NEWS

Radar line-of-sight analysis is a good indication of whether a wind turbine will be visible to aradar. The most important result of the analysis is the visible height - the extent to which aturbine is visible, or hidden, from the radar beam.

The building block of a line-of-sight is the terrain profile between the turbine and the radar.For computer generated charts this terrain profile is built from a series of height data alongthe path between the radar and the wind turbine. This height data is often derived fromDigital Surface Models (DSM) or Digital Terrain Models (DTM).

Accuracy of the chart and the resulting visible height figure is dependent on the accuracy ofthe location and height data for the radar and wind turbine; the accuracy of the DSM/DTMdata and of the computing algorithm used to calculate profile height from the DSM/DTMdata.

DTM and DSM data points are normally equally spaced in a rectangular grid formation withthe distance between points known as post spacing – a typical value being 50 metres.However, when undertaking analysis neither the radar nor the wind turbines will necessarilybe located on the corner points of a fixed grid whose heights are known. Therefore, theneed arises for calculating the unknown ground height at a particular location from theheight of the surrounding DTM/DSM points.

A number of interpolation algorithms exist for doing this. Two commonly used methods bycommercial applications are the nearest-neighbour and “centring type” algorithms. Thediagram below shows a location of unknown height (i.e. turbine, radar, obstruction) markedX and the surrounding DTM data points marked DTM 1, DTM 2, DTM 3 and DTM 4. Thenearest-neighbour simply returns the value of the nearest point – DTM 1 in this example.

The centring algorithmeffectively repositions thelocation to the centre ofthe square and returns theaverage height of each ofthe four data points.However, the nearestneighbouralgorithm,although simple tocompute, has a reducedaccuracy because itassumes the terrain is flat.The centring algorithm hasalso reduced accuracy asthe location used for theassessment is wrong.

A more accuratecalculation method, usedby Pager Power’sbespoke software,Sightlines, is a weighted average algorithm which calculates the unknown height from theheights of the nearby points, in our example DTM 1, DTM 2, DTM 3 and DTM 4.

In conclusion, it is very important to understand which algorithm a radar line-of-sightsoftware application actually uses. Of the three mentioned methods the weighted averagealgorithm gives the most accurate results.