Avionics News January 2021 - 50

A look inside a panel-mounted DME

THEORY & PRACTICE
Continued from page 49

subsystem, DME/P, died as a result of the promises
made by the king of time-based navigation - the global
positioning system.
The United States Department of Defense began
developing a navigation system with timing and ranging
(NAVSTAR) in the 1970s. The DoD launched its first
NAVSTAR satellite in 1978. The NAVSTAR system became
known as GPS, which became fully operational in 1993.
The timing was perfect for the aviation industry.
Already, thanks to LORAN-C, we had databases and
microprocessors. GPS promised much greater accuracy than
LORAN-C and could be used anywhere on the globe with a
view of the sky. Overnight, LORAN-C was obsolete. Even
though GPS-based precision approaches were decades in
the future, deployment of MLS came to a stop.
At a basic level, GPS is LORAN-C with stations located
in the sky instead of on the ground. One of the things
improving accuracy is the fact that several satellites are
viewable at any given time, whereas an aircraft would only
receive three LORAN-C stations simultaneously. Thanks
to faster and better microprocessors, GPS has the ability
to keep track of the location of all the satellites, which is
something LORAN-C never had to do. LORAN-C stations
never moved.
Keeping track of satellite locations is quite a job.
Satellites react to a number of forces, which will move
them about slightly in their orbits. Real-time information
about actual satellite locations is collected into a
document known as GPS almanac. A GPS receiver must
download almanac information constantly. GPS users may
notice that when the aircraft has been parked for a while,
the GPS takes longer to lock on. This is because it must

download more of the almanac before it can measure time
differences accurately and determine its position. If the
unit has been turned off for less than two hours, position
determination seems nearly instantaneous.
Like LORAN-C, GPS receivers measure time
differences between the pulses received from satellites,
plot lines of position, and determine our position in
latitude and longitude. Then the position can be plotted
on an air chart, approach chart, or a road map.
Today, GPS is one of several global satellite navigation
systems. The Russians use GLONASS; the Europeans
have Galileo; the Chinese have BeiDou; the Japanese
have QZSS; and the Indians have NavIC.
Surprisingly, another LORAN system is in
development and will be called eLORAN, with the " e "
for enhanced. Expect the chains and coverage to be
similar to the LORAN-C of the past, but eLORAN will
support enhancements similar to WAAS and LAAS. All
of these systems use time to navigate, and today, using
any of these satellite navigation systems, both latitude
and longitude is easy to determine. It's all about time. q

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50

avionics news

*

january

2021


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Avionics News January 2021

Table of Contents for the Digital Edition of Avionics News January 2021

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Avionics News January 2021 - Intro
Avionics News January 2021 - No label
Avionics News January 2021 - Cover2
Avionics News January 2021 - 1
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Avionics News January 2021 - Cover3
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