The Electronic Age - The Science of Navigation - page 215

A and B send out signals modulated in a manner that permits a receiver to
estimate the time di√erence between them. From the detected time di√er-
ence, a navigator can construct an LoP: it is a hyperbola. 12 (Several hyper-
bolas are shown in fig. 8.6, corresponding to various time di√erences.)
Consequently, an LoP derived from LORAN was known as a TD line (time-
di√erence line). A third transmitter leads to a second LoP; the intersection
of these two hyperbolic LoPs yields a position fix. This method of deter-
mining position is not triangulation but instead is a two-dimensional ver-
sion of the trilateration technique later used by GPS.
The first LORAN system was developed in World War II from a British
system for guiding bombers onto German cities. LORAN—an American
development, more powerful and sophisticated than its predecessor—had
a longer range; and eventually British as well as American bombers used
LORAN to help them navigate to their target areas. Unlike the German
Lorenz beam rider method, this hyperbolic system did not reveal to the
enemy the probable target city of the bomber planes. This first version
transmitted signals in the frequency range of 1.8 to 2.0 MHz. LORAN-A
lasted well beyond the war years. Its transmitter network was extended,
and the system became very popular among commercial fishing fleets and
other marine vessels because it worked quite well in most weather condi-
tions and because a LORAN-A receiver was inexpensive.
The LORAN-A transmitters were maintained until 1980, but long be-
fore then, an improved version, LORAN-C, had been introduced. This
version operated at lower frequency (around 100 kHz) and consequently
was able to see over the horizon. Many hundreds of LORAN-C transmitter
towers were constructed around the world, some of them very powerful
(fig. 8.7), with ranges exceeding 5,000 km. LORAN-C was the mainstay of
airline navigation for four decades, from about 1970 to 2010, when the U.S.
transmitters were switched o√. It was a reliable system that provided navi-
gation fixes for airplanes or ships anywhere within the coverage area. The
coverage was not universal, however, and the system required expensive
infrastructure. Moreover, it was susceptible to ionospheric fluctuations at
dusk and dawn, or to fluctuations in atmospheric ionization caused by
12. Because the speed of light (and of a radio signal) is constant, a measured time
di√erence converts into a well-defined di√erence in path length from the receiver to the
two transmitters. Mathematics tells us that the receiver must lie somewhere on a hyper-
bolic line passing between the transmitters; which hyperbola depends upon the size of the
path length di√erence.