What makes up the GPS 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
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 the GPS satellites
(also called NAVSTAR, the official U.S. Department of Defence name for
GPS):
- The first GPS satellite was launched in 1978.
- A full constellation of 24 satellites was achieved in 1994.
- There are 27 satellites in orbit in total, 3 of which are back
up units
- 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.
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