EEE381B Aerospace Systems

1
EEE381BAerospace Systems Avionics

• Communications Systems
• Ref Moir Seabridge, Chapter 7
• Dr Ron Smith

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Outline

• Introduction
• RF propagation modulation techniques
• Radio communications
• Data link
• 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 communication
  systems has become a complex task complicated by
• aircraft speeds, air traffic density and the wide
  range of communication types.
• The communication control function is
  increasingly becoming integrated with the flight
  management system,
• automatically selecting and tuning the
  communications required for each lag of the
  flight.

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1.2 RF Spectrum1

• Communication equipment
• 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 redundancy
  requirements.
• 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 waves1
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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 a
  frequency comparable to voice, such as those used
  by a loudspeaker, or a telephone.
• However, to transmit such a signal through the
  atmosphere would require an antenna 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 f )
• it is possible to add an information signal to
  the carrier in amplitude, frequency, or phase
• s(t) A(t) sin (?(t) t f(t) ),
• where ? 2pf

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2.2.4 Modulation Possibilities
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2.3 Amplitude modulation

• Amplitude modulation is perhaps the simplest
  technique for modulating a signal. It works by
  varying the strength (amplitude) of the carrier
  in relation to the signal being sent.
• Message m(t)
• Carrier cos (2?fct)
• sAM(t) m(t) cos (2?fct) A cos (2?fct)

Extra carrier added to make detection easier.
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2.3.1 Amplitude modulation
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2.3.2 Amplitude modulation
M(f)
m(t)
2A
f
t
B
-B
m(t) Message
m(t)cos ?ct Modulated signal
cos ?ct Carrier
m(t)cos ?ct
A
f
t
fc-fm
fcfm
2B
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2.3.3 Demodulation of AM
m(t)cos(?ct)cos(?ct)
Low Pass Filter
m(t)cos ?ct Received signal
m(t)/2 Message
cos ?ct Carrier
A
-2?c
?
2?c
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 / Demodulator5
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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
  with the same phase and frequency as the incoming
  carrier
• needs much less energy (50)
• SSB (single side band)
• more sophisticated receiver circuitry required,
  but much 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 frequency of a carrier
  (sinusoidal wave) is varied 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
  least twice the bandwidth of the message.

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2.4.2 FM detector / demodulator5
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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 of waves off the
  ionosphere.

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3.1.1 High Frequency (HF) 1

• A number of factors affect 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 best performance.

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 with
  accompanying error-correction bits.
• Employing advanced modulation and frequency
  management systems, HFDL permit communications
  under adverse conditions when HF voice would be
  incomprehensible.

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3.2 Very High Frequency (VHF)

• The most common voice communications band used by
  civil aviation is VHF. For aeronautical
  applications the band ranges from 118.000 to
  135.975 MHz, with 25kHz wide channels.
• Recently, the channel spacing has been reduced to
  8.33kHz to help decongest the spectrum and to
  better support digital communications (data
  link).
• 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 R is range in km
• H1 and H2 are the heights of the antennas in
  m.

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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
  negotiated between 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 use the
  UHF band for communications.
• The band covers 225 to 400 MHz.
• In general, civil aviation does not use UHF.

243.0 2 x 121.5 Cest une fréquence harmonique
de 121.5 MHz
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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)

243.0 2 x 121.5 Cest une fréquence harmonique
de 121.5 MHz
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3.4.1 SATCOM principles of operation1
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 typically includes
• 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, this is
  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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5.1 Quick response exercise 1

• 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?

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5.2 Quick response exercise 2

• Why was 243.0 MHz selected as the UHF
  international distress frequency?
• Hint do you recall what the VHF distress
  frequency is?

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References

• Moir Seabridge, Military Avionics Systems,
  American Institute of Aeronautics Astronautics,
  2006. Sections 2.6 2.7
• Wikipedia
• Military Communication Systems, LFTSP course
  notes, ECE Dept, RMC, 2007
• Air Power Australia, http//www.ausairpower.net/TE
  -NCW-JanFeb-05.html.
• Georgia State University, hyperphsyics,
  http//hyperphysics.phy-astr.gsu.edu/hbase/audio/b
  cast.htmlc3
• Mark A. Hicks, "Clip art licensed from the Clip
  Art Gallery on DiscoverySchool.com"