**3.4. Tracking systems and feedforward control in the presence of disturbance currents**

Simple feedforward control elements proved effective against known or estimated constant lateral disturbance currents by allowing the vehicle to autonomously perform "set-and-drift" principles where a highly trained helmsman would turn the bow of a ship into a current, but the simple feedforward elements were ineffective at countering currents with sinusoidal variation. In the set and drift principle the heading is de facto non-zero, so the vehicle cannot simultaneously maintain center-pointing while countering the disturbance. If such a requirement were added, designers must decouple the scissored-pair rudder constraint and design the rudder commands separately to simultaneously counter the disturbance while maintaining centerline pointing.
