  The paper presents a new, robust control algorithm for position trajectory
tracking in a 3D space, dedicated to underactuated airships. In order to take
into account real characteristics of such vehicles, and to reflect practically
motivated constraints, the algorithm assumes a highly uncertain system dynamics
model. The tracking problem is solved in a uniform way, without dividing it
into subtasks considered in 2D spaces, thanks to the introduction of an
auxiliary tracking error.
  The proposed controller is based on the sliding mode approach. Its stability
is investigated using Lyapunov theorem. Numerical simulations are conducted in
order to verify properties of a closed-loop system for a generic model of the
airship.
  Performance of the control system is examined via experiments in various
scenarios using a prototype airship. The obtained results indicate that the
control objectives are satisfied in practice with a reasonable accuracy.
Moreover, it is shown that the controller is robust to some bounded additive
measurement perturbations and delays in the control loop.
