01 general communication (1)

7/30/2019 01 General Communication (1)

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TMT 1243

Maritime Communications

General Communications


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Overview

A communication system is made up of devices that employone of two communication methods (wireless or wired ),different types of equipment ( portable radios, mobile radios,base/fixed station radios, and repeaters ), and various

accessories (speaker microphones, battery eliminators, and carrying cases ) and/or enhancements (encryption, digital communications, security measures, and networking ) to meetthe user needs.

01 Jan 2013 Lt Kdr Ir Hardy Azmir Anuar TLDM 2


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Technologies

Wired:hard-line system.localised.uses wires to operate over limited area.

Wireless:uses radio frequencies (RF) to connect users.capable of operating over a much larger geographical area.

Advantages of RF communication systems:provide communications over large distances.through some obstacles.to an almost unlimited number of users.



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Technologies

Range of the signal:defined as the distance between the transmitter and the receiver atwhich the amplitude of the signal received by the receiver is less thanthe amplitude of the background noise.may also be affected by interference from atmospheric disturbances,

such as electrical storms, and high-power RF sources (such as radar equipment and broadcast equipment ).RF signals do not pass through water.

Subject to saturation by usersamount and quality of information able to be transferred over thesystem.



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Radio Frequency

Common radio messages (data and voice ) are transmittedover the RF band between 0.05 MHz and 900 MHz.

Cell phones, global positioning receivers and remote sensing,

transmit in the microwave band between 1 GHz and 20 GHz.



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Conventional Radio System

User is assigned a discrete radio channel independent of other user group channels.

Transmissions may occur with or without the assistance of a

repeater.

Communications without a repeater are considered to besimplex communications used for a small coverage area.

Users equipped with radios programmed with samefrequency and CTCSS/DCS can communicate with oneanother.



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Trunked Radio Systems

Typically allocate large group of users to a few radiofrequency channel.

User is assigned a logical grouping (talkgroup ) instead of

channel grouping.

Computer-controlled two-way radio system.

It selects the channel and monitors the repeater beforetransmitting.



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Types of Equipment

Portable radios:small, lightweight, handheld, wireless communication units thatcontain both a transmitter and a receiver, a self-containedmicrophone and speaker, an attached power supply (typically arechargeable battery ), and antenna.

have relatively low-powered transmitters.

Mobile Radios:larger than portable radios and are designed to be mounted in a fixedlocation inside a vehicle.

mobile radios connect to the vehicles power supply.External antenna.Medium-powered transmitter.



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Types of Equipment

Base/Fixed Station Radios:also contains a transmitter and a receiver.powered by an external electrical system.have the most powerful transmitters.have the most sensitive receivers.

Repeaters:specialized radio that contains both a receiver and a transmitter.used to increase the effective communications coverage area.repeaters receiver is tuned to the users transmitted frequency. repeaters transmitter is tuned to the users received frequency. incoming signal is rebroadcast back to the radio network usually withhigher power and from a better location.



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Enhancements

Enhancements are those items or applications available tothe customer for modification of the communication systemfor a specific purpose.

Encryption - RF radios may allow for the encryption of sensitivecommunications for security purposes.Digital Communications - radio massages is converted into binary codeusing electronic or electromagnetic signals.Security Measures - improved by the use of spread spectrumtechniques, voice inversion, digitizing of voice and data and etc.Interoperability and Networking - process of connecting differentgroups using different radio systems and communication technologies.



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Transmission Methods

Radiotelegraph:referred to as continuous wave (CW) telegraphy.system of transmitting signals using a wave of RF energy.separates a continuously transmitted wave into dots and dashes basedon the Morse code.

first means of radio communications military/commercial.slow speed of transmission.require experienced operators.reliable.



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Radiotelephoneone of the most useful military/commercial communications methods.directness, convenience, and ease of operation.possible to communicate around the world by radiotelephone.short-range tactical communications allows commanders to

communicate directly with other ships.operated on frequencies that are high enough to have line-of-sightcharacteristics.it reduces the chances of the enemy intercepting the message.



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Teletypewriteris an electromechanical typewriter that can be used to send andreceive typed messages from point to point and point to multipointover various types of communications channels.mainly for high-speed automatic communications across ocean areas.

equipped with a keyboard similar to a typewriter.when a key is pressed, a sequence of signal is transmitted.at receiving station, the sequence of signal is translates into letters,figures, and symbols automatically.efficient and reliable for communications between ships and from

ship-to shore.Common speed of operation is 100 WPM.



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Facsimileprocess used to transmit photographs, charts, and other graphicinformation electronically.image to be transmitted is scanned by a photoelectric cellcorresponding to the light and dark areas on the picture.

the signal is then reproduce by a recorder at receiving station.suffer distortion from all of the common sources of interference.picture quality will be downgraded by any noise bursts.it is an important communications supplement.handling certain types of graphic and pictorial intelligence.



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Radio Frequency Band

Frequency Description Wavelength30 GHZ - 300 GHZ extremely high frequency

(EHF)1 cm - 1 mm

3 GHZ - 30 GHZ superhigh frequency (SHF) 10 cm - 1 cm

300 MHZ - 3 GHZ ultrahigh frequency (UHF) 1 m - 10 cm

30 MHZ - 300 MHZ very high frequency (VHF) 10 m - 1 m

3 MHZ - 30 MHZ high frequency (HF) 100 m - 10 m

300 KHZ - 3 MHZ medium frequency (MF) 1000 m - 100 m

30 KHZ - 300 KHZ low frequency (LF) 10 km - 1 km

3 KHZ - 30 KHZ very low frequency (VLF) 100 km - 10 km300 HZ - 3 KHZ voice frequency 1000 km - 100 km

Up to 300 HZ extremely low frequency(ELF)

> 1000 km



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Extremely Low-Frequency Communications (Up to 300 HZ ):to send short "phonetic letter spelled out" (PLSO) messages fromsurface to submarines.has the ability to penetrate ocean depths to several hundred feet withlittle signal loss.

Impractical due to large size of transmitters and antennas.

Very-Low-Frequency Communications (3 KHZ - 30 KHZ ):also known as the myriametre band voice cannot be transmitted and only low data rate coded signals are

used.used for a few radio navigation services, worldwide transmission of standard frequency and time signals, and for secure militarycommunication with submarines.as a backup to shortwave communications.



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Low-Frequency Communications (30 KHZ - 300 KHZ):also known as the kilometre band .not so seriously affected by ionospheric condition as with HF.this band allows long distance communication.AM broadcasting, aircraft beacon, LORAN, and information and

weather systems.

Medium-Frequency Communications (300 KHZ - 3 MHZ):also known as the hectometre band .maritime distress and emergency frequency and AM broadcasting.

non-directional (radio) beacon (NDB) signals follow the curvature of the earth.lower MF band - long distance transmission over water.upper MF band - short-haul communications (400 miles or less).



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High-Frequency Communications (3 MHZ - 30 MHZ):also known as the decameter band .extensively used due to its variable nature of the propagationmedium.long-distance communications depend upon refraction of radio waves

by layers of ionospehere.economic and technical advantages of HF have led to increasing use of the band and is approaching saturation.HF general types of services includes point-to-point, ship-to-shore,ground-to-air and fleet broadcast.



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Very-High-Frequency and Above Communications:not normally refracted by the atmosphere.ground-wave range is minimal.limits our use of this frequency spectrum to line of sight.increased range through the use of tropospheric scatter techniques.

certain atmospheric can also cause the range to be extended.capable of overcoming the shielding effects of hills and structures tosome degree.free from atmospheric and man-made static.the transmitting antenna is in a direct line with the receiving antenna

and not over the horizon.VHF ideal for amphibious operations or marine band.UHF well suited for tactical voice transmission.SHF used for radar and satellite communications.EHF used only in the experimental stage.



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AM Broadcast Band


Designation Name FrequencyLW long wave 153 - 279 kHz

MW medium wave 531 - 1620 kHz

SW short wave 2310 - 25820 kHz


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Microwave Letter Band Designations



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Modes of Operation

Simplex:the simplest method of electronic communication.uses a single channel or frequency.this type of communication is one-way.

Half duplex:two-way communication.only one party transmits at a time.Same antenna and circuitry may be used for both transmission andreception.

Full duplex:communications take place simultaneously in both direction.using two separate channels or frequencies.



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Voice Signal



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Data Signal



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Carrier Signal



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Sine Wave



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Sine Wave



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Modulation



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Amplitude Modulation Concept

Encoding information onto a carrier signal by varying theamplitude of the carrier signal



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AM Spectrum

= 2 2 2 ( )

the first term is the carrierthe second term, containing the difference, is the lower

sidebandthe third term, containing the sum, is the upper sideband



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Modulation Index & Bandwidth

measure of the amplitude variation surrounding anunmodulated carrierindicates how much the modulation varies around its"original" level

= = = +

bandwidth of the signal can be seen to be twice that of thehighest f m transmitted

= 2



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Modulation Index



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Amplitude Shift Keying (Digital)



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AM Transmitter



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AM Transmitter



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Crystal Radio



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Frequency Modulation

Encoding information onto a carrier signal by varying thefrequency of the carrier signal



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FM Spectrum

= (2 2 ) = cos (2 )



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Modulation Index & Bandwidth

equal to the peak deviation caused when the signal ismodulated by the frequency of the modulating signal

= Bandwidth of the signal is given by

= 2 1 If h 1 the modulation is called narrowband FM , and itsbandwidth is approximately 2 f mIf h 1 the modulation is called wideband FM and its

bandwidth is approximately 2 f While wideband FM uses more bandwidth, it can improve thesignal-to-noise ratio significantly



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Modulation Index



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Frequency Shift Keying



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FM Transmitter



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FM Receiver



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Phase Modulation

Encoding information onto a carrier signal by varying thephase of the carrier signal

= (2 2 )


= = 2 1


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Phase Shift Keying



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Behaviour Of Radio Waves

Reflectionwhen waves strike another medium

Refractionanother effect when waves strike another medium

Diffractionhow waves can be bent

In all cases absorption is always occuring



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Reflection

When an electromagnetic wave encounters a change in ordifferent medium (as in air to water), some or all of the wavemay propagate into the new medium (transmitted portion)and some may be reflected from it (reflected portion). In thiscase, the angle of incidence is equal to the angle of reflectionand is known as Fresnels Law .

Angle of incidence = Angle of reflection


Incident Wave Reflected Wave


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Reflection

Media boundaries with dissimilar propagation result inreflection.


Specular reflection is reflected atequal but opposite angle from

smooth surface.

Diffuse reflection results from wavesstriking an irregular surface and

reflecting over a broad range of angles
http://www.kettering.edu/~drussell/Demos/reflect/reflect.htmlhttp://www.phy.ntnu.edu.tw/java/sound/sound.html


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Reflection

The reflected wave will be 180 degrees out of phase with theoriginal wave of incident.The only way to get reflected energy back to the source (asneed for radar to work) is to hit the material perpendicular tothe surface.If the surface is not flat then reflected rays will not go in thesame direction.Many other things besides targets can cause reflections (dust,rain, snow, and water vapor).

Can get reflections off ionosphere and when there areinversions in the atmosphere.



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Refraction

When an electromagnetic wave enters a new medium, thespeed of propagation will change and as a direct result, theincident wave will change its direction of propagation. Theincident wave will bend more towards the perpendicularwhen entering a medium with higher Index of Refraction (n).This phenomenon is known as Snells Law and can berepresented by the following: n 1sin 1= n2sin 2


f


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Refraction

Incident wave passes through two transparent media inwhich the velocity of light differsIncident wave divides into a reflected wave and a refractedwave Incident wave divides into a reflected wave and arefracted waveAlthough there are no clear boundaries of medium in air,there are different densities. The result is that the energy raywill bend toward the area of higher density.


f


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Refraction

As the angle of incidence increases the angle of refractionincreases as wellThe critical angle can be calculated from Snell's law by settingthe refraction angle equal to 90For an incidence angle greater than c there is no refractedray known as total internal reflection

No internal reflection when starting in a lower n because sin1


1

21sinn

n

c

f


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Refraction


R f i


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Refraction


R f i


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Refraction


Diff i


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Diffraction

Wave bending around an object because of different speedsof wavelet at medium interface. Can detect signals in spite of LOS limitations.Classic example of diffraction is waves around a break water.


island not detected

detected

EM Si l L


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EM Signal Losses

Spreading - energy distributed over an increasingly largerarea. Energy per unit area proportional to 1/R 2.Absorption - energy dissipated into medium. Molecules of medium absorb some of the energy as it passes through.Scattering - energy bouncing off suspended particles within a

medium


I t f


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Interference

When two or more waves meet, amplitudes add to produce anew wave.Can be described as either constructive or destructiveinterference, depending on phase shift between waves.

Constructive phase difference between 0 and 120 or between 240 and 360 .Destructive phase difference between 120 and 240 .


I t f


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Interference


W P ti


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Wave Propagation

There is a relationship between distance and frequencyPropagation Modes

Ground WavesSky WavesSpace Waves

Tropospheric Scatter


G d W


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Ground Wave

Very low frequencies (5-10Khz)Vertical polarizationWaves travel along earths surface. Very long wavelengths - unsuitable for ships & aircraft, butused for sub comsShore-based installations (HF-DF)


Sk W


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Sky Wave

E-M energy refracts back towards the earths surface in upperionosphere layer.E-M energy then reflects back toward upper layer again.Frequencies used up to 550 KHz effectivelyWavelengths still too long for anything but comms by aircraft

and ships.


Space Wa e


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Space Wave

Higher frequency signals that penetrate the ionosphere andtravel through space.Above 30 MHz, ionosphere will not refract E-M waves backtoward earth.Energy tends to travel in straight line.




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Method of transmitting and receiving microwave radio signalsover considerable distances often up to 400 milesRadio waves at particular frequencies are randomly scatteredas they pass through the upper layers of the troposphere2 GHz are best suited - interacts well with the moist.



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ITU Emission Code


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ITU Emission Code

International Telecommunication Union (ITU) uses aninternationally agreed system for classifying radio frequencysignals.

first symbol - type of modulation of the main carrier.second symbol - nature of signal(s) modulating the main carrier.

third symbol - type of information to be transmitted.fourth symbol - details of signal(s) (optional).fifth symbol - nature of multiplexing (optional).


Type of Modulation


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Type of Modulation


Type of Modulating Signal


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Type of Modulating Signal


Type of Transmitted Information


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Type of Transmitted Information


Details of Information


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Details of Information


Multiplexing


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Multiplexing

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