The LOng RAnge Navigation system is constituted by a chain of stations that allow a wide coverage range using low frequency radio signals. It is an evolution of its precursor: Loran-A, developed during World War II. LORAN is based on measuring the time difference between the receipt of signals from a pair of radio transmitters. A given constant time difference between the signals from the two stations can be represented by a hyperbolic line of position. If the positions of the two synchronized stations are known, then the position of the raircraft can be determined as being somewhere on a particular hyperbolic curve where the time difference between the received signals is constant. In ideal conditions, this is proportionally equivalent to the difference of the distances from the aircraft to each of the two stations.
An aircraft which only receives signals form a pair of LORAN stations cannot fully fix its position. The aircraft must receive and calculate the time difference between a second pair of stations. This allows to be calculated a second hyperbolic line on which the aircraft is located. In practice, one of the stations in the second pair also may be (and frequently is) in the first pair. This means signals must be received from at least three LORAN transmitters to locate exactly the aircraft. By determining the intersection of the two hyperbolic curves, the location of the aircraft can be determined.
LORAN has been widely used to navigate when overflying oceans, where DME and VOR coverage ranges are insufficient. In recent decades LORAN use has been in steep decline, with the GNSS systems as primary replacement. However, there have been attempts to enhance LORAN, mainly to serve as a backup to GNSS systems.
Figure 11.10: LORAN.