GPS Time
GPS time from the 24 orbiting satelites

GPS time server

Time sync products

Accurate time is transmitted around the world using satellite navigation technology, with the Global Positioning System (GPS) as the foremost example. This works by a worldwide MEO satellite navigational system formed by 24 satellites orbiting the earth and their corresponding receivers on the earth. The satellites orbit the earth at approximately 12,000 miles above the surface and make two complete orbits every 24 hours. The GPS satellites continuously transmit digital radio signals that contain data on the satellites location and the exact time to the earth -bound receivers. The satellites are equipped with atomic clocks that are precise to within a billionth of a second.

Based on this information the receivers know how long it takes for the signal to reach the receiver on earth. As each signal travels at the speed of light, the longer it takes the receiver to get the signal, the farther away the satellite is. By knowing how far away a satellite is, the receiver knows that it is located somewhere on the surface of an imaginary sphere centered at the satellite. By using three satellites, GPS can calculate the longitude and latitude of the receiver based on where the three spheres intersect. By using four satellites, GPS can also determine altitude. 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.

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.

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