A windfarm is being constructed, consisting of 45 turbines along several rows extending 3-7 km roughly eastward from the LOFAR Core area.
All the "special transports" of major components such as rotor blades have concluded. The turbines are being taken into operation one-by-one. Construction is expected to finish soon (summer/fall) this year.
The construction work itself will, from time to time, lead to RFI, but the magnitude and extent is unpredictable (at the time of the Cycle 17 proposal call). Consultation with the windfarm operators is set up with the aim to restrict RFI impact.
In accordance with conditions laid down in the co-existence covenant between the windfarm operators and ASTRON, the turbines have been specially designed to limit their electromagnetic emissions on the LOFAR Core in the relevant frequency bands (“RFI”). Significant adaptations to the standard design have led to substantial improvements.
A measurement campaign on the fully operational test turbine has confirmed that there is no excess RFI emitted above the ceiling agreed in the covenant. That limit can be globally summarised in astronomical terms as leading to less than 10% increase above the thermal noise in an image from a 4-hour integration (LBA or HBA). Note that long-baseline imaging will be largely unaffected, because the noise is not correlated.
It is likely that in some frequency ranges, the actual RFI emitted by the windfarm will be below that ceiling, but the measurement campaign was not designed to yield reliable measurements below the agreed upper limit.
Adherence to the limit will be actively monitored when the windfarm is operational; in case of transgression, the windfarm operators have the obligation to solve the issue.
The measurement campaign on the test turbine has confirmed prior expectations of increased RFI from external sources reflected by the windfarm onto LOFAR. The most important impact has been measured in the band 174-230 MHz, which has Europe-wide frequency allocations for Digital Audio Broadcasting. DAB broadcasters close to LOFAR stations are known sources of RFI already; with the windfarm, DAB broadcasters from a wider area will impact the LOFAR Core. A few narrow bands are unaffected in this frequency range, and certain types of broad-band science may still be possible; proposers should contact Radio Observatory staff for advice in advance. Current information indicates that the HBA below 174 MHz is also largely free of RFI by reflection.
As part of the covenant conditions, the operators will put their turbines into “EMC Shutdown Mode” for at least 600 hours per year, during low-wind conditions. All electrical equipment will be off, the turbines will be parked and not turning.
The measurement campaign has not been designed to yield reliable measurements below the agreed emission upper limit. Arguments suggest that standstill may yield a 10-15 dB reduction of RFI emitted, but only upper limits have been measured to date. Users interested in deep observing (several tens of hours or more in a field), are encouraged to contact the ASTRON well ahead of the proposal deadline, to explore setting up a shared-risks project, in which their exposure time could be carefully built up with the dual goals of gradually yielding science, and eventually measuring the practical noise levels in standstill conditions. The science validity and allocation of such a project will be subject to ILT-PC review. It could be considered to carry out such a project throughout the windfarm building phase.
During standstill time reflection RFI will be stationary, because the turbines do not turn. Transient and pulsar projects may therefore benefit from standstill time.
Proposers should include careful argumentation why their project would greatly benefit from standstill time. Most standstill time slots will come at unpredictable moments, each lasting 24 hours only. It may be possible to schedule at short notice observations requiring standstill RFI conditions at any LST during these time slots, although this cannot be guaranteed. Since the LOFAR does not have dynamic scheduling in place, standstill time cannot be an absolute requirement for a project, and will not be guaranteed to any project. Instead, earmarking for eligibility will be conferred to scientifically highly ranked projects that clearly argue their benefit from windfarm standstill time.