It can be said of the oldest form of radio position-fixing for ships and aircraft, by direction finding, that two or more position lines in the form of the bearings of known radio stations provide a ‘common-reference’ fix: within the accuracy limits of the technique, all craft visiting a given spot will get the same bearings, and hence the same fix, at that spot. Similarly, the modern hyperbolic systems Decca, Loran and Omega each provide, within their respective accuracy limits, a common frame of navigational reference for all users. As the term implies, the users can fix position at will within this frame of reference without first having to undertake a calibration or similar procedure at some accurately known position.