously indicated by the arrow head of the pointer.
¾ The QDR is continuously indicated under the tail.

30 33 N
W3

24 6 V
O
V R

O 21 E
R
S 15 12

ADF ADF

This is now the most common type of presentation.

If the double pointer represents the ADF then the QDM is 300° and the QDR is 120°.

DIRECT WAVE LIMITATIONS

The Direct Wave follows the line of sight. Its range can be determined using the line of sight
formula. In most cases, the direct wave range is considerably less than that of the Ground Wave.
Height may become significant when it is desirable to receive the direct wave, to minimise the risk
of ADF error, when flying in mountainous areas, or when using coastal NDBs.

Radio Navigation 3-9

Chapter 3 Non Directional Beacons and Automatic Direction Finding

SKY WAVE LIMITATIONS

F1
E

TX

Skip Distance

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 sky
wave returns at 1000 miles. The name for this gap is the dead space.

The exact length of the dead space depends on frequency and the state of ionisation of the
atmosphere. At frequencies in the lower MF and the LF bands, intense ionisation by day
attenuates (absorbs) RF signals and no sky wave return is noticeable. By night, the ionisation
levels fall and returning sky waves can be detected.

NIGHT EFFECT

At short range (30 to 80 miles), the sky waves mix with the ground wave signal (there is no dead
space). Because the returning sky waves travel over a different path, they have a different phase
from the ground wave. This has the effect of suppressing or displacing the aerial null signal, in a
random manner. The needle on the RMI or RBI wanders. This effect is at its most variable during
twilight at dusk and dawn.

A further effect occurs due to the design of the loop aerial system. The loop uses a vertically
polarised signal. As the radio wave travels through the ionosphere, the vertical polarisation
changes as the wave refracts back toward the Earth, so the returning wave has a horizontal
polarisation component.

A current is now induced in the horizontal members of the loop:

¾ The horizontal member AB, and
¾ The two smaller feeds to the bottom of the aerial

AB

The resultant current flow further degrades the null position and an accurate reading is
impossible.

3-10 Radio Navigation

Non Directional Beacons and Automatic Direction Finding Chapter 3

At longer ranges, the sky wave signal becomes progressively stronger. Ionospheric refraction
may cause the plane of polarisation of the signal to shift randomly. This can cause the random
introduction of a horizontally polarised component into the loop aerial, which causes displacement
of the null signal.

In summary, the airborne ADF is designed and optimised for use in conjunction with the more
predictable ground wave signal from the selected NDB.

ERRORS OF THE ADF

The ADF bearing is subject to a number of error sources including any or all of the following.

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 then re-
radiate as weak signals but, being close to the ADF aerial, are strong enough to detect.

The effect of this signal is to displace the measured null toward the major electrical axis of the
aeroplane creating an error that is maximum on relative bearings 045°, 135°, 225°, 315° (the
quadrantals). Calibration and electro-mechanical compensation at installation minimises this
error.

MAJOR ELECTRICAL
AXIS OF AIRCRAFT

POSITIVE
CORRECTION

REQUIRED

CORRECT
BEARING OF
TRANSMITTER

INCOMING
RADIO WAVE

BEARING
ACTUALLY
MEASURED

DIP (BANK) ERROR

During turns, the horizontal member of the loop aerial detects a signal. This causes displacement
of the null and the display of a short-term erroneous bearing.

Radio Navigation 3-11

Chapter 3 Non Directional Beacons and Automatic Direction Finding

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 causes displacement of the wave front. This results in
a bearing error.

CORRECT BEARING
OF BEACON

NDB ACTUAL PATH
OF RADIO WAVE
LAND (INDICATED BEARING)
SEA REFRACTION
TOWARDS COAST

WAVE CROSSING
AT 90°

NO REFRACTION

Coastal refraction errors may be minimised by any or all of the following:

¾ Do not use beacons unless they are situated on islands or near to the coast.
¾ If using an inland NDB, only use bearings at or near 90º to the coast.
¾ Remember that coastal refraction decreases as height increases.

MULTIPATH SIGNALS

When flying in mountainous regions, signals may refract (bend) around and/or reflect from
mountains. Such multipath signals may affect the ADF, making the bearings unreliable.

TRANSMITTER

3-12 Radio Navigation

Non Directional Beacons and Automatic Direction Finding Chapter 3

NOISE

The definition of noise is 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 adjacent frequencies. Remember
that, from sunset to sunrise, sky wave propagation of signals in the LF and MF bands is
possible. This causes the signal to noise ratio to decrease and results in errors as null
displacement occurs, 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 occur. The rule is use with extreme caution if unsure.

Lightning
There are an average of 44 000 thunderstorms over the Earth’s surface in every 24 hour
period and more than half of these occur over or near land surfaces within 30º latitude of
the Equator.

Each thunderstorm generates electro-magnetic signals and these radiate in all directions
from that storm. When flying near one of these storms, the aircraft’s ADF detects the
signal and the bearing indication may well deflect toward that storm. Such noise levels
are normally quite low, but they do increase under the following conditions:

¾ In temperate latitudes during the summer
¾ As one moves toward the tropics
¾ At night as a result of sky wave propagation

Charged Water Droplets
Water droplets held in a cloud have an electric charge. As an aircraft flies through the
cloud, the water droplets that contact the aircraft discharge on the metal surface. The
collective effort of the water droplet discharge can distort and blur the polar diagram
enough to displace the null position.

Noise effects are indicated by:

¾ Random wandering of the bearing indication
¾ 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. The
aircraft could be at half the published range before finding a reliable signal.

Radio Navigation 3-13

Chapter 3 Non Directional Beacons and Automatic Direction Finding

SYNCHRONOUS TRANSMISSION

Where two or more beacons are transmitting on the same frequency, the measured bearing
becomes the resultant of the two received signals.

As long as the NDB is used within its promulgated range, the effects of synchronous transmission
should be a minimum.

PROMULGATED RANGE
Most NDBs are given a daytime-only protection range where the unwanted signals are limited to
± 5°. Outside this range the error increases. The propagation conditions at night also increase the
bearing errors.

ABSENCE OF FAILURE WARNING

There is no visible indication to the user that there is a system failure.

ACCURACY

When used within the published range by day, the ADF should give a bearing accuracy
within ± 6°.

3-14 Radio Navigation

INTRODUCTION

This chapter discusses the uses of the NDB. Even though pilots are unlikely to use the instrument
for plotting position lines, they need to understand this procedure as it helps a General Navigation
course. The process of homing and understanding the Jeppesen plate is essential in instrument
flying for the Instrument Rating.

ADF BEARING

The procedure for obtaining an ADF bearing is:

¾ Determine the frequency, identification, and modulation of the required beacon and
ensure that the aircraft is within the published (promulgated) range.

¾ Switch on the ADF and adjust volume.
¾ Tune the frequency and identify the station using ANT and BFO as necessary.
¾ Select ADF on the control panel and note the bearing on the indicator.

LINE OF POSITION (LOP) USING THE RBI

With the help of the information provided by instruments, the pilot is now able to determine the
line of position along which the aircraft is situated. To draw this LOP on the chart, the pilot needs
the QDR or the QTE. Assume the aircraft is on a heading of 015°M:

33 0 3

24 27 30 6 9 12

15 18 21

HDG 4-1

The relative bearing from the indicator is 340°.
The QDM is the relative bearing plus the heading.
340 + 015 = 355°
The QDR is the reciprocal, or 175°.

Radio Navigation

Chapter 4 NDB Navigation

LINE OF POSITION (LOP) USING THE RMI

An RMI solves the bearing automatically. The RMI continuously provides QDMs and QDRs.
Magnetic Bearings can only be used on charts that are oriented to magnetic north. The beacons
on most instrument charts have the direction of magnetic north attached with an arrow.

30 33 N
W3

24 6 V
O
V R

O 21 E
R
S 15 12

ADF ADF

Assuming that the single pointer is the ADF:

¾ The QDM is 017
¾ The QDR is 197

HOMING

The ADF needle always points toward the station, and 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 toward the
station.

Once aimed at the station, any crosswind component displaces the aircraft to either side of the
straight track to the station and the ADF needle swings away from the top of the indicator.

The pilot must then make a correction of the heading toward the needle in order to continue
heading to the station.

This process must be repeated again and again, since the crosswind continues to push the
aircraft away from the straight track. As a result, the path to the station is a curved one.

4-2 Radio Navigation

NDB Navigation Chapter 4

33 0 3

12 15 18
12 15 18

9 12 15
6 9 12
24 27 3015 18 21
27 30 33
30 33 036
30 33 00

18 21 24 3 69 369

21 24 27 21 24 27

The crosswind component requires the aircraft to turn further and further into the wind in order to
continue toward the station. The aircraft must turn until eventually reaching a point where the
aircraft faces directly into the wind. At that point, the aircraft no longer drifts off the direct track
and is now heading straight to the station. The actual curved path that results is different for each
combination of crosswind and TAS. A strong crosswind component and low TAS results in a large
deviation. A weak crosswind component and a high TAS results in a small deviation. Since the
actual track over the ground varies with every wind and airspeed combination, there is no way to
ensure that any given aircraft stays within the boundaries of an airway or approach path when
homing. Homing is a very simple but extremely inefficient procedure. Because of the uncertain
demands on airspace, it is not commonly used.

INTERCEPTING A COURSE

To navigate with the help of ADF and NDB:

¾ Visualise the aircraft’s position
¾ Intercept the desired course
¾ Maintain the course to or from the station

The first step is to visualise the aircraft’s position. Once this is completed, intercept the desired
course, which in this case is 035° inbound.

The second step is to make any turn necessary to the heading that provides a suitable intercept.
Observe the instrument readings during the turn.

Radio Navigation 4-3

Chapter 4 NDB Navigation

INBOUND TO THE BEACON

Now look at the corresponding plan view.
The heading of 090° provides an intercept angle of 55°. Since the desired QDM is 035°, the
aircraft is on track when the RBI indicates a relative bearing of 305° as shown in the diagram on
the next page.

4-4 Radio Navigation

NDB Navigation Chapter 4

33 0 3
30 6

27 9

24 12
21 15
18

When the needle nears the desired relative bearing, in this case 305°, begin the turn toward the
station. This places the aircraft on the desired inbound track. Compare with the instrument
indications.

OUTBOUND FROM THE BEACON

To intercept a track outbound, follow the same procedures. First of all, visualise the aircraft’s
position.

33 0 3

24 27 30 6 9 12

15 18 21

HDG

The relative bearing of 100° combined with the magnetic heading of 125° indicates a location
North and East of the NDB.

Radio Navigation 4-5

Chapter 4 NDB Navigation

The desired track is 050° outbound. The intercept angle is 40°. When the relative bearing is 140°
the aircraft has reached its outbound course. Observe the instruments’ indications.

When the needle reaches 135° degrees, start turning to intercept the outbound course. Look at
the instrument.

Heading 050°, with relative bearing 180°, places the aircraft on course. This heading will only
maintain the course in still air. A crosswind component will require a drift correction.

Example 1: In order to intercept a specific course:

¾ First determine the aircraft’s position relative to the desired course.
¾ Then establish a suitable interception angle.

Consider the following situation. To assist in visualising the situation, draw a plan and the
instrument indications.

¾ The aircraft is on a heading of 340°.
¾ The relative bearing to the NDB is 080°.
¾ The required course is 090° inbound.

By maintaining a heading of 340°, the aircraft eventually intercepts the 090 course. This would be
a rather untidy intercept because a turn of 110° would be required when the aircraft is on track.

A more efficient intercept can be achieved by turning onto an initial heading of 360°, for a 90°
intercept. A heading of 030° leads to a 60° intercept with the required inbound course.

4-6 Radio Navigation

NDB Navigation Chapter 4

Since the aircraft is on QDR 240, a heading of 060° would turn the aircraft directly toward the
station, and the QDM 090 would never be intercepted.

Once the aircraft is on a correct intercept heading, the rule is a simple one. When the angle
formed by the aircraft’s heading and the desired course 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 course
(QDR or QDM).

When intercepting OUTBOUND, the aircraft is on the desired course 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 course from an assigned heading using this technique requires knowing
the interception angle. For instance, with a heading of 220° and a clearance to intercept QDM
180, the intercept angle is 40°.

When the needle is 40° to the left of zero, the track has been intercepted.

Example 2: The aircraft heading is 265° and the RBI indicates 005°. The aircraft must join
QDM 240 at an intercept angle of 60°.

The first step is always to visualise the aircraft’s position:

¾ What is the QDR? In this instance, it is east of the station.
¾ Which way must the aircraft turn to make the intercept, left or right?

The course is to the right of the aircraft, so a right turn has to be made for the interception.

Which heading is required in order to intercept the QDM 240 with an intercept angle of 60°?

To intercept QDM 240 at 60°, the aircraft should turn to a heading of 300°.

Maintain a heading of 300°, and observe the RBI needle.

Since this is a 60° intercept, wait until the pointer falls 60° to the left of the zero indication on RBI.
Mentally superimposing the RBI needle on the directional gyro always provides a good
crosscheck for the calculations. To avoid overshooting the intended course, turn a few degrees
before reaching the desired QDM. Observe the instruments and initiate the turn 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 becomes a lot easier if the pilot maintains a
mental picture of where the aircraft is and where it should be.

TRACKING

With no crosswind, a direct inbound course is achieved by:

¾ Heading the aircraft directly at the NDB
¾ Maintaining the ADF needle on the nose of the aircraft.

If there is no drift, the aircraft homes straight to the NDB. 4-7
Radio Navigation

Chapter 4 NDB Navigation

Any crosswind will cause the aircraft to be blown off track. In the cockpit, the ADF needle
indicates this as it starts to move away from the top of the indicator.

To fly a straight course to the station is called TRACKING. To track to the station requires
establishing a wind correction angle (WCA) to compensate for 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, pilot reports, etc.). Remember that the higher the crosswind, the greater
the WCA and, for the same crosswind, slower aircraft must establish a greater WCA than faster
aircraft.

Once the aircraft is on a course with a wind correction angle to compensate for drift, observe the
instruments and look at where the crosswind is coming from and what the impact is on the
aircraft.

If the ADF needle indicates a constant relative bearing while maintaining a constant magnetic
heading, the current wind correction angle is correct and the aircraft is tracking directly to or from
the station. A wind correction angle that does not compensate for the present wind allows the
aircraft to drift off course, and the ADF needle shows a gradually changing relative bearing.

If the head of the ADF needle moves to the right, it indicates that a turn to the right is necessary
to maintain the course to the NDB and, conversely, if the head of the needle moves to the left, a
left turn is required.

How large each correcting turn should be depends upon the deviation from the course. A simple
method is to double the angle of bearing change. Observe that if the aircraft deviates 10° to the
left, the needle has moved 10° to the right. Doubling the angle of bearing change simply means
the pilot must alter the heading 20 degrees to the right.

Having regained the course, turn left by half of the correcting turn of 20°. In other words, turn left
10° to maintain the track. This WCA should provide reasonable tracking.

In real life, the perfect track is difficult to achieve and the pilot makes a number of minor
corrections to the heading, a technique known as bracketing the track.

The ADF needle becomes more and more sensitive as the aircraft nears the NDB station. Minor
displacements to the left or right of the track cause larger and larger changes in the relative
bearings and the QDM. When passing overhead the NDB, the ADF needle oscillates then moves
toward the bottom of the dial and settles down. When close to the NDB, do not change heading.
Maintain the heading until obtaining accurate readings on the outbound flight.

To facilitate the QDR calculations when tracking outbound, 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° to the
right to counteract the wind from the right.

To fix the QDR 040 in a no wind condition fly heading 040°. With a right crosswind that requires a
10° WCA, the heading is 050°.

4-8 Radio Navigation

NDB Navigation Chapter 4

NDB APPROACH

The approach chart (plate) shown below is for the Coventry NDB(L) Runway 23.

Radio Navigation 4-9

Chapter 4 NDB Navigation

To facilitate discussion, the plate is split into 3 parts in a similar method to the Cranfield VDF
discussed in an earlier chapter.

PART 1 – ADMINISTRATION

The top of the chart follows the same profile as the VDF chart. In the top right:

¾ The name of the airfield Coventry

¾ The approach to be flown NDB(L)

¾ The L indicates that the beacon being used is a locater beacon. Normally, this
beacon is associated with the ILS.

¾ The frequency and identification CT 363.50 kHz

On the left:

¾ The minimum safe altitude sectors.

In the centre:

¾ The four letter identifier for Coventry
¾ The categories of aircraft that can use the approach
¾ Airfield elevation, threshold elevation, transition altitude and variation
¾ Frequencies in use at Coventry

4-10 Radio Navigation

NDB Navigation Chapter 4

PART 2 - THE PLAN VIEW

The plan view shows information around the environs of Coventry:

¾ Coventry airfield
¾ Coventry NDB (CT 363.5), which is also the Initial Approach Fix (IAF)
¾ The prominent obstacles
¾ Major airfields (Birmingham)
¾ Minor airfields (Baxterly, Bruntingthorpe)
¾ Disused airfields

The plan view shows a circle radius 10 nm from Coventry. Superimposed on the chart are the
tracks to fly:

¾ From the VOR at Daventry (DTY)
¾ From the NDB at Lichfield (LIC)
¾ The tracks to fly for category A, B, C, and D aircraft. Note that the two tracks are

different
¾ The missed approach
¾ Warnings

Radio Navigation 4-11

Chapter 4 NDB Navigation

PART 3 - THE ELEVATION VIEW

The profile is complementary to the plan view. The altitude (height in brackets) and tracks appear
on the elevation view. Timing for the outbound (OUBD) legs is in a box to the right of the diagram.

The missed approach is a written commentary from the Minimum Descent Altitude (MDA).

PART 4 - LIMITS AND OTHER INFORMATION

The bottom part of the chart is split into four columns:

¾ The recommended profile
¾ The rate of descent and time to the MAPt from the Final Approach Fix (FAF) for

different groundspeeds
¾ The Obstacle Clearance Altitude (OCA) or Obstacle Clearance Height (OCH) which

is used in calculating the MDA.
¾ The Visual Manoeuvring (Circling) Altitude (VM(C) OCA)

The bottom two rows show an alternate procedure and a note as to the lowest altitude from which
the aircraft can begin a procedure.

4-12 Radio Navigation

INTRODUCTION

Recognised in 1949 by ICAO as the international standard for short-range navigation, VOR is the
most commonly used beacon in radio navigation. As opposed to the NDB, which transmits a non-
directional signal, the signal transmitted by the VOR contains directional information.

This chapter discusses two types of VOR; Conventional VOR (CVOR) and Doppler VOR (DVOR).
For the user in the aircraft there is no apparent difference in the indications.

PRINCIPLE OF OPERATION

Two independent modulations are placed on a VHF frequency, known as the reference and
variable phase. The aircraft equipment measures the magnetic bearing of the station by phase
comparison of these two waves.

FREQUENCY
VHF – 108 to 117.95 MHz

It is prudent to discuss the allocation of the VHF frequency band in this section. VORs are used
for two separate purposes, as Terminal VORs (TVOR) and Airway VORs. These beacons occupy
different parts of the frequency band. Further complicating the allocation, Instrument Landing
System (ILS) occupies frequencies in the same range as well.

TVOR uses the first even decimal and first even decimal + 50 kHz up to 112 MHz (e.g.
108.00 MHz, 108.05 MHz, 108.20 MHz, 108.25 MHz etc.).

ILS uses the first odd decimal and first odd decimal + 50 kHz up to 112 MHz (e.g.
108.10 MHz, 108.15 MHz, 108.30 MHz, 108.35 MHz etc.).

Airways VOR occupies the remainder of the frequency band 112 MHz to 117.95 MHz at
50 kHz spacing.

POLARISATION
Horizontal

EMISSION CHARACTERISTICS
A9W

Radio Navigation 5-1

Chapter 5 VHF Omnidirectional Radio Range (VOR)

CONVENTIONAL VOR

REFERENCE SIGNAL

Conventional VOR is an omni-directional continuous wave transmission on the VOR frequency.
The signal is frequency modulated (FM) at 30 Hz. The polar diagram of this omni-directional
signal is circular, meaning that the phase detected by the aircraft’s receiver is the same on all
bearings. This reference signal effectively tells the VOR receiver where magnetic north is.

VARIABLE SIGNAL

The variable signal is transmitted from an aerial that is effectively a loop and is amplitude
modulated (AM) at 30 Hz. As with the ADF, this produces a figure 8 polar diagram. Unlike the
ADF, the “loop” aerial electronically rotates at 30 revolutions per second. By combining the
reference and variable signal, the resulting polar diagram is similar to a cardioid with the
exception being it does not have a null position.

This polar diagram is called a limacon, the French for edible snail.

AIRCRAFT RECEIVER

The aircraft VOR receiver splits the received signal into three parts. The first connects to the
aircraft communications system so that the beacon can be identified. The second and third parts
pass through a filter that separates the AM and FM reference signals.

The 30 Hz FM reference signal is then electronically changed so that it can be compared with the
AM variable signal. Thus the principle of operation of VOR is bearing measurement by phase
comparison.

BEARING MEASUREMENT

The absence of a null position is compensated for by varying the power relationship between the
reference and variable signals. This difference in field strength is graphically illustrated below.

A FM REFERENCE
BOTH SIGNALS 30 HZ
ARE IN PHASE
ON A BEARING AM VARIABLE
OF MAGNETIC SIGNAL 30 HZ

NORTH ROTATING
LIMACON
REFERENCE A REFERENCE

B 7 4 1D

B TX D
270° PHASE DIFFERENCE
4 90° PHASE DIFFERENCE
= 270 RADIAL = 090 RADIAL
C

REFERENCE

C
180° PHASE DIFFERENCE

= 180 RADIAL

5-2 Radio Navigation

VHF Omnidirectional Radio Range (VOR) Chapter 5

The resultant shape is that of an elongated cardioid, called a limacon. In the diagram above, the
reference signal appears at the four cardinal headings. If the line of the variable phase plots the
power curve, then it can be seen that at North it is 4 units of power, West 7 units, South 4 units
and East 1 unit. The variable phase is rotated at the same rate as the reference signal is
modulated so that at:

North
The limacon is set in the start position (remember that the aerial is rotating at 30
revolutions per second). By comparing the variable signal wave for magnetic north with
the reference signal wave it is obvious that the wave diagrams are the same. By looking
at the limacon signal strength start North 4, West 7, South 4, and East 1. Comparing this
with the reference signal whose amplitude varies 4, 7, 4, and 1 shows that the two waves
are in phase.

East
On East, the sequence starts East 1, North 4, West 7, and South 4. When drawn as a
sine wave and compared with the reference wave, it shows that the variable signal lags
the reference signal by 90°. Hence, the aircraft is on a bearing of 090°M.

South
On South, the sequence starts South 4, East 1, North 4, and West 7. When drawn as a
sine wave and compared with the reference wave, it shows that the variable signal lags
the reference signal by 180°. Hence, the aircraft is on a bearing of 180°M.

West
On West, the sequence starts West 7, South 4, East 1, and North 4. When drawn as a
sine wave and compared with the reference wave, it shows that the variable signal lags
the reference signal by 270°. Hence, the aircraft is on a bearing of 270°M.

AIRCRAFT EQUIPMENT

AERIAL
The aerial is a small, horizontal dipole, designed to receive the horizontally polarised signals
transmitted from the ground station. Designed for the frequency band of 108 MHz – 118 MHz, the
aerial has to be mounted so that it offers 360° reception. It must also be shielded from
transmissions from the VHF communication radio aerial. Aerials are frequently mounted on the fin
of an aircraft.

RECEIVER
The receiver compares the reference signal and the variable signal in order to detect the phase
difference. The signal from the aerial is filtered through the high-frequency part of the receiver
and only the signals from the desired VOR station pass through to the detectors and filters.

FREQUENCY SELECTOR
The frequency selector switch on the control panel selects the desired station. The phase
comparator compares the phase of the two signals and the difference is fed to the indicator.
Special circuitry within the receiver detects the identification signal and amplifies it for a speaker
or headphones. Some VORs can also transmit “voice”, either radio communication, identification,
met-information, or other voice transmissions. The receiver panel has a frequency selector knob,
a dial indicating the selected frequency and a selector switch with a position for IDENT and Voice.

Radio Navigation 5-3

Chapter 5 VHF Omnidirectional Radio Range (VOR)

Select the IDENT position to hear the identification signal of the VOR. The identification transmits
according to ICAO recommendations and consists of a two or three letter Morse code transmitted
at a rate of:

¾ Seven words a minute
¾ Repeated at least once every 30 seconds
¾ Modulated at 1020 Hz

Selecting the VOICE position improves the reproduction of speech, and is selected when the
transmission contains voice messages (for instance ATIS), or if the station serves as a regular
voice transmitter.

INDICATORS
The indicator can be in many different forms, from the simplest to the most complex electronic
flight information system. The parts and functions of the basic indicator are covered in the next
chapter.

MONITORING

All VOR stations are monitored by automatic equipment located approximately 50 ft from the
transmitter. The monitor performs functions when it detects any malfunction:

¾ It warns the control point.
¾ It removes the identification and navigation component of the beacon.
¾ It switches the beacon off in extreme cases.

The monitor limits are:

¾ A change in bearing information of >1°
¾ A reduction of more than 15% in the signal strength of either of the 30 Hz

modulations
¾ The monitor failing

If the beacon is switched off, the standby system comes into operation. The standby beacon
requires time to become operational, so no transmission of identification happens until a full
changeover occurs.

TERRAIN

Uneven ground, high ground, or man made obstacles nearby the VHF wave can affect VOR
signals. If the terrain causes erroneous indications, they are listed in the AIP under the
Designated Operational Coverage.

DESIGNATED OPERATIONAL COVERAGE (DOC)

VOR operates in a range where the signals are line of sight, so the line of site formula can be
used to calculate the maximum range in which a signal is receivable. The AIP lists the VORs and
provides the maximum range, altitude, and bearings where reliable signals are obtainable. As
with the promulgated range for an NDB, only use the VOR with confidence within the DOC. VORs
on the same frequency must be spaced at least 500 nm apart to ensure there is no mutual
interference. The DOC is applicable for both day and night operations, as returning sky waves do
not affect the VHF wave as they do the NDB.

5-4 Radio Navigation

VHF Omnidirectional Radio Range (VOR) Chapter 5

CONE OF CONFUSION OR CONE OF SILENCE

Unlike the NDB, which has a cone of confusion of 40° from the vertical, the VOR cone of
confusion is 50° from the vertical. The area above the VOR gives no signal, which causes
problems with the indicators in the aircraft. Rapid bearing changes may be displayed near the
beacon, making it impractical to home or follow a radial. The easy option is to fly the required
heading through the overhead until receiving reliable indications.

The cone of confusion is easily calculated by trigonometry.

50° 50°

Radius of the cone of silence = altitude (nm) x Tan 50°

Example: An aircraft is flying at 30 000 ft overhead a VOR, what is the diameter
of the cone of confusion?

30 000 ft = 30 000 ÷ 6080 ft = 4.93 nm

Radius = 4.93 x tan 50° = 4.93 x 1.19 = 5.9 nm

The diameter is twice the radius = 11.8 nm

ACCURACY

A number of sources account for the total accuracy of a VOR:

Site Error is due to the nature of the terrain or obstacles in the vicinity of the transmitter.
The limitation for course displacement errors is ± 1°. This site error is monitored as stated
earlier.

Propagation Error occurs due to signal travel over terrain or obstructions. These errors
are in the region of ± 1°.

Airborne Equipment Error occurs due to the tolerances of the equipment in the aircraft.
These errors are normally no more than ± 3°.

The normal accuracy of the VOR is ± 5°.

Radio Navigation 5-5

Chapter 5 VHF Omnidirectional Radio Range (VOR)

AIRWAY NAVIGATION

The student may be asked to calculate the maximum distance between two VORs. Again, this is
a matter of using simple trigonometry.

Distance = width of the airway ÷ tan 5° (accuracy of the VOR) x 2

width
5°

Airway Centre Line

Example: What is the maximum distance between two VORs on an airway which is
10 nm wide (5 nm from the centreline to the edge of the airway)?

5 / tan 5° x 2 = 5 / .087 x 2 = 115 nm

Where the figures are easy, try the 1 in 60 rule:

There exists an effective track error of 5 nm caused by a 5° angle error.

With the knowledge that 1° in 60 nm causes a 1 nm error,

5° must cause 5 nm, so the distance is twice this = 120 nm.

TEST VOR

Certain airports have VOT transmitters installed. These are VOR test transmitters and allow a
pilot to check the airborne equipment on the ground. The test can be conducted at any position
on the aerodrome:

¾ Tune the VOT frequency

¾ Centre the needle on the Course Deviation Indicator (discussed in the next chapter)

¾ The bearing indicates 180° with a TO flag

¾ 000° with a FROM flag

¾ If the indications are not within ± 4° of 180°/000°, the aircraft installation should be
repaired.

5-6 Radio Navigation

VHF Omnidirectional Radio Range (VOR) Chapter 5

DOPPLER VOR

CVORs suffer from reflections originating with objects in the vicinity of the site. It was found that
reducing these errors is possible by increasing the horizontal aerial dimensions. This was
impractical as the CVOR uses a mechanical rotating aerial, so a new system was devised.

The Doppler VOR is the second generation VOR, providing improved signal quality and accuracy.
A fundamental change is that the reference signal of the DVOR is amplitude modulated, while the
variable signal is frequency modulated. This means that the modulations are the opposite of
conventional VORs, so the variable signal rotates anticlockwise to maintain the same phase
relationship at the receiver. Because the frequency-modulated signal is less subject to
interference than the amplitude modulated signal, the received signals provide a more accurate
bearing determination.

The Doppler effect is created by “electronically rotating” the variable signal. Circular placed
aerials (diameter 44 ft), rotate at a speed of 30 revolutions per second. The diameter of the circle
is 13.4 metres, making the radial velocity of the variable signal 1264 m/s. This causes a Doppler
shift, increasing the frequency as the signal rotates toward the observer and decreasing as it
rotates away, with 30 full cycles of frequency variation per second. This results in an effective FM
of 30 Hz. A receiver situated at some distance in the radiation field continuously monitors the
transmitter. When certain prescribed deviations are exceeded, either the identification is switched
off, or the complete transmitter is taken off the air. This is similar to the CVOR.

The VOR receiver does not know if it is receiving a signal from a CVOR or a DVOR. As a result,
the pilot treats both types in the same way.

CVOR Reference Signal Variable Signal
DVOR Frequency Modulated Amplitude Modulated
Amplitude Modulated Frequency Modulated

Radio Navigation 5-7

Chapter 5 VHF Omnidirectional Radio Range (VOR)

5-8 Radio Navigation

INTRODUCTION

This chapter discusses the uses of VOR. Three types of VOR indicators are covered:

¾ Radio Magnetic Indicator (RMI)
¾ Omni-Bearing Selector (OBS)
¾ Horizontal Situation Indicator (HSI)

RADIO MAGNETIC INDICATOR

The RMI combines the information from the radio navigation instruments with the directional
information from the directional gyro. The RMI has two needles, which can indicate both ADF and
VOR information.

The two needles usually have single and double lines to make it easier for the pilot to identify the
stations.

30 33 N
W3

24 6 V
O
V R
21
O E
R S 15 12

ADF ADF

There are two small buttons on the bottom of the instrument. These enable the pilot to select
either VOR or ADF information to be displayed by each needle. The indicator needles constantly
point toward the tuned station. The RMI card is slaved to the directional gyro, so that the heading
of the aircraft can be read directly off the lubber line at the top. In this way, the needles show the
bearing to the ground stations continuously:

¾ The tip of the needles indicate a magnetic bearing to the ground station, the QDM.
¾ The tail indicates magnetic bearing from the ground station to the aircraft, the QDR.

Radio Navigation 6-1

Chapter 6 VOR Navigation

When tuned to a VOR, the tail of the needle indicates the VOR radial. In our example, the single
needle points to a VOR station, indicating that the aircraft is on radial 195°. The needle marked
with a double line indicates QDM 302° (QDR 122°).

Be aware that the bearing registered from the ADF is a magnetic bearing against the magnetic
meridian passing through the aeroplane. If there is a significant variation change or meridian
convergence between the station and the aeroplane, the bearing indicated is not the same as a
QDM.

Consider a flight at high latitudes, and tuning a station with both a VOR and an NDB where there
is a marked difference of variation and longitude between the station and the aeroplane. If the
VOR is fed to the single needle and the NDB to the twin needle of the RMI, will they have the
same bearing indications?

OMNI-BEARING SELECTOR

The indicator shown below has three components:

¾ The Omni Bearing Selector
¾ A TO/FROM Indicator
¾ A LEFT/RIGHT Course Deviation Indicator

TO
FR

When using this instrument the pilot has certain selections to make:

OBS Selector
The control knob selects the desired magnetic track that a pilot wishes to fly TO or FROM
a VOR beacon. In the case above, the pilot has selected a track of 100°M.

TO/FROM Indicator
When the required magnetic track is set, the TO or FROM arrow appears showing where
the aircraft is relative to the beacon. The case above shows the TO arrow. The indication
changes as the aircraft passes over the beacon.

6-2 Radio Navigation

VOR Navigation Chapter 6

Course Deviation Indicator
The indicator has 4 small dots and one large central dot. Each of these dots represents
2°, with a full-scale deflection of the needle being 10°. The vertical bar moves left or right
according to the relative position of the aircraft to the magnetic track selected. With the
vertical bar centred, the aircraft is on the magnetic track selected. In the instrument
above, the bar shows 3½ dots left. This means that the aircraft has a deviation of 7° from
the selected course. To get back to track the aircraft must fly toward the needle, in this
case left.

Warning Flag: A warning flag appears when:

¾ There is a failure of the aircraft’s receiving equipment.
¾ There is a failure of the ground station.
¾ There is a failure of the indicator.
¾ Signals received are too weak or the aircraft is out of range of the beacon.

The indications are totally independent of aircraft heading. The instrument shows the aircraft
position in relation to the course selected.

TO Flag
If the TO flag is visible, the number shown by the arrow at the bottom of the instrument (if
the vertical bar is central) is the radial of the aircraft.

FROM Flag
If the FROM flag is visible, the number selected at the top of the instrument (if the vertical
bar is central) is the radial of the aircraft.

Radio Navigation 6-3

Chapter 6 VOR Navigation

USING THE OBS

The diagram below shows three aircraft at different positions. The OBS indication for each is
discussed in turn. (Note that the angles shown are exaggerated.)

Aircraft 1

TO
FR

The aircraft in position 1 has the indications shown above. The aircraft is flying TO the beacon
and must fly left to regain the track. Note that when centralising the needle, the aircraft homes to
the beacon. When flying on an airway, an aircraft must regain track as quickly as possible.

With TO showing, if the aircraft were on track (vertical bar central) it would be on the 280° radial.
The lubber line shows this at the bottom of the OBS indicator.

In this case, the vertical bar shows a fly left situation of 3½ dots, which is the amount of deviation
from the selected track. 3½ dots are equal to 7°.

6-4 Radio Navigation

VOR Navigation Chapter 6

Because the displacement of the vertical bar provides an angular measurement of deviation, it is
easy to determine the radial the aircraft is on. With TO in the window, the deviation shown by the
vertical bar should be:

¾ ADDED to the OBS radial if the aircraft is left of track
¾ SUBTRACTED from the OBS radial if the aircraft is right of track

Subtracting the deviation from the selected radial (because the aircraft is right of track) reveals
the radial the aircraft is now on: 280 – 7 = 273°.

In the case of Aircraft 2, where the aircraft is within the area of ambiguity, 10° either side of the
perpendicular cutting track, no positive indications exist.

Aircraft 3

TO
FR

The aircraft in position 3 has the indications shown above. The aircraft is flying FROM the beacon
and must fly left to regain the track.

With FROM showing, if the aircraft were on track (vertical bar central) it would be on the 100°
radial. The top of the OBS indicator shows the radial.

As with Aircraft 1, the vertical bar shows a fly left situation of 3½ dots, which is the amount of
deviation from the selected track. 3½ dots are equal to 7°.

To determine which radial the aircraft is on with FROM in the window, the deviation shown by the
vertical bar should be:

¾ ADDED to the OBS radial if the aircraft is right of track
¾ SUBTRACTED if the aircraft is left of track

Taking the radial plus the deviation (because the aircraft is right of track) allows the pilot to work
out the radial of the aircraft (100 + 7) = 107°.

Radio Navigation 6-5

Chapter 6 VOR Navigation

HORIZONTAL SITUATION INDICATOR (HSI)

A more modern derivative of the CDI, this instrument is widely used and pilots need to be familiar
with its presentation and interpretation. As the name suggests, the HSI (shown below) provides
the pilot with a pictorial presentation of the aeroplane’s navigational situation in relation to a
selected course, as defined by a VOR radial or ILS localiser beam. It also displays glide slope
information, a heading reference and, on many units, a DME range indication.

The instrument consists of a number of discrete elements:

1. The HORIZONTAL SITUATION INDICATOR (HSI) provides a pictorial presentation of
aircraft deviation relative to VOR radials or localiser beams. It also displays glide slope
deviations and gives heading reference with respect to magnetic north.

2. The NAV FLAG is in view when the NAV receiver signal is inadequate. When a NAV flag
is present, the navigation indicator of the autopilot operation is not affected. The pilot
must monitor the navigation indicators for NAV flags to ensure that the Autopilot and/or
Flight Director are tracking valid navigation information.

3. The LUBBER LINE indicates the aircraft magnetic heading on the compass card (10).

6-6 Radio Navigation

VOR Navigation Chapter 6

4. The HEADING WARNING FLAG (HDG) is in view when the heading display is invalid. If
a HDG flag appears and a lateral mode (HDG, NAV, APR or APR BC) is selected, the
autopilot disengages. It is possible to re-engage the autopilot in the basic wings-level
mode along with any vertical mode.

5. The COURSE BEARING POINTER indicates the selected VOR course or localiser
course on the compass card (10). The selected VOR radial or localiser heading remains
set on the compass card when the compass card rotates.

6. The TO/ FROM INDICATOR FLAG indicates direction of the VOR station relative to the
selected course.

7. The DUAL GLIDE SLOPE POINTERS indicate, on the glide slope scale (8), aircraft
displacement from the glide slope beam centre. Glide slope pointers in view indicate the
reception of a usable glide slope signal. The glide slope pointers bias out of view if the
glide slope signal is lost.

8. The GLIDE SLOPE SCALES indicate displacement from the glide slope beam centre. A
glide slope deviation bar displacement of 2 dots, represents full scale (0.7°) deviation
above or below the glide slope beam centre line.

9. The HEADING SELECTOR KNOB positions the heading bug (14) on the compass card
(10). The bug rotates with the compass card.

10. The COMPASS CARD rotates to display the aeroplane’s heading with reference to
lubber line (3) on HSI.

11. The COURSE SELECTOR KNOB positions the course-bearing pointer (5) on the
compass card (10) by rotating the course selector knob.

12. The COURSE DEVIATION BAR (D-BAR) moves laterally to pictorially indicate the
relationship of aircraft to the selected course utilising the centre portion of the omni-
bearing pointer. It indicates degrees of angular displacement from VOR radials and
localiser beams, or displacement in nautical miles from RNAV courses.

13. The COURSE DEVIATION SCALE functions with a course deviation bar displacement of
5 dots representing full scale (VOR = ± 10°, LOC = ± 2.5°; RNAV = 5 nm, RNAV APR =
1 nm) deviation from the beam centre line.

14. The HEADING BUG shows the desired heading, as selected using the heading bug
knob (9).

Radio Navigation 6-7

Chapter 6 VOR Navigation

VOR NAVIGATION

The VOR is a very versatile navigational aid, and forms the basis of the Airway routes structure. It
can assist VFR pilots, as the main navigational aid for enroute navigation, a holding aid, or also
as an approach to landing aid.

Before looking at the different ways of using the VOR, there are a few important things that must
be done prior to using the information indicated by the instruments;

Always make sure the aircraft is within the coverage area of the VOR stations
planned for use. Do this by checking the official AIP (Aeronautical information
publication) or other published enroute manuals.

After having turned the receiver on, dial the frequency of the aid and listen to the
identification signal to ensure the aircraft is receiving the correct and desired station,
and that it is “on the air”.

Make sure that the warning flag (NAV or OFF) is not visible, indicating reception of a
satisfactory signal and that the aircraft installation is working properly.

ESTABLISHING POSITION

Using the VOR to find the aircraft’s present position requires either a VOR in combination with
DME, or two VOR stations. Turning the OBS to centre the needle with a FROM indication
determines the radial on which the aircraft is located. Performing this procedure using two
different VOR stations provides two crossing position lines, good enough to determine a fix
position.

TRACKING A RADIAL INBOUND FROM A PRESENT POSITION

Flying to a VOR station from the aircraft’s present position requires turning the OBS to centre the
CDI needle with a TO indication and flying the heading indicated in the selected course window.
The inbound track is the reciprocal the aircraft’s present radial. In a no-wind condition, a heading
equal to the inbound track takes the aircraft to the VOR with the CDI needle centred.

Any crosswind calls for heading corrections in order to keep the CDI needle centred. Initially
make a small heading correction. If the needle drifts to one side, turn toward the needle, since the
needle actually indicates the position of the desired track. Use only small changes in heading at
any one time and wait for the needle to move back to centre position. This procedure of changing
the heading to stabilise the needle in centre is called “bracketing”. Use small changes of heading
and keep the new heading for a while to await needle movement. If the needle remains still at a
position off centre, the aircraft is using the correct WCA, but still requires a correction in order to
regain the desired track.

INTERCEPTING A RADIAL

To plan an intercept and follow a specific radial, first determine the aircraft’s position in relation to
the desired track. If tracking a radial outbound, set the CDI to the desired radial. CDI deflection
now shows which way to turn in order to make an intercept. The intercept angle depends on
different factors. If ATC wants the pilot to join the new track as soon as possible, the pilot should
make an initial intercept of up to 90° and, when the CDI starts to move, start leading the turn to
establish on the new radial. If the aircraft is close to the VOR station, the needle moves quite fast.
Conversely, if the aircraft is far from the station the needle moves more slowly. Aircraft speed
also affects the needle movement.

6-8 Radio Navigation

VOR Navigation Chapter 6

If there are no restrictions regarding the intercept, an intercept angle of 30° or 45° is normally a
good alternative. This is part of a pilot’s practical training.

If the intent is to intercept a radial and track it inbound, the procedure is as above, except that the
pilot sets the reciprocal of the radial, which is the inbound course. The procedure is otherwise the
same. Upon intercepting the selected course, the procedure is the same as described earlier.

If tracking TO a VOR station and the aircraft is to continue on the same course after passing the
station, needle-movement becomes very erratic when close to the station. The TO/FROM flags
flicker during the passage of the station and the warning flag (NAV/ OFF) appears momentarily.
This is due to the cone of confusion that is directly overhead the VOR.

When tracking along an airway between two VORs, the normal procedure is to switch from
tracking FROM one VOR to tracking TO the next VOR when midway between the two facilities.
Sometimes the changeover point is specified elsewhere on the route segment. This is usually
because of signals being restricted by terrain or by frequency interference. When this is the case,
the changeover point will be specified on the appropriate instrument chart.

When tracking from one VOR to another, the published radial for the inbound track to one VOR
should match the outbound track or radial from the other, but this is not always so. Radials are
magnetic tracks from the VOR and the directions of the radials depend on the magnetic variation
at the different VORs. The difference between the corresponding radials equals the difference in
variation at the position of the two VORs. At high latitudes, convergence of the meridians also
contributes noticeable differences.

VOR APPROACHES

VOR approaches may be published for an approach to an aerodrome. The example shown on
the next page is for Cranfield.

Radio Navigation 6-9

Chapter 6 VOR Navigation

6-10 Radio Navigation

VOR Navigation Chapter 6

PART 1 – ADMINISTRATION

22 180° 20 EGTC CRANFIELD
CAT A,B,C
080° 270° VOR RWY 22
AD ELEV THR ELEV TRANSITION ALT VAR CFD 116.50
22 20 4°W
360° 364FT 357FT 3000 000 20W

MSA 25 NM ARP APPROACH TOWER
122.850 122.850

000 50W 000 40W 000 30W

As with the NDB, the top part of the chart contains the details of:

¾ The airfield and the type of approach
¾ The categories of aircraft that can fly the approach
¾ The Minimum Safe Altitude

PART 2 – PLAN VIEW

Part 2 shows a plan view of the airfield out to 10 nm. Restricted areas, danger areas, and
significant obstacles are marked. In the centre of the plan is the airfield with a plan view of the
track to be flown.

Radio Navigation 6-11

Chapter 6 VOR Navigation
PART 3 – ELEVATION VIEW

The elevation view is used in co-ordination with the plan view. Timings and altitudes to fly are
included (as with all charts QFE heights appear in brackets). The MAPt is the IAF and a
commentary of the missed approach is given in a box to the left of the diagram.

PART 4 – NOTES

The notes given include:
¾ The rate of descent for a given groundspeed
¾ The OCA for the approach
¾ The OCA for visual Manoeuvring (Circling)

The holding procedure for the approach is included. The notes box at the bottom gives the
restrictions applicable to the approach.

6-12 Radio Navigation