Reducing the amount of energy reflected back to the radar

Wind turbines have the potential to impact air navigation service provider's electronic infrastructure in a number of ways.

Such infrastructure consists of radar, communication systems and navigational aids. It is said that any of these could be compromised by the establishment of a windfarm in the wrong place.

In order to safeguard this infrastructure, an entity in NATS En Route Plc, NERL, assesses the potential impact of every proposed windfarm development in the UK. Inevitably these assessments have led to NERL objecting to a number of proposals and potentially becoming a roadblock to the realisation of hundreds of megawatts worth of renewable energy. The potential for primary surveillance radar, PSR, to confuse turbines with aircraft is the reason for the majority of NERL objections.

The function of an ATC surveillance technology, such as radar, is to accurately determine an aircraft's position and provide this information, in a timely fashion, to an air traffic controller in order that one can guide it towards its destination whilst maintaining separation from other aircraft.

Radar returns from wind turbines can distract and confuse air traffic controllers.

One of the main problem of wind turbines, apart from their sheer size, is that they are extremely reflective to radar energy. This energy can not only cause problems for the radar, but also for the radar operators watching the display.

The QinetiQ advanced radar modelling software has been used to predict how well the new radar absorbent material is likely to perform, and so far the research has suggested that the turbines are likely to be much less visible to the radar operators, which should help to significantly reduce the effects of the turbines on radar and aviation services, according to Samantha Dearman, team leader - wind farm radar impact assessment team, QinetiQ, and Chris New, the technical lead in the team.

According to Samantha, who is scheduled to speak during European Wind Farm Site Selection Summit (Hamburg, 1-2 April) QinetiQ's aim with respect to impact assessments is to get involved as early as possible in the planning process.

In the early stages, simple techniques can be used to identify major issues with a potential development and can aid the developer in deciding which projects are worth pursuing.

In this type of situation, a line of sight assessment can be ideal for identifying early radar objections. Radar propagation modelling, whilst accurate, can make these early assessments expensive.

QinetiQ has developed a tool into which the user enters turbine coordinates, and then the tool returns all radars in the country that are in line of sight to the development, which could potentially raise an objection. Also, a list of all aerodromes (military and civil) and controlled airspace regions are returned, providing a snapshot of the areas of operational significance. This method of analysis, which is all displayed in Google Earth, allows a quick assessment of a site to be made.

A similar type of analysis can also be undertaken for microwave fixed links, another potential stumbling block for many developments early in the planning process.

As the number of wind turbines erected on and off-shore increases, the impact of wind turbines on radar has become more evident. In this context, it is vital to assess the utility of tools used to quantify and predict the impact of wind farms on radar and communication systems.

"The view of the QinetiQ Wind Farm Radar Impact Assessment Team is that accurate modelling combined with advanced visualisation techniques is crucial to fully understanding the impact that wind turbines can have on radar, aviation and telecommunications," said Samantha.

According to Samantha and her colleague Chris, simplistic tools, such as line of sight, often provide a good approximation to the potential impact of wind turbines but to understand in more detail how the turbines would impact on a particular radar or communication facility sometimes requires more complex modelling.

Citing an example, they said, line of sight relies purely on the curvature of the earth, the geometry of the radar, the terrain and the height of the turbine. Radar propagation analysis, however, takes into account advanced radar parameters that describe how the energy is transmitted by the radar, how that energy propagates through the atmosphere, over the terrain (including effects like diffraction, reflection and refraction), and how much of that energy is reflected back by the turbine structures, and ultimately detected by the radar. More often than not, objects that are out of line of sight to the radar can still be detected by the radar because of effects such as diffraction over hills, etc.

This is why a propagation assessment can be preferential to a basic line of sight. Even though some of the turbines may not be in line of sight, the propagation analysis can determine if the turbines are still likely to be detected. This analysis can be taken a step further by undertaking a full air traffic control simulation, which considers how the turbines will move and rotate in time, creating the familiar 'twinkling' effect that radar operators often see. It is this kind of modelling evidence that can allow an air traffic controller to make a better informed decision about the impact of a potential development on their air traffic control operations.

Another challenge faced by QinetiQ in undertaking impact assessments, is conveying information to a wealth of different stakeholders, some whom have minimal knowledge in the domain, others who have a great deal. Thus, utilising the visualisation techniques to convey the information from the models is just as critical as the modelling techniques themselves.

European Wind Farm Site Selection Summit

QinetiQ's Samantha Dearman is scheduled to speak during European Wind Farm Site Selection Summit, which is scheduled to take place in Hamburg in April this year.

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or contact: Ian Evans by email