A Doppler radar is a specialized radar that employs the use of Doppler Effect to produce velocity data on objects that are positioned at a certain distance. This is achieved by bouncing a microwave signal off from a desired target and then calculating how the motion of the object has affected or altered the frequency of the reflected signal. This variation is responsible for the provision of highly accurate measurements of the radial component of the target’s velocity in relation to the radar.
The Doppler Effect
The Doppler Effect was named after an Australian physicist called Christian Doppler who came up with the idea in 1842. According to Christian Doppler, the Doppler Effect is the difference between the observed frequency and the emitted frequency of a wave for an observer moving in relation to the wave source. The most practical example or illustration of a Doppler effect can be related to a vehicle passing with a siren on. The siren will be heard as the vehicle approaches the observer, passes and recedes from the observer. As the vehicle approaches the observer, the received frequency will be higher when compared to the emitted frequency. As the vehicle passes by, the frequency will be identical to the emitted frequency. During recession, the frequency will be lower.
The variation of frequency when explaining the Doppler Effect is dependent on the direction of the wave source with respect to the direction of the observer. The relationship is such that it is maximum when the source is moving directly towards or away from the observer. However, as the angle between the direction of motion and the direction of the waves increase, the frequency of the wave will also reduce. When the source of the wave begins moving or is moving at right angles to the observer, there will be no shift in frequency.
The Doppler Effect can best be explained using a baseball pitcher and catcher. Assuming the pitcher throws baseballs to a catcher at a constant velocity of one ball per second. This means that the catcher will be catching one ball per second. However, assuming the pitcher was running towards the catcher as he throws the balls, then the catcher will be catching more balls per second because the balls will be less spaced out in between the throws. This increases the frequency of the throws. The inverse is true if the pitcher is jogging or running away from the catcher as he throws the balls. The balls will be more spaced out in between the throws and this will decrease the frequency of the throws. Likewise, if the pitcher decides to move at an angle as he throws the balls at the catcher even though he maintains the same speed, then the variation in the frequency of the throws will be much less as the distance between the two people changes at a much slower rate.
From the pitcher’s point of view, the frequency with which he throws the ball will remain the same. The same can be said of the transmitter of electromagnetic waves. Hence the Doppler Effect is given by the relative difference in velocity between the source and the observer.
Marine Doppler radar works with the same principle in that it uses the relative distance between the object and the vessel to determine the position of an object or an observer, whether it is moving or stationary. The radar transmitter on the vessel will transmit radar signals at a constant rate and this will be transmitted to a moving object and will be reflected back at a varying frequency. The reflected signal will be transmitted back at a varying frequency that is different from the transmitted signal from the vessel.
Marine Doppler radar is important in the determination of relative distance of both stationary and moving objects at sea and plays an important role in the prevention of collision or accidents between vessels.