Impact of wind farms on low-frequency radio astronomy observations with LOFAR.

The LOw Frequency ARray (LOFAR} – developed by ASTRON, the Netherlands Institute for Radio Astronomy – is the world’s largest and most sensitive low-frequency radio telescope. It is a network of geographically distributed antenna stations, each containing hundreds of 'low-band’ (LBA} and 'high-band’ (HBA) dipole antennas. LOFAR stretches across Europe, from Ireland to Poland, with a dense `core’ in Exloo, the Netherlands. LOFAR is opening one of the last unexplored regions of the spectrum for astronomy – very low radio frequencies, from 30 to 240 MHz. There are plans to erect a large wind turbine farm near this central core, at distances down to 2 km from the LOFAR core area. The windfarm will have a negative impact on LOFAR’s measurements. Several mechanisms have been identified that affect the LOFAR data: direct interference from the turbine; diffraction of external signals on the turbine’s blades and structure; reflection of external signals on the turbine. Measurements on existing remote wind farms have been done to quantify the impact of the planned nearby wind farm, both with external equipment as well as with LOFAR itself. Together with the Agentschap Telecom a system is proposed to independently measure the impact of wind turbines. In the presentation we will present the effects of the wind turbines on the astronomical measurements with LOFAR; the mechanism which will impact these measurements and the system to analyse the impact of wind turbines