WWV is the callsign of NIST's shortwave radio station located in Fort Collins, Colorado. WWV's main function is the continuous dissemination of official U.S. Government time signals. The station broadcasts simultaneously on five distinct frequencies - 2.5 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. These carrier frequencies, as well as the time signals, are derived from a set of atomic clocks located at the transmitter site, which themselves are traceable to NIST's primary frequency standard in Boulder, Colorado using such techniques as GPS common-view observations. WWV is partnered with radio station WWVH, located in Hawaii. Onsite with WWV in Fort Collins, Colorado is also radio station WWVB, which operates on the low frequency of 60 kHz.
WWV transmits the exact time-of-day in two ways at the same time -- through male voice announcements every minute followed by long beeps that serve as on-time markers for those announcements, and through a continuously-transmitted sub-audible digital time code. The transmitted time is given in Coordinated Universal Time (UTC). WWV also "ticks" each second (except at 29 and 59 seconds past the minute), to allow for accurate manual synchronization of clocks. Between seconds one and sixteen inclusive past the minute, the current difference between UTC and UT1 is transmitted by doubling some of the once-per-second ticks. The absolute value of this difference, in tenths of a second, is determined by the number of doubled ticks. If the doubled ticks are between seconds one and eight inclusive past the minute, UT1 is ahead of UTC. If the doubled ticks fall between seconds nine and sixteen inclusive, UT1 is behind UTC.
In most minutes of the hour, WWV transmits standard audio frequencies of 500 and 600 Hz, switching between the two frequencies each minute. These two frequencies are used to synchronize 50 Hz and 60 Hz electrical power supplies respectively. WWV also transmits a 440 Hz tone every hour at two minutes past, with the exception of the first hour of the UTC day, when it transmits just the "ticks" and time code.
WWV's broadcasts are not limited to time-of-day and standard time intervals. At 8, 9 and 10 minutes past the hour, WWV stops transmitting its standard audio frequencies to broadcast regular high-seas weather warnings. At 14 and 15 minutes past the hour, reports relating to the health of the U.S. Department of Defense's GPS service are transmitted. And at 18 minutes past the hour, a special "geophysical alert" report is transmitted, containing information on solar activity and shortwave radio propagation conditions. No audio tones are transmitted from WWV between 43 and 51 minutes inclusive past the hour.