The Global Positioning System is playing a very important role in the Cosmic Ray Telescope Project. It has two purposes in the project: determine the positions of the detectors and the arrival time cosmic rays.
By using the GPS, we can determine the positions of our detectors to within a few meters and record our arrival times to about 100 nanoseconds.
Without the GPS, it is not possible to run this project in high school for two reasons.

1. The relative positions of the detectors are very hard to determine.
2. It is not possible to determine the arrival time accurate enough.

There are more than one Global Positioning Systems in the world. Most of them are developed for military purposes. The most popular one is developed and controlled by the U. S. Department of Defense. It was developed for military purposes and released to civil users in recent years. The Global Positioning System can be divided into three segments.

1. Space Segment: The space segment consists of more than 24 GPS satellites. All of them emit radio signals constantly such that Global Positioning is possible by decoding these signals. More than eight of the satellites can always be seen from any place and on the earth if there is no obstacle in between. At least four of them must be detected in order to determine the position and time.

2. User Segment: Our user segment consists of the Motorola's UT Plus Oncore Timing Module and Oncore Timing 2000 Antenna. The antenna receives the radio signals from the GPS satellites, amplifies them and send them to the timing module. The timing module decodes the radio signals and determine the position of the antenna. Beside of the position information, the timing module can also determine the time to an accuracy of 50 nanoseconds and a timing signal is available at the begining of each second.
     
3. Control Segment: The control segment consists of a master control station and a few monitor stations located in USA. The positions of the satellites are determined by the signals received at the monitor stations. The master control station will then upload the necessary corrections to the satellites.

Each GPS satellite has an atomic clock and computers in it such that it can track its position and time accurately. In addition, any deviation in position and time will be corrected by the information uploaded from the master control station. These informations (together with a timing signal) are emitted to the user segment at the beginning of each second.
If the position of the user segment is known, the user segment can determine the time by adding the time delay of the radio signal to the timing signal received from the GPS satellites. In principle, one GPS satellite is enough to determine the time to an accuracy of about 110 nanoseconds. The inaccuracy is mainly due the deviation of the satellite tracks from the broadcasted position. It can be improved if more GPS satellites are detected and the accuracy is about 50 nanoseconds when eight satellites are detected.

It is possible to know the position of the user segment in advance such that the time can be determined by using only one satellite. It is not true the other way around. The timing accuracy needed for GPS positioning has to be better than 100 nanoseconds. This is not possible even with an atomic clock which has an error of 2 microsecond in about a year.
Therefore, the time must be determined together with the position. There are four variables have to be determined: x, y, z, t (or latitude, longtitude, gps height and time). At least four satellites have to be detected in order to determine these four variables. The position accuracy is about 10 meters when eight satellites are detected. This accuracy can be improved by taking the time average if the user segement is not moving.