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10 years of LOFAR highlights: The use of GPS receivers and rubidium modules to sync the stations

By André Gunst

One of the important aspects of radio telescopes, in general, is the synchronisation in between antennas and for LOFAR in particular the synchronization between stations. Normally a clock is distributed from a central location to antennas. However, the stations are far apart and the distribution of a clock signal over distances of 100’s and even 1000’s of kilometres is not trivial. Therefore, it was decided not to distribute the clock but generate the clock locally near each station.

Published by the editorial team, 12 June 2020

For this a three-staged approach was taken to ensure stability on:

1. the long-term by the usage of GPS signals,

2. the mid-term by a Rubidium module and

3. the short-term by applying a crystal.

Soon, it turned out that the differential clock drifts between stations required an additional real-time calibration for the Tied-Array mode, which could be prevented by a central clock distribution. Since in the early days the Tied-Array mode used primarily the superterp stations, a central clock distribution was implemented for the superterp stations. The clock source was installed in the concentrator node and from there on the clock signals were distributed via a fibre link to the six superterp stations in the year 2010. Due to the great success of this, the central clock solution was extended to all 24 core stations in 2012. Currently, the optical clock distribution system results in a timing accuracy between the core stations within one nanosecond, which equals 0.000000001 seconds. In LOFAR2.0 also the remote stations will be synchronized via the same central clock source in the concentrator node which is located in the heart of LOFAR. In the future the ambition is, to include also the international stations in the central clock distribution such that at the end all stations are synchronized via the same central clock source.

Synchoptics board, credit: ASTRON

On 12 June 2020, LOFAR celebrates its tenth anniversary. The radio telescope is the world’s largest low frequency instrument and is one of the pathfinders of the Square Kilometre Array (SKA), which is currently being developed. Throughout its ten years of operation, LOFAR has made some amazing discoveries. It has been a key part of groundbreaking research, both in astronomy and engineering. Here we feature some – but definitely not all – of these past highlights, with surely more to come in the future.



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