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ELEC4504 Avionics Systems
91
CHAPTER 7. Communications Systems
7.1. Uses
7.1.1 Air Traffic Control
The primary use of aeronautical communications systems is in air traffic control. In fact the air traffic control
system could not function without it.
A typical flight starts with the submission of a flight plan by the pilot. This includes such things as the requested
route, the expected time of departure, the alternate airport(s), the requested altitude and the navigation equipment available on board the aircraft. This is passed, usually by telephone to the air traffic control centre which
fits the requested route into its schedule.
Before starting the engines, the pilot requests his clearance via the Clearance Delivery frequency. The clearance is the official approval to fly a prescribed route at a prescribed altitude. The approved route is usually the
one requested in the flight plan but there may be changes due to conflicts with other aircraft.
After starting the engines, the pilot talks to Ground Control who control the traffic (including vehicles) on the
airport with the exception of the runways.
Just before reaching the runway the pilot changes to the Tower frequency for takeoff clearance.
Once airborne, the pilot changes to Departure frequency and the departure controller provides guidance (called
“vectors”) to avoid local traffic until the aircraft is established on its approved route.
The pilot then changes to Centre (e.g. Toronto Centre) frequency and reports the progress of the flight at regular intervals. At this stage there is usually not much activity on the channel unless, for some reason, the pilot
wishes to change flight plan.
Near the destination the pilot changes to Arrival frequency and is given vectors to the final approach, at which
point the pilot changes to Tower frequency for landing clearance.
As well as talking to the air traffic controllers, the pilots listen to transmissions from other aircraft which keeps
them aware of the traffic in their immediate vicinity.
7.1.2 Emergencies
A special frequencies (121.5MHz in the VHF band and 243.0 MHz in the UHF band) are reserved for emergency transmissions. Emergency radio beacons also transmit on these frequencies.
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7.1.3 Airline Operations
In order to keep track of the location and status of their aircraft, airlines make use of communications links for
both verbal and digital transmission of information. For example, if a piece of equipment had failed, the crew
could contact the maintenance staff at the next destination to have replacement parts available thus saving considerable time. Some if not all airlines are now using communications links to transmit aircraft performance
data such as engine temperatures, vibration measurements, automatically without the need for action by the
crew.
7.1.4 Differential GPS Data Link
As was mentioned in the section on GPS, some sort of data link is required to transmit local area differential
GPS corrections and integrity information to the aircraft.
7.1.5 Passenger Communication
Due to the demand, primarily from business passengers, airlines are starting to provide telephone and data
transmission services on board their aircraft. This is also a requirement for the larger business jets such as the
Gulfstream IV and V and the Canadair Challenger
7.1.6 Automatic Dependent Surveillance
Air Traffic Control agencies are working to develop a means by which the positions of aircraft outside of radar
range can be shown on the controllers’ displays. This feature depends on the aircraft to determine its position
by GPS and then relay the position along with other pertinent information to the air traffic control centre. Since
this system is aimed at air routes in remote locations, the data must be transmitted by a long distance link; either
satellite or HF.
7.1.7 Requirements
The above applications place widely varying demands on the communications systems and thus the techniques
used will vary as well
7.1.8 Elements of Radio Communications
7.1.8.1 Modulation
In order to transmit information on a particular carrier signal it is necessary to modify, or modulate the carrier
in accordance with the information to be transmitted
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In the previous lecture on the radio spectrum the three major modulation techniques were mentioned: AM (amplitude modulation), FM (frequency modulation) and PM (phase modulation). in air communications the modulation used for voice transmissions is AM, primarily because it requires the smallest bandwidth. For digital
data transmissions phase modulation is used because it is relatively easy to introduce discrete phase changes
into the carrier e.g. +/- 180˚ (binary phase shift keying, BPSK) or +/-90˚ and +/- 180˚ (quad phase shift keying,
QPSK)
7.1.8.2 The Communications Channel Losses and Noise
The fundamental communications system configuration is a shown below:
Transmitter
Channel
Receiver
In the case of aeronautical communications, the channel is the path that the electromagnetic waves take as
they travel from the transmitting antenna to the receiving antenna. Inherent in this process is the presence
of losses. i.e. the received signal power is much less than the transmitted power. The primary reason for
this loss is that the signal power which arrives at the receiver is proportional to the receiver antenna area
and the power density (power per unit area) of the signal. Since, for a given square angle, the area increases
as the square of the distance, the power density at the receiver is proportional to
Pt
----------where Pt is the
2
4πr
transmitted power and r is the distance between the transmitter and receiver. This is called the path loss
and is the primary loss in the channel. Other losses are caused by mismatch between the transmitter and its
antenna, mismatch between the receiver and its antenna and attenuation in the signal path due to rain or
obstructions.
Because of the path loss, the signal level at the input to the receiver is very small, usually less than -100 dBm.
Note: dBm stands for decibels referred to a reference level of 1 milliwatt.
a decibel is a logarithmic measure of the ratio between two power levels:
P
dB = 10 log -----1P2
Thus -100 dBm is 10-10 milliwatts or 10-13 Watts
In order to extract the information from such a low level signal it is necessary to amplify it which, not surprisingly, is done with devices called amplifiers.
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On the surface it would appear that any signal level could be amplified to usable levels simply be adding more
amplifiers, and thus, for a given transmitter power, the system could be used at any range.
Unfortunately this is not possible due to a phenomenon called noise which is a random electrical signal which,
in certain circumstances, can mask the desired signal.
The sources of noise are many and varied. One major source is the sun and the rest of the universe. Another is
meteorological phenomena such as lightning. A third is manufacturing processes such as arc welding.
These are noise sources external to the receiver, but there are inescapable noise sources within the receiver
circuitry itself:
Ohms law states that the voltage across a resistor is equal to the current flowing in the resistor times its
resistance.
V = IR
Since the current consists of the total electron flow in the resistor, it would appear at first that a resistor by
itself, not connected to anything, would have a voltage of zero across its terminals. However, due to the
fact that the resistor is at a temperature above absolute zero, the electrons are all moving in a random
fashion. Thus, while the average current is zero, at any given time there are more electrons going in one
direction than in the other and this gives rise to a random voltage which is noise when the resistor is
inserted into a circuit.
Since it is impossible to build an amplifier without using resistors, any amplifier will contribute to the
noise level of the signal it is amplifying.
The presence of noise places a lower level on the signal power that can be processed usefully by the receiver.
This gives rise to the expression signal to noise ratio (SNR or S/N) which is used to describe the relative levels
of signal and noise in a system. It is usually expressed in dB.
Since the external noise is beyond our control, the only way to increase the communication range is to increase
transmitter power or decrease the internal receiver noise. (Low noise amplifiers or LNA)
7.1.8.3 Transmitter Design
A generic transmitter block diagram is shown below:
Source
Baseband
Converter
Frequency
translate
and
amplify
Carrier
Modulator
Synchronization
subsystem
Frequency
generator
Electromagnetic
Field
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- Source. The information source generates the electronic signals to be sent to the receiver. They are
classed as analog or digital. An analog source produces a time continuous signal. A digital source
produces sequences of data symbols.
- Baseband Converter. Source signals are generally converted into a baseband waveform to prepare it for
the carrier transmission. The principle objective is to insert suitable control and proper formulation of the
source output prior to carrier modulation. Conversion operation may take a variety of forms depending on
the application. For example, prefiltering, band limiting. preemphasis, encoding, multiplexing.
In simple systems this is not used and the analog source is connected directly to the modulator.
- Carrier Modulator. The baseband waveform is modulated onto the carrier to form the carrier waveform.
The objective is to generate a transmission waveform which is best suited for the electromagnetic
propagation and communication reception.
- Frequency translation and amplification. The modulated carrier is often frequency translated and
amplified to the desired level. Frequency translation involves a direct shift from one frequency to another.
Filtering is also performed on the frequency translated and amplified signal to ensure that spurious out-ofband emissions are minimized.
- Frequency Generator. Frequency shifting is accomplished with the aid of devices called frequency
synthesizers, which may also provide the carrier waveforms for modulation.
- Synchronization subsystem. In addition to the subsystems devoted to the processing of the source signal,
it may be necessary to provide auxiliary transmitter waveforms that aid in the transmission and recovery of
the desired information. The basic objective of these signals is to keep the link operationally aligned and
properly interfaced with the transmitter. These signals can take on a variety of forms and may be
superimposed on the source, baseband or carrier waveforms.
7.1.8.4 Receiver Design
A generic receiver block diagram is shown below:
Received
Carrier
Field
Receiver
Front end
Frequency
translate
Frequency
generator
IF
Amplifier and
Demodulator
Baseband
processor
Synchronization
system
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- Receiver Front End. The front end electronics tunes and filters the radio frequency (RF) signal to select
the desired communication link (determined by the frequency of the carrier wave). Filtering is done to
reduce the interference from adjacent carrier channels and to reduce the noise power. Sometimes the signal
is amplified at the carrier frequency before further processing.
- Frequency Translation. The RF signal is down-converted either directly to baseband frequency or to in
intermediate frequency (IF) for the demodulation process. Converting to an IF simplifies the design of the
amplifiers and reduces the cost.
Frequency conversion is done by multiplying the input signal by the frequency generator (or Local
Oscillator as it is usually called)
as follows:
A cos (ωc t) * B cos (ωLO t) = (AB/2)[cos (ωc - ωLO)t + cos (ωc + ωLO)t]
i.e. the result has two components, one whose frequency is equal to the difference between the two input
frequencies and the another whose frequency is equal to their sum. Since the objective is usually to
decrease the frequency of operation, the sum component is filtered out.
- Demodulator. The RF or IF signal is demodulated to recover the baseband waveform.
- Baseband processor. The effects of channel irregularities (noise, distortion) usually show up at this point
and the demodulated baseband no longer closely resembles the transmitted baseband signal Here the task
is to invert the baseband conversion processes done in the transmitter. Some examples are baseband
filtering, de-emphasis, decoding and demultiplexing.
- Synchronization system. If synchronization signals were provided at the transmitter, they are recovered
here to be used in processing the data signal.
- Frequency Synthesizer . One of the main means of generating electrical signals with stable frequencies
has been the quartz crystal. Quartz exhibits the piezoelectric effect by which physical deformation e.g.
compression, will induce a voltage across the crystal, while a voltage impressed across the crystal will
cause physical deformation to occur. Since the mechanical oscillation frequency of a given crystal is
almost entirely determined by its physical dimensions, connecting it to an electrical feedback circuits leads
to a very stable frequency generator.
However, since crystals are not infinitely small, it is difficult to satisfy the requirements for a large number
of channels.
This problem is solved by the use of a frequency synthesizer which is based on the phase locked loop
(PLL).
The PLL consists of three components, a voltage controlled oscillator (VCO), a phase comparator and a
filter.
The voltage controlled oscillator, as the name implies, is an oscillator whose output frequency can be
controlled by an input voltage.
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The phase comparator produces an output voltage whose amplitude is proportional to the difference in the
phase of two radio frequency input signals.
VCO
Output = N x Reference
Frequency
÷N
Reference
Frequency In
e.g. 1MHz
Programmable
Frequency Divider
Phase
Comparator
Frequency Synthesizer
In the PLL, the job of the phase comparator is to lock the two input signals to the same frequency. i.e. if the
VCO starts to drift off frequency, the phase between the two signals changes. The phase comparator then
adjusts the frequency of the VCO so that the phase moves back to the stable point. Thus, if one of the
inputs to the phase comparator is the VCO divided by N, the output of the VCO is N x the reference
frequency.
7.1.8.5 Digital Communications
One big advantage of digital communications over analog voice is that the various parts of the message do
not have to arrive in the same order in which they were sent. i.e. the transmitter can divide the message up
into “packets”, each labelled with the destination and a sequence number. Once these have been
transmitted into a communications network, they can each travel a different route to the destination
system. Once received, they are reassembled in the correct order and presented to the operator. This
scheme has several advantages:
a) The communications system can spread the load over the available resources.
b) In case of a link failure, it is easy to bypass the failed part
c) Messages will not be held up by temporary overloads or failures
7.1.8.6 Coding
Signal to noise ratio in a digital link has a direct effect on the number of data bits which are interpreted
incorrectly by the receiver. i.e. reading a 1 instead of a 0 or vice versa. The rate at which this occurs is
called the bit error rate (BER) and is a measure of quality of a digital data channel. according to the text a
BER of 10-3 is adequate for digitized voice while much lower rates are required for data transmission.
Although the BER is fixed for a given S/N and a given modulation technique, the effective BER may be
increased dramatically by using error correcting codes. This method uses coding theory to add specially
coded bits by means of which the receiver can detect and correct bit errors. With a relatively small
overhead of extra bits a BER of 10-3 can be decreased to 10-8.
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7.1.9 VLF Comm
As with navigation systems, VLF is used for very long distance communication and, because of the costs and
operational requirements, it is almost entirely restricted to military uses. Also, due to the low bandwidth, the
rate of data transmission is low.
A remarkable feature of the system installed on the E-6A the Airborne Command Post for Fleet Ballistic Missile Submarines (shown below) is the 28,000 Ft. trailing wire antenna which is fed from a second, 5000 Ft. antenna. The transmitter power is 200 kilowatts This would hardly be practical for general use!
The operation of this system requires the aircraft to follow a circular flight path (orbit)
7.1.10 HF Comm
7.1.10.1 General
Up until recently, HF has been the standard long range communications system for aeronautical use
especially over the North Atlantic routes.
However, due to the variable nature of HF propagation, this mode is not as responsive as others.
For a given period of time, several frequencies are assigned for HF communication over the Atlantic. This
is because the “skip” distance is different for each frequency, and aircraft are a different distances from the
transmitter sites. Thus, if an pilot wishes to contact air traffic control for some reason e.g. to change
altitude or report position, it is necessary to try different assigned frequencies until contact is established.
This is possible because the receiver site has several receivers each tuned to a different assigned frequency.
Even when contact has been established, the quality of the channel may be bad and several repetitions may
be required to complete the conversation.
On the other hand it is more difficult for the radio operator to contact a given aircraft because it usually has
only one receiver and the operator has no way of knowing to which frequency it is tuned although, with
experience, and a knowledge of the frequencies being used by aircraft in the same area, it may be possible
to make an informed guess.
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Aircraft separation standards are affected by the state of long range communications as well as by the
accuracy of the navigation systems.
In spite of the growing interest in satellite communications in this area, HF is not dead.
By using a frequency hopping technique to determine the optimum frequency and by using digital coding
instead of voice, HF communications efficiency could be improved considerably and could yet prove to be
competitive with satellite-based methods
7.1.10.2 Single Sideband Modulation
In normal AM modulation, two sidebands and the carrier are produced. Each sideband carries all of the
information and contains 25% of the signal energy. The carrier has no information and contains 50% of the
signal energy. Thus 75% of the transmitted energy is not used. To improve efficiency, and to reduce
bandwidth requirements, HF transceivers use single sideband or SSB modulation. In this mode, the
transmitter filters out the carrier and one of the sidebands. Thus there are two choices: upper or lower
sideband modes.
In the receiver, it is necessary to reintroduce the carrier signal otherwise the signal could not be
demodulated. Therefore a beat frequency oscillator (BFO) is used to lock on to the signal. This oscillator
can be adjusted to give the correct relationship with the sideband.
Some transmitters leave some of the carrier in the signal to assist the receiver.
7.1.10.3 Aircraft Installations
Airborne HF transceivers tend to be larger than other communications equipment primarily because of the
power output requirements (400 Watts typical). Usually the higher the power generated, the larger the
equipment due mainly to the requirement to dissipate more heat. A typical VHF transmitter on the other
hand, has a power output of about 25-50 Watts.
The HF antenna in older and lower speed aircraft consisted of a long wire reaching from the top of the
vertical stabilizer to the top of the cockpit. With the advent of jet aircraft a “boom” type antenna was used.
This type can be seen on some of the earlier Boeing 707s at the top of the vertical stabilizer
HF Boom antenna
on early 747
HF Boom antenna on
early 707
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On early 747s the boom antenna was mounted on the trailing edge of the wingtip. The first airline to install HF
on a 747 was British Airways. On the initial testing it was found that, when the HF transmitter was activated,
the engine instruments, such as RPM and temperature gauges, became unusable. This was traced to the fact that
the RF cable was routed along the leading edge of the wing along with the engine instrument wiring. This
caused significant interference.
At the present time, the most popular HF antenna is the slot, usually located along the leading edge of the vertical stabilizer. Antennas are usually made of a conducting material surrounded by a dielectric (a non conducting material, normally air). However, it is possible to make an antenna with the reverse configuration. i.e. a
dielectric surrounded by a conductor. This is what is done with the slot antenna. A length of dielectric material
is inserted in the leading edge of the stabilizer and an electrical connection is made to its edges. This has the
great advantage of not requiring any external protrusions.
7.1.11 VHF/UHF Comm
VHF/UHF Comm is the most common means of airborne communication. The frequencies used are 118.0
MHz to 135.975MHz with 25 kHz spacing between channels for the VHF band (civilian use) and 225MHz to
400MHz for the UHF band (military use).
Because transmission is line of sight, the range depends on the altitudes of the aircraft and the ground station
and is approximately
2H t + 2H r miles where Ht is the height of the transmitter in feet and Hr is the height
of the receiver in feet. Also transmission can be blocked by intervening objects such as buildings and hills.
Power output is typically in the 25 to 50 Watt range.
At the present time most VHF/UHF communication is voice using AM. However there are certain applications
in which digital transmissions are used. In particular, Transport Canada has been experimenting with digital
transmission of ATIS (automatic terminal information system), a recorded message which provides such general purpose information as current weather, runway in use, arrival and departure procedures in use and special
notices or warnings such as the presence of migratory birds in the area or taxiway closures. Work is also being
done on using digital means to provide clearance delivery.
Digital VHF is also used by airlines to transmit aircraft data as was mentioned earlier.
Due to the restricted bandwidth (less than 12.5 kHz i.e one half of the 25kHz channel spacing) data rates are
relatively low, but are still much higher than the rates that voice communications can provide.
7.1.11.1 Aircraft Installations
Due to their low power requirements and relative simplicity, VHF transceivers are relatively small with the
most expensive airline units being about 5 inches wide, 8 inches high and 18 inches deep.
Antennas are almost always quarter wave stubs and are matched to the transceiver.
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Most aircraft carry two VHF transceivers with one tuned to the current station and the other tuned to the
next anticipated station. Airliner installations usually consist of the transceivers mounted in the avionics
bay with only a control heads installed in the cockpit.
Transceivers for light aircraft usually built as a single unit which is mounted in the instrument panel.
7.1.12 Satellite Comm
Satellite communications offer the potential for more reliable long distance airborne communication. However
this reliability involves considerable technical complexity and cost.
(about $500,000 today)
A simplified block diagram of the system is shown below
Satellite
1.5 - 1.6 GHz
Aircraft
System
4 - 6 GHz
Ground
Station
Figure 46:
Satellite Communication Network
Telephone
Network
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7.1.12.1 Antenna Gain
Since the performance of the satellite link depends in part on the characteristics of the antennas used, the
following definition of antenna gain is required:Antenna gain in a particular direction is the ratio of the
power density in that direction to the power density which would be produced by an isotropic
(omnidirectional) antenna. e.g.
isotropic
B
A
The gain in the direction of the arrow is the magnitude of A divided by the magnitude of B.
Thus for an isotropic antenna the power density would be
Pt
----------(the power output divided by the area of
2
4πr
the sphere).
For a directional antenna the power density in a given direction is
Pt
G ⋅ -----------2 where G is the gain in that
4πr
direction. Note that the average gain over the sphere must be 1 (0 dB)
7.1.12.2 Antenna Beam Width
In order to describe how directional an antenna is, the term beam width is often used.
This is defined as the angle between the points at which the gain is one half (or -3dB) of the maximum gain
Generally, the higher the gain of the antenna, the narrower the beamwidth.
7.1.12.3 Satellite Comm Channel and Antennas
All aeronautical satellite communication at the present time uses the INMARSAT series of
communications satellites. These are geostationary and thus have an orbit radius of about 36000 miles.
Given the 1/r2 path loss, the received power density is therefore very low as is the received signal to noise
ratio. This restricts the data rates which can be used.
One way to overcome this is to use antennas with large areas. In ground-based systems this is relatively
easy since large parabolic reflecting dishes can be used
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In the airborne environment, this is not a viable solution.
Antennas with high gain can be used but they have an inherently narrow beamwidth. Since the aircraft is
moving relative to the satellite and is also changing attitude randomly, some means of pointing and
stabilizing the antenna beam is required.
Two approaches to this problem are being used: phased array antennas and mechanically steered antennas,
either small dishes or phased arrays.
The phased arrays are either mounted as a single unit on the top of the fuselage or as two units at the 45˚
points. The mechanically steered antennas are more common in the corporate jet fleet (Challengers, GV
etc.) where they are mounted inside the fin cap.
Omnidirectional antennas are also installed for the low data rate service
7.1.12.4 Data Rates
The high data rate, which requires high gain antennas operates at 9.6 kb/s for voice and 10.5 kb/s for data.
The low data rate is about 400b/s
7.1.12.5 Users
Satellites are now economically feasible only for areas where line of sight systems are unavailable (oceans,
polar region). The main users of satellite communications at the present time are passengers using voice
and fax facilities although the cost is about $10 a minute.
In the near future, however, satellites are expected to provide the data links for Automatic Dependent
Surveillance (ADS) by which the aircraft will be interrogated periodically by the air traffic management
system to provide its position, and for the Wide Area GPS Augmentation System.
Other users might be the airlines themselves to transmit scheduling and maintenance information
7.1.12.6 Aircraft Installation
Aside from the antennas which have been mentioned, multichannel transceiver/processors are installed in
the aircraft. Currently the largest of these have 6 channels, with 2 data and 4 voice channels. Weight is
about 50 pounds.
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7.1.12.7 Current Developments
7.1.12.7.1 INMARSAT 3
At the present time, the INMARSAT 3 series of satellites are operational.
These have a couple of features which are aimed at the aeronautical world; a GPS transponder
and spot beams which provide a much higher power density in areas of high air traffic density
The first of these is to implement the GPS WAAS
The second reduces the demands on the airborne equipment since lower gain antennas can be
used and the steering requirements are much looser.
Data rates for the new level of service are about 4.8 kb/s
7.1.12.7.2 MSAT
MSAT is a joint US-Canadian comm satellite for the mobile communications. This could possibly used for aeronautical comm, and is expected to be much cheaper than INMAR SAT but it
has the drawback of being limited to the North American Continent
7.1.12.7.3 Cellular Satellites (Iridium)
In the next few years, several projects to launch systems of low earth orbit communications satellites are planned. These will undoubtedly be considered for future aircraft communications
systems.