  We propose a nonlinear observer to estimate the state (orientation and
in-plane velocity vector) of the quadrotor, based on a drag-force-enhanced
model. It is an alternative to recent works using a similar model together with
an Extended Kalman Filter (EKF). But while enjoying the benefits of an enhanced
model, it does not have the usual drawbacks of an EKF: indeed, the
computational cost is much lower, the tuning is easier, and above all the
guaranteed domain of convergence is very large.
