  We propose a simple nonlinear observer for estimating the attitude and
velocity of a rigid body from the measurements of specific acceleration,
angular velocity, magnetic field (in body axes), and linear velocity (in body
axes). It is uniformly globally exponentially convergent, and also enjoys other
nice properties: global decoupling of pitch and roll estimation from magnetic
measurements, good local behavior, and easy tuning. The observer is
"geometry-free", in the sense that it respects only asymptotically the
rotational geometry. The good behavior of the observer, even when the
measurements are noisy and biased is illustrated in simulation.
