Parameter, associated with the result of a measurement, that characterizes the dispersion of values that could reasonably be attributed to the measurand. By convention, two standard deviations are normally used for uncertainty numbers.

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 (by mutual agreement) within 100 nanoseconds of UTC(NIST). Both NIST and the USNO can be considered official sources of time and frequency in the United States.

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:

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

and conversely

The table shows the frequency and wavelength ranges for the frequency bands in the radio spectrum.

Noise having a frequency spectrum that is continuous and uniform over a specified frequency band. White noise is independent of frequency, and its spectrum looks flat on a spectrum analyzer's display. It has equal power per hertz over the specified frequency band.

The NIST radio station located near Fort Collins, Colorado. WWV broadcasts time and frequency information 24 hours per day, 7 days per week to millions of listeners worldwide on carrier frequencies of 2.5, 5, 10, 15, and 20 MHz. Please visit the WWV web pages for a complete description of the station.

The NIST radio station located on the same site as WWV near Ft. Collins, Colorado. WWVB broadcasts on a carrier frequency of 60 kHz. The WWVB broadcasts are used by millions of people throughout North America to synchronize consumer electronic products such as wall clocks, clock radios, and wristwatches. In addition, WWVB is used for high level applications such as network time synchronization and frequency calibrations. Please visit the WWVB web pages for a complete description of the station.

The NIST radio station located on the Island of Kauai, Hawaii. WWVH broadcasts time and frequency information 24 hours per day, 7 days per week to listeners worldwide on carrier frequencies of 2.5, 5, 10, and 15 MHz. Please visit the WWVH web pages for a complete description of the station.