15. Television interference, radar and aviation

 

Telecommunications and television    

 

Wind turbines can interfere with telecommunications signals including TV and radio, mainly by the multi-path effect, where there is corruption or distortion of the received signal by the secondary signal. Uniquely with wind turbines this may 'chop' the signal causing variable 'ghosting' or 'jittering' on the TV picture.

 

The effects of wind power fall into two main categories: effects on broadcast television and effects on fixed radio links, mostly at microwave frequencies.     Wind turbine effects on television reception are generally found where the TV is situated between a wind farm and the TV transmitter. Modern composite blades have less effect than older metal rotors but embedded lightning conductor strips may negate the advantage.    

 

Reception solutions may require the use of a more sensitive aerial or aiming it at a different transmitter. More expensive remediation may need a re-broadcasting mast, satellite or cable supply to affected householders. Once analogue TV is replaced by digital it is possible that transmission will be less vulnerable to interference.     

 

The SDC (2005) report includes a useful case study of the Blaen Bowi wind ‘farm’ in Carmarthenshire where it is claimed the initial problems with TV reception have been solved. 

 

Aviation and military considerations  

 

"Aviation and radar issues have long been a major source of complaint for the wind industry. This is because wind turbines can interfere with radar systems and be a collision risk for low-flying aircraft. These concerns have resulted in a significant number of planning objections, particularly from the Ministry of Defence" (SDC 2005).

 

The main effect of wind turbines on air-traffic control radar is due to the rotation of the blades. The radar may 'illuminate' one turbine on one sweep, then a different one on the next sweep, producing shifting radar returns sometimes referred to as 'twinkling' on the radar screen. Usually this only occurs when the wind development is within line of sight of the radar. A planning objection is likely for any wind energy project within 67 km (37 nm) on such a line from air-traffic radar. Ground-based air-defence radar installations may lodge similar line of sight objections.

 

Interference with radar and remote sensing is not the only problem for military aircraft. Wind generators are now reaching 140m (500 feet) above ground level. This is not high compared to the normal flying height of most aircraft but for some it is. The military practise low flying for operational reasons (ground support and flying "under the radar"). Standard heights are 250 or 300 feet (Civil aircraft typically operate at 1000 feet around aerodromes except for approach and departure).

 

Developers are able to submit pre-planning enquiries to Defence Estates and out of the 4,000 pre-planning consultation requests received since 1996, around 2,000 have received “no objections” advice.    In total nearly half of the wind farms proposed so far in Britain have been successfully opposed by the MoD because of their proximity to air-defence according to David Wallace, vice-president, Royal Society (Nature, 428, 2004).

 

Another link with aviation is the need for warning lights which are required on onshore structures exceeding 150 m in height (TAN 8 2005). There are currently none in the UK but turbine development may soon break this barrier. Offshore wind up to the seaward limits of the territorial water and which is 60 m or more above the level of the sea at the highest tide  shall be fitted with at least one medium intensity steady red light as close as possible to the top of the fixed structure. Arrays of turbines require only peripheral machines to be lit (Directorate of Airspace Policy, 2003, The Lighting of Offshore Wind Turbines)

 

Military remote sensing may also be compromised in the context of the seismic monitoring of international compliance with the Comprehensive Test Ban Treaty. Wind turbines may generate vibration which can mask the seismic signals from nuclear weapons tests (see Section 13).

 

The report on aviation for the DTI, edited by Jago and Taylor (2002) is a useful source of information.