The paper describes an automatic position-finding system for aircraft, making use of the signals radiated in the course of ordinary communication with the ground, and not requiring any special equipment in the air. A network of automatic direction-finders on the ground feeds bearings to a control centre where they are displayed virtually instantaneously on a map.The operational requirements which have influenced the development of the system are discussed, and a brief reference to the history of position-finding by d.f. triangulation is followed by an account of the main technical features of the present system, with particular reference to the method of bearing transmission to the control centre and the problems of display.A practical approach to the planning of d.f. triangulation networks for specified accuracy and coverage is described, followed by details of some systems which have gone into service and of the methods used in testing them on signals from aircraft. Typical results are given.The paper concludes with an indication of the need for further development of display methods.