GPS
Time
GPS time from the 24 orbiting satelites
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GPS ( Global Positioning System ) time | |
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Accurate time is transmitted around the world using satellite navigation technology, with the Global Positioning System (GPS) as the foremost example. This is a system of 24 satellites orbiting the Earth, each with several synchronised atomic clocks on board. Effectively, it is a highly accurate time-transfer system available to anyone with the technology to use it. At any moment on any point on Earth at least 4 satellites can be seen. A signal from one satellite is enough to determine the time accurately, however, signals from four satellites are necessary to calculate time and positional information for navigation. The accuracy of time signals from GPS is limited to ±340 nanoseconds (where 1 nanosecond = 0.000 000 001 seconds) by a deliberate distortion of the satellite signal (for military security) called Selective Availability. The clocks that make up the world time system are compared by using GPS satellites as 'transfer' standards. Laboratories in the same region measure the time difference between themselves and individual GPS satellites at the same instant. By taking account of the signal delays, these measurements can be used to calculate the time difference between laboratories with an accuracy of approximately ±3 nanoseconds. Current research is directed at reducing the time transfer errors even further, with the promise of improvements by factors of 100 to 1000, by future atomic clocks based on the 'caesium fountain' and possibly even 'ion trapping' techniques. UTC is designed to be a compromise between the time defined by atomic clocks, and the time based on the earth's rotation about its axis. While the seconds of UTC are counted by atomic clocks, allowance is made to keep UTC within 0.9 seconds of Earth's rotation- time by inserting leap seconds (to take account of the speeding up or slowing down of the Earth) at the end of each quarter. Twenty two leap seconds have been added between January 1st 1972 and January 1st 1999 either at the end of June or December. Without the addition of leap seconds, the sun would be seen overhead at midnight (rather than noon) after approximately 50 000 years. Acknowledgements NPL, National
Physical Laboratory
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