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 users 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 users position and display it on the unit's electronic map.
How Accurate is GPS?
Today's GPS receivers are extremely accurate thanks to their parallel multi-channel
design. Certain atmospheric factors and other sources of error can affect the
accuracy of GPS receivers. GPS receivers are accurate to within 15 meters on
average. Newer GPS receivers with WAAS (wide area augmentation system) capability
can improve accuracy to less than 3 meters on average. No additional equipment
or fees are required to take advantage of WAAS. Users can also get better accuracy
with Differential GPS (DGPS), which corrects GPS signals to within an average
of 3-5 meters. The U.S. Coast Guard operates the most common DGPS correction
service. This system consists of a network of towers that receive GPS signals
and transmit a corrected signal by beacon transmitters. In order to get the
corrected signal, users must have a differential beacon receiver and beacon antenna
in addition to their GPS.
The GPS Satellite System
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 2 complete orbits
in less than 24 hours. These satellites are traveling at speeds of roughly 7,000
miles per 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 is no solar power. Small rocket boosters on each satellite keep them flying
in the correct path. 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.
- Replacement satellites are constantly being built
and launched.
- A GPS satellite is about 2,000 lbs. and about 17
ft. across.
- Transmitter power is only 50 watts or less.
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