# Picosecond-precision optical time transfer in free space using flexible binary offset carrier modulation

## TL;DR

This paper introduces a flexible optical time transfer method using binary offset carrier modulation that achieves picosecond precision with significantly reduced bandwidth, eliminating multi-path effects and enabling outdoor timing over free space.

## Contribution

The proposed FlexBOC modulation technique offers high-precision free-space optical time transfer with at least 97.5% bandwidth reduction and improved robustness against physical obstructions.

## Key findings

- Time deviation of 2.3 ps over 15 hours
- Frequency instability of 4.0E-12 at 1 s
- Approaching 2.6E-15 at 10,000 s

## Abstract

Free-space optical time transfer that features high precision and flexibility will act a crucial role in near-future ground-to-satellite/inter-satellite clock networks and outdoor timing services. Here we propose a free-space optical flexible-binary-offset-carrier-modulated (FlexBOC-modulated) time transfer method. The utilized FlexBOC modulation could yield a comparative precision, although its occupied bandwidth is tremendously reduced by at least 97.5% compared to optical binary phase modulation. Meanwhile, the adoption of optical techniques eliminates the multi-path effect that is major limit in the current microwave satellite time transfer system. What's more, the time interval measurement avoids a continuous link that may be routinely broken by physical obstructions. For verification, a time transfer experiment with our home-built system between two sites separated by a 30-m free-space path outside the laboratory was conducted. Over a 15 h period, the time deviation is 2.3 ps in a 1-s averaging time, and averages down to 1.0 ps until ~60 s. The fractional frequency instability exhibits 4.0E-12 at a gate time of 1 s, and approaches to 2.6E10-15 at 10000 s.

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Source: https://tomesphere.com/paper/1905.13553