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Global Navigation Satellite Systems (GNSS)
Published in Leonid Nadolinets, Eugene Levin, Daulet Akhmedov, Surveying Instruments and Technology, 2017
Leonid Nadolinets, Eugene Levin, Daulet Akhmedov
Satellite Navigation Systems employ satellites orbiting high above the Earth and which are spread out in such a way that there is line-of-sight connections to at least four satellites from any point on the ground. Each one of these satellites is equipped with onboard atomic clocks. Atomic clocks are the most precise time measurement instruments known, losing a maximum of one second every 30,000 to 1,000,000 years. In order to make them even more accurate, they are regularly adjusted or synchronized from various control points on Earth. GNSS satellites transmit their exact position and onboard clock time to Earth. These signals are transmitted at the speed of light (300,000 km/s) and therefore require approximately 67.3 ms to reach a position on Earth’s surface directly below the satellite. The signals require another 3.33 s for each additional kilometer of travel. To establish the position, all that is required is a receiver and an accurate clock. By comparing the satellite’s signal arrival time with the onboard clock at the moment of signal transmission, it is possible to determine the signal travel time (Figure 6.5).
Clock synchronisation: the establishment of Coordinated Universal Time (UTC) through the work of Louis Essen
Published in The International Journal for the History of Engineering & Technology, 2023
The atomic clock is a frequency generator of exceptional accuracy and stability that can also function as a clock. Sixty years ago, the invention of the atomic clock enabled the International Bureau of Weights and Measures (BIPM) in Paris to approve a change to the international system of measurement (SI) whereby the unit of time based on the solar system and astronomy was replaced by a definition based on the atom and physics.3,4