21 aprile 2009

WWV Fort Collins, uno sguardo sul tempo atomico

Un bell'articolo del sito del quotidiano Fort Collins Now offre un inedito ritratto divulgativo del funzionamento della stazione di tempo e frequenza campione WWV. Scritto per un pubblico generale, il pezzo di Rebecca Boyle è molto accurato e accompagnato da una splendida fotografia dell'antenna VLF di WWVB.

Atomic Synchronicity in Fort Collins

On a wind-swept field northeast of Fort Collins, a compound of computers, radios and 400-foot antennas — manned by a handful of dedicated engineers — transmits the precise time to everything from the stock market to traffic signals.
By Rebecca Boyle

Quick — look at your cell phone, or glance your watch. What time is it right now? Oh, really? How can you be so sure?
That timepiece has to base its claims on something. When you set it, you told your microwave or your watch what time it was. You got that time from a different clock somewhere else. Meanwhile, a little computer or a piece of quartz has been keeping track for you, but what does it actually track? What is a minute, anyway? What’s a second?
At some point, somebody had to decide what those things mean. And somebody has to keep an eye on that definition so seconds don’t get too short or too long, messing up everything from GPS satellites to the stock market to your paycheck. (Did you really work eight hours yesterday? According to whose clock?)
You can thank a bunch of radio engineers in Fort Collins for helping keep all that straight. Four engineers in a compound northeast of the city keep watch, as it were, over the official atomic clock, and therefore the official time, of the United States.
The antennas at radio stations WWV and WWVB broadcast radio waves that synchronize everything from the stock market to traffic lights to your alarm clock. WWV broadcasts a signal you can hear; WWVB broadcasts a signal only your radio-controlled clock will recognize. “Your computer clock has a crystal — it has the ability to keep your time,” said Douglas Sutton, an engineer with the National Institute of Standards and Technology. But, he pointed out, it has its limitations. Eventually, the clock will be fast or slow, and you’ll have to correct it. “Say, in one week, you lose a second,” Sutton said. “Well, it’s our job to make sure you can correct that back to the proper time, because you’re referencing the standard.”
The windswept home of the nation’s timekeepers — where that standard is maintained — seems like any other rural site, complete with antelope tracks through the snow. But that’s before you see the copper-lined doors, the flickering, unplugged fluorescent lights powered by ambient energy, and the massive antenna towers.
Before the proliferation of radio-controlled clocks — there are between 3 or 4 million such clocks in the U.S. — government agencies were the atomic clock’s main users. Civilian use started in the early 1990s, and now, radio-controlled clocks (commonly but inaccurately called “atomic”) are widely available, often for a few dollars. But they wouldn’t work — whether in Hilton Head, S.C. ,or Walla Walla, Wash. — without the radio towers based in Fort Collins.

The National Institute of Standards and Technology, formerly known as the National Bureau of Standards, keeps track of a lot of official standards — the pound, the ounce, the cup, the inch, and, of course, the second.
Though the main time-standard-setting atomic clock, NIST-F1, is in Boulder, the Fort Collins facility has four of its own Hewlett-Packard cesium clocks that synchronize to the main system.
The antennas that broadcast the time to clocks around the country are in a remote location, off Colo. 1 in eastern Larimer County. That was partly out of necessity; when they were built in 1962, the area was even less populated than it is now. “It would be hard to have this site in the middle of Boulder,” Sutton said.
The site was also chosen for its electrical properties. The ground chemistry, which includes several metals that are found with the uranium that lies a few miles east, allows for good conductivity; the chemical make-up and the high groundwater level helps the radio waves travel farther.
The NIST site encompasses 340 acres and is surrounded by private land. It’s not like living under power lines, but living next door to the NIST antennas has its own complications. “People will call us from Wellington and say, ‘My home entertainment system announces the time every minute,’” Sutton said. At least they can accurately program their DVD players.
Still, Sutton said it’s good that not too many people live around the antennas. There is plenty of wildlife, however. Partly because of the flora and fauna, the facility resembles a summer camp for circuit engineers, or even a survivalist compound.
In the main building, the engineers have a break room and kitchen, and message boards for notes and sketches. Last week, the chalkboard was full of intricate server schematics and a reminder to spray the salt cedars this spring. The rooms containing radio signal generators are full of stacked water bottles, which add extra mass to keep things cool.
The facility used to employ dozens of people, but funding cuts have reduced the staff down to Sutton, Matt Deutch, Glenn Nelson and Bill Yates. They all wear many hats — they’re electrical engineers, plow drivers, firefighters who conduct controlled burns, tower-climbers who shimmy up 400 feet to change light bulbs, and all-around renaissance men. Sutton has been at NIST for 14 years. And that’s an accurate measurement.

Like many developments in 19th-century America, standard time came about because of railroads. Until the mid-1880s, most cities and towns set their own times according to high noon — when the sun was at its highest point in the sky. But with the advent of railroads, people traveling from place to place needed a standard. Railroads came up with one in November 1883, and the following year, delegates from 27 nations met in Washington, D.C., for a Meridian Conference, to set the world’s time. They agreed on a system that’s basically the same as the one we still use: Time zones were divided along 24 standard meridians of longitude, 15 degrees apart. The standard starts in Greenwich, England; all other time zones are either ahead or behind by one hour (or a half-hour in some spots).
The U.S. government started keeping standard time a few years later. In 1904, the U.S. Naval Observatory broadcast time signals from Boston as an aid to navigation. By 1923, NIST station WWV was broadcasting over the public airwaves — at the time, it was located in Washington, D.C., which is why it has the east-of-the-Mississippi “W” appellation. The name doesn’t actually mean anything — it’s just a series of letters that conforms to the radio station naming standard. It stuck when the station moved to Colorado in the 1950s, when President Dwight Eisenhower placed many of the nation’s research facilities on the Front Range.
Keeping time in the atomic age is a little more complicated than that first Navy message 105 years ago. To measure the standard second, a computer frequency-counter measures the cycles of a type of energy change that takes place in a cesium atom. One second equals 9,192,631,770 cycles. NIST’s main clock, the F1, is accurate to within 0.1 nanoseconds per day. That means it ends up being off by one second once every 80 million years. Last year, NIST developed an even more accurate clock, which would neither gain nor lose one second for 200 million years.
Such highly precise clocks are important for telecommunications, deep-space communications, navigation and other purposes. The Federal Aviation Administration must accurately monitor air traffic, for instance. Traffic signals, which must stay green, yellow and red for a given length of time, have to measure that time against some standard. Emergency 911 operators have to know what time someone calls for help. Satellites and space probes have to know where and when to communicate with Earth.
The time standard is even important for financial markets — stock prices can fluctuate by mere seconds, so a buyer or seller wants the most accurate information. “You need to make sure you bought this stock at nine hours, 10 minutes and 58 seconds, and they’ll probably put 10ths or 100ths of a second on there,” Sutton said. “Wherever a time stamp is used, it eventually references back to the nation’s time standard.”
Time code generators at WWVB and WWV use binary code to convert the time into information that can be broadcast via radio waves. It’s all in Universal Coordinated Time, formerly known as Greenwich Mean Time. Mountain Daylight Time is six hours behind UTC. Mountain Standard Time is seven hours behind. The time code contains the year, day of year, hour, minute, second and flags that indicate the status of Daylight Saving Time, leap years and leap seconds.
Each generator is constantly tested for accuracy — “If you are the standard, you cannot broadcast the wrong time,” Sutton points out — and the data is sent to a radio frequency transmitter, which sends the signal to the antennas.
NIST uses power from Poudre Valley Rural Electric Association, but has a backup generator that fires up in 30 seconds in case of an outage. Four thousand gallons of diesel fuel stored on site can keep the time for two and a half weeks.
The low frequency station, WWVB, broadcasts at 60 kHz. Its sine wave — the curvy line with equal hills and valleys — is 3.5 miles long; it’s at such a low frequency that it can penetrate the Earth. That’s another reason why WWVB is in Colorado instead of a more central location like, say, Omaha — it’s closer to the Rockies to facilitate broadcasting all the way to California.
As the signal propagates from Fort Collins, the wave tends to follow the Earth’s curvature, making it an efficient frequency for huge distances. The sun inhibits radio waves, but the antennae are powerful enough to cover the continental U.S. at all times. At night, the signal can reach Santiago, Chile. The size of an antenna corresponds to the wave of a frequency — the higher the frequency, the smaller the antenna needed.
Gigahertz antennas are for ultra-high frequencies; users of GPS devices and satellite radio services are familiar with their small, stubby antennas. Kilohertz antennas are a lot bigger; to broadcast a very low-frequency radio wave like 60 kHz, a dish antenna would have to be two miles wide. (AM radio is also broadcast in the kHz range, although at much higher frequencies than the 60 kHz used at WWV. The low end of the AM spectrum starts around 530 kHz and reaches about 1800 kHz; FM radio uses about 88 megahertz to about 108 MHz.)
Thankfully for Northern Colorado residents, eastern Wellington is not covered by mammoth dish antennas. Two cable-connected arrays in a “top-hat” design do the job in a smaller area. Each array consists of four 400-foot towers arranged in a diamond shape, with a system of cables suspended among the four towers. The wires meet in the middle, where a cable connects the array to a helix house on the ground, which transmits the signal. Each array covers 32 acres. WWVB is just one way in which NIST tells you the time, however. Radio station WWV actually comes with sound; signals are broadcast on five frequencies, from 2 MHz to 20 MHz.
At the end of every minute, a computerized voice will announce the time, effective at the tone. The station also broadcasts a 440 Hz tone to help with piano tuning; an update on the world’s GPS satellites; and space weather forecasts, including news of solar flux, solar flares and geomagnetic activity. WWV’s building also hosts the four HP clocks, the heart of the station. NIST also provides an Internet time service, a telephone number, a Web site and even a subscription service, which has an inaccuracy of less than 20 nanoseconds.
Next time you set your watch, call NIST and listen for the tone. That way, when you look at your watch, you’ll be sure it’s right.

What time is it?

Listen to the radio:
WWV can be heard with any shortwave radio. A typical shortwave radio provides coverage from about 150 kHz to about 30 MHz. WWV broadcasts the time at 2.5, 5, 10, 15 and 20 MHz.

Call NIST:
Call 303-499-7111 for WWV and call 808-335-4363 for WWVH, the atomic clock broadcast in Hawaii.

Go online:
The NIST Internet Time Service allows users to synchronize their computer clocks via the Internet. Most of the servers are in Boulder. The service requires a download that you can configure to access a specific server. Visit nist.time.gov to learn more.

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