Does the velocity remain constant in the doppler effect?

The doppler effect is all about the effect of relative velocity on the frequency observation....so as long as relative velocity is constant the doppler shift is constant for same frequency. while studying doppler radars, we were quizzed with problems involving change in speed of the plane....one more thing, that relative velocity is the component of object's velocity in the direction of line joining object to observer for a sattionary observer, so even change in direction by the object....would also make a difference in doppler shift.