1. LORAN
LOng RAnge Navigation

2. LORAN (LOng RAnge Navigation)
Is a terrestrial radio navigation system which enables ships and aircrafts to determine their position and speed from low frequency radio signals transmitted by fixed land based radio beacons, using a receiver unit.
The most recent version of LORAN in use is LORAN-C, which operates in the low frequency (LF) portion of the radio spectrum from 90 to 110 kHz. 

3. The LORAN-C concept is based on groups or “chains” of stations around the world.
Worldwide there are 28 chains.
Each chain consists of one master station and 3 – 6 secondary or slave stations in a geographic area.
The unique time between the start and stop of each pulse identifies the particular chain known as the Group Repetition Interval (GRI).

4. The master transmits a set of 8 plus 1 pulses.
Ninth pulse enables the Loran receiver to identify the master station.
When the secondary stations receive the first master pulse, they wait for a short time known as a coding delay and then each transmits a similar set of 8 pulses.

6. Receivers require a data base which provides
the location (Lat/Lon) of the Master and Slave stations
the GRI of the chains to be used
the Time Delays for the individual stations
The receiver measures the time difference or time delay between signal transmission and reception relative to the towers to calculate distance or the position.