  We study the problem of estimating and tracking an unknown trajectory with a
multi-rotor UAV in the presence of modeling error and external disturbances.
The reference trajectory is unknown and generated from a reference system with
unknown or partially known dynamics. We assume the only measurements that are
available are the position and orientation of the multi-rotor and the position
of the reference system. We adopt an extended high-gain observer (EHGO)
estimation framework to estimate the unmeasured multi-rotor states, modeling
error, external disturbances, and the reference trajectory. We design a robust
output feedback controller for trajectory tracking that comprises a feedback
linearizing controller and the EHGO. The proposed control method is rigorously
analyzed to establish its stability properties. Finally, we illustrate our
theoretical results through numerical simulation and experimental validation in
which a multi-rotor tracks a moving ground vehicle with unknown trajectory and
dynamics and successfully lands on the vehicle while in motion.
