Precise time measurements are also essential for accurate navigation and the support of communications on earth and in space. Scientific organizations such as NASA depend on reliable and consistent time measurement for projects such as interplanetary space travel. Fractional disparities in times between a space probe and tracking stations on Earth can dramatically affect the positions of spacecraft. By synchronizing the satellite clocks within nanoseconds of each other, it makes it possible for a receiver to know its position on earth within a few meters. Other physical quantities, like the volt and meter, also rely on the definition of the second as part of their own definitions.
Atomic clocks are quite complex, but the basic theory is simple.
Like all clocks, they are intended to make the same event happen over and over. The repetition of this event produces a frequency, which is intended to be as stable as possible. For example, the pendulum in a grandfather clock swings back and forth at the same rate, over and over. The swings of the pendulum are counted to keep time. In a cesium oscillator, the transitions of the cesium atom as it moves back and forth between two energy levels are counted to keep time.