Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Time and Frequency from A to Z, U to W


The parameter, associated with the result of a measurement, that characterizes the dispersion or range of values that could reasonably be attributed to the measurand.  By convention, a “coverage area” of 2σ, known to metrologists as k = 2, is normally used for uncertainty numbers, indicating that there is an approximate 95% probability that the true value lies somewhere within the coverage area.

United States Naval Observatory (USNO)

Established in 1830, the USNO is one of the oldest scientific agencies in the United States. The USNO determines and distributes the timing and astronomical data required for accurate navigation and fundamental astronomy. It maintains a UTC time scale that is typically within 20 nanoseconds of UTC(NIST). Both NIST and the USNO can be considered official sources of time and frequency in the United States.

Universal Time (UT) Family

Before the acceptance of atomic time scales such as TAI and UTC in the 1960s, astronomical time scales were used for everyday timekeeping. These time scales are still used today, but mostly for applications related to astronomy. They are based on mean solar time. The mean solar second is defined as 1/86,400 of the mean solar day, where 86,400 is the number of seconds in the mean solar day. This mean solar second provides the basis for Universal Time (UT). Several variations of UT have been defined:

  • UT0 - The original mean solar time scale, based on the rotation of the Earth on its axis. UT0 was first kept by pendulum clocks. As better clocks based on quartz oscillators became available, astronomers noticed errors in UT0 due to polar motion, which led to the UT1 time scale.
  • UT1 - The most widely used astronomical time scale, UT1 is an improved version of UT0 that corrects for the shift in longitude of the observing station due to polar motion. Since the Earth’s rate of rotation is not uniform, UT1 is not completely predictable, and has an uncertainty of ±3 milliseconds per day.
  • UT2 - Mostly of historical interest, UT2 is a smoothed version of UT1 that corrects for known deviations in the Earth’s rotation caused by angular momenta of the Earth’s core, mantle, oceans, and atmosphere.


An acronym for voltage controlled crystal oscillator.  A VCXO is a quartz oscillator whose frequency is adjusted by varying its input voltage.


The distance between identical points in the adjacent cycles of a waveform that is traveling in free space or in a guide structure such as a coaxial cable. The wavelength of radio signals is usually specified in meters, centimeters, or millimeters. In the case of infrared, visible light, ultraviolet, and gamma radiation, the wavelength is more often specified in nanometers (units of 10-9 meter) or Angstrom units (units of 10-10 meter). The wavelengths of the various frequency bands in the radio spectrum are shown in the table.

Wavelength is inversely related to frequency. The higher the frequency of the signal, the shorter the wavelength.  If f is the frequency of the signal as measured in megahertz, and w is the wavelength as measured in meters, then

w = 300 / f ,

and conversely

f = 300 / w .

The table shows the frequency and wavelength ranges for the various frequency bands.

Band Description Frequency Wavelength
VLF Very Low 3 to 30 kHz 100 to 10 km
LF Low 30 to 300 kHz 10 to 1 km
MF Medium 300 to 3000 kHz 1 km to 100 m
HF High 3 to 30 MHz 100 to 10 m
VHF Very High 30 to 300 MHz 10 to 1 m
UHF Ultra High 300 to 3000 MHz 1 m to 10 cm
SHF Super High 3 to 30 GHz 10 to 1 cm
EHF Extremely High 30 to 300 GHz 1 cm to 1 mm

White Noise

Noise having a frequency spectrum that is continuous and uniform over a specified frequency band.  White noise is independent of frequency.  Its spectrum looks flat because there is equal power per hertz over the specified frequency band.


The NIST radio station located near Fort Collins, Colorado. WWV broadcasts time and frequency information, including voice announcements of time, on carrier frequencies of 2.5, 5, 10, 15, 20, and 25 MHz.


The NIST radio station located on the same site as WWV near Ft. Collins, Colorado. WWVB broadcasts a digital time on a carrier frequency of 60 kHz. The WWVB time code is used throughout North America to synchronize radio controlled clocks.


The NIST radio station located on the Island of Kauai, Hawaii. WWVH broadcasts time and frequency information, including voice announcements of time, on carrier frequencies of 2.5, 5, 10, and 15 MHz.

Created May 12, 2010, Updated March 1, 2023