Abstract: In view of the deficiencies of conventional ultrasonic ranging systems,Quasi-GPS ultrasonic rang-ing system and its work principle are introduced.After the estimated distances between the object to be located and the base-stations are obtained with a multiple base-stations distance estimation algorithm,the Gauss-Newton iterative location algorithm and its location precision are analyzed.It is proved by theoretical analysis that the location precision is relevant to the object′s location and the geometrical layout of the base-stations.In order to obtain better location precision after the geometrical layout of the base-sta-tions has been decided,we should select different location base-stations according to the object′s location in ranging space.The location base-stations selection principle using three or more base-stations for location is presented based on the PDOP geometrical distributing figure.Simulation shows that the above analysis is reliable.