| Global Positioning System (GPS) satellites send | | | | are repeated are transmitted at precisely the |
| signals all over the world to the GPS receiving | | | | same instant. However, though the signals are |
| equipment located on the ground. These GPS | | | | travelling at light-speed, they reach the GPS |
| receivers accept these satellite signals passively, | | | | receivers at slightly different times due to the |
| and do not transmit any of their own signals. | | | | distortion in distance between the receiver and |
| They require a view of the sky that is completely | | | | each of the individual satellites. The distance to |
| unobstructed. Therefore, they must be used only | | | | the GPS satellites can be calculated by estimating |
| outdoors, and they cannot perform with too | | | | how much time the signal has taken to reach the |
| much accuracy when they are surrounded by | | | | receiver from the satellite. Once the receiver has |
| trees or tall buildings. In such situations, their | | | | estimated the distance to at least four different |
| performance is notably depleted. GPS operations | | | | satellites, then it is able to determine a three |
| rely on the accuracy of time references, and so | | | | dimensional position. |
| they depend on the U.S. Naval Observatory | | | | At any given time, there are a minimum of 24 |
| atomic clocks. Furthermore, each of the GPS | | | | operational GPS satellites. Each of them is |
| satellites contains its own onboard atomic clock. | | | | operated by the U.S. Air Force, and orbits the |
| Among the data sent by GPS satellites in their | | | | earth about twice per day. The ground stations |
| signal is their current location and the accurate | | | | are utilized in order to track the orbit of each |
| time. All of the GPS satellites synchronize each of | | | | satellite with great precision. |
| their operations so that any of the signals that | | | | |