  We consider the classical problem of estimating the attitude and gyro biases
of a rigid body from vector measurements and a triaxial rate gyro. We propose a
simple "geometry-free" nonlinear observer with guaranteed uniform global
asymptotic convergence and local exponential convergence; the stability
analysis, which relies on a strict Lyapunov function, is rather simple. The
excellent behavior of the observer is illustrated through a detailed numerical
simulation.
