MAIN PUBLICATION :

• APPENDIX
• ABREVIATIONS
• GLOSSARY
• REFERENCES
• EVENTS

Radars and radio signals

The wind turbines may impact on aviation activity, both civil and military, due to interference with radars that manage aircraft operations. The radar is a system for detecting the presence or position or movement of objects by transmitting radio waves, which are reflected back to a receiver. The radio wave transmitted by radar can be interrupted by an object (also called target), then part of the energy is reflected back (called echo or return) to a radio receiver located near the transmitter.

The wind turbines are vertical structures that can potentially interfere with certain electromagnetic transmissions. Mobile structures such as rotating blades may generate more interference on the radars. The effects depend on type of radar, specific characteristics of wind turbines and the distribution of wind turbines. Air traffic management is susceptible to being negatively affected by wind turbine installations. The systems managed by radars are: Air traffic control, military air defence and meteorological radars .

The table 2.5 summarises the functions and the mitigation measures according to the different types of radars and wind turbines effects in the UK

Table 2.5. Effects and Mitigation measures by radar types. (Based on DTI, 2002).

The impacts associated with wind turbines are masking, returns/clutter and scattering.

Masking

The radars work at high radio frequencies and therefore depend on a clear "line of sight" to the target object for successful detection. When any structure or geographical feature is located between the radar and the target, it will cause a shadowing or masking effect. The interference varies according to turbine dimensions, type of radar and the aspect of the turbine relative to the radar. The masking of a real aircraft can occur by reflecting or deflecting the returns when the aircraft is flying in the "shadow" of wind turbines and it is not detected. Also the masking can occur when returns from the towers and blades of the wind turbines are so large that returns from real aircraft are lost in the "clutter" (radar returns from targets considered irrelevant to the purpose of the radar.)

Returns/clutter

Radar returns may be received from any radar-reflective surface. In certain geographical areas, or under particular meteorological conditions, radar performance may be adversely affected by unwanted returns, which may mask those of interest. Such unwanted returns are known as radar clutter. Clutter is displayed to a controller as "interference" and is of concern primarily to ASACS and aerodrome radar operators, because it occurs more often at lower altitudes.

The combination of blades from turbines at a wind farm can give an appearance of a moving object, which could be considered as an unidentified aircraft requiring controllers take action to avoid a crash with another aircraft.

Scattering, refraction and/or false returns

Scattering occurs when the rotating wind turbine blades reflect, or refract radar waves in the atmosphere. The source radar system or another system can absorb the waves and provide false information to that system. This effect is not well known, but it has been reported in Copenhagen airport as a result of the Middelgrunden offshore wind farm.

The possible effects are:

• Multiple, false radar returns are displayed to the radar operator such as blade reflections.
• Radar returns from genuine aircraft are recorded but in an incorrect location.
• Garbling or loss of information.

Marine radars and communication and navigation systems may suffer interference from nearby wind farms. Howard & Braun, 2004 stated that most of the effects of Hoyle offshore wind farm do not significantly compromise marine navigation or safety. Mitigation measures in open water include the definition of vessel routes distant from wind farms, while in restricted areas the boundaries of wind farms must be kept at appropriate distances from navigation routes or port approaches.