Unit 2Radar communication

Prepared by Chandra Shekhar KTNTC

2.1   Explain radar working principle and Radar applications 1 Hr2.2   List the Radar frequencies bands 1 Hr2.3   Define pulse, Pulse width, PRF, Duty cycles, 1 Hr

  2.3.1 Peak power, Average power 1 Hr  2.3.2 Simple problems 1 Hr

2.4   Write and explain the Radar range equation      Discuss the factors affecting radar range 1 Hr

2.5   Discuss Duplexer and Display systems - 1 Hr  2.5.1 and List the types 1 Hr

2.6   Explain type of Radar:    2.6.1 Pulse and 1 Hr  2.6.2 CW 1 Hr  2.6.3 FMCW 1 Hr  2.6.4 MTI 1 Hr

Syllabus

 2.6.5 Secondary RADAR - IFF 1 Hr

2.7 

Different landing systems ILS & GCA 1 Hr

  

Different landing systems ILS & GCA 1 Hr

2.8 

Introduction to SONAR 1 Hr

What is radar?

Radar is an acronym for Radio Detection and Ranging.

It detects the presence of objects by electromagnetic energy.

It measures the direction , height and distance of the objects in three co-ordinates (x,y,z).

The frequency of electromagnetic energy used for radar is unaffected by darkness and penetrates fog and clouds to determine the position of objects.

2.1 Radar working principle

The radar is working under the concept of reflection of electromagnetic energy.

The radar signal is generated by a powerful transmitter and antenna illuminates the target with the RF waves , then reflected back and received as an echo/ return signal by a highly sensitive receiver.

Block diagram of a primary radar

Transmitter It produces the short duration high-power RF pulses of energy that are into space by the antenna.

Duplexer It alternately switches the antenna between the transmitter and receiver so that only one antenna need to be used.

This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver point.

Block diagram explanation

ReceiverIt will analyze the received echoes and present desired information in the suitable form.

Radar Antenna Antenna transfers the transmitter energy to signals in space with the required distribution and efficiency. This process is applied in an identical way on reception.

Radar Displays

It presents the observer a continuous, easily understandable, graphic picture of the relative position of radar

AApplications of Radar

1. Air Traffic control

2. Aircraft Navigation

3. Ship Safety

4. Space

5. Automobile traffic measurement

6. Military

Band name Frequency range Wavelength rangeHF 3–30 MHZ 10–100 m

VHF 30–300 MHz 1–10 mUHF 300–1000 MHz 0.3–1 m

L 1–2 GHZ 15–30 cmS 2–4 GHz 7.5–15 cmC 4–8 GHz 3.75–7.5 cmX 8–12 GHz 2.5–3.75 cmKU 12–18 GHz 1.67–2.5 cmK 18–24 GHz 1.11–1.67 cm

Ka 24–40 GHz 0.75–1.11 cmmm 40–300 GHz 7.5 mm – 1 mm

2.2 Radar Frequency bands

2.3 Define of pulse related termsPulse width:The duration for which transmitter is on is called Pulse-width.

Pulse Repetition frequency (PRF):Rate of transmission of consecutive pulses or number of consecutive pulses transmitted per second is called Pulse Repetition frequency.

Pulse Repetition interval (PRI):Time duration between onset of two successive pulses is called pulse repetition interval.Duty cycle:It is the ratio of pulse width to the pulse repetition interval.

2.3.1 Peak power , Average power

Peak power:The power produced by the transmitter during pulse width is known as peak power of the transmitter.

Average power:Average power is the power transmitted over the one PRI. Its typical values are ranging from 50 to 300 watts.

2.4 Radar Range EquationRadar Range equation is an interesting tool for the Radar system design .

It is an important relation between range of the target & characteristic parameters of various components that forms Radar .

Radar Range equation is given by

4/1

min4max )4(

ec

t AGAp

Pr

2.4 Radar Range Equation parameters.

4/1

min4max )4(

ec

t AGAp

Pr

Where,

Pt = Transmitted power in watts.G = Antenna Gain Ae = Antenna effective apertureAc = Radar cross section.Pmin = minimum detectable signal

Factors affecting the Radar Range:

1. Transmitted power.

2. Antenna Gain

3. Antenna effective aperture

4. Radar cross section.

5. Minimum detectable signal

2.5 Duplexer

Duplexer

It alternately switches the antenna between the transmitter and receiver so that only one antenna need to be used.

This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver point.

2.5.1 Types of Display systemsRadar can be classified into different types depending on their application & requirement , they are:1. Based on Video Display system

a. Raw Video display systemsb. Synthetic video display systems

2. Based on operating conditions:a. Conventional CRTb. Direct View storage tube

3. Based on CRT displaya. A-scope d. D-scopeb. B-scope e. E-scopec. C-scope

4. Plan position Indicator display

The PPI display provides a 2-D "all round" display of the airspace around a radar site. The distance out from the center of the display indicates range, and the angle around the display is the azimuth to the target.

Plan Position Indicator

2.6.Types of Radar

1. Pulse Radar

2. Continues wave Radar

3. FMCW

4. MTI

5. Secondary Radar

2.6.1 Pulse Radar block diagram

Synchronizer Transmitter

Display Unit Receiver

PowerSupply

ANT.Duplexer

RF Out

Echo

InAntenna Control

1. Synchronizer: a. Coordinates the entire system b. Determines the timing of the transmitted pulse c. Includes timers, modulator and central control.

2. Transmitter: a. Generate the pulse (RF) at the proper frequency and amplify.

3. Antenna: A. Receives energy from the transmitter, radiates it in the form of a highly directional beam. B. Receives the echoes for pulse radars.

Explanations

4. Duplexer: a. Allows one antenna to be used to transmit and receive. b. Prevents transmitted RF energy from going directly to the receiver. c. Tells the antenna to radiate or receive.

5.Receiver: receives incoming echoes from antenna, detects and amplifies the signal, and sends them to the display.

6. Display: Displays the received video to the operator.

7. Power Supply: Provides power to all the components of the system.

Special purpose RADARS

Special purpose RADARS: RADARS which are used for special purpose such as to find Relative velocity of the targets is called Special purspose Radars.

Types of Special RADARS:1. CW Radar2. FMCW Radar3. MTI Radar

All the above RADARs are based on application of Doppler Effect.

Doppler effect

Statement:“The apparent frequency of electromagnetic waves depends on the relative radial motion of the source and observer . If the source and observer are moving away from each other , the apparent frequency will decrease. If they are moving towards each other, the apparent frequency will increase.”This is called Doppler effect.Doppler shift is given by

Fd= (2Vr)/λ

Doppler Frequency Shifts

Motion Away: Echo Frequency Decreases

Motion Towards:Echo Frequency Increases

Example of Doppler effect

The radars (Special purpose RADARs) works on the Doppler effect principle .

If the aircraft approaches radars and moves away from the radar, correspondingly frequency of reflected signal will increase or decrease than the actual transmitted frequency

i.e, speed of moving objects alters the EM frequency .

The difference in transmitted frequency and reflected frequency is called doppler shift frequency (fd)

2.6.2 Continuous Wave Radar

Employs continual RADAR transmission

Separate transmit and receive antennas

Relies on the “DOPPLER SHIFT”

Block Diagram

Discriminator AMP Mixer

CW RFOscillator

Indicator

OUT

IN

Transmitter Antenna

Antenna

Amplifier. Increases strength of signal before sending it to the indicator. Discriminator. a. Selects desired frequency bands for Doppler shifts, eliminates impossible signals. b. The unit will only allow certain frequency bands so won’t process stray signals.Indicator. Displays data. Displays velocity or the component directly inbound or directly outbound. Range is not measured

Fm Transmitter

Modulation

Mixer Amplifier Limiter Frequencycounter Indicator

Receiving antenna

Transmitting antenna

Reference signal

2.6.3 FMCW

2.6.3 FMCWCW radars have the disadvantage that they cannot measure distance, because it lacks the timing mark necessary to allow the system to time accurately the transmit and receive cycle and convert the measured round-trip-time into range.

In order to correct for this problem, phase or frequency shifting methods can be used. In the frequency shifting method, a signal that constantly changes in frequency around a fixed reference is used to detect stationary objects and to measure the rage

In the frequency shifting method, a signal that constantly changes in frequency around a fixed reference is used to detect stationary objects and to measure the rage.

In Frequency-Modulated Continuous Wave radars (FMCW), the frequency is generally changed in a linear fashion, so that there is an up-and-down or a saw tooth-like alternation in frequency.

If the frequency is continually changed with time, the frequency of the echo signal will differ from that transmitted and the difference Δf will be proportional to round trip time Δt and so the range R of the target too.

When a reflection is received, the frequencies can be examined, and by comparing the received echo with the actual step of transmitted frequency, you can calculate range

Advantages:1.There is no limit on the minimum range.2.Simple low power equipment can be used.3.Small antennas can be used to reduce the size of the equipment.

2.6.4 MOVING TARGET INDICATOR (MTI) RADAR

IntroductionIt is possible to remove clutter (echoes from stationary targets) from the radar display and show only the moving targets.MTI Radar is capable of measuring the range and radial velocity of moving targets even in the presence of strong clutter.

The range is measured on the basis of time lapse between the transmitted signal and recived echo.

Velocity of moving targets is measured on the basis of Doppler shift imparted to the transmitted signal.

Block diagram explanation1. Coho(Coherent Oscillator): It is a stable oscillator whose frequency is same as

intermediate frequency used in the receiver (Fc=Fi). In addition to providing the reference signal, the output of

the coho (Fc) is mixed with the local oscillator frequency (Fo) in the transmitter mixer to produce RF signal (Fs=Fc+Fo).

2. Stalo (Stable Local oscillator):The function of stalo is to provide necessary frequency translation in the transmitter as well as receiver.The transmitter mixer converts IF to RF (Fs=Fc+Fo).In the Receiver mixer, RF echo Fs is heterodyned with the stalo signal Fo to produce the IF signal.

Block diagram explanation3. Power Amplifier:

Power amplifier such as Klystron are most commonly used to amplify the RF signal produced by the transmitter mixer. The waveform amplified by the power amplifier travels to the antenna through duplexer via a transmission line, where it is radiated into free space.

4. Modulator:The power amplifier is “pulsed” (turned on or off ) by the modulator to generator a repetitive train of pulses.

5. Duplexer:A single antenna is generally used for both transmitting and receiving. The function of the duplexer is to protect the receiver from damage caused by the high power from transmitter.

6. Phase Detector:The inputs to the phase detector are Fc and Fi. Its output is a voltage proportional to the phase difference between the two input signals. Since the output of this detector is phase sensitive, the phase difference between the transmitted and recived signals will be constant for fixed targets.But it will vary for moving targets due to Doppler shift. The output of the phase detector is used for producing the video signal.

7. Video: The output of phase detector and delay line are amplified to suitable level

by Amplifier 1 and Amplifier 2 respectively before applying to subtractor. Subtractor compares received echo with that received during the previous

scan. For stationary targeget result will be zero hence no display.

For moving targets it produces butterfly effect on the A scope indicator.

If the target happens to have a velocity whose radial component results in a phase difference of exactly 2π rad between successive pulses,the target thus appears stationary. Echoes from it are canceled by the MTI action. A radial velocity corresponding to this situation is known as a blind speed.This can be mathematically expressed as

Vb=(PRF * nλ)/2Where Vb= Blind speed

λ= Wavelength of transmitted signaln= any integer including 0

Blind Speeds

2.6.5 Secondary radar

The RADARS discussed in previous sections, Pulsed Radar, CW Radar ,FMCW radar and MTI Radars are classified as Primary Radars.these RADARs are used for detection, ranging or velocity measurements. Transmitter and receiver frequencies are the same because Radar operation depends on echo.

Secondary radars are also called as Secondary Surveillance Radar(SSR).

It is used for identification . It differs from the primary radar in the sense that it does not make use of reflection of the transmitted energy.

A typical IFF system of SSR is as shown below,

IFF SYSTEM

IFF system explanation

IFF stands for Identify Friend or Foe

In a SSR system, pulse transmission from ground is received at the target such as an aircraft.

It is detected and decoded in the aircraft transponder (beacon) shown in figure b.

The aircraft transponder then transmits coded pulses back to ground after a certain known delay. Thus identify of the aircraft can be established on ground.

Applications

Used in air traffic control (ATC)

Used for Military purposes.

To track aircraft

2.7 Different landing systems ILS & GCAIntroduction:While landing, the aircraft has to take a path in line with the runway. when the visibility is good, landing operation is carried out by visual observation of ground and landing lights.when the visibility is poor, help of Landing instruments is taken.

Landing instruments provide information about horizontal and vertical deviation of the aircraft. Two types of landing aids are commonly used:

1. Instrument Landing system (ILS)2. Ground Controlled approach (GCA)

It is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway,

Instrument landing system (ILS)

Typical ILS system

ILS ExplanationsILS comprises of the following 3 units:1.Localizer:

It gives a vertical equisignal plane which passes through centre line of the runway.

It is located at about 300 meters from the end of runway.

It operates in the VHF band (108-112 MHz). When the aircraft is flying along the correct path, the

meter remains in the centre. If the aircraft deviates , the indicator bar on the panel

meter shifts in direction accordingly so that corrective action can be taken.

2. Glide slope: It gives an equisignal plane inclined to the horizontal at

the desired angle of descent which generally lies between 2 degree to 5 degree.

This system operates at about 330 MHz.

Transmitter of glide slope is located on one side of the runway near the touch down point at a sufficient distance.

The intersection of two equisignal paths provided by the localizer and the glide scope gives the approach path.

3. Marker Beacons: Are used to indicate the position of the aircraft along the

localizer path from the touch down point. Three beacons operating at 75 MHz are used for each

runway. Outer marker is located at about 8-10km from the touch

down point of runway. Middle marker is located at about 1km from touch downpoint. Inner marker is placed at about 60 meters from touch downpoint.

In the aircraft , a single receiver is used to receive the marker signals and the output is available as an audio tone as well as three lamps with colour code.

Ground-controlled approach

Ground-controlled approach is the oldest air traffic technique to fully implement radar to service a plane - it was largely used during the Berlin airlift in 1948-49.

This approach is runway navigational aid for the aircraft landing in the low or zero visibility and ceiling conditions.

It consists a ground based radar system, having two separate radars; one for searching aircraft & other for tracking it. 

The two separate radars are a surveillance radar element (SRE) and other precision approach radar (PAR).

SRE: to locate and identify the approaching aircraft before it is acquired by the PAR for tracking.PAR: it keeps track of aircrafts parameters like azimuth, elevation and range.

1. An aircraft to be landed with this system is first brought into the proper position for starting its descent by means of surveillance radar.

2. A controller at the indicators of PAR set then takes over.

3. Instruction to pilot must be done through glide path 4. If for any reason, the aircraft cannot be talked through

proper glide path ,it is instructed to discontinue the landing and turn back for a second attempt.

Procedure of aircraft landing:

2.8 Introduction to SONAR

Sound Navigation And Ranging (SONAR)– Sound pulses emitted reflected off metal objects with

characteristic ping– Like Radar and Lidar time of flight is measured to

determine distance– Early sonar gave only distance and direction to target– Modern sonar used for mapping

Principles of SONAR Sound waves are a mechanical vibration

Can only travel through an elastic medium (air, water, earth)

Measured by frequency instead of wavelength (λ)How is frequency related to λ?

Types of SONAR

1.Single beam SONAR

2.Multi beam SONAR

Sonar

Problems:

1. Calculate the dutycycle of radar if the pulse width is 1µs and TRF is 500pps.

Solution:Given data Pulse width = 1µs ; TRF= 500 ppsWe know that,1.PRI=1/(TRF);

therefore substituting we getPRI=1/(500)= 2ms

2. Duty cycle =(Pulse width )/ (PRI) therefore substituting we get

Duty cycle =(1* 10^-6 )/ (2*10^-3) = 0.5Result : Duty cycle for a given RADAR is 500µ

Problems:

2. Calculate the dutycycle of radar if the pulse width is 20µs and TRF is 1000pps.

Procedure is same as 1st Problem

Result : Duty cycle for a given RADAR is 20m

Problems:

3. Calculate the Maximum range of a radar system which operates at 3cm with a peak pulse power of 500kW, if its minimum receivable power is 10^-13 W, the capture area of its antenna is 5meter square and the cross-sectional area of the target is 20meter square.

Solution: List out given data Substitute in Radar range equation formula

Result: Maximum range of Radar is found to be 685.68 Km

Reference Books:

1. Introduction to Radar Systems - M.I. Skolnik

2. Radar, Principles,Technology, applications – BYRON

EDGE

3. Electronic communication systems – George Kennedy

4. Advanced communication system- Veeranna