Loran

land-based system of radio navigation, first developed at the Massachusetts Institute of Technology during World War II for military ships and aircraft located within 600 miles (about 970 km) of the American coast. In the 1950s a more accurate (within 0.3 mile [0.5 km]), longer-range system (over 2,000 miles [3,200 km]), known as Loran-C, operating in the 90–110 kilohertz range, was developed for civilian use, and the original loran (renamed Loran-A) was phased out. Eventually, Loran-C was extended to cover most of the continental United States and, in cooperation with Canada and Russia, Canadian waters and the Bering Sea. Numerous other countries have deployed loran-like systems as well. It is still used by many marine craft, but the precision (typically within 30 feet, or 10 metres) of satellite-based navigational aides, such as the global positioning system (GPS), is increasingly relegating land-based navigational systems to the status of backup systems.

Loran is a pulsed hyperbolic system.

This means that hyperbolic lines of position are determined by noting differences in time of reception of synchronized pulses from widely spaced transmitting stations, primary and secondary. A primary station broadcasts an uninterrupted series of pulses of fixed duration and at a fixed rate (e.g., of 50 microseconds' duration at a rate of 25 pulses per second). A secondary station, 200–300 miles (320–480 km) away, automatically transmits its own signals, maintaining a frequency and pulse duration in accord with those of the primary station. The secondary station maintains a fixed time difference between its reception of the primary signal pulse and the sending out of its own. The noted time difference of arrival of the two pulses locates the craft somewhere on a curve (hyperbola) every point of which is located at a constant difference in distance between the stations (e.g., is three miles farther from the primary than from the secondary). Tuning in another secondary station locates the craft on another hyperbola, so that its position can be fixed at the intersection of the two.