  This paper investigates the coordinated trajectory tracking problem of
multiple vertical takeooff and landing (VTOL) unmanned aerial vehicles (UAVs).
The case of unidirectional information flow is considered and the objective is
to drive all the follower VTOL UAVs to accurately track the trajectory of the
leader. Firstly, a novel distributed estimator is developed for each VTOL UAV
to obtain the leader's desired information asymptotically. With the outputs of
the estimators, the solution to the coordinated trajectory tracking problem of
multiple VTOL UAVs is transformed to individually solving the tracking problem
of each VTOL UAV. Due to the under-actuated nature of the VTOL UAV, a
hierarchical framework is introduced for each VTOL UAV such that a command
force and an applied torque are exploited in sequence, then the position
tracking to the estimated desired position and the attitude tracking to the
command attitude are achieved. Moreover, an auxiliary system with proper
parameters is implemented to guarantee the singularity-free command attitude
extraction and to obviate the use of the unavailable desired information. The
stability analysis and simulations effectively validate the achievement of the
coordinated trajectory tracking of multiple VTOL UAVs with the proposed control
approach.
