| If you watch much television, you probably | | | | signals. |
| believe that GPS (Satellite Navigation) was | | | | The receivers for aircraft, ships, ground vehicles, |
| invented by General Motors, which is not the | | | | and for portable units provide information in the |
| truth. They have just capitalized on it and sold a | | | | three dimensions. However, four satellites are |
| lot of cars because of it. | | | | required to since they compute three positions as |
| The Department of Defense created this | | | | well as time. The receivers at reference locations |
| navigating tool for use by the US military. It | | | | provide corrections and relative positioning data |
| provides specially coded satellite signals that can | | | | making precise positioning possible. |
| be captured and read by an appropriate receiver. | | | | Since the 1980s, civilian users worldwide have |
| There are many thousands of non-military, | | | | been able to use the navigation services without |
| non-government GPS users in the world today | | | | charge or restriction. Today, the receivers that |
| even though it wasn’t created for that use. A | | | | civilians use are extremely accurate. |
| GPS receiver can compute position, velocity, and | | | | Some facts about GPS satellites: |
| time. Position may be computed in three | | | | 1.Each satellite weighs about 2,000 pounds. |
| dimensions. | | | | 2.Each satellite is approximately 17 feet across. |
| The constellation consists of 24 satellites that | | | | 3.In 1994, 24 satellites were put together to form |
| orbit the earth in 12 hours. Sometimes there are | | | | the first constellation. |
| more than 24 when new ones are launched to | | | | 4.1978 was the year of the first launching of a |
| replace older ones. These satellites are powered | | | | GPS satellite. |
| by solar energy but have backup batteries so | | | | 5.Satellites are replaced about every 10 years. |
| they will continue to operate in the event of a | | | | 6.Power for the transmitter is only 50 watts. |
| solar eclipse. | | | | While the system is very accurate, some factors |
| The orbits repeat almost the same ground track | | | | can degrade the signal. Passing through the |
| as the earth turns beneath them once each day. | | | | atmosphere slows a satellite a little bit; the |
| The altitude is planned to that the satellites repeat | | | | system has a built-in correction for this. The signal |
| the same track and configuration of any particular | | | | can reflect off of tall buildings or large rock |
| point 24 hours less 4 minutes each day. There | | | | surfaces. A receiver’s clock may not be as |
| are six orbital planes spaced 60 degrees apart and | | | | accurate as the atomic clocks onboard the |
| with a fifty-five degree incline with respect to the | | | | satellites. Sometimes there are inaccuracies in the |
| equatorial plane. The constellation makes possible | | | | reported location. The more satellites a receiver |
| between five and eight views from any point on | | | | can see, the more accurate the signal. GPS units |
| earth. | | | | don’t work indoors, underwater, or |
| Schriever Air Force Base in Colorado houses the | | | | underground. |
| master controls, which measure signals from the | | | | The system is constantly being upgraded and |
| satellites and become orbital models for each | | | | improved and more and more commercial |
| satellite. The models calculate orbital data and | | | | receivers are being sold and installed in vehicles. |
| satellite clock corrections, which the master | | | | No longer must you purchase a General Motors |
| control uploads to the satellites. They then send | | | | car to have your own GPS receiver. |
| subsets of that date to GPS receivers over radio | | | | |