A change or shift in frequency in the reflected signal is known as?

The correct answer is Doppler Shift, which refers to the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. This phenomenon occurs when there is relative motion between the radar source and the target. In radar applications, as a target moves towards the radar, the frequency of the reflected signal increases, resulting in a higher pitch. Conversely, if the target is moving away from the radar, the frequency decreases.

Understanding the Doppler Shift is crucial for radar operators, particularly in applications such as tracking moving objects, weather radar, and aviation radar. It allows operators to determine the speed and direction of moving targets based on the frequency shift observed in the returning radar signal.

The other choices, although related to concepts of signal processing and wave behavior, do not specifically refer to the effect of motion on frequency in the manner that the Doppler Shift does. For example, Frequency Modulation relates to varying the frequency of a signal to encode information, Amplitude Shift involves changes in signal strength rather than frequency, and Wavelength Modulation indicates changes in the wavelength of a wave without addressing the motion factor that characterizes the Doppler effect.