In order to receive radio signals from across the Universe, LOFAR needs to be very sensitive. The downside of that sensitivity is susceptibility to radio interference: other sources that produce radio signals that LOFAR detects, but does not want to measure. ASTRON staff members are working out technologies to make LOFAR more robust against interference.
A big source of interference are wind turbines. Wind farms around LOFAR produce radio waves that are detected by LOFAR. By knowing the source and frequency of this interference, they can be filtered out of the data, but that creates a blind spot.
“In 2016, we, together with the Ministry of Economic Affairs and the wind farm builders, drew up a covenant,’ says ASTRON’s senior researcher Stefan Wijnholds. ‘Part of that covenant is that we will be allowed to ask the wind farm owners to turn off their turbines at the moment that we want to make our most sensitive observations. As compensation we will make our telescope more robust against the radio interference of wind turbines. For this, we have received government funding.”
Wijnholds: “We have selected the most promising techniques, which are now being worked out further. The most promising method is a technique called ‘spatial filtering.’” LOFAR looks into the sky in a single direction. All radio signals that LOFAR receives from that particular direction are added together, so that these signals strengthen each other. “You can do the exact opposite for signals that you do not want to observe,” Wijnholds says. “You can make these signals counteract one another, nulling them out completely. In 2004 we successfully demonstrated this technique with a LOFAR prototype station.”
An example of nulling. Radio interference (left) can be removed by nulling (right). (Copyright: Albert-Jan Boonstra, from Radio Frequency Interference Mitigation in Radio Astronomy, 2005)
Another measure that could be taken is expanding the wire meshes under the Low Band Antennas (LBA). Wijnholds: “Each LBA is built on a grid of 3-by-3 metres of wire mesh. Without that, the antennas would be sensitive for radio signals from the horizon, which are signals you do not want to measure.” The wind turbines are positioned at such a distance that from the antenna stations of LOFAR, they appear to be close to the horizon, Wijnholds explains. “The wire meshes that are currently placed under the antennas, already filter out much of this ‘horizon interference’, but due to cost reductions, the wire mesh does not cover the entire area covered by the station. If we cover the ground below the antenna field entirely with wire mesh, horizon interference will be even further suppressed. We are already implementing this with SKA, the Square Kilometre Array.”