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GPS Frequently Asked Questions
Mar 12, 2007
What is GPS?
The Global Positioning System (GPS) is a
satellite-based navigation system made up of a network of 24 satellites
placed into orbit by the U.S. Department of Defense, and their ground
stations. GPS was originally intended for military applications, but in
the 1980s, the government made the system available for civilian use.
GPS works in any weather conditions, anywhere in the world, 24 hours a
day.
GPS uses these satellites as reference points to
calculate positions accurate to a matter of meters. In fact, with
advanced forms of GPS you can make measurements to better than a
centimeter!In a sense it's like giving every square meter on the planet
a unique address.
GPS receivers have been miniaturized to
just a few integrated circuits and so are becoming very economical. And
that makes the technology accessible to virtually everyone.
These days GPS is finding its way into cars, motorcycles, boats,
recreational vehicles, planes, construction equipment, and now, even
people for medical emergencies.
How it works?
GPS satellites circle the earth twice a day in a very precise orbit and
transmit signal information to earth. GPS receivers take this
information and use triangulation to calculate the user's exact
location. Essentially, the GPS receiver compares the time a signal was
transmitted by a satellite with the time it was received. The time
difference tells the GPS receiver how far away the satellite is. Now,
with distance measurements from a few more satellites, the receiver can
determine the user's position and display it on the unit's electronic
map.
A GPS receiver must be locked on to the signal of at
least three satellites to calculate a 2D position (latitude and
longitude) and track movement. With four or more satellites in view,
the receiver can determine the user's 3D position (latitude, longitude
and altitude).
How accurate is GPS?
Today's
GPS receivers are extremely accurate, thanks to their parallel
multi-channel design. They are quick to lock onto satellites when first
turned on and they maintain strong locks, even in dense foliage or
urban settings with tall buildings. Certain atmospheric factors and
other sources of error can affect the accuracy of GPS receivers, but in
general, GPS receivers are accurate to within 15 meters on average.
The GPS satellite system
Here are some other interesting facts about the GPS satellites (also
called NAVSTAR, the official U.S. Department of Defense name for GPS):
The first GPS satellite was launched in 1978.
A full constellation of 24 satellites was achieved in 1994.
Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
Transmitter power is only 50 watts or less.