Radio Station WWV: All Time, All the Time [Hackaday]

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Of all the rabbit holes we technical types tend to fall down, perhaps the one with the most twists and turns is: time. Some of this is due to the curiously mysterious nature of time itself, but more has to do with the various ways we’ve decided to slice and dice time to suit our needs. Most of those methods are (wisely) based upon the rhythms of nature, but maddeningly, the divisions we decided upon when the most precise instrument we had was our eyes are just a little bit off. And for a true time junkie, “a little bit off” can be a big, big problem.

Luckily, even the most dedicated timekeepers — those of us who feel physically ill when the clock on the stove and the clock on the microwave don’t match — have a place to go that’s a haven of temporal correctness: radio station WWV. Along with sister stations WWVB and WWVH, these stations are the voice of the US National Institutes for Standards and Technology’s Time and Frequency Division, broadcasting the official time for the country over shortwave radio.

Some might say the programming coming from these stations is a bit on the dry side, and it’s true that you can only listen to the seconds slip by for so long before realizing that there are probably better things to do with your day. But the WWV signals pack a surprising amount of information into their signals, some of it only tangentially related to our reckoning of time. This makes these stations and the services they provide essential infrastructure for our technological society, which in turn makes it worth your time to look into just how they do it.

First On The Air

Callsign WWV has been around and active longer than US commercial radio itself. WWV was assigned as an experimental license to the National Bureau of Standards, the predecessor of the NIST, back in 1919. By September of 1920, WWV was broadcasting weekly concerts on 600 kHz, beating Pittsburgh station KDKA to the airwaves by a couple of weeks.

In those early days, WWV was very much a solution looking for a problem, alternating between music and farm market reports, and focusing on the Washington, DC area, where the NBS offices were located. In 1922, someone must have noticed that the “S” in NBS stood for “standards,” and WWV’s signal became a reference frequency standard for other broadcasters in the burgeoning industry. A succession of technological advances gradually increased the accuracy of WWV’s signals from a few tenths of a percent to the parts-per-million level, which was vital for allocating spectrum in the gold-rush years of the 20s and 30s.

WWV’s signature time programming didn’t start until nearly the end of WWII. Shortly thereafter, In 1948, station WWVH went on the air from the Hawaii Territory, in a nod to the country’s growing interest in events in the Pacific Basin. It wasn’t until 1950 that Morse time announcements gave way to the now-familiar voice time announcements; by the late 50s, frequency control on the station’s 5 MHz, 10 MHz, and 15 MHz frequencies was better than 200 parts per billion.

In 1966, the station moved to its current location in Fort Collins, Colorado, only about 80 km away from the NIST laboratories in Boulder, Colorado that house the US standards for time and frequency. The relocation put WWV on the same site as WWVB, a low-frequency (60 kHz) station that transmits nothing but encoded time signals at a scorching 70,000 watts effective radiated power (ERP). WWVB is designed to reach the entire United States for at least part of every day, and if you’ve got a “radio-controlled” clock or watch, chances are it’s listening to WWVB.

Atomically Accurate

The heart of WWV and WWVB operation in Fort Collins centers around the “Screen Room,” a Faraday cage-shielded room housing the station’s cesium frequency standards and time code generators. The three redundant oscillators derive their time and frequency information from the NIST Time Scale, called “UTC(NIST),” maintained at the Boulder NIST lab using a suite of hydrogen maser and cesium oscillators, which in turn are calibrated against a cesium fountain oscillator. The station oscillators are compared to UTC(NIST) every day and corrected as needed. Only one oscillator serves as the station master at a time; a supervisory system monitors the output of each oscillator and automatically promotes one of the backups to master status if anything goes wrong with the time signal.

The station master cesium oscillator is the heartbeat of the entire system. Its 5 MHz signal — actually, at 1 part in 10¹⁴, that’s 5.00000000000000 MHz — gets divided down to multiple reference frequency signals that control both time code generators and transmitter carrier frequencies. WWV currently broadcasts on 2.5 MHz, 5.0 MHz, 10.0 MHz, 15.0 MHz, and 20.0 MHz, with an experimental signal at 25.0 MHz; WWVB still operates at 60 kHz.

Audio time signals and voice announcements are generated by the time code generators in the Screen Room. WWV uses a male voice for time announcements while WWVH, which transmits on some of the same frequencies, uses a female voice to avoid confusion. The announcements are concatenated from digitally recorded phrases spoken by professional announcers; WWV currently uses the golden voice of Atlanta’s John Doyle.

Sounds Around The Clock

While the voice announcements on WWV are certainly its biggest draw, there’s so much more going on in these signals. The audio signals are carefully engineered to relay the maximum amount of information in the most flexible way possible, giving users access to all sorts of valuable information. The most obvious component of the audio signal is the constant tick of seconds. Each tick is really a 1,000 Hz sine wave that lasts for five milliseconds — a mere five cycles — which sounds like a tick. The seconds signal sounds every second of each minute, except for the 29th and 59th seconds, and for whenever a leap second is called for. At the top of each minute, the seconds pulse is changed to a 1,5000 Hz tone and extended to 800 ms. For the first sixteen seconds of every minute, you might hear double clicks for certain seconds, which serve as correction indicators between Coordinated Universal Time (UTC) and UT1, a time standard based on the rotation of the Earth. The number of doubled clicks tell you how many tenths of a second UTC and UT1 differ by; if the clicks are doubled from seconds 1 through 8, that means UT1 is ahead of UTC, whiles seconds 9 through 16 indicate that UT1 is lagging.

In addition to the ticks, the audio signal contains an audio tone that changes depending on the minute of the hour. Tones alternate between 500 Hz for the even-numbered minutes and 600 Hz for odd, with a 440 Hz “A440” tone used for the second minute of each hour. Certain other minutes are blocked out as reserved as well, but generally contain the tone normally designated for that minute. The audio tones, which can be used to calibrate audio equipment, are suppressed for one minute during the station identification announcements at the top of the hour and 30 minutes, and are suppressed entirely from 43 to 51 minutes and again for minute 59. The idea behind these blocks of silence is to prevent interference with WWVH’s signal, while the switch to A440 once each hour is meant to be used as a signal by systems that can receive WWV signals but don’t have the equipment to decode the subaudible time signals discussed below.