The concept of radial velocity is proposed for high frequency (HF) radar for sea surface current measurement. However, in a spaceborne synthetic aperture radar (SAR) system, the meaning of this concept has changed greatly. Through the evidence and analysis presented in this paper, the radial velocity of the sea surface observed via spaceborne SAR is revealed from the projection of the satellite’s velocity at different points along the range direction, and its distribution on the image presents a strip-like texture, which is very different from the central radial distribution on the velocity image measured via HF radar. The retrieved sea surface velocity field does not exhibit any radial characteristics, and its value is solely the magnitude of the sea surface velocity.