Gps-monitoring the World

By: David Wood

If you watch much television, you probably believe that GPS (Satellite Navigation) was invented by General Motors, which is not the truth. They have just capitalized on it and sold a lot of cars because of it.

The Department of Defense created this navigating tool for use by the US military. It provides specially coded satellite signals that can be captured and read by an appropriate receiver. There are many thousands of non-military, non-government GPS users in the world today even though it wasn't created for that use. A GPS receiver can compute position, velocity, and time. Position may be computed in three dimensions.

The constellation consists of 24 satellites that orbit the earth in 12 hours. Sometimes there are more than 24 when new ones are launched to replace older ones. These satellites are powered by solar energy but have backup batteries so they will continue to operate in the event of a solar eclipse.

The orbits repeat almost the same ground track as the earth turns beneath them once each day. The altitude is planned to that the satellites repeat the same track and configuration of any particular point 24 hours less 4 minutes each day. There are six orbital planes spaced 60 degrees apart and with a fifty-five degree incline with respect to the equatorial plane. The constellation makes possible between five and eight views from any point on earth.

Schriever Air Force Base in Colorado houses the master controls, which measure signals from the satellites and become orbital models for each satellite. The models calculate orbital data and satellite clock corrections, which the master control uploads to the satellites. They then send subsets of that date to GPS receivers over radio signals.

The receivers for aircraft, ships, ground vehicles, and for portable units provide information in the three dimensions. However, four satellites are required to since they compute three positions as well as time. The receivers at reference locations provide corrections and relative positioning data making precise positioning possible.

Since the 1980s, civilian users worldwide have been able to use the navigation services without charge or restriction. Today, the receivers that civilians use are extremely accurate.

Some facts about GPS satellites:

1.Each satellite weighs about 2,000 pounds.
2.Each satellite is approximately 17 feet across.
3.In 1994, 24 satellites were put together to form the first constellation.
4.1978 was the year of the first launching of a GPS satellite.
5.Satellites are replaced about every 10 years.
6.Power for the transmitter is only 50 watts.

While the system is very accurate, some factors can degrade the signal. Passing through the atmosphere slows a satellite a little bit; the system has a built-in correction for this. The signal can reflect off of tall buildings or large rock surfaces. A receiver's clock may not be as accurate as the atomic clocks onboard the satellites. Sometimes there are inaccuracies in the reported location. The more satellites a receiver can see, the more accurate the signal. GPS units don't work indoors, underwater, or underground.

The system is constantly being upgraded and improved and more and more commercial receivers are being sold and installed in vehicles. No longer must you purchase a General Motors car to have your own GPS receiver.

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