Introduction To Radar - Basic Electrical Engineering

Introduction

1.         Radar is basically a means of gathering information about distant object or targets by streaming electromagnetic waves at them and analyzing their echoes (reflection). It was evolved during the years just before World War II independently and more or less simultaneously in Great Britain, the United States, Germany and France. At first it was used as an all weather method of detecting an approaching aircraft and for detecting anti-aircraft weapons and later for many other purposes. The words ‘RADAR’ is an acronym coined from the words Radio Detection And Ranging (means target is detected with the help of radio waves and also it measures distance). It was radar that gave birth of microwave technology. It was seen that the higher frequencies gave the most accurate results.


2.         Basic Types of Radar.      There are mainly two types of radar systems, Primary and secondary radar. The Primary radar is an independent radar works on echo principle. In this the transmitter transmit EM energy in the space. If there is any object in the direction of the energy transmitted the object simply reflects the incident RF energy. The reflected energy known as echo pulse is being received by the highly sensitive radar receivers and processed. In the case of secondary radar object is caused automatically re-transmit the radio wave on the same frequency or some other frequency. Further the radar system is classified into two main category i.e., Pulse Radar and Continuous Wave radar (CW Radar).


Primary Radar

3.         Search Radar.         These are mainly primary pulse radar sets. These sets need not to have exceptional accuracy or resolution but are expected to detect enemy aircrafts at the greatest possible range to present a picture of the naval or air situation on which tactical decisions can be made. These sets have high power output and large antennae, which are rotated 360° for search purposes.  (For example Ground Radars i.e. THD – 1955, TRS, P-40, etc.)

4.         Tracking (Fire Control) Radar.    These are also primary pulse radars, but these sets emphasis on the accuracy of range and angle information. These sets operate on high frequencies and have short pulse with the receivers having high bandwidth. In these sets a single object is tracked. The tracking is either manual or automatic. In auto tracking mode the error signal is derived from the echo pulse which actuates servo-mechanism that causes the antenna and range gate to follow the object co-ordinates faithfully for the purpose of aiming anti-aircraft missiles and guns. Airborne fire control radars are for interception and automatic searching and tracking of targets and launching air-to-air missiles. For Example Missile Guidance Radar,R2L (MiG), etc.

Secondary Radars

5.         This refers to radars, which works along with the primary radar i.e., IFF, Beacons etc. Secondary radars are extensively used in navigational aids i.e., Homing Equipment and IFF Equipment (Identifying Friend or Foe).

6.         Pulse Radar.            The essential function of radar set is to fix the co-ordinate of a material object in space with reference to the radar set.  The range, azimuth height and speed of the target can be determined with this.  Position of an object can be completely determined if the radar slant range ‘r’, bearing and the angle of elevation from the radar set are accurately measured. For this purpose the radar set sends a narrow beam of EM energy in the form of short bursts of pulses. The target, which is illuminated by radio wave, reflects some of these radio waves in all possible directions.  A small portion of this reflected energy called Echo Pulse, is being received and processed by highly sensitive receivers located at the radar sets.  If the time interval between the instances when the RF energy leaves the transmitter and the echo pulse arrival at the receiver is measured accurately the range ‘r’ of the target can be computed from the formula:-


Where C is velocity of the EM energy in free space, which is equal to 1,86,000 miles per second or 3 X 108 metre per second.

Angular / Rotation Information
Fig 1 Block Diagram of Pulse Radar

7.         The components of the pulse radar and their function in brief are in the following paragraphs:-

(a)         Timer.               This unit provides sharp trigger or timing pulses of fixed frequency for transmitter and indicators.
(b)         Pulse Modulator.    Once trigger pulse is received from the timer the pulse modulator produces sharp rectangular pulses of HT voltage for fixed duration to bring the transmitter(Tx) Oscillator into conduction.
(c)          Transmitter. This may is a high power oscillator like magnetrons. This transmitter produces RF energy during the pulse period. The RF energy is fed to antenna via duplexer.
(d)         Duplexer.      Normally a single antenna is used in radar sets for transmitting and receiving. Duplexer is nothing but a electronic switch, which consists of TR and ATR. During the transmission the TR prevents RF energy entering in to receiver and during reception the ATR switch takes care to see that whole RF energy in the echo pulse is directed into the receiver.

(e)         Antenna.       It is a mechanical device, which acts as the transducer between free space and trans-receiver equipment.  The RF energy is concentrated into a narrow beam by a highly directional antenna. It is rotated 360° for search purpose. The rotation of antenna is synchronised with the rotation of Time base in indicators.

(f)           Receiver.      A highly sensitive Super Heterodyne receiver is used in the radar sets. The first stage may be a low noise RF amplifier. Generally RF amplifier is avoided. Therefore the first stage of radar receiver is a mixer stage.

(g)         IF Amplifier.  These may have a centre frequency of     30 M Hz or 60 M Hz and a bandwidth of 1 M Hz. The purpose of this unit is to amplify the received signal at IF and give its output to the next stage i.e., detector.

(h)         Indicator.      This unit combines information received from three different units of radar sets. They are :-

(i)          Timing information received from timer for synchronising start of time base with start of transmission for measuring accurate range.

(ii)         Rotation information received from antenna.

(iii)        And also the actual video from the detector and video amplifiers.

Study on Pulse Diagram
a –  Pulse Width   b – Pulse Repetition Period   c – Peak Power  d – Average Power

Fig 2 Wave Form of Pulse Radar

8.         Pulse Width (PW).    It is the duration “T” for which the radar Tx sends the RF Energy into the space. It is represented in microseconds.

9.         Pulse Repetition Frequency (PRF).       The number of pulses transmitted by the transmitter in one second is called PRF or the pulse repetition frequency. It is represented in Hz and it is normally between 200 to 10000 Hz.

10.       Pulse Repetition Period (PRP).     It is the time interval between the start of one pulse to the start of next pulse. In other words, it is the inverse of PRF i.e., 1/PRF will give you PRP.

11.         Peak Power (PP).     The instantaneous power of the transmitter during the pulse is defined as the peak power.

12.       Average Power (AP).         The amount of power transmitted in one second is called average power.

            P Av                =          PP x PW X PRF
=          PP x Duty cycle

13.       Duty Cycle.     It is the product of PRF and pulse width. It gives the total time the transmitter was on in one second.

            Duty Cycle    =          PW x PRF

14.       Peace Time / War Time Applications of Radar.          Radar has been employed on the ground, in the air, on the sea and in the space. Ground based radars are applied chiefly to the detection and tracking of a/c or space targets. Ship borne radars are used for navigation aid and safety device to locate buoys shorelines and other ships and also observing the aircrafts. Airborne radar may be used to detect targets and ships or land targets. Space radar has assisted in the guidance of spacecraft and remote sensing of the land and sea. Some of the important applications of radar are described in brief in the following paragraphs:

(a)          Air Traffic Control.   Radar is employed throughout the world for the purpose of safety controlling of aircraft en route and in the vicinity of airport.  Radar has been used in GCA (Ground Controlled Approach) to guide the a/c for safe landing in bad weather conditions and in nighttime.

(b)      Aircraft Navigation.  The weather avoidance radar used in the a/c to outline the regions of charged clouds.  Airborne radars are also used for terrain avoidance and terrain following.  Ground mapping radars with moderately high resolution are used for a/c navigation purposes.

(c)       Ship Safety.    Radars used in ships for enhancing the safety of ship travel by warnings of potential coalition with the others ships and for detecting navigation buoys especially in poor visibility. Shore based radars is also used for surveillance of harbors as an aid to navigation.

(d)      Space.    Space vehicles have used radars for docking and for landing on the moon.  Some of the largest ground based radars have the capability of detecting and tracking of satellites. Satellite borne radars have also been used for remote sensing as mentioned below.
(e)      Remote Sensing.      Remote sensing radars are concerned with earth resources, which include the measurement and mapping of sea conditions water resources, ice cover, agriculture & forestry conditions, geological formation and environmental pollution.
(f)        Law Enforcement.     Radars used to measure the speed of automobile traffic on highways by Transport Dept and Traffic Police.
(g)       Military.          Military applications are detection, identification and interception of airborne targets Air Traffic Control, Navigation, tracking, weaponry guidance, AWACS, UAV guidance etc.
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