NIST distributes time-of-day and frequency information to the public using radio broadcasts. The low frequency (LF) station WWVB is located in Fort Collins, Colorado and broadcasts on 60 kHz. The high frequency (HF) stations WWV and WWVH are located in Colorado and Hawaii respectively, and broadcast on five different frequencies ranging from 2.5 to 20 MHz. At least one HF signal should always be usable at any given time anywhere in the United States.

The method used to broadcast the time is similar at all three stations, and is outlined below. You can click any of the images to view a larger image.

	1. Start with an atomic clock. This unassuming device is a cesium frequency standard, sometimes referred to as an atomic clock. There are several cesium standards at the radio station sites. The radio station standards are continuously compared to the national frequency standard, located in the NIST laboratories in Boulder, Colorado.
	2. Generate "time codes." The signal from the atomic clock is fed to a unit called a Time Code Generator. This produces the time code which is distributed by each station.
	3. Amplify signal. The signal must now be amplified by powerful radio transmitters. Since HF radio transmissions can be affected by solar activity, time-of-year, and time-of-day, the NIST HF broadcasts are simulcast on five different frequencies. At least one frequency should be usable at all times.
	4. Transmit to antenna. The high power encoded signal is now sent to the antenna using a 50 Ohm transmission line. This thick cable connects the transmitters in the station house with the antenna towers.
	5. Match the signal (for 60 kHz). In the case of the 60 kHz antenna, the transmission line must be matched to the antenna through a coil called a variometer. A variometer is a "tunable transformer" that helps maximize the amount of energy radiated from the antenna.
	6. Broadcast to the country. The antenna radiates the time signal. The HF signals are broadcast from single antenna towers. The LF antenna used by WWVB is actually a network of wires connected to eight different towers.
	7. Transmit through the atmosphere. The radio signal travels through the atmosphere, shown in this graphic. The LF signal (green) propagates by groundwave, following the curvature of the earth; the HF signal (pink) propagates by skywave, and bounces off the ionosphere. All signals travel at the speed of light.
	8. Receive and use signal. Millions of people receive the NIST signals every day, using equipment ranging from low cost wall clocks and wrist watches, to sophisticated laboratory quality receivers.

The NIST broadcasts provide an essential service to the American public. They are essential to the nation's space and defense programs, to manufacturers, and to transportation and communications. In addition, the NIST broadcasts keep millions of radio clocks on time in homes and offices throughout the United States.