White Rabbit in radio interferometry

Radio interferometry relies on distributed telescopes having precise time and frequency sources that allow them to operate coherently over timescales up to several hours. As radio telescopes are being connected to fiber-based high-speed communication networks, it is of interest to make use of these for time and frequency distribution. The White Rabbit protocol enables the accurate and precise distribution of time and frequency signals over telecommunication optical fibers. We set out to evaluate the quantitative limits for interferometers over a range of observing frequencies when synchronized through White Rabbit. We develop a method to quantify the possible loss of sensitivity due to the phase noise contribution of a White Rabbit link. Our findings include a new expression for the coherence loss due to flicker phase noise. As this type of noise is common in frequency transfer links, its use extends beyond the case of White Rabbit. Furthermore, we designed a calibration procedure to measure the dispersion on already deployed fiber networks. We demonstrate adding a White Rabbit signal to an existing high-speed production network, together with data traffic on other wavelengths on the same fiber. Finally we built a VLBI setup with fiber links of 35 and 169 km, connecting two radio telescopes together. The agreement between our predicted and measured coherence loss indicates the usefulness of our approach, and that White Rabbit is suitable for clock distribution in radio interferometry instruments. We find that regular White Rabbit v3 switches support observing frequencies up to 3.5 GHz, and their low-jitter version up to 15 GHz.