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EXAMINING TECHNOLOGY AND PUTTING IT TO USEI
n the immortal words of Buckaroo Bonzai,
“Wherever you go, there you are.” But if
you want to know precisely where “there”
is, you need a GPS device. Let’s examine how
this technology operates.
The fundamental idea of a satellite-based
navigation system was conceived prior to
Word War II, but no one pursued the idea
aggressively until the Russians launched
Sputnik, the fi rst artifi cial satellite. Research
continued through the 1960s, and the U.S.
Department of Defense settled on the fi rst
design in 1973.
The fi rst developmental GPS satellite—
Navstar 1—was launched in 1978, the fi rst
fully operational GPS satellite was put into
orbit in 1989, and the system was declared
fully operational in 1995. Although GPS
remains an indispensable military tool (and
is maintained by the U.S. Department of De-
fense), the technology was made available toconsumers in the 1980s and can now be found
in relatively inexpensive devices ranging from
cellphones and PDAs to dedicated handheld
GPS receivers.THE INFRASTRUCTURE
The Global Positioning System consists of three
segments: a network of satellites (24 in the
original system, 31 today) orbiting 12,600
miles above Earth (the
space segment), a series
of ground stations (the
control segment), and
individual GPS receivers
(the user segment). The
satellites are positioned
in space so that a GPS
receiver anywhere in the world can receive
signals from at least four simultaneously
(i.e., at least four satellites are above the
horizon at any point on the planet). We’llexplain the importance of having access to
four satellites at the same time shortly.
Each satellite transmits two coded radio
signals, designated L1 and L2, to Earth. The L1
signal operates at a frequency of 1,575.42MHz,
and the L2 signal operates at 1,227.60MHz.
These signals are of low power (between 20
and 50 watts each), and they travel by line
of sight, which means they can pass throughclouds, glass, or plastic on their way to a
receiver, but they’re obstructed by more solid
objects, such as buildings and mountains.
The L1 signal contains two pieces of infor-
mation: a coarse-acquisition code (a pseudo-
random number that identifi es a particular
satellite) and a navigation message. A pseudo-
random number exhibits all the properties of
a random sequence, but it’s actually generated
by a complex algorithm and can therefore be
repeated. The L2 signal contains an encrypted
precision code that can be decrypted only by
military-grade GPS receivers. The navigation
message in the L1 signal contains the date and
time the signal originated, information related
to the satellite’s status and health, ephemeris
data (the satellite’s precise location at a given
time, which the receiver uses to calculate the
satellite’s exact position based on the speed at
which the satellite is traveling and the current
time), and almanac data (coarse orbital param-
eters for all the satellites in the constellation).
Ephemeris data is highly detailed and
is considered valid for only four hours
after receipt; almanac data is more general
and remains valid for 180 days after being
downloaded to the receiver. The receiver
uses almanac data to determine which
satellites it should search for, based on the
current time and their last known position
(as reported in the almanac).How satellite technology pinpoints your location anywhere
on Earth —MICHAEL BROWN
The Global Positioning System
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The Global Positioning System
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HOW IT WORKSUsing a GPS to Pinpoint Your Location
EACH SATELLITE’S FLIGHT PATH
IS MONITORED BY A NETWORK
OF SIX U.S. AIR FORCE STATIONS
LOCATED AROUND THE WORLD.If the GPS receiver calculates that it is 13,000 miles from one satellite, it knows that it is located
somewhere on an imaginary sphere with a radius of 13,000 miles. The satellite is in the center of this
sphere, and the receiver is at the outer edge. The receiver then measures its distance from the other
two satellites and generates two more imaginary spheres. The receiver will be located at the precise
point at which all three spheres intersect. A GPS receiver able to communicate with a fourth satellite
can determine your current altitude.If the GPS receiver calculates that it is 13,000 miles from one satellite, it knows that it is located
somewhere on an imaginary sphere with a radius of 13,000 miles. The satellite is in the center of this
sphere, and the receiver is at the outer edge. The receiver then measures its distance from the otherIf the GPS receiver calculates that it is 13,000 miles from one satellite, it knows that it is located
somewhere on an imaginary sphere with a radius of 13,000 miles. The satellite is in the center of thisX
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