# The correlation of co-located hydrogen masers

## TL;DR

This paper investigates the correlation between co-located hydrogen masers using a fiber-based frequency transfer network, revealing how correlation varies with averaging time through experimental and simulation methods.

## Contribution

It introduces a method to directly measure co-located hydrogen maser correlation using fiber-based frequency transfer and provides empirical data on correlation behavior over time.

## Key findings

- Correlation is negligible below 10^3 seconds.
- Correlation coefficient increases rapidly between 10^3 and 10^5 seconds.
- Correlation decreases beyond 10^5 seconds up to 5 days.

## Abstract

The correlation of co-located hydrogen masers (H-masers) is difficult to measure because their common-mode noise induced by the environment will be cancelled out during the comparison measurement. With the development of fibre-based high-precision time and frequency transfer technique, the correlation of co-located hydrogen masers can be directly measured with the help of remote H-masers. Recently, a fiber-based frequency synchronization network was constructed in the Beijing region by connecting 5 H-masers from 4 institutions. The correlation coefficient of atomic clocks is defined and the correlation between two co-located H-masers is measured using both experimental and simulative methods. The results show that the correlation is not prominent until the averaging time is larger than $\sim10^3$s; then, the coefficient grows rapidly for averaging times ranging from $\sim10^3$s to $\sim10^5$s and decreases beyond $\sim10^5$s up to 5 days.

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Source: https://tomesphere.com/paper/1705.06580