US Patent 10136576 Navigation for a robotic working tool

A robotic work tool system (200) comprising a charging station (210) and a robotic work tool (100), said robotic work tool (100) comprising a position determining device (190) for determining a current position and at least one deduced reckoning navigation sensor (195), the robotic work tool (100) being configured to determine that a reliable current position is possible to determine and in response thereto calibrate at least one of the at least one deduced reckoning navigation sensor (195).