Communication-Org 10 Oct
Transcript of Communication-Org 10 Oct
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EEE381B
Aerospace Systems & Avionics
Communications SystemsRef: Moir & Seabridge, Chapter 7
Dr Ron Smith
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Outline
1. Introduction
2. RF propagation & modulation techniques
3. Radio communications4. Data link
5. In-class exercises
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1. Introduction
Communications: The
ability to communicate by
either voice or data link
with friendly forces
(wingman, airborne
command centre, ground
troops ).
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1.1 Communication Control Systems
Control of the aircraft suite of communicationsystems has become a complex taskcomplicated by: aircraft speeds, air traffic density and the wide range
of communication types.
The communication control function isincreasingly becoming integrated with the flightmanagement system, automatically selecting and tuning the
communications required for each lag of the flight.
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1.2 RF Spectrum[1]
Communicationequipment: High frequency radio
(HF)
Very high frequency(VHF)
Ultra high frequency(UHF)
Satellite (SATCOM) Data (Data links)
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2. Propagation of radio waves
The number of antennas required to support
communications on military and civilian aircraft is
considerable.
This is further complicated by redundancyrequirements.
The antennas must be strategically located so
as to minimize interference and to optimize
reception / transmission for all aircraft attitudes.
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2.1 Propagation of radio waves[1]
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2.2 Modulation techniques
Modulation is the process by which an
underlying RF signal (carrier wave) is
transformed so as to convey information.
This forms a communications channel.
Modulation is accomplished by varying a
parameter of the carrier wave such as the
amplitude, frequency or phase.
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2.2.1 Why modulation?
A signal can quite easily be generated at afrequency comparable to voice, such asthose used by a loudspeaker, or a
telephone.
However, to transmit such a signal
through the atmosphere would require anantenna with the appropriate dimensions.
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2.2.2 Why modulation?
For example, for an audio signal at 3kHz,
the wavelength will be:
= c/f = 1 x 105 m = 100 km
Even using a quarter wavelength whip antenna, you would
need to drag a wire 25 km long behind your aircraft!
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2.2.3 Modulated carrier waves
With a carrier wave defined as:
s(t) = A sin (t + )
it is possible to add an information signal
to the carrier in amplitude, frequency, orphase:
s(t) =A(t) sin ((t) t + (t) ),
where = 2f
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2.2.4 Modulation Possibilities
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2.3.1 Amplitude modulation
http://fr.wikipedia.org/wiki/Image:Modulation_d%27amplitude_figure_2.2.1.3.pnghttp://fr.wikipedia.org/wiki/Image:Modulation_d%27amplitude_figure_2.2.1.2.pnghttp://fr.wikipedia.org/wiki/Image:Modulation_d%27amplitude_figure_2.2.1.1.png -
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2.3.2 Amplitude modulation
cos ct
Carrier
m(t)
Message
m(t)cos ct
Modulated signal
m(t)
M(f)
ft
m(t)cos ct
ft
2A
-B
A
B
2B
fc-fm fc+fm
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2.3.3 Demodulation of AM
cos ct
Carrier
m(t)cos ct
Received signal
A
-2c
m(t)cos(ct)cos(ct) Low PassFilter
m(t)/2
Message
2c
The Low Pass Filter allows the low-frequency message
through, and stops the high-frequency side bands.
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2.3.4 AM Detector / Demodulator[5]
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2.3.5 Other types of AM modulation
DSB-SC(double side band - suppressed carrier)
easiest to produce, but requires a local carrier withthe same phase and frequency as the incomingcarrier
needs much less energy (~50%) SSB (single side band)
more sophisticated receiver circuitry required, butmuch more energy is used to carry the message.
as above - but susceptible to noise and uses 1/2 the bandwidth and ~1/6 the power
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2.4 Frequency modulation
A process by which the
frequencyof a carrier
(sinusoidal wave) isvaried in accordance with
a modulating wave (data
or analog message).
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2.4.1 Frequency modulation
The bandwidth of an FM signal is given by:
BFM = 2(f +B)
f is the frequency deviation (how much the carrier
deviates from the carrier frequency)
B is the bandwidth of the modulating signal
(message)
So, the bandwidth required of an FM signal is at
leasttwice the bandwidth of the message.
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2.5 FM versus AM?
FM modulation is more resistant to noise and
jamming (spread spectrum).
Spread spectrum also makes FM a better choice for
low probability of intercept operations. FM does require a wider frequency band, but it
achieves a higher efficiency as more of the
energy is concentrated in the signal as opposed
to the carrier.
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3. Radio communications
HF
VHF
UHF Satellite
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3.1 High Frequency (HF)
Covers the communication band between
2 and 30 MHz with 1kHz channel spacing.
Very common communication band for air,
land and sea.
Long range due to
the reflection ofwaves off the
ionosphere.
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3.1.1 High Frequency (HF) [1]
A number of factorsaffect transmission: Solar radiation
activity (sun spots)Atmospheric
conditions
Day / night
Season
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3.1.2 High Frequency (HF)
Maps are produced
that help predict
which frequencies
might give the bestperformance.
[http://www.ips.gov.au]
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3.1.3 High Frequency (HF)
The performance of HF communications can be
improved by transmitting the information in a
digital form. Known as HF data link (HFDL),
these digital systems encode the message withaccompanying error-correction bits.
Employing advanced modulation and frequency
management systems, HFDL permit
communications under adverse conditions whenHF voice would be incomprehensible.
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3.2 Very High Frequency (VHF)
The most common voice communications bandused by civil aviation is VHF. For aeronauticalapplications the band ranges from 118.000 to135.975 MHz, with 25kHz wide channels.
Recently, the channel spacing has been reducedto 8.33kHz to help decongest the spectrum andto better support digital communications (datalink).
The international distress frequency (VHF) is:121.5 MHz
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3.2.1 Very High Frequency (VHF) [1,3]
For all bands higher than HF line of sight propagation
applies, and maximal theoretical range is given by:
where: Ris range in km
H1and H
2are the heights of the antennas in m.
217.127.12 HHR +=
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3.2.2 Very High Frequency (VHF)
Some systems evaluate each channel in real-
time, automatically selecting the best frequency
to use. In practice the system measures the losses and the
noise between the receiver and the other station
continuously sweeping across all frequencies.
The best frequency is then selected and negotiatedbetween the sender and receiver.
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3.2.3 Very High Frequency (VHF)
AN/ARC-210
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3.3 Ultra high frequency (UHF)
Instead of VHF, most military aircraft usethe UHF band for communications.
The band covers 225 to 400 MHz.
In general, civil aviation does not useUHF.
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3.3.1 Ultra high frequency (UHF)
AN/ARC-164
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3.4 Satellite communications (SATCOM)
International Maritime Satellite Organisation
(INMARSAT)
11 geostationary satellites (2005)
Improved coverage over the original 4 satellites
Used for voice or data communications
SwiftBroadBand (432 kbps per channel)
Swift 64 (64 kbps per channel)
Aero (600 bps to 10.5 kbps per channel)
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3.4.1 SATCOM principles of operation[1]
Inmarsat-3
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3.4.2 SATCOM coverage [1]
Inmarsat-3
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3.4.3 Satellite communications
Inmarsat-3
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4. Data link
Provides faster, more precise communications than voice
Provides encryption and built-in error-correction
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4.1 Data link
Data link transmissions (packets) may include: Present position reporting Surveillance results EW and intelligence
Information management Mission management status
Two primary airborne data links include:
Link 16 (JTIDS) Link 11 (used primarily in naval operations)
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4.2.1 JTIDS frequencies
Shares the same frequencies as UHF
51 channels at 3MHz spacing
Employs frequency hopping (jam-resistant)
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4.2.2 JTIDS architecture
URC-138
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4.2.3 JTIDS equipment
A system typicallyincludes: Secure voice
Encrypted data Interfaced to the
onboard 1553 bus
Interacts with the radar,
electro-optics, EW,
URC-138
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4.3 Other data links
SATCOM HF data links (HFDL) Used extensively by maritime and civil
aviation
Supplemented with encryption equipment, thisis also used in military avionics
Local cooperative data links Used for close proximity data link (formation)
Example: F-22 Raptor
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5. In-class exercises
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At what height do you have to install a
tower antenna to maintain VHF/UHF
communications up to a range of 250 km
with airplanes at 10,000 feet or above?
5.1 Quick response exercise # 1
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Why was 243.0 MHz selected as the UHF
international distress frequency?
Hint: do you recall what the VHF distress
frequency is?
5.2 Quick response exercise # 2
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References
1) Moir & Seabridge, Military Avionics Systems, American Instituteof Aeronautics & Astronautics, 2006. [Sections 2.6 & 2.7]
2) Wikipedia
3) Military Communication Systems, LFTSP course notes, ECEDept, RMC, 2007
4) Air Power Australia, http://www.ausairpower.net/TE-NCW-JanFeb-05.html.
5) Georgia State University, hyperphsyics, http://hyperphysics.phy-astr.gsu.edu/hbase/audio/bcast.html#c3
6) Mark A. Hicks, "Clip art licensed from the Clip Art Gallery onDiscoverySchool.com"