We all are fans of cricket. We always talk about Bret lee’s tremendous fast bowling. But have you ever wondered how they calculated his bowling speed with in seconds of time?
Suppose we are travelling at a speed of 50kmph on a road where the speed limit is 45kmph. Though the speed variation between 45 and 50kmph cannot be found by human senses that much accurately, traffic police will still catch our tail. Have you ever wondered how he found out we are over speeding?
Answer to both the questions above is given by simple Continuous wave radar or simply a CW-radar.
What is a radar and how does it work?
Radar is something that is in use all around us. Air traffic control uses radar to track planes both on the ground and in the air, and also to guide planes in for smooth landings. Police use radar to detect the speed of passing motorists. NASA uses radar to map the Earth and other planets, to track satellites and space debris and to help with things like docking and maneuvering. The military uses it to detect the enemy and to guide weapons.
A radar consists of a transmitter which releases a wave respective to its type. Say we consider a pulse radar. Here, the transmitted waves are pulses with an amplitude of kilo volts. These pulses are nothing but high frequency sine waves. Means, a series of high frequency sine wave forms a pulse with low frequency. This pulse is transmitted by an antenna and hits the target. After hitting, the pulse gets scattered and a small part of that scattered pulse receives radar. If the target is in motion, then that received part of scattered pulse shows a variation in frequency from its original transmitted signal due to Doppler Effect. Upon finding the difference between frequencies of transmitted and received pulses, the distance between target and radar is found.
Actually, the change in frequency is not that of pulse. But, the sine wave which forms a pulse changes its frequency upon colliding with target. This change in frequency of sine wave obviously results in change of frequency of pulse wave.
What is Doppler Effect?
Consider two objects Source and Observer, source producing a continuous sound like a horn. When source or observer starts moving towards or away from each other, the sound becomes more audible to observer when he is coming near to source and the sound becomes less when he moves away from the source. The change in sound due to the movement of source or observer or both is called as Doppler shift and the phenomenon is called as Doppler Effect.
Here as the police van moves away from the girl, the distance between girl and car increases and the wavelength increases thus resulting in decrease of frequency of sound wave. As frequency decreases, the sound becomes less audible than the previous second. Hence it becomes less audible for us to listen to a sound that is going away from us.
And to the boy, the wavelength decreases as car approaches him and the resultant frequency increases, there by increasing the sound level of the car to his ear.
Continuous wave radar:
The name itself gives us the idea that continuous wave is transmitted instead of pulse which is explained above. Implies Continuous-wave radar is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. It uses the same Doppler Effect, which shows the difference between a moving target and stationary object.
As opposed to pulsed radar systems, continuous wave (CW) radar systems emit electromagnetic radiation at all times. Conventional CW radar cannot measure range because there is no basis for the measurement of the time delay. Recall that the basic radar system created pulses and used the time interval between transmission and reception to determine the target’s range. If the energy is transmitted continuously then this will not be possible.
CW radar can measure the instantaneous rate-of-change in the target’s range. This is accomplished by a direct measurement of the Doppler shift of the returned signal. The Doppler shift is a change in the frequency of the electromagnetic wave caused by motion of the transmitter, target or both. For example, if the transmitter is moving, the wavelength is reduced by a fraction proportional to the speed it is moving in the direction of propagation. Since the speed of propagation is a constant, the frequency must increase as the wavelength shortens. The net result is an upwards shift in the transmitted frequency, called the Doppler shift.
To explain CW radar more perfectly, we can use a block diagram of it
The transmitter transmits the sine continuous wave from the transmitter and through the antenna. The transmission is continuous, that means there is no point where the radar is off. The transmitted signal will have an RMS power of kilo watts. This amount of energy is required for the waves to travel a particular large amount of distance. This wave gets scattered in the space and by the time it reaches target, we can say 80% of the transmitted energy is lost upon scattering. When the wave hits the target, it gets scattered back again and only a 0.1% of the transmitted wave energy only returns back to the radar. If we say a 1KW wave is sent, we receive only a 1W wave. This means to sense that small amount of wave, the receiver section have to be sensitive enough. If the receiver section is that much sensitive, then as the transmitted energy pass through the receiver, the receiver definitely passes the transmitted energy through it and gets burnt. Hence to protect the receiver from the transmitted energy of transmitter, we use duplexer.
Now the duplexer turns receiver section OFF when the antenna is transmitting the signal. And it turns transmitter section OFF during reception of the signal. The received signal is then received by the antenna and then given to the receiver section. The receiver then amplifies the wave and finds out the difference between transmitted signal’s frequency and received signal’s frequency. A part of transmitted signal is taken before hand for this purpose. After finding the difference in frequency, the velocity of the target is found using the formula
Velocity of target = (F.T~F.R) (operating frequency) / 2
F.T = frequency of transmitted wave
F.R = frequency of received wave.
Using the above formula one can find the velocity of target that is moving to or away from the radar.
The main disadvantage of a CW radar is that it cannot measure the range of the target.
As the time transmitted and received signal’s time duration cannot be found, it cannot find out the range of target.