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The question reads:
When using active radar, where radio waves are emitted from the same point at which the reflected waves are detected, all of the following should be considered EXCEPT:
A. the reflected wave is double Doppler-shifted by the moving object.
B. a stationary reference beam is needed.
C. the gun operates when stationary and in motion.
D. the best electromagnetic radiation is in the ultraviolet range.
Choice D is the best answer. When using active radar (such as a radar gun), the wave reflected from a moving object is in fact doubly compressed, which leads to a double Doppler shift. This makes choice A valid, so eliminate it. A reference beam is needed to compare how much the wave has shifted. This makes choice B valid, so eliminate it. The gun operates while stationary or in motion. But when in motion, the speed of the emitter must be factored in. If the speed is analyzed by comparing the wave reflected from a moving object with the wave reflected from a stationary object, then the motion of the gun subtracts out. This makes choice C valid, so eliminate it. The best electromagnetic radiation is one that can travel far and not be much affected by absorbance, refraction, or dispersion. The longer the EM wavelength, the better it is for use in radar.
This means that UV, with a short wavelength, would not work well. In addition, it would be expensive and dangerous to send out a signal of such high energy. For practical purposes, communication bands and radar are found at the low-energy/long- wavelength end of the electromagnetic spectrum. The best answer is D.
I was under the assumption that if the emitter is stationary it won't be "double Doppler-shifted". Can someone please explain why it is double shifted?
When using active radar, where radio waves are emitted from the same point at which the reflected waves are detected, all of the following should be considered EXCEPT:
A. the reflected wave is double Doppler-shifted by the moving object.
B. a stationary reference beam is needed.
C. the gun operates when stationary and in motion.
D. the best electromagnetic radiation is in the ultraviolet range.
Choice D is the best answer. When using active radar (such as a radar gun), the wave reflected from a moving object is in fact doubly compressed, which leads to a double Doppler shift. This makes choice A valid, so eliminate it. A reference beam is needed to compare how much the wave has shifted. This makes choice B valid, so eliminate it. The gun operates while stationary or in motion. But when in motion, the speed of the emitter must be factored in. If the speed is analyzed by comparing the wave reflected from a moving object with the wave reflected from a stationary object, then the motion of the gun subtracts out. This makes choice C valid, so eliminate it. The best electromagnetic radiation is one that can travel far and not be much affected by absorbance, refraction, or dispersion. The longer the EM wavelength, the better it is for use in radar.
This means that UV, with a short wavelength, would not work well. In addition, it would be expensive and dangerous to send out a signal of such high energy. For practical purposes, communication bands and radar are found at the low-energy/long- wavelength end of the electromagnetic spectrum. The best answer is D.
I was under the assumption that if the emitter is stationary it won't be "double Doppler-shifted". Can someone please explain why it is double shifted?
