Abstract: Based on the radar return model of the target with a collection of scattering centers,target motion induced Doppler effect of UWB (ultra wideband) radars is analyzed.Doppler time shift,a time domain expression of Doppler effect in carrier free UWB radars is proposed.A simulation is carried out for a kind of time domain target motion Doppler processor based on the Doppler shift,with the result that target velocities can be detected by estimating the Doppler time shifts.As an advantage, there is no blind velocity phenomenon which exists in conventional carrier radars.