Avionics News January 2021 - 48

series
B Y

T H O M A S

I N M A N

It's about time
N

avigators have been using time to navigate
since the beginning of navigation. First, in
the form of deduced reckoning, or as we
call it today, dead-reckoning. If one can calculate their
speed and keep track of how long they have traveled,
they can tell how far they've gone. Dead-reckoning has
worked extremely well for pilots, and it is still part of the
private pilot curriculum.
Navigators can determine latitude easily by looking at
the stars. Longitude isn't so easy. Early mariners would
keep track of time at the prime meridian, which now passes
through Greenwich, England. They would take note of the
difference in time, as measured from the sun, between the
time in Greenwich and the time at their location. By doing
some math, they could calculate their longitude.

A collection of GPS units

48

avionics news

*

january

2021

Fast forward to the 20th century and navigators had
more time-based options. One of the first options was
Decca, which the United States developed, and the
United Kingdom implemented in the 1940s. Strictly
speaking, Decca wasn't time based as much as phase
based. Still, a phase change of a signal is a change in
time. The range and accuracy of the Decca system varied
depending on the time of day and radio propagation
characteristics. During daylight, the system had a useable
range of approximately 400 nautical miles. After the sun
set, the range could decrease to as much as 200 nautical
miles. Decca was more accurate during the day than at
night, and more accurate when the navigator was close
to the transmitting stations. Decca used radio frequencies
between 70 kHz and 128 kHz. The system lasted with
various improvements until this century when the last
Decca system shut down in March 2001.
Another system of the same age is known as Gee.
Developed during World War II, Gee measured the
time delay between two signals. The Gee system was
developed for aviation and was accurate to within 200
meters at a range of over 300 miles. It operated on
frequencies between 20 MHz and 85 MHz. For large,
unmoving targets, Gee offered enough accuracy for
bombers to strike without using a bombsite. A Gee
system master station would send a pulse, which would
trigger a secondary or slave station to send its own pulse.
The navigator would receive both pulses, measure the
time difference between them, and determine how far the
aircraft was from both stations. These distances would be
represented on a map as hyperbolic lines. Typically, the



Avionics News January 2021

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Avionics News January 2021 - Intro
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Avionics News January 2021 - Cover2
Avionics News January 2021 - 1
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