United Kingdom

United Kingdom

Flight trials test solution to radar interference -- Srong indications that technical solutions are available

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Britain's Royal Air Force (RAF) aircraft are engaged in trials around four working wind farms in Wales to test pioneering new technology which the wind industry hopes will reduce the potential for interference with civil and military air traffic control radar from wind turbines.

The trials, near Newtown in Powys, are a collaboration between the Ministry of Defence (MOD), the Department of Trade and Industry (DTI) and the British Wind Energy Association (BWEA), with support from National Air Traffic Services and the Civil Aviation Authority. The aim is to test two technologies -- the Advanced Digital Tracker (ADT) developed by BAE Systems and the Sensis SPE-3000 by Selex SI, a subsidiary of Finmeccanica of Italy. Both systems are designed to track the flight of aircraft to distinguish it on radar screens from similar images -- or radar clutter -- which are caused by the rotating blades of wind turbines.

If the technology proves successful it could free up vast areas of the British countryside for wind development. Chris Tomlinson from the BWEA points out that UK skies are among the busiest in the world. He says that 50% of potential wind farm sites are affected by civil and military air traffic control radar. "There are huge geographic swathes of the UK that are omitted from wind development as a result of radar," he says.

Radar clutter

Currently, the MOD objects to all wind farms within line of sight of its radar. Interference on radar from wind turbines with blade tips moving at speeds of up to 200 miles per hour can take several forms: clutter, false tracks, shadowing and overhead obscuration.

Clutter is unwanted returns on radar screens which may obscure returns from aircraft. The rotating blades of wind turbines create an intermittent return and many turbines in a wind farm may be interpreted as a moving object.

False returns -- or scattering -- occur when rotating blades reflect radar waves in the atmosphere and may give false information to the radar operator. The effect has been observed at Copenhagen airport as a result of the offshore Middelgrunden wind farm which lies within ten kilometres of the runway.

Shadowing is caused when wind turbines lie in line of sight between a radar and an airborne target, thereby masking the target from detection. Overhead obscuration is where a "dead zone" is created above a wind farm which obscures small aircraft on radar screens. Both shadowing and overhead obscuration are mainly a problem for air defence radar.

For the trials, an MOD mobile Watchman radar was sited near four wind farms of varying scale with different types and sizes of turbine: Bryn Titli, Llandinam, Carno in Powys and Cefn Croes in Ceredigion. The RAF and an independently appointed safety team are conducting calibrated flights over the wind farms using a selection of different RAF aircraft.

The tests mark the second round of trials during the development of the tracking technology. In July 2005, the first flight trials took place, initially with the aim of testing the ADT technology, but Selex joined in to test its Sensis software for the last two days of the trial. The data from the earlier trials has been used to refine the technologies. "In order for the project to go to the next stage there needs to be a second trial in the same location to assess progress," says Tomlinson.

The trials will also build up a safety case for the technology, he says. "A safety case will need to be produced for each proposed wind farm on a site by site basis, but to increase confidence it is considered essential that we have a generic safety case for both products. It will give confidence to aviation regulators that the technology works and can meet their baseline safety requirements, and will enable developers to go ahead and invest and take the risk associated with developing projects."

Pioneering work

British energy Minister Malcolm Wicks points out that wind power is expected to make a major contribution to the UK target of 10% of electricity from renewable energy by 2010. "There are still hurdles to be overcome but one by one we are knocking down the barriers that could prevent us reaching that target," he says. Radar interference is a problem that has to be overcome all over the world, he says. "The pioneering work being done in Wales puts the UK at the forefront of the technology, with the inherent business benefits that can bring. It is encouraging to see government, industry and the military working closely together to find a solution."

Early signals from the trials are very positive, says Tomlinson. "We may have a solution, but it will never be a 100% solution," he adds. He points out that some years ago radar interference was perceived as a potential show stopper for further wind power development in the UK as the MOD took a more cautious approach to siting wind farms than its counterparts on the continent.

Now, however, the issue is surfacing in more countries around the world as air traffic controllers increasingly regard interference from wind farms as a problem for radar. In France, vast swathes of the countryside have become potential no-go areas for wind turbines and a ban on installations within 30 miles of radar stations is effectively in place (Windpower Monthly, April 2006). Concerns from the military also stopped a large offshore wind farm in Sweden last year (Windpower Monthly, January 2006) and in the United States hundreds of megawatts of wind projects are in limbo for the same reason (next story). "The UK is leading the technological development of mitigation solutions for wind turbines and radar to make sure they can live in harmony," says Tomlinson.

Issues remaining

While the trials look close to finding a solution that could satisfy air traffic controllers, there is still an issue with wind turbines and air defence radar. Six offshore wind farms in advanced stages of planning for the Greater Wash off England's east coast are caught up in the radar net. Last October the MOD told the wind industry it had found potential solutions to the problem in the Wash and, indeed, to projects throughout the country. "So you can build as many wind farms as you like within sight of air defence radar," Wing Commander Nicky Loveday told developers.

MOD trials and a study by BAE systems had revealed two different routes to resolving the problem. The industry's preferred option is a technical solution that would involve developing new hardware and software to change the way that radar signal processing works. According to the MOD's Wing Commander, Trevor Catmull, the technical solution looks promising. "But it is tied into radar replacement and upgrades," he says. And there is still a long way to go in terms of development. He accepts that timing of a technical solution will be tight for developers wishing to proceed with their offshore projects.

Additional radar

Alternatively, additional radar or radars could be deployed using e-tilt (electronic tilting of the beam) technology to bring obscuration down to manageable levels; this would supplement existing radar to compensate for interference from wind farms. If new signal processing technology cannot be deployed in time for the development of the offshore plant in the Wash, the use of additional radar could be a "back-stop solution," says Tomlinson.

But there are practical hurdles associated with both solutions that need to be crossed, warns Catmull. "Trials would have to be undertaken. The MOD needs to be 100 per cent convinced having done trials and testing that the effects of wind turbines were overcome before we could go ahead and give our consent," he says. "I understand that everyone wants a solution. We are not at that stage yet, but we are engaged with all stakeholders to energise that decision-making process and we are hoping within the next few months to make a decision. At this stage it is much too early to say what it will cost and how it will be delivered."

Catmull, like all the parties involved in resolving the radar issue, stresses the collaborative approach to the problem. "One of the positives is that everyone is sharing information. The DTI, MOD and industry associations like BWEA are all working hard in one place."

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