TELECOMMUNICATION

Introduction to RADAR system

Radar-like radio detection and ranging is used to assist the pilots while flying through fog
because the pilot cannot notice that where they are traveling. The radar used in the airplanes is
similar to a torchlight that works with radio waves in place of light. The airplane transmits a
blinking radar signal and listens out for any indications of that signal from nearby objects. Once
the indications are noticed, then the airplane identifies something is near & it uses the time taken
for the indications to reach for discovering how distant away it is. This article discusses an
overview of Radar and its working.

What is a Radar System?

RADAR stands for Radio Detection and Ranging System. It is basically an electromagnetic
system used to detect the location and distance of an object from the point where the RADAR is
placed. It works by radiating energy into space and monitoring the echo or reflected signal from
the objects. It operates in the UHF and microwave range. RADAR is an electromagnetic sensor,
used to notice, track, locate, and identify different objects which are at certain distances. The
working of radar is, it transmits electromagnetic energy in the direction of targets to observe the
echoes and returns from them. Here the targets are nothing but ships, aircraft, astronomical
bodies, automotive vehicles, spacecraft, rain, birds, insects, etc. Instead of noticing the target’s
location and velocity, it also obtains their shape and size sometimes.

The main objective of radar as compared with infrared and optical sensing devices is to discover
faraway targets under difficult climate conditions & determines their distance, range, through
precision. Radar has its own transmitter which is known as a source of illumination for placing
targets. Generally, it works in the microwave area of the electromagnetic spectrum that is
calculated in hertz when frequencies extend from 400 MHz to 40 GHz. Radar undergoes quick
development during the years 1930-the 40s to reach the requirements of the military. It is still
broadly used through the armed forces, wherever several technological advances have created.
Simultaneously, radar is also utilized in civilian applications particularly in controlling air traffic,
observation of weather, navigation of ship, environment, sensing from remote areas, observation
of planetary, measurement of speed in industrial applications, space surveillance, law
enforcement, etc.

Working Principle

The radar working principle is very simple. When a signal is transmitted electromagnetically,
the signals are reflected back to the source. The reflected signal is then examined to ascertain the
possibility of the presence of an obstacle an obstacle, the distance of the obstacle and the nature
of the obstacle in the transmission way. This is the working principle of radar.

Fundamentals of Radar

The RADAR system generally consists of a transmitter that produces an electromagnetic signal
which is radiated into space by an antenna. When this signal strikes an object, it gets reflected or
reradiated in many directions. This reflected or echo signal is received by the radar antenna
which delivers it to the receiver, where it is processed to determine the geographical statistics of
the object.

The range is determined by calculating the time taken by the signal to travel from the RADAR to
the target and back. The target’s location is measured in angle, from the direction of the
maximum amplitude echo signal, the antenna points to. To measure the range and location of
moving objects, the Doppler Effect is used.

The essential parts of this system include the following:

 A Transmitter: It can be a power amplifier like a Klystron, Travelling Wave Tube, or a

power Oscillator like a Magnetron. The signal is first generated using a waveform generator
and then amplified in the power amplifier.

 Waveguides: The waveguides are transmission lines for transmission of the RADAR signals.

 Antenna: The antenna used can be a parabolic reflector, planar arrays, or electronically
steered phased arrays.

 Duplexer: A duplexer allows the antenna to be used as a transmitter or a receiver. It can be a

gaseous device that would produce a short circuit at the input to the receiver when the
transmitter is working.

 Receiver: It can be a superheterodyne receiver or any other receiver which consists of a

processor to process the signal and detect it.

 Threshold Decision: The output of the receiver is compared with a threshold to detect the
presence of any object. If the output is below any threshold, the presence of noise is assumed.

How Does Radar use Radio?

Once the radar is placed on a ship or plane, then it requires a certain essential set of components
to produce radio signals, transmit them into space and receive them by something, and finally
display the information to understand it. A magnetron is one kind of device, used to generate
radio signals which are used through radio. These signals are similar to light signals because they
travel at the same speed but their signals are much longer with fewer frequencies. The light
signals wavelength is 500 nanometers, whereas the radio signals used by radar normally range
from centimeters to meters. In an electromagnetic spectrum, both the signals like radio and light
are made with variable designs of magnetic and electrical energy throughout the air. The
magnetron in radar generates microwaves the same as a microwave oven. The main disparity is
that the magnetron within radar has to transmit the signals several miles, rather than just small
distances, so it is more powerful as well as much larger.

Whenever the radio signals have been transmitted, then an antenna functions as a transmitter to
transmit them into the air. Generally, the antenna shape is bent so it mainly focuses the signals
into an exact and narrow signal; however radar antennas also normally revolve so they can notice
actions over a huge area. The radio signals travel outside from the antenna with 300,000 km per
second speed until they strike something and some of them return back to the antenna. In a radar
system, there is an essential device namely a duplexer. This device is used to make the antenna
change from side to side in between a transmitter & a receiver.

Types of Radar

The following are different types of radars:

Bistatic Radar

This type of radar system includes a Tx-transmitter & an Rx- receiver that is divided through a
distance that is equivalent to the distance of the estimated object. The transmitter & the receiver
are situated at a similar position is called a monastic radar whereas the very long-range surface to
air & air to air military hardware uses the bistatic radar.

Doppler Radar

It is a special type of radar that uses the Doppler Effect to generate data velocity regarding a
target at a particular distance. This can be obtained by transmitting electromagnetic signals in the
direction of an object so that it analyzes how the action of the object has affected the returned
signal’s frequency.

This change will give very precise measurements for the radial component of an object’s velocity
within relation toward the radar. The applications of these radars involve different industries like
meteorology, aviation, healthcare, etc.
Monopulse Radar

This kind of radar system compares the obtained signal using a particular radar pulse next to it
by contrasting the signal as observed in numerous directions otherwise polarizations. The most
frequent type of monopulse radar is the conical scanning radar. This kind of radar evaluates the
return from two ways to measure the position of the object directly. It is significant to note that
the radars which are developed in the year 1960 are monopulse radars.

Passive Radar

This kind of radar is mainly designed to notice as well as follow the targets through processing
indications from illumination within the surroundings. These sources comprise communication
signals as well as commercial broadcasts. The categorization of this radar can be done in the
same category of bistatic radar.

Instrumentation Radar

These radars are designed for testing aircraft, missiles, rockets, etc. They give different
information including space, position, and time both in the analysis of post-processing & real-
time.

Weather Radars

These are used to detect the direction and weather by using radio signals through circular or
horizontal polarization. The frequency choice of weather radar mainly depends on a compromise
of performance among attenuation as well as precipitation refection as an outcome of
atmospheric water steam. Some types of radars are mainly designed to employ Doppler shifts to
calculate the wind speed as well as dual-polarization to recognize the types of rainfall.

Mapping Radar

These radars are mainly used to examine a large geographical area for the applications of remote
sensing & geography. As a result of synthetic aperture radar, these are restricted to quite
stationary targets. There are some particular radar systems used to detect humans after walls that
are more different as compared with the ones found within construction materials.

Navigational Radars

Generally, these are the same to search radars but, they available with small wavelengths that are
capable of replicating from the ground & from stones. These are commonly used on commercial
ships as well as long-distance airplanes. There are different navigational radars like marine
radars which are placed commonly on ships to avoid a collision as well as navigational purposes.

Pulsed RADAR

Pulsed RADAR sends high power and high-frequency pulses towards the target object. It then
waits for the echo signal from the object before another pulse is sent. The range and resolution of
the RADAR depend on the pulse repetition frequency. It uses the Doppler shift method. The
principle of RADAR detecting moving objects using the Doppler shift works on the fact that
echo signals from stationary objects are in the same phase and hence get canceled while echo
signals from moving objects will have some changes in phase. These radars are classified into
two types.

Pulse-Doppler

It transmits high pulse repetition frequency to avoid Doppler ambiguities. The transmitted signal
and the received echo signal are mixed in a detector to get the Doppler shift and the difference
signal is filtered using a Doppler filter where the unwanted noise signals are rejected.

Moving Target Indicator

It transmits low pulse repetition frequency to avoid range ambiguities. In an MTI RADAR
system, the received echo signals from the object are directed towards the mixer, where they are
mixed with the signal from a stable local oscillator (STALO) to produce the IF signal.

This IF signal is amplified and then given to the phase detector where its phase is compared with
the phase of the signal from the Coherent Oscillator (COHO) and the difference signal is
produced. The Coherent signal has the same phase as the transmitter signal. The coherent signal
and the STALO signal are mixed and given to the power amplifier which is switched on and off
using the pulse modulator.

Continuous Wave

The continuous wave RADAR doesn’t measure the range of the target but rather the rate of
change of range by measuring the Doppler shift of the return signal. In a CW RADAR
electromagnetic radiation is emitted instead of pulses. It is basically used for speed measurement.
The RF signal and the IF signal are mixed in the mixer stage to generate the local oscillator
frequency. The RF signal is then transmitted signal and the received signal by the RADAR
antenna consist of the RF frequency plus the Doppler shift frequency. The received signal is
mixed with the local oscillator frequency in the second mixture stage to generate the IF
frequency signal.
This signal is amplified and given to the third mixture stage where it is mixed with the IF signal
to get the signal with Doppler frequency. This Doppler frequency or Doppler shift gives the rate
of change of range of the target and thus the velocity of the target is measured.

Radar Range Equation

There are different kinds of versions available for the radar range equations. Here, the following
equation is one of the fundamental types for an only antenna system. When the object is assumed
to be in the middle of the antenna signal, then the highest radar detection range can be written as
Radar range equation and is useful in knowing the range of the target theoretically.

The standard form of Radar range equation is also called as simple form of Radar range
equation. Now, let us derive the standard form of Radar range equation.

We know that power density is nothing but the ratio of transmitter power to the surface area of
a sphere. So, the power density, Pdi at a distance, R from the Radar can be mathematically
represented as –

Pt
Pdi = 2 (1)
4π R

Where,

Pt is the amount of power transmitted by the Radar transmitter

The above power density is valid for an isotropic Antenna. In general, Radars use directional
Antennas. Therefore, the power density, Pdd due to directional Antenna will be

GPt
Pdd = 2 (2)
4πR

Target radiates the power in different directions from the received input power. The amount of
power, which is reflected back towards the radar depends on its cross section. So, the power
density Pde of echo signal at Radar can be mathematically represented as

σ
Pde =Pdd ( 2
) (3)
4π R

Substitute, Equation (2) in Equation (3), we obtain

GPt σ
Pde =( 2
)( 2
) (4)
4π R 4πR
The amount of power, Pr received by the Radar depends on the effective aperture, Ae of the
receiving Antenna.

Pr=PdeAe (5)

Substitute equation (4) in equation (5):

GPt σ
Pr =( 2
)( 2
) Ae
4π R 4π R

Pt Gσ A e
Pr = 2 4
(4 π) R

4 P t Gσ Ae
R= 2
4 π Pr

1/ 4
Pt Gσ A e
R=[ 2
] (6)
( 4 π ) Pr

Standard Form of Radar Range Equation

If the echo signal is having the power less than the power of the minimum detectable signal,
then Radar cannot detect the target since it is beyond the maximum limit of the Radar's range.

Therefore, we can say that the range of the target is said to be maximum range when the
received echo signal is having the power equal to that of minimum detectable signal. We will
get the following equation, by substituting R=R max ∧P r =S min

1/ 4
Pt Gσ A e
Rmax=[ 2
] (7)
( 4 π ) Smin

Equation 7 represents the standard form of Radar range equation. By using the above
equation, we can find the maximum range of the target.

Radar Applications

The applications of radar include the following.

Military Applications

It has 3 major applications in the Military:

 In air defense, it is used for target detection, target recognition, and weapon control (directing
the weapon to the tracked targets).

 In a missile system to guide the weapon.

 Identifying enemy locations on the map.

Air Traffic Control

It has 3 major applications in Air Traffic control:

 To control air traffic near airports. The Air Surveillance RADAR is used to detect and display
the aircraft’s position in the airport terminals.

 To guide the aircraft to land in bad weather using Precision Approach RADAR.

 To scan the airport surface for aircraft and ground vehicle positions

Remote Sensing

It can be used for observing whether or observing planetary positions and monitoring sea ice to
ensure a smooth route for ships.

Ground Traffic Control

It can also be used by traffic police to determine the speed of the vehicle, controlling the
movement of vehicles by giving warnings about the presence of other vehicles or any other
obstacles behind them.

Space

It has 4 major applications

 To guide the space vehicle for a safe landing on the moon

 To observe the planetary systems

 To detect and track satellites

 To monitor the meteors