On completion of this module, you will be able to:

• To become familiar with basics of Radar
• Explain the Blind speed and Doppler effect in Radar
• Understand the operation of CW Radar
• Understand the working and application of MTI Radar
• Explain Automatic target detection Radar

Introduction to Radar

• RADAR is an abbreviation for Radio Detection And Ranging.
• Radar is an electromagnetic system for the detection and location of objects. Detection refers to whether the target is present or not, the target can be stationary or moving.
• Ranging refers to whether the distance between the Radar and the target.
• A system used for detecting and locating the presence of objects like ships, vehicles, aircraft etc. by radiating electromagnetic signal in space is known as the Radar system.
• Basically, radar is used to collect the information related to the object or target like its range and location by radiating electromagnetic energy and examining the echo received from the distant object.
• Radar was invented for military purpose before world war - II in order to secretly detect the presence of unknown objects and it has given birth to microwave technologies.
• Radar can see through condition such as:darkness, haze, fog, rain, and snow which is not possible for human vision.

Basic Principle of Radar

• 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.
• Radar mainly consists of a transmitter, receiver, antenna and display devices.
• It uses the same Antenna for both transmitting and receiving the signals.
• Radar is used for detecting the objects and finding their location.



• The distance between Radar and target is called Range of the target, R.
• Let the time taken for the signal to travel from radar to target and back to radar be T.
• The distance between the radar and the target is R.
• The two way distance between the radar and the target will be 2R.
2R = C T_R
C = (C T)/2
Where C = Speed of light.
• The duration between the two clock pulses must be selected in such a way that the echo signal corresponding to present clock pulse should be received before the next clock pulse.


• The number of radar pulses transmitted per sec is known as Pulse Repetition Frequency(PRF)
• The time interval between the successive clock pulses is called Pulse Repetition Time(PRT)

Types of Radar

• Basically there are two major radar systems first one is Monostatic Radar System and second one is Bistatic Radar System
• Monostatic Radar System: A monostatic radar system uses a single antenna for transmission as well as reception purpose.


• Bistatic Radar System: A bistatic radar system utilizes independent antennas for transmission and reception of the signal.

The Basic Terminology used for Radar

• Range:
• The range of the target is observed bu measuring the time(T_R) takes for the radar signal to travel to the target and return back to the radar
• Thus the time for the signal to travel to the target logated at range(R) and return back to the radar is 2R/C. The range of the target can be given as:
  R = (CT_R)/2
• With the range in Kilometers or in nautical miles and T in microseconds.
• Maximum Unambiguous Range:
• Once the radar transmitter transmits a pulse, sufficient time should be allocated so that the echo signal due to this pulse may be received and detected before the next pulse is transmitted.
• If the PRF is too high echoes may arrive after the transmission of next pulse. such echoes are called "second time around echoes"
R_un = (CT_P)/2 = (C/2F_P)
Duty Cycle = τ /T_P
• Pulse Repetition Frequency(PRF):
• The rate at which the pulse are transmitted towards the target from the radar is called as the pulse repetation frequency.
  F_P = 1 / T_P
• Pulse Repetition Period:
• The time interval at which the pulses are periodically transmitted towards the target from the radar is called as the pulse repetition period, T_P is given in terms of PRF.
  T_P = 1 / f_P
• Duty Cycle:
• The duty cycle of the radar waveform is described as the ratio of the total time the radar is radiating to the total time it could have radiated.
  Duty Cycle = P_av /P_T
  Duty Cycle = τ /T_P = τ f_P
• Where τ is pulse width of the transmitted pulse and T_P is the pulse repetition period
• The Doppler Effect:
• If the target is not stationary, then there will be change in the frequency of the signal that is transmitted from the Radar and that is received by the radar, this effect is known as the Doppler effect.
• So the shift in frequency of the received echo signal from a moving target compared to the transmitted frequency of a radar station is called Doppler frequency shift and phenomena is called Doppler Effect
• The Doppler Effect refers to the change in wave frequency during the relative motion between a wave source and its observer
• According to Doppler Effect, the frequency of the received signal will increase if the target is moving towards the direction of radar. Similarly the frequency of the the received signal will decrease if the target is moving away from the radar.
• Duplexer:
• Duplexer is the device that allows a single antenna to serve both transmitter and receiver. It switch the antenna for transmitter or receiver.
• During transmission it protects the receiver from burnout or damage and on reception it channels the echo signal to the receiver.

Radar Range Equation

• Radar range equation is used for determining the maximum range at which a particular radar can detect a target with their performance parameter.
• It relates the range of a radar to the characteristics of the transmitter, receiver, antenna, target and environment.
• If the transmitted power P_t is radiated by an isotropic antenna, then the power density P_d at range R is given by.
  P_d = P_t /(4πR²)
• Above equation is valid for lossless isotropic antenna.
• The maximum gain of the antenna can be defined as
  G = (Max. Power Density Radiated by a given Antenna)/ (Power Density Radiated by a isotropic antenna)
• Radar generaly uses directional antenna, so power density P_dd at range R from a directive antenna whose gain is G is given by
  P_dd = P_t.G /(4πR² )
• The target receives a portion of the incident energy and reflected it in various direction.The reflected power from the target through its cross section can be given as
  Reflected Power from the target towards the radar = (P_tG /4πR²).( σ/4πR²)
• The radar antenna receives a portion of the reflected power from the target cross section. It depends on the effective aperture A_e of the received antenna and it can be given as:
  P_r = (P_tG /4πR²).( σ/4πR²).A_e
  A_e = σ . A
• Where, σ is the radar cross section of the target, A_e is the effective area of the receiving antenna, A is the physical antenna area and σ _a is the antenna aperture efficiency.
• The maximum range of the radar can be defined as the maximum distance beyound which radar cannot detect the target. So the recieved signal power can be given as the minimum detectable signal.
  S_min = (P_tG /4πR²).( σ/4πR²).A_e
  R_max = [(P_tG /4π).( σ/4π).A_e/S_min]^1/4

Moving Target Indication (MTI) Radar

• The radars discussed till now were required to detect targets in the presence of noise. But in practical radar have to deal with more than receiver noise when detecting targets they can also receives echoes from the natural environment such as land, sea and weather. These echoes are called cluter, since they tend to cluter the radar display with unwanted informations. Cluter echoes signal has greater magnitude then echo signal receives from the aircraft.
• When an aircraft echo and a cluter echo appear in the same radar resolution cell, the aircraft might not be detected. But the doppler effect permits the radar to distinguish moving targets in the presence of fixed target even the echo signal from fixed targets has comparatively greater magnitude than the moving target such as aircraft.
• MTI radar is to reject signals from fixed or slow-moving unwanted targets, such as buildings, hills, trees, sea, and rain, and retain for detection or display signals from moving targets such as aircraft.
• Echo signal from fixed target are not shifted in frequency, but the echo from a target moving with a relative velocity V_r will be shifted in frequency by an given Doppler formula
f_d = (2V_r) / λ
Where, λ is wave lengtth of the transmitted signal.
• The MTI radar has Pulse Repition Frequency (PRF) low enough to not have any range ambiguities R_amg = C/(f_r) (such as multiple around echoes. It does however have many ambiguities in the Dopler domain such as blind speed
• There is a type of MTI which does use the informations directly from the doppler but it observing the area map scan to scan. Only moving objects which are changes their position scan to scan from mapping operations are displayed. This type of MTI is known as Area MTI.
• The figure given below shows the 5 succesive return on MTI radar. This sweep shown several fixed targets and a moving target. on the basis of single sweep, moving targets cannot be distinguished from fixed targets. Echoes from fixed targets remain constant throughout but echoes from moving targets varies sweep to sweep at a rate corresponding to the doppler frequency.


• The block diagram of MTI Radar that uses a Power Amplifier as the Transmitter is shown below

• The local oscillator of an MTI radar's superheterodyne receiver must be more stable than the local oscillator for a radar that does not employ doppler. If the phase of the local oscillator were to change significantly between pulses an uncancelled cluter residue can result at the output of the delay line canceler which might be mistaken for a moving target even though only cluter were present. to recognize the need for high stability, the local oscillator of an MTI receiver is called the Stalo, which stands for stable local oscillator. the IF stage is designed as a matched filter as is usually the case in radar. Instead of an amplitude detector, there is a phase detector following the IF stage. this is a mixer like device that combines the received signal (at If) and the reference signal from the coho so as to produce the difference between the received signal and the reference signal frequencies. this difierence is the doppler frequency.
• The name Coho stands for coherent oscillator to signify that it is the reference signal that the phase of the transmitter signal Coherency with the transmitted signal is obtained by using the sum of the Coho and the stalo signals as the input signal to the power amplifier. thus the transmitter frequency is the sum of the stalo frequency f_l and the Coho frequency f_c.The combination of the stalo and coho sometimes is called the receiver exciter partion of the MTI radar. Using the receiver stalo and coho to also generate the transimtter signalinsures better stability than if the function were performed with two different sets of oscillator.
• The output of the phase detector is the input to the delay line canceler. the delay line canceler acts as a high pass filter to separate the doppler-shifted echo signals of moving targets from unwanted echoes of stationary cluter. The doppler filter might be a single delay line canceler, but it is more likely to be one of several other more elaborate filters with greater capability.
• The power amplifier is a good transmitter for MTI radar since it can have high stability and is capable of high power. The pulse modulator turns the amplifier on and off to generate the radar pulses. the Klystron and Travelling wve tube have usually been preferred type of Vaccum-tube amplifier for MTI radar. The crossed-field amplifier has also been used but it is generally less stable than other devices. Hence it might not be capable of canceling of large cluter echoes.
• MTI Radar can be classified into two types based on the type of transmitter that has been used (1.) MTI Radar with Power Amplifier Transmitter
  (2.) MTI Radar with Power Oscillator Transmitter

Applications of Radar

• Military Appications: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 Control(ATC):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.
• Aircraft Safety
• 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.
• Navigation
• Space
• 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.