The Exciting World of GPS –
Part I
By Ron Whitelock
Where in the world are we? With GPS or a Global Positioning System, it
is easy to answer that question.
In our high-tech, modern world, you cannot escape the acronym GPS.
No matter your occupation or your station in life, GPS is everywhere: in
your car; in your cell phone; they may eventually be implanted in your
body at birth.
In public safety, we accept GPS as a context of our work-life. It is a tool to
support us to be more efficient and accurate at our tasks. So what is this
omnipresent technology we have come to call GPS?
GPS, for its modern sounding name, has deep roots. Long before satellites,
ancient travelers were able to navigate the world with acceptable accuracy – they got there; then were able to get back home. From Christopher
Columbus to James Cook, they developed methods to plot their travels.
These methods used a form of Latitude and Longitude.
Longitude
Vertical mapping lines on Earth are
lines of longitude, also known as
“meridians”.
Longitude lines are a numerical
way to show/measure how far
you are east or west of a line called
the Prime Meridian – which runs
vertically, north and south, directly
over the British Royal Observatory
in Greenwich England. As the vertical starting point for longitude,
the Prime Meridian is numbered 0
degrees longitude.
There are 180 vertical longitude
lines east and west and 180 vertical
longitude lines west of the Prime
Meridian dividing the Earth into
360 parts. Interestingly, going west
is negative number and east is a
positive number. (Now, who came
up with the concept?)
Any point on Earth can be represented by a Lat/Long value. For
example, the Lat/Long for Chicago,
IL equals 41.8500° N, 87.6500° W or
41.8500°, -87.6500°.
The Earth
Figure 1: Source - http://www.learner.org/jnorth/tm/LongitudeIntro.html (Credit
to Illinois State University)
Referring to Figure 1, Latitude and Longitude (Lat/Long for short) lines
are a grid map system. Unlike the ‘intersection of Main Street and Cross
Avenue’ that are straight lines of intersection, Lat/Long lines encircle the
Earth.
Latitude
Horizontal mapping lines on Earth are lines of latitude. They are known
as “parallels” of latitude, because they run parallel to the equator.
Latitude lines are a numerical way to measure how far north or south of
the equator a place is located. The equator is the starting point for measuring latitude. It starts at 0 degrees latitude. The larger the number the
further you are away from the equator. Plus 90 degrees (or North) places
you at the North Pole; similarly minus 90 degrees (or South) places you
at the South Pole.
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The Earth is not completely round.
It is 7,926 miles (12,715.43 km) in
diameter at the equator and 7901
miles at the poles. Each degree of
Latitude then is approximately
21.95 miles while each degree of
Longitude is 22.02 miles.
Thus, a four decimal place Lat/
Long value yields an accuracy of
approximately 12 feet or 3.5 m.
Greater accuracies are possible as
we will see later.
A Short History of GPS
From Wikipedia, “In 1956, the
German-American physicist Friedwardt Winterberg proposed a test of
general relativity (for time slowing
Continued on page 45
IMSA Journal
The Exciting World of GPS – Part I . . .
in a strong gravitational field) using accurate atomic clocks placed
in orbit inside artificial satellites.
(To achieve accuracy requirements,
GPS uses principles of general
relativity to correct the satellites’
atomic clocks) Additional inspiration for GPS came when the Soviet
Union launched the first man-made
satellite, Sputnik in 1957. Two
American physicists, William Guier
and George Weiffenbach, at Johns
Hopkins’s Applied Physics Laboratory (APL), decided on their own to
monitor Sputnik’s radio transmissions Within hours they realized
that, because of the Doppler effect,
they could pinpoint where the satellite was along its orbit from the
Doppler shift. The Director of the
APL gave them access to their UNIVAC to do the heavy calculations
required. The following spring,
Frank McClure, the deputy director
of the APL, asked Guier and Weiffenbach to investigate the inverse
problem – pinpointing the user’s
location given that of the satellite.
(The Navy was developing the
submarine-launched Polaris missile, which required them to know
the submarine’s location.) This led
them and APL to develop the Transit system. In 1959, ARPA (renamed
DARPA in 1972) also played a role
in Transit. The GPS project was
developed in 1973 to overcome the
limitations of previous navigation
systems, integrating ideas from
several predecessors, including a
number of classified engineering
design studies from the 1960s. GPS
was created and realized by the U.S.
Department of Defense (DoD) and
was originally run with 24 satellites. It became fully operational in
1994. Roger L. Easton is generally
credited as its inventor.
Advances in technology and new
demands on the existing system
have now led to efforts to modernize
the GPS system and implement the
next generation of GPS III satellites
July/August 2013
and Next Generation Operational
Control System (OCX). Announcements from the Vice President and
the White House in 1998 initiated
these changes.
In 2000, U.S. Congress authorized
the modernization effort, referred
to as GPS III. The first step in GPS
modernization took place in May
2000, when President Bill Clinton
directed the Department of Defense
to turn off the GPS Selective Availability (SA) feature.
Continued from page 44
for traffic signal timings; all make
use of accurate GPS data.
Consequently, it would be beneficial
to have a fundamental understanding of this data and how to use it
effectively. Part II in the next issue
will focus on how we get this location data and how it can be used to
make our work and lives easier.
Something to Think About!
SA was an intentional degradation
of civilian GPS accuracy, implemented on a global basis through
the GPS satellites. (Didn’t want the
competition to have too much information.) During the 1990s, civil GPS
readings could be incorrect by as
much as a football field (100 meters).
On the day SA was deactivated, civil
GPS accuracy improved tenfold,
benefiting civil and commercial users worldwide.
In 2007, the government announced plans
to permanently eliminate SA by building
the GPS III satellites
without the 2005 SA
feature.”
The GPS system will
continue to evolve.
www.gps.gov is a
wonderful website for
information.
GPS serves many
military and commercial purposes. It
has become embedded in our everyday
personal lives. Our
smartphones, our cars,
our commercial fleets,
our personal navigation system, our asset
management systems,
even the tools we use
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