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Chenguttuvan
Communication system process

Elements of Communication Systems

Information
Message or information is the entity that is to be transmitted. It can be in the form of
audio, video, temperature, picture, pressure, etc.

Signal
The single-valued function of time carries the information. The information is
converted into an electrical form for transmission.

Transducer
It is a device or an arrangement that converts one form of energy to the other. An
electrical transducer converts physical variables such as pressure, force, and temperature into
corresponding electrical signal variations.

Amplifier
The electronic circuit or device that increases the amplitude or the strength of the
transmitted signal is called an amplifier. When the signal strength becomes less than the
required value, amplification can be done anywhere between the transmitter and receiver. A
DC power source will be provided for the amplification.

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Modulator
As the original message signal cannot be transmitted over a large distance because of
their low frequency and amplitude, they are superimposed with high frequency and amplitude
waves called carrier waves. This phenomenon of superimposing of message signals with a
carrier wave is called modulation, and the resultant wave is a modulated wave which is to be
transmitted.

Transmitter
It is the arrangement that processes the message signal into a suitable form for
transmission and, subsequently, reception.

Antenna
An antenna is a structure or a device that will radiate and receive electromagnetic
waves. So, they are used in both transmitters and receivers. An antenna is basically a metallic
object, often a collection of wires. The electromagnetic waves are polarised according to the
position of the antenna.

Channel
A channel refers to a physical medium such as wire, cables, or space through which the
signal is passed from the transmitter to the receiver. There are many channel impairments that
affect channel performance to a pronounced level. Noise, attenuation and distortion, to mention
the major impairments.

Noise
Noise is one of the channel imperfections or impairments in the received signal at the
destination. There are external and internal sources that cause noise. External sources include
interference, i.e. interference from nearby transmitted signals (cross talk), interference
generated by a natural source such as lightning, solar or cosmic radiation, automobile-
generated radiation, etc. The external noise can be minimised and eliminated by the appropriate
design of the channel and shielding of cables. Also, by digital transmission, external noise can
be minimised.
Internal sources include noise due to random motion and collision of electrons in the
conductors and thermal noise due to diffusion and recombination of charge carriers in other
electronic devices.

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Internal noise can be minimised by cooling and using digital technology for transmission.
• A different cable design.
• Proper design of the channel.
• Use digital transmission
• Using BPF or LPF at the receiver side.

Receiver
An arrangement that extracts the message or information from the transmitted signal at
the output end of the channel and reproduces it in a suitable form as the original message signal
is a receiver.

Demodulator
It is the inverse phenomenon of modulation, i.e., the process of separation of the
message signal from the carrier wave takes place in the demodulator. The information is
retrieved from the modulated wave.

Repeaters
Repeaters are placed at different locations in between the transmitter and receiver. A
repeater receives the transmitted signal, amplifies it and sends it to the next repeater without
distorting the original signal.

Modulation:
The process by which data/information is converted into electrical/digital signals for
transferring that signal over a medium is called modulation.
The process of extracting information/data from the transmitted signal is
called demodulation.
The most commonly altered characteristics of modulation include amplitude, frequency, and
phase.
Carrier signal: The signals which contain no information but have a certain phase, frequency,
and amplitude are called carrier signals.
Modulated signals: The signals which are the combination of the carrier signals and
modulation signals are modulated signals. The modulated signal is obtained after the
modulation of the signals.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

Types of modulation:
1. Amplitude modulation: It is a type of modulation in which only the amplitude of the carrier
signal is varied to represent the data being added to the signals whereas the phase and the
frequency of the signal are kept unchanged.

Amplitude Modulation

2. Frequency modulation: It is a type of modulation in which only the frequency of the carrier
signal is varied to represent the frequency of the data whereas the phase and the amplitude of
the signals are kept unchanged.

3. Phase modulation: It is a type of modulation in which the phase of the carrier signal is
varied to represent the data being added to the signal. Different information values are
represented by different phases. For example: ‘1’ may be represented by 0° while ‘0’ by 180°.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

What is the need for modulation?
Size of antenna:
The size of the antenna is inversely proportional to the frequency of the radiated signal
and antenna size must be 1/10th of the wavelength. If the frequency signals are more than 5KHz
in that case it is quite impossible to set up an antenna of that size. So, by using the modulation
technique the size of the antenna is reduced.

Wireless communication:
Modulation provides a wireless connection to transmit the signals to a longer distance.
Earlier we used wire systems (like the telephone) to transfer information with the help of
telephonic wires but it was not possible to spread the wires all over the world for
communication. By using the modulation technique, the cost of wire is saved and even
information can be transferred to longer distances faster.

Advantages of modulation:
• It reduces the size of the antenna.
• It reduces the cost of wires.
• It prohibits the mixing of signals.
• It increases the range of communication.
• It improves the reception quality.
• It easily multiplexes the signals.
• It also allows the adjustment of the bandwidth.

Disadvantages of modulation:
• The cost of the equipment is higher.
• The receiver and the transmitter are very complicated.
• For better communication, the antennas for the FM system must be kept closed.
• It is not efficient for large bandwidth.
• Power wastage takes place.

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Demodulation

Demodulation is the technique to recover the original signal from the modulated signal.
The demodulation is done with the help of a demodulator. A demodulator will convert the
carrier variation of amplitude, frequency, or phase back to the message signal.
There are three different types of demodulators for converting the AM (amplitude
modulation), FM (frequency modulation), and PM (phase modulation) modulation schemes.

Demodulation Techniques

• Diode rectifier envelope detector

• Product detector
• Synchronous detection

Diode Rectifier Envelope Detector

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 The signals from the previous stages is applied to the diode. The capacitor helps in
removing the unwanted radio frequency (RF) elements.

When the radio signal is applied to the diode, only half the waveform passes through it
and half of the waveform is removed. This is due to the property of diode i.e., current can flow
only in one direction through the diode.

The resulting signal is applied to the audio output which is seen in the above diagram.
This resulting signal is the demodulated signal.

Disadvantages :

• It is not linear due to the diode characteristic which leads to the distortion in the signal.
• It is less sensitive and its performance is affected by the selective fading.

Product Detector
The product detector circuit is used to demodulate the SSB (single side band) signal. In
SSB amplitude modulation, the first carrier signal is suppressed, and then either the upper-side
band or lower-side band is suppressed.

The SSB signal is passed from

the IF transformer. It uses a crystal
oscillator which produces the local
oscillator signal. The generated local
oscillator signal has a frequency
similar to the carrier signal.

The SSB signal is then

multiplied by the local oscillator signal. This leads to the creation of different frequencies i.e.,
sum and difference of the frequencies.The resulting signal is then passed to the low-pass filter.
It will pass only the low frequency signal which is the baseband signal. Hence, in this way, we
extract the original signal.

This technique is used in the various communication application during demodulation. Also,
this circuit can be used to intercept Morse Code signal.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

Synchronous Detection
It is also known as coherent detection and is used to demodulate the amplitude-
modulated signal. It also comprises the oscillator which generates the local oscillator signal. It
is then multiplied with the incoming modulated signal. The resulting signal is then passed
through the low-pass filter to remove the high-frequency components.

The filtered signal contains the original baseband signal. The received signal is
then decoded to extract the original information. It helps in providing better performance. It is
used in various applications of broadcast receivers like radio communication equipment or
walkie-talkies because it is easy to incorporate.

Difference between Modulation and Demodulation

Modulation Demodulation

It is the process where properties of the Demodulation is the technique to recover

carrier signal like amplitude, frequency, or the original signal from the modulated
phase change according to the baseband or signal
message signal.

Modulation of the signal is done at the Demodulation of the signal is done at the
transmitter side. receiver side.

It is performed to transmit the signal over It is performed to extract the original

the long distance. information from the modulated signal.

It is a simple process as compared to It is a complex process.

demodulation.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

 Modulation Demodulation

It converts low-frequency signals into high- It converts high-frequency signal into low-
frequency signals. frequency signal.

To perform modulation, modulator To perform demodulation, demodulator or

(electronic circuit) is required. detector (electronic circuit) is required.

It is the mixing of two signals of different It is the process of recovery of original

parameters. signal from the mixed signal.

Advantages and Disadvantages of Demodulation

Advantages

• It helps in the efficient use of the bandwidth by allowing multiple signals to pass
through a single channel.
• It helps in the recovery of the original signal which is essential to retrieve the original
information from the signal.
• It improves the noise immunity which helps in improving the reliability of the signal.
• It can demodulate the AM, FM, and PM signals. This helps in improving the
diversification of demodulation and can be used in a wide range of applications.

Disadvantages

• Some of the demodulation techniques, like coherent demodulation, are a very complex
process. Due to the complexity of the process, the cost of the system increases.
• It is sensitive to channel conditions like attenuation, and distortion. It can lead to poor
quality of the obtained signal.
• Power consumption is high in some of the demodulation techniques like digital
demodulation.
• When the frequency spectra are crowded, it might lead to crosstalk and interference.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

Applications of Demodulation
Radar System: The demodulation technique is used in the radar system to extract information
about range, velocity, angle, and many more. It helps in navigation, weather monitoring, and
defence.

Television: The demodulation technique is used to recover the video and audio signals which
helps in proper displaying of the television programs.

Radio broadcasting: Demodulation is used to extract the audio signals which is to be

broadcast. It can retrieve AM and FM signals to broadcast over a wide range of frequency
spectra.

Underwater communication: For underwater communication, acoustic signals are used.

These acoustic signals needed to be demodulated, to receive the original information coming
from the deep sea.

Medical: The demodulation technique is used to generate the image of internal organs. It is
widely used in MRI and ultrasound.

Difference between Amplitude and frequency modulations

Amplitude Modulation Frequency Modulation

The radio wave is defined as a carrier signal The radio wave is defined as a carrier signal
and both phase and frequency are maintained and both phase and amplitude are maintained
at the same at the same

The AM signals can be transmitted to long The FM signals can be transmitted to long
distances but have lesser sound quality distances and have good sound quality

These waves lie in the frequency range of 535 These waves lie in the frequency range of 88
to 1705 kHz to 108 MHz

Highly suspected of noise signals Less suspected to noise signals

Applications of Amplitude Modulation

• Air traffic control radios

• Keyless remotes

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

 • Broadcasting of TV signals
• Electronic communications

Advantages & Disadvantages of Amplitude Modulation

Advantages

• Both modulation and demodulation processes are simply implemented

• Cost is also economical as fewer circuits and diodes are used
• Can transmit signals to longer distances
• The waves have lesser bandwidth
• Less complicated

Disadvantages

• Minimal efficient for power utilization as a DSB-SC modulation process consumes

more power
• Even not efficient in bandwidth utilization too.
• Less sensitivity to noise signals, so more prone to noise disturbances
• Reproduction is not greatly reliable.

Types of wireless communication:

1. Infrared Communication

This wireless network can only be used for short-range, i.e. 300GHz – 400THz. The IR
communication functions only when the sender and the receiver have an exchange of a light
beam. Any disruption in it will cause the photoreceiver not to receive the signal. This also
means that any object between the receiver and the transmitter will cause a non-operation.

2. Wi-Fi

Wi-Fi also works as two-way communication and is used by several electronic devices
like smartphones, laptops and smart TV. In this process, the signal transmission works based
on a router. So, the network allows uninterrupted usage when in close proximity to a router. A
range of devices can be connected to one network at high speed. The only concern is that the
Wi-Fi network must be password protected for security reasons.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

3. Radio Frequency

Also popularly known as broadcast radio, here’s the oldest type of wireless
communication. It can generate signals easily ranging from 3kHz to 300GHz. Unlike infrared
communication, RF transmission can happen through objects and travel long distances. Gladly,
radio transmission can penetrate through buildings, walls or anything else. And that’s how the
popular radio stations function.

4. Bluetooth

The functioning of the Bluetooth communication allows two devices to connect for the
data transfer (files, messages, music, images). Mobile phones can easily be connected to
Bluetooth headphones and have a wireless experience. This wireless communication
technology has a major drawback of distance. It can function smoothly at a distance of a max
of 30 feet.

5. Satellite communication

It is extensively used to connect people anywhere in the world. This technology requires
two dominant integrants: the space and ground segments. Once the satellite receives a signal,
it amplifies it and sends it back to the receiver’s antenna, which is on Earth’s surface. The
ground segment comprises a mobile transmitter, receiver and the space segment (the satellite
itself).However, this crucial communication mode requires major testing before application.

6. Cellular communication

It’s one of the most commonly used in our day-to-day life. Cellular communication
allows transmission through any mobile phone. The preset mode permits a dual way wherein
the transmitter and receiver can obtain signals simultaneously. Cellular communication scores
very high on convenience, and that’s why the use of video and voice calls is growing by the
day.

7. Microwave

Microwave wireless communication technology has a frequency range between 1GHz-

300GHz. This communication is routinely used in mobile phones and TV distribution. It is one
of the most rapid technologies and can carry about 2500 voice channels at one time. This type
of wireless communication can be used in the terrestrial or satellite method. However, it is
unidirectional and hence used in point-to-point communication. One weighty downside is that
it can massively be affected by bad weather.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

Advantages of Wireless Communication:

1. Speed

Speed is one of the greatest advantages of wireless communication. The advanced

mechanization allows the transfer of data within a few seconds only. A wired network can easily
be affected by external interruptions, which is not the case in wireless.

2. Mobility

You will obviously get restricted to one single location with a wired connection. But
that’s simply not the case with wireless communication. With good grace, you have the liberty
to move around and continue to stay connected.

3. Cost-saving

Wireless connection will keep you away from the additional cost of entailing cables and
all unnecessary drilling. There is no need for any maintenance when there is no wire involved.
With fewer connection points and the absence of cables, wireless communication helps in cost
savings.

4. High Connectivity

Having access to wireless connections is a big advantage in this digital era. Since there
is no fixed installation, you can move around and be connected to emails and messages at any
point. Constant connectivity also allows you to respond to emergencies speedily.

AM Transmitter
High-Level AM Transmitters

Block Diagram of High Level AM Transmitter

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Carrier Oscillator

The carrier oscillator generates the carrier signal, which lies in the RF range. The
frequency of the carrier is always very high. Because it is very difficult to generate high
frequencies with good frequency stability, the carrier oscillator generates a sub multiple with
the required carrier frequency.

This sub multiple frequency is multiplied by the frequency multiplier stage to get the
required carrier frequency. Further, a crystal oscillator can be used in this stage to generate a
low frequency carrier with the best frequency stability. The frequency multiplier stage then
increases the frequency of the carrier to its required value.

Buffer Amplifier

• It first matches the output impedance of the carrier oscillator with the input impedance
of the frequency multiplier, the next stage of the carrier oscillator.
• It then isolates the carrier oscillator and frequency multiplier.
• This is required so that the multiplier does not draw a large current from the carrier
oscillator. If this occurs, the frequency of the carrier oscillator will not remain stable.

Frequency Multiplier

The sub-multiple frequency of the carrier signal, generated by the carrier oscillator , is
now applied to the frequency multiplier through the buffer amplifier. This stage is also known
as harmonic generator. The frequency multiplier generates higher harmonics of carrier
oscillator frequency. The frequency multiplier is a tuned circuit that can be tuned to the requisite
carrier frequency that is to be transmitted.

Power Amplifier

The power of the carrier signal is then amplified in the power amplifier stage. This is
the basic requirement of a high-level transmitter. A class C power amplifier gives high power
current pulses of the carrier signal at its output.

Audio Chain

The audio signal to be transmitted is obtained from the microphone. The audio driver
amplifier amplifies the voltage of this signal. This amplification is necessary to drive the audio
power amplifier. Next, a class A or a class B power amplifier amplifies the power of the audio
signal.

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Modulated Class C Amplifier

This is the output stage of the transmitter. The modulating audio signal and the carrier
signal, after power amplification, are applied to this modulating stage. The modulation takes
place at this stage. The class C amplifier also amplifies the power of the AM signal to the
required transmitting power. This signal is finally passed to the antenna., which radiates the
signal into space of transmission.

FM Transmitter

Using Reactance modulator direct method

The FM transmitter has three basic sections.

The Exciter

The function of the carrier oscillator is to generate a stable sine wave signal at the rest
frequency when no modulation is applied. It must be able to linearly change frequency when
fully modulated, with no measurable change in amplitude.

The buffer amplifier acts as a constant high- impedance load on the oscillator to help
stabilize the oscillator frequency. The buffer amplifier may have a small gain.

The modulator acts to change the carrier oscillator frequency by application of the
message signal. The positive peak of the message signal generally lowers the oscillator's
frequency to a point below the rest frequency, and the negative message peak raises the
oscillator frequency to a value above the rest frequency. The greater the peak-to-peak message
signal, the larger the oscillator deviation.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

Frequency Multiplier

Frequency multipliers are tuned-input, tuned-output RF amplifiers in which the output

resonant circuit is tuned to a multiple of the input frequency. Common frequency multipliers
are 2x, 3x, and 4x multiplication.

Power Output Section

The final power section develops the carrier power, to be transmitted and often has a
low-power amplifier driven by the final power amplifier. The impedance matching network is
the same as for the AM transmitter and matches the antenna impedance to the correct load on
the final power-amplifier.

Reactance Modulator

The reactance modulator takes its name from the fact that the impedance of the circuit
acts as a reactance (capacitive or inductive) that is connected in parallel with the resonant
circuit of the Oscillator. The varicap can only appear as a capacitance that becomes part of the
frequency-determining branch of the oscillator circuit.

Application of FM Transmitter

• The FM transmitters are used in the homes like sound systems in halls to fill the sound
with the audio source.
• These are also used in cars and fitness centers.
• The correctional facilities have used in the FM transmitters to reduce the prison noise
in common areas.

Advantages of the FM Transmitters

• The FM transmitters are easy to use and the price is low

• The efficiency of the transmitter is very high
• It has a large operating range
• This transmitter will reject the noise signal from an amplitude variation.

Disadvantages of the FM Transmitter

• In the FM transmitter, the huge wider channel is required.

• The FM transmitter and receiver will tend to be more complex.
• Due to some interference, there is poor quality in the received signals

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

Working of CRT TV
1. An antenna (aerial) on your roof
picks up radio waves from the
transmitter. With satellite TV, the
signals come from a satellite dish
mounted on your wall or roof. With
cable TV, the signal comes to you
via an underground fibre-
optic cable.
2. The incoming signal feeds into
the antenna socket on the back of
the TV.
3. The incoming signal is carrying picture and sound for more than one station (program).
An electronic circuit inside the TV selects only the station you want to watch and splits the
signal for this station into separate audio (sound) and video (picture) information, passing
each to a separate circuit for further processing.
4. The electron gun circuit splits the video part of the signal into separate red, blue, and green
signals to drive the three electron guns.
5. The circuit fires three electron guns (one red, one blue, and one green) down a cathode-ray
tube, like a fat glass bottle from which the air has been removed.
6. The electron beams pass through a ring of electromagnets. Electrons can be steered
by magnets because they have a negative electrical charge. The electromagnets steer the
electron beams so they sweep back and forth across the screen, line by line.
7. The electron beams pass through a grid of holes called a mask, which directs them so they
hit exact places on the TV screen. Where the beams hit the phosphors (coloured chemicals)
on the screen, they make red, blue, or green dots. Elsewhere, the screen remains dark. The
pattern of red, blue, and green dots builds up a coloured picture very quickly.
8. Meanwhile, audio (sound) information from the incoming signal passes to a separate audio
circuit.
9. The audio circuit drives the loudspeaker (or loudspeakers, since there are at least two in a
stereo TV) so they recreate the sound exactly in time with the moving picture.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

RADAR
The word "radar" stands for radio detection and ranging. A radar system operates in a
way that it radiates electromagnetic energy into space and detects various aspects related to
objects by analysing the echo generated when the radiated energy gets re-radiated by the object.

The radio waves used by radar are produced by a piece of equipment called
a magnetron. Radio waves are similar to light waves: they travel at the same speed—but their
waves are much longer and have much lower frequencies. Light waves have wavelengths of
about 500 nanometers, whereas the radio waves used by radar typically range from about a few
centimeters to a meter.

Transmitter Section: The transmitter section is composed of the following units:

1. Waveform Generator: The waveform generator (usually a magnetron) generates a radar

signal at low power which is to be transmitted into space.
2. Transmitter: The signal generated by the waveform generator is fed to the transmitter. The
transmitter section can be a magnetron, travelling wave tube or a transistor amplifier.
In the case of pulse radar, magnetrons are widely used as transmitters but whenever there
exists a need for high average power then amplifiers are used.
3. Pulse modulator: A pulse modulator is used to build synchronization between the
waveform generator and transmitter. The pulse modulator causes the turning on and off of the
power amplifier according to the input pulses generated by the waveform generator.
4. Duplexer: A duplexer is basically used to form isolation between transmitter and receiver
section. A duplexer allows the use of a single antenna for both transmission and reception

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purpose. However, both the sections operate at different power level, therefore, a duplexer is
used to isolate the two section.Thus the signal from the transmitter is provided to the antenna
through the duplexer. As the duplexer short circuits the input of the receiver section.Also, the
re-radiated signal received by the common antenna is fed to the receiver section using
duplexer.

Receiver Section: The following components are present inside the receiver section:
5. Low noise RF amplifier: The receiver must be superheterodyne. The unit acts as the input
stage for the receiver section. The RF amplifier generates an RF pulse which is proportional
to the echo of the transmitted signal.
6. Mixer and Local Oscillator: The RF pulse received from the low noise RF amplifier is
converted into an IF pulse. Usually, the RF amplifier acts at the input stage of the receiver
section but sometimes the mixer acts at the input stage by eliminating the RF amplifier.
But this leads to a less sensitive receiving section due to the high noise figure of the mixer.
7. IF amplifier: The IF pulse generated by the mixer circuit is amplified by the IF amplifier.
It acts as a matched filter and increases the SNR of the received signal. Also, it enhances the
echo detecting ability of the receiver section by reducing the effects of unwanted signals.
The receiver’s bandwidth is associated with the bandwidth of the IF stage.
8. 2nd Detector or Demodulator: This unit is nothing but a crystal diode that performs
demodulation of the signal by separating the transmitted signal from the carrier.
9. Video Amplifier: This unit amplifies the received signal to a level that can be displayed on
the screen.
10. Threshold decision: This unit makes the decision about the existence of the target in
space. Basically, it has some threshold limit set which is compared with the magnitude of the
received signal. If the threshold value is surpassed by the output signal, then this shows that
presence of the target. Otherwise, it is assumed that only the noise component is present in
the space.
11. Display: The display unit shows the final output of the receiver section. PPI i.e., plan
position indication is typically used as the radar display unit.It presents the range and location
of the object by mapping it in polar coordinates. PPI is implemented with CRT.The output
signal modulates the electron beam of the cathode ray tube in order to permit the electron
beam to sweep from the centre in the outward direction of the tube. And this sweep shows
rotation in synchronization with the pointing of the antenna.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

Applications of Radar
• Military: It is the major application of radar and is one of the most important parts of the
air defence system. Radar is used for the purpose of navigation and surveillance in the
military for secure operations.
• Air traffic controlling: Radar is used to control the air traffic in the air routes and
airports. High-resolution radars are used for analysing the aircraft and ground vehicular
traffic at the airports.
• Ship safety: Radars are used to provide safety measures to the ships in bad visibility
conditions by giving alerts about the existence of other ships in the route.
• Remote sensing: Radar is a remote sensor by nature as they can sense the geophysical
objects. And these are used forecasting of weather conditions along with agricultural
conditions and environmental pollution.

Radar Classifications
1. Classification based on specific function
Classification based on the primary function of radar is shown in the following figure

Primary Radar:
A Primary Radar transmits high-frequency signals toward the targets. The transmitted
pulses are reflected by the target and then received by the same radar. The reflected energy or
the echoes are further processed to extract target information.

Secondary Radar:

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 Secondary radar units work with active answer signals. In addition to primary radar,
this type of radar uses a transponder on the airborne target/object.
A simple block diagram of secondary radar is shown below

The ground unit, called interrogator, transmits coded pulses (after modulation) towards
the target. The transponder on the airborne object receives the pulse, decodes it, induces the
coder to prepare the suitable answer, and then transmits the interrogated information back to
the ground unit. The interrogator/ground unit demodulates the answer. The information is
displayed on the display of the primary radar.
The secondary radar unit transmits and also receives high-frequency impulses, the so
called interrogation. This isn’t simply reflected, but received by the target by means of a
transponder which receives and processes. After this the target answers at another frequency.
Various kinds of information like, the identity of aircraft, position of aircraft, etc. are
interrogated using the secondary radar. The type of information required defines the MODE of
the secondary radar.

Pulsed Radar:
Pulse radars can be used to measure target velocities. Two broad categories of pulsed
radar employing Doppler shifts are

• MTI (Moving Target Indicator) Radar

The MTI radar uses low pulse repetition frequency (PRF) to avoid range ambiguities, but these
radars can have Doppler ambiguities.
• Pulse Doppler Radar
Contrary to MTI radar, pulse Doppler radar uses high PRF to avoid Doppler ambiguities, but
it can have numerous range ambiguities.

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 Doppler Radars make it possible to distinguish moving target in the presence of echoes
from the stationary objects. These radars compare the received echoes with those received in
previous sweep. The echoes from stationary objects will have same phase and hence will be
cancelled, while moving targets will have some phase change.
If the Doppler shifted echo coincides with any of the frequency components in the frequency
domain of the received signal, the radar will not be able to measure target velocity. Such
velocities are called blind speeds.

Where, fo = radar operating frequency.

Continuous Wave Radar:

CW radars continuously transmit a high-frequency signal and the reflected energy is
also received and processed continuously. These radars have to ensure that the transmitted
energy doesn’t leak into the receiver (feedback connection). CW radars may be bistatic or
monostatic; measures radial velocity of the target using Doppler Effect.

Types of RADAR based on applications

1. Surveillance Radars
Primary application of radar is surveillance. These radars typically use high power,
scanning antenna and have moderate resolution. They are deployed for
• Detection and Tracking of Aircraft, Missiles or Space Objects
• Detection of Fixed or Moving Surface Targets
• Moderate precision tracking of multiple targets

Some of the important applications of Surveillance radars are

a. Air Traffic Management Radars
Radars commonly used for air traffic management are
• En-route radar systems
These radars usually operate in L-Band, detect and determine the position, course, and
speed of air targets in an area up to 250 nautical miles.
• Air Surveillance Radar systems

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

 These radars usually operate in E-Band, and are used to detect and display an aircraft’s
position in the terminal area. They can reliably detect and track aircrafts at altitudes
below 25,000 feet and within 40-60 nautical miles of the airport
• Precision Approach Radar (PAR) systems
This radar helps the aircraft to land in bad weather. Using Precision Approach radar, the
guidance information is obtained by the radar operator and passed to the aircraft.
• Surface movement radars,
Surface Movement Radar (SMR) uses very narrow pulse widths and is used to scan the
airport surface and locate the positions of aircraft as well as ground vehicles.

b. Air-defence Radars
Air-Defence Radars are employed to detect air targets and to determine target range,
velocity, etc. in a relatively large area. They are able to detect threats at great distances and
hence act as early warning devices. Typical range of an Air-Defence Radar is 300 miles, and
azimuth coverage is full 360 degrees.
Range and bearing information provided by these radars are used to initially position
tracking radars.

2. Tracking Radars
Also called fire control radars, they are used to provide range and bearing information
of a single target continuously. These radars use very high PRF, very narrow pulsewidth as well
as beamwidth. This allows these radars to have high accuracy, limited range, and initial
detection of the target a bit difficult.
They typically take range and bearing information from the search radars. Until a target
is located, they remain in acquisition phase searching for the target. Once a target is located,
they enters track phase and automatically follow target motions.

3. Meteorological Radars
Also known as weather radars, they are primarily used to observe hydrometeors in the
atmosphere. Radar is probably the only way to map the spatial distribution of precipitation over
large areas. Radar can be used to forecast flash flooding and severe thunderstorms.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

4. Imaging Radar
In contrast to non-imaging radars(which create linear 1D measurement), imaging radar sensors
measure two dimensions of co-ordinates to create a map-like picture of the observed object or
area. Imaging radars have been used to map the Earth, other planets, and celestial bodies and
also to classify military targets. Imaging radar, monostatic radar, is an active illumination
system and is mounted on airborne objects like aircraft, satellites. Radar transmits a signal
towards the Earth’s surface and then waits for the reflected signal or echo. Backscattered signal
from the surface is processed to construct the image.

VHF Radio
VHF radios are sophisticated communication devices widely used in the maritime
industry for ship-to-ship and ship-to-shore communication. They operate on frequencies
between 156.000 MHz to 174.000 MHz, offering an effective line of communication over a
considerable range in open water. VHF radios are preferred for marine use due to their ability
to penetrate fog and transmit signals over the horizon, making them indispensable tools for
seafarers.

VHF is one of the primary forms of communication used by ships at sea. It is used for
various purposes, including navigation, fishing, security/surveillance, and vessel
communication. It is beneficial for communicating with other ships in the same vicinity since
it has a limited range (up to about 10 miles) and requires line-of-sight communication.

Basics Steps for Using a VHF Radio

1. Turn on the VHF unit and adjust the squelch by turning the knob until the static stops.
2. Tune to channel 16, the channel monitored by the U.S. Coast Guard.
3. Perform a radio check to ensure your unit is functioning properly—do not use channel
16 .
4. Use an "open channel" to performance the check (channels 68, 69, 71, 72 and 78A).
5. Turn radio to one-watt power setting, and key the microphone.
6. Call "radio check" three times, followed by your boat name and location.
7. Wait for a reply confirming someone has heard your transmission.
8. For general communications, always use channel 16.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan

Using a VHF Radio in an Emergency
• Tune the radio the channel 16 and full power.
• If lives are in danger transmit “Mayday Mayday Mayday” and your vessel name.
• Wait for the Coast Guard to respond and be ready to reply with your location, ideally
with latitude and longitude from GPS.
• If your situation is bad but not life-threatening, use the call “pan-pan.”

Types of VHF Marine Radios

A VHF marine radio may be hand-held or fixed-mount.
1. A fixed-mount VHF unit is permanently installed on the boat and powered by the
boat’s electrical system.
2. The hand-held version is portable and is battery-powered, so it will still function if the
boat’s battery is dead or the electrical system fails, and can be used if you are forced to
abandon ship.

Advantages:
VHF radios offer several benefits over other radio communications systems, such as
UHF (ultra-high-frequency) or HF (high-frequency). They are less susceptible to interference
caused by power lines and construction sites because their signals travel at higher frequencies
than those used by other radios.
Additionally, they have a much more extensive range than most other radio systems—
up to 100 miles in some cases—which makes them ideal for operations that require
communication over large distances. VHF radios are relatively affordable compared to other
radio systems and require minimal maintenance.

VELS UNIVERSITY-School of Maritime Studies-Compiled by J.Chenguttuvan