%AMena-Parra, J.%ALeung, C.%ACary, S.%AMasui, K.%AKaczmarek, J.%AAmiri, M.%ABandura, K.%ABoyle, P.%ACassanelli, T.%ACliche, J.%ADobbs, M.%AKaspi, V.%ALandecker, T.%ALanman, A.%ASievers, J.%Anull%BJournal Name: The Astronomical Journal; Journal Volume: 163; Journal Issue: 2; Related Information: CHORUS Timestamp: 2024-01-09 15:19:56 %D2022%IDOI PREFIX: 10.3847 %JJournal Name: The Astronomical Journal; Journal Volume: 163; Journal Issue: 2; Related Information: CHORUS Timestamp: 2024-01-09 15:19:56 %K %MOSTI ID: 10363054 %PMedium: X; Size: Article No. 48 %TA Clock Stabilization System for CHIME/FRB Outriggers %XAbstract

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has emerged as the prime telescope for detecting fast radio bursts (FRBs). CHIME/FRB Outriggers will be a dedicated very-long-baseline interferometry (VLBI) instrument consisting of outrigger telescopes at continental baselines working with CHIME and its specialized real-time transient-search backend (CHIME/FRB) to detect and localize FRBs with 50 mas precision. In this paper, we present a minimally invasive clock stabilization system that effectively transfers the CHIME digital backend reference clock from its original GPS-disciplined ovenized crystal oscillator to a passive hydrogen maser. This enables us to combine the long-term stability and absolute time tagging of the GPS clock with the short- and intermediate-term stability of the maser to reduce the clock timing errors between VLBI calibration observations. We validate the system with VLBI-style observations of Cygnus A over a 400 m baseline between CHIME and the CHIME Pathfinder, demonstrating agreement between sky-based and maser-based timing measurements at the 30 ps rms level on timescales ranging from one minute to up to nine days, and meeting the stability requirements for CHIME/FRB Outriggers. In addition, we present an alternate reference clock solution for outrigger stations that lack the infrastructure to support a passive hydrogen maser.

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