A robotic work tool for movable operation within a work area. The robotic lawnmower comprises a satellite navigation device; and a controller, the controller being configured to cause the robotic work tool to movably operate within the work area based on positions partly or in whole determined from satellite signals received by the satellite navigation device being a differential satellite navigation device and on phase information received from a reference station, by: receiving phase information from the reference station for a satellite signal received by the reference station; storing the phase information; determining that further phase information from the reference station for the satellite signal received by the reference station is not available or erroneous, and in response thereto; extrapolating phase information based on the stored phase information; receiving a satellite signal received by the robotic work tool; comparing the extrapolated phase information to phase information of the satellite signal received by the robotic work tool; determining a position based on the comparison for movably operating the robotic work tool.