Masterclock GMR5000 - GPS Cable Delay and Planning; Understanding GPS Cable Delay

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MasterclockGMRUserManualv2–2016.12
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LIGHTNINGARRESTOR
In‐linelightningarrestorsaremountedonalowimpedancegroundbetweentheantennaandthepointwherethe
cableentersthebuilding.Theyrequirenoadditionalpowerorwiringexceptthegroundlead.
CABLEDELAY
GPSpositionaswellaspreciseUTCtimeisdeterminedatthepointtheGPSsignalsarereceivedattheantenna.
SincetheantennaistypicallyattachedtotheGPStimingreceiverviaacable,signalpropagationdelaysthroughthe
cablecausethetimecalculatedbythereceivertobeslightly
behindUTC.InGPSclockswithnanosecondand
microsecondaccuraciesthisisacriticalfactor.Inproductssuchasnetworktimeserverscabledelayisnot
importantbecausetimetransferoverIPnetworksdegradesthetimetothemillisecondlevel.
Cabledelayisafunctionofthecabletype.RG‐
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cableforexampletypicallydelaysthesignal1.24
ns/ft.For50feetofcable(15m)thedelaywould
be62nanoseconds.Cabledelayisremovedby
advancingtheantennasignalinsidetheGPS
receiver.Inthisexample,advancingthesignal+62
nanosecondsremovesallcablelatency.Solutions
suchas
theGPSDown/upconverteralso
introducesignallatencybutthislatencycanalso
beremovedbyadjustingthesignal.Allprecision
GPStimingreceiverswithnanosecondor
microsecondtimingaccuracyhavetheabilityto
compensateforcabledelay.
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ADVANCEDPLANNING
ItistimewellspenttoestimateinadvancethecablelengthfromtheGPSantennatothereceiverforanyplanned
installation.Cablelengthsthataretooshortortoolongcaneachintroduceproblems.Insomecases,addingan
inlineamplifierandsomeextracablemaybeaquick
andeconomicalsolution.Inotherinstances,retrofittingfora
GPSdown/upconvertermaybenecessarywhichwillrequireinstallingadifferentcabletype.Keepinmindthat
someextracablecoiledinaceilingleftoverfromoverestimatingthecablelengthisnotnecessarilynegative.
Providedyouknowthelength
ofthetotalcable,thecabledelaycanbeaccountedforandthetimingaccuracy
maintained.
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