A wireless tracking system comprises mobile devices and stationary devices at known locations. The system receives a first radio measurement indicative of a first propagation path length and receives sensor data indicative of the movement of the first mobile device. The sensor data is of a type different to the radio measurement. The system then determines a historical location of the first mobile device based on the first radio measurement and the sensor data and determines error data indicative of a difference between a historical radio measurement and the historical location and stores the error data associated with the historical location. The system can then receive a second radio measurement indicative of a second propagation path length of radio frequency radiation and determine an estimated location of the second mobile device based on the second radio measurement and the stored error data.