Jeppesen-Radio-Navigation

INTRODUCTION

DME is a secondary radar providing the pilot with an accurate slant range from a ground
transmitter. Normally paired with VOR, the combination provides the standard for ICAO short-
range navigation systems (know as rho-theta). More recent uses see the DME paired with ILS
and MLS to give range from touchdown during a precision approach.

PRINCIPLE OF OPERATION

The system works on the principle of secondary radar:

¾ The interrogator on board the aircraft transmits an interrogation signal.
¾ The ground-based transponder (transponder meaning a transmitter that is

responding to an interrogation) transmits to the aircraft.

The interrogation signal from the aircraft and the response are on different frequencies.

Aircraft transmits the
interrogation signal

DME Ground Aerial Replies

Frequency
UHF – 960 to 1215 MHz

Emission Characteristics
P0N

Radio Navigation 7-1

Chapter 7 Distance Measuring Equipment (DME)

AIRCRAFT EQUIPMENT

The airborne unit (interrogator) consists of:

¾ An omni-directional blade aerial
¾ A transmitter
¾ A receiver
¾ A time measuring device
¾ A tracking unit

The interrogator transmits pulse pairs on the selected frequency. These pulse pairs are spaced
by either 12µ seconds (X channel) or 36µ seconds (Y channel). Discussion regarding the X and Y
channels occurs later.

Note: For the X channel, the transponder replies with a pulse pair spacing of 12µ
seconds. The Y channel reply is at 30µ seconds.

When the pilot switches the equipment on, or when a new DME channel is selected, the pulse
pairs transmit at 150 pulse pairs per second (pps). This is the search mode. The equipment stays
in search mode until the equipment either:

¾ Locks on (normally 4 to 5 seconds)
¾ 15 000 pulse pairs have been transmitted

If lock-on occurs, then the transmitter reduces the pulse recurrence frequency (PRF) of pulse
pairs to 24 to 30 pps, known as tracking mode. If the system transmits 15 000 pulse pairs, the
PRF drops to 60 pps until the system locks on.

TRANSPONDER

The ground transponder consists of a receiver and a transmitter. When it detects an interrogating
signal, it transmits a response after a delay of either 50 µs or 74 µs, depending on the channel.
Response is at a different frequency to that received, with the transponder being capable of
generating up to 2700 pps. When replying to a signal, the ground transponder replies at a rate of
24 – 30 pps.

FREQUENCY ALLOCATION

Interrogator and transponder operating frequencies are grouped into pairs, the two frequencies
being 63 MHz apart. The airborne interrogator uses frequencies from 1025 MHz to 1150 MHz for
transmissions, while the ground-based transponder answers on frequencies in two groups, 962
MHz to 1024 MHz (low) and from 1051 MHz to 1213 MHz (high).

The range of the interrogator frequencies from 1025 to 1150 provides 126 channels. To double
the available response channels to 252, the response frequencies are split into groups 63 MHz
higher and lower than the corresponding interrogation channel. For the interrogation channels
from 1 to 63, an X beacon would prompt a response 63 MHz lower and a Y beacon, 63 MHz
higher. For interrogation channels 64 to 126, the X beacon would obtain a response 63 MHz
higher and the Y beacon, 63 MHz lower.

To differentiate between the two beacon types, the interrogation pulse pairs are separated by
12µs for an X beacon and 36µs for a Y beacon. The response to an X beacon has the same 12µs

7-2 Radio Navigation

Distance Measuring Equipment (DME) Chapter 7

separation whereas the response to a Y beacon has a 30µs separation. The delay at the
transponder (50µs for the X beacon and 74µs for the Y beacon) provides further differentiation.

ICAO recommends the pairing of DME channels with VOR/ILS or MLS. Consequently, a VOR on
112.30 MHz is always paired with the DME on Channel 70X (1094 MHz interrogation – 1157 MHz
reply). A VOR on 112.35 MHz would pair with the DME on Channel 70Y (1094 MHz interrogation
– 1031 MHz reply). Each DME channel has an identifiable number and a letter (X or Y). The
following table is an illustration of some of the available channels with their paired frequencies.

DME Channel VOR/ILS/MLS
Paired frequency
20X
20Y 108.3
21X 108.35
21Y 108.4
108.45
-
70X -
70Y 112.3
112.35
-
126X -
126Y 117.9
117.95

The channel numbers and paired frequencies can be found in the relevant communications
documents.

Pilots never select a DME frequency because of the pairing. Even though the system works in the
UHF band, the frequency is selected by tuning the paired VHF frequency. The major reason for
this procedure is to reduce the workload on the flight deck.

JITTERED PRF

If two aircraft transmit to a DME at the same time, the replies are on the same frequency. If both
signals received by the aircraft are the same, how can the receiver differentiate the correct reply?
To which aircraft is each reply being directed?

Radio Navigation 7-3

Chapter 7 Distance Measuring Equipment (DME)

Same frequency
used for the reply

?

The equipment in the aircraft “jitters” the PRF before transmission. This random PRF is unique to
the aircraft. When the ground station replies, it manufactures exactly the same PRF reply for the
aircraft. Any reply taken by the airborne equipment that does not match the PRF of the initial
transmission is rejected. The responder now responds to the new rate, and since the interrogator
PRF randomly varies, only the responses to that interrogation will have the same random
variation of PRF. Within the airborne receiver, the tracking unit looks for responses around the
anticipated time interval compatible with the current range from the ground responder. This
effectively creates a gate, and only responses that arrive within that gate receive consideration.
The receiver then determines a match between the PRF of the response and those that were
transmitted. Once matched, the time difference is measured, and, allowing for responder fixed
delay, a range is derived. This is tracking mode.

REFLECTED TRANSMISSIONS

The advantage of using secondary radar is that the aircraft equipment does not process reflected
transmissions from the ground or cloud, as the frequency of reply is incorrect.

MEMORY

Interruption of the responding signals whilst the system is in tracking mode activates a memory
circuit. The system holds the following:

¾ The last measured range value
¾ The receiver gate at the last measured time interval

The system returns to the search mode after holding the memory mode for 8 to 10 seconds.

BEACON SATURATION

The ground-based responder beacon has a limit of a maximum PRF of 2700 pps and
interrogations occur at 24 - 30 pps (27 pps average). This means that one DME beacon can
handle up to 100 aircraft.

7-4 Radio Navigation

Distance Measuring Equipment (DME) Chapter 7

Saturation Gain
Level

Normal Gain Level

AB C D

The ground transponder has a set gain level that a signal must exceed in order to receive a reply.
This ensures that receiver noise or other weak returns are ignored. In the diagram above:

Signal A is too weak to break the normal gain level and so receives no reply.

Signals B & D exceed the normal gain level and consequently receive a reply from the
transponder as long as the beacon is not saturated.

Signal C exceeds the normal gain level the most. If the beacon is saturated, the normal
gain level elevates to a saturation gain level and only the strongest 100 signals receive
replies.

CO-LOCATION OF BEACONS

As stated in the introduction, the DME is usually paired with a VOR to provide the primary short
range fixing required by ICAO. Where a VOR and DME transmit the same callsign in a
synchronised manner the stations are considered “associated”. This means:

¾ The VOR and DME transmitter are co-located.

¾ The aerials are a maximum of 100 ft apart in a terminal area that uses the VOR/DME
for approach purposes.

¾ The aerials are at a maximum of 2000 ft apart in instances not using VOR and DME
for approach purposes.

Where co-location occurs, the identification is synchronised and transmitted 7½ seconds apart. In
a 30 second period:

¾ The VOR idents 3 times
¾ The DME idents once

Where a VOR and DME serve the same area (within 7 nm) they may be frequency paired but the
DME generally uses Z as the last letter of the ident.

¾ VOR ident MAC
¾ DME ident MAZ

Where a VOR and TACAN are co-located, the system is called VORTAC. The VOR uses the
DME portion of the TACAN.

Radio Navigation 7-5

Chapter 7 Distance Measuring Equipment (DME)

Where DME works with an ILS or MLS, the 50 µsecond time delay is gradually reduced to a
minimum to allow the DME to read zero when the aircraft passes the runway threshold.

SLANT RANGE

All aircraft displays show the value of the measured slant range while some contain an arithmetic
unit, which calculates the instant ground speed and time to the station. It is possible on most
modern installations to select “GS” or “TIME” for this purpose. Some indicators show distance,
ground speed, and time simultaneously.

It is important to note that the indications of ground speed and time are only correct when flying
directly toward the ground station. If the aircraft is flown in any other direction, both the DME
indicated ground speed and time to the station would be too low. In this case, only slant distance
is correct.

DME NAVIGATION

All navigational aids provide the pilot with a position line, depending on the type of radio aid. The
position line resulting from the DME is a circle. When an aircraft’s DME indicator shows 55 nm,
the pilot knows the aircraft is at a slant range of 55 nm from the station, but not whether it is
south, east, north, or west of the station.

As a result of this, the position line from one DME station alone is of little help. A radial from a
VOR at some distance from the DME station provides a second position line that could intersect
the DME circle at two places. This results in an ambiguity situation as seen in the diagram below.

If the VOR and DME are associated, the pilot has one clearly-defined fix position.
As an approach aid, the DME provides, together with the tracking facility, positions like initial
approach fix (IAF), final approach fix (FAF), and missed approach point (MAPt).

7-6 Radio Navigation

Distance Measuring Equipment (DME) Chapter 7

DME PROCEDURES

A DME procedure is one that is published for use with a particular ground facility. The most
common type of DME procedure is flying the arc. This procedure requires the pilot to maintain a
specific range from a DME, generally between two stated VOR radials.

SLANT RANGE

The DME indicates a slant range, that is, the straight line from the aircraft to the ground station,
not the distance along the ground. The true range can be calculated by using Pythagoras’s
theorem.

A2 + B2 = C2

Example An aircraft is flying at 45 000 ft with an indicated DME of 175 nm.
What is the true range?

45 000 ft = 7.4 nm
Range = 1752 – 7.42

Distance = 174.84 nm

This results in a slant range error of 0.16 nm or 0.09%, which is
negligible. Problems start when the aircraft is closer than 20 nm to the
beacon.

Example The aircraft is at 30 000 ft, with an indicated DME of 20 nm.
What is the true range?

30 000 ft = 4.9 nm
Ground distance = 19.39 nm
The error is 0.61 nm or 3%

The slant error is almost negligible at long distances, but increases both with altitude and with
decreasing DME distance. When using a DME at less than 20 nm range, a pilot must apply a
slant range correction.

Radio Navigation 7-7

Chapter 7 Distance Measuring Equipment (DME)

FLIGHT OVERHEAD THE DME

When an aircraft passes directly overhead a DME station, the DME indicates the altitude of the
aircraft in nautical miles. For instance, if the aircraft passes at an altitude of 40 000 ft, the
indication is about 6.6 nm.

There is a cone of silence directly above the ground station. However, the arithmetic unit in the
aircraft remembers the last computed data and continues to indicate the altitude for some time.

FAILURE INDICATIONS

If the system receives no signals of an acceptable strength it unlocks and only regains the
tracking mode upon receiving the correct signal of an acceptable strength. The unlock condition is
indicated by:

¾ An off flag on the rotary indicator
¾ A red bar across the face of the digital indicator

Failure indications also appear upon switching the equipment on, and:

¾ No signal is being received
¾ The received signal is below the minimum strength required
¾ The aircraft is out of range of the transponder

Switching the equipment off also results in an off indication.

ACCURACY

The DME is extremely accurate. ICAO prescribes a maximum system error of
± (0.25 nm + 1.25% of the slant range) on 95% of occasions.

7-8 Radio Navigation

Chapter 8 Instrument Landing System (ILS)

INTRODUCTION

The instrument landing system is the primary precision approach facility for civil aviation. The
definition of a precision approach is an approach that provides both glideslope and track
guidance. ILS signals are transmitted continuously, and provide pilot-interpreted approach
guidance. When flying the ILS approach, the ILS provides approach guidance for the descent to
the decision height (DH), at which point the pilot makes the final decision to land or go around. All
installations must conform to the standards laid down in ICAO Annex 10 and must include
allocation of an appropriate performance category. Exceptions to these standards are published
in NOTAMs.

Many ILS installations use an associated DME to provide a more accurate and continuous
ranging facility than that provided by the markers. A low power NDB, known as a locator beacon,
may also complement ILS installations to provide guidance during the intermediate approach and
into the final approach path marked by the ILS. The ideal flight path on an ILS approach is where
the localiser plane and the glide slope plane intersect. To fly this flight path, the pilot follows the
ILS cockpit indications.

PRINCIPLE OF OPERATION

The ILS consists of the following components:

Localiser
The localiser transmitter and aerial system provide the azimuth guidance along the
extended runway centerline.

Glidepath
The glidepath and its aerial system provide the approach guidance in the vertical pane.

Marker Beacons
Separate beacons (up to three) along the approach path provide the aircraft with range
check points on the approach (discussed in Chapter 9).

Radio Navigation 8-1

Chapter 8 Instrument Landing System (ILS)

OUTER MIDDLE
MARKER MARKER

5 MILES INNER
MARKER

3250 FT . GLIDEPATH TRANSMITTER
(MAY BE EITHER SIDE)
RUNWAY

LOCALISER
TRANSMITTER

Frequency
The localiser and glidepath operate on separate frequencies:

Localiser
VHF – 108 to 112 MHz using the odd first decimals and the odd first decimals plus 50
kHz: 108.10 MHz, 108.15 MHz, 108.30 MHz, 108.35 MHz, etc.

Glidepath
UHF – 329.15 MHz to 335 MHz at 150 kHz spacing: 329.15 MHz, 329.3 MHz, 329.45
MHz, etc.
(The frequency band allocated is 328.6 MHz to 335.4 MHz. It is not necessary to
remember these figures.)

The localiser and glidepath frequencies are paired. The glidepath is automatically selected upon
selection of the ILS VHF frequency (the localiser).

Marker Beacons VHF – 75 MHz

Emission Characteristics
A8W

The localiser and marker beacons also radiate an A2A identifier.

LOCALISER

The localiser transmitter aerial is located in line with the runway centre line, at a distance of
approximately 300 metres from the “up-wind” end of the runway. The aerial, which is of frangible
construction, may be 20 metres wide and 3 metres high, and consists of a number of dipole and
reflector elements. The radio signal transmitted by the localiser aerial produces a composite field
pattern consisting of two overlapping lobes. The two lobes are transmitted on a single ILS
frequency, but are modulated differently so that the receiver can distinguish between them.

8-2 Radio Navigation

Instrument Landing System (ILS) Chapter8

COURSE SIDE LOBES MAIN LOBE APPROX
PRODUCING SPURIOUS 20°
150 Hz
EQUISIGNALS RUNWAY EXTENDED CENTRE LINE NIL DDM

1000 FT 90 Hz
RUNWAY

THE COURSE RADIATION PATTERN

The lobe on the left-hand side of the approach has a 90 Hz tone. The lobe on the right hand side
has a 150 Hz tone.

A receiver located to the left of the centre line detects more of the 90 Hz modulation tone and
relatively less of the 150 Hz modulation. This difference is called DDM (Difference in Depth of
Modulation) and causes the vertical indicator needle of the ILS to indicate that the centerline is to
the left. Conversely, a receiver right of the centre line receives more 150 Hz than 90 Hz
modulation and, as a result, the needle indicates that a correction to the left is necessary.

With the needle in the centre the difference in depth of modulation is zero.

Because the beam of the ILS localiser is very directional, unwanted side lobes are produced. To
ensure that the aircraft does not pick up a false localiser signal the basic pattern shown above is
covered with a clearance pattern. This changes the localiser signal to the one shown below.

CLEARANCE
SIDE LOBES

RUNWAY NIL DDM

APPROX 70° APPROX 20°

COMBINED COURSE AND CLEARANCE RADIATION PATTERNS

Radio Navigation 8-3

Chapter 8 Instrument Landing System (ILS)

LOCALISER COVERAGE

The ILS localiser covers:
¾ ± 10° of the centreline to 25 nm range
¾ ± 35° of the centreline to 17 nm range

Centre of Localiser 25° 10° Course Line
Antenna System 10°
17 NM
25 NM

25°

Localisers paired with steep-angle glideslopes provide coverage from the centre of the localiser to
distances of:

¾ ± 10° of the centreline to 18 nm range
¾ ± 35° of the centreline to 10 nm range

P

300 m

7°

The coverage of the localiser in elevation is determined as follows:

¾ First calculate point “P”, which is the higher of a point 600 metres above the threshold
and a point 300 metres above the highest point within the approach area.

¾ Connect this point to the threshold.
¾ Draw a line 7° above the horizontal.
¾ The resulting shaded area corresponds to the localiser vertical coverage within its

horizontal coverage.

The vertical coverage extends from ¾° above the surface up to 7°.

The maximum field strength is directed along the centreline out to a range of 10 nm. If an aircraft
is outside the coverage of the course and clearance patterns, it can lead to receipt of false
localiser signals. In some cases these signals, caused by side lobes, can give reverse
indications.

8-4 Radio Navigation

Instrument Landing System (ILS) Chapter8

The localiser signals are protected out to a range of 25 nm and up to a height of 6250 ft. The
localiser is checked for accuracy out to a range of 10 nm.

The above criteria should enable the aircraft to undertake the manoeuvres that are necessary to
capture the localiser course at the outer limit of the coverage pattern and to carry out the
subsequent descent on the glide path.

GLIDEPATH

Placement of the glidepath aerial is 300 metres upwind from the threshold and 150 metres from
the centre line. Placement is at the optimum touch down point at which the extension of the glide
path intersects the runway. This ensures adequate wheel clearance over the threshold and over
any other object or terrain during landing approach.

Glide path transmission is in the UHF band on 40 spot frequencies from 329.15 to 335 MHz. UHF
is used to produce an accurate beam. The transmission is beamed in the vertical plane in two
lobes similar to that of the localiser. The upper lobe has a 90 Hz modulation, while the lower lobe
has a 150 Hz modulation.

The DDM (Difference in Depth of Modulation) energises the horizontal needle of the instrument,
in order to indicate whether the aircraft is in the 90 Hz lobe or in the 150 Hz lobe. In this way, it
gives the position of the centre line of the glide path. The line along which the two modulations
are equal in depth, defines the centre line of the glide path. It is generally 3° from the horizontal,
but could be adjusted between 2° and 4° to suit the particular local conditions. A glide slope much
in excess of 3° requires a high rate of descent and is not common in public transport operations.

In the vicinity of the landing threshold, the glide path becomes curved and gradually flattens. This
is of consequence when considering a fully automatic landing. It is one of the reasons a Category
III landing requires the use of a radio altimeter. The siting of the glide path aerial and the choice
of the glide path angle depend on many interrelated factors:

¾ Acceptable rates of descent and approach speeds for aircraft using the airfields
¾ Position of obstacles and obstacle clearance limits resulting
¾ Horizontal coverage
¾ Technical siting problems
¾ The desirability of attaining the ILS reference datum 50 ft above the threshold on the

centre line
¾ Runway length

Radio Navigation 8-5

Chapter 8 Instrument Landing System (ILS)

GLIDEPATH COVERAGE

The coverage in azimuth extends 8 degrees on either side of the localiser centreline, to a
distance of 10 nm.

Runway 8°
Centre Line

8°

10 NM

The coverage in the vertical plane extends from 0.45 θ to 1.75 θ where θ is the nominal glidepath
angle above the surface (1.35° to 5.25° for a 3° glidepath). Remember that correct signals are
guaranteed only within the approved coverage zones, and false indications can be received
outside these zones.

Runway 1.75 θ θ
0.45 θ

(or to such lower angle, down to 0.30 θ, as required to safeguard the promilgated Glide Path Procedure).

Use of the glidepath below 0.45 θ that is below 1500 ft QFE at 10 nm range (for a 3° glidepath)
should only be attempted when the Promulgated Glide Path Intercept Procedures requires the
aircraft to fly at this level. The aircraft should never fly below 0.3 θ (0.9° for a 3° glidepath) which
is 1000 ft at 10 nm range. When the procedure is designed to join a glidepath from above the pilot
must bear in mind that a false glidepath may exist at approximately 2 θ (discussed later in this
chapter).

AIRBORNE EQUIPMENT

The ILS airborne equipment consists of:

¾ A frequency control box
¾ A VHF localiser receiver
¾ A UHF glide path receiver
¾ A 75 MHz marker beacon receiver
¾ An ILS indicator

The installation includes three separate aerials, one for each receiver.

Since all the marker beacons transmit on the same frequency, there is no need for a marker
beacon control box. An audio coded signal automatically identifies markers, by the related
transmission audio tone and a coloured light.

FREQUENCY PAIRING

Both localiser and glidepath are tuned using the same unit. This is possible because of
international agreement under ICAO standards. Every localiser frequency has a paired glidepath
frequency. Since the frequencies come in pairs, only the correct localiser frequency needs to be
selected. The glide path receiver is then automatically tuned to the appropriate UHF channel. The
VHF navigation receiver panel tunes the ILS frequency.

8-6 Radio Navigation

Instrument Landing System (ILS) Chapter8

LOCALISER AND GLIDEPATH RECEIVERS

Once the localiser frequency has been tuned, both the localiser and the glide path receiver are
activated, and they send the received signal to the indicator. The two receivers are similar to each
other in that they both detect the modulations on the carrier wave. The modulations (90 Hz and
150 Hz) are compared and the Difference in Depth of Modulation (DDM) is measured. This
output, which is in the form of a DC electrical signal, is used to drive the pointers on the display. If
the aeroplane is on the centre line, the 90 Hz and the 150 Hz signals will have the same
amplitude and the indicator needle will be centred. If the aircraft is not on the centre line, one
signal will be stronger than the other and the resultant DC output energises the needle
displacement. To indicate whether the received signals are adequate or not, a warning system is
incorporated into the receivers. A red warning flag appears on the ILS indicator if the signal is not
reliable. There are separate flags for the localiser and for the glide path signals.

ILS INDICATOR

The indicator consists of a dial, similar to the VOR indicator, but with an additional needle.
Localiser signals displace the vertical needle, while glide path signals displace the horizontal
needle. The same indicator normally provides for both ILS and VOR guidance.

When the localiser receiver detects that the 150 Hz signal is stronger, it feeds a voltage to the
localiser needle moving it to the left. This indicates that the localiser centre line is to the left of the
aircraft on approach. If the 90 Hz signal is stronger, the voltage moves the localiser needle to the
right, indicating that the pilot must turn right to get back on centre line.

Full-scale deflection occurs when the aircraft is displaced 2.5° or more from the centre line. In
other words, when tuned to a localiser frequency, the indicator is four times more sensitive than it
is when tuned to a VOR (full scale deflection for a VOR corresponds to 10°). Unlike the VOR,
which gives the pilot a choice of 360 radials using the omni-bearing selector (OBS), the localiser
course is a single, fixed beam. Once a localiser frequency is selected, all the needle indications
refer exclusively to the localiser centre line. Consequently, the fact that the instrument also
contains the OBS selector has absolutely no significance and rotating it has no effect on the ILS
indications. However, pilots should always turn the OBS to the correct inbound course for the
sake of reference when flying the ILS. The localiser indicator does not provide any heading
information. It only gives information regarding the geographical position of the aircraft. It displays
how many degrees the aircraft is displaced from the localiser centre line.

Localiser Pointer
Shows 3 Dots
Fly Right

Warning Flag
Appears if Signal
is Unusable

Radio Navigation 8-7

Chapter 8 Instrument Landing System (ILS)

The diagram on the previous page shows only the vertical localiser needle. A full illustration with
the glidepath needle is below. The important features of the localiser needle are:

¾ Full scale deflection is 2.5°.
¾ Each dot represents a deviation of 0.5°.
¾ A warning flag appears when the signal is unusable.

The horizontal needle of the indicator indicates the position of the glide path, relative to the
aircraft. The vertical glide path scale on the usual cockpit indicator consists of 5 dots above and
below the central position.

Glide Path Pointer
Shows 21/2 Dots
Fly Up

Warning Flag
Appears if Signal
is Unusable

The full instrument has the horizontal glidepath needle with important features being:

¾ Full scale deflection is only 0.7°.
¾ Each dot represents a deviation of 0.14°.
¾ As with the localiser, a warning flag appears if the signal is unusable.

If the 90 Hz signal is the stronger one, the aircraft is above the glide path and the indicator needle
deflects down. This indicates that the aircraft must fly down to recapture the glide path.
Conversely, if the receiver detects a stronger 150 Hz signal, the needle moves up, known as a
fly-up indication. The glide path has a total depth of 1½°, making the glide path indicator
considerably more sensitive than the localiser indicator. This means that for a full-scale deflection
of the needle the aircraft must be at least 0.7° above or below the glide path. A “half-scale”
(2½ dots) fly-up indication indicates the maximum safe deviation below the glide path.

Deviation from the glide path is referred to in terms of dots instead of degrees, in that there are 5
dots above it and 5 dots below it on the instrument. Very accurate control is necessary when
flying down a glide path. A more sophisticated instrument, used to fly an ILS approach, is the
horizontal situation indicator.

HORIZONTAL SITUATION INDICATOR (HSI)

With the HSI, the course arrow must be manually aligned with the localiser inbound course. Using
the deviation bar provides localiser guidance. A scale alongside the instrument provides the glide
path position.

8-8 Radio Navigation

Instrument Landing System (ILS) Chapter8

ILS ACCURACY

Up to now, the ILS has been viewed as an instrument that provides assistance in landing
approaches. This means that the ILS provides guidance down to a specified height above the
threshold. If the visibility at this point is good enough for landing, then the pilot may legally land
the aircraft. If the existing weather does not permit the pilot to see the visual references at the
prescribed minima, the aircraft cannot land. Operators were not happy about the prospect of
delaying a flight or wasting time and fuel while holding overhead an aerodrome and waiting for
the weather to clear. This required an improved ILS system. Obtaining these improvements
requires considering certain limitations of the system. The main problems come from bends and
scallops in the beams.

Bending

Scalloping

¾ Bending of the beam is a single angular displacement from the approach path.
¾ Scalloping is where the guidance beam direction varies from side to side of the

intended approach path.

Reflections from obstacles on and around the aerodrome produce these bends, such as airport
structures, vehicles, aircraft flying overhead the localiser aerial, etc. The ability to use ILS
installations for fully automatic landing has necessitated that ICAO lay down stringent
requirements and that both ground and airborne equipment are constantly improved. These
requirements concern the quality of the transmitted signal data and the suppression of bending of
the radio beams by improvement in aerial design in order to reduce unwanted reflections.

Radio Navigation 8-9

Chapter 8 Instrument Landing System (ILS)

FALSE BEAMS

Even with strict monitoring of all the ILS ground equipment, there are unavoidable factors to
consider. The first of these is false signals. This problem is particularly associated with the glide
path transmission and occurs because of the aerial’s propagation characteristics. The number of
such false glidepaths produced at any ILS site depends on several factors, such as the design of
the aerial, transmission power, obstacles and other such factors. These false glide paths occur at
multiples of the nominal glide path. As a result, the first occurs at approximately 6° above the
horizontal for a glide path of 3°. There is never a false glide path below the true one.
Consequently, the recommended practice when carrying out an ILS approach is to lock onto the
localiser first and then intercept the glide path from below.

VERTICAL COVERAGE
AT LEAST FROM 0.45 η BELOW
TO 1.75 ηABOVE HORIZONTAL

SIDE-LOBES PRODUCING
FALSE GLIDE PATHS

GP NOT CORRECTLY G.9L10.5HG0zLHIPDzREPERPDEAODTMHOINM- ANINTILAETDSEDSM
DELINEATED IN η BETWEEN 2° AND 4°
FINAL STAGES (NORMALLY 3°)

ILS REFERENCE

RUNWAY

50 FT APPROX. ABOVE THRESHOLD
1000 FT

GP AERIAL OFFSET
A SAFE DISTANCE
FROM RUNWAY

SIMPLIFIED DIAGRAM OF GLIDE PATH RADIATION PATTERN

Outside the localiser ‘protected area’, it is possible to encounter false localiser beams. The angle
from the actual centre line to the false beams varies with the number of aerial elements. Six
elements produce a false beam at approximately 40° and 12 elements at 50° to 60°.

LOCALISER BACK BEAM

Some localisers transmit in the opposite direction of the ILS inbound course and the signal is
receivable when flying behind the aerial. This signal, called the back beam, should normally not
be used.

Some transmitters, however, are designed to radiate a back beam. This beam can provide a back
course approach to the reciprocal runway. Note that when using a back course pilots do not have
the benefit of a glide path. Usually, back beams are less accurate than front beams. They are not
checked for accuracy unless they are a part of published procedure. Do not use a back beam
unless it has a published procedure.

8-10 Radio Navigation

Instrument Landing System (ILS) Chapter8

Note that, when flying the localiser back beam approach, pilots must be very careful in
interpreting the course selector. When using a conventional ILS indicator for a back beam
approach, the localiser needle gives a “fly left” indication when the aircraft is left of the centre line
and vice versa. In other words, the pilot experiences a reverse sensing. Such reverse sensing
occurs regardless of course selector setting.

Conversely, when flying with an HSI display, the pilot receives normal indications (i.e. “fly left”
when the aircraft is to the right of the centre line) if the course selector is set to inbound track on
the localiser front beam. When the course selector is set to the back beam course, it provides
reverse sensing.

Radio Navigation 8-11

Chapter 8 Instrument Landing System (ILS)

8-12 Radio Navigation

Instrument Landing System (ILS) Chapter8

The graphic above shows a plate for a back beam approach at Lista in Norway. Back beam
procedures are not common in Europe.

ILS PERFORMANCE CATEGORIES

A system of facilities performance categories defines the capability of a particular ILS system.
These categories state that the ILS must be capable of providing guidance from the coverage
limit as follows for:

Category I to a height of 60 metres above the horizontal plane containing the
threshold

Category II to a height of 15 metres above the horizontal plane containing the
threshold

Category III with the aid of ancillary equipment when necessary, down to and along
the runway.

ILS OPERATIONAL PERFORMANCE CATEGORIES

A similar categorisation exists for operational purposes, and it is to these limits that the pilot flies.
These categories establish practical weather minima for an approach. Pilots must be familiar with
the following ILS operational minima.

ILS Cat I: DH down to 200 ft, RVR 550 metres
ILS Cat II:
ILS Cat IIIA: DH down to 100 ft, RVR 350 m
ILS Cat IIIB:
ILS Cat IIIC: DH 0 to 100 ft, RVR 200 m

DH 0 to 50 ft, RVR 50 – 200 m

No external visual reference

When flying a Category I approach, reference is to the barometric altitude, but for a Category II or
III approach reference is to the radio altimeter.

In a Category I ILS approach, also called CAT I, the pilot may manually follow the ILS indications
down to the decision height (DH), which is not less than 200 ft. At that point, if visual contact has
been established, the landing can be made. If not, the pilot must initiate a go-around. Note that
the ILS coverage, described earlier in this chapter, refers to ILS Category I.

Category II and III requirements are more stringent. ILS CAT I, although still widely used, is
gradually being replaced by CAT II & CAT III facilities. On a CAT II approach, the aircraft must be
flown by the autopilot down to the DH. From there, if visual contact has been made, the pilots can
make the landing. Otherwise, they must initiate a go-around. A CAT II approach can only be
made at a Category II certified airport. The localiser and glide path transmissions must meet
stricter standards than for a CAT I system. The transmissions must be monitored and failure
indications must be available in the control tower. In addition, two RVR (Runway Visual Range)
transmissometers must be operating on the runway, and extensive airport lighting requirements
must be met. A CAT II approach requires an aeroplane with CAT II equipment certified by the
regulatory authority. Additionally, special training programmes must be certified and conducted
for the flight crews.

Category III approaches require the same criteria as those for CAT II but with additional, more
stringent, requirements. This is because the aircraft must have guidance all the way down to the
runway. The autopilot must perform the CAT III approach.

Radio Navigation 8-13

Chapter 8 Instrument Landing System (ILS)

The performance of the airborne equipment must match the improvement in the ground
equipment. To this end, operational performance categories have been established. They
correspond to the facility performance categories.

In accordance with its airborne equipment, an aircraft is certified in one of the listed categories.
Naturally, aircraft and airport performance categories are required to conduct any of the
approaches. Remember that in order to perform a CAT II or CAT III approach, not only must the
airport and the aircraft be properly equipped and checked, but so must the flight crew. Unqualified
and untrained pilots may not carry out a precision approach.

PROTECTION RANGE AND MONITORING

National and regional frequency plans have been established by the ICAO and are adhered to by
contracting states. These plans take many factors into account, such as the sensitivity and
selectivity of receivers, and the channel spacing and geographical proximity of transmitters. In
this way, interference between facilities is reduced to negligible proportions. Within Europe, the
congested radio frequencies have resulted in FM transmissions from aerials that are close
enough to allow side band interference to spill over into the ILS frequencies. These can cause
random displacement of the localiser, so be aware. Monitoring equipment automatically and
continuously checks both localiser and glide path transmitters. The monitors take action
whenever they sense a shift or change in the basic transmission. If the ILS is Category II or III the
transmissions are stopped within 2 seconds. If Category I, the transmissions are stopped within 6
seconds.

The localiser and glidepath monitors take action when:

¾ The mean course line shifts by: ± 35 ft
Category I: ± 25 ft
Category II: ± 10 ft
Category III:

¾ The glidepath angle changes by > 0.075θ 0.225°
3° Glidepath:

¾ A reduction in power of 50% or more in any transmission

If a monitor operates, the standby unit will be used. Before this happens:

¾ All radiation stops

¾ The identification stops

¾ For a Category II or III operation, the system may allow degradation to a lower
category operation

8-14 Radio Navigation

Instrument Landing System (ILS) Chapter8

USE OF ILS

ILS IDENTIFICATION

Because the localiser and glide path frequencies are paired, selecting a localiser frequency
automatically activates the glide path receiver so that the corresponding glide path signal is
automatically received. The ILS must be identified before use. The identification is transmitted on
the localiser frequency and is amplitude modulated by a 1020 Hz (A2A transmission) tone. A two
or three letter Morse code transmitted at a rate of seven words per minute. The letter “I” may
precede the identification. When an ILS is undergoing maintenance or is being used for test
purposes, the identification is completely removed and a continuous tone replaces the coding. In
both cases, the ILS must not be used.

FLYING THE LOCALISER

When initiating the approach, the localiser indicator shows the position of the aircraft in relation to
the centre line and provides no heading information. Thus the term “follow the needle” is only
valid when flying inbound within the coverage area. For an aircraft on approach, the localiser
needle indicates which way the aeroplane should move to regain the centre line. If the localiser
needle is to the right, then the aircraft should be flown to the right. To regain the centre line, fly
toward the needle. The aim is to fly a heading that maintains the aircraft on the centre line. If a
crosswind exists, a wind correction angle (WCA) is required and the aircraft heading differs
slightly from the published inbound course.

The localiser beam narrows as the aircraft approaches the runway. As a result, corrections
should become smaller and smaller.

TOTAL COURSE WIDTH 5 DOTS OR FULL SCALE
IS 700 FT WIDE AT THRESHOLD DEFLECTION

1000 FT NOMINAL

LOCALIZER
TRANSMITTER

(100 WATTS)

4° ON LONG RUNWAY
6° ON SHORT RUNWAY

FLYING THE GLIDEPATH

The horizontal glide path needle should be flown in the same way as the localiser needle. To
regain the glide path, fly toward the needle. The needle is the pilot’s glide path. If the glide path
needle is below centre, the aircraft is too high and requires a steeper descent. Remember that
the closer the aircraft is to the threshold, the more dangerous a high descent rate is. If the aircraft
is not properly established on the glide path, stop the approach. Do not continue to hunt the glide,
and if in doubt, carry out a missed approach.

With an angular depth of only 1.4° (0.7° above and below), the glide path needle is three times
more sensitive than the localiser and 15 times more sensitive than the VOR. When following a
glide path, the rate of descent is the pilot’s reference, so the vertical speed indicator becomes
important. Determine the proper vertical speed before starting the descent on the glide path.
Instrument approach plates usually give the rate of descent related to the ground speed of the
aircraft during the approach. As a rule of thumb or guide, the rate of descent (in fpm) may be
calculated as half the ground speed, in knots, with a zero added. This is valid for a glide path of
3°. A ground speed of 120 kt during final approach requires a 600 fpm descent rate.

Radio Navigation 8-15

Chapter 8 Instrument Landing System (ILS)

As a general rule, use the pitch attitude to control the glide path and the throttle to control the
airspeed. When flying the ILS procedure the pilot must continuously monitor all the instruments,
and follow both ILS needles at the same time. This phase of flight requires a great deal of
accuracy and attention. Look at the indications of the needles relative to the aircraft’s position.
The same rules apply when using the horizontal situation indicator, but naturally the localiser
tracking is simpler. Normally the HSI has a scale along either side of the instrument, on which
glide path information appears. As shown, many instruments are involved in an ILS approach:
The localiser and glide path indicators, the directional gyro, the airspeed indicator, the vertical
speed indicator, and the altimeter. Recently, there has been a tendency to group all these
instruments into a single display.

ILS WITHOUT GLIDEPATH

If glide path information is not available, either because of equipment failure on the ground or
glide path receiver failure in the aircraft, the ILS automatically becomes a localiser approach. In
some cases, ILS installations may be purposely commissioned without glide slope.

Since no vertical guidance is provided, localiser approaches are non-precision approaches. They
are similar to VOR approaches except for the fact that a localiser course is four times more
sensitive than a VOR course. In addition, localiser approaches normally include marker range
indicators. The minimum descent height (MDH) for a localiser approach is never lower than 250
ft, whereas the DH for ILS with glide path can be 200 ft (ILS Category I).

The next two pages show the approaches to Coventry on Runway 23. The first plate is for a
normal ILS approach. The second plate is for the localiser-only approach.

8-16 Radio Navigation

Instrument Landing System (ILS) Chapter8

Note: For a Category A aircraft the OCA is 431 ft. 8-17
Radio Navigation

Chapter 8 Instrument Landing System (ILS)

8-18 Radio Navigation

Instrument Landing System (ILS) Chapter8

The OCA on the localiser only approach for a Category A aircraft is 635 ft. The Category A
aircraft OCA for the ILS on the previous page is 431 ft. The difference occurs because a precision
approach, such as an ILS, allows the aircraft to a lower DH than the localiser only non-precision
approach where a MDA is used.

DISTANCE MEASURING EQUIPMENT

It is common to find a DME paired with an ILS frequency to supplement the marker beacons or
very often, to replace them.

All information is zero-referenced to the threshold. The ILS DME is protected from other DME
services only within the localiser service area described earlier up to a height of 25 000 ft.

RATE OF DESCENT (ROD)

The rule of thumb is only valid for a 3° glidepath. To calculate the ROD for any other glidepath,
use the 1 in 60 rule:

ROD = θ x groundspeed (miles per minute) x 101.3

Example: For a 4° glidepath at a groundspeed of 120 knots, the rate of descent
should be:

4 x 2 x 101.3 = 810 fpm

HEIGHT PASSING ON THE APPROACH

Using a similar method, the height an aircraft should be passing at any given range from either
the threshold or touchdown can be calculated.

Height Passing = θ x Range from touchdown (nm) x 101.3

Example: For a 4° glidepath, an aircraft is 4.5 nm from touchdown. What height
should it be passing?

4 x 4.5 x 101.3 = 1824 ft

If range from the threshold is provided, add 50 ft to the answer as the aircraft is assumed to be
passing 50 ft as it passes over the threshold.

Radio Navigation 8-19

Chapter 8 Instrument Landing System (ILS)

8-20 Radio Navigation

INTRODUCTION

Marker beacons are radio beacons transmitting their power vertically toward the sky. All marker
beacons transmit on the same frequency. The marker beacon is only received when the aircraft is
directly overhead. The marker beacon cannot be used as a tracking aid for navigation purposes.

PRINCIPLE OF OPERATION

The transmitted beam forms a polar diagram in the shape of a vertical fan or funnel-shaped lobe.

There are four types of markers:

¾ Airway marker (fan marker)
¾ Outer marker
¾ Middle marker
¾ Inner marker

The indicators for marker beacons are small coloured lights on the instrument panel marked:

A Airways Marker
O Outer Marker
M Middle Marker

No inner marker indicator is shown. The inner marker is now part of military installations only. The
beacon and its limitations are included since military airfield systems are used for both training
and commercial practices.

Frequency
VHF - 75 MHz

Emission Characteristics
Amplitude modulated, A2A emission, with different audio frequencies in order to distinguish
between the different types of markers.

AIRBORNE EQUIPMENT

All marker beacons transmit on the same frequency and there is no need for a frequency selector
in the receiver. Besides the receiver unit, the airborne equipment consists of three coloured
lamps and a sensitivity switch. Audio output is channelled through the audio panel of the nav/com
installation of the aircraft.

Radio Navigation 9-1

Chapter 9 Marker Beacons

Signals received from the marker beacons first pass through a 75 MHz filter, then to the RF
amplifier and detector. The sensitivity switch (HI/LO) acts as a gain control on the RF-amplifier
input. Whichever audio tone is modulating the carrier triggers detector output. Three audio filters
discriminate audio tones. The diagram below shows passage of the different markers.

500 FT. 1000 FT. 2000 FT.
(±160 FT.) (±325 FT.) (±650 FT.)

Inner Middle Glide Path (Exaggerated) Outer
Marker Marker Marker

Runway

250 FT.
(±25 FT.)

3500 FT.
(±500 FT.)

31/2 to 5 NM

The outer, middle, and inner markers are parts of the ILS installation and are used when
conducting an ILS approach to an airfield. Remember that the inner marker is only used at
military installations.

The Inner Marker has a modulating signal frequency of 3000 Hz with an audio tone of 6
dots per second. The white indicator lamp flashes on the indicator panel.

The Middle Marker has a modulating signal frequency of 1300 Hz with an audio tone of
alternate dots and dashes at a rate of 2 dashes per second. The amber indicator lamp
flashes on the indicator panel.

The Outer Marker has a modulating signal frequency of 400 Hz with an audio tone of
continuous dashes at a rate of 2 dashes per second. The blue indicator lamp flashes on
the indicator panel

Marker Position Light Signal Modulation

Outer Distance From Deviation Blue Continuous 400 Hz
Middle Threshold Allowed Amber dashes 1300 Hz

3.5 to 5 nm - Alternate 3000 Hz
dots and
3500 ft ± 500 ft dashes

Inner 250 ft ± 25 ft White Continuous
dots

Since the transmission pattern is fan-shaped, the horizontal area of reception depends on aircraft
altitude. Where the aircraft is on the glidepath, the distance of travel where the beacon is received
is:

Outer Marker 2000 ft ± 650 ft

Middle Marker 1000 ft ± 325 ft

Inner Marker 500 ft ± 160 ft

9-2 Radio Navigation

Marker Beacons Chapter 9

With airways markers, in order to get a more precise indication and to avoid receiving ILS
markers at high altitude, the ILS Marker sensitivity switch on the panel is set to low.

AIRWAY MARKER

As the name indicates, the airway marker is used while route-flying along airways as follows:

¾ To identify certain fixes along routes where there are no other means of establishing
the fix

¾ When flying over mountainous areas, it works where it is difficult or impossible to
receive other navigation aids aside from the one being tracked

¾ To supplement an NDB by providing vertical cover above the cone of silence

Some markers, usually modulated with 3000 Hz, function to mark important points such as
significant positions in a noise abatement procedure.

Airway markers are becoming rare, as are the inner markers of ILS.

GROUND INSTALLATION

Airway marker usage is decreasing; however, they still appear along airways in order to establish
accurate reporting points. In areas with poor radio-coverage, such as mountainous areas, airway
markers can provide a point source fix. With the sensitivity switch set to high, these markers can
reach as high as 50 000 ft. The outer, middle, and inner markers are parts of the ILS installation,
and are all installed along the extended centre line from the approach end of the runway.

The outer and middle markers are no more than 75 metres from the extended centreline. The
inner marker is installed no more than 30 metres from the extended centreline.

Glidepath crossing heights at the different markers are published on the appropriate approach
plate for the actual ILS approaches. When on the glide slope, the outer marker crossing height is
approximately 1500 to 2000 ft dependent on the glideslope angle. The purpose of the outer
marker is to provide height, distance, and ‘equipment functioning’ checks to aircraft on
intermediate or final approach. The middle marker is crossed at approximately 200 ft AAL, which
is close to decision height for a Category I ILS approach. The purpose of the middle marker is to
indicate the imminence, in low visibility conditions, of visual approach guidance. The purpose of
the inner marker is to indicate to the pilot that the aircraft is about to pass the threshold. The
height is the lowest decision height applicable in Category II operations.

Radio Navigation 9-3

Chapter 9 Marker Beacons

9-4 Radio Navigation

INTRODUCTION

ILS has served as the primary precision approach and landing aid since the last war. Since the
mid 1980s, the limitations of the system prompted ICAO to develop a replacement system to fulfill
the needs for future aviation. The limitations of the ILS are:

¾ Procedurally, ILS limits aircraft to long straight-in final approaches of at least 7 miles.
This creates potential airspace conflicts in multi-airport environments and constrains
the number of possible approach paths. Each ILS provides only one approach path
for one aircraft at a time.

¾ Direct and ground-reflected signals form part of the ILS guidance signal, and require
a significant level of site preparation. ILS can only be installed at locations where site
preparation is practical.

¾ ILS is limited to 40 frequency channels constraining the number of sites that can be
allocated a frequency in a given geographical area.

¾ The ILS frequency band suffers from interference from high power FM transmitters
operating in adjacent bands. Aircraft receivers contain FM filters, which narrow the
band of reception and reduce this ‘noise’ interference but this is still a limitation. At
some airports, interference can cause the ILS to receive local radio broadcasts.

¾ ILS is sensitive to signal diffraction and blockages caused by ground traffic,
necessitating the use of large protected areas on the airport surface (critical areas).
Within these areas, the ground movement of vehicles and aeroplanes must be
prohibited. This reduces the effective capacity of the airfield during low visibility
operations.

These limitations led to the development of a microwave landing system. Parallel to the
development of MLS, the civilian use of satellite-based Global Positioning System (GPS) was
also under development, both as an enroute navigation aid and, with augmentation systems, as
an approach aid. The development of GPS has advanced so that, in some countries, further
development and installation of MLS has been abandoned. In practice, this chapter describes a
system pilots may never encounter in their careers.

PRINCIPLE OF OPERATION

The MLS system is a precision approach system that provides the pilot with highly accurate
azimuth and elevation information. It also utilises a precision DME (DME/P) which provides highly
accurate ranging information. The MLS system can also transmit other types of information to the
aircraft such as station identification, system status, runway information, and weather.

Frequency
SHF – 5031 to 5090.7 MHz. Spacing 300 kHz, giving 200 channels

Polarisation 10-1
Vertical

Radio Navigation

Chapter 10 Microwave Landing Systems

GROUND INSTALLATION

This is a completely digital system that is not influenced by weather or other common sources of
disturbances. The system allows for several approach paths, both in azimuth and elevation at the
same time. As with visual approaches, MLS lets the air traffic controller clear the aircraft for
curved approach paths, with a straight-in final segment being as short as 1.5 nm. This leads to a
significant reduction in air traffic delays.

The ground installation consists of the following three main elements:

¾ Azimuth (AZ)
¾ Elevation (EL)
¾ Precision DME (DME/P)
¾ Some installations may also contain Back Azimuth (BAZ)

AZIMUTH COVERAGE

The azimuth (AZ) part of the installation is comparable to the localiser of the ILS except that it
provides a much wider area of information, up to 40° on each side of the extended centre line.

APPROACH
ELEVATION
ANTENNA

45 m (150 ft) 40° 37 km (20 NM) C
APPROACH 40° L
AZIMUTH
ANTENNA APPROACH
DIRECTION
45 m (150 ft)

MLS DATUM
POINT

THRESHOLD

LATERAL COVERAGE ADDITIONAL
6000 m (20 000 ft) COVERAGE
RECOMMENDED

600 m (2 000 ft) 15°

20° 0.9°
30° HORIZONTAL
2.5 m (8 ft)

APPROACH 37 km (20 NM)
AZ IM U T H VERTICAL COVERAGE
ANTENNA

COVERAGE OF LOCALIZER EQUIVALENT

10-2 Radio Navigation

Microwave Landing Systems Chapter 10

The system provides the AZ out to 20 nm and the BAZ to 5 nm (ICAO minimum).
The azimuth coverage is:

¾ ± 40° of the runway centreline out to 20 nm
¾ Vertical coverage of the beam is 0.9° to 15°
¾ Has a beam no more than 4° wide

ELEVATION COVERAGE

The elevation (EL) part is comparable to the glide path of the ILS. The main difference is that the
pilot can choose the desired glide path angle (up to 15°).

The elevation coverage is:

¾ ± 40° of the centreline to 20 nm
¾ The aerial scans vertically from 0.9° to 7.5° above the horizontal (most systems can

scan to 15°)
¾ The system has a beam no more than 2.5° wide

Radio Navigation 10-3

Chapter 10 Microwave Landing Systems

DME/P

The DME/P is an integral part of the MLS system. The DME/P signal defines two operating
modes, Initial Approach (IA) and Final Approach (FA).

The IA mode design improves accuracy for the initial stages of approach and landing. The FA
mode provides substantially improved accuracy in the final approach area.

The DME/P coverage is at least 22 nm from the ground transponder. The interrogator does not
operate in the FA mode at ranges greater than 7 nm from the transponder site, although a
transition from the IA mode may begin at 8 nm from the transponder. These ranges assume that
the transponder location is beyond the stop-end of the runway at a distance of 2 nm from the
threshold.

BACK AZIMUTH

The back azimuth (BAZ) provides navigational guidance for precision departures and for missed
approach procedures.

In practical installations, the coverage in the horizontal plane can vary according to local
conditions and needs and does not have to be symmetrical on each side of the centre line.

SIGNAL TRANSMISSION FORMAT

The AZ and EL elements transmit on the same frequency, while the DME uses a paired channel
in the UHF band. The format of the digital signal is very flexible and the information from the
different elements can be sent in any desired order. A preamble precedes each group, which tells
the processor in the receiver which functions are being sent. As soon as decoding is complete for
one group, the processor is ready and waiting for the next element.

There are two types of signals sent, basic data and auxiliary data:

Basic Data directly relate to the operation of the landing guidance system. Station
identification is a part of the basic data.

Auxiliary Data is other data used for siting information and other information not directly
related to the guidance system.

Measuring the time difference between successive passes of the highly directional fan-shaped
beams derives the angular measurements required by the aircraft. The DME/P provides the
distance measurement. Using a system called time division multiplexing allows accommodation
of all information required by the aircraft on the same channel. This means that an aircraft can
decode the incoming signal in a sequential manner. Each function is a separate entity within the
format and a preamble identifies all functions This preamble sets up the receiver processing
circuits, which then decode the remainder of the function transmission. Once the decode is
complete, the receiver waits for the next function preamble and the process repeats itself. The
diagram below explains the approach azimuth signal.

As well as information regarding the position of the aircraft, this signal can carry extra information.

10-4 Radio Navigation

Microwave Landing Systems Chapter 10

Each angle function transmission consists of four elements:

¾ The preamble consisting of a synchronising code plus a function identity code
¾ A series of pulses for azimuth guidance
¾ The “TO” and “FRO” angle scan
¾ Two pulses which provide a system check

If the aircraft is close to the ground, the process then begins for the flare information.

TIME REFERENCE SCANNING BEAM

ANGULAR MEASUREMENT IN AZIMUTH AND ELEVATION
The aerial transmitting the AZ beam, forms a vertical narrow fan-shaped beam, which scans from
one side to the other and back at a constant angular velocity.

-40° -40°

'To' Scan

Azimuth
Aerial

+40° +40°

Received Measurement
Signals Threshold
Amplitude
Time interval is 'Fro' Scan
'To' Scan Directly Related to Ends
Begins
Azimuth Angle
Radio Navigation
10-5

Chapter 10 Microwave Landing Systems

The total scan of the beam lasts 9000 microseconds (µs):

¾ 4000 µs for the “TO” scan
¾ 1000 µs resting time
¾ 4000 µs for the “FRO” scan

In the diagram above, the aircraft is 15° left of the centreline. The system receives the “TO” scan
after x microseconds. The “FRO” scan occurs after y µs. The measured intervals of time provide
the aircraft’s azimuth.

Calculating the vertical position (EL) occurs exactly in the same way as the horizontal with one
exception. The scanning beam moves in the vertical plane, first up and then down. Normally the
horizontal AZ- scan repeats 13 times per second, while the vertical EL- scan repeats 39 times per
second.

AIRBORNE EQUIPMENT

The aircraft receiver measures the time passage between the “TO” and “FRO” scans of both the
AZ and the EL elements. These time measurements allow calculation of both the azimuth and
elevation angles. When coupled with a range measurement from the DME/P, they allow the pilot
to establish a three-dimensional aircraft position.

In its simplest form, this position can be compared with a planned approach path and, if not on
that path, can be used to create an error signal, which can drive the conventional ILS indicator to
show displacement from the selected azimuth and glide path approach.

The conventional ILS indicator is used since it is also required for conventional ILS approaches.
Therefore, the indicator is multi-mode. More sophisticated computerised systems would allow the
full potential of MLS to be realised, making it possible to follow curved and segmented
approaches.

If the DME/P is not available, the system still provides an ILS look-alike approach.

ACCURACY

When used with a Category III system, accuracy is ± 20 ft in azimuth and ± 2 ft in elevation.

10-6 Radio Navigation

PULSE TECHNIQUES AND ASSOCIATED TERMS

This chapter discusses the uses of radar. To understand how these various types of radar
operate requires refreshing and expanding some fundamentals and expressions related to radar.
Chapter 1 gives some of the basic principles of radar. This chapter expands on those principles.
The following is a review of some of the terms used:

Frequency f Hertz Hz
Wavelength λ metres m
Speed of light C metres per second m/s
Time intervals – t milliseconds ms
t microseconds µs
Pulse Width PW microseconds µs
Pulse repetition interval PRI microseconds µs
Pulse repetition frequency PRF pulse per second pps
Duty cycle DC no units
Radar mile microseconds

THE COMPONENTS OF A RADAR UNIT

A radar unit consists of:

¾ A transmitter
¾ An aerial
¾ A receiver (and possibly a receiver aerial)
¾ A timebase
¾ A display

Radio Navigation 11-1

Chapter 11 Radar Principles and the Cathode Ray Tube
Look at each of those elements in simple terms:

Magnetron T/R Switch Receiver

Modulator Timebase Display

Within the transmitter, the timer unit triggers a supply of RF signals to the modulator. The
modulator forms the pulses and passes these to the magnetron where they are amplified to a
very high energy level. This high-energy pulse travels through a waveguide via the T/R switch to
the aerial where it radiates outward. The receiver is isolated from the aerial to ensure that there is
no damage from this high-energy pulse. After the pulse passes the T/R switch, the switch
recovers and the receiver, using the same aerial, waits for an echo of the energy returning, after it
bounces off a target.

THE TIMEBASE

The timebase connects to the transmitter and receiver and knows when a pulse is sent and when
an echo received. Measuring time between these two events and using the speed of an
electromagnetic wave determines a range.

THE DISPLAY

In simple radar systems, the timer and display are a single unit known as a plan position
indicator or PPI. In many modern applications, the information from this timer is processed and
sent, with other information, to a suitable display. This is particularly evident in modern Air Traffic
Control systems.

THE TRANSMITTER

Like other transmitters, the radar transmitter consists of a RF generator, a modulator, and an
amplifier. The RF generator creates the transmission frequency and the modulator creates the
pulses of energy.

CHOICE OF FREQUENCY
A number of factors govern the choice of frequency (wavelength):

Attenuation
Attenuation due to intervening weather can be high so returning signals are very weak.

11-2 Radio Navigation

Radar Principles and the Cathode Ray Tube Chapter 11

Target Size

The relative size of the desired target must be considered. Smaller targets require shorter
wavelengths in order to create reflection of energy.

Aerial Size
If space is limited and aerial size is restricted, shorter wavelengths give narrower beams.

Pulse Length

The energy content of a pulse increases with the number of cycles transmitted during the
pulse.

The following are commonly used frequencies:

1000 MHz 30 - 50 cm Long Range Surveillance
3000 MHz 10 cm Surveillance Radar & Approach Radar
10 000 MHz 3 cm Approach Radar

The modulator creates pulses of the desired length and at the desired rate.

Factors affecting the choice of pulse length include:

Minimum Detection Range

Since the aerial is common to both transmitter and receiver, it is important to protect the
sensitive receiver from the high power pulse. As a result, the receiver is disconnected
from the aerial during the transmission of the pulse (and for a short interval after).

Range Discrimination

The ability to detect separate targets that are on the same bearing (azimuth) and are
close together depends on pulse length. For example, if a pulse length is 4 microseconds
(4 µs) its physical length is 1200 m. If two targets lie on the same bearing and are within
600 m of each other, they both illuminate at the same time and their echoes merged at
the receiver. If the two targets are so close that the beamwidth covers both aircraft then
only one target is visible.

Range

The longer the pulse the greater the energy content, permitting the pulse to travel farther
before attenuation and increasing the return of detectable energy from distant targets.

Choice of PRF is also affected by a number of factors including:

Design Maximum Range

The transmitter must remain ‘silent’ while the receiver is ‘listening’ for echoes. If the
design maximum range is 200 nm from the receiver it is necessary to allow it to listen for
the period of time from when a pulse transmits until it can go 200 nm and then return.
That is a round trip of 400 nm, which would require a silent period of 2473.3 ms. This
would be the minimum PRI (pulse repetition interval). In practice, the minimum PRI would
be increased to allow for receiver recovery time. The PRF is the inverse of the PRI so
that, if the PRI was 2500 ms the PRF is 400 pps.

Radio Navigation 11-3

Chapter 11 Radar Principles and the Cathode Ray Tube

Data Acquisition
If the pulses are of short duration (1 m or less) it takes up to six echoes to cause the
radar display to show the target echo. The PRF must be sufficiently high to allow this to
occur in the period of time that the effective part of the aerial is pointing toward the target.

THE AERIAL

A highly directional aerial system is necessary in order to concentrate the transmitter power and
increase the effective pulse power and provide azimuth information.

The systems most commonly used consist of either a wave-guide horn and parabolic dish or a flat
plate “phased array” system. Both systems are designed to focus the radiated energy into a
narrow beam.

The radar pulses go through the wave-guide horn and reflect from the parabolic dish. This acts
very much as the reflector in a car headlamp.

BEAMWIDTH
The polar diagram from such an aerial assembly appears with a main lobe and a number of
smaller side-lobes. The side-lobes are not desirable and may, on some systems, be suppressed
by modification of the reflector or by use of suppression systems.

Beamwidth Main Beam

The beam width is the angle contained between the ½ power points on the polar diagram. This
determines the radar’s ability to discriminate between targets that are close together and at the
same range. If, for example, two targets are at a range of 60 nm and are separated by 1 nm, a
beam of more than 1° allows both targets to be ‘illuminated’ at the same time (1 in 60 rule). The
reflections from these targets merge at the receiver and they appear as one echo on the display.
As long as a target is within the beam, it remains illuminated and continues to paint an echo. All
targets therefore appear to have an azimuth dimension equal to the beam width irrespective of
the physical size of the target.

11-4 Radio Navigation

Radar Principles and the Cathode Ray Tube Chapter 11

For a parabolic reflector, the beam width can be calculated from the following relationship:

Beam width° = 70λ ÷ d

In this formula, λ is the wavelength of the transmission and d is the dish diameter. Both λ and d
must have the same units.

Example: What dish diameter is needed to get a 1° beamwidth at a 25 cm
wavelength?

1 = (70 x 25) ÷ d
d = 70 x 25
d = 1750 cm = 17.5 m

On 10 cm radar the same beam width is possible with a 7 metre dish.

1 = (70 x 10) ÷ d
d = 70 x 10
d = 700 cm = 7 m

In order to provide coverage, the aerial is rotated in azimuth so that the beam rotates. The
direction in which the aerial points when illuminating a target provides the azimuth (or bearing)
information. An electronic link relays this direction to the display. The display may be either
analogue or digital. The rate of the aerial rotation must be matched to the criteria that affect the
PRF selection. It must be sufficiently high to allow for renewal of target information at short
enough intervals of time, while at the same time, it must be slow enough to allow for sufficient
echo comparison in order to allow the display to show the target.

THE RECEIVER

This unit detects the extremely low energy signals reflected from a target. The receiver is,
consequently, extremely sensitive and has a very powerful amplifier circuit. It must be protected
from the very high energy of transmissions from the same aerial, so it is electronically
disconnected during transmission. This means that the receiver is dead during transmission and
for a short period afterward, known as receiver recovery time. The duration of time that the
receiver is inactive governs the minimum range at which a target is detectable.

After detection and amplification, echoes from the target go to the display. In the case of ATC
radar, the echoes include returns from fixed objects on the ground, and these may well hide the
returns from aeroplane targets.

In order to remove these stationary targets, signals are filtered by a circuit known as Moving
Target Indicator (MTI). If the target is moving radially toward or away from the aerial, the
frequency of the echo pulseschange due to Doppler effect, increasing if the target is moving
toward the aerial and decreasing if moving away. There is no frequency change from stationary
targets, so the receiver circuit can be designed to reject all signals that do not exhibit a change in
frequency. This introduces another problem if a moving target happens to fly so as to stay at the
same distance from the radar. In that case, there is no radial motion and no Doppler effect. The
MTI circuit could reject such targets unless measures occur to counteract this problem. Varying
the transmissions achieves this in modern units.

Radio Navigation 11-5

Chapter 11 Radar Principles and the Cathode Ray Tube

TIMER – CATHODE RAY TUBE

In its simplest form, the timer is combined with the display, and consists of an electron gun and
screen assembled into a unit known as a Cathode Ray Tube (CRT), as illustrated below.

FIRST THIRD AQUADAG-WALL ANODE
ANODE ANODE (INTERNALLY CONNECTED

TO THIRD ANODE)

X PLATES

HEATER/CATHODE SECOND Y PLATES
/GRID ASSEMBLY ANODE

FLUORESCENT
SCREEN

The elements of this unit have the following functions:

CATHODE
When heated, the cathode emits electrons. Due to the great heat required to expel the electrons
from the parent metal, tungsten is generally used.

GRID
The annular grid restricts the flow of electrons. By applying a negative voltage the electron flow
can be:

¾ Cut off if the grid is at a maximum negative voltage
¾ A maximum if the grid has no voltage applied

The grid controls the brilliance of the CRT.

FOCUSSING ASSEMBLY
The focussing assembly consists of 3 anodes at different potentials. These anodes both focus
and accelerate the electrons. The electrons are attracted at high speed to this group of annular
anodes which are all at a high positive potential with respect to the cathode. The higher the
potential used the higher the electron velocity.

Due to this high velocity, large numbers pass through the apertures. This means the emerging
beam has enough energy to strike the screen at the end of the tube.

The potential in anodes 1 and 3 are constant with the potential in anode 2 being variable. By
using a variable potential in anode 2 the focal length of the beam can be adjusted.

11-6 Radio Navigation

Radar Principles and the Cathode Ray Tube Chapter 11

ELECTRON BEAM DEFLECTION
To write usable information on the screen the beam must be deflected in a controlled manner.
Two plates are set in the CRT, the X for horizontal scanning and the Y plates for vertical
scanning.

SCREEN
The screen is coated with a chemical salt that gives a fluorescent glow when the electron beam
strikes it. When the electrons strike the screen, the heat produced allows electrons from the
chemical screen to be freed (called secondary emission). This forms a cloud of electrons which
can inhibit the energy of the electron beam and cause a loss of brilliance. Covering the screen
wall with a carbon coating – Aquadag, removes the secondary emission. The aquadag is charged
to a high positive potential which attracts the secondary emissions. The screen wall is also known
as the final wall anode.

TIME BASE

In the CRT, the stream of electrons striking the interior surface of the screen causes the screen to
fluoresce at the point of impact. If the electron stream is deflected, the spot leaves a trace image.

Y1

0 200
X2 X1

T

Y2

In the illustration above, if a high negative electrical potential is applied to plate X1, the electron
beam is held to the left at plate X2. This equates to zero range. Changing this negative potential
at X1 at a constant rate from negative to positive causes the electron beam to move toward X1.
The spot moves from the left to the right at a constant speed, leaving a straight line traced
behind.

Knowing the speed of travel allows us to calibrate across the scope for range. This can also be
done for the Y plates allowing the possibility of moving the beam around any part of the screen.

If the frequency at which the potential applied to the plate is made to match the PRF then the time
taken for a single movement of the spot will match the interval between pulses and is therefore
compatible with the maximum range. The trace left by the spot is called a time-base.

Radio Navigation 11-7

Chapter 11 Radar Principles and the Cathode Ray Tube

SAW TOOTH VOLTAGE

Once the electron beam is fully displaced, the right hand plate X1 is made rapidly negative. This
causes the electron beam to “fly back” to X2. If nothing happens in this fly back period, a visible
trace would remain on the screen.

During the fly back, a high negative voltage is applied to the grid, stopping any electron flow. This
ensures that no visible trace is left on the screen. This is known as a saw tooth voltage and is
illustrated below.

+

SAW-TOOTH 0
VOLTAGE

- TIME

GRID VARIABLE BIAS
BIAS 0 (BRILLIANCE)
VOLTS
ELECTRONS FLOW
-VE
CUT-OFF BIAS

ELECTRONS DO NOT FLOW
DURING FLY-BACK PERIODS

Most radar timer/display CRTs use a rotating time base. The time base is rotated by varying the
potentials, in sequence, to the deflector arrangement. By connecting the receiver output to the
grid and causing the flow of electrons to be increased momentarily when an echo is detected,
targets are shown by causing the spot to become brighter.

Rotating the time base in sympathy with the aerial allows the derivation of bearing information.

RADAR PERFORMANCE

Radar uses frequencies that are normally in the higher bands of the electromagnetic spectrum.
Propagation typically follows a direct wave path, so the range of radar is generally line of sight.
Certain factors, other than those of design, affect radar’s performance:

Atmospheric Conditions
At the very high frequencies used, super refraction caused by inversions of temperature
and/or humidity (Anaprop) may cause a considerable increase in the direct wave range. It
is possible for echoes to return from a range greater than the design maximum range,
and to appear on the screen as false targets at any range.

Sub-refraction causes poor radar performance at the upper range limits.

11-8 Radio Navigation

Radar Principles and the Cathode Ray Tube Chapter 11

Weather
Rain and snow attenuates the radar signal and causes at least decreased performance
and possibly even blind sectors.

Target Size, Shape and Aspect
As previously mentioned, the size and shape of the target have a tremendous effect on
its ability to reflect radar signals. How it appears to the radar is also important. A B747
head-on is much harder to see than one that is side-on. Radar has the same difficulty
with targets of differing size.

By the same reasoning, a flat surface at right angles (to the direction to the radar aerial)
produces a much stronger return than a similar sized curved surface.

SECONDARY RADAR

The principles of primary radar operation and some of the factors that affect a radar’s
performance have been illustrated. Using Secondary Radar techniques can minimise some of the
effects. The principle of measuring range from a time delay is still applicable, but the target plays
an active role.

The interrogating radar unit sends out a pulse (interrogation pulse). When the target detects this
pulse, it triggers a transmitter to respond, sending a signal back to the interrogator. This signal is
stronger than an echo, is not dependent on how well the target has reflected the energy and
could be coded with additional information.

Radio Navigation 11-9

Chapter 11 Radar Principles and the Cathode Ray Tube

11-10 Radio Navigation

INTRODUCTION

In today’s Air Traffic Control systems, the role of radar is crucial for the safe and efficient
controlling of an ever-increasing air traffic density. To provide for the needs of this task, the
differing Air Traffic Control environments demand different performance parameters from radar.
The main categories of ground radar are:

¾ Surveillance Radar

¾ Long range surveillance radar
¾ TMA surveillance radar
¾ Aerodrome surveillance radar

¾ Secondary surveillance radar (SSR) – discussed in Chapter 13
¾ Precision approach radar
¾ Surface movement radar
¾ Weather radar – a meteorological service

LONG RANGE SURVEILLANCE RADAR

The radar has the following properties:

¾ A range capability of 200 nm to 300 nm
¾ An ability to penetrate intervening weather
¾ The ability to detect small targets out to maximum range
¾ Moderate target discrimination capability in range and bearing

Using two radar systems generally fulfils these needs:

Primary Radar Wavelength of 30 - 50 cm
Pulse length 4 µs
PRF 270 pps
Horizontal beamwidth 1.7°
Aerial rotation 5 rpm

Secondary Radar This serves as a complement to the primary radar, improving the
possibility of detecting targets at long ranges and allowing for the
identification of co-operating targets.

Radio Navigation 12-1

Chapter 12 Ground Radar

TERMINAL SURVEILLANCE RADAR

This radar provides separation between aircraft within the terminal area during transit, approach,
and departure. It may be used to provide a radar approach. The service is provided by primary
radar with the following characteristics:

¾ Ranges up to about 60 or 80 nm
¾ Ability to refresh the target information at short intervals
¾ Ability to penetrate intervening weather
¾ Good target discrimination properties
¾ Good accuracy

The radar has the following properties:

¾ Wavelength of 25 cm
¾ Pulse length 3.9 µs
¾ PRF 350 pps
¾ Beamwidth 1.2°
¾ Aerial rotation rate 8 rpm

An SSR element is also normally used in the terminal surveillance radar environment.

Surveillance radar displays for long range and terminal radars are normally processed and
combined with the information from the primary radar. The superimposing of the SSR information
on the primary display shows the controller a complete situational picture of the relevant airspace
on an easily viewed screen.

AERODROME SURVEILLANCE (APPROACH) RADAR

Where provided, the aerodrome surveillance radar is normally a short-range (25 nm) primary
radar that is capable of providing guidance during the initial, intermediate, and even final
approach phases of the flight. As such it requires the following:

¾ Very accurate range and bearing
¾ Excellent target discrimination
¾ Rapid refreshing of the information.

The properties of the radar are:

¾ Wavelength 10 cm which allows a very short pulse length to be produced (pulse
length1 µs)

¾ PRF of 700 pps
¾ Beamwidth 1°
¾ Aerial rotation rate increased to 15 rpm

The shorter pulse length and narrower beam width improve both accuracy and target
discrimination. The increased speed of aerial rotation allows for an increase in the rate of target
information renewal. This type of radar can provide a Surveillance Radar Approach.

12-2 Radio Navigation