Performance impacts related to primary surveillance radar (PSR) within the line of sight of wind farms is termed as the most common cause of concern to the aviation stakeholders.

These typically fall into the three categories: Unwanted turbine reports on the radar displays; Reduced probability of detecting the wanted aircraft reports in the vicinity of turbines; Tracking performance degradation.

There has been a lot of effort over the past few years to address these effects on radar. These range from very simple changes (such as simply blanking areas of radar displays), through high performance tracker designs, hardware/software modification of existing radars, fusion of data from new gap filling radars, novel gap filling radar designs and even stealthy wind turbine designs.

In orde to know more, windenergyupdate.com spoke to Samantha Dearman, team leader - wind farm radar impact assessment team, QinetiQ, and Chris New, the technical lead in the team about the impact of turbines on PSR, use of stealth materials and much more. Excerpts:

windenergyupdate.com: In the past, it has been acknowledged that the impact of turbines on primary surveillance radar or PSR is currently a roadblock to numerous developments and a technological solution mitigating this impact could potentially release hundreds of megawatts of renewable energy. How has this been taken care of to an extent?

QinetiQ: Ultimately, this is down to the developers and whether they wish to open up old projects, with the intent of using new mitigation techniques.

This is where advanced modelling techniques become crucial, in showing to the relevant stakeholders how successful a particular mitigation may be. It is generally accepted that there is unlikely to be a 'silver bullet' mitigation technique but a 'toolbox' of solutions may be the answer. The modelling techniques used by QinetiQ identify which mitigation solutions are likely to work best, with the agreement of all the stakeholders. As suitable land for wind energy development becomes increasingly scarce, opening up old, abandoned projects could be crucial for the UK in hitting its renewable energy targets.

windenergyupdate.com: How can one use stealth materials, radar upgrade and tracking solutions to mitigate the effect of wind farm on radar? What do you think are the major challenges associated with the same?

QinetiQ: The problems caused by wind turbines to radar and aviation facilities are complex.

One of the main problems is that the rotating blades can appear on a radar operator’s screen just like the aircraft they are trying to track. Many of the solutions on the radar upgrade side are attempting to implement techniques to remove the turbines from the display, however, turbine signals are extremely difficult to distinguish from aircraft signals, and consequently, sometimes these techniques can remove the aircraft signals too.

The air traffic control environment is a highly regulated one, rightly so, as safety is of the utmost importance. Thus, any mitigation solution has to work to an acceptable performance criteria, as outlined in the regulations. This is one of the main reasons why a technology solution is taking a long time to come to the fore, and is why it is thought that a toolbox of solutions is the way forward.

Each solution can focus on a particular problem, and so combining the solutions could mitigate the effects of the wind farms on radar. The greatest challenge is getting such solutions to the market, and through the regulations, with a safe implementation into the UK air traffic control environment.

windenergyupdate.com: It is said that a radar designer/optimiser has a variety of techniques at his disposal when attempting to ensure a high probability of detection, pd, for wanted targets whist minimising the number of false alarms. Can you provide an insight into the latest advanced modelling techniques to assess mitigation method effectiveness? What new trends have you witnessed in this arena?

QinetiQ: The advanced modelling tools that QinetiQ uses in day to day work allows us to essentially simulate a radar in its entirety, from transmission, to propagation and through to detection and display. We are able to test the performance of that radar in a simulated environment, and perform measurements on how well the performance matches expectations. This allows us to see changing subtle radar parameters, e.g. improving the constant false alarm rate algorithms, increasing the moving target indication cancellation or the use of moving target detection, altering the radar beam shape, higher resolution clutter mapping, etc.

All of these factors can have an effect on the ability of the radar to distinguish wind turbines from aircraft. This also allows for development and testing of mitigation techniques early in the design process. Advances in the QinetiQ modelling techniques have lead to a better understanding of what causes the problems, and exactly where those problems lie. Some of the newer mitigation techniques being proposed show some very innovative thinking to a complex problem.

windenergyupdate.com: It is said that Moving target indication, MTI, and its subsequent evolution into moving target detection, MTD, was the radar design breakthrough allowing the greatest step towards the goal of detection without clutter. What's your opinion regarding the same?

QinetiQ: Radar energy propagates out into the environment, and reflects off of almost everything surrounding it. Hence, the radar detects the reflected energy from objects such as terrain, buildings, cars, and also any aircraft in the sky, as well as rain.  Clutter, in terms of a radar display, is everything that the radar operator is not interested in. For instance, in an air traffic control environment, the operator is only interested in aircraft; therefore, everything else is deemed clutter. Most of this clutter is either from stationary or slower moving objects than aircraft. Moving target indication is a process that can detect if a signal is from a moving or stationary object; if the object is stationary, it can be filtered out. This process effectively removes most of the clutter from an air traffic controller’s display, assisting in controlling the aircraft without the clutter interference.

However, not all clutter is removed; clutter from strong signals is difficult to filter out. With wind turbines send strong reflections back to the radar, as well as having fast moving blades, and consequently, the moving target indication filters are often unable to remove the effects. Moving target detection is a more advanced form of moving target indication, which allows the radar to determine some information on how fast an object is moving. Moving target indication can only determine if the object is moving, and not its speed. Traditionally, moving target indication would detect a whole blade as one moving object, but moving target detection can determine what power is coming from different parts of a turbine’s blades (as different parts of the blades move at different speeds).

Therefore, rather than processing the blade as a single, high powered reflection, the radar can process the blade as a number of different objects, each with a lower power than the whole blade. This can help reduce many of the problems associated with the effects of wind turbines on radar processing. However, the radar will still detect moving objects, which can be confused with aircraft. Thus, whilst moving target detection is not a silver bullet solution, it is a big step forward, and a major contribution to the mitigation solutions toolbox.

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.

For more information click here: http://www.windenergyupdate.com/eurosite09/programme.shtml

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Contact: Pete Carkeek by email pcarkeek@windenergyupdate.com