| There currently are 24 operational satellites | | | | everything else have limited lifespan. The |
| as part of the GPS satellite constellation, | | | | GPS satellites have been designed to provide |
| that orbit the earth with a orbital radius of | | | | a lifespan of ~8 years. New satellites are |
| ~16,000 miles (from the center of the earth). | | | | being launched to space at regular intervals |
| The satellite orbits are spaced out in 6 | | | | of time to make up for satellites that have |
| orbital planes (orbital plane is the plane | | | | lived out their lifespan. |
| surface formed by the path traced by the | | | | |
| orbit of the satellite - it is nearly | | | | When the satellites are launched into space, |
| circular). The six orbital planes are | | | | they are inserted to pre-calculated positions |
| separated by an angle of 60 degrees. Refer to | | | | in their orbital planes. The pre-calculations |
| the constellation figure below. Four to six | | | | ascertain that a GPS receiver located |
| satellites occupy each orbital plane. | | | | anywhere on earth and at all times, can |
| | | | receive signals from at least four |
| In addition to the 24 operational satellites, | | | | satellites. This is required because earth |
| there currently are 6 satellites in orbit | | | | blocks the GPS signals from passing through |
| that are maintained as spares, should any of | | | | it. For a GPS receiver to be able to receive |
| the 24 operational satellites malfunction. | | | | a signal from a satellite, the straight |
| The six spare satellites are kept operational | | | | line-of-sight line should not have any |
| for users to receive signals from them just | | | | obstructions in between. Since the satellites |
| like from the 24 satellites. However, the | | | | are flying around the earth, not all |
| U.S. Government does not guarantee their | | | | satellites are visible to a GPS receiver |
| availability at all times. | | | | located somewhere on earth. Thus, though |
| | | | there are 30 operational satellites, not all |
| Remember that the GPS satellites are being | | | | of them can be visible at a location on |
| launched since 1974. Satellites like | | | | earth, at any given time. |