Time standards and frequency standards can include cesium or rubidium atomic clocks, quartz oscillators, hydrogen masers and other master clocks. Time reference sources are used in networks & telecommunications, observatories, utilities and navigation (GPS). Atomic clocks use the atomic resonant frequency of a gas or lasing medium as basis of it's timekeeping mechanism. Atomic are not radioactive. The signal used is derived from the electromagnetic or microwave radiation released when electrons in atoms change energy levels (hyperfine transitions). Masers and atomic fountain clocks are the two main types available today. The current masers or microwave lasers used for atomic timekeeping are based on a hydrogen lasing medium. Hydrogen masers are very low cost and short term stability, but may not have the long term accuracy compared to other atomic clocks. Rubidium and caesium or cesium atomic fountain clocks use the absorption of microwaves in a gas stream or microwave cavity to provide a timekeeping element. Caesium or cesium oscillator or atomic clocks are the primary standard for time and frequency measurements, but these systems can be very costly. Rubidium clocks are low cost and small in size, but they do not have long term stability equivalent to national standards. The lower cost hydrogen maser or Rubidium time standards are often updated to the national standard through a GPS signal.
Additional time standards or references include quartz oscillators that are periodically updated to the national standard or atomic clock through a GPS signal. NIST offers a remote calibration service (TMAS - Time Measurement and Analysis Service) to help laboratories around the country or world maintain an accurate local time. NIST's Internet Time Service (ITS) allows users to synchronize computer clocks through a web connection. The time information is traceable back to NIST. Accurate time standards and frequency standards are important in synchronizing network processes, long baseline interferometry, radio astronomy, GPS navigation, and certain laboratory or analytical applications.