Fig 1.
Modified FURUNO FAR 2117 marine radar setup used in this study.
The red oval indicates the modified part of the radar–the waveguide in between aluminum panels for a custom adjustable vertical angle of the antenna. The clutter shielding fence is visible behind the radar, with three towers of the wind turbines in the background.
Fig 2.
Schematical representation of the measurement setup in the wind park.
The location of the radar setup (radar and clutter shielding fence) is indicated by the small blue rectangle at the origin of the coordinate system. The simulated detection volume of our marine radar (RCS = 12.7cm2, antenna inclination angle 15°) is in turquoise, with more saturated shade for the threshold of -74 dBm and less saturated for -89 dBm. The simplified shape of the detection volume of the ABD (for bat calls with frequencies ≤ 20 kHz) is in red.
Fig 3.
Measured distance from the radar antenna to an object.
x—horizontal distance of an object to the radar antenna; z—elevation of an object over the radar antenna; r—object’s distance along the radar beam, which is traced directly on the 2D radar image.
Fig 4.
Field measurements of intensity levels of metal spheres with known RCS.
Tripod, made from thin, dry bamboo rods, holding a polished hollow aluminum sphere of 150 mm diameter.
Fig 5.
Procedure to identify a bat event.
Fig 6.
Two areas of interest around the microphone, with radii of 40 and 80 m.
(A) Location of the areas of interest on the radar image, (B) vertical projection of boundaries of the areas of interest.
Fig 7.
Calculated regression of measured intensity level over the simulated received power.
Grey dotted line illustrates the robust regression line y = 1.4666∙x+132.3656. Horizontal whiskers are the range of the simulated values considering the inaccuracy of the measurement position. The orange dot is the theoretical value taken from [49] for comparison.
Fig 8.
Study data from the measurement season 2020.
Fig 9.
Radar image of a typical bat event with detected tracks.
The large radar echo in the center of the image is the wind turbine. The red dot on the radar echo is the estimated instantaneous location of the ABD in the nacelle. The red circumference indicates the 40 m detection range of the ABD for signals with frequency f ≤ 20 kHz. The orange circumference indicates the 80 m range from the ABD. The detected tracks are plotted in red with their pathway indicated by the increasing order of enumeration. Identified clutter on the image: a—trees, b–transmission towers, c–vegetation alongside long ditches.
Fig 10.
Distance over time diagram of the aligned typical bat event.
Tracks are represented as a distance of a radar echo from the ABD at the timestamp of a corresponding radar image. Acoustic data is represented as points in time of the new aligned timestamp at the estimated detection distance of the bat detector for a corresponding frequency.
Fig 11.
Relative frequency graph of the average track speed of the identified tracks (n = 323).
Tracks with same boundary integer values were grouped together in one category.
Fig 12.
The number of tracks within outer circumference during active time of a bat event.
Orange dashed line illustrates the robust regression line y = 0.1169∙x+0.5146.
Fig 13.
Expected intensity levels for an object with an RCS of 12.7cm2 near a wind turbine.
Expected intensity levels calculated from simulated reflected power using a robust regression line. Two half-circles (r = 40m) represent the two extreme positions (the closest to and furthest from the radar) of the simplified detection volume of the ABD for bat calls with frequencies f ≤ 20 kHz. The vertical line gives reference to the tower position and height of the WT.
Fig 14.
Expected and measured intensity levels within the detection volume of the bat detector.
Simulated values sampled in 5 m intervals for vertical and horizontal distances. Horizontal whiskers indicate the inaccuracy of the measurement position. Vertical whiskers consider the measurement uncertainty of ± 3 intensity levels (± 2 dBm).