Abstract: Sea surface current observation is of great importance for the study of ocean dynamic processes, and Doppler scatterometers provide an effective approach for measuring sea surface radial velocities. To compare the pulse-pair velocity measurement performance of different waveforms for a spaceborne Doppler scatterometer, a unified signal-level simulation model for distributed surface targets was developed, incorporating sea surface scattering echoes, thermal noise, speckle noise, and ideal platform-motion phase compensation. Under identical system parameters and within a 7.5-km common range window, the root-mean-square error (RMSE) of radial velocity retrieval using single-frequency (SF) and linear frequency-modulated (LFM) signals was quantitatively analyzed. The results show that, in the simulation scenario with both speckle noise and thermal noise, the radial-velocity RMSE of both waveforms generally decreases as the input signal-to-noise ratio (SNR) increases. When the input SNR exceeds approximately −9 dB, the LFM signal reduces the influence of speckle-induced phase fluctuations on phase estimation through range compression and multi-look complex correlation averaging, yielding a lower radial-velocity RMSE than the SF signal. These findings provide a useful reference for waveform configuration analysis of spaceborne Doppler scatterometers.