Unmanned Aerial Vehicles (UAVs), often referred as drones, have been widely implemented for civilian, commercial, search and rescue, and military operations with the advantages of easy deployment, low cost, automation, as well as, most importantly, allowing the execution of dangerous or difficult tasks remotely and safely. However, current UAVs are equipped with a skid or wheel landing gear that limits the application of UAVs to an even and flat ground for safe landing and taking off; this constraint impedes the development of UAVs for application in extreme environments, such as war fields and remote wilderness where proximate level ground is inaccessible. The ability of UAVs to land on un-level ground would help broaden the application of UAVs; in particular, the ability to go beyond thermal imaging to locate a lost hiker with the ability to land and deliver life-sustaining resources in a more timely manner offers a benefit to human rescue missions. This paper presents an innovative robotic landing system consisting of three slanted legs, each individually controlled by a motor. The footpad of each leg has an integrated force sensor for detecting ground touch. An inclinometer is installed on the platform of the landing system to sense the UAVs orientation during landing. Thus, the landing system can keep the platform horizontal when it lands on the ground by extending or retracting the legs. The feasibility and effectiveness of the robotic method have been demonstrated by several indoor and outdoor experiments.

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