This paper presents a new ultrasonic sensor for 3D co-ordinate estimation, which has been especially designed to localize and sketch findings after they are extracted by archaeologists. Classical tasks at paleo-archaeological excavations are: measuring position with metric tapes, drawing a sketch of found object, and introducing all information into a database manually; operations that are not efficient and prone to errors. The positioning system we have designed allows simultaneous characterization of several findings (absolute position, shape, size and orientation) using as a tool a wireless 2-metre-long rod, whose lower tip has to be placed on the object under study. The system contains two ultrasonic emitters and employs the time-of-flight (TOF) the ultrasonic signal takes to reach several fixed receivers, and a robust trilateration algorithm to determine the position of the rod tip with 10 mm accuracy. Object position and contour information are automatically transferred to a database in a central computer avoiding manual typewriting. Airflow is the main source of positioning error in outdoor environments, so a strategy based on a differential emitter fixed at a known position is used, which permits to cancel out the effects of uniform air motion.Fundación Atapuerca, y el Ministerio Ciencia y TecnologíaPeer reviewed