A navigation system developed in the 1950s based on the time displacement between signals from two or more fixed shore based antennas. Two types of Loran (an acronym for LOng RAnge Navigation) were developed. A low resolution Loran-A was first. Cumbersome and difficult to use, Loran-A required the user to tune a receiver and align signal peaks on a scope. A more efficient system, Loran-C, was developed for use in the 1960s and provided the user with a readout of numbers representing time differences in microseconds. American charts were produced with these “TD” lines overprinted on them. Although Loran-C was very repeatable (often to within 20 metres) the system was not accurately tied to any datum. Further, since the radio transmissions propagated over land, conversion to latitude and longitude was only approximate, at best. For several decades since the late 1950s, Loran and a similar British system, Decca, were the major worldwide land-based navigation systems. Offshore beyond the 600-mile range of these systems, dead reckoning with occasional fixes from transit satellites was a large part of ocean navigation. The Global Positioning System is expected to completely replace the need for Loran transmitting stations worldwide.