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.
A Basic Idea of RADAR
The RADAR system generally consists of a
transmitter which produces an electromagnetic signal which is radiated
into space by an antenna. When this signal strikes any 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 the 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 maximum
amplitude echo signal, the antenna points to. To measure range and
location of moving objects, Doppler Effect is used.
A Basic RADAR System
Given below are 6 major parts of a RADAR System:
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 transmitter is
working.
Receiver: It can be super heterodyne 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.
A RADAR System
A Overview about the 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 send. The range and
resolution of the RADAR depends 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 same phase and hence get cancelled while echo
signals from moving object will have some changes in phase.
Two types of Pulsed RADAR are:
Pulse Doppler RADAR: 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.
Block Diagram of Pulsed Doppler RADAR
Moving Target Indicator RADAR: It
transmits low pulse repetition frequency to avoid range ambiguities. In
a 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.
Block Diagram Showing MTI RADAR by Edgefx Kits
An Overview about the Continuous Wave RADAR
The continuous wave RADAR doesn’t
measures 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 the transmitted signal and the received signal by the
RADAR antenna consists 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.
Block Diagram Showing CW RADAR
RADAR Applications in 5 Areas:
Military Applications:
The RADAR has 3 major applications in Military:
In air defense it is used for target detection, target recognition
and weapon control (directing the weapon to the tracked targets).
In missile system to guide the weapon.
Identifying enemy locations in map.
Air Traffic Control:
The RADAR 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: RADAR can be used for observing weather or
observing planetary positions and monitoring sea ice to ensure smooth
route for ships. Ground Traffic Control: RADAR can also be used by traffic
police to determine speed of the vehicle, controlling the movement of
vehicles by giving warnings about presence of other vehicles or any
other obstacles behind them. Space:
RADAR has 3 major applications:
To guide the space vehicle for safe landing on moon
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