Fig. 1.

Difference between speed and velocity. The motion of an object can be described in two exactly equivalent ways: speed-and-direction or velocity components. The left panel (A) shows speed and direction. The right panel (B) shows the same motion decomposed into horizontal velocity (v_horizontal) and vertical velocity (v_vertical). For movement in a plane, speed² = v_horizontal² + v_vertical². Doppler shows only movement towards/away from the probe, which in this figure is approximately vertical.