Spaceborne SAR and HF radar are two instruments for detecting the sea surface current field. Research on HF radar began much earlier than research on spaceborne SAR, so it is generally believed that the velocity measured via spaceborne SAR is also the radial component of the sea surface velocity, which is consistent with the radial sea surface velocity measured via HF radar. Through sufficient evidence, in this paper, it is shown that the sea surface velocity measured via spaceborne SAR is not the radial velocity but rather the magnitude of the sea surface velocity. Only the range satellite velocity measured via spaceborne SAR presents a strip-like radial feature. Based on the explanation presented in this paper, spaceborne SAR will become the most important technique for sea surface current remote sensing since it can directly obtain the velocity field of the sea surface current.