<math> f_d = f_{Rx} - f_{Tx} = 2 f_{Tx}\frac{v_r}{c-v_r} \approx \frac{2v_r}{\lambda_0} </math>
where and λ<sub>0</sub> = c/f is the free=space wavelength, and it was assumed that v<sub>r</sub> << c.For example, at an operating frequency of f<sub>0</sub> = 10GHz, λ<sub>0</sub> = 30mm. A moving car target driving at a speed of 20m/s (or 72km/hr) towards the radar generates a frequency shift of f<sub>d</sub> = 1.33kHz. When driving away from the radar, the same car produces a Doppler shift of -1.33kHz. The Doppler frequency shift f<sub>d</sub> caused by a moving target is superposed with the frequency shift of the echo signal due to reflection from a stationary target. When the target is approaching the radar, f<sub>d</sub> is positive and it slightly lifts up the reflected ramp signal. When the target is moving away from the radar, f<sub>d</sub> is negative and it slightly lowers down the reflected ramp signal. This leads to generation of two different frequency beat signals during the up-ramp and down-ramp sweeps:
<math> f_{bu} = f_b - f_d \\