GPS or Global Positioning Systems
Part Two

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 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 on board 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.

Other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS) include:

• 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.

What is the signal?

• GPS satellites transmit two low power radio signals, designated L1 and L2.
• Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band.
• The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.
H3
• a pseudorandom code, ephemeris data and almanac data.
• The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information.
• Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time.
• This part of the signal is essential for determining a position.
• The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day.
• Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.