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Swapan Basu, Ajay Kumar Debnath, in Power Plant Instrumentation and Control Handbook (Second Edition), 2019 4.3.1.2.2 Doppler Frequency Shift Methodįlow of fluid can also be measured by the help of the Doppler “frequency shift” method. In this case, the speed differential is −300 mph, or −134 m/s.
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The radar that is airborne, traveling at 500 mph, is tracking a target ahead moving at 800 mph in the same direction. Also, this allows easy discrimination between moving objects, such as an aircraft, and the background clutter, which is generally stationary.įor example, imagine we have a radar operating in the X band at 10 GHz (λ = 0.03 m or 3 cm). By binning the receive echoes both over range and Doppler frequency offset, target speed as well as range can be determined. This can be of great advantage in a radar system. The police radar will need to subtract the speed of the police car to display your speed. For example, if you are driving on the highway at 70 mph and an approaching police car is traveling at 50 mph, the radar will show a Doppler shift corresponding to 120 mph. If the radar is vehicle or airborne based, then the Doppler frequency shifts will be due to the relative motion between the radar and target object. If the radar is ground based, then all Doppler frequency shifts will be due to the target object motion. There is no change in the distance between the plane and ground. Assuming level terrain and the aircraft is at a constant altitude, the Doppler shift will be zero, even though the plane is moving relative to the ground. An example of this would be an airborne radar directed at the ground immediately below the aircraft. If the object is moving at right angles to the radar, there will be no Doppler frequency shift. This effect only applies to the motion relative to the radar and the target object. Therefore, Doppler frequency shifts are more easily detected when using higher frequency waves, as the percentage change in the frequency will be larger. Then the effect of the receive wavelength being shorted or lengthened due to the Doppler effect is more noticeable. This effect becomes more pronounced when the frequency of the transmitted sinusoid is high (short wavelength). Each successive wave crest has a longer round-trip distance to travel, so the time between arrival of receive wave crests is lengthened, resulting in a longer (larger) wavelength and a lower frequency. If the target object is moving away from the radar system, then the opposite happens. Since frequency is inversely proportional to wavelength, the frequency of the sinusoidal wave appears to have increased. As long as this motion continues, the distance between the arriving wave crests is shorter than the distance between the transmitted wave crests. This is because the target has moved closer in the interval of time between the previous and current wave crest. When this object is moving toward the radar system, the next wave crest reflected has a shorter round-trip distance to travel, from the radar to the target and back to the radar. Each successive wave is reflected from the target object of interest.
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The distance from the crest of each wave to the next is the wavelength, which is inversely proportional to the frequency. Consider the transmission of a sinusoidal wave. What is happening in a radar system is that the frequency is modified by the process of being reflected by a moving object. Technically this is true only in a vacuum, but the effect of the medium such as our atmosphere can be ignored in radar discussions. The speed of light is constant-Einstein proved this. Where f = wave frequency (Hz or cycles per second), λ = wavelength (meters), v = speed of light (approximately 3 × 10 8 m/s).