  This paper presents a simple control approach for global trajectory tracking
and formation control of underactuated surface vessels equipped with only two
propellers. The control approach exploits the inherent cascaded structure of
the vehicle dynamics and is divided into control designs at the kinematics
level and the kinetics level. A controller with a low-gain feature is designed
at the kinematics level by incorporating the cascaded system method,
persistency of excitation, and the small-gain theorem. Furthermore, a PD+
controller is designed to achieve the velocity tracking at the kinetics level.
The proposed control laws are partially linear and saturated linear and easy to
implement. Based on a leader-follower scheme, our control approach applies to
the formation tracking control problem of multi-vehicle systems under a
directed spanning tree topology. Our main results guarantee uniform global
asymptotic stability for the closed-loop system, which implies robustness with
respect to perturbations.
