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WIND TURBINE CLUTTER


FEATURE SPONSOR


HOW RADAR CLUTTER CAN HELP MITIGATE THE IMPACT OF WIND TURBINES


The term ‘clutter’ is often used by aviation stakeholders when referring to wind turbines, and the potential impact they can have on Primary Surveillance Radar (PSR) systems. You may be forgiven therefore for thinking that clutter is somehow a modern problem that is unique to wind turbines.


HISTORY


In fact, the problem of clutter has existed from the time of the first radar experiments in the 1930s. Clutter is by definition any unwanted reflected signals of sufficient magnitude to impede the job of detecting targets (i.e. aircraft), and in the real world an awful lot of things reflect radar signals.


Furthermore, it’s not just large structures that cause clutter. High ground, buildings, road vehicles and even birds all have the potential to produce clutter, which the radar has to manage.


AVIATION RADAR


It’s not surprising to learn that various innovative techniques have been developed over the years to allow primary radar systems to dynamically manage clutter and reduce the impact it has on radar performance. For aviation radar, a useful technique is the ability to separate the clutter according to whether or not it is from a moving object.


This is possible by looking for small changes in the frequency of reflected signals as a result of the Doppler Effect, and is referred to as Moving Target Indictor (MTI) filtering. As the majority of clutter is from non-moving objects, this technique vastly reduces the amount of clutter the radar has to deal with. However, this technique is far from perfect as the filtering process does not occur at the front end of the receiver chain, and hence strong signal clutter (from non-moving objects) can swamp the MTI filter.


REDUCING CLUTTER


For this reason it is essential to reduce the amount of clutter in the first place, and the only way to control this is through the design of the antenna. So, as well as producing a beam that is narrow in azimuth, radar antennas will also have a vertical pattern that is aligned to be above the horizon and reject (to as greater extent as possible) signals that are at low elevation angles.


Furthermore, the two-way nature of the radar antenna means that any reduction in the amount of energy transmitted by the antenna at low elevation angles is effectively doubled in so far as clutter reflections from objects located on the earth’s surface.


SO WHY IS THIS IMPORTANT? Many wind turbines are sited at low elevation angles relative to radar antennas, and are far from the only source of clutter that the radar will see. Due to the local clutter environment (irrespective of wind turbines) most radar antennas will operate with a upward tilt angle to reduce the amount of clutter they ‘see’ in order to ensure they perform efficiently.


The tilt angles and artefacts of the radar beam pattern are too detailed for airports and radar operators to take account of in their desktop assessments, so very often wind turbine planning applications receive objections that appear to be clear cut and indisputable. The reality is often very different.


With our ability to measure radar signals in the airspace immediately above a proposed wind turbine site we can clearly see the variation that occurs with height due to the antenna’s vertical beam pattern. The results consistently show what has been described here, i.e. the need to focus the radar’s energy into the sky not the ground to avoid clutter.


Furthermore, by being able to measure the strength of the actual radar signal at blade- tip height we can more accurately predict the amount of reflected signal that will be returned to the radar receiver, which will almost certainly be less than assumed in any desktop assessment.


REASSURANCE


This level of information can reassure the radar operator about the likelihood of detection, and help overcome objections.


So, if you have a wind turbine that has received an objection due to radar issues, it is entirely possible that clutter turns out to be the thing that helps rather than hinders you.


Aerostat


96


www.windenergynetwork.co.uk


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