Glossary of Abbreviations

Timekeeping has spawned its own lexicon of abbreviations andacronyms. The following are the most common examples used inthis book.

BIH Bureau International de l’Heure, the organisationformerly responsible for coodinating world timescales, now superseded by BIPM and IERS.

BIPM Bureau International des Poids et Mesures, the or-ganisation responsible for international standards ofmeasurement. The Time Section disseminates TAIand UTC.

CGPM Conference Generale des Poids et Mesures, theintergovernmental body that oversees the work ofCIPM and BIPM.

CIPM Comite International des Poids et Mesures, the com-mittee of experts that supervises BIPM.

DCF77 A standard time and frequency service broadcastfrom Germany.

EAL Echelle Atomique Libre (Free Atomic Time), a free-running time scale formed by averaging the readingsof numerous atomic clocks worldwide.

ET Ephemeris Time, a time scale based on the move-ments of bodies in the Solar System.

GLONASS Global Navigation Satellite System, a Russian net-work of navigational satellites.

GMAT Greenwich Mean Astronomical Time, a former timescale based on GMT with the date changing at noonrather than midnight.

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190 GLOSSARY OF ABBREVIATIONS

GMT Greenwich Mean Time, a former time scale for civiltimekeeping based on mean solar time at Greenwich,now superseded by UTC.

GPS Global Positioning System, a US network of naviga-tional satellites.

IAU International Astronomical Union, the organisationformerly responsible for world timekeeping throughBIH.

IERS International Earth Rotation Service, the organisa-tion that monitors the Earth’s rotation and decideswhen leap seconds are required.

ITU International Telecommunication Union, the organ-isation responsible for the agreements that defineUTC in terms of TAI.

LPTF Laboratoire Primaire du Temps et des Frequences,the French time standards laboratory.

MSF A standard time and frequency service broadcastfrom the UK.

NBS National Bureau of Standards (US), now renamed asNIST.

NIST National Institute of Standards and Technology, theUS standards laboratory.

NPL National Physical Laboratory, the UK standardslaboratory.

OP Observatoire de Paris, the national observatory ofFrance and the home of LPTF and IERS.

PTB Physikalisch-Technische Bundesanstalt, the Germanstandards laboratory.

PZT Photographic Zenith Tube, a specialised telescopefor timing star transits.

RGO Royal Greenwich Observatory, the former nationalobservatory of England.

GLOSSARY OF ABBREVIATIONS 191

SI Systeme International des Unites, the internationalsystem of units.

TAI Temps Atomique International (InternationalAtomic Time), a time scale formed by calibratingEAL against a small number of primary caesiumstandards that generate SI seconds.

USNO United States Naval Observatory, the US nationalobservatory.

UT Universal Time, originally the scientific name forGreenwich Mean Time but now redefined as UT0,UT1, UT2 and UTC.

UT0 UT determined by astronomical observations at agiven observatory.

UT1 UT0 corrected for polar motion.

UT2 UT1 corrected for seasonal variations.

UTC Coordinated Universal Time, a time scale formedfrom TAI but kept close to UT1 by the inclusion ofleap seconds. The basis of all civil timekeeping.

VLBI Very Long Baseline Interferometry, a method fordetermining UT1 and also used for research in radioastronomy.

WWV, WWVB, Standard time and frequency services broadcast

WWVH from the US.