In these equations, a refers to the linear acceleration of the
aerial vehicle, a = (ax,ay,az), ax and ay are x and y components
of the acceleration vector defined in (35) respectively.
The third component of the acceleration vector az is the
acceleration of the vehicle along z axis which is computed
as az = u1/m. Note that θ is the instantaneous pitch angle
measured by IMU. ||.|| symbol refers to Euclidean norm and
for the acceleration vector a it is defined as