Page 122 - Electronic Navigation Cyber Book

LORAN

Loran is an acronym for long range navigation. It is an electronic system
of land-based transmitters broadcasting low frequency pulsed signals
that enable ships and aircraft to determine their position. A system that
used this concept was first proposed in the 1930s and implemented as
the British Gee system early in the Second World War. The Gee system
used master and slave transmitters sited approximately 100 miles apart
and used frequencies between 30 and 80 MHz. The use of frequencies
in the VHF band constrained the system to 'line-of-sight' distance for
coverage but this was not a problem at the time since the system was
designed to aid bomber navigation on raids over Germany.
Loran-A chains operate by measuring the difference in time arrival of the
pulses from the master and the slave stations. Every time difference
produces a line of position (LOP) for a master-slave pair and a positional
fix is obtained by the intersection of two such LOPs using two suitable
master-slave pairs. Two adjacent chains usually have a common master
transmitter station. For each chain the slave station transmission is
retarded in time compared to that of the master station. Such retardation
is known as the coding delay and has a value such that within the
coverage area of the chain the master pulse is always received at a
receiver before the slave pulse. Known unreliable signals can be
indicated by the master or slave signals, or both, being made to blink.
Loran-A chains are identified by an alphanumeric which specifies the
transmission frequency and the pulse repetition rate (determined by the
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