Clutter Free

October 8, 2012 by  
Filed under Wind Energy Tips

Much effort is being invested in addressing the problems of radar clutter caused by wind turbines. David Crisp, chief executive of Aveillant, offers a guide to co-existence with no compromise and no degradation

The aviation and wind industries face a mutual problem. Governments have committed to renewable energy targets that are driving wind energy development. To meet these targets, energy companies have often selected sites in the vicinity of airports or remote radar sites.

Unfortunately, wind turbines are difficult to differentiate from aircraft when detected by air traffic control primary radars. This can make it problematic for Air Navigation Service Providers (ANSPs) to provide a safe air traffic control service when aircraft are operating above or in the vicinity of a wind farm. 

Planning objections will only hold for so long. The global imperative for renewable energy is just too great.  There are two converging issues that make this an even more pressing problem. EUROCONTROL predicts that air traffic levels in Europe will almost double by 2030 at the same time the UK Department of Energy and Climate Change forecasts onshore wind farm deployment will increase by 250 per cent by 2020.

To ensure that airports continue to operate safely, while providing a safe and expeditious air traffic control service, a solution must be found. The UK Civil Aviation Authority has published CAP 764, Policy and Guidelines on Wind Turbines to assist aviation stakeholders understand and address wind energy related issues in their consideration of the potential impact and viability of proposed wind turbine developments.

With several technologies at various stages of development, all claiming to have the answer, ANSPs need to understand the functions and characteristics, strengths and weaknesses of each so that they can agree on a solution that will allow them to operate safely and at a reasonable cost.

The Problem

The issue lies with conventional primary surveillance radar (PSR) systems. Rotating at between 12-15 rpm, the primary purpose of a PSR is to detect non-cooperating targets and report their range and bearing.  Wind farms, the turbines of which are constantly rotating at different speeds and aspect angles (with respect to the direction of the radar beam); appear as ‘clutter’ on the radar display. Aircraft flying over the wind farm are indistinguishable from this clutter, thereby increasing the risk that separation between aircraft could be compromised.

A number of solutions have begun to emerge. As manufacturers promote their technologies to the wind farm industry, the developers are becoming more confident that they can win their case with the planning authorities.

Airports are not the only objectors, but they are in a strong position, though this may not last forever. Lodging an objection that makes development conditional on paying for a mitigation solution will seem reasonable to planning authorities who think in terms of ‘polluter pays’. The airport’s problem is solved, at no cost to the airport, and energy companies can get on with building wind farms and everyone can feel good about the proliferation of clean energy.

Cutting Through

Getting the wind farm operators to pay is half the battle. The other half is identifying and approving the right solution for an individual aerodrome and its surrounding region.

Our advice is to be thorough when investigating the solution. Wind farm operators may be seduced by technologies and promises that offer the prospect of overcoming planning objections for their particular wind farm. However, such solutions may or may not be suitable for other wind farms and consequently the airport’s region as a whole.

Our conversations with the industry suggest that airport operators and ANSPs are increasingly seeking advice on wind turbine mitigation. There is a lot of uncertainty, as well as conflicting interests. There is a clear need for unambiguous information with regard to the capabilities of these emerging technologies and a clear route to operational regulatory approval by the Civil Aviation Authority.

Mitigation or Solution?

There are a few organisations looking at this problem, including our own. I will outline the existing mitigation methodologies and where we believe their strengths and limits lie. It is worth pointing out that all the offerings in the market today can be split into two basic categories:, those that attempt to reduce the problem, that is ‘mitigate’ it,  and those that actually solve the core problem and completely eliminate it at source (radar detection).

Blanking and non-automatic initiation zones: Blanking simply means that primary radar returns over the whole area of the wind farm are suppressed or excluded from use in the radar tracker. The use of this approach requires careful assessment of the position of the wind farm relative to aircraft trajectories. Buffer zones around the blanked areas have to be established to allow sufficient time to recognize and react to unexpected movement as aircraft emerge from the blanked area. This method is often considered as an interim solution and is generally limited to areas where the consequences of missed information do not impact on safety, i.e. where procedures are in place to ensure aircraft separation.

High Range Resolution Radar: One mitigation technology under investigation by a number of companies utilises increased bandwidth to deliver high range resolution radar. Range resolution is the ability of a radar system to distinguish between two or more targets on the same bearing, but at different ranges, e.g. one behind the other. In theory, because the range resolution is enhanced, clear detection space is created between individual wind turbines rather than, as happens now, the individual turbines merging to form an area of impenetrable ‘clutter’. The idea being that as an aircraft passes over the wind farm the radar is able to detect it, albeit intermittently, as it should be visible  between the turbines and hence the radar tracker is able to maintain a continuous track on the radar display. 

Whilst the system is able to differentiate between targets at different ranges, it is unable to provide an equivalent improvement in bearing resolution, i.e. the ability to separate objects at the same range, but on slightly different bearings, e.g. side by side, on the circumference of a circle. A certain area is still subject to turbine interference. Consequently, it is difficult to provide the necessary assurance that aircraft can always be detected in normal operating conditions; the risk being that radar contact could be lost and endanger aircraft safety.

This problem is exacerbated with increasing distance of the wind farm from the radar; it is also exacerbated as more wind turbines are deployed. These issues limit the potential for this solution to low wind turbine densities deployed in low air traffic areas, what’s more this approach can only be effective at the expense of radio spectrum. A conventional airport PSR utilises ~1MHz of bandwidth. High Range Resolution Radar will require a significantly greater bandwidth of 30 to 50MHz to be effective. Demand for radio spectrum is intense so a solution that uses over 30 times the bandwidth of a standard PSR will be difficult to licence when other methods can be used equally or more effectively at a lower bandwidth, especially in the S-band.

Terrain Shielding Infill: This method involves siting an additional, typically S-Band, radar in such a position that the problem wind farm is shielded from the radar’s line of sight (LOS) by geographical features such as hills.  The additional Infill radar is left with a clear LOS of the airspace above the wind farm and since it cannot see the wind farm no interference arises.  Data from the Infill radar is fed to the airport’s ATM system and used to replace the area of ’clutter’ previously observed on the radar display.

This approach has been used on a couple of occasions but with mixed results. Merging data from two 2D radars, located at different positions, has proven problematic and causes positional inaccuracies leading to an increase in aircraft separation. There are two further problems:  firstly, lower airspace coverage below the line of sight of the hill will be lost in the area of the infill patch, meaning that aircraft at low altitude will not be detected. Secondly, this approach can only be effective for the area that the hills have ’shielded’ from the Infill radar; it is not a regional solution. If a wind farm is proposed at a site in another direction (where terrain provides no such shielding assistance) the airport is no nearer finding a solution.

Dual Beam or 2½D Radar: Dual beam radar, as its name implies, has two radar beams. It concurrently processes both high and low beams to discriminate turbines on the ground from higher altitude aircraft. Recent test results from UK En Route applications

Of such a system show acceptable performance when the elevation of an aircraft is 1.2 degrees above the wind turbine tips. This means that aircraft cannot be separated from turbine returns when they are less than 2,700ft above turbine tips at distances of 20 nautical miles. This limits the dual beam approach applicability for terminal approach applications, particularly when local topography is taken into consideration.  Again, it is not a practical regional solution since increasing amounts of lower airspace are lost as more wind farms are deployed. Use of a system like this requires replacement of the airport’s PSR.

3D Holographic Radar – the Aveillant solution: All of the previously described approaches have a common flaw; they use the very same radar technology as the original PSR whose problem they are trying to fix. Remember that the problem is the radar’s inability to differentiate between wind turbines and aircraft. Consequently these approaches have limitations, some severe enough to render them impractical.

In Holographic Radar Aveillant has developed a solution that overcomes the limitations of current mitigation technologies. Instead of increasing the resolution of existing radar or removing areas of concern, our 3D Holographic RadarTM actually differentiates between the returns produced by wind turbine  and those of aircraft.  The radar data processor is able to intelligently filter out wind turbine signals leaving just aircraft returns to be passed to the airport ATM system.

How do we do this? Whereas conventional radar rotates and sweeps a focussed beam 360° around the horizon once every four seconds, Aveillant technology employs a stationary planar array. Its transmitter illuminates the whole field of view all the time, while forming receiving beams in all directions and reports 4 times per second. To the non-expert we explain the former as analogous to sitting in a darkened room whilst swivelling around in an office chair holding a torch. The beam only sees a brief glimpse of a target once per rotation. With the Aveillant system – you uncap the torch bulb and allow it to light up the whole room – seeing the same 360°, but in three dimensions, all the time. As the reader can imagine identifying someone from a brief glimpse is very difficult compared to staring at them in dim light for a long time.  Rather  like the difference between a cartoon flip book and 3D cinema.

“Holographic Radar does not saturate and so can future proof an airport against future expansion of wind turbine deployment. Unlike conventional 2D radar, Holographic Radar’s 3D data is fused with the airport ATM system without any performance degradation”

As a result of the continuous staring  the Holographic Radar acquires a vastly better data sample of the targets it sees and consequently aircraft and wind turbine behaviours appear with very  different characteristics and are directly distinguishable.  Holographic Radar uses just 1MHz of bandwidth, vastly less bandwidth than high range resolution radar.  The operating range of our first commercial unit is 20 nautical miles enabling it to be located at the airport. Any wind farm within a 20nm range can be ‘processed out’, and can cope with 1 or 1,000 turbines. 

Holographic Radar does not saturate and so can future proof an airport against future expansion of wind turbine deployment. Unlike conventional 2D radar, Holographic Radar’s 3D data is fused with the airport ATM system without any performance degradation.  In the event an extended range is required, a second remote Holographic Radar can be deployed (off the airport) and its data also fused into the ATM system with zero degradation of the PSR performance.  Similarly  if there is a second overlooking radar (e.g. Ministry of Defence) the 3D Holographic Radar data can be shared and fused with that or an air defence radar network. 

We are currently at an advanced stage of development. Our development unit is performing fully as expected in field trials. Demonstrations will be held this year, and we will be conducting further field trials with our pre-production unit in Q2 of 2013. We are interested in hearing from any airport or wind farm operator who is interested in deploying this technology on a reduced cost, evaluation basis, ahead of the full product launch towards the end of 2013.

Crisp: Aveillant’s intention is to act as a long term service partner to ANSPs, dealing directly with the wind farms and allowing airports to proceed with confidence both financially and operationally

Naturally, we would talk up our own technology but we believe this is an area where there is much uncertainly and we hope to bring clarity to the issue. We believe that it’s vitally important

for airports to understand the different solutions on offer, and for them to take a considered and informed decision.

Aveillant innovation is not just limited to our technology; we are also taking an fresh approach to deployment where we will offer Holographic Radar as part of a turnkey service solution. 

Our intention is to act as a long term service partner to ANSPs, dealing directly with the wind farms and allowing airports to proceed with confidence both financially and operationally. We can provide support to current planning objections and help ‘future-proof’ airports with a long term solution rather than a mitigating “patch”.

 

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