Avionics News January 2021 - 49

Microprocessors and databases allow
amazing advances in instrument displays.

Gee system used multiple secondary stations that would
allow the navigator to plot several crossing lines, thus
determining position. This system was shut down in the
1960s.
Next to come along was LORAN, which is short for
long range navigation. It was also developed during
World War II. In principle, it worked like the Gee
system. But LORAN worked on lower frequencies, 1.85
MHz and 1.95 MHz. The lower frequencies allowed for
greater range, and the LORAN system had a range of
over 1,500 miles. The U.S. Navy developed a LORAN-B
system, which it hoped would provide better accuracy,
but abandoned the effort. The original LORAN system
was phased out in the 1970s by the United States, but
Japanese systems stayed on the air until 1991.
In 1957, LORAN-C was introduced. LORAN-C used
100 kHz exclusively, and all stations transmitted on
the same frequency. The system used a master station,
and toward the end of its life, up to four secondary
stations. Each master station and its related secondary
stations were known as a chain. To get a position fix, a
receiver would need to receive the master and at least
two secondary stations. The principle of operation was
the same as Gee and the original LORAN. LORAN-C
transmitted pulse groups at precise timing intervals
known as group repetition intervals. Chains were
identified by their unique GRI. This system offered the
user a range of around 2,000 nautical miles and accuracy
rivaling that of today's GPS.
With the arrival of microprocessors, LORAN-C
became attractive to aircraft owners. The combination
of the microprocessor and database started a navigation
revolution. During the 1980s, LORAN-C was the thing
to have in an aircraft and was capable of anything except
precision approaches. Since the original purpose of
LORAN-C was to serve ocean-going vessels, all of the
coverage areas focused on coasts. As a result, the middle
of the United States wasn't covered well. Not long before
LORAN-C was shut down, engineers had filled in the
middle of the country, and aircraft could use the system
to navigate from coast to coast. LORAN-C in the United
States shut down in February 2010.
At roughly the same time period as LORAN, the
United States developed the military tactical navigation
system. TACAN used pulses broadcast from a TACAN
station. The receiving aircraft could use these pulses

to determine a bearing to the TACAN station and its
location on a line from the TACAN station. In addition,
the TACAN system in the aircraft could transmit to
the TACAN station and get a reply. By measuring the
length of time it took to receive the reply, the aircraft
could determine its range from the station. Avionics
manufacturers built relatively low-cost radios called
distance measuring equipment, which could interrogate
the TACAN and get a range readout. This was extremely
useful because many TACAN stations were located on the
same site as a VHF Omnirange transmitter. As a result,
civilian aircraft could also determine both range and
bearing to a navigation station.
This system was useful, and the government
installed devices called DME transponders at other
VOR locations. These systems are still in use today.
Sophisticated navigation systems such as those found in
flight management systems on larger aircraft can create
complicated navigation solutions by scanning several
DME or TACAN stations and measuring the time from
each. By measuring the times, the system can determine
its distance from each of the stations, and navigate, much
like LORAN-C.
DME and TACAN work on UHF around 1 GHz. These
frequencies will only travel in a line of site, so from an
aviation standpoint, they are short range. They can't cover
thousands of square miles like LORAN-C could.
Notable is the brief appearance of DME/P, which came
about in the 1980s as a part of the microwave landing
system. Although DME/P used the same frequencies as
DME and TACAN, the DME/P system utilized lessons
learned in the development of LORAN-C, namely
teardrop-shaped pulses. The result was an improvement
in accuracy from 600 feet to 60 feet. MLS and its
Continued on following page
avionics news

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january

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Avionics News January 2021

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Avionics News January 2021 - Cover2
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