Table of Contents
RADAR stands for radio detection and ranging or radio direction ranging. Radar used radio frequency for object detection and with the help of the radar we can detect the target unknown object velocity, range and angle. The radio frequency in the radar system is generated through magnetron and can be transmitted through antenna. The duplex is used for the isolation of the receiving and transmitting antenna. The transmitting antenna will have high power and receiving antenna have low power. Therefore the isolation of both antennas is very necessary. Single antenna is used for both transmitting and receiving signal.
History of the Radar:
Radar was invented by Henrich Hertz in 1880-1885. RADAR as a ship detector was used first by Huelsmeyer in 1904. Concept of side looking airborne Radar came in 1940’s but with sophisticated techniques in 1950’s. Civilian applications in the geoscience started since 1960. Imaging radar satellites started in the 1970’s.
Principal of the radar system:
In radar system we transmit some electromagnetic wave from the radar and it goes to the target or any object it intercept with the target and it reflects back in all the direction the electromagnetic wave which is coming back to the radar is of our interest. It means that the wave has been travelled to wave that is from the radar station to the target and from the target to the radar that means it has been travels about twice of our if the distance between the target and the radar is up and the time taken by the wave to travel from the source to radar. So from radar to the object and object to the radar is given by distance upon velocity. The velocity we will consider is nothing but the velocity of light because we are talking about the free space. So in this way we can calculate the overall time and on the basis of that time duration we can have the range of the target. it not only tells the range of the target it also tells the object is present or not. so in the radar system we have the two advantages first the we can have the information about the object or the target and also the range of the target that’s why it’s name is radio detection that it detects the object or the target and it also gives the range of the target that is radio detection and ranging and it can work in all type of weather cast weathers like it can be working the rainy season or the if there is a very much fog there then then also this system can be used to detect the target.
Radar gives information about object or target. Radar gives information about the object or target by sending electromagnetic wave by sending electromagnetic wave to them and observing the Eco signal or the return signal. So the wave which is reflected by the target is known as the Eco signal which is coming back to the radar system. So to get the information about the target or to get the range of the target we transmit electromagnetic wave and that is a tentative intercept with the target and some part will be radiated by the target and received at the radar station that is known as eco signal and reradiated signal.
So we have studied the electromagnetic wave or the plane wave uniform plane wave and we have seen that whenever there is a change in the characteristics of the medium there will be some part of the incident wave that is absorbed by the medium and some part is reflected by the medium. So here also it will happen because we are transmitting the electromagnetic wave and we don’t change the conductivity, permeability and permittivity of the medium. Some part of incident wave is absorbed by the medium and some part will be radiated or reflected by the medium and this reflected wave is of our interest and by observing this re-radiated signal we can get the information about the target. So this re-radiated energy gives information about the location of target and its range.
Range of the Radar:
Range is distance between radar and target which is determined by the time taken for the signal to travel from radar to target and back to target.
Radar equation relates the range of the radar to the characteristics of transmitter, receiver, target, antenna and environment. Theoretically we can say that the radar equation is useful to know the range of the target. So let us consider a radar system where:
Pt= Power transmitted by the radar
Pr= Power received by the radar
Pdt= Power density of isotropic antenna
Pde= Power density of echo signal at Rx of radar
Ϭ = cross section area of EM wave
Ae= Aperture of the receiving antenna effective aperture
Power density at a distance R due to Pt is given by:
Pdt= Pt/(4πR2 )
Generally radars use the directional antennas which are given by:
Pdd= Pt/(4πR2 ) × G
Where “G” is the power received by antenna in a specific direction.
Power density of the echo signal at radar is given by:
Pde= Pdd (ϭ/(4πR2 ))
Pde = Pt/(4πR2 ) × G × (ϭ/(4πR2 ))
Pde = (Pt Gϭ)/(4π)2 R4
Now amount of the power received by the radar depends upon on the affective aperture Ae of the antenna.
Pr= Pde Ae
Pr= (Pt Gϭ)/(4π)2 R4 Ae
R4 = (Pt Gϭ)/((4π)2 Pr ) Ae
As R represents range and if want to maximize the range. If want the range detect the object then we will follow the condition which states that the intensity of the radar is of minimum power which can reach the object. The echo signal is known by Pr.
Rmax= [(Pt Gϭ Ae)/((4π)2 Smin )](1/4)
For parabolic antenna:
As we know that the gain of the parabolic antenna is given by:
G= (4π Ae)/λ2
Now putting the value of the gain in the standard radar range equation
Significance of the radar equation:
- Radar range is directly proportional to the transmitting power , antenna gain G, cross section area and antenna aperture.
- It is inversely proportional to the wavelength
- Range is directly proportional to the frequency
Radar performance factors:
From radar range equation
Rmax= [(Pt ϭ 〖Ae〗2)/(4π λ2 Smin )](1/4)
From above equation for maximum range of the radar system following factors are responsible:
Transmitting power (Pt ):
From equation we have:
If Pt increases then Tmax will increases and vice versa.
Gain of the transmitting antenna:
From the equation we know that
So from this equation we conclude that if we increase the gain the range of the transmitter will increase.
Cross section area:
So the range of the radar will increase if we increase the area.
Power of minimum detectable signal (Smin):
Types of Radar system:
In bi-static radar we have two antennas in which receiver and transmitter have separate antennas. The transmitter transmits the radio waves by the help of transmitting antenna. In bi-static radar we have a transmitter and this transmitter transmits a high power with the help of this transmitting antenna. When this electromagnetic wave intercept with this target some part of it is absorbed and some part of it will be reflected and received by the receiving antenna. So bi-static radar means we are having the separate antenna for transmitter and receiver or the transmission and receive reception because we are transmitting high-power by the help of this antenna transmitting antenna and after transmitting this we are having the re-radiated signal which is known as echo signal. This echo signal is received by the receiving antenna and it is processed in the receiver. The receiver will then display the behaviour of the signal in the indicator and we will understand the speed, velocity and range of the signal these factor of the target will be known. There is no need of duplexer because duplexer is need when we are using single antenna for transmitting and receiving. It is low cast system but needs more space.
Mono static Radar:
In mono static radar system we use a single antenna for the transmission and reception purpose and how it is possible so it is possible with the help of a device called duplexer. Transmitter has power range of 106 watt and the receiver have power range of 10-6 watt . So first of all we will have a transmitter at which we will be generating a high power. When the radar transmits signal the duplexer will connect the transmitter and when receiving signal the duplexer will connect the receiver. The duplexer acts as a switch that allows bidirectional communication over a single path. The system is complex and costly. We have generated a high power in the transmitter and after generating this high power we have to transmit it. So for this transmission of high power we are using a duplexer and this duplexer is connected to an antenna which is used for transmitting an electromagnetic wave to the target. So we have transmitted this electromagnetic wave with the help of this antenna and this electromagnetic wave travels through the free space and go into the target and in the target. It is some part of it is received and some part of it is reradiating and this Re-radiated signal which is known as echo signal this echo signal is again received by the antenna and after this receiving. We pass it through the duplexer and then it goes into the receiver. So in this way with the help of this duplexer only a single antenna can be used for the transmission and reception purpose. So whenever we use the power in the radar system so the transmitted power is very high. So during the high power reception it isolates the receiver. So that whatever you want to transmit it is directly connected to the antenna and the reception is now disconnected from the circuit so this is a function of duplexer. In radar duplexer isolates the receiver from the transmitter while permitting them to share common antenna. The power level in a radar system can be very large at the transmitter and very small at the receiver.
Uses of Radars:
Radar can be used to detect aircraft, ships, space crafts, guided missiles, motor vehicles, weather formation and terrain. Radar was developed secretly for military use by several nations in the period before and during World War 2.
Advantage of Radar system:
- Radar can see through condition such as darkness, haze, fog, rain and snow which is not possible for human eye.
- It can measures:
- Range of target
- Location of target
- Velocity of target
- Angular position of target
- Radar signal can penetrate insulators
- Radar can tell about the difference between the moving and stationary target
- Low power required to radiate the signal and thus reduce the cost
Disadvantage of the Radar system:
- It cannot recognize the colour of target
- It cannot tell the internal aspect of target
- It cannot resolve the target which are placed in the deep sea
- It cannot resolve targets which are placed behind some conducting sheets
- Radar cannot resolve in details like human eye mainly at short distance
Application of Radar:
- Radars altimeters used to calculate the height of the planes above the ground
- Radar blind landers for helping aircrafts to land under a poor visibility
- Police radars for measuring the speed of moving vehicles
- Radars are used for weather forecasting basically Doppler radar used in weather forecasting measures the speed of object such as drops of precipitation.
- Airborne radar for satellite surveillance
- It is used for the detection and ranging of enemy targets even at night
- Bombing ships, air crafts or cities even during night
- Directing guided missiles
- Aiming guns at aircrafts and ships
- Early warning regarding approaching aircrafts or ships