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How it’s done – Assessing Wind Farm Radar Interference – France (military)

How it’s done – Assessing Wind Farm Radar Interference – France (military)
November 2, 2017 Kai Frolic


Radar interference due to wind farms is managed differently in various countries. A previous article gave an overview of the approaches that are the most common. This article presents the criteria that are generally applied in France for Primary Surveillance Radar concerns.

Figure 1

Figure 1: The French military applies specific rules for siting turbines.

Wind Farm Rules in France (Primary Surveillance Radar)

The challenges for wind farm developments are dynamic, and safeguarding rules change over time for various reasons. For more on the current picture in France, please see this article.

Presently, the military’s stance on wind farm radar interference can be summarised as follows:

  • The primary concern is shadowing – i.e. reduced probability of detection for aircraft beyond the wind farm. This is illustrated within Figure 2.
  • Wind turbines within a wind farm should not occupy more than 1.5 degrees horizontally, relative to the radar (more on this below).
  • Adjacent wind farm should be separated by at least 5 degrees horizontally (more on this below).

Figure 2

Figure 2: Shadowing illustration.

Sector Width

A radar generally provides coverage through 360 degrees. A target due exactly north of the radar can be described by a bearing of 0 degrees. A target due east would have a bearing of 90 degrees, and so on.

A wind turbine that is detectable to a radar will have an associated bearing. A wind farm comprising multiple turbines will have a range of associated bearing. The angular width of the wind farm from the radar’s perspective is defined as the difference between the maximum bearing and the minimum bearing of detectable turbines within the wind farm (if the wind farm is spread either side of the 0/360 degree radial this must be accounted for within the calculation of angular width).

The French military requires the angular width of a wind farm to be within 1.5 degrees. This is illustrated in Figure 3.

Figure 3

Figure 3: Angular width of a wind farm

Under the current rules, if the angle theta is 1.5 degrees or less, the wind farm’s footprint is sufficiently small and can be accepted by the French military. If the angle exceeds 1.5 degrees, an objection is much more likely.

Separation Between Wind Farms

The other siting rule that is related to angular width is the separation between adjacent wind developments. This is illustrated in Figure 4.

wind farm radar interference

Figure 4: Angular separation between wind farms

Under the current rules, if the angle gamma is 5 degrees or more, the separation is sufficiently large and can be accepted by the French military. If the angle is below 5 degrees, an objection is much more likely.

Things to Consider

In order to understand the likely acceptability of a wind farm, and manage mitigation options appropriately, it is important to ascertain:

The third point is relevant if, for example, parts of the wind farm area are below radar line of sight. The footprint that matters should be based on turbines that have the potential to cause an impact, which isn’t necessarily all turbines within a development.

For any advice or queries regarding a specific project, please feel free to contact Pager Power (info@pagerpower.co.uk).


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