The atomic clocks onboard the GPS satellites are synchronized to within nanoseconds of each other. Precise time measurements are also essential to radio navigation systems like the Global Positioning System (GPS). Fractional disparities in times between a space probe and ground-based tracking stations can dramatically affect the control and position of spacecraft. Scientific organizations such as NASA depend on reliable and consistent time measurement for projects such as interplanetary space travel and transmissions. Precise time measurements are also essential for accurate navigation and the support of communications on Earth and in space. Mobile phone base stations must have stable and accurate oscillators in order to handle the massive amount of data being transmitted and received. Radio and television stations require both precise time-of-day and frequency in order to broadcast programs. Power companies use precise time to regulate power system grids and reduce power losses. Synchronization between two or more locations is necessary for high-speed communication systems, banking and stock transactions and transmitting everything from e-mail to sonar signals in a submarine. Precise time and frequency synchronization have many uses in everyday life. Other physical quantities, like the volt and meter, also rely on the definition of the second as part of their own definitions. This definition makes the cesium oscillator (sometimes referred to generically as an atomic clock) the primary standard for time and frequency measurements. Since 1967, the International System of Units (SI) has defined the second as the period equal to 9,192,631,770 cycles of the radiation, which corresponds to the transition between two energy levels of the ground state of the Cesium-133 atom. The best cesium oscillators (such as NIST-F1) can produce frequency with an uncertainty of about 3 x 10 -16, which translates to a time error of about 0.03 nanoseconds per day, or about one second in 100 million years. If an atomic clock was off by 1 Hz and the frequency was 1 GHz (1 billion Hz), then it would be off by one second in 31.7 years or, roughly, 86 microseconds (0.000086 s) per day. The benefits of an atomic clock are that the resonant frequencies are natural properties (not human-made) and that they are very high frequencies, in the billions of Hertz. In this case, the generated frequency is the 'tick'. If the correct frequency can be generated to make the atoms change, then that frequency can be counted or divided down and compared. With an atomic clock, there is a natural tendency of atoms to change energy levels when they are exposed to very specific ("resonant") frequencies. If the quartz frequency on a watch is 0.1 Hz off, the watch will be off by one second in 327,680 seconds or, roughly, 0.26 seconds per day. For instance, if a half-swing of a pendulum is actually 0.1 Hz off, then the grandfather clock will be off by one second in ten seconds. By dividing a high frequency down to a low frequency, the accuracy can be increased. The ticks are used to advance the seconds on the clock. The frequency is "counted" by dividing it by 32,768 to equal one second 'ticks'. With a quartz clock (like most wristwatches), a piece of quartz crystal is cut and used in an electronic circuit where it vibrates at a certain frequency (usually 32,768 Hz). One cycle per second equals 1 Hertz (Hz). The arm of the pendulum is adjusted in order to make each half-swing take one second. For example, the pendulum in a grandfather clock swings back and forth at the same rate, over and over, and the gears "count" the swings. What are some sources for further reading about clocks and timekeeping?Ĭlocks work by counting a periodic event with a known frequency. What are Julian Date and Modified Julian Date (MJD)? What is the origin of hours, minutes and seconds? How are stopwatches and timers calibrated? Why must time and frequency be measured so precisely? Stopwatch calibrations, calendars and history. Cesium clocks, why we need precise time and frequency.
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