GLOBAL POSITIONING SYSTEM (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. 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. There are no
subscription fees or setup charges to use GPS.
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
A GPS receiver must be locked on to the signal of at least three satellites to calculate a
position (latitude and longitude) and track movement. With four or more satellites in view,
the receiver can determine the user's position (latitude, longitude and altitude). Once the
user's position has been determined, the GPS unit can calculate other information, such as
speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and
The GPS satellite system (NAVSTAR)
The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us.
They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling
at speeds of roughly 7,000 miles an hour.
GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in
the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep
them flying in the correct path.
Here are some other interesting facts about NAVSTAR:
- The first GPS satellite was launched in 1978.
- The number of satellites was increased to 24 in 1994.
- Each satellite is built to last about 10 years. Replacements are constantly being built
and launched into orbit.
- An average GPS satellite weighs around 2,000 pounds and is about 17 feet across with the
solar panels extended.
- Transmitter power is only 50 watts or