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Transcript

RADAR THEORY

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Scope

SCOPE

  • What is Radar?
  • Purpose of Radar
  • Components of Radar
  • Working Principle
  • Terminlogy
  • Classification/Types of Radars
  • Doppler Radars
  • Continuous Wave Radar

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WHAT IS RADAR?

  • The word radar is an acronym derived from the phrase RAdio Detection And Ranging.

  • Equipment to detecting or tracking objects.

  • It is characterize by radio waves to be reflected from them and analyzing the reflected waves. The waves can be converted into images.

  • Radar, was one of the most important scientific discoveries and technological developments that emerged from WWII.

Radar

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1. Purpose of Radar

1. Purpose of Radar

  • To allocate object/target
  • To identify distance of an approaching object/target
  • To identify the speed of approaching object/target
  • To possible identify (type) of an object

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1.1 Purpose of Radar

  • A target or an object can be determined through AZIMUTH AND ELEVATION from a Radar.
  • AZIMUTH; is an arc of the horizon measured between a fixed point (such as true north) and the vertical circle passing through the center of an object usually in astronomy and navigation clockwise from the north point through 360 degrees.
  • ELEVATION; the height to which something is elevated.

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1.2 Radar in a system

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3. Component of Radar

2. Component of Radar

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Transmitter

Transmitter

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A set of equipment used to generate and transmit electromagnetic waves carrying messages or signals, especially those of radio or television.

Antenna

Antenna

An antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver.

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Receiver

Receiver

An electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna.

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Indicator

Indicator

One common “auxiliary” instrument, the purpose of which is to provide a human readable indication of an instrument signal.

An indicator gives a human operator or Artificial Intelligent (AI) a convenient way of seeing what the output of the transmitter.

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How RADAR works

3. How Radar Works

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Explanation

How RADAR work

The radar transmits a focused pulse of microwave energy at an object, most likely a cloud. Part of this beam of energy bounces back and is measured by the radar, providing information about the object.

(a) Transmitter: The source of the radio pulse.

(b) Antenna: Needed to send the pulse out into the ether and receive it when it is reflected back.

(c) Switch: This tells the antenna when to transmit or receive the pulses.

(d) Receiver: Required to detect and turn the pulses, which come back into a visual format to be read by an operator.

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4. Working Principles

Working Principles

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RADAR Clutter

Clutter

weather clutter

fixed clutter

targets

Plan Position Indicator (PPI) of ATC Radar

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Clutter?

Clutter

Definition: Radar Clutter is defined as the unwanted back-scattered signals or echoes generated from physical objects in the natural environment.

This makes detection of targets more difficult.

Simply put: radar noise.

Surface Clutter - Returns from geographical land masses. Generally stationary

Volume Clutter - Weather (rain, snow, clouds), Chaff

Point Clutter - Birds, and individual tall buildings. On radar, very similar to aircraft

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weather clutter

fixed clutter

targets

Main and Side Lobes

Main lobe is the lobe of the radiation pattern of a directional antenna which contains the direction of maximum radiation. This is the lobe that exhibits the greatest field strength.

Side lobes represent unwanted radiation in undesired directions

directional antenna radiation pattern

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Directional Antenna

An antenna is a metallic structure that captures and/or transmits radio electromagnetic waves.

A directional antenna is an antenna that radiates or receives greater power in specific directions.

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Main and Side Lobe Clutter

Parasitic returns that enter the radar through the antenna’s main lobe are called main lobe clutters; otherwise they are called side lobe clutters

  • Main lobe clutters are inevitable
  • Side lobe clutters are energy inefficient, and wastes power
  • Eliminate side lobe clutters by using side lobe cancellers

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Minimising Clutter

Surface Clutter:

Reducing the ground area illuminated by the antenna

Volume Clutter:

Reducing the pulse duration

Modulation

Modulation

Definition: The variation of wavelength and frequency of a periodic waveform

This is how a message signal is transmitted (digital bit stream or an analogue audio signal)

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5. Terminology

Terminology

Classification

Radars are designed around its intended use and purpose.

As such, they are classified differently based on their capabilities and functions.

Classification

2-dimensional: range, azimuth

3-dimensional: range, azimuth, elevation

4-dimensional: range, azimuth, elevation, velocity

ARSR: Air Route Surveillance Radar

ASR: Airport Surveillance Radar

Airborne Radars: G550 Airborne Early Warning

Radar Bands & Usage

Radar Bands/Usage

Considerations in Selecting Frequency

Why?

Physical Size. The dimensions of the hardware used to generate and transmit radio frequency power are, in general, proportional to wavelength

Transmitted Power. Higher wavelengths transmit more power. This affects hardware requirements for Radar equipment

Beamwidth. At low frequencies, large antennas must generally be used to achieve acceptably narrow beams. At higher frequencies, small antennas will suffice.

Atmospheric Attenuation. At high frequencies (>10GHz), radio waves are absorped and scattered by atmospheric oxygen and water vapour

Factors Affecting Radar Design

How?

Categories of Radar

  • Continuous Wave (CW)‏
  • Frequency Modulated, Continuous Wave (FM-CW)
  • Pulsed
  • Pulsed Doppler
  • Moving Target Indication (MTI)‏
  • Monopulse Radar

Categories

Basic Radar Types

Doppler Radar

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Pulsed Radar

Pulsed Doppler Radar

Moving Target Indication(MTI) Radar

Monopulse Radar

Objective

Understand what is :

  • Pulsed Radar
  • Pulsed Doppler Radar
  • Moving Target Indication(MTI) Radar
  • Monopulse Radar

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Pulsed Radar

  • Pulse radars are designed mainly for long distances.
  • Radar systems are classified on the basis of Pulse Repetition Frequency (PRF)
  • Low, medium and high PRF radar

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Pulsed Radar

  • Determine target range by measuring round trip time of microwave signal
  • Low PRF waveforms produces accurate, long, unambigous range measurement

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What information does it provides?

  • Presence of target
  • Range to target
  • Direction to target
  • Velocity

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Doppler Radars

  • The Doppler effect is a shift in the frequency of a wave caused by the motion of the transmitting source, the reflecting object, or the receiving system.

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Coherence

  • A radar is said to be coherent if the phase of any two transmitted pulses is consistent, i.e. there is a continuity in the signal phase from one pulse to the next.
  • Coherency can be achieved by using a Stable Local Oscillator (STALO).

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Doppler Sensors

  • Using both ultrasound and electromagnetic waves make use of the Doppler principle to measure target motion effects
  • Either use continuous wave (CW) or pulsed waveforms
  • CW sources generally determine velocity only
  • Pulsed sources can discriminate range and velocity

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Pulsed Doppler Radar

  • Speed ranges through the use of filters
  • Range and relative speed with only one antenna

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Moving Target Indicator (MTI) Radar

  • Mode of operation of a radar
  • Pulse radar that uses the Doppler frequency shift of the received signal to detect moving targets such as aircraft.
  • Reject the large unwanted echoes from stationary clutter that do not have a Doppler shift.
  • Received signals are delayed and compared to subsequent signals.
  • Signals without a relative phase shift (0 speed) are cancelled.
  • Almost all ground-based aircraft surveillance radar systems use some form of MTI.

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Doppler Shift

  • The Doppler shift describes the changes in the frequency of any sound or light wave produced by a moving source with respect to an observer.
  • A common example we have all experienced is standing beside a train track or highway. As a train or truck approaches, we hear a certain frequency sound. As a high speed train or truck passes, the sound immediately drops several octaves. This is caused by a frequency shift caused by the Doppler effect.

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Moving Target Indicator (MTI) Radar

  • For moving targets: amplitude varies on a pulse to pulse basis due to the changing phase between the transmitted and received signals.
  • For static targets: the phase will remain unchanged, and the amplitude will remain constant.
  • An MTI based on a delay-line canceller operates by taking the difference of the amplitudes of successive pulses to identify.

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Monopulse Radar

  • A monopulse radar uses multiple beams to track the direction of a target, with an angular accuracy equal to a fraction of its antenna beam
  • This allows the radar to be less subject to difficulties caused by sudden changes in the strength of the signal.
  • More difficult to jam

Two types:

  • Amplitude-comparison monopulse
  • Phase-comparison monopulse

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Monopulse Radar

Amplitude-Comparison Monopulse :

  • The tracking information is extracted from the amplitudes of the echoes received by the four horns

Phase-Comparison Monopulse :

  • Relies on the phases of the received signals

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Continuous Wave Radar

Continuous Wave Radar

  • Radar which operates with continuous signal wave
  • Use Doppler Effect to detect moving targets
  • Classified into:
  • Unmodulated continuous wave
  • Frequency modulated, continuous wave (FMCW)

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Unmodulated Continuous Wave

Unmodulated Continuous Wave Radar

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Frequency Modulated, Continuous Wave

Frequency Modulated, Continuous Wave Radar

Takes into consideration of time factor

  • Determine the target distance and speed.

Working Principle

1. Transmitter antenna emits frequency modulated continuous radio waves

2. Reflected signal from the target is received by the receiving antenna

3. Output of the receiving antenna is given to the mixer stage of the receiver via a pre-amplifier

  • In the mixer circuit, a part of the frequency-modulated transmitted signal is mixed with the received signal
  • Producing a new signal (Diff in transmitted and echo)
  • Multiplied within mixer and filtering out clutters produce imtermediate frequency signal ("Beat Frequency")

4. Beat frequency has to be amplified and limited for eliminating any amplitude fluctuations

  • Used to determine the distance (R) and/or velocity of the moving object
  • Pass through a cycle-counting frequency meter (calibrated in distance)

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Frequency Modulated, Continuous Wave Radar

Application

  • Radar altimeter used on Aircraft
  • Target’s range is calculated based on the measured delay Δt
  • Frequency offset Δf gives the velocity
  • From Δt and Δf (height of the aircraft can be calculated)
  • Mixer output gives the frequency difference which is amplified and limited
  • Output is then fed to a frequency counter which in turn is fed to an indicator
  • can be calibrated in feet or meters.

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Conclusion

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