  In this work we solve the position-aided 3D navigation problem using a
nonlinear estimation scheme. More precisely, we propose a nonlinear observer to
estimate the full state of the vehicle (position, velocity, orientation and
gyro bias) from IMU and position measurements. The proposed observer does not
introduce additional auxiliary states and is shown to guarantee semi-global
exponential stability without any assumption on the acceleration of the
vehicle. The performance of the observer is shown, through simulation, to
overcome the state-of-the-art approach that assumes negligible accelerations.
