In January 2023, Creotech delivered White Rabbit switches to the Dutch radio astronomy institute ASTRON, operator of the LOFAR network. LOFAR is the world’s largest radio telescope operating at the lowest frequencies observable from Earth. It is a highly distributed instrument, with 38 stations in the northern Netherlands: 24 located in a dense core and 14 spread over distances of up to 120 km. Another 14 stations are located across Europe — six in Germany, three in Poland, and one each in the UK, Sweden, France, Ireland, and Latvia — effectively forming a telescope the size of Western Europe.
Thanks to extremely long baselines of up to 2,000 km, with antennas spread across the continent, LOFAR achieves high angular resolution despite operating at exceptionally low frequencies. This opens a window into regions of the radio sky that other instruments can barely access, enabling studies of the early Universe, galaxies, radio emission, magnetic fields, and more.
LOFAR2.0 relies on White Rabbit switches to synchronize timing signals from all antennas in the Netherlands with sub-nanosecond precision. This precise synchronization improves ionospheric calibration and imaging, while enabling the distribution of time and frequency over hundreds of kilometres — a critical component of the LOFAR2.0 timing system. Interestingly, the link path between the master clock and the furthest slave switch represents a unique application compared to other radio telescopes.
Upgrading from LOFAR1.0 to LOFAR2.0 required, among other improvements, a major increase in timing stability. A perfectly synchronized, modern radio telescope network spread across the Netherlands? Challenge accepted.
Another fun fact: in typical White Rabbit networks, distances between elements are around 10 km. ASTRON pushed the limits, deploying the longest direct White Rabbit link to date — 65 km between two switches.
Delve deeper into the details:
Context / Problem
Random clock offsets between remote stations and the core require frequent calibrations, reducing the telescope’s operational time. Clock drift also increased pointing errors.
Goal
A distributed clock system with small, fixed offsets across all Dutch LOFAR stations. By distributing a central clock, all stations operate on the same time base, reducing systematic effects in the scientific data.
Solution
• Installation of a White Rabbit clock distribution network with precisely calibrated links to all Dutch LOFAR stations.
• Use of DWDM SFPs to improve link stability.
• Use of adjacent DWDM channels to minimize dispersion effects on forward and reverse paths.
Result
Left: LOFAR1.0
The plot shows the clock offset of individual stations relative to a reference clock. In addition to static offsets between stations, a clock drift of approximately 10–20 ns over an 8-hour observation is visible.
Right: LOFAR2.0
The plot shows the clock offset between two stations using the new White Rabbit–based clock distribution. Red dots represent individual observations (1 hour and 10 minutes each), while the black line is a fit to the data. The clock offset is reduced from hundreds of nanoseconds to just a few nanoseconds, and the clock drift drops to approximately 0.05 ns/hour.
Key takeaway
The stable hardware enabled the deployment of the clock distribution network with high reliability, drastically improving timing accuracy and overall radio telescope performance.
