“…This problem arises in many industrial applications such as navigation of flying robots through underground mines and connected tunnels, navigating small aerial vehicles in cluttered indoor environments, 3D mapping of cave networks, interior inspection of pipeline networks, search & rescue missions during disaster events in underground rail networks, etc. For example, some variants of these applications that have gained a great interest by researchers recently are dam penstocks inspection and/or mapping [1,2,3,4], chimney inspection [5], hazardous deep tunnels inspection [6,7], mapping and navigation in underground mines/tunnels [8,9,10,11,12,13,14,15,16], search & rescue in underground mines [17,18], inspection of ventilation systems [18], inspection of narrow sewer tunnels [19] and inspection tasks in the oil industry [20]. In all these applications, a UAV should navigate through a tunnel-like unknown environment while avoiding collisions with the tunnel walls.…”