Overview of the B737NG Communication systems.

1. B 737 NG Ground School.
See the aircraft study guide at www.theorycentre.com
The information contained here is for training purposes only. It is of a general nature it is
unamended and does not relate to any individual aircraft. The FCOM must be consulted for
up to date information on any particular aircraft.

2. COMMUNICATIONS

3. GENERAL EXTERNAL ANTENNA LOCATIONS

4. Introduction
The communication system includes:
• radio communication system
• interphone communication system
• cockpit voice recorder system
• communication crew alerting system
The communication systems are controlled using the:
• audio control panels
• radio tuning panels
• radio communication panels

5. Radio Tuning Panel
Navigation radio
Audio Control panel
SELCAL panel
AFT Electronic panel

6. Audio Systems and Audio Control Panels
An ACP is installed at the Captain, First Officer, and Observer stations. Each
panel controls an independent crew station audio system and allows the
crewmember to select the desired radios, navigation aids, interphones, and PA
systems for monitoring and transmission.

7. Transmitter selectors on each ACP select one radio or system for transmission
by that crewmember. Any microphone at that crew station may then be keyed to
transmit on the selected system.
Receiver switches select the systems to be monitored. Any combination of
systems may be selected. Receiver switches also control the volume for the
headset and speaker at the related crew stations. Audio from each ACP is
monitored using a headset/headphones or the related pilot’s speaker.
Audio warnings for altitude alert, ground proximity warning, collision avoidance,
and windshear are also heard through the speakers and headsets at preset
volumes. They cannot be controlled or turned off by the crew.

8. AUDIO CONTROL PANEL (Typical)
Transmitter Selector (MIC SELECTOR) Switches
Illuminated – related switch is active
Push –
• selects related communication
system for subsequent transmission
• only one switch may be selected at a
time; pushing a second switch
deselects first switch
• reception possible over selected
system regardless of whether related
receiver switch is on.

9. AUDIO CONTROL PANEL (Typical)
Receiver Switches
Illuminated (white) – related switch
is active
Rotate – adjusts volume
Push –
• allows reception of related
communication system or
navigation receiver
• multiple switches may be selected
Push again – deselects related
system or receiver.
The light may also be integral with
the volume control

10. AUDIO CONTROL PANEL (Typical)
Push–to–Talk Switch
(spring–loaded to neutral position)
RADIO (radio–transmit) –
keys oxygen mask or boom
microphone for transmission as
selected by transmitter selector.
INT (Intercom) – keys oxygen mask
or boom microphone for direct
Transmission over flight interphone
and bypasses transmitter selector.

11. AUDIO CONTROL PANEL (Typical)
Filter Switch
V (Voice) – receive NAV and ADF voice
audio.
B (Both) – receive NAV and ADF voice
and range audio.
R (Range) – receive NAV and ADF
station identifier range (code) audio.

12. AUDIO CONTROL PANEL (Typical)
Speaker (SPKR) Switch
Illuminated (white) – SPKR switch is
active.
Push – audio from selected receiver is
heard on overhead speaker.
Rotate – adjusts overhead speaker
volume.
Push again – deselects audio from
selected receiver to be heard on
Overhead speaker.

13. AUDIO CONTROL PANEL (Typical)
Alternate–Normal Switch
NORM (Normal) – ACP operates
normally.
ALT (Alternate) – ACP operates in
degraded mode.

14. AUDIO CONTROL PANEL (Typical)
CALL Light
Illuminated (white) – Accompanied
by a chime, indicates call received by
SELCAL, ACARS (if installed),
ground crew (INT),
SATCOM (if installed), or
flight crew (CABIN).
Resets when transmitter is selected
and microphone is keyed.
PA does not have a CALL indication.

15. Speakers and Headsets
Each crew station has a headset or headphone jack. The Captain and First Officer
have speakers on the ceiling above their seats. There is no speaker at the
observer station. Headset volume is controlled by the receiver switches. Speaker
volume is controlled by the receiver switches and also the speaker switch.

16. Microphones
Hand microphones and boom microphones may be plugged into the related
jacks at the flight deck crew stations. Each oxygen mask also has an integral
microphone.
An oxygen mask microphone is enabled and the boom microphone is disabled
when the left oxygen mask panel door is open. The oxygen mask microphone is
disabled and the boom microphone is enabled when the left oxygen mask panel
door is closed and the RESET/TEST Switch is pushed.
Each hand microphone has a PTT switch to key the selected audio system. The
PTT switches on the control wheel or ACP are used to key the oxygen mask or
boom microphone, as selected by the RADIO/INT switch. The RADIO/INT
switch does not affect the operation of the hand microphone.
A MIC switch may be mounted on the captain’s and first officer’s glareshield
panel and is used to key the oxygen mask or boom microphone, as selected by
the transmitter switch on that pilot’s ACP. Pushing the glareshield MIC switch is
the same as pushing the control wheel MIC switch.

18. RADIO TUNING PANELS
VHF Communications Transfer (TFR) Switch
Push – selects standby frequency as active frequency for transceiver.
Active frequency becomes standby frequency.
Frequency Selector
Rotate – selects frequency in standby indicator:
• outer selector changes three left digits
• inner selector changes two right digits.
NOTE: There are a number of different RTP’s in use depending on the
aircraft radio fit.

19. RADIO TUNING PANELS
Older style ACP is directly connected to
the onside radio.
Captain Left
F/O right.

20. RADIO TUNING PANELS
With Newer style ACP all
communications are passed through a
Remote Electronics Unit. This enables
connection to any radio from any crew
position.

21. B737 Typical Radio tuning Panel
Each VHF radio has an Active and Standby frequency.
Radio Tuning Switch light indicates which radio is
selected on this panel

22. B737 Typical Radio tuning Panel
Panel OFF light illuminated White. Not connected to any Radio.

23. B737 Typical Radio tuning Panel
HF Sensitivity Knob.
Hard wire connected to ON SIDE HF Radio.
Captain HF 1 and F/O HF 2 as installed.

24. B737 Typical Radio tuning Panel
AM Switch
Push – light on sets the AM (amplitude modulation)
or light off USB (upper side band) mode for the
selected HF radio.

25. B737 Typical Radio tuning Panel
VHF TEST Switch
Push –
• removes automatic squelch feature, permitting
reception of background noise and thereby testing
receiver operation
• improves reception of weak signals.

26. The selected Radio is not available
The Panel is not able to communicate with the selected Radio.
It does not mean a Radio failure has occurred.
Selecting on another panel may work.

27. This RTP has failed.

28. The Selected Radio has failed.
Option on some aircraft only.

29. Normal Audio System Operation
The Captain, First Officer, and Observer audio systems are located in a common
remote electronics unit in the E/E compartment. They function independently and
have separate circuit breakers. The audio systems are normally controlled by the
related ACPs through digital or computerized control circuits.
Degraded Audio System Operation
If the remote electronics unit or ACP malfunctions, the ACP cannot control the remote
electronics unit. Audio system operation can be switched to a degraded mode by placing
the ALT–NORM switch to ALT. In this mode, the ACP at that station is inoperative and
the crewmember can only communicate on one radio.
The ACP transmitter selectors are not functional. Any transmission from that
station must be from the radio shown on the chart below. The transmitter selector
for the usable radio illuminates when a station is operating in the degraded mode.
The receiver switches are not functional, and only the usable radio is heard at a
Preset volume, through the headset. The speaker and speaker switch are not
functional at that station. In addition, the flight interphone and service interphone
cannot be used. The control wheel PTT switch INT position and the ACP PTT
switch INT position are not functional since the flight interphone is not functional.
Audio warnings for altitude alert, GPWS, and windshear are not heard on an audio
system operating in the degraded mode.
An audio system operating in the degraded mode cannot access the passenger
address system through the audio control panel. The crewmember can still use the
service interphone handset and PA microphone if they are installed on the control
stand.

30. Degraded Audio System Operation
If the remote electronics unit or ACP malfunctions . Audio system operation can be
switched to a degraded mode . ALT–NORM switch to ALT. ACP is inoperative and the
crewmember can only communicate on one radio.

32. LIMITATIONS
Do not use VHF–3 (if installed) for voice communication with ATC VHF- 3 is for
ACARS use.
Note: The following items are not AFM limitations, but are provided for flight
crew information.
Use the VHF radio connected to the top of fuselage antenna for primary ATC
communications on the ground.
Note: VHF antennae located on the lower fuselage are susceptible to multipath
interference from nearby structures or vehicles. This may disrupt VHF
communications. VHF antennae located on the upper fuselage are not as
susceptible to this interference.
Note; Be cautious with VHF & UHF transmitting antennas
No person should ever be near a transmitting antenna
VHF & UHF has the most effect on soft tissue
WARNING: MAKE SURE THAT PERSONNEL STAY A MINIMUM OF 10 FT (3 M)
AWAY FROM THE VERTICAL STABILIZER WHEN THE HF SYSTEM TRANSMITS.
RF ENERGY FROM THE HF ANTENNA CAN CAUSE INJURIES TO PERSONNEL.

33. Flight Interphone System
The flight interphone system is an independent communication network. Its primary
purpose is to provide private communication between flight deck crewmembers without
intrusion from the service interphone system. The ground crew may also use the flight
interphone through a jack at the external power receptacle.
The pilots can transmit directly over the flight interphone by using the control
wheel PTT switch. Alternately, any crewmember with an ACP can transmit/receive over
the flight interphone by using their related ACP and normal PTT switches. Any standard
microphone may be used with the flight interphone system.
the flight interphone cannot be used when the audio system is operating in the degraded
mode.
Audio warnings for altitude alert, GPWS, TCAS and windshear are not heard on an
audio system operating in the degraded mode as they are relayed via the flight
interphone speakers.

34. FLIGHT INTERPHONE SYSTEM
For communication between the flight crew.
Can be used from the External power panel by the Ground crew.

35. Service (Attendant) Interphone System
The service interphone system provides intercommunication between the flight
deck, Flight Attendants, and ground personnel. Flight deck crewmembers
communicate using either a separate handset (if installed) or their related ACP and
any standard microphone.
The Flight Attendants communicate between flight attendant stations or with the
flight deck using any of the attendant handsets. Anyone who picks up a
handset/microphone is automatically connected to the system.
External jacks for use by maintenance or service personnel can be added to the
system by use of the service interphone switch.

36. SERVICE INTERPHONE
Used for communication between Flight and cabin crew.
Used Cabin crew to cabin crew.
Service Interphone switch (Aft Overhead panel.)
Turns ON additional interphone connection at service
points around the aircraft. Off does not affect Flight
crew and cabin crew operation.

37. All external jacks turned
Off by switch.
Jack Locations
These locations have service interphone jacks:
A. P19 external power panel
B. Electronic equipment compartment
C. Fueling station, door on the right wing
D. Right wheel well,
E. Left wheel well,
F Aft cabin, on the ceiling above the attendant station
G. APU service area.

38. Flight
interphone
Service
Interphone
External Power panel

39. Passenger Address System
The passenger address (PA) system allows flight deck crewmembers and flight
attendants to make announcements to the passengers. Announcements are heard
through speakers located in the cabin and in the lavatories.
The flight deck crewmembers can make announcements using a PA handset or by
using any standard microphone and the related ACP. Flight Attendants make
announcements using PA handset located at their stations. The attendants use the
PA to play recorded music for passenger entertainment.
PA system use is prioritized. Flight deck announcements have first priority and
override all others. Flight Attendant announcements override the music system.
Chimes
The chime circuits of the PA system supply these chime signals:
* High chime
* Low chime
* High/low chime.
High chimes operate when there is an attendant call signal from a passenger service
unit or lavatory.
Low chimes operate when the no smoking or fasten seat belts signs come on.
High/low chimes operate when there is an attendant call from the flight compartment
or another attendant station.

40. Call System
The call system is used as a means for various crewmembers to gain the attention
of other crewmembers and to indicate that interphone communication is desired.
Attention is gained through the use of lights and aural signals (chimes or horn).
The system can be activated from the flight deck, either flight attendant station, or
from the external power receptacle. Passengers may also use the system to call an
attendant, through the use of individual call switches at each seat.
The flight deck may be called from either flight attendant station or by the ground
crew. The ground crew may only be called from the flight deck. Flight Attendants
may be called from the flight deck, the other attendant station, or from any
passenger seat or lavatory. Master call lights in the passenger cabin identify the
source of incoming calls to the attendants.
Call system chime signals are audible in the passenger cabin through the PA
system speakers. The PA speakers also provide an alerting chime signal whenever
the NO SMOKING or FASTEN SEAT BELT signs illuminate or extinguish.

41. Master Call Lights
Illuminated –
• amber – a lavatory call switch is activated or smoke is detected in a lavatory.
• pink – flight deck or other flight attendant station is calling.
• blue – a passenger seat call switch is activated.

42. CALL SYSTEM
Ground crew can be
called only from the flight
deck. Call sounds the
ground crew call horn in
the nose wheel well
The Ground crew can call the flight deck
from the External power panel.
C light illuminates on ACP INT switch.
Tone Aural and CALL light if fitted.

43. VHF Communications
Primary short–range voice communications is provided in the VHF range by two
or three independent radios. Each radio provides for selection of an active
frequency and an inactive (preselected) frequency. Voice transmission and
reception are controlled at the related ACP.
HF Communications
There are one or two independent HF communication radios, designated HF 1
and HF 2.
Each HF radio can be tuned by any radio tuning panel. HF radio sensitivity can
only be set on the on-side radio tuning panel.
Some aircraft have a separate HF radio tuning panel.
When an HF transmitter is keyed after a frequency change, the antenna tunes. A
steady or intermittent tone may be heard through the audio system. While tuning,
the tone can last as long as 7 seconds. If the system fails to tune, the tone will
last more than 7 seconds, to a maximum of 15 seconds. The antenna is located
in the vertical stabilizer. Both HF radios use a common antenna. When either HF
radio is transmitting, the antenna is disconnected from the other HF radio, and it
cannot be used to transmit or receive. However, both HF radios can receive
simultaneously if neither is being used for transmitting.
Note: Data for the last 100 tuned frequencies is stored in memory. Tuning
duration for these stored frequencies will be very short and a tune tone may
not be noticeable. (Aircraft fit)

44. Selective Calling (SELCAL)
In international aviation, SELCAL or SelCal is a selective-calling radio system that
can alert an aircraft's crew that a ground radio station wishes to communicate with
the aircraft. SELCAL uses a ground-based encoder and radio transmitter to
broadcast an audio signal that is picked up by a decoder and radio receiver on an
aircraft. The use of SELCAL allows an aircraft crew to be notified of incoming
communications even when the aircraft's radio has been muted. Thus,
crewmembers need not devote their attention to continuous radio listening.

45. SELCAL operates on the high frequency (HF) or very high frequency (VHF)
radio frequency bands used for aircraft communications. HF radio often has
extremely high levels of background noise and can be difficult or distracting to
listen to for long periods of time. As a result, it is common practice for crews to
keep the radio volume low unless the radio is immediately needed. A SELCAL
notification activates a signal to the crew that they are about to receive a voice
transmission.
An individual aircraft has its own assigned SELCAL code. To initiate a SELCAL
transmission, a ground station radio operator enters an aircraft's SELCAL code
into a SELCAL encoder. The encoder converts the four-letter code into four
designated audio tones. The radio operator's transmitter then broadcasts the
audio tones on the aircraft's company radio frequency channel in sequence: the
first pair of tones are transmitted simultaneously, lasting about one second; a
silence of about 0.2 seconds; followed by the second pair of tones, lasting about
one second.
The code is received by any aircraft receiver monitoring the radio frequency on
which the SELCAL code is broadcast. A SELCAL decoder is connected to each
aircraft's radio receiver. When a SELCAL decoder on an aircraft receives a
signal containing its own assigned SELCAL code, it alerts the aircraft's crew by
sounding a chime, activating a light, or both.
The crew next turns up the volume on the aircraft radio to hear the incoming
voice transmission.

46. An individual aircraft is given a SELCAL code upon application to the SELCAL code
registrar, Aviation Spectrum Resources, Inc. (ASRI). The code is technically
assigned to the owner-operator of the aircraft rather than the aircraft itself; if an
aircraft is sold, the new owners-operators must apply for a new code.
The code is a sequence of four letters, written or transmitted as an ordered two sets
of two letters each (e.g., AB-CD). The letters are chosen from a subset of the Latin
script comprising A through S, excluding I, N and O. The letters within a given pair
are written or transmitted in alphabetical order (e.g., AB-CD is an allowable distinct
SELCAL code, as is CD-AB, but CD-BA is not). A given letter can be used only
once in a SELCAL code; letters may not be repeated (e.g., AB-CD is allowable,
but AA-BC and AB-BC are not).
Each letter designates a specific audio tone frequency.

47. The current rules for SELCAL code assignment, with sixteen available
letters/tones, limit the number of possible allowable codes to 10,920.
Additionally, SELCAL codes assigned previously use a subset of only twelve
letters/tones. Therefore, more than one aircraft may be designated by the same
code.
To avoid confusion from two or more aircraft using the same SELCAL code,
ASRI tries to assign code duplicates to aircraft that do not usually operate in the
same region of the world or on the same HF radio frequencies. However, aircraft
commonly move between different geographical regions and it is now routine for
two aircraft with the same SELCAL code to be found flying in the same region.
Therefore, air crew always verify both SELCAL and call sign (i.e., aircraft tail
registration, or telephony designator and flight identification) to be sure their
aircraft is the intended recipient.

48. SELECTIVE CALLING
Selective calling is automatic.
Each aircraft has a four letter SELCAL code. Each letter has a tone. The aircraft listens to
the airwaves for its own tone using the VHF & HF radios.
An aural tone sounds and a SELCAL light illuminates for the radio receiving the message.
HF RADIO
VHF RADIO
SELCAL
DECODER
SELCAL
AURAL
RELAY
Remote
Electronics Unit

49. SELCAL PANEL

50. Cockpit Voice Recorder
The cockpit voice recorder uses four independent channels to record flight deck
audio for 30 minutes or 120 minutes. Recordings older than 30 or 120 minutes
are automatically erased. One channel records flight deck area conversations
using the area microphone. The other channels record individual ACP output
(headset) audio and transmissions for the pilots and observer.
(Aircraft fit dependent)
The RIPS (Recorder Independent Power Supply) provides power to the cockpit
voice recorder for 10 minutes after aircraft power is interrupted either by normal
shutdown or by any other loss of power.
Normal power supply 115V transfer bus 2 and 28 V from DC bus 2.

51. VOICE RECORDER Switch (when fitted)
AUTO - powers the cockpit voice recorder from first
engine start until 5 minutes after last engine shutdown
ON - powers the cockpit voice recorder until first engine
start, then trips the switch to AUTO.
On aircraft with no switch the voice recorder is active
anytime 115V AC is applied to airplane.

52. Area Microphone
Active anytime the voice recorder is active.
Records one channel
The voice recorder unit collects these audio from the R.E.U.at the
same time:
* Captain microphone and headphone
* First officer (F/O) microphone and headphone
* First observer (F/OBS) microphone and headphone
* The voice recorder unit also receives time from the Captains clock.

53. STATUS Light
Illuminated (momentary green) – no faults are detected
during recorder TEST.

54. ERASE Switch (red)
Push (2 seconds) –
• all four channels are erased
• operative only when airplane is on ground and parking
brake is set.

55. TEST Switch
Push – after a slight delay and no faults are detected:
• status light illuminates momentarily
• a tone may be heard through a headset plugged into
HEADPHONE jack.

56. HEADPHONE Jack
Headphone may be plugged into jack to monitor tone
transmission during test, or to monitor playback of voice audio.

57. Aircraft Communications Addressing and Reporting System
(ACARS)
The ARINC Communications Addressing and Reporting System (ACARS) is an
addressable digital data link system which permits exchange of data and
messages between an airplane and a ground-based operation centre utilizing an
onboard VHF communications system.
The ACARS airborne subsystem provides for the manual entry of routine data
such as departure/arrival information. Also possible is manual entry of addresses
(telephone codes) of parties on the ground for voice communications.
The airborne system consists of a management unit in the E/E compartment,
either a interactive display unit or multipurpose control display unit (MCDU), and
frequently a printer. Data is entered and transmitted to the ground operations
centre.
A worldwide ground data network of radio stations provides ACARS service.
Over the next 20 years, ACARS will be superseded by the
Aeronautical Telecommunications Network (ATN) protocol for Air Traffic Control
communications and by the Internet Protocol for airline communications.

58. One of the applications for ACARS is to automatically detect and report
changes to the major flight phases (Out of the gate, Off the ground, On the
ground, and Into the gate), referred to in the industry as OOOI. These OOOI
events are determined by algorithms that use aircraft sensors (such as doors,
parking brake and air ground sensors) as inputs. At the start of each flight
phase, a digital message is transmitted to the ground containing the flight
phase, the time at which it occurred, and other related information such as the
amount of fuel on board or the flight origin and destination. These messages
are used to track the status of aircraft and crews.
In addition to detecting events on the aircraft and sending messages
automatically to the ground, There may be interfaces with other on-board
avionics. A data link interface between the ACARS management units and
flight management systems. This interface enables flight plans and weather
information to be sent from the ground to the ACARS management unit, for
forwarding to the flight management system. This feature gives the airline the
capability to update flight management systems while in flight, and allows the
flight crew to evaluate new weather conditions or alternative flight plans.

59. The ACARS management unit can be linked to the flight data acquisition and
management system and the aircraft condition monitoring system systems which
analyze engine, aircraft and operational performance conditions to provide
performance data to the airlines on the ground in real time using the ACARS
network. This reduces the need for airline personnel to go to the aircraft to off-load
the data from these systems. These systems are capable of identifying abnormal
flight conditions and automatically sending real-time messages to an airline.
Transmission of maintenance-related information in real-time through ACARS.
enables airline maintenance personnel to receive real-time data associated with
maintenance faults on the aircraft. When coupled with the flight data acquisition
and management system data, airline maintenance personnel can start planning
repair and maintenance activities while the aircraft is still in flight.
Detailed engine reports can also be transmitted to the ground via ACARS. The
airlines use these reports to automate engine trending activities.

60. ACARS
Aeronautical Radio, Incorporated (ARINC)
Communicating Addressing & Reporting System.
VHF based digital data link.
Used to pass data from Air to Ground and Ground to Air.
Can be used to send automatic reports like Brake release for
push back and transition to Air mode for T/O time reporting.
OOOI reports

61. ACARS
Reports can be sent from the MCDU.
Flight information can be sent to the printer.
Can be used for voice communication.
ACARS is accessed from the CDU Main Menu.
There are many possible reports which are
configured by the airline.
Includes.
Report type
Flight number
Destination and Origin.
Aircraft registration number.
Date and time sent.

62. The ACARS management unit receives flight
information including flight number from the FMC

63. ACARS can be used to send both Manual and
automatic reports. The Airline can use the
information for future performance and flight
planning.

64. Airlines decide how to use ACARS and
the ACARS menu on an aircraft dictates
what the crew can use it for.

65. On the Captains Audio Control Panel the ALT/NORM switch is selected to ALT which radios
are available for the Captain?

66. On the Captains Audio Control Panel the ALT/NORM switch is selected to ALT which radios
are available for the Captain?
The Captain can transmit and receive only on VHF 1.

67. How is the volume of an Aural warning adjusted?

68. How is the volume of an Aural warning adjusted?
Audio warnings for altitude alert, ground proximity warning, collision
avoidance, and windshear are heard through the speakers and headsets at
preset volumes. They cannot be controlled or turned off by the crew.

69. List the PTT switches that each pilot has;

70. 1.
2.
3.
4.
Audio control panel.
Hand Microphone.
Glare shield.
Control wheel.

71. INOP is shown in both the ACTIVE and STANDBY windows of the Radio Tuning Panel;
a) Both VHF radios have failed
b) The selected radio has failed.
c) The radio tuning panel has failed.
d) The selected radio is not available.

72. INOP is shown in both the ACTIVE and STANDBY windows of the Radio Tuning Panel;
a) Both VHF radios have failed
b) The selected radio has failed.
c) The radio tuning panel has failed.
d) The selected radio is not available.

73. AIR/GROUND PUBLIC TELEPHONE SYSTEM - GENERAL DESCRIPTION
(OPTION)

74. AIR/GROUND PUBLIC TELEPHONE SYSTEM - GENERAL DESCRIPTION
(OPTION)
The air/ground public telephone system lets the passengers make telephone calls
to persons on the ground.
The cabin telecommunication unit is the central processor for the air/ground public
telephone system. The CTU receives calls from the telephone handsets and sends
them to the ground telephone network via the terrestrial flight telecommunication
system.
The air traffic control system sends the airplane identification and airplane altitude
to the CTU.
The air/ground relay tells the CTU when the airplane is in the air.
The terrestrial flight telecommunication system (TFTS) is a UHF radio system. The
TFTS provides line-of-sight connections for telephone calls from the airplane to
ground stations.

76. Satellite Communications (SATCOM) System (OPTION)
The SATCOM system provides both data and voice communications. The
satellite data unit is controlled through the control display units (CDUs). Voice
transmission is controlled using CDUs and the audio control panels.
Calls can be initiated using the CDU. Directories of airline defined numbers
are line selectable or manual numbers can be entered. The SATCOM CDU
control pages are displayed by selecting SAT on the MENU page.
SATCOM menu configuration is defined by the individual airline.

77. Satellite Communications (SATCOM) System

78. RADIO THEORY FOR THOSE NOT FAMILIAR WITH HF
High Frequency (HF) radio is used for first-line and backup communications
over long distances, mainly in remote regions.
Whilst VHF is used for short-range line-of sight (LOS) communications, only
HF is capable of communicating over distances of 3000 km or more.
In HF radio transmission there are two modes of operation.
amplitude-modulated wave consists of a carrier and two identical sidebands
which are spaced above and below the carrier by an amount equal to the
modulating frequency. These are called the upper and lower
sidebands. One sideband contains all the elements necessary for voice
transmission.
carrier wave
Radio a wave of fixed amplitude and frequency that is
modulated in amplitude, frequency, or phase in order to carry a signal in radio
transmission,
Upper sideband
Unmodulated
carrier
Lower sideband
Modulated carrier

79. RADIO THEORY FOR THOSE NOT FAMILIAR WITH HF
Amplitude modulation, AM is the most straightforward way of modulating a signal.
Demodulation, or the process where the radio frequency signal is converted into an
audio frequency signal is also very simple. An amplitude modulation signal only
requires a simple diode detector circuit.
The second mode is single-sideband modulation (SSB) this is a refinement
of amplitude modulation that more efficiently uses transmitter power and bandwidth.
Amplitude modulation produces an output signal that has twice the bandwidth of the
original baseband signal. Single-sideband modulation avoids this bandwidth
doubling, and the power wasted on a carrier, at the cost of increased device
complexity and more difficult tuning at the receiver. The upper sideband USB is
used in this form of transmission.
USB
SSB AM TRANSMISSION

80. Radio propagation is the behaviour of radio waves when they are transmitted,
or propagated from one point on the Earth to another, or into various parts of
the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are
affected by the phenomena of reflection,
refraction, diffraction, absorption, polarization and scattering.
Radio propagation is affected by the daily changes of water vapour in the troposphere and
ionization in the upper atmosphere, due to the Sun. Understanding the effects of varying
conditions on radio propagation has many practical applications, from choosing
frequencies for international shortwave broadcasters, to designing reliable mobile
telephone systems, to radio navigation, to operation of radar systems.
Radio propagation is also affected by several other factors determined by its path from
point to point. This path can be a direct line of sight path or an over-the-horizon path aided
by refraction in the ionosphere, which is a region between approximately 60 and 600 km
above the Earth. Factors influencing ionospheric radio signal propagation can
include solar flares, geomagnetic storms, ionospheric layer tilts, and solar proton events.

82. Surface mode (ground wave)
Lower frequencies (between 30 and 3,000 kHz) have the property of following the
curvature of the earth via ground wave propagation in the majority of occurrences.
In this mode the radio wave propagates by interacting with the semi-conductive
surface of the earth. The wave "clings" to the surface and thus follows the
curvature of the earth.
Direct mode (line-of-sight)
Line-of-sight is the direct propagation of radio waves between antennas that are
visible to each other. This is probably the most common of the radio propagation
modes at VHF and higher frequencies. Because radio signals can travel through
many non-metallic objects, radio can be picked up through walls. This is still lineof-sight propagation. Examples would include propagation between a satellite and
a ground antenna or reception of television signals from a local TV transmitter.
Ionospheric mode (skywave)
Skywave propagation, also referred to as skip, is any of the modes that rely
on refraction of radio waves in the ionosphere, which is made up of one or more
ionized layers in the upper atmosphere. F2-layer is the most important
ionospheric layer for long-distance, multiple-hop HF propagation.
The layers, or more appropriately "regions", are directly affected by the sun on a
daily diurnal cycle, a seasonal cycle and the 11-year sunspot cycle and determine
the effect of these modes

83. H.F. RADIO
Ionosphere
ADVANTAGES.
Good for transmitting
information over long distances.
Relatively simple equipment.
DISADVANTAGES.
Requires high power to transmit.
Suffers from many forms of
interference.
Sky wave
Direct wave
Ground wave
Dead Zone
The size of the dead zone is
called the skip distance.
Skip distance is dependent on
the height of the transmitting
antenna and the height of the
receiving antenna.

84. This is Amplitude Modulation. It
uses a large band width limiting
the number of stations which
can transmit at any one time.
It uses more power to transmit.
It is more prone to interference.

85. USB
SSB AM TRANSMISSION
This is USB. All of the information is still
carried on the carrier, but only half the band
width is used. This allows twice as many
transmission without interference.

86. Radio HF/VHF
HF (High frequency) radio frequencies are between 3 and 30 MHz.. Frequencies
immediately below HF are denoted Medium-frequency (MF)
VHF (Very high frequency) is the radio frequency range from 30 MHz to 300 MHz..
Common uses for VHF are FM radio broadcast, television broadcast, air traffic control
communications and air navigation systems (e.g. VOR, DME & ILS).
HF radio use
The high frequency band is very popular with radio operators, who can take
advantage of direct, long-distance (often inter-continental) communications
VHF radio use
VHF propagation characteristics are ideal for short-distance terrestrial
communication, with a range generally somewhat farther than line-of-sight from the
transmitter. Unlike high frequencies (HF), the ionosphere does not usually reflect VHF
radio and thus transmissions are restricted to the local area. VHF is also less affected
by atmospheric noise and interference from electrical equipment than lower
frequencies. Whilst it is more easily blocked by land features than HF, it is less
affected by buildings and other less substantial objects
Universal use
Certain subparts of the VHF band have the same use around the world.
108–118 MHz: Air navigation beacons VOR and Instrument Landing System localiser.
118–137 MHz: Air band for air traffic control, 121.5 MHz is the emergency frequency

87. VHF FM Transmissions.
VHF is line of sight only. The higher the
frequency the shorter the range.
For VHF we do not add amplitude to the
carrier we change or MODULATE the
frequency of the carrier.
This is FM radio.

88. Which VHF radio should be used on the Ground?
WHY?

89. Which VHF radio should be used on the Ground?
Note: The following items are not AFM limitations, but
are provided for flight crew information.
Use the VHF radio connected to the top of fuselage
antenna for primary ATC communications on the ground.
WHY?
SAFETY!

90. Communications.
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