This chapter provides an overview of clock technology and typical clocks (Cs, Rb, H-Maser) in use today for onboard and ground systems and identifies future trends such as fountain clocks, etc. Concepts such as clock drift, trend, random variations and the statistical methods for their characterization (Allan deviation (ADEV ), etc.) are introduced and performance characteristics of global navigation satellite system (GNSS ) onboard clocks are presented. The handling and impact of special and general relativity on timing measurements are discussed. Finally, the generation of a GNSS time from an ensemble of ground clocks is described.


atom clock ensemble in space


Allan deviation


amplitude modulation


bulk acoustic wave


barycentric celestial reference system


BeiDou time


Bureau International de l’Heure


Bureau International des Poids et Mesures


Comité Consultatif International des Radiocommunications


clock monitoring and comparison unit


coherent population trapping


chip scale atomic clock


digital cesium beam frequency standard


Echelle atomique libre (free atomic scale)


Earth-centered Earth-fixed


Earth-centered inertial


flicker drift


flicker frequency (noise)


flicker phase (noise)


Geocentric Celestial Reference System


Global’naya Navigatsionnaya Sputnikova Sistema (Russian Global Navigation Satellite System)


global navigation satellite system


Global Positioning System


GPS Time


Galileo System Time


Geocentric Terrestrial Reference System


International Astronomical Union


Institute of Electrical and Electronics Engineers


International Earth Rotation and Reference Systems Service


intensity optical pumping


Indian Regional Navigation Satellite System


International Terrestrial Reference System


International Telecommunication Union


International Union of Geodesy and Geophysics


Jet Propulsion Laboratory


micro-electromechanical system


modified Julian day/date


magneto-optical trap


National Aeronautics and Space Administration


National Geospatial-Intelligence Agency


National Institute of Standards and Technology


Naval Research Lab


oven controlled crystal oscillator


one-way carrier-phase technique


passive hydrogen maser


phase modulation


pseudo-random noise


Quasi-Zenith Satellite System


rubidium atomic frequency standard


radio frequency


random walk drift


random walk frequency (noise)


random walk phase (noise)


surface acoustic wave


signal-to-noise ratio


system time


space vehicle number


International Atomic Time


barycentric coordinate time


Geocentric Coordinate Time


temperature compensated crystal oscillator


time keeping system


terrestrial time


two-way satellite time and frequency transfer


International Union of Radio Science


United States Naval Observatory


Coordinated Universal Time


white phase (noise)



The authors would like to thank Dr. Joseph White, a colleague at the US Naval Research Laboratory, as well as Dr. Bob Nelson. Publication restrictions did not permit including Dr. White or Dr. Nelson in the authorship though many of the sections on individual atomic frequency standards and relativity were either contributed or edited by them.


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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  1. 1.Advanced Space PNT BranchUS Naval Research LaboratoryWashingtonUSA

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