**Acknowledgements**

This work was supported by the Robotics and Cybernetics Group at the *Centre for Automation and Robotics (CAR) UPM-CSIC*, and funded under the project *ROBOCITY 2030* sponsored by the Community of Madrid (S-0505/DPI/000235) and The project *ROTOS (MultiRobot Systems for Protection of large outdoor infrastructures)* sponsored by the Ministry of Science And Innovation of Spain (DPI2010-17998)
