Most types of clocks rely on the oscillation of a slid body, be it a pendulum, a balance-wheel, or a quartz crystal, but each suffers from the effects of temperature, pressure, and gravity. Time measuring devices depended on the spin of the earth, but these suffer from seasonal effects and tidal friction. The moon causes tides to occur on earth and it causes friction between moon and the earth. This friction slows down the earth’s rotation by few milliseconds. This is called tidal friction. The atoms, however, vibrate a fixed number of times per second. Both the U.S. National Bureau of Standards and the United Kingdom’s National Physics Laboratory tried to take advantage of these vibrations.
In 1949 the Americans built a quartz clock that was synchronized by the 24-GHz vibrations of low pressure gaseous ammonium molecules. The British, under the leadership of physicist Louis Essen (1908-1997), used the oscillations of an electrical circuit synchronized to the vibrations of caesium atoms, the first caesium was kept in a tunable microwave cavity and the clock relied on the fact that were 9,192,631,770 transitions between two hyperfine ground state energy levels every second. This number defined the second, as opposed to the old definition of there being 86,400 seconds in one day. A good atomic clock was accurate to one part in 1,014, and therefore would take about 3 million years to lose or gain a second.
Four atomic clocks are used in each of the many satellites of the global positioning system and comparisons of electromagnetic-wave travel times enable positions of earth to be measured very precisely. The clocks are also used by geophysicists to monitor variations in the spin rate of earth, and the drifting of the continents. Since record began, earth recorded the shortest day on July 19, 2020, when the day was 1.4602 milliseconds shorter than 24 hours.
Why atomic clocks is used in GPS?
The Global Positioning System (GPS) consists of 24 satellites orbiting the earth. A GPS receiver uses the position of four of these satellites to locate itself. One to correct the time on the receiver, and three to locate its position. A signal is sent to the receiver from the first satellite that contains the satellites location and the signal’s time of departure. The receiver then multiplies the signal’s travel time by the speed of light to calculate its distance from the satellite. With one satellite the receiver knows that it’s located o a sphere around that satellite with a radius equal to the calculated distance. So, it does the same calculation with a second satellite. The intersection of these two spheres narrows the location to the circumference of a circle. Then with a third satellite, the receiver can reduce the location to a single point. Since signals are travelling at the speed of light, being off by even a millisecond means an error off about a million feet, or 300 kilometres. But with atomic accuracy, the receiver can locate itself to about 3 feet. Global Positioning System (GPS) satellites fly in medium earth orbit (MEO- Medium Earth Orbit) at an altitude of approximately 20,200 kilometres from ground.
The NIST-F1 is one of the most accurate time standards based on microwave atomic clocks. The most accurate atomic clocks lose about a second over 138 million years.
“Time isn’t the main thing. It’s the only thing.” – Miles Davis