Masterclock GMR5000 - GPS Antenna Location and Installation; GPS Antennas and Cables; Antenna Placement and Mounting Guidelines

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MasterclockGMRUserManualv2–2016.12
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GPSAntennaLocationandInstallation
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GLOBALPOSITIONINGSYSTEMANDPRECISETIMEANDFREQUENCY
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TheGlobalPositioningSystem(GPS)isaworldwideradio‐navigationsystemformedfromaconstellationof24
satellitesthatcontinuouslyorbittheearth.EachGPSsatellitehasonboardseveralatomicclocksthatareprecisely
synchronizedtoUniversalTimeCoordinated(UTC)providedbytheU.S.NavalObservatory(USNO).Codedsignals
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arebroadcastbyeachofthesatelliteswiththeexacttimeandpositionofthesatellite.AllGPSreceiversusean
antennatoreceivethesesignals.ByusingaGPSreceiveroptimizedfortimeandnotpositionitispossibletoget
extremelyprecisetimesynchronizationwiththesatellite’satomic
clocks.
GPSANTENNASANDCABLES
ThesignalsfromtheGPSsatellitesoperateinthe“semi‐visible”spectrumoftheL1band(1575.42MHz)witha
minimumsignallevelof‐162.0dBW.WiththisverylowsignalstrengththeGPSantennamustbeableto“see”the
skytoacquirethesignals.Practicallyspeakingtheantennamust
haveaclearviewoftheskyandthusbemounted
onaroof,orinsomecasesinawindow.Theantennasarerelativelysmall,coffeecupsizeorsmaller,andare
connectedtotheGPSreceivertypicallyviacoaxialcable.
SincetheGPSsignalisveryweakthe
antennausuallyamplifiesthesignaltodriveitthroughthecabletothe
receiver.AntennacablehoweverofferssomeresistanceandtheGPSsignalstrengthwillattenuateasittravels
downthecable.GPSreceiversensitivityisfinitesoifthecablelengthistoolongthesignalwillbetoo
weakfor
thereceivertodetectit.Consequently,itisveryimportanttoknowthedistanceinadvancebetweentheantenna
andthereceiversothatthepropercablesolutioncanbeinstalled.
ANTENNATYPES
TherearetwobasicantennatypesusedwithGPStimingreceivers;roofmountedandwindowmounted.Theroof
mountedantennaisrequiredforthemoreaccurateGPSclockssinceatleastthreesatellitesarerequiredtobein
viewatalltimestomaintaintimingaccuracy,typicallynanosecondstoUTC.The
windowmountedantennais
applicableforthenetworktimeserverswhichoperatewithalesserdegreeofaccuracy,typicallymicrosecondsor
insomecaseslowmillisecondstoUTC,andcanfunctionwithasfewasoneintermittentsatelliteinview.Theroof
mountedantennaisalwayspreferablesincebynature
ofitslocationhasthebestviewofthesky.Avariationon
theroofantennaistheGPSDown/Upconverterusedforverylongcableruns.ThisisaspecialGPSantennathat
receivestheGPSsignalanddownconvertsittoalowerfrequencythatisthensentdown
thecable.Nexttothe
GPSreceiverisanupconverterthatconvertsthesignalbacktotheoriginalfrequencyanddeliversittotheGPS
receiver.ThisprocessistransparenttotheGPSreceiver.
ANTENNAPLACEMENTANDMOUNTING
RoofAntennaPlacement:Whenselectingasitefortheroofantenna,findanoutdoorlocationthatprovidesfull
360‐degreevisibilityofthehorizon.Inmostcases,thismeanslocatingtheantennaashighaspossible,suchason
theroof.Anyobstructionsmaydegradeunitperformancebyblockingthesatellite
signals.Blockedsignalscan
increasethetimeforsatelliteacquisition,orpreventacquisitionaltogether.
Ashortmountingmastandhoseclampsareprovidedwiththeroofantennatomounttheantennatoapoleorthe
peakofabuilding.Theantennamountingmastandclampsarewellsuitedto
attachtheantennatoaventpipeor
mastaffixedtotheroof.Thepipemustberigidandabletowithstandhighwindswithoutflexing.
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TYPICALROOFANTENNAMOUNTING
GPSReceiverscanbesusceptibletoreflectedGPSsignalscalledmultipath.MultiPathinterferenceis
causedbyreflectedsignalsthatarriveattheantennaoutofphasewiththedirectsignal.Thisinterference
ismostpronouncedatlowelevationanglesfrom10to20degreesabovethehorizon.Theheightofthe
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mast/antennamaybeextendedupwardtopreventmultipathinterference.Theantennashouldalsobeat
leastthreetosixfeet(1‐2m)fromareflectingsurface.
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