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Frequently asked questions (FAQ)


 What time is it? 

 NIST radio broadcasts 

 NIST time 

 Local time 

 Leap seconds 

 Timekeeping and clocks 

 Times of day 

 Glossary 


Descriptions and uses of radio signals from WWV, WWVB and WWVH.

What frequencies do WWV and WWVH use?
When I listen to WWV or WWVH, how accurate is the time?
What other information is broadcast on WWV and WWVH?
What do the call letters stand for?
How do I convert WWV or WWVH announcements to my local time?
Is it possible to tour the WWV/WWVB facilities in Colorado or the WWVH facilities in Hawaii?
Why are both male and female voices used for the time announcements?
How does WWVB set clocks?


What frequencies do WWV and WWVH use?

WWV broadcasts on 2.5, 5, 10, 15 and 20 MHz from a location near Fort Collins, Colorado. WWVH broadcasts on 2.5, 5, 10 and 15 MHz from Kauai, Hawaii. Both stations broadcast a timing signal 24 hours per day, 7 days per week, to listeners all over the world.

All broadcast frequencies used by WWV and WWVH are in the high frequency (HF) radio spectrum which extends from 3 to 30 MHz. This part of the spectrum is commonly referred to as "shortwave". General coverage shortwave receivers typically receive all frequencies from 530 kHz (the beginning of the AM broadcast band) to 30 MHz and are capable of receiving WWV and WWVH on all of the available frequencies.

Multiple frequencies are used because shortwave propagation varies with many factors, including time of year, time of day, geographical location, solar and geomagnetic activity, weather conditions and antenna type and configuration. In general, the 2.5 MHz signal works best for locations within a few hundred miles of the broadcast sites. The 15 and 20 MHz frequencies often work best during the daytime hours. The 5 and 10 MHz frequencies are probably the best signals to continually receive, and also the best signals to receive at night. The 5, 10 and 15 MHz transmissions are broadcast at a higher power than the other frequencies.

When I listen to WWV or WWVH, how accurate is the time?

The time is kept to within less than 0.0001 milliseconds of Coordinated Universal Time (UTC) at the transmitter site, but the signal is delayed as it travels from the radio station to your location. This delay increases the further that you are from the station, and the delay can vary by as much as 1 millisecond if the signal is bouncing between Earth and the ionosphere. However, for most users in the United States, the received accuracy should be less than 10 milliseconds (1/100 of a second).

Listening to the signals by telephone, the delays will be larger, but the time is usually accurate to within 30 milliseconds if you are using a landline. If you are using a cellular phone or a voice over Internet protocol (VOIP) connection, the delay can be much larger, but should still not exceed 150 milliseconds, based on International Telecommunication Union (ITU) recommendations for voice transmissions.

What other information is broadcast on WWV and WWVH?

WWV and WWVH broadcast much more than just the time. Please visit the station web pages to read about the information transmitted by WWV and WWVH.

What do the call letters stand for?

They are merely a unique combination of letters, just as for a commercial radio or TV station, that meet certain requirements as spelled out in the International Telecommunication Union's Radio Regulations (for example, call letters for U.S. radio and TV stations begin with a "W" or a "K"). Normally, call signs that begin with a "W" are located east of the Mississippi River, but WWV, WWVB, and WWVH are exceptions, because WWV was originally located in Washington, DC.

How do I convert WWV or WWVH announcements to my local time?

The time announced on WWV and WWVH is Coordinated Universal Time or UTC, which is a 24-hour time system based at the prime meridian (0° longitude) located near Greenwich, England. An hour correction must be made to UTC to determine your local time.  

For example, if you are in the eastern time zone, then subtract 5 hours from the UTC announcement during standard time and 4 hours during DST. If the announcement states the time is 1830 UTC, eastern standard time would be 1330, or 1:30 p.m. in the 12-hour time system. Similarly, central standard time would be 1230, or 12:30 p.m., mountain standard time would be 1130, or 11:30 a.m., and Pacific standard time would be 1030, or 10:30 a.m.

For more information, see the sections on Coordinated Universal Time and time zones.

Is it possible to tour the WWV/WWVB facilities in Colorado or the WWVH facilities in Hawaii?

Unfortunately, due to security reasons and the limited staff at each station, it is impossible to provide tours. However, you can view images of the facilities on the station web pages.

Why are both male and female voices used for the time announcements?

Since the two stations broadcast on several of the same frequencies, using different voices enables listeners to distinguish which station they are receiving. WWV uses a male voice and WWVH uses a female voice. In areas where both signals are strong, such as California, it is often possible to hear both voice announcements. However, the two announcements never occur at the same time.

How does WWVB set clocks?

WWVB sets clocks by broadcasting a digital time code on a frequency of 60 kHz. The time code bits are sent by raising and lowering the power of the signal. A radio-controlled clock has an antenna and a radio receiver inside, which decodes the bits to get the time (UTC), day of year, daylight saving time, leap year and leap second indicators. The user of a radio-controlled clock must select the time zone, so that the clock can convert UTC to local time. It typically takes several minutes for a radio controlled clock to synchronize when it is first turned on. After that, most radio controlled clocks will synchronize once per day, usually in the middle of the night when the signal is the strongest. Please see the WWVB web page for more information.