  This paper introduces an $\mathcal{L}_1$ adaptive control augmentation for
geometric tracking control of quadrotors. In the proposed design, the
$\mathcal{L}_1$ augmentation handles nonlinear (time- and state-dependent)
uncertainties in the quadrotor dynamics without assuming or enforcing
parametric structures, while the baseline geometric controller achieves
stabilization of the known nonlinear model of the system dynamics. The
$\mathcal{L}_1$ augmentation applies to both the rotational and the
translational dynamics. Experimental results demonstrate that the augmented
geometric controller shows consistent and (on average five times) smaller
trajectory tracking errors compared with the geometric controller alone when
tested for different trajectories and under various types of
uncertainties/disturbances.
