Running Fix

A running fix is a technique for establishing position when only a single fixed object is available for bearing observation. Two or more bearings of the same object are taken at known times and the ship's track between observations is used to advance the earlier bearing line forward to the time of the later bearing. Where the advanced and later bearing lines intersect, the vessel is positioned. Bowditch (American Practical Navigator) provides the mathematical basis, noting that accuracy depends critically on the quality of the ship's track data used in the advance—that is, the course and distance made good, accounting for current. The running fix is commonly employed in coastal waters where only one lighthouse or radar conspicuous object is within range. The OOW must ensure that the course and speed used to advance the earlier bearing are accurate; any error in the track advance propagates directly into the fix. A current flowing at right angles to the course line can shift the advanced position significantly. Despite being less precise than a simultaneous fix from multiple objects, a carefully executed running fix is a valid and entirely practical navigational technique, and its calculation is a STCW-assessed competency.