3-27
To ensure reliable coverage of the United States and extensive coverage of other parts of the world,
radio stations WWV and WWVH provide the primary standard radio frequencies listed in table 3-1. The
transmission of WWV and WWVH are interrupted for 5 minutes of each hour. The silent period begins at
15 minutes past the hour for station WWVH and 45 minutes past the hour for station WWV. These silent
periods are provided to eliminate errors caused by interference.
Table 3-1.—NBS Frequency Standards and Time Transmission
TRANSMISSION
WWV
WWVH
RF Signal Frequency MHz
5, 10, and 15
5, 10, and 15
Frequency Stability
1 part in 1011
1 part in 1011
Frequency Deviation
1 part in 1012 per day
1 part in 1012 per day
Seconds Frequency and Duration
5 cycles of 1000Hz for .005
seconds
6 cycles of 1200Hz for .005
seconds
Audio Tones
600Hz and 500Hz with 440Hz to
mark the hour
600Hz and 500Hz with 440Hz to
mark the hour
Frequency Accuracy
1 part in 1012
1 part in 1012
Propagation Forecast
14 min. past the hour (in voice)
None
Two primary standard audio-frequency tones (440 Hz and 600 Hz) are broadcast on all WWV and
WWVH carrier frequencies. In the absence of a message, a 500-Hz tone is broadcast during the message
interval. The 440-Hz signal that denotes the 1-hour mark is the standard musical pitch, A above middle C.
The 600-Hz tone provides a frequency standard for checking the 60-Hz power-line frequency.
The standard time pulse marking interval of 1 second consists of five cycles of a 1,000-Hz tone at
WWV and six cycles of a 1,200-Hz tone at WWVH. These marker pulses are heard as clock ticks.
Intervals of 1 minute are marked by a 0.8-second, 100-Hz tone for WWV and a 0.8-second, 1,200-Hz
tone for WWVH. Each hour is marked by a 0.8-second, 1,500-Hz tone on both stations. Universal Time
Coordinated (UTC) is announced on WWVH between the 45 and 52.5 seconds of each minute and on
WWV between the 52.5 and 60 seconds of each minute.
An announcement of radio propagation conditions (geophysical alert) for the North Atlantic area is
broadcast by station WWV in voice at 18 minutes after each hour. For example, these short-term
announcements might state, "The radio propagation quality forecast at ... (normal, unsettled, disturbed)."
The propagation format is repeated phonetically and in numerical code to ensure clarity. The letter
designations N, U, and W, signifying "normal," unsettled," and "disturbed," respectively, classify the
radio propagation conditions at the time of the broadcast. The digits from 1 to 9 indicate the expected
radio propagation conditions during the next 6 hours; refer to table 3-2 for code interpretations. The
National Bureau of Standards forecasts are based on information obtained from a worldwide network of
geophysical and solar observations.
