# Fibre-optic delivery of time and frequency to VLBI station

## TL;DR

This paper introduces a fibre-optic system for delivering precise time and frequency signals to VLBI stations, eliminating the need for local hydrogen masers and enabling remote synchronization with advanced optical clocks.

## Contribution

It presents a novel fibre-optic delivery method for T&F signals and demonstrates remote synchronization with a strontium optical lattice clock.

## Key findings

- Successful delivery of T&F signals via fibre-optic link
- Routine VLBI operation without local hydrogen masers
- Proof-of-concept remote synchronization with optical lattice clock

## Abstract

The quality of Very Long Baseline Interferometry (VLBI) radio observations predominantly relies on precise and ultra-stable time and frequency (T&F) standards, usually hydrogen masers (HM), maintained locally at each VLBI station. Here, we present an operational solution in which the VLBI observations are routinely carried out without use of a local HM, but using remote synchronization via a stabilized, long-distance fibre-optic link. The T&F reference signals, traceable to international atomic timescale (TAI), are delivered to the VLBI station from a dedicated timekeeping laboratory. Moreover, we describe a proof-of-concept experiment where the VLBI station is synchronized to a remote strontium optical lattice clock during the observation.