![]() ![]() Leap seconds are used to keep the difference between UT1 and UTC to within ☐.9 s. ![]() The first leap second was inserted into the UTC time scale on June 30, 1972. The current difference between UTC and TAI is 37 seconds. This correction keeps UTC in conjunction with the apparent position of the Sun and the stars, and it is the standard used for all general timekeeping applications. Leap SecondsĬoordinated Universal Time (UTC) is based on International Atomic Time (TAI), but it is adjusted by leap seconds to account for the difference between the definition of the second and the rotation of Earth. See information about why we need leap seconds. Its purpose is to keep the UTC time scale within ☐.9 s of the UT1 astronomical time scale, which changes slightly due to variations in the rotation of the Earth. Occasionally, 1 s is added to the UTC time scale. The master clock pulses used by the WWV, WWVH, WWVB, ACTS and Internet Time Service (ITS) time code transmissions are referenced to the UTC(NIST) time scale. Listener comments and reception reports may be emailed to: wwv nist.This page is updated monthly and contains a table of leap seconds, the current difference between the UT1 and UTC time scales, and the current UT1 - UTC difference that is being broadcast by NIST (called the DUT1 correction). As an experimental broadcast, the 25 MHz signal may be interrupted or suspended without notice.Īntenna: half-wave vertical dipole. The broadcast consists of the normal WWV signal heard on all other WWV frequencies, at the same level of accuracy. WWV has resumed broadcasting on 25 MHz on an experimental basis. The modulation level is 50 percent for the steady tones, 50 percent for the BCD time code, 100 percent for the second pulses and the minute and hour markers, and 75 percent for the voice announcements. The signals broadcast by WWV use double sideband amplitude modulation. This sloping skirt functions as the lower half of the radiating system and also guys the antenna. The bottom half of each antenna consists of nine quarter-wavelength wires that connect to the center of the tower and slope downwards to the ground at a 45 degree angle. The top half of each antenna is a quarter-wavelength radiating element. The shortest tower, for 20 MHz, is about 7.5 m tall. ![]() The tallest tower, for 2.5 MHz, is about 60 m tall. Each antenna is mounted on a tower that is approximately one half-wavelength tall. Each antenna is connected to a single transmitter using a rigid coaxial line, and the site is designed so that no two coaxial lines cross. There are antennas at the station site for each frequency. The WWV antennas are half-wave vertical antennas that radiate omnidirectional patterns. The variety of frequencies makes it likely that at least one frequency will be usable at all times. Although each frequency carries the same information, multiple frequencies are used because the quality of HF reception depends on many factors such as location, time of year, time of day, the frequency being used, and atmospheric and ionospheric propagation conditions. Each frequency is broadcast from a separate transmitter. The station radiates 10 000 W on 5 MHz, 10 MHz, and 15 MHz and 2500 W on 2.5 MHz and 20 MHz. WWV operates in the high frequency (HF) portion of the radio spectrum. The broadcast information includes time announcements, standard time intervals, standard frequencies, UT1 time corrections, a BCD time code, and geophysical alerts. WWV is located near Fort Collins, Colorado, about 100 kilometers north of Denver. NIST radio station WWV broadcasts time and frequency information 24 hours per day, 7 days per week to millions of listeners worldwide. ![]()
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