Essay on Radar, Sonar and The Doppler Effect

Radar transmits a signal to a target, interacts with the target, and returns back to the radar. The change in the properties of the returned signal contains characteristics of interest of the target. When the transmitted signal of a coherent radar system hits moving targets, the carrier frequency of the signal will be shifted, known as the Doppler effect. The

Doppler frequency shift reflects the velocity of the moving target. Mechanical vibration or rotation of a target, or structures on the target, may induce additional frequency modulations on the returned radar signal, which generate sidebands about the target’s Doppler frequency, called the micro-Doppler effect. Micro-Doppler signatures enable us to determine some properties of the target. The micro-Doppler effect was originally introduced in coherent laser radar systems. In a coherent system, the phase of a signal returned from a target is sensitive to the variation in range. In many cases, a target or structures on the target may have vibrations or rotations in addition to target translation, such as a rotor on a helicopter or a rotating radar antenna on a ship. Motion dynamics of the rotating rotor or antenna will produce frequency modulation on the backscattered signals and induce additional Doppler variations to the translation Doppler shift. From the electromagnetic point of view, when a target has vibration, rotation or other non uniform motions, the radar backscattering is subject to modulations that constitute features in the signature. Micro-Doppler can be regarded as a unique signature of the target and provides additional information that is complementary to existing methods. To exploit these unique micro-Doppler features, traditional analysis, such as the Fourier transform, or the sliding window or short time Fourier transform, may not possess the necessary resolution for extracting these features. Therefore, high-resolution time–frequency analysis is necessary for extracting the time-varying...............

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