Tether-guided landing of unmanned helicopters without GPS sensors

Landing of unmanned helicopters is a challenging maneuver which can be seriously affected by GPS sensor reliability. This work explores an alternative approach to GPS sensors to avoid typical drawbacks, such as inaccuracies of standalone sensors or weight/cost for Real Time Kinematic (RTK) setups. To this end, the paper studies a setup consisting of a tether linking the helicopter to the landing point. Appropriate sensing of tether attitude relative to the helicopter together with an altitude measurement, allows estimation of the helicopter´s linear position relative to the landing point. Additionally, the tension exerted on the tether provides a stabilizing effect on helicopter translational dynamics. Taking into account all these considerations, a model-based control strategy is developed and implemented in the real platform. The experimental results endorse the validity of the proposed approach.