1
Q
Radio waves are used in the following areas of ATC
A
Voice Communication VHF/ UHF
Navigation VOR /NDB
Surveillance systems
Weather radar
Data transmission
2
Q
According to Ampère’s law, an
electric current produces:
A
a magnetic field perpendicular to the
flow direction
3
Q
Radio waves are a form of:
A
electromagnetic energy
that are similar in behaviour to light waves
4
Q
What is the speed of light?
A
300,000,000 meters per second (c)
5
Q
Radio waves are capable of:
A
passing through a vacuum
Travel in straight lines (generally)
Invisible
Intangible (Can’t touch it)
Inaudible without specialist equipment
6
Q
What is Audio?
A
These movements of air make the eardrum
vibrate and mimic the air which is moving, thus
the receiving person hears the propagated tone
7
Q
Why is Audio limited in range?
A
how loud one can speak and absorption by materials within its range.
8
Q
How do we overcome absorbtion?
A
very high
frequencies are used which can be propagated
over large distances, but are not within the
audio spectrum
9
Q
What is osscilation?
A
how a wave changes from a
maximum to a minimum. Shown as a sine
wave.
10
Q
What is amplitude?
A
“The maximum displacement or value attained
by the wave from it’s mean value during a cycle”
11
Q
Wavelength
A
The distance in meters or part of a meter between
corresponding points in consecutive waves
Scientifically represented by lambda
12
Q
Frequency
A
“The rate of repetition of the cycle in one second
where one cycle per second is known as one Hertz.”
13
Q
Hertz numbers
A
Kilohertz (KHz) - where 1 KHz = 1,000Hz
Megahertz (MHz)- where 1 Mhz = 1,000,000Hz
Gigahertz (GHz) - where 1 GHz = 1,000,000,000Hz
14
Q
Velocity definition
A
“The speed in a given direction.”
15
Q
Velocity of electromagnetic waves in a vacuum is?
A
a constant 300,000,000
meters per second
16
Q
Velocity =
A
frequency x wavelength
17
Q
“The Local Oscillator”
A
A Carrier Wave is an electro-magnetic wave which can be modulated (varied) and is produced by a device
18
Q
carrier wave is modulated (varied) in direct proportion to…
A
the signal that is to be transmitted
19
Q
modulation can change either:
A
the amplitude or the frequency of the carrier wave.
20
Q
How does modulation work?
A
The audio signal is fed into a Modulator, which
combines the carrier wave with the audio wave.
21
Q
AM-
A
amplitude modulation
22
Q
FM-
A
frequency modulation
23
Q
Advantages of FM:
A
Resilience to noise
easy to apply modulation at low power
use of efficient RF amplifiers
24
Q
Advantages of AM:
A
Stronger stations can override weaker
Control tower can “talk over”
heterodyne will be heard
25
What is a quarter wave aerial?
Using the top of the positive part of this cycle is
known as a half-wave aerial, or using the first
90deg of the cycle
26
The EMF radiates perpendicular to...
the antenna, so with a vertical antenna, the EMF is horizontal; with a horizontal antenna the EMF radiates vertically.
27
A radio wave reduces in strength with range or time
from the point of transmission.
This is due to:
The ever-expanding wave front.
The medium through which the wave is passing resists the passage of energy passing through it.
28
As a signal is attenuated, it’s amplitude...
decreases, but the wavelength and frequency remain unchanged
29
Higher frequency, greater attenuation,
shorter range
30
The radio waves are divided according to:
the frequency of the transmission into
internationally recognised bands
31
Upper and lower end of waveband scale:
Electrical energy
(lower end of scale)
Infra red energy
(upper end of scale)
32
Very low frequency:
Very Low Frequency
VLF 3 - 30 KHz 100km - 10km
Long range communications.
Requires immense aerials and high transmitter power.
Very prone to static interference.
Very long range navigation aids.
33
Low and Medium frequency:
Reliable, long range communications.
Requires large aerials and high transmitter power.
Prone to static interference and night effect.
Very congested waveband.
NDB.
Some radio broadcasts.
34
High frequency:
Long range communications by day and night limited
by diurnal and seasonal variation of the ionosphere.
Requires smaller aerials and transmitter power.
Suffers from static interference and fading.
Optimum operating frequency varies diurnally.
Long distance wireless telegraphy.
RTF communications.
35
Very high frequency and ultra high frquency
Line of sight communications.
Maximum range dependent upon aircraft height
and aerial height.
Small aerial and transmission power.
Free from static interference and easy to suppress
on aircraft.
Prone to ducting.
Wavebands become congested.
RTF, ILS, VOR, VDF, Surveillance Radar.
36
Super high frequency and Extremely high frequency
Short range communications.
Severe attenuation.
Precision, Surveillance and Airborne weather Radar.
Radio altimeter.
37
VHF and higher frequencies propagate on:
straight paths
38
DUCTED WAVE
A marked temperature inversion plus a rapid decrease in humidity may form a duct within which VHF, UHF and SHF wavebands can unusually travel long distances
39
What is an indirect wave?
the low frequencies will bend
around objects and be heard a great distance from the source. The low frequency means less attenuation
40
Where do we use Radar?
Area control
Approach control
Aerodrome control (AIR & GROUND)
41
Speed of sound in m/s:
330m/s
42
RADAR stands for:
Radio detection and ranging
43
Primary radar
‘A SYSTEM THAT USES REFLECTED RADIO SIGNALS'
44
The position of the object that reflected the energy is
determined from:
The direction that the radar aerial was pointing, and
The time between transmitting the pulse of energy and receiving an echo.
45
ATC Radar is in that part of the waveband spectrum between:
1mm to 100cm
46
PSR Blip
The visual indication, in non symbolic form,
on a situation display of the position of an
aircraft obtained by primary radar.
47
Wavelength for surface movement radar:
2-3cm
48
Wavelength for primary approach radar:
3cm-10cm
49
Wavelength for primary area radar:
23cm-50cm
50
A typical approach radar may
transmit around...
1200 pulses per second
51
Position indication
A generic term for the visual indication, in nonsymbolic and/or symbolic form, on a situation
display of the position of an aircraft, aerodrome
vehicle or other object.
52
POSITION SYMBOL
A visual indication in symbolic form, on a situation
display, of the position of an aircraft, aerodrome
vehicle or other object obtained after automatic
processing of positional data derived from any
source.
53
RADAR CONTACT
The situation which exists when the radar position of a particular aircraft is seen and identified on a situation display. (ICAO
54
RADAR CONTROL
Term used to indicate that radar-derived information is employed directly in the provision of air traffic control service.
55
SITUATION DISPLAY
An electronic display depicting the position and movement of aircraft and other information as required. (ICAO)
56
Every primary radar system must be capable of:
transmitting energy in a suitable form, (transmission)
receiving energy which has been reflected by objects within the operational range of the system, (Reception)
displaying information to the controller. (Display)
57
Trigger Unit (Master Timer)
device whose output is used to initiate action.
58
Transmitter Unit
The output of the Transmitter is a series of pulses of
radar energy. Each pulse thus produced is delivered
to the aerial.
59
The Modulator Unit
Every time the modulator unit is fired by the
trigger unit it sends a high power, high voltage
pulse to the transmitter.
60
RECEPTION BLOCK
The signal reception block detects energy
reflected from objects. The weak signal must be
amplified and suitably treated by the HIGH GAIN
LOW NOISE RECEIVER
61
SECONDARY SURVEILLANCE RADAR
A SYSTEM OF RADAR USING GROUND
INTERROGATORS AND AIRBORNE
TRANSPONDERS TO DETERMINE THE POSITION
OF AIRCRAFT IN RANGE AND AZIMUTH, AND
WHEN AGREED MODES AND CODES ARE USED,
HEIGHT AND IDENTITY ALSO.
62
what are the frequencies of secondary radar?
interrogator- 1030MHz
transponder- 1090MHz
63
The Interrogator Process
The interrogator communicates a request for
identification information by transmitting MODE A
pairs of pulses.
For vertical position information the interrogator
transmits MODE C pairs of pulses.
The difference between Mode A + C is the timing
between the pulses
64
The Reply Process
The aircraft transponder recognises the
interrogator Mode by the time interval
The transponder response to an interrogation
consists of a train of pulses containing binary
bits of information.
65
How many possible squawk codes are there?
4096
66
CCAMS
Centralised Code Assignment and Management System
67
Types of squawk code
discrete code
non discrete code
special code
68
Conspicuity codes
These are codes assigned to individual positions to identify aircraft being controlled by a particular unit to another unit
69
GARBLING
False codes may be displayed if aircraft are so
close to each other that their responses to
Mode A interrogation overlap
70
F.R.U.I.T
False Replies Un-synchronised In
Time
71
MONOPULSE SSR
A single pulse used, and accuracy is improved by
averaging measurements made on several or all of
the pulses received in a reply from the aircraft.
72
How is Antenna shadowing mitigated?
placing more than one
antenna.
73
The accuracy of a radar is called?
Resolution
74
Factors that determine radar coverage:
Aerial size, shape and height above the ground.
Size of the target - Primary Radar only.
Atmospheric conditions.
Transmitter power.
Receiver efficiency.
Pulse Recurrence Frequency (PRF) - Primary Radar only.
Pulse Length - Primary Radar only
75
What is an Aerial?
A narrow beam in azimuth combined with a
wide beam in elevation is the usual requirement
in Air Traffic Control.
The usual beam width is 2-3 degrees which at
60nm would give a coverage of 2-3nm
76
The height of the aerial above the
ground must be considered with
reference to wavelength because...
For a given aerial height, the shorter
the wavelength the more gaps in high
coverage but the better will be the low
coverage
77
The optimal service volume of a single radar
installation would cover a cylindrical area
extending up to...
about 60,000ft with a radius of
about 200 NM
78
A Vertical Coverage Diagram will display the...
theoretical coverage of the radar display being
described
79
What does a VCD take into account?
the inherent design factors
of the radar together with general
environmental and atmospheric conditions
80
What does a UCD show?
actual coverage of the radar system after it has
been installed at a specific location
81
What does a UCD take into account?
the local terrain, hills,
valleys etc. and thus it is unique to each
particular installation
82
When is atmospheric effect greatest?
shorter wavelengths
83
what can anomalous propagation cause?
radar echoes from below normal cover and
from ranges in excess of those allowed for in the
design of the radar
84
For secondary radar the transmitted power
must be sufficient for?
attenuation, for the interrogation signal to reach an aircraft at maximum range.
85
What does the PRF do?
The higher the PRF (together with beam width
and scanning rate) the more ‘hits’ on a target
and hence the stronger and more recognisable
will be the return.
86
Two main methods of removing unwanted
returns are used:
Moving Target Indicator.
Circular Polarisation
87
How does circular polarisation work?
The EMF is reflected
and is reversed by reflection from aspherical
object
88
When does tangential fading occur?
if an aircraft is moving tangentially (90
degrees) to the radar beam the aircraft movement will not have a radial component
89
When will MTI cancellation occur?
when the distance is a multiple of half a wavelength
90
When does blind velocity fading occur?
when the aircraft is flying directly towards, or
directly away from, the radar scanner
91
COAXIAL CABLES
normally limited to short distances (e.g., from the perimeter of an airfield to the control tower).
They are expensive and the signal deteriorates
rapidly
92
RADIO LINK
operates in the UHF - SHF frequency band
To avoid possible interference between links the
frequency is changed at each radio link repeater
93
TELEPHONE LINK
may be used if they are of sufficiently high
grade and the information is being sent in a processed form. (e.g. digital from a plot extractor)
can be used over considerable distances and they are comparatively cheap to maintain
The use of plot extractors at radar sites, high grade
transmission lines and suitable error checking processes
enables a high standard of reliability to be maintained
94
FIBRE OPTICS
Used over relatively short distances e.g.
between airfield radar site and the control
tower
95
what is the coverage of a radar sort box?
16nm x 16nm
96
What does a PLOT EXTRACTOR do?
Decides validity of targets.
Calculates correct azimuth and range data.
Decides if target is primary only, secondary only or combined.
Correlates code data in both identity and height modes.
Checks for emergency, radio fail, SPI indications, etc.
97
What does a plot extractor produce?
Produces one complete target report per
aircraft per aerial revolution and outputs this in
digital form.
Produces synchronisation messages.
Produces north mark messages
98
Data from the flight plan database can be
processed with the radar signal data enabling
useful information to be presented on the radar
display:
Conversion of Mode A squawk code to aircraft
callsign.
Aircraft destination included in the radar
response label.
99
What happens during the track fusion process?
Processor determines an average position for
each aircraft (each “track”) based on returns
from each radar antenna.
Reduces inaccuracies in displayed position.
Allows uninterrupted display of aircraft even if
one radar antenna fails or loses contact.
100
What are the advantages of surveillance data networking?
Each unit in the network therefore has access to
all radar/ADS sources in the network.
Provides enhanced coverage and extra
redundancy and allows multi radar processing
and track fusion
101
Moveable data includes?
DF BEARING LINES
ELECTRONIC RANGE & BEARING MARKER - ERBM
ELECTRONIC CURSOR
SYMBOLS
ALPHA NUMERICS
102
Video mapping includes?
SIGNIFICANT POINTS - nav. aids, holding points, etc.
FINAL APPROACH TRACKS with ranges
AIRSPACE RESTRICTIONS
CONTROLLED AIRSPACE BOUNDARIES
COASTLINES & RIVERS
FIR &NATIONAL BOUNDARIES
LATITUDE & LONGITUDE
103
What does mode S employ?
ground based sensors, and airborne
transponders
104
What year did transponders need to allow Mode S?
1992
105
Aircraft Operators will be required to equip their
aircraft with Mode S airborne equipment that
supports...
Mode S Elementary Surveillance
functionality
106
Benefits of Mode S
The use of selective interrogation of aircraft
reduces FRUITING and GARBLING
Mode S also offers a data link facility, which will
be used to enhance safety by providing
Downlink Aircraft Parameters (DAPS)
107
Mode S coverage limitations
SSR coverage is limited by Line of Sight.
Cone of Silence (or ‘Overhead Gap’)
Min & Max Elevations (e.g. 0 to 60 degrees)
Depends on antenna design and configuration.
108
Operation: Acquisition Phase
The interrogator tracks conventional
transponders and looks for new Mode S
transponders and retains in memory the
individual address
109
Addressed Surveillance Phase
Mode S aircraft are interrogated individually for
ATC identification, altitude information and
exchange of other data using longer
communication messages
110
2 stages of Mode S
Mode S Elementary Surveillance
(ELS) and Mode S Enhanced Surveillance (EHS)
111
Basic Surveillance functionality (elementary surveillance)
24-bit technical identification.
Mode A code.
Altitude reporting to 25ft (Mode C).
Transponder capability reports.
Datalink capability report.
Common usage GICB report (BDS 1,7).
Aircraft Identification - call sign (BDS 2,0).
Flight status (airborne / on the ground).
Including Emergency situations + SPI.
SI-Code functionality.
112
Mode S Interrogations and replies also pass more data between air and ground, such as:
Altitude coded in 25ft increments.
Indication whether aircraft is airborne or on the ground.
Aircraft Identification (call sign) used for Elementary
Surveillance.
113
What code is not compatible with Mode S?
IATA
114
What is selective interrogation?
Using the unique 24-bit address, attached to an
individual airframe, ATCOs can select each
aircraft individually, rather than having a Radar
screen cluttered with labels.
115
What does ELS allow the controller to do?
co-ordinate with an adjacent centre without referring to Mode-A codes
116
8 DAPs-
1. Selected Altitude
2. Roll Angle
3. Track Angle Rate
4. True Track Angle
5. Ground Speed
6. Magnetic Heading
7. Indicated Airspeed (IAS) /
Mach no.
8. Vertical Rate
117
Enhanced Surveillance can provide the controller with:
ELS DAPs plus Heading + Speeds + Selected Altitude.
118
The provision of Mode S EHS will support the following
operational improvements:
Automatic provision of airborne derived data to enhance
ground systems functions including surveillance.
Use of data link to improve efficiency of communications.
Use aircraft derived data for ground based safety nets.
Enhance ATC decision support by using aircraft derived data.
Maintain and improve the quality of surveillance.
119
The safety benefits of Mode S EHS accrue through the
improvement of the controller’s situational awareness by:
Early recognition of aircraft manoeuvres.
Detection of level busts.
Reduction in risk of communication errors.
Improvement of safety by provision of a more precise
prediction of horizontal and vertical aircraft behaviour
Improvement of safety in high density traffic areas
through increased situational awareness
120
The capacity benefits of Mode S EHS accrue through:
Reduction of both controller and aircrew workload.
Direct provision of up-to-date aircraft parameters to the controller.
Extension of the domain of common reference for aircrew and
controller.
Reduction of voice channel occupancy.
Improvement of the capacity of pre-regulation (e.g. sequencing) in
terminal sectors.
Reduction of the controller workload by reducing uncertainty
concerning expected behaviour of the aircraft.
Improvement of efficiency by allowing for more anticipation in
planning of traffic.
121
What does Mode S Multilateration provide?
accurate surveillance and identification of all transponder equipped aircraft on the airport surface
122
ACTIVE MULTILATERATION SYSTEMS
Independent of existing infrastructure.
Improved detection of Mode A/C only aircraft.
Complementary information to position.
(Mode A, Mode C, A/C ID)
Increase accuracy at long range.
123
ADS-B
(Automatic Dependant Surveillance Broadcast)
A means by which aircraft, aerodrome vehicles
and other objects can automatically transmit
and/or receive data such as identification,
position and additional data, as appropriate, in a
broadcast mode via a data link.
124
ADS-B overview-
Broadcasts; horizontal position, vertical position,
and velocity; as well as other information.
Can be used by aircraft or vehicles.
Also can be received by ground facilities.
Enables a passive system to monitor, uniquely identify, and track all targets in range.
Suitable for ground, and airborne application.
(For sole-source surveillance)
Is reliant on ALL users providing accurate information ALL of the time
125
Aircraft requirements for ADS-B OUT-
GPS navigation capability.
Mode-S Transponder.
Avionics capable of providing correct data (usually comes with Enhanced Surveillance capability).
Correct connections between the three
126
What are the typical comms links?
satellite and microwave
127
ADS-B Message Types
Three Primary Types – Regular Broadcast
Position (Surface and Airborne)
Airborne Velocity
Aircraft Identity
Various Additional Types – ‘On Event’
Trajectory
Status
Test
128
ADS-B Permits High Update Rates of:
2 Position Messages Per Second.
2 Velocity Messages Per Second.
2 ‘On Event’ Messages Per Second.
0.2 Flight Identity (i.e. 5s Update)
129
ADS-B Maximum and minimum accuracy?
Maximum Accuracy Category is 7.5m.
Minimum Accuracy Category is >20NM.
130
What does the "C" is ADS-C mean?
the ground system provides the
aircraft with a list of reports to send and
specifies when they are required to be sent
131
three types of ADS contracts:
Periodic contract.
Demand contract.
Event contract.
132
What process determines the accurate position of aircraft or vehicle?
Time Difference of Arrival
(TDOA)
133
Benefit of MLAT-
Cheaper
Easier to install
Wide coverage
134
ADVANTAGES PRIMARY RADAR OVER SECONDARY RADAR
SELF CONTAINED
INDEPENDENT OPERATION
NON–RELIANT
PERMANENT ECHOES
WEATHER INFORMATION
135
DISADVANTAGES OF PRIMARY OVER SECONDARY RADAR
NO LEVEL INFORMATION
IDENTIFICATION
WEATHER CLUTTER
PERMANENT ECHOES
RANGE DEPENDENT ON
NO EMERGENCY INDICATION
136
Secondary radar ADVANTAGES (over primary radar)
Less transmitter power needed for a similar range.
No ground or weather clutter.
Identification of aircraft simplified and displayed.
Vertical position information displayed.
Emergency squawk codes.
Independent of the equivalent echoing area.
137
Secondary radar DISADVANTAGES (over primary radar)
All Aircraft require a transponder.
No weather returns.
Fruit.
Garbling.
Fading.
No permanent echoes for display alignment.
(calibration)
138
AUTOMATIC SAFETY NETS
SSR system automatically interprets aircraft
position symbols and Downlinked Airborne
Parameters (DAPs) to detect and alert
controller to potential dangers
139
Short Term Conflict (STCA)
ground-based safety net intended to assist
the controller in preventing collision between
aircraft by generating, in a timely manner, an
alert of a potential or actual infringement
of separation minima
140
Medium-Term Conflict Detection (MTCD)
flight data processing system designed to warn the
controller of potential conflict between flights in his area of
responsibility in a time horizon extending up to 20 minutes
ahead.
141
Features of MTCD
Trajectory prediction: responsible for creating, in the system, future trajectories for each aircraft/target.
Conflict detection: responsible for identifying in the system potentially conflicting trajectories. (not necessary the separation minima)
Trajectory update: responsible for updating in the system the predicted trajectories whenever this occurs.
Trajectory edition: responsible for allowing the human interaction with the predicted trajectory of one or more aircraft/targets.
142
Minimum Safe Altitude Warning (MSAW)
A ground-based safety net intended to warn the controller about increased risk of controlled flight into terrain accidents by generating, in a timely manner, an alert of aircraft proximity to terrain or obstacles.
143
What is the main purpose of MSAW?
enhance safety and not to monitor adherence to any specified minima. In practice MSAW is a part of the ATC system and from this perspective it can be regarded as a “function”.
144
Runway Incursion Monitor and Collision Avoidance System
(RIMCAS)
Fitted to some surface movement surveillance systems.
Provides Tower controllers with audio and visual warnings of
potential conflictions on runways.
(PSR and SSR returns can both trigger RIMCAS)
145
Approach Monitoring Aid
Also known as Approach Funnel Deviation Alerting System. (AFDAS)
Automatically monitors aircraft on final approach and provides visual and aural alert to Tower controller of any significant lateral or vertical deviation from final approach track or glide path
146
Controlled Airspace Infringement Tool (CAIT)
Radar system monitors squawks and mode C readouts to alert radar controller to any unauthorised entries into controlled airspace.
147
What is primary CAIT?
A version of CAIT can also detect non squawking aircraft entering CAS which extends upwards from the surface with no upper limit
148
What is an FMS?
a computerised avionics component found on most commercial and business aircraft to assist pilots in
navigation, flight planning, and aircraft control functions.
149
FMS 3 main components:
FMC (Flight Management Computer)
AFS (Auto Flight System)
Navigation System including IRS and GPS.
150
The FMC is taken as the core of FMS whose primary
function is to:
Compute real-time navigation information
Calculate Performance Data
151
CPDLC
Controller pilot data link
152
CPDLC adds a number of benefits such as:
Allow flight crew to print messages, to give better
understanding to foreign pilots.
Allowing auto load of specific uplink messages into the FMS reducing crew-input errors.
Allowing crew to downlink complex route clearances
and requests, which ATCO’s can re-send when approved without having to type a long string of coordinates
153
Who can initiate and cancel an emergency reporting mode in ADS-B?
The pilot
154
A clearance delivered by CPDLC Requires no specific readback. True or False?
True
155
The flight crew can set the data-link emergency mode by
two methods:
The sending of a CPDLC MAYDAY message automatically places the CPDLC and the ADS functionality into Emergency Mode.
The crew can also select ADS Emergency Mode
independently of CPDLC.
156
What is ACARS?
AIRCRAFT COMMUNICATIONS, ADDRESSING AND REPORTING SYSTEM
A digital data link system for the transmission of messages between aircraft and ground stations, which has been in use since 1978.
157
ACARS messages may be of three types based upon
their content:
Air Traffic Control (ATC)
Aeronautical Operational Control (AOC)
Airline Administrative Control (AAC)
158
Burst transmissions are used with a limit of:
220 characters per message. Transmissions often last less than one second
159
ATC ACARS messages:
Pre-Departure
Datalink ATIS
En-route Oceanic Clearances
160
AOC & AAC ACARS messages:
Upload to the aircraft of final load and trim sheets;
Upload of weather or NOTAM information;
Download from the aircraft of status, position, eta,
and any diversion;
Download of spot weather observations from aircraft sensors;
Download of technical performance data including
automatically triggered exceedance or abnormal
aircraft system status information;
‘Housekeeping’ Information such as catering uplift
requirements, special passenger advice and ETA.
161
Benefits of ACARS
It automatically sends messages about each phase
of flight OOOI msgs (out of the gate, off the
ground, on the ground, and into the gate).
Information on aircraft systems in real time.
Interfaces with the FMS system for flightplans and
weather information.
162
ICAO has proposed commercial aircraft
report their position every...
15 minutes
163
What can ACARS be used for outside of aviation?
MET observations
164
Voice communications available in ATC:
Direct Controller to Controller Communication (Talking)
Normal Telephone Communication.
Interphone (via Controllers Headset)
Intercom system.
Mobile telephones
165
SELCAL
signalling method which can alert an individual
aircraft that a ground station wishes to communicate with it
166
ATOTN
Air traffic operational telephone network
167
Who's responsible for ensuring that all necessary stations know its SELCAL code?
aircraft company/pilot
168
In the event the SELCAL signal remains unanswered
after two calls on the primary frequency and two calls
on the secondary frequency, the ATCO should...
Revert to voice calling
169
BTC 5-24
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