Device That Revolutionized Timekeeping Receives an IEEE Milestone (ieee.org)

An anonymous reader writes: The invention of the atomic clock fundamentally altered the way that time is measured and kept. The clock helped redefine the duration of a single second, and its groundbreaking accuracy contributed to technologies we rely on today, including cellphones and GPS receivers. Building on the accomplishments of previous researchers, Harold Lyons and his colleagues at the U.S. National Bureau of Standards (now the National Institute of Standards and Technology), in Washington, D.C., began working in 1947 on developing an atomic clock and demonstrated it to the public two years later. Its design was based on atomic physics. The clock kept time by tracking the microwave signals that electrons in atoms emit when they change energy levels. This month the atomic clock received an IEEE Milestone. Administered by the IEEE History Center and supported by donors, the milestone program recognizes outstanding technical developments around the world.

Comments

Re:Atomic clock operation

[msauve]

"Atomic clocks are based on the periodic decay of radioactive isotopes."

No, they aren't. Radioactive decay is random and would make a terrible clock. The second is defined in terms of a electron transitions of the cesium-133 atom, which is the only stable isotope. Hydrogen maser clocks are even more stable timekeepers, but again, not based on a radioactive isotope.

Re:More information

[taiwanjohn]

Well, I was going to mod you up +1 Informative, but then I notice the goatse link... Now, since I'm writing this, I can't mod you down either. But at least I can warn others to check the links before clicking. (Note: The rest of the links appear to be legit.)

Re:Atomic clock operation

[sl3xd]

Hydrogen maser clocks are even more stable timekeepers, but again, not based on a radioactive isotope.

Don't forget rubidium! - much cheaper than cesium or hydrogen, but also not as accurate. (Also not radioactive)

Hydrogen masers become unstable and drift ('aging effect') while Cesium is consistent. Applications where very accurate time over both the short and long term are required (such as Galileo GNSS) use a combination of Hydrogen and Cesium standards, using each to overcome the weakness of the other.

Cesium also benefits from the fact its oscillation is literally the definition of a second.

Re:Why do phone (cell) systems need accurate timin

[Strider-]

The GPS receiver in your phone (and GPS/GNSS systems in general) utterly depend on hyper-accurate timekeeping to work. In a nutshell, the orbiting satellites are transmitting perfect "true" time continuously, as well as information on its orbit. Your GPS receiver uses that information to work out the precise local time, and using that it can calculate the exact position of the satellite around the earth, the distance to each of the satellites, and thus its own position.

The ephemeris data (precise information on its orbit) transmitted by the satellites is only good for about 6 hours, before it has to be updated both on the satellite and on your phone, and takes time to download. In the GPS world, this is known as a "Warm Start" and can take up to a few minutes. If you've moved your device more than a few hundred km while off, or left it powered off for several days, it needs to download an entirely new Almanac, which takes up to 12 minutes (a cold start).

With assisted GPS, as used on cell networks, the phone will get the almanac and relevant ephemeris from the cell network, greatly improving the time to fix.

That's just the GPS side of things though... As others have mentioned, the actual radio interface is utterly dependent on precise timing in order to function. The cheapest and most reliable way to achieve this is with timing derived from GPS.

Of course, the software clock on a phone is completely independent of these two things, it's just that normally it gets its settings from GPS or the network.