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]
http://www.ips.gov.au/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"

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Communication-Org 10 Oct