US Patent 9720417 Navigation for a robotic working tool

A robotic work tool system includes a charging station and a robotic work tool. The robotic work tool includes a position determining device for determining a current position. The robotic work tool may be configured to determine that reliable navigation through the position determining device is no longer possible, such as when satellite signal reception is not possible, at a time point and position and in response thereto generate an obstacle map which gives information on the position of at least one obstacle, determine when an area will be shadowed with regards to satellite reception based on the obstacle map, and schedule operation of the robotic work tool accordingly.