Technology Today

2012 Issue 2

Air Traffic Control Wind Farm Interference Mitigation at Raytheon - The past decade has seen a tremendous growth of wind turbine farms. While the benefits of wind energy are apparent, the potential hazards are less onvious. These hazards include the inherent tendency of wind turbines to severely interfere with air traffic control radars. Raytheon, a world leader in air traffic control radar technology, is applying its resources to solve this problem.

Air Traffic Control Wind Farm Interference Mitigation at Raytheon - The past decade has seen a tremendous growth of wind turbine farms. While the benefits of wind energy are apparent, the potential hazards are less onvious. These hazards include the inherent tendency of wind turbines to severely interfere with air traffic control radars. Raytheon, a world leader in air traffic control radar technology, is applying its resources to solve this problem.

The wind turbine interference problem has drawn considerable attention in recent years. A number of mitigation approaches have been proposed and pursued, ranging from special anti-reflective coatings on wind turbine blades, to the fielding of additional gap-fill radars in the vicinity of the air traffic control (ATC) radar site, to data suppression algorithms that edit out target detections or inhibit track formation in wind farm areas. These solutions have demonstrated only limited success to date, as they tend to mask the problem instead of solving it; moreover, most involve excessive cost as well as further technology development.

ATC radars generally fall into two categories: primary surveillance radars (PSRs) that detect echoes of transmitted electromagnetic pulses to identify targets in the surveillance area, and secondary surveillance radars (SSRs) that send out coded messages and receive replies from aircraft equipped with appropriate electronic transponders. The data collected by PSRs and SSRs is usually combined in an automation system that generates an airspace picture used by controllers to maintain separation between aircraft. Raytheon is a world supplier of PSRs, SSRs and automation systems.

Table 1

The wind turbine RF reflection problem significantly affects PSRs. PSRs find targets principally by discriminating moving objects within the imaged space using the Doppler shift imparted to the radar pulse echo by the target. Rotating wind turbine blades can produce echoes with the same Doppler frequency offsets as aircraft. As a result, the radar echo from a wind turbine looks like a real aircraft to the radar. This may potentially result in the generation of many false tracks, the dropping of real tracks and the displacement of real tracks to false locations on the air traffic control display. Unless this problem is solved, these effects can result in dramatic restrictions being imposed on air traffic. The construction of new wind farms may even compromise flight safety. Large tracts of airspace above wind farms are currently designated as "no-fly zones" because local ATC radars are effectively blind in these areas. In the U.K. alone, more than 30 gigawatts of wind power are currently prevented from coming on-line because of objections raised by air navigation service providers.

The Raytheon Solution for Wind Turbine Interference

Over the past two years, under contract to the U.K. National Air Traffic Services (NATS), Raytheon has developed a solution to the wind farm interference problem for ATC radars — one that cures the problem instead of masking it. This solution relies on four major enhancements in the PSR signal processing chain that help the radar differentiate valid aircraft returns from false turbine returns. The solution has been incorporated into a prototype radar modification kit that was fielded and tested over extended time intervals at three Raytheon ATC radar sites located in dense wind turbine environments. These tests produced consistently excellent results (high detection probability and low false alarm rate).

The processing enhancements (Table 1) span the full extent of the radar data processing chain. The first key enhancement is the simultaneous processing of data from multiple receiver beams to improve the probability of detection. In a traditional PSR design, radar processing uses returns from one antenna beam feed for near-range processing, and then transitions to a second beam for far-range processing. This transition facilitates ground clutter rejection for near-range targets while maximizing energy on target for distant objects. Raytheon has demonstrated that by processing data from both beams throughout the instrumented range of the radar, and optimally combining detection information from the two data streams, significant performance gains can be achieved, especially in high-clutter areas.

The second key enhancement involves advances in the real-time characterization of the local radar clutter environment. PSRs typically generate and continually update a clutter map of the imaged space. This map represents the strength of radar returns in any given range/azimuth cell from stationary objects. The prevailing clutter levels in a given area are used to establish detection thresholds such that when detection thresholds are exceeded, a high likelihood exists that a target is located in the area of interest. The Raytheon wind farm mitigation radar upgrade extends this technique to characterize not only clutter from stationary objects, but also clutter from moving objects such as wind turbine blades. By doing so, many more appropriate detection thresholds can be produced for cells containing wind turbines, which, in turn, dramatically reduces false detections caused by turbine blades.

Figure 1

The third processing improvement also relates to the calculation of better detection thresholds. Part of the calculation to establish a detection threshold for a range/azimuth cell involves generating an estimate of not only the clutter level at that cell location in the recent past (as referred to above), but also the noise and clutter level in the near vicinity of the cell at the present time. This component of the detection threshold calculation can be thrown off by the proximity of a wind turbine to the cell of interest; the returns from the turbine can bias the detection threshold to a detrimentally high level. This results in the radar becoming desensitized in the vicinity of wind turbines, losing the ability to identify small targets in these areas. The Raytheon solution detects and suppresses the influence of strong reflectors, in particular wind turbines, to significantly reduce the desensitization of the radar in these areas.

The final enhancement provides powerful mitigation effects at the back end of the radar processing chain. Drawing from extensive experience in the tracking of small targets in high-clutter environments for maritime and over-the-horizon applications, Raytheon has developed a sophisticated target tracker that forms a final line of defense against wind turbine interference. Based on both live real-time radar target detections and a priori information about the radar environment, this tracker intelligently identifies areas of high clutter and high wind turbine activity, and then modifies processing algorithms to maximize the likelihood of preserving real targets and rejecting false ones. As an example, the near proximity of a wind turbine may cause the PSR tracker to require additional corroboration (e.g., additional detections in subsequent radar scans) before declaring a valid target, or to tighten up the boundaries within which a target echo can be associated with a track. The tracker also simultaneously maintains multiple models of target characteristics for each tracked target, and based on error measurements, combines the outputs of these models in a statistically optimal way to yield a more accurate estimate of target position and velocity than is possible with conventional tracking algorithms.

Table 2

Field Test Results for the Raytheon Solution

In 2010–2011, Raytheon's wind farm mitigation kit was installed at three active ATC radar sites in close proximity to large wind turbine farms in the U.S., Holland (Figure 1) and Scotland. The site in Holland had more than 1,450 wind turbines within the instrumented range of the radar, while the Scotland radar illuminates Whitelee, the second largest capacity wind farm in Europe. Typical performance test results at these sites, located directly over dense wind farm areas with and without mitigation, are summarized in Table 2. In each case, radar probability of detection was below acceptable specifications without mitigation, but was improved to well above specification with mitigation. Typical ATC customer specifications require a probability of detection of 80 to 90 percent.

A qualitative illustration of the degree of improvement in the radar picture with wind farm mitigation applied is shown in Figure 2. This snapshot shows the live data radar track output at Soesterberg, Holland, with conventional ATC radar processing in the left panel and enhanced processing in the right panel. Current target positions are shown by dark color dots, with the trails behind these dots indicating track history for the target. Known wind turbine locations are depicted as green circles.

The panel on the left contains an overwhelming number of false tracks generated by wind turbine returns that obscure real tracks and clutter the display. Close examination shows that several of the real tracks passing smoothly and continuously through the wind turbine area in the enhanced processing snapshot (right panel) suffer track loss, track discontinuity, or track seduction to false return locations with conventional processing (left panel). Two such instances, identified as Track A and Track B, are highlighted in Figure 2.

Figure 2

Deployment of the Raytheon Wind Farm Mitigation Solution

The Raytheon wind farm interference mitigation solution is now in the deployment phase. The first production deployment modification kit was installed for the Royal Netherlands Air Force at Woensdrecht Air Force Base in Holland in the summer of 2012. Four additional systems are expected to be fielded in Holland in 2013, and discussions are under way with defense and civil aviation agencies in numerous countries (including the U.S., U.K. and Canada) for upgrades and new radar system installations. Four live radar demonstrations of this technology are being conducted at ATC radar sites in the U.S. for the Federal Aviation Administration and the U.S. Departments of Defense, Homeland Security and Energy.

The Future of ATC Radar and Wind Energy

Having successfully proven this technology at Woensdrecht Air Force Base, Raytheon is preparing to deliver it in newly deployed PSRs as an upgrade kit to already installed Raytheon radars, and even (in limited form) as an add-on to third-party ATC radars. Raytheon is building upon the success of the wind farm solution by adding concurrent weather radar processing capabilities, with plans for deploying mitigation-equipped systems to extend the solution to new application areas throughout this decade and beyond. This is expected to relax the current restrictions on the expansion of alternative energy initiatives, while improving flight safety and facilitating air travel.

Andrew Shchuka, Inderbir Sandhu
Contributors: Oliver Hubbard, Derek Yee,
Jian Wang, Jonathan VanVeen,
Mike Waters, Brad Fournier

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