  This paper considers a collaborative tracking control problem using a group
of fixed-wing unmanned aerial vehicles (UAVs) with constant and non-identical
speeds. The dynamics of fixed-wing UAVs are modelled by unicycle-type equations
with nonholonomic constraints, assuming that UAVs fly at constant altitudes in
the nominal operation mode. The controller is designed such that all fixed-wing
UAVs as a group can collaboratively track a desired target's position and
velocity. We first present conditions on the relative speeds of tracking UAVs
and the target to ensure that the tracking objective can be achieved when UAVs
are subject to constant speed constraints. We construct a reference velocity
that includes both the target's velocity and position as feedback, which is to
be tracked by the group centroid. In this way, all vehicles' headings are
controlled such that the group centroid follows a reference trajectory that
successfully tracks the target's trajectory. A spacing controller is further
devised to ensure that all vehicles stay close to the group centroid
trajectory. Trade-offs in the controller design and performance limitations of
the target tracking control due to the constant-speed constraint are also
discussed in detail. Experimental results with three fixed-wing UAVs tracking a
target rotorcraft are provided.
