Conditions for LOFAR Radio Telescope and Wind Farm Co-existence

The LOFAR radio telescope is a low-frequency radio interferometer in the north of the Netherlands and across Europe. The location selection of the core area of LOFAR was based on its relatively large distance to urban areas, and to industrial and logistics infrastructures. Recently, to meet the demand for renewable energy in the Netherlands, new wind farms were projected throughout the country. This includes a large wind farm to be built close to the core site of LOFAR. The farm is projected to include 45 turbines of approximately 4 MW and a wing-tip height of about 200 m, threatening to deteriorate the relatively radio quiet environment. Several experiments with remote LOFAR stations close to existing wind farms have shown increased interference levels due to interference reflected from wind mill blades and also intrinsic emissions from the wind turbines. . To protect the LOFAR core area from hampering or even crippling interference, a covenant was signed between ASTRON and the wind farm developers, aimed at defining operational conditions that allow mutual co-existence. One part of the covenant considers measures for making wind turbines more quiet, and also includes agreements on building restrictions and turbine operational restrictions, depending on interference levels. Several categories have been defined, ranging from 35 to 50 dB improvement compared to the EN55011 class A norm. At the projected distances to the LOFAR core stations, these levels lie well below the galactic noise, but exceed the ITU RA769 recommendation in the LOFAR frequency range by far. When the recommendation was written a sensitive low frequency instrument such as LOFAR was not considered possible or realistic. . In order to determine which category a wind turbine belongs to and which restriction type is applicable, an emission measurement protocol has been jointly defined by Agentschap Telecom (Dutch radio regulations authority), ASTRON (LOFAR radio telescope operator), INAF, and the wind farm project developers. Given the relatively low interfering levels that need to be detectable, the measurement protocol is based on radio interferometric imaging, imaging, including of subtraction of celestial sources that may otherwise influence the detection of wind turbine emissions. This contribution will present an overview of the protocol and measurement approach.