1
INTRODUCTION TO LORAN-C
LORAN-C is an accurate navigation system that is maintained by the
U.S. Coast Guard (LORAN stands for Long Range Navigation). A
receiver that measures the arrival times of the signals from three
LORAN stations can determine its position with an accuracy of about
1000 feet at a range of over 1000 miles. Because of the desire for
good long range position accuracy, the frequency and transmission
time of each LORAN transmitter is controlled by a set of Cesium clocks
or Hydrogen masers whose frequency accuracy is maintained by the
U.S. Naval Observatory. Because the timing characteristics of the
LORAN transmission are so tightly controlled, a receiver measuring the
signal from a single LORAN station can produce a very accurate
frequency output that is traceable to the U.S. Naval Observatory and
NIST. LORAN-C transmissions are also highly reliable. The stations
are functional more than 99% of the time and signal errors usually last
only a few minutes.
Signal Characteristics
Signal LORAN-C stations transmit a pulsed signal at a carrier
frequency of 100 kHz. This frequency was chosen for stable
propagation characteristics and low ground wave attenuation. The
transmissions of the various stations are differentiated by the timing of
their pulses. The LORAN transmitters in a specific geographical region
are arranged in groups of at least three (the minimum number needed to
establish position) to at most 6 stations called chains. The chains are
differentiated by the repetition rate of the pulses transmitted by the
stations in the chain. This rate is called the Group Repetition Interval, or
GRI. For example, the U.S. West Coast chain has a GRI of 99400
µ
s
and each station in that chain (4) will transmit its signal once every
99400
µ
s. If the receiver synchronizes its timing with the desired GRI,
only stations in that GRI will produce a stable signal. Each chain has a
master station (labelled "M") and up to four secondary stations
(labelled "V", "W", "X", "Y", and "Z"). Once every GRI, each station will
transmit a group of eight pulses at the 100 kHz carrier frequency. Each
pulse is about 250
µ
s long, and the pulses are separated by exactly 1
ms. The master station has a ninth pulse that is transmitted 2 ms after
the eighth pulse. This ninth pulse identifies the station as the master.
The master station always transmits first in the GRI followed by each
Figure 1
Transmission of LORAN
Signals During a GRI
