However, a Doppler shift will be apparent if the target is rotating, and the resolution of the radar is sufficient to distinguish its leading edge from its trailing edge. Thus no Doppler Effect in Radar will be noticed if a target moves across the field of view of a radar. Note also that the Doppler Effect in Radar is observed only for radial motion, not for tangential motion. However, it will represent an increase in frequency in the former case and. The same magnitude of Doppler shift is observed regardless of whether a target is moving toward the radar or away from it, with a given velocity. Since f t/ν c =1/λ, where λ is the transmitted wavelength. The two are still approaching each other, and so the Doppler Effect in Radar is encountered a second time, and the overall effect is thus double. This target becomes the “source” of the reflected waves, so that we now have a moving source and a stationary observer (the radar receiver). In radar involving a moving target, the signal undergoes the Doppler shift when impinging upon the target. Equation (16-19) was calculated for a positive radial velocity, but if ν r is negative, f′ d in Equation (16-19) merely acquires a negative sign. The principle still holds under those conditions, and it holds equally well if the observer is stationary and the source is in motion. If the relative velocity is higher than that (most unlikely in practical cases), relativistic effects must be taken into account, and a somewhat more complex formula must be applied. Note that the foregoing holds if the relative velocity, ν r, is less than about 10 percent of the velocity of light, ν c.
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