1.Locate the Satellites
The Global Positioning System (GPS) comprises at least 24 satellites
orbiting the Earth twice a day at an altitude of about 12,500 miles and
an inclination of about 55 degrees. Each satellite, equipped with a
highly accurate atomic clock, beams out three pieces of information
every millisecond: a code identifying which satellite it is, its
location and the exact time.
2.Pick Up a Signal
Back on Earth, the GPS unit in your hand or your dashboard doesn’t
broadcast tracking information about where it is. Instead it receives
the satellites’ data, which is sent through low-power radio waves. (It
takes about .06 second for the receiver to get a transmission from a
satellite directly overhead.) The receiver has an internal quartz clock
that, on its own, is less accurate than the satellites’ atomic clocks,
but it maintains accuracy by constantly resetting itself based on time
data it receives from multiple satellites, enabling it to make a
precise calculation of the time delay of the signal.
3.Do the Math
This is crucial, because the receiver calculates the distance to a
given satellite based on the exact length of that delay (how long it
took the signal to get there), multiplied by the speed of light, which
is the speed at which radio waves travel.
4.Translate Your Position
Once the receiver knows how far it is from the satellite, it can
calculate a sphere that represents all the locations where it, the
receiver, could possibly be located—a process called trilateration. To
figure out precisely where it actually is, the receiver needs to find
the point of intersection of those spheres from at least three
satellites (more satellites equals more accuracy). Depending on how
it’s programmed, the GPS unit translates that position into latitude
and longitude coordinates, or onto a street map preloaded in its
memory. The whole process happens once per second, which is why it can
steer you down a highway at 70 mph.
PICKING YOUR SPOT
A GPS reciever finds its location by measuring its distance
from each
of at least three satellites—which define three spheres—and then
calculating the one point on Earth where the spheres intersect.
NAVIGATION AROUND THE WORLD
GPS refers only to the U.S. system of satellite navigation, the only
positioning signal available to the public. Other countries are
following suit with their own systems, but the technology will be
nearly identical.
Name: GPS Country: U.S. First Launched: 1978 Number of Sats: 24 What's It For?
Initially launched by the Department of Defense for military
applications, the signal was opened in 1996 for civilians anywhere in
the world to use. Improved satellites and a better civilian signal were
added to the system in 2005.
Name: Galileo Country: European Union First Launched: 2005 Number of Sats: 30 What's It For?
When completed in 2010, Galileo will be an open system free from
military control. Most Galileo receivers will also receive GPS signals,
increasing precision in northern latitudes and urban areas where errors
are common.
Name: Beidou Country: China First Launched: 2000 Number of Sats: 35 What's It For?
The Chinese government invested in the Galileo project but recently
announced that it would open the formerly military-only Beidou system
for civilian use, potentially damaging Galileo’s economic prospects in
the Far East.
Name: GLONASS Country: Russia First Launched: 1982 Number of Sats: 24 What's It For?
GLONASS (“Global Navigation Satellite System”) was originally a
Soviet-era military project, but the Russian government plans to expand
and upgrade the system, including opening the signal for civilian use
this year.
