Direction finding is the oldest, simplest, and probably most reliable radio aid yet offered to the mariner. In some ways it may be likened to a magnetic compass and, if its range is very much smaller, it has the inestimable advantage over a compass of giving bearings on a very large number of stations instead of one only. It is the only navigational aid, other than radar, in which the performance is controlled by the shipborne installation and is not subject to technical failures which may occur on land-based transmitters.